Artificial Radioactivity Citation Ridenour, Louis Nicot (1936) Artificial Radioactivity. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/h3xf-m563. https://resolver.caltech.edu/CaltechTHESIS:11132020-185208297 Abstract No abstract. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Physics Degree Grantor: California Institute of Technology Division: Physics, Mathematics and Astronomy Major Option: Physics Thesis Availability: Public (worldwide access) Research Advisor(s): Yost, Don M. Thesis Committee: Yost, Don M. (chair) Defense Date: 1 January 1936 Funders: Funding Agency Grant Number Seeley W. Mudd Fund UNSPECIFIED Record Number: CaltechTHESIS:11132020-185208297 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:11132020-185208297 DOI: 10.7907/h3xf-m563 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 14000 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 16 Nov 2020 19:29 Last Modified: 16 Nov 2020 19:29 Thesis Files PDF - Final Version See Usage Policy. 31MB Repository Staff Only: item control page

ARTIFICIAL RADIOACTIVITY A Thesis by LOUIS NICOT RIDENOUR In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosopey CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 19:,6 ARTIFICIAL RADIOACTIVITY ********* HISTORICAL lli'TRODUCTION On January 15, 19;4, I. Curie and F. Jol iot 1 presented the first good evidence for the phenomenon which, under the name of II induced II or 11 artificial II radioactivity, had been sought since the discovery of the natural activity of uranium by Becquerel in 1896. In the course of their investigations of the positive electron discovered leas than two years previously, they found that when certain light elements ( B, Mg, Al) were bombarded with the alphaparticles from polonium, they emitted positrons in large numbers. Their discovery of artificial radioactivity was made when they observed that this positron emission did not cease immediately on the removal of the source of alphaparticles, but instead decayed according to an exponential law, with a half-life characteristic of the substance bombarded, and of several minutes duration. They found that the growth of the activity under alpha-particle bombardmenit followed the usual exponential curve, the transformation constant beingthe same for growth and decay. After irradiation for a period long compared with the half-life, the induced activity observed in a sample reached a saturation value which could not be increased by further irradiation. Curie and Joliet ascribed the radioactivity induced in boron to Nl3, an isotope of nitrogen not known in nature, and predicted that the same isotope could be formed by the bombardment of carbon by deuterons, according to the reaction Thia prediction proved to be correct, and led to the first production of radioelements by totally artificial means, without the use of a naturally radioactive source of alpha-particles. Crane and Lauritsen, 2 Henderson, Living- aton, , and Lawrence,3 and later Cockcrort, 4 verified that an artificial radioelement -1- was produced by the deuteron bombardment of carbon, but the half-life as measured by them differed markedly from that reported by Curie and Joliot for the product of the alpha-particle bombardment of boron. Thie discrepancy was later shown to have been false, the radioelement being N15 in each case. Cockcroft, Gilbert, and Walton 5 , and Crane and Lauritsen, 6 had meanwhile demonstrated that the same N15 was formed by the proton bombardment of carbon, while Crane and Lauritsen had also found that boron displayed a radioactivity under deuteron 2 and proton 6 bombardment which they correctly attributed to c11 • It was apparent that at the voltages available (up to about 5 mev.) for accelerating protons and deuterons, and with the energies possessed by _alphaparticles obtainable from naturally radioactive sources, the nuclear transformations necessary to produce artificial radioelements could be stimulated only in the lightest elements; viz, those having the lowest potential barriers opposed to the entry of charged particles into the nucleus. Fermi, 7 therefore, tried the effect of neutron bombardment of various elements in the hope of producing artificial radioactivity. From the first, this method waa successful, and he and hie co-workers were able to produce transmutations leading to artificial radioelements in many elements, including even uranium. In a communication8 dated October 22, 1934, Fermi and his co-workers reported a great increase in the activation of silver under neutron bombardment when the source and the silver were surrounded by quantities of water or paraffin. This was shown to be a specific effect of a substance containing hydrogen, although exhibited in a lesser degree by carbon, iron, lead, and silicon. It was correctly attributed by Fermi to the slowing down of the neutrons by collision with hydrogen nuclei, the resulting slow neutrons having a greater probability of capture than fast neutrons. This phenomenon has proved to be of great importance in the study of nuclear reactions. During the past year, the investigations in the field of artificial radioactivity have been concerned with the discoveFJ of new radioelements made -2- in the three ways mentioned above, and the cor reiation of data concerning them. Advances in our understanding of the effects due to slow neutrons have also , been made, al though this subject is by no means closed as yet. The present paper is an attempt to treat in detain the knowledge of the whole subject down to the end of August, 1955. THE ARTIFICIAL RADIOELEMENTS 1. General Stability Considerations: It has long been a subject of remark that in a diagram whose ordinate is atomic weight, or the difference between atomic weight and atomic number, and whose abscissa is atomic number, the points representing the stable isotopes found in nature and measured in the massspectrograph lie in a well-defined region (Figure L) As this can hardly be regarded as being fortuitous, we should consider the stable isotopes as defining a region of nuclear stability outside of which lie nuclei which can be formed by the expenditure of energy, but which, once formed, tend to return to the region of stability by the expulsion of an alpha-particle, by the transformation of a nuclear neutron into a proton with the consequent expulsion of a betaparticle, or by the reverse transformation of a nuclear proton into a neutron, with th~mission of a positron. The rules of isotopic statistics, 9 particularly those concerning the paucity of isotopes of elements with odd atomic number and that of isotopes with odd A - Z, where A is mass-number and Z is charge-number, doubtless have an important bearing, whose meaning has not been completely elucidated as yet, on the matter of nuclear stability. The problems of stability 11 of nuclei have been examined in detail by Heiaenberg, 10 and by Gamow; here a brief and completely qualitative discussion will suffice. In Figure 2 the stable isotopes up to 01 are plotted as circles, and the artificial radioelements in this region of the periodic table are plotted as squares, the arrows indicating their mode of' formation. The diagram will be better understood after reference to Section;, below, which it is designed to illustrate, but here it should be noted that the stable isotopes lie in an extremely regular grouping, and that all of the known artificial radioelements lie in such a position that they can transform themselves into stable isotopes by the emission of a positron or of an electron, and that in every case they do emit the appropriate particle. It is also to be noted that those unstable nuclei which lie above and to the left of the line defining the stable isotopes emit electrons to return to the region of stability, while those belov, and to the right emit positrons, as is indeed demanded by their positions on the diagram. Among the heavier elements are known cases in which the emission either of a positron or an electron would return the artificial radioelement to a stable form; such cases are pointed out in Section 5. 2. Chemical Identification of the Radioactive Isotope: It is obviously of the utmost importance to determine the chemical nature of an artificial radioelemert. Manifestly, it must be an isotope of an already known element in almo st all cases, for the chemical properties of an atom are determined almost entirely by the charge on its nucleus, without regard to the structural stability of this nucleus. It is equally clear that the moment a disintegration leading to the formation of a new and unstable nucleus takes place, the newly-born atom has properties determined by its new charge-number and will behave in all chemical respects like the element of which it is an isotope. The chemical identification of artificial radioelements is based on the well-known fact that isotopes are not appreciably separated by ordinary chemical reactions. Thus, if an element consists of the fraction A of isotope ~, and (1 - A) of isotope b, and if the fraction B of the total enters into a chemical reaction, then BA and B(l - A) will be the fractions of each isotope that have reacted. While the chemical behavior of the few hundred thousands or few millions of atoms of an artificial radioelement formed in a nuclear reaction may be erratic and uncertain, if a quantity of the element with which the radioele:msnt -4- is suspected of being isotopic is added, and then completely separated out chemically and examined for activity, it can be definitely established whether or not the radioelement is isotopic with the element in question; for, if the isotopy exists, all of the activity will have been separated out in the removal of the element under examination. 'Ihe procedures involved may best be illustrated by means of examples; the radioactive products of the bombardment of carbon by deuterons and of boron by deuterons were carefully investigated by Yost, Ridenour, and Shinohara, 12 and their results are given below. Experimental Method Carbon was bombarded in the form of graphite and boron in the form of fused boric oxide by 0.8 and 0.9 mev. deuterons accelerated in the tube developed by Crane and Lauritsen for nuclear investigations. amperes were usually employed. 15 currents of about 40 micro- The presence, amount, and half-life of the radiO- active material were determined by quartz-fiber electroscopes constructed and kindly loaned by Professor Lauritsen, or by means of a Geiger counterwhose cylindrical electrode was of aluminum about 0.2 mm. thick. The experiments were always conducted on a gaseous fonn of the radioelement; in the case of boron this was prepared simply by heating the boric oxide target to about 600° in a closed vessel and washing the resulting gases into a gasometer with air. In the case of carbon, a thin layer containing practically all the activity was scraped off the upper surface of the target and burned with a bout 200 cc. of air in a platinum boat in a quartz combustion tube. the combustion was supplied fran a specially constructed gas pipette. Air for Small amounts of other gases could be added to the air used to burn the carbon or to the gases present in the gasometer in the experiments on boron. The following experiments were carried out; when any gas other than air waa present, the fact is so stated. Carbon bombarded by deuterons: The gases from the combustion of the carbon were passed first over heated copper oxide in all cases. '.L'h ey were suosequently passed through a train of solid or liquid absorbents, or they were colJ.ected over concentrated su1f'uric acid in a second gas pipette s1milar to the first. tl) The exit gas was passed through a train consisting of solid I OH tor soda lime}, potash bulbs filled with 50 percent KOH, OaCl not absorbed was active. The solid KOH and CaC1 and P O • The gas 2 2 5 showed no activity. This 2 experiment eliminates H, Li, Be, Band C as the active substances. The hydrogen would have been burned to H 0 and this would have mixed with the water of the 2 KOH· solution. The small amount of water vapor from the KOH solution would have been absorbed by 0ao1 room temperatures. or P o • The oxides of Li, Be and Bare not volatile at 2 2 5 The hydrides of boron are bumed by heated air and decomposed o, by alkali with the formation of B or its salts. 16 Carbon was present as CO 2 2 and this is readily absorbed by the alkalis present in the train. (2) The exit gases were passed through alkaline pprogallol and then col- lected in the pipette. 'lhe resulting gas was active. This result eliminates oxygen as o , since it is absorbed by the alkaline pyrogallol. 2 If the oxygen were present as N0 , it would undergo complete absorption in the potash. 2 (,) The gases resulting from the combustion of the graphite by a heliun- . air mixture were collected over concentrated sulfuric acid in the pipette. By passing the gas repeatedly (10 to 15 times) through alkaline pyrogallol it was treed from o and 00 • 1he gas so treated was strongly radioactive. It was 2 2 next passed i4flto an evacuated tube containing metallic calcium shavings and the calcium was then heated with a Bunsen burner. took place. Extensive absorption of the gases The tube was again evacuated to remove the helium and any other gases not absorbed by the calcium. After the contents of the tube had cooled and air had been admitted, the solid contents were removed and ground to a powder in a mortar. The powder showed strong activity. -6- '!his result eliminates helium as the source of' the activity, since it would be removed with the ordinary helium when the tube was evacuated. The powder was then placed in a glass tube equipped with a dropping funnel containing water. The gases evolved on adding water were radioactive. This result eliminates oxygen ( if originally present as NO) and fluorine if present as the very unreactive CF or c F • The oxygen would not be liberated 4 2 6 as a gaseous substance 1n this procedure. It' the fluorides of carbon we-re originally present, they would have been pmiped out if they had not reacted with calcium, and they would have been converted to calcium fluoride and carbide had they so reactedJ the calcium fluoride does not yield a volatile product in the alkaline solution such as existed on addition of water to the powder. Nitrogen would have absorbed as Oaf , and this substance, on treatment with water, yields ammonia. 2 '!he combined results of t~e chemical experiments eliminate not only all the elements of the first row of the periodic system except nitrogen as the source of the radioactivity, but also all others. On physical grounds it is possibB to state that the activity must be due to one of the element·s H, He, Li, ·Be, B, C or N (unleee the possibility of the emission of an electron from the carbon nucleus during bombardment is admitted, 1n which case O must be a possibility; no example of such a reaction has been established), since the total nuclear charge entering into the reaction producing the radioactive subst~ce is 7• The total mass entering into the reaction is probably 14 and possibly 15 (although the abundance of c13 is very small), and we may certainly presume from the experiments on recoil radioactive nuclei4 that the mass number of the radioactive product is surely not larger than 14 and is probably 1;. J'naamuch as the only particles emitted from carbon under deuteron bombardment in observable numbers are known to be neutrons and protons, the radioactivity is probably due to C or N. Since we may assume in analogy with all cases of artificial radioactivity that the active substance is an isotope of mass number different fran those known -7- by mass spectra, it is likely to be 014 or N1;, and the latter is the more probable on the ground of the rarity of c1;, as well as from the fact that it emits positrons. The present experiment seems definitely to have established that it is nitrogen, and we may then infer that it is likely to be Nl;. Measurements on the half-life of the active gas at various stages in the chemical treatment yielded concordant values around 10.5 minutes, indicating that the activity is due to a single substance. Boron bombarded by deuterons: The total mass entering into this reaction 11 may be 12 or 1;, corresponding to a10 or a • The total charge is 6, and we rt/3..y again for safety .admit the possibility of the emission of a negative electron from the nucleus. On physical grounds, the activity may, then, be due to H, He, Li, Be, B, O or N. Since a heavy particle must be emitted in the creation of 5 the radioelement, the total mass of the product must be 12 or smaller. The chemical experiments were as follCtRs: (1) A small amount of carbon dioxide was added to a sample of gas collected in the gascmeter. One-half of the gas was tested and found to be active. The other half was passed repeatedly through KOH solution, and the resulting gas showed an activity of about one-third that of the first half, after correcting for known decay which took place while the operation was carried out. the active materials evidently had been absorbed by the potash. Some of. Since Li and Be compounds are involatUe, these elements are eliminated as possible sources of all the activity. (2) To a 150 cc. sample of gas in the gasometer about 10 cc. of carbon monoxide were added, and the mixture was then passed over heated cupric oxide. Half of the resulting gas was tested with an electroscope and found to be active. The other half was passed several times through 50 percent KOH solution and then into the electroscope. The resulting gas was inactive. These two results eliminate B, N and o as possibilities, since the volatile hydrides of B would 2 2 burn to B o; on the hot OuO or be decomposed by the alkali\6 and N and o would 2 2 2 -8- not be absorbed by KOH. (;) Experiment 2 was repeated af'ter 10 cc. of helium had been added to the gas sample in the gasometer. The activity was absorbed by KOH. 'lhia result excludes He as the source of the activity since it is not absorbed by alkalies. (4) A sample of gas to which 00 and NO had been added was passed over hot 2 CuO and through a strong solution of KMno gas was active. 4 in ; N sulfuric acid. The resulting The experiment was repeated but without passing over hot euo. The result was the same. '.these results excluded H , and N as NO or N0 , for 2 2 hydrogen would burn to water and be mixed with the water in the permanganate solution; NO or NO would be oxidized completely to HNO by the acidified per2 ; manganate. 17 Fluorine, if present as BF;, would be completely hydrolyzed and absorbed by the aqueous solution. Boron, if present as volatile hydrides, would be oxidized and decomposed by the heated OuO or hydrolyzed by the solution. The results of the chemical exper:bnents eliminate all elements in the first row of the periodic system except carbon. The gas driven from the boric oxide target has the properties of a mixture of carbon dioxide and carbon monoxide. Just why the newly formed carbon atoms should be oxidized partly to monotCide and partly to dioxide is not evident. It was not detennined whether the two compounds were always formed in the same proportions. Measurenents on the half-life of the radioactive gas both before and aft.er chemical treatment yielded the value 20.5 minutes 1n all cases, indicating 12 that the activity is due to a single substance. Since 0 is stable, on the physical grounds outlined above, this substance is probably 011 • When the artificial radioelement produced in a nuclear reaction is an isotope of a completely unknown element, or of one which is so rare that none can be added to the radioelement to stabilize its chemical properties, the matter of chemical identification is a great deal more difficult. -9- Such a case occurs in the bombardment of uranium by neutrons; Fermi., who first studied this reaction., early concluded 18 that at least one of the half-lives observed belonged to an elE111ent of charge-number 9~., for its chemical reactions did not seem to be those of any of the known elements fran 86 to 92 incluside. 'lhie point of view was criticized by v. Grosse and AgruS'S, l9 who thought that the experiments of' Fermi did not exclude the possibility that the ra:dioelement in question might be isotopic with element 91., whose isotopes occuring in nature -protactinium and UX2-- are both veey rare. New experiments by Ferrni~and Hahn . and Meitner 21 seemed to indicate that not only element 9, was formed., but also element.a 94 and 95 were formed in the disintegration of uranium by neutrons., a cha:ln of radioactive products apparently being :initiated• . Interesting result, e have been obtained also in the bombardment of thorium by neutrona, 20122 and our whole knowledge of the effects of neutron bombardment of the natural radioactive element.a, sketchy as yet ( see Section ( 5), where the reactions for each element. are treated :in detail ),may be expected to shed some light on the problsns of the uranium.,· actinium, and thorium series, and perhaps on the missing elements 85 and 87. 3. Concentration of the Artificial Radioelements: It is evident that if the radioelement produced in a nuclear reaction is not isotopic with the bombarded element., ave~ effective concentration of the radioactivity can be performed by separating out the active element by means of an appropriate chsnical reaction, a few milligrams of the element with which the radioelement is isotopic having been added to stabilize the chemical behavior of the small number of active atoms. Thie self-evident procedure has often been snployed. Another useful mans of concentration of a radioelement differing chemically f'rom its parent, which has a much more limited range of application, is that of electrochemical deposition. Thie has been employed by Haisainelcy 2; in the concentration of radiocopper, produced by neutron bombardment of' zinc. 1h order that this means may be applicable, the radioelement must be electro- chemically more noble than its parent. In the case which has occurred moat frequently to date in the pro- duction of artificial radioelements -- that in which the radioelement is isotopic with its parent -- the situation is by no means so hopeless as ie the chemical separation of ordinary isotopes. Szilard and Chalmers 24 first showed that active iodine could be separated from ordimry iodine by making use of the fact that when 1128 is formed by the capture of a neutron by 11 27, the struck nuclei may have been removed f'rom the compound containing the I being irradiated. Whether the atoms f'reed in this way will interchange with their isotopes bound in the irradiated chemical compound will depend on the nature of the chemical compourd with which we are dealing. To prevent such interchange, Szilard and Chalmers first worked with non-ionizing organic compounds, such as ethyl iodide and bromoform, with a trace of :£4-ee halogen added to protect the radioactive isotope. Af"ter the irradiation, the free iodine or bromine was reduced and precipitated as a silver 20 halide. Fermi has prevented the interchange of radioactive and ordinary chloriIB atoms by irradiating a compound in which the chlorine is bound in a radical which, once destroyed., has a negligible probability of being rebuilt. If sodium chlorate is irradiated., a trace of Cl- added, and AgCl precipitated, care being taken to prevent the precipitation of AgClO~, from 70'/o to 90'/o of the activity is concen= trated in the precipitate. Similar results can be obtained with bromates and iodates. Manganese dioxide precipitated from irradiated potassium permanganate solution carries a large part. of the activity due to radiomanganese. Physical methods of separation depending on the fact that the a tom is likely to have lost one or more extl'anuclear electrons at the moment of forn:ation, and hence can be collected by an electrostatic field, have also been employed by Fermi 20 and by Paneth 25 . for the concentration of radioactive isotopes of the bombarded element. The phenomenon of recoil of the struck nucleus at the moment of formation of an artificially radioactive nucleus was first observed by Werten- 11 - ste:m26 in the case of radio~uorine formed fran atmospheric nitrogen by alpha-particle bombardment, and has since been observed in many cases where a heavy particle is emitted in the formation of an artificial radioelement.4, 2 7 The possibility of concentrating an artificial radioelement makes artificial radioactivity useful as a detector of neutrons which can be observed only in the presence of gamma-radiation so intense that electrical methods of neutron detection are not feasible. hara, and Yost ~ This procedure was used by Ridenour, Shino- . 9 in their experiments on the disintegratiop of Be by x-rays; their results are given below in illustration of the value of the method. The Disintegration of Berylli,:im by Photons and its Possible Bearing on the Mass of Be9 Szilard and Chalmers29 found that when beryllium was b~barded with the gamma-rays of radium in equilibrium with its decay products, neutrons were liberated, and could be detected by muns of the radioactivity they excited in iodine. Meitner;o showed that these neutrons excited radioactivity in I, Au and Ag, but not in ~, Si and Al. The former reactions involve the capture of a neutron, which, as Fenn1 20 has shown, is most probable when the neutrons have 1 ittle energy; while the latter three reactions involve the emission of an alphaparticle or proton, and probably have a higher probability when the bombarding neutrons have greater energ:ie s. Brasch and others, jl working with x-rays, have obtained this disintegration at voltages between 1.5 and 2 m:ev. ln the present experiment, beryllium was bombarded with the x-rays from the tube in the Kellogg Radiation Laboratory. The tube is self-rectifying, and was supplied with 50 cycle a.c. at 0.9 mev. peak. target was 2 m.a. The electron current to the The beryllium was located directly behind the tungsten target, in a bomb which was lOflered inside the electrode. 'lhe ethyl iodide used as a detector for the neutrons was also in the bomb, just above the beryllium and in - 12 - a Dewar vessel to prevent the heat generated at the target fran reaching the ethyl iodide in the course of the irradiation, which lasted 40 minutes. The active iodine was separated from the ethyl iodide by the method of Szilard and Chalmers. 24 200 g. of beryllium and '575 cc of ethyl iodide were employed; the inside diameter of the bomb was 6.2 cm., and its overall length 25 cm. The in- 4 tensity of radiation at 1 cm. behind the target is calculated to be 2.5 x 10 r/min., on the basis of the measured intensity at 50 cm. from the target and the known filtration in both cases. Thee amples of iodine separated after irradiation were tested for activity with a quartz-fiber electroscope loaned by Professor c. c. Lauritsen. There was no increase over the backgrrund on the introduction of a sample. To check the sensitivity of the method, a test run was made with ;70 mg. of radium placed under the banb in the position occupied by the target of the x-ray tube in the first experiment. The activation of the iodine in this case was easily JDl9asur- able, initially more than doubling the cosmic-ray background. This poor yield in the case of the radium is to be attributed to the unfavorable geometrical conditions of necessity offered by the bomb; a much larger effect can be obtained by increasitg the solid angles of irradiation. Taking into consideration the relative intensities of the two sources of radiation, it may be said that an activation produced by the x-rays of one-onethousandth the magnitude of that produced by the gamma-rays of radi~ would have been detected in this experment. Thie may be perhaps reconciled with the statement 2 of Gentner,' that 0.9 mev. radiation is most effective in the disintegration of beryllium, if one presumes tmt the efficiency of the process falls off sharply on the low-energy side of O.? mev., for the number of 0.9 mev. quanta in the radist. ion produced in an x-ray tube excited with a.c. at 0.9 mev. peak is very small. Of one is to suppose that beryllium is disintegrated into two alpha- particles and a neutron by a high energy photon, and that the enormously reduced yield just below a million volts found in-this experiment indicates the existence of a genuine threshold for the phenom~non, then it seems possible that the mass of Be9 is too high. If we fix the threshold at 0.9 mev. and take the mass of' the neutron as 1.0080, then, on our assumptions, the mass of Be9 must be 9.0114, a result which explains the anOt111ly of the stability of' Be9 , as well as Bonner and Brubaker's ;; results on the energy of' the neutrons from Be bombarded with deuterons. **************** 4. The Reactions Leading to the Formation of Artificial Radioelement.s: There are only eight type-reactions which need be adduced to explain the formation of about 100 artificial radioelements known to dat~, with one possible exception which will be considered under the paragraph on bromine in the next Section. Each reaction has been studied in sufficient detail so that there can be little doubt that it is an established one. It should be always explicitly borne in mind tm t in all these reactions the conservation. of energy in the broad sense is satisfied, so that in reaction VIII, for example, a gamma. ray of the appropriate energy must always be emitted. A. REAOT IONS PRODUCED BY ALPHA PART IOLE B~BARIMENT: I. Capture of the alpha particle with emission of a neutron. xA + He4~ · , yA+?+ n1 followed by Z 2 Z+ 2 0 yA+:5➔ Z+2 Z+l wA~+ e l This is historically the first reaction of artificial radioactivity, and being that observed by Curie and Joliot1 and aluminum bombar~d by alpha-particles. in the cases of' boron They thought that this reaction also led to the 2.; minute period observed in magnesium bombarded with alpha-particles, and although this reaction doubtless does take place in Mg+ n, the period of the positron emission is unknown, . the ·2.;minute period being due to Al 28 , formed in accordance with reaction II, belOlf. (See the paragraph on magnesium in Section 5.) In all the cases known in which reaction I takes place, the al- . terna.tive emission of a pr-otGn, leading at once to the stable isotope - 14 - Z+l wA -r;, also takes place, and with a much greater probability. The successive emission of a neutron and a positron, or the immediate emission of a proton on alpha-particle bombardment may be thought of as alternative routes between the two stable isotopes z-x_A and Z + wA +;, 1 must be equal. II. and the energy expended along both routes Capture of the alpha particle with emission of a proton. This reaction which was discussed in tm last paragraph tor the case where it gives rise to a stable isotope, is known in one case to give rise to an artificial radioelement. B. REACTIONS PRODUCED BY DEUTERON BCMBARDMENT III. capture of the deuteron with emission of a neutron. 2 __A zx-- + 1 H ➔ Z+lW A+l 1 + 0 n, followed by Z+lW A+l ~ ZX .A+1 +e + This is historically the earliest reaction involving deuterons. It gives rise 1n tm case of carbon bombarded by deuterons to Nl;, the same radioelement which is produced in boron by alpha-particle banbardment. Only two cases of this reaction may be regarded as certain, the radioactivity 2 reported by early investigators, ' ; in Be, Al, etc., being probably due to contamination. rv. capture of the deuteron with Ellliesion of a prat.on. xA + H2 ~ xA+l +- z 1 Z H1 1 ' followed bv xA+l_,,,, ~ Z z+l wA+l -t- e- '!his reaction, first studied by Lawrence, 28 gives rise, in the elements of snaller charge-number, to the same radioelements that are formed by reaction VIII, below. There is no reason for doubting that, given sufficiently high voltages, it would do so tor all elements. Reaction IV has been examined theoretically by Oppenheimer.;5 Elements as high 1n the periodic - 15 - table as copper ( Z : 29) have been disintegrated 1n this manner, while our information regarding the beta-radioactive elements of the shortest half-lives and most energetic spectra known,,6 , ,1 also comes from this reaction. C. THE REACTION PRODUCED BY POOTON BOMBARDMENT v. Radiative capture of the proton. -,!- + Z l gl__,,. Z +l WA +l , followed by wA+l ~ ~+ l + e+ Z + 1 Z There has in the past been considerable confusion concerning this reaction in the reports emanating from different laboratores. 41 61 ,a This confusion is to be attributed to the failure to recognize that we have here a case of 11 resonance 1 penetration of the target nucleus by the proton, with no third particle to carry off any excesses of energy of momentum. An appreciable yield in the reaction will result only near certain very definite resonance levels which may be supposed to correspond roughly to quantized states of the proton in the field of the target nucleus. This fact, when taken in connection with the differences in the voltage scales at different laboratories, makes it by no meaus surprising . 4 that Cockcrof't, Gilbert, and Waltcn have failed to find an effect in boron bombarded by protons, and th;t Hafstad and Tuve'8 at first failed completely to find the activity, due ;tor', induced in carbon by proton bombardment. This reaction has been treated theoretically by Breit and Yost.'9 D. REACTIONS PRODUCED BY NEUTRON BOMBARDMENT VI. Capture of the neutron with emission of an alpha-particle. xA-t- n1 - - ? z o Qf-, + He4, followed by QA-,~ aA,-, + ez-z 2 z-z Z-1 This reaction, where _it leads to the formation of an artificial radioelement, is usually endothermic, so that it will take place only when the neutron has a certain minimum energy. - 16 - There are cases (B I Li) in which a stable nucleus is formed in a similar reaction, and here the reactions are exothennic and seem to recpire neutrons of almost zero energy. 20 , 40 Such cases will be discussed more fully later. VII. Capture of the neutron with emission of a proton zx_A + 0n 1~ z-l~ + 1 H1 , followed by z-t ~ zXA + e- This reaction, like VI, involvesthe emission of a heavy charged parlicle from the struck nucleus, and hence as a rule requires energetic neutrons for its production. It should al.so be remarked in connection with re= actions VI and VII that, since the emergent heavy charged particle must escape from the attractive field at the center of the nucleus, which becomes stronger with increasing charge-number of the nucleus involved, these reactions are found only among the light elements; viz., those of charge-number less than about ;o. VIII. Radiative capture of the neutron xA + z n1 --} xA + l;-hv followed by xA·t-1 ~ o Z ' Z Z-1 ~-+l ...+ e- This is, to date, the most frequently occuring reaction encountered in the production of artificial radioactivity, and it takes place in a larger number of elements than any other nuclear reaction. When it was first ob- served that a radioelement isotopic with its parent could be fonned by neutron bombardment, there were theoretical difficulties in accepting the idea of simple capture of a neutron of an initial energy of several million volts by a stationary nucleus, without the emission of any heavy particles whatever. Incontrovertible evidence that this process was indeed occurring, however, made it necessary to abandon thes.e objections. 'Ihe discovery by Fermi81 20 that this reaction was greatly enhanced by slowing down the incident neutrons in paraffin surrounding the source enabled the theoretical treatment of the phenomenon by Bethe41 and Perrin and Elsasser42 to be made on the basis that only very slow neutrons could be captured with the - 17 - emission of a gamma ray. Reaction VIII is by no means limited to cases in which the product nucleus is unstable, as is shown by the enormous absorption of' slow neutrons by such substances as cadmium and mercury, which are only feebly activated by neutron bombardment. In such eases., the next heavier stable isotope of Cd, Hg., et al • ., is formed by the radiative capture of the neutron. Gamma rays emitted in such neutron capture, in the production of both stable and unstable nuclei, have been observed by Ferm1 20 and others.42 , 44 It should be remembered that, while the emission of gamma rays is demanded in react ions V and VIII , it may also occur for any of the other reactions. Until such gamma rays have been sought for and their energy deter- mined, if they exist, it will not be possible unambiguously to balance mass and energy in any of the reactions just discussed. In attempting to strike such a balance, one should also bear in mind that ordinarily the tables of isotopic masses are tables of atanic masses, and hence include the masses of all the extranuclear electrons. The reason for this is obscure, but this defect in the tables does not make trouble save in cases of artificial radioactivity. In -the ease of - beta-particle emission, the mass of the emitted electron should not be taken into account in the computation, for its loss from the radioactive atom is compensated for by the gain of a new extra-mclear electron demanded by the fact that the charge-number of the nucleus has been raised by one owing to the loss of the disintegration electron. In the case of positron emission, on the other hand, •ice the electron rest-mass energy should be added to the energy of disintegration, for not only has the positron escaped, but an extra-nuclear electron will be lost owing to the reduction of the nuclear charge-number by one. It seems clear from the mveetigations of Henderson, 45 and of Crane, Delsaseo, Fowler, and Lauritsen,;, 6 that the maximum energy of the continuous electron spectrum, and ~ the mean energy, should be taken as the energy of - 18 ~ electron or positron disintegration. This result, which ia a natural consequence of the neutrino hypothesis, seems now to rest on a fairly finn experimental foundation. 5. Detailed Discussion of the Artificial Re.dioelementst In this section the reactions producing artificial radioactivity are treated for the case of each element. 'lhe listing has been made according to the bombarded element, although it wil 1 be noted that in many cases the same artificial radioelement can be made in a variety of ways; for example., Al 28 is produced from Mg25 by reaction II, from Al 27 by reaction IV and reaction VIII, from P~1 by reaction VI, and from s1 28 by reaction VII. The number of the reaction which is probably the one responsible for the formation of the radioelement being discussed will be found in the margin to the left of' the discussion. 'lhe sign of the disintegration electron emitted from each radioelement., where not explicitly stated, can be determined fran the type of nuclear reaction involved; we have seen that reactions I, III, and V lead to radioelements which emit positrons in their decay, while reactions II, IV, VI., VII, and VIII yield electron-emitting bodiese' No case is known in which bombardment by neutrons produces a radioactive emission of positrons. At the end of the paragraph on each element is a complete list of references to the work bearing on that element; the most :important ones have been mentioned specifically in the paragraph. The half-value thickness for the absorption of the beta-rays emitted from an artificial radioelement is o:f'ten given below 1n g/cm.2 of aluminum; the mean energy of the electron or positron spectrum can be obtained rough%ly in mev. by multiplying the half-value thickness by 8. 1. ) It7drogen No artificial radioactivity has been excited in this element by neutron, proton, deuteron, or alpha-particle bombardment. (7,20) 2.) Helium 'Ihe technical di:f'ficulties-.Qj' irradiating and testing for subsequent activity sufficiently large volumes of the noble gases has prevented their careful investigation for artificial radioactivity. - 19 - Ridenour , Shinohara, am Yost46 bombarded ;500 cc of argon at atmospheric pressure with neutrons from Be bombarded by deuterons, but failed to find any detectable activity. The noble gases will be omitted from this list • .?•) Lithium I(?) II(?) Meitner47 reported observ:mg the radioactive emiesion of positrons from Li bombarded w 1th alpha-particles. '!he upper energy limit was o.;; ~ v., and the half-life several minutes. These she attributed to B. She also reports a beta-particle emisaiof of energy and half-life not stated. This was attributed to Be 0 • This work has not been confirmed. Orane, Delsasso, Fowler, and Lauriteen.?7 have found a beta-activity in Li bombarded with deuterons. The half-11:f'e is about 0.5 seconds, am the upper limit of the electron energy apec!rum .about 10 mev. The activity is probably to be attribut'd to Li and possibly to H4 • Strong bombardment of Li with neutrons, , 20 has yieldgd no detectable activation, although one might expect the same Li mentioned above to be fonned in reaction VIII. IV 4.) Beryllium An extremely weak activi~ probably due to impuriti5s was exhibited by Be a:f'ter strong irradiation with slow neutrons. 2 5.) Boron The formation ot: N1 .? by alpha-particle bombardment of boron is one of' the three original reactions of artificial radioactivity. The half-life first measured by Curie and Joliot1 and Ellis and Henderson14 was wrong, being given as 14 minutes instead of about 10.5. More accurate measurements due to Ellis and Henderson 50 cleared up the · difficult~, mentioned earlier in this paper, regarding the identit y of the Nl:, f'onned by B and alpha-particles with that formed by bombarding carbon with deuterons or protons; Nl) emits positrons, the upper limit of whose energy spectrum is about 1.;; mev. References: I 1;5, 51, 52, 82, 85. IV III V Emission of beta-particles fran B following deuteron bombardment has been observed by Crane, Delsasso, Fowler, and Lauritsen • .?6 The he.lf'life is about 0.02 seconds, and the upper limit of' the energy spectrum in the neighborhood of 11 mev. The activity is presumed to be due to B12 • Orane and Lauritsen2 and Cockcrort4 have reported a positrC11 activity with half-life 20 minutes, produced by the deuteron bombardment of boron. The radioelement involved was shown to be 01 1 by Yost, Ridenour, and Shinohara.12 The upper limit of the positronaspectrum is about 1.~ mev., aecording to Neddermeyer and Anderson. Orane and Lauritsen6 found that 011 could also ~e forim d in boron by proton bombardment. The reason for Oockcro:f't 1 s disagreement with this finding has been discussed in Section 4. Ridenour and Sh1nohara49 have measured the voltage excitation curve for the reaction (Figure ,) - 20 - but the evidence for resonance is doubtful owing to lack of extrane accuracy 1n the measurements and the fact that the proton beam contains particles of all energies. References: 48. 6.) III V Carbon Crane and Lauritsen,2 003kcroft, Gilbert, and Walton, 4 and Henderson, Livingston, and Lawrence have found a positron activity of about 10.1 minutes half-life in carbon bombarded by deuterons. Yost, Ridenour, · and Shinohara 2 showed that the radioelement was Nl~. Lauritsen and Orane 6 found that the same element was formed in the bo2tbardment of carbon by protons, as had Cockcroft, Gilbert, and Walton, , 5 and Henderson, Livingston, and Lawrence., Hafstad and Tuve~ could net at first find evidence of such a~tivity, but the discrepancy has been cleared up, and Hafstad and Tuve 0~ have now ma.de the best measurements of the voltaga excitation function for this resonance reaction. The radiative capture of protons by carbon has been treated theoretically by Breit and Yost.'9 7.) Nitrogen I The phenomenon of recoil of active nuclei formed by alpha-particles bombardment of atmosph~rio nitrogen, leading to a slight radioactive emission of positrons in all targets when bombarded by Q.lpha-particles in air, was first interpreted correctly by Wettenstein26 as being due to the formation probably of pJ. 7 accord~ to reaction I. The halflife is 1.1 minutes. Ellie and Henderson 4 have also studied this radioelement. References, 8~, 85. III Livingston and McMillan27 have observed the collection by recoil on to a pt target bombarded by deuterons 1n air of a radioelement whose parent is nitrogen. This radioelement emits positrons with a half-life of lfg seconds, and has been chemically shown to be o:xygen; it is no doubt O • The upper limit of the positron energy spectrum is 1.7 mev. 8.) O:xygen No radioelement has been formed from oxygen by bombardment with al.phaparticles, protons, deuterons, or neutrons. 9.) I(?) Fluorine The emission of neutrcns on the alpha-particle bombardment of fluorine is well known. This reaction would lead to Na22 , which might be xpected to deoay to Ne22 with the emission of positrons. Meitner has observed the emission of positrons from bombarded fluorine, and gives the half-life as "very short," and the upper limit of the spectrum as o.4 DBV• Fahlenbrach85 attributes this activity to recoil of F17 produced in atmospheric nitrogen between source and target. 47 IV o ~ne, Delsasso, Fowler, and Lauritsen;7 have observed the formation of F2 in the bombardment of F with deuterons. The upper limit of the beta-particle spectrum is about 5 mev., and the half-life 12 seconds. - 21 - ? Fermi et a1.7, 20 have observed a 9 second and a 40 second activity in F bombarded with neutrons. Neither is sensitive to hydrogen ( see Section 6.) Both emit negative electrons •. Neither can be identified with certainty; one is probably Nl6, but the identity of the other radioelement is at present obscure. References: 54, 55. 11.) I IV VII ? VIII Sodium Frisch 56 has observed a positron emission with a half-lif'e of 7 s ecmds, produced in Na by alpha-particle bombardment. It is doubtless to be attributed to Al 26. The upper spectral limit is about 1.8 mev. Lawrence;4 has produced Na 24 by the deuteron bombardment of sodium. Its half-life is 15 hours, and the upper limit of its electron spectrum about .2 mev. It gives off about 1 gamma-ray per disintegration, the energy of the gamma rays being tentatively given as 5.5 mev. A 4o-second beta-activity found by Ferm17, 20 in sodium after neutron bombardment is- possibly to be attributed to Ne 2 , . References: ,o. A fi:f'teen-hour half-life f~{ ~ emission of electrons is fiund in Na a:f'ter neutron bombardment, , and is due to the same Na2 discussed above. 12.) Magnesium I Positrons, tncmn to be emitted :f'r2m Mg after bombardment with alphaparticles,1 probably come fran Si~,. The half-life for positron emission is not known, however,51, 57 because of the fact that the negative electrons emitted are four times as numerous. These come from reaction II. References: 1}, 14. II Alichanca, Alichanian, and Dzelepow57 were the first to point out that the measured half-life of about 2.; minutes for Mg- was that that of electron emission. The electrons have a maximum energy of, mev. 51 , 57 and the radioelement involved is Al28. References: 82. ? Fahlenbrach!j:J has found an half-life of 7 or 8 minutes in magnesium bombarded with the alpha-particles of Th0 1 , in addition to that o~ minutes mentioned above• This has been confirmed by Eckardt. 6 ' 87 This may be attributed to the positrons from Si2 f ( see above,) in which ease we have an exaqiple of reaction I; alternatively, the activity can be attributed to A13J formed in accordance with reaction II, the pa.rent nucleus being Mg20: A determination of the sign of the electrons emitted would serve to decide which is the true reaction. VI Mg, when bombarded with neutrons, 4 , 20 exhibits a 40 second period which ma.Y be tentatively identified with that shown by Na and attributed to 2., Ne 2 ,. References: 78. VII A fift~en-hour period observed in Mg41 20 is doubtless to be attributed VIII A weak and water-sensitive (Vide infra) period 7 ' 20 of ten minutes to Na. 24 • observed in Mg is probably that of Mg27. - 22 - 1;.) Aluminum One of the three elements in which artificial radioactivity was first I discovered by Curie and JQliot1 is Al. The positron emission from this element is due to p;!IO, as has been shown chemically.l;!, The halflife is about ;.; minutes, and the upper limit of the energy spectrt.1D_ is 2.74 mev. Ellis and Hendersonl4 have studied the yield of this p.?U and as a function of alpha-particle energy, and find a pronounced flattening beyond 8 mev., this corresponding to the height of the potential barrier of the Al nucleus for alpha-particles. References: 51, 81, 82, 85, 86. r.v Lawrence;4, 58 has reported that bombardment of Al with high-energy deuterons gives a beta-particle radioactivity with a half gife of 2.5 minutes, which is without question to be attributed to Al 2 • VI Under neutron bombardment, Fermi 7 , 20 has found a period of 15 hours excited in Al. This he chemically showed to be due t~ an isotope of Na, so it is clear that we have again to deal with Na 4 • VII Another period for electron emission obaer!8d 1n Al bombarded with neutrons is a 10 minute one which Fermi f I has shown to be due to an isotope of Mg, :moeasarily Mg27. References: ;o, 4;, 55, 78. VIlI Al bombarded with neutrons exhibits a period of 2.; minutea 20 which is quite strong when the Al is irradiated under water. This is clearly due to Al 2ts. 14.) Silicon n Fahlenbrach59 has found that an electron-emitting radioelement of about 17 days half-life is formed in the bombardment of silicon with the alphaparticles from a source of ThB+ o. An effect of double his background was found after 18 days• bombardment with a source of 6 me. which was renewed each morning. The radioelement is probably p.?2 , owing to the agreement in half-lifes. VII Si b0D1barded with neutrons has been found by Ferm17, 20 to have a m1nut~ beta-activity which has been chemically demonstrated to be due to Al 8 • References: ;o, 4;, 41, 60, 78. VIII Under neutron bombardment Si also exhibits a weak and water-sensitive activi~~ of half-life several hours. Thia is probably due, according to Fermi, to Si' • 2., 15.) I Phosphorus Frisch 56 has reported poeitrm emission from P bombarded witha lphaparticles. Its half-life is about 40 minutes and the upper limit of the energy spectrum about 1.8 mev. The activity be.a been chemically shown to be due to an isotope of 01, doubtless 01)4. 0 VI Curie, Joliot, and Prieswer~O first not ced a decay period of 2.; minuta:e in P bombard~g by neutrons, which Fermi 2 has chemically demonstrated to belong to Al • Reference: 51. VII Ferm:t:7' 20 showed by chemical tests that a 2.4 hour period found in P - neutrons was that of Si;1 • Half-value thickness of the beta-rays in Al is 0.15 g/cm. 2 • References: 44, 60. 16.) Sulphur P;2 , formed in the 1mbardment of S with neutrons, has been chemicallyidentified by Fermi. It has a half-life o:f' 14 days, and t~ half-value thickness for absorption of the electrcns in Al is 0.10 g/cm • Reference: 80 VII 17 •) Chlorine VI P;2 may also be :fbrmed by bombardment of Cl by neutrons. 7, 20, 80. VIII A water-sensitive period of ;5 ~inutes_bas been shown by Fermi 2 0 tcll be due to a Cl isotope, either Cl; or c1Jtl. References: 79, References: 19.) Potassium I Alpha-particle bombardmell~ of~otassium gives a positron emitter of; hour half-life,4lhich Zyw 1 , and demonstrated to be an isotope of Sc, probably Sc • . VIII An indiced activity found by Fermi 2 0 in I\'. irradiated by neutrcas is strongly water-sensitive 1 and chemical tests show it to be an isotope of potassium, probably ir2 • The half-life is 16 hours. 20.) Calcium VII Hevesy and Levi 62 report he.vi~ found and identified chemically a 16hour activity belonging to ~2, produced in Ca by neutrcm bombardment. Fermi on the other hand, 20 using sources of neutrons twice aa strong as that employed by the former investigators, finds no activity. VIII Hevesy and Lev1 62 also report a 4 aour activity due to an isotope of Ca. This was not found by Fermi. 2 21.) VI Hevesy6 ; reports that K42 was shown by his chemical tests to be the radioelement of 16 hour period fonned by neutron bombardment of Sc. The maximum energy of the beta-rays emitted is given as about 1.2 mav. 22 •) ! Titanium A ver~ weak effect, with a period of a few minutes, was observed by Fermi' after neutron bombardment, and may possibly be accounted for by the presence of impurities. 2;. ) VIII Scandium Vanadium The half-value period of the activity7, 20 produced in V by neutrm bombardme~t is ;. 75 minutes; the half-value thickness for the beta-rays 0.17 g/cm. Al. The beta rays are accompanied by a gamma radiation. The activation is strongly sensitive to the presence of substances containing hydrogen. The radioelement is v3 2 • - 24 - 24.) vn Chromium The ~~tivity7' 20 induced in Or by the action of neutrons is also due to V • 25.) Manganese VI v52 is also formed by the neutron bombardment of manganese.7, 20 VIII A water-sensitive activity of 2.5 hours half-life is fonned in Mn by irradiation with neutrons; this cen be shotrn ~emically to be due to an isotope of Mn, and it must therefore be Mn • References: 20, 78. 26.) VII Iron 56 is formed in the action of neutrons on Fe. 7, 20 References: 4~, 60, 78. Mn 27 •) Oobal·t VI Oo bombarded with neutrons is transformed into Mn56 • 7, 20 VII ( ?) Oo also ehOlfs a strong absorption for the slOI' neutrons~ this absorption being accompanied by the Elllission of' a gamma-radiation. O Since there is only one stable isotope of' Co knorm, and since this absorstion of slow neutrons must then be concerned in the formation of 00°, the fact that no beta-radiation corresponding to this unknown isoto~ is observed should be investigated. It may be that the half-life of 0060 is so short as not to have been observed, or so long that two/ ft!III atoms disintegrate per second to permit their observation. On the other hand, it is possible, though not likely, tm.t co60 is stable. 28.) ? Nickel Fenni reports a dubious trace of activity in Ni strongly irradiated with neutrons.20 29.) CopPer rv Lawrence64 ha~obse~ed the production of the two radioactive isotopes of copper ( Cu , mu ) by the banbar<hnent of copper with d euterons. This is the heaviest nucleus in which transmutation has been accanplished by means of artifically accelerated particles. '!he periode are 5 minutes and 10 hours. VIII Both copper neutron-capture reactions giving rise to the radioelements mentioned above are strongly Ellhanced bJ the presence of water. The periods §lven above are those of FenniJ O for the long period Bjerge and Westcott report 6 hours. References: 2,, 55, 78• ,o.) Zinc VII The long period due to radiocopper ( see above) is found in zinc 54 which has been subjected to neutron bombardment. The active copper has been separated chemica11y20 end electrochemically~ 2' The same remarks apply to the short-period radiocopper. References: 60, 71. - 25 - McLennan., Grimmet!~ and Read report a 100 minute period in zinc banbarded with neutrons. The origin of this is not known. ? jl • ) VIII TWo half-lives corresponding to neutron capture by the two known isotopes of Ge. are observed. 20 '!he stronger ie the shorter; its half-~ife is 20 minutes. Half-value thickness for the beta-rays is 0.17 g/am Al. The longer period is 2) hours., and the beta-particles are accompanied by a rather strong gamma-radiation. ,2.) 'l ? Selenium The ;5 minute activity produced in this elem, nt by irradiation w1 th neutrons has been shown to be due to an isotope of Se~O. j5. ) VIII Arsenic Neutron capture in this element results in the formation of As76, whose half-life is 26 hours. The bet~~particles are half absorlred in 0.16 g/C111 2 of Al. 2 0 References: 25, ;4.) VIII Germanim A ver-/ weak activity produced by neutron bombardment of Gg having a half-life of around 2 hours., has been reported by Sugden. 6 The identity of the rad ioel ement is not known. jj.) VIII Ga 11 ium Branine Two neutron-capt~5e react ~n s have been tnown f'or bromine for s' ome t :ime • These ~SS..d to Br and Br ., whose half-lives are 18 minutes and 4.2 hours. ·· · Ha;r-value thickness for the beta-rays of both radioelements is 0.12 g/C111 Al, and each radioelement emits fairly strong ganma radiation. References: jl, 79• 8 Recently, Kourtscbltow., Kourtschatow., Myssowsky, and Roussinow67 have reported a longer-period activation of Br by neutron bombardment., which they have shown by chemical tests to be carried by an isotope ~ Bromine. The half-life is about hours., and the new radiobranine emits very feebly-penetrating beta-particles 1 together with a strong gamma radiation. The upper limit of 'the electron spectrum is o.6 mev. and the energy of the gamma radiation is abffl!t 0.65 mev. The existence of this radioelemerrt. has been checked by Femi and the fact that only two stable isotopes exist makes the identity of the 'third type of radiobromine an interesting puzzle. The original discoverers regarded this as a case where the incident neutron., without being captured, expelle6 another neutron from the nucleus of Br79, so that the radioelement in question is Br78. Thie seems very unlikely indeed, as all three reactions involving Br and neutrons go best at small neutron energies., and the likelihood of a very slow neutrm expelling another fran a nucleus would seem small. A possibility which does not seem to have been suggested is thEt one of' the periods of Br represents that for positron decay., since both oft he possible radiobromine isotopes are so located in the periodic table that either the emission of an electrm or that of a positron would return them to stable isotopes, of Kr or Se., respectively. ,6 - 26 - j7.) Rubidium ? Fermi7 has reported a very weak activity with a period of about 20 minutes in Rb bombarded with neutrons. It.a source is unknown. )8•) Fenn1 20 ~eporte finding no activity att.er strong neutron irradiat. ion. j9.) VII(?) Yttrium This element has one known stable isotope; namely., yB9 • It strC11gly absorbs slow neutrons with the emission of gamna-radiation. Since only a weak and doubtful activity has been observed in Yt bombarded with neutrons., the case of this element presents a problem similar to that of Co. (q.v.). Either a heavy particle is emitted in the neutron capture, which seems not likely because of the Emission of ganma rays at the moment of neutron capture., or the decay period of yt90 is aich that its activity has not been observed., or Yt90 is stable. The last alternative seems doubtful because of the observed arrangement of isotopes of all other elements of odd charge-number. 40. ) VIII Strontium Z ire onim Hevesy and Levi62 report an activity of 4o ho~, half-life in Zr bombarded by neutrais. 'Ibis they attr~bute to Z~ • Half-value thickness tor the beta-particles is O.lj g/cm Al. 41.) ? strong irradiation with neut.rons 20 produces only a doubtful activity probably due to impurities. 42.) 1 Col umbium Molybden\.111 A very weak activity is exhibited after neutrm bombardment. There are at least two periods, one of' 15 minutes and one longer than a day. The radioelements responsible have not been identified.7 McLennan, Grimmett., and Read65 give the half-lives as 25 minutes and ~ hours. ()Wing to its rarity, this element has never been tested for artificial radioactivity. 44.) ? Ruthenium Kourtschatcm, Nemenow., and Selinow90 report that on bombardment of Ru with slow neutrons, they were able to excite activity showing at least four periods of decay, these being 40 seconds, 100 seconds, 11 hours., and 75 hours. The intensities in equilibrium of these activities are respectively 100, 100, 10, and 4o. All periods are extremely water-sensitive, the activity of Ru irradiated in air not being observable. The identity :In intensity of the 'bro short periods suggests that they a re pro.ducts of successive decay, blt none of the radioelements concerned has been identified. - 27 - 45.) VIII Rhodium Rhodium can be strongly activated 20 by bombardment with slow neutrons. It has two periods, of which the $ort.er, 44 seconds, is water-sensitive and hence is probably that of Rh1 • Half-value th ckness for the electrcne anitted by this radioelement is 0.15 g/cm Al. 2 ? 46.) VIII 20 is observed (0.10 g/cm 2 ); this may indicate the ex.i.stence of an unlmown isotope of Rh, or ~~y possibly be a case of alternative beta- and positron-decay of Rh104• The shorter period is much the stronger. A longer period of ~.9 minutes Palladium This element displays at least two periods under neutron bombardment. Both are sensitive to water. 'Ihe half-lives are given as abrut 15 minutes, and about 12 hours by Fermi. 20 McLennan, Grimmett and Read65 give 14 hours for the longer period. The ra.dioelementa are probably isotopes of Pd, whose ma.ss-numbera~e not known because of the existence of several stable isotopes of Pd. 47.) Silver VIII Neutron capture in the two known isotopes of this element gives rise to ra.diosilver isotopes with half-lives of 22 seconds and 2.3 minutes. They are both veey sensitive to water. The chemical nature of the longer pe:d. od has been shown to be that of silver. 7, ~8 References: ~O, 44, 55, 60, 66, 78. 48.) Cadmium The great absorption of slow neutrons in Od apparently does not correspond to a strong activation. Several weak activities of dwerent periods have been reported by Fermi, b.lt none has been identified. ? 49.) VIII Inditm A 1, second period and a 55 minute period in Indium irrad!iated by neutrons are very wate~ sensitive and doubtless correspond to the disintegration of the radioelements 1nl14 and 1nll6. The electrons from the 54-minute isotope are known to be negative. A t:175d activity of about* hours was discovered by Szilard and Chalmers; it is either not sensitive to water or only moderately so. Fenn120 has performed experiments which seem to indicate that the two longer periods of In are due to radioactive isotopes of Jn, so that the radioelement responsible for the 3f hour period is not knORn. Thia is another case in which e. ternative positron and bet.a decay might occur in the case of either In1 1 or 1nll6, so :far as the positrons of stable isotopes are concerned. 4 50.) Tin Fenn1 2 0 reports that Sn strongly bombarded with slow neutrons remained inactive. 51. ) VIII Antimony An activity in Sb induced by neutron bombardment and decaying with a - 28 - period of 2.5 days has been found b~ Fermi. 20 The half-value thiclt:rB es for the beta-particles is 0.09 g/cm Al. Chemical t.eete have shown the activity t.o be carried by an isotope of Sb. 52.) VIII Tellurium A weak activity in Te irradiated with neutrons has a period of 45 minutes and is water-sensitive. 5;.) VIII It. is probably due to an isotope of Te. 20 Iodine 1128 is almost certainly responsible for the period excited in I by neutron bombardment, as numerous chemical tests have shown. The period is given by Fermi as 25 minutes; it is about 2~ minutes according to Ridenour and Yost. Half-value thick?B ea 0.11 g/cm Al. References: 24, ,O, 44, 60, 66, 78, 79• 54.) Caesium T The half-life of a weak activity produced by neutron bomirdment of Cs has been given by McLennan, Grmett, and Read as 75 minutes. 1 Ferm17 indicates that other periods may exist. 56.) ? Barium A weak activity found by Fermi in Ba aft.er neutron bombardment has a period of ) minutes and is not water-sensitive. VIII A water-sensitive 80 minute period has been shown by chemical tests to be due to an isotope of Ba. It is no doubt aa1;9 formed by neutron capture.20 57 • VIll Its identity is not known. 7 Lanthanum Although ~armi ha.a been unable to find any activity in La irradiated with neutrons, Sugden7 2 reports an activity f~half-life 1.9 days wh~ h is water-sensitive and no doubt belongs to La • It could have been missed by Fermi because of the brevity of his irradist. ion. 58.) Caril.Ill 20 Marsh and Sugden72 and Ferm1 agree in finding no activity in Ce after neutron bombardment:.. 59. ) Praseodymhm VIII ? Fermi, 7 Sugden, 7 2 and Hevesy and Levi7J agree in finding a 19-hour period which is water-sensitive and doubtless correpponds to neutron capture in Pr. Half-value thic.km as for the beta-rays 0.12 g/cm2 • An activity of' 5 minutes half-life was early reported by Fermi.7 not water-sensitive, and its carrier is not known. It is 60.) Neodymium ? Fermi 7 reports a very weak actf~ity of 1 hour half-life in this element, which was not f01md by Sug.den, but was by Hevesy and Levi.7;, - 29 - 61. ) 111 inium This element has never been tested for artificial radioactivity. 62.) Samarium A weak activity with a period of about 40 minutea has been found af'ter 1 neutr~u bombardment by Ferm.i,7 and confirmed by Sugden72 and Hevesy and Levi. ' An addition, Sugden reports a much longer period. The identity of the radioelements is unknown. 6;.) Europium VIII An extremely intemie activation apparently due to neutron-capture to form Eul52 or Eul54, which has a half'-li:f'e of 9.2 hours, has been fa.md by Sugden66, 72 and confirmed by Hevesy and Levi. 7; Half-value thicloe ss for the beta-particles is 0.11 g/cm. 2 • 64.) Gadolinium Although this element absorbe slow neutrons very strongly, only a very weak activity has been fou~ br . Fermi7 and Hevesy and Levi. 7; The period was 8 hours. Since Sugden , 72 found no activity, the possibility of Eu contamination of the former authors• Gd samples should be borne in mind. ? 65.) VIII 'lhe ;.9 hour activity discovered by Sugden66, 72 and confirmed by Hevesy and Levi 7; is apparently to be attributed to Tbl60 formed by neutron capture. 66.) VIII Terbium Dysprosium The strongest artificial radioactivity found to date in reactions involving neutron bombardment is the 2e5 hour activity of Ds. The reaction is strongly water-sensitive, and no drubt is due to Dsl65• Thia radioelement has been investigated by Hevesy and Levi7; and by Marsh and Sugden.72 Half-value thickness for the emitted electrons is 0.07 g/em.2 Al. 67. ) Holmium VIII ? Hevesy and Levi 7; report an activity of ;5 hours shown by one of their samples which had undergone neutron bombardment. Since dysprosium is one of the most abundant of the rare earths, and since it shows such an enormous activity under neutron bombardment, one must regard with caution the statement of Marsh and Sugden72 that carefully purified holmium, free from De, exhibited a period of 2.6 hours, since it may so readily have been caused by contamination of the Ho with a felll hundredths of a milligram of De. The intensity of the :;5 hour period is very high, and the half-value thick:nesg of Al for the beta-particles ia 0.11 g/am2 • The radioelement is Hol 0 6 • 68.) Erbium ? Reports on the neutron-produced radioactivity of this element are in c Cllf'lict. Marsh and Sugden72 report periods of about 7 minutes and l .6 days, the latter being about ten times as intense as the former. Hevesy and Lev173, on the other hand, report a period of 12 hours. is knovn about the carriers cf the activity. cf).) Nothing Thulium Artificial radioactivity has never been sought in this element. 70.) Ytterbium Marsh and Sugden72 and Hevesy and Levi 73 are in agreement in finding an activity of 3.5 hour half-life in Yb bombarded with neutrons. The identity of the radioelement is not la1own. 'Ihe activity is weak. ? 71.) ? Here another serious disagreement exists. Hevesy and Levi 73 report an activity induced in Lu by ne~~ron bombardment whose period ie about 5 days. Marsh and SUgden report finding a 4-hour period, and no other. 72.) VIII Lutecium Hafnium 62 Hevesy and Levi have reported an activity with a half-life of several months to be excited in Hf by neutron bombardment. This is probably a reaction involving neutron capture, the radioelement being Hfl81. Thie is the longest period reported todate. 73•) Tantalum Fermi 20 reports having found only a dubious activity after 12 hours• irradiation of Ta with 500 lllC• of Rn-Be. This is confirmed by McLennan, Grimmitt, and Read. 05 74.) VIII Tungsten Fenni 20 found an activity excited in W. by neutron bombardment. It is watereensitive and the carrier of the activity has been chemically shown to be a tungsten isotope. McLennan, Grimmett, and Read65 give 23 hours for the period. 75. ) Rhenium VIII 20 Ferm1 discovered a water-sensitive activity produced in Re by neutron bombardment, and showed chemically that it ie borne by an isotope of rheium. Half-life is 20 hours, and the emitted beta particles are half absorbed in 0.12 g/cm2 /Al. 76.) Osmium Fermi 7 found that this element, irradiated 15 hours with 4:0 me of Rn-Be, was inactive. 77 •) VIII Iridium A 19-hour period reported by Fermi 20 in Ir bombarded with neutrons is much enhanced byw ate r, and is probably due to an isotope of' Ir. Sosnowski,74 on the other hand, finds periods of 50 minutes and; days, the shorter one being accompanied by the emission of gamma rays. 78•) Platinum 2 Fermi 0 found a ve r., weak 50-minute activity in pt bombarded with neutrons. The half-life ~gcording to McLennan, Grjmmett, and Read 65 is~ minutes. Sosnowski( reports a half-life of 100 minutes, this value being probably the least accurate. VIII 79•) Gold 20 , Fermi found a water-sensitive period of 2.7 days in Au bombarded by neutrons. The half-value thickness f~r the beta-rays, which have been shown to be electrona, ~s o.04 g/cm Al. The radioelement is doubtless Au1 98 • Sosnowski7 has confi:nned the half-life and penetration measurements on the electrons emitted, and has claimed to find a gamma-radiation with a half-life of about 5 hours, to which corresponds no known period of beta-decay. This surprising result may be readily explained by the skeptical as being a consequence of the very weak initial intensity of his gamna rays. References: ;o, 78• 80. ) Mercury 20 Fermi has failed to find any activity after strong irradiation with slow neutrons, in spite of the large absorption (v. Sect. 8) shown by Hg. for slow neutrons. 81 • ) ? Thallium McLennan, Grimmett, and Read71 report an activity with a 97-minute half-life to produced in the bombardment of Ti with neutrons. 82.) The same as Thallium. 20 a,.) Lead Bisrauth a' A weak activity of 100 minute period observed by Sosnowsld.77 in Bi bombarded with ~5utrons must be regarded as doubtful, it has been denied by :ermi . ·, and by Mc~ennan,t Grilllmett, and Read. 1 Bi has only one known isotope, Bi209, while Bi~lO is RaE, a beta-active body of 5 days half-life. The existence of the lClO-minute activity in bombarded Bi was regarded by Sosnowski as being a case of nuclear isomerism. 90.) Thorium Fermi20 reports periods of 1 minute and 24 minutes in Th bombarded with neutrons, and states that these activites are "scarcely sensitive to water." Hahn and Meitner22 report that the period of 1 minute is not watersensitive, and that it is the parent of an 11-minute activity. They have further found a period of about :$0 minutes which is water-sensitive, which is identical with Femi 1 s 24-minute body, and has been chemically shown to be to an isotope of Th. Hahn and Meitner regard the 1 mirute body as being a pr~~ct of reaction V~AA The radioel~,,nta are then: 1 minute, Ra ; 11 minutes, Ac c::,,; jO minute, Th • OUrie, v. Halban, and Preiswerk89 report that activities of halflives 1 minute, 15 minutes, 25 minutes, and j.5 hours are produced on neutron bombardment of Th. The 25 minute activity is due to an isotope of Th, and its production is enhanced by the presence of paraffin. It is doubtless Th 275. The j.5 hour body is not chemically similar to Th, Pa, Ra, or U, but behaves like La, and is hence probably an isotope of Ac. The 15 minute activity s eeme also to be carried by an isotope of Ac. They agree with Hahn and Meitner that the 1 minute activity is probably due to an isotope of Ra. In a later cawnunication, the same authors have chemically shown that the activity of 1 minute period is due to an isotope of Ra, and have found an isotope of Pa having a period of 2.5 minutes which they consider to be a dec9¥ product of the active isotope whose half-life is 25 minutes. See also Reference 88. 92.) Uranium Fenn1 20 has found periods of activity, induced in U by neutrons, of 15 seconds, 40 seconds, lj minutes, and 100 minutes. The reactions creating all these radioelements eave the one of 40-second half-life seem to be water-sensitive. For the 15 second, 1) minute, and 100 minute activities, the increase in activation produced by irradiation under water instead of in air is the same in each case. This led Fermi to the conclusion that the 40-second period is due to one prima-y process, while the other three are chat.nor branching products arising from another primary phenomenon. The identity of the radioelement with the 40-second period is unknown, but the 15-second, 13-minute and the 100-minute products are regarded by Fermi as rad,ioelementa of mass-number 239, and charge-number 92, 93, and 94, respectively. 21 concluded that the 13-minute and 100-minute activity were probably due to elements beyond uranium and different fran one another. In a later comnunication, they reported 21 that the longest-lived activity reported by Fermi was in fact due to a mixture of the two radioelements of periods about 1 hour and 2 to 5 days. All the three radioelements studied by Hahn and Meitner were shown by their chemical tests to have properties different from any of the known elements. The properties one would predict for elements 93 and 94 have been stated by v. Grosse.19 Hahn and Meitner SLOW NEUTRONS 6. Reactions Involving Slow Neutrons: No treatment of artificial radioactivity would be complete if it did not include a discussion of the effects due to "slow" neutrons. In October, 1934, Fenni and his co-workers 20 found that the activation of silver by the neutrons from a Rn-Be source could be increased several fold by surrounding source and detector with la.l\ge quantities of water or paraffin. The effect was shown to be specifically due to the hydrogen content of the water or paraffin. The most obvious explanation for the phenomenon was that the neutrom from the source, initially having all energies from zero to about 7 mev.91 1ost energy in collisions with the hydrogen nuclei present in water or paraffin, and that these slowed-down neutrons were much more effective in producing radioactivity in silver than were the neutrons of higher velocities. When the effect of sub- stituting such materials as silica and iron for the paraffin or waterwe.s examined, it was found that a small increase in the ~ctivation was produced by surrounding the source and detector with these materials, but the magnitude of the effect was not comparable with that attained by the disposition of large masses of paraffin about the soµrce during irradiation. This lends added weight to the hypothesis that the slowing-down of the neutrons is responsible for the observed effect, as a neutron can lose mbre energy in a collision with a hydrogen nucleus than in one with a heavier nucleus. other detectors were substituted for the silver, and the striking discovery was made that all the reactions of type VIIl and only these reactions, showed an enhancement when the incident neutrons were slowed down. The assumption was early made by Ferm1 20 that the energies of the neutrons conventionally called "slow" were of the order of magnitude of thermal energies of agitation; that is, about 1/40 of a volt. It can be shown that the impact of a neutron against a proton reduces the energy of the neutron, on the average, by a factor of 1/e. From this it follows that in the course of ten im- pacts, the energy of a neutron is reduced to about 1/20,000 of its original value. W1th a neutron of an initial energy of 4 mev., less than 20 impacts would be necessary to reduce the energy to thermal values. Experiments designed to measure the energy of slow neutrons will be discussed in Section 8. The intense ·activation observed in many elements bombarded with slow neutrons suggests that the absorption of slow neutrons in these elements must be correspondingly large. This proved to be the case, 20 but the discovery was also made that many elements which did not show pronounced radioactivity under neutron bombardment exhibited large absorption for slow neutrons. In the cases of. heavier elements, (Cd, Hg, and others) this is to be attributed to the capture of a slow neutron with the emission of a gamma ray, in accordance with reaction VIII, except that the nucleus f'onned by such neutron capture is in this case a stable isotope of the bombarded element, so that no radioactivity is observable. The emission of the gamma rays accompanying the capture of slow neutrons has been observed by Fermi2 0 for several of the elements showing large absorption, both with and without the consequent production of radioactivity. In his studies of the absorption of slor, neutrons, FErmi 20 further found that certain light elements (B and Li) exhibited large absorption f'or slow neutrons, without becaning radioactive or emitting ganma rays at the moment of neutron capture. Thie has been shown to be due to the capture o:f' the neutroo in the B or Li nucleus, and the subsequent disintegration of the product nucleus with the emission of heavy particles. 40 This latter phenomenon (viz., the emission o:f' heavy particles) may be regarded as an alternative to the emission of a gamma ray at the moment of neutron capture; the energy balance in the reaction can be preserved either way. 7. Absorption of Slow· Neutrons: The earliest measurements of absorption of slow neutrons were made by Fermi,-20 with the experimental arrangement shown :in Figure 4. The absorption curves he obtained with this arrangement were not exponentials, as indeed the geometry would lead one to expect, even though the absorption measured in a parallel beam might follow an exponential law. A theoretical treatment of the absorption curve to be expected with such a distribution of source, absorbers, and detectors is to be found in Appendix A, where the assumption is made that the absorption is exponential when measured in a parallel beam. Experimental evidence the. t this is the case has been obtained fer absorptioo in Cd by Dunning9 2 and Ehrenberg and Hu Chien Shan, 9) and for absorption in Ag by the latter investigators.95 Table I, below, shows the cross sections for absorption of slow neutrons obtained by Dunning92 with the experimental arrangement shown in Figure 5. It will be ob- served that Dunning used for tP.e detection of slow neutrons the alpha-particles emitted in the Li reaction mentioned in Section 6. Atomic Element Noe H 1 1 D Li Be B C N 0 F Na Mg Al ; 4 5 6 7 8 9 11 Ca Ti 12 13 14 15 16 17 19 20 22 V 2; Cr Mn 24 25 26 27 28 29 30 ;2 53 ;4 35 38 .59 40 41 42 Si p s Cl K Fe Co Ni Ou Zn Ge As Se Br Sr y Zr Ob Mo - Neutron-nucleus collision cross sections ComFraction Or~~ See~ion Atomic pound Xl CM-.c. TransWt. used G/cm 2 mitted P. $1 ow Faet 1.008 2.014 6.94 9.02 10.82 12.00 14.01 16.00 19.00 25.00 24.52 26.97 28.06 51.05 52.06 55.46 59.10 40.07 48.1 50.96 52.0l 54.9; 55.84 '5 8.94 58.69 63.57 65.;8 72.60 74.96 79.2 79.92 87.6; 88.9 91 93.1 96.0 (OH2)n D20 LiF Be B c 4 a NaN; Si02 NaF Na Mg Al Si p s NaCl K CaO Ti0 2 V205 Or Mn Fe Co Ni Ou Zn Ge02 As Se KBr SrOr0 4 ; o, Zr02 Cb 2o5 Mo 0.131 1.;6 .24 • 70; .0557 2.86 1 • .30 5.78 1.94 1.48 4.67 7.24 2.2; 1.69 6.6 1.35 1.60 1.87 1.97 2.41 5.82 5.69 3.87 1.41 ;.o; 6.22 10.7 .08 5.28 3.47 3.4; 2.21 .18 1.87 1.01 1.70 0.67; .6;1 .765 .780 .418 .549 ;5 4.o 45 5.; 560 4.1 11., .650 ;.; .7~ 2 s:_ .827 4.2 .847 ,.5 .669 1.5 .788 2.5 .867 .615 14.7 .840 1.4 .541 59 .816 8.2 11.0 .751 .760 11.9 10 .751 • 718 4.9 .558 14.; .605 12.0 .602 55 15.4 .657 .642 7.5 .627 4.7 .9; est. 75 8.6 .692 .606 19 .705 11.9 .8;2 est. 9 800 .46; 16.7 .807 .90 est. 14 .926 7.1 1.68 1.71 1.84 1.65 1.60 1.65 1.76 ·- 2.4 2.6 ;.o 5.2 ;.; Atomic No. Atanic wt. La 44 45 46 47 48 50 51 52 5:; 56 57 Ce 58 Pr Nd Sm(Sa) 59 60 62 6; 64 65 66 67 68 69 70 71 101.7 102.9 106.7 107.88 112.41 na.70 121.77 127.5 126.9; 1:;7.57 1,a.90 14o.25 140.9 144.27 150.43 152 157.6 159.2 162.5 163.4 167.7 169.4 17'5.6 175.0 181.5 184.o 186.;l 190.6 19;.1 195.2 197.2 200.61 204.39 207;20 20).00 2:;2.15 2:;a.17 Element Ru Rh Pd Ag Cd Sn Sb Te I Ba Eu Gd Tb Dy Ho Er Tm Yb lJ.1 Ta w Re Os Ir pt Au Hg Tl Pb Bi Th Ur 1, 74 75 76 77 78 79 80 81 82 8:; 90 92 Table I ( con •t.) Compound used G/cm2 6.1 0.62 Rh Pd 6.2 Ag 1.88 Cd .0416 Sn 18.8 Sb 7.26 5.52 Te 6.78 I .662 BaO .214 La203 Ce0 2 .98 2.4:; Pr20, .72 Nd 20:; .04 Sa20:; Eu-Gd-Al .02 .0068 Gd 2o 3 .06 Tb-Al Dy-Al .27 Ho-Al .144 Er o .666 2 3 Tm-Al .07 .315 Yb20; Lu-Al .045 Ta 4.56 w 7.05 Re 2.;9 Od 5.:;5 Iif .785 Ru pt 4.;o Auu 2.52 .545 10.5 22.7 9.07 2.75 2.52 HgO Tl Pb Bi Th02 Ur0 2 - .?7 - Fraction transmitted P, .647 .664 .715 .557 .524 .678 .745 .790 .7,a .680 .933 .896 .77 .559 .525 .647 .590 .919 .685 .877 .780 .987 .912 .9:; .66:, .592 .501 .64:; • 5()'.J .769 .508 .556 .710 .568 .805 .772 .8:, Croat Section no2 em-2 Slow Fast 12.5 115 1055 ;:500 4.o 8.1 8.2 9.4 140 80 est. 25 25 220 4700 est.1000 4.; 4.6 ,o,a>oo est.1000 700 est. 400 120 est. 500 90 est. 400 27 2:; 89 27 285 25 5.3 88 380 5.8 11 8.6 8.2 :;2 4:; 5.7 In Bethe•a 41 theoretical treatment of the absorption of slow neutrons, it is predicted that the elastic scattering by the elements showing large &baorption for slow neutrons will be at least as important a factor as neutron capture in removing slow neutrons from a beam. 'Ihe experiments of I:>i.mning9 2 in Cd, and of Ridenour and Yost (see below) in Ag, indicate that elastic scattering of slow neutrons in these elements is negligible compared with capture. This indicates that the extremely large cross sections for capture exhibited by certain elements are not adequately explained by the assumptions made by Bethe. It seemed possible that the measured absorption of an element for slow neutrons might depend on the reaction whereby the slow neutrons are detected. Experiments on this point were performed by Ridenour and Yost,9 4 and show that such an effect does exist. Their technique and results are given below. 'Ihe geometrical ccnditions under which irradiation of samples was conducted is shown in Figure 6. The detectors of slow neutrons were discs 5 om. in diameter, the absorbers discs 7 om. in diameter. The Ag and Cu detectors were metal sheets; the I, Br, and V detectors consisted of compounds (cf. Table II) held on stiff paper with thin collodion. The erurce of' neutrons was beryllium ( or boron) and radon in amoonts usually between 100 and 200 millicuries. The activity of the detectors art.er irradiation was measured with a quartz-fiber electroscope of the Lauritsen type, provided with a window of thin aluminum. The procedure was to irradiate the detector, either bare or between tao equal layers of an absorber, for a length of time standard for that detector; then to commence readings one minute after the removal of the detector from the neutron source. The reading was terminated at a time, also standard for each detector, depending on the half-life of the activity under measurement. In Br, for example, their- radiation lasted for 25 minutes I and the measurement for 15 minutes, so that only the short-lifed activity was measured in these experiments. Frequent readings on the background were taken in precisely the way the measurements were made, except that the detector had not been exposed to the neutron source. The activities found were always easily measurable, being from twice to about twenty-five times the background due to comnic and local radiation. The results are shown in Table II. The absorber was in all cases ail ver, and the numbers in the table under the heading Percent Activity are referred in each case to the no-absorber value fur that detector as 10~, so that they are measures of the transmission through the absorber of neutrons having the proper energy to activate the detector. Table II. Absorption of slow neutrons in silver. PERCENT ACTIVITY ( TRANSMISSION) Wrr H ABSORBER OF 0 0.81 o.4~ 1.722 2 g/cm g/cm g/cm2 cm SOURCE OF NEUTRONS Element B-Rn Silver 1.195 100 60 1: 1 42 !. ; 26 't 4 Be-Rn Silver 1.195 100 59 :t:: 1 44t 1 27:!:: l Be-Rn Be-Rn Silver Copper 0.094 46 ±: 4 0.717 100 100 42 t 2 ;o :r- 1 28 t ; 17 t 2 12! 2 Be-Rn Vanadium (as NH vo ) 4 3 Bromine (as NH Br) 0.146 100 47 t.. 2 ;5 !.1 17 t 2 0.305 100 62 ! l 56 + 1 38 t l Iodine (as CHI?) 0.14; 100 57 ! .. 2 48 1. 1 37 t 2 Be-Rn DETECTOR Thickn~ae I cm A A 4 Be-Rn The fact, already mentioned, that the absorption curves obtained with such an arrangement of paraffin as was used :1n the present experiment are not exponential, rut that the absorption decreases with increasing thickness of' absorber, would seem to explain the result, clearly shown in the Table, that the absorption measured with a thick detector is much smaller than that measu1·ed with a thin detector. The disintegration electrons measured in the case of a thick detector come from a layer on the surface whose thickness is equal to the maximum range, in the material of the detector I of the electrons emitted by the radioelement fonned in the reaction concerned. The thickness of this layer is seen to depend on the density of' the bombarded mate rial and the electron spectrum of the radioelement. For an effect of detector t.1-iicknese on measured absorption to be ob- servable, it is necessary tha t the absorption of' neutrons in the material composj_ng the detector be appreciable. The optimum detector thickness, if the absorption of neutrons in the material concerned is less than that of electrons, will be that which ia just equal to the range of the beta-particles in the substance. If a detector of this optimum thickness and one of greater thickness be irradiated equallj' on both sides by neutrons, the thinner detector will display the greater activity, because the neutrons from the opposite side of the thick detector reaching the aide under neasurement have suffered an appreciable absorption in the material composing the detector, whereas all the neutrons absorbed in the detector of opt:imum thickness give rise to electrons which may escape from the detector on the aide under measurement. Since, as we have seen, the absorption becanee less with increasing absorber thickness, and since the effect of a thick detector is to commence the absorption measurements farther out on the absorption curve in the direction of increasing absorber thickre ea, the observed absorption with a thick detector will be leas than that with a thm detector, as is indeed found experi- mentally. So long as the absorption of neutrons in the thickness of the detector can be neglected, the effect of detector thickness will not complicate the results., and the other detectors are apparently thin enough, relative to the half-value thickness of each for slow neutron abaorption, 20 so that the results obtained with them can be intercompared, and compared wit h those for a thin silver detector. None of the detectors employed in the present experiment gave a half-value thickness of ail var as large as that (1.2 g/cm2 ) measured by Fenui with a Rh detector, but his value cannot properly be compared with t he present ones because of diffe ranees 'il\Which may have been introduced by detector thickness and the disposition of' paraffm about the source and detector. The differences between Br, I, and the other detectors employed in the - 40 - present experiment seems necessarily to be due to the fact that neutrons ca pill red by different detectors lie in different ranges of energy. 'Ihe sense of the dif- ferences among the detectors in the energy range of neutrons moat likely to be captured by each cannot be detennined fran these results alon~, but further investigations using various absorbers might serve for such a detennination. Tn order to gain an insight into the distribution of neutrons in the paraffin during irradiation of a detector, exper:iments were performed with the thin and the thick silver detector under the conditions of irradiation and measurement shown in Figure 2. The resul ta are sha-vn on the figure, and demonstrate (1) that the numbers of slow neutrons incident on the top and bottom surfaces of the detector are the same; i.e. that the distribution of neutrons inside the paraffin is uniform;(2) that the absorption of neutrons in the thick detector is quite %; (3) that there is no appreciable elastic appreciable, being of the order of wcattering of slow neutrons from silver, the placing of a sheet of silver absorber on top of the detector actually r educing the intensity of activation of the detector, owing to the reduction in the number of slow neutrons scattered to the back of the detector by the paraffin still above it ( compare 7c and 7e in the case of the thick detector); and (4) that the detector thickness has a pronrunced effect on the total activation of tre detector ( see 5a). The selective effect found here in the absorption of slow neutrons has also been observed. by Moon and Tillman,9 5 and indications of such an effect have been reported in connection with other experiments by Bjerge and Westcott9 6 and Artsimovitch and others.9 7 8. Velocities of the Slow Neutrons: The idea that al~, neutrons possess thermal velocities seems to be susceptible to an experimental test, and such a teat was made first by Fermi, 20 with negative results. He compared the activity of a sheet of rhodium activated first in a mixture of liquid hydrocarbons at room temperature, and then at 200° in another mixture of hydrocarbons which had, at that temperature, the same density and elementary composition as the former DJ1Bdium. - 41 - The geometrical conditions of irrad:iation were the same in the tmo experiments. activity could be observed. No di:f'ference in Another inconclusive experiment was performed by Mc- Lennan and others,98 first at room temperature, and then at liquid hydrogen temperature. The first positive evidence of a temperature effect on the properties of slow neutrons was offered by Dunning and others, 921 99 the absorption of Ucoldn ( 90° K.) neutrons in Cd. being about 4% greater than that of t "warm" neutrons (room temperature.) The effect, though anall, is regarded by funning as being well above probable error. A much larger effect of temperature has been announced by Moon and Tillman951100 and confirmed in part by Ferm1. 68 The activation of silver, rhodium, iodine, and copper was found by the former investigators to increase by about 10 - 20% when a layer of paraffin 2.5 cm. thick surrounding the detector was cooled from room te:nperature to liquid air temperature, the temperature of the neutronsource, which was located outside of a Dewar flask containing the cetector and the paraffin being cooled, rema.in:ing constant at that of the room. Ridenour and Yost sought to meaaure thiseffect in silver, bromine, and indium. 'Ihe experimental setup for irradiation is shown in Figure 8, and measurement of the activated sample was carried with the same electroscope and by the same procedure used in the previous experiments. Measurements using the same Rn-Be neutron source were carried out, first with the paraff:in warm, and then with the Dewar flask and its contents at liquid air tanperature. A layer of Cd 1 mm. thick was placed behind the detector, with the intention of preventing neutrons from passing through the detector more than once. results for silver and bromine a re shown in Table III. - 42 - The Moon and Tillman951 lOO report a 17% effect in the case of' silver, while the effect observed by Ridenour and Yost is certainly much smaller, and cannot be said to be a great deal larger than the errors of measurement. The results for Br seem more definitely to show that there is an effect of thee ort observed by Moonand Tillman. The moat precise measurements made by Ridenour and Yost on the temperature effect employed the 55 minute activity of' In for the detection of' the slow neutrons. Eight simile.r circular sheets of' 7 cm. diameter and 0.125 mm. thickness were prepared f~om pure indium and irradiated in a stack. Since the thickness of of' each sheet was the same, the activities f'oWld in the sheets could be intercompared. After the stack of eight sheets had been radiated for an appropriate length of time, the sheets were separated and measured one by one, the readings on each being subsequently corrected for decay of the activity. A carefully measured decay curve from which the corrections were made is shown in Figure 9, demonstrating that the errors introduced by correcting for the decay cannot be large, and that only the 55-minute activity was involved in these measurements. The first run with these sheets indicated that there might possibly be an effect of temperature on the activation of the one closest to the detector (see Figure 10). Accordingly, two sheets of a thickness of 0.05 mm. were prepared and placed closest the source. Behind them were two of the 0.125 mm. sheets used in the previous experiment. The results of this experiment, which was made with a stronger neutron source than that used in the previous one, and is hence more accurate, seem conslusively to show that in indium there is no effect of temperature either on the abS> lute value of the activation or on the absorption exhibited for slow neutrons. The situation regarding our knowledge of the energies of the slow neutrons is still far from satisfactory, principallyfor the reason that these energies lie in a region which is extremely difficult to investigate. Dunning92 and Bonner and Brubaker101 have demonstrated that the energies of slow neutrons are below those measurable with an amplifier and counter or with a cloud chamber; viz., lower than about 50,000 or 100,000 electron volts. Measurements made by means of neutron- produced zadioactivity are still extreirely inconclusive. It does not seem to have been pointed out that since the concentration of an active isotope by the method of Szilard and Chalmers (see Section 3) depends on the removal from its chemical bond of t.i'1e atom struck and made radioactive by a neutron, the neutron must have a certain minimum energy in order so to remove the struck atom and make it available for the subsequent separation of the radioactive atoms from the inactive balance of the compound irradiated. Jn ethyl iodide, for example, the energy required to remove the I atom frornthe ethyl radical ie about 1 electron volt, and if the collision between the neutron and the I nucleus is regarded as elastic, fran a classical point of view the I atom can take only 1.6% of the energy of the incident neutron. Thia means that for the separation of active iodine from ethyl iodide to be effective, the neutrons activating the iodine atoms must have energies of at least fif'ty volts, or two hundred times the velocities of thermal agitation. It would seEm that careful measurements of the yield of isotopic separations made by the method of Szilard and Chalmers on compounds in which the I is bound with various energies would give valuable information about the energies of' the "slow" neutrons effective. - 44 - ACKNOWLEDGEMENTS The wor~ described in this paper in which the author has had a share was done in collaboration with Professor Don M. Yost or Mr. Ken 1 1ch1 Shinohara, or both, and to them the author is extremely grateful. A great many of the measurements made on the selective absorption of slow neutrons were ta.ken by Mr. Ken 1 ichi Watanabe, without whose assistance the work would have been very tedious. Professor c. c. Lauritsen constructed and loaned the electroscopes used in all this work, and was alv,ays a source of helpful advice and discouragement. To the staff of the Kellogg Labora:t;ory the author is in- debted for favors too numerous to mention in detail. 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Rev., in press Mass and A tom ic Numbers ot the Isotopes Naturally Occurring Stable Nuclei Known December, 1934 Mass numbers LAtom,c numbers Tl T 210--I~l 208---<> 206--Au_o_; 204-1-0-0 202--~°l 1 200 Tlr - -; Pb 1 9 8 - -?~ 196-- 1-0 194-- · l 192- 0 190-~ ?1 1e2 H& 188-0~ 186--o ;;o Pt 184-~ 182-o 0 180-- 180 78 Fl6t./Rt= .I r//£ SrAZJL.E /soro?£5 r. n r + tt :t J.+ 20 + L l±U. ' ' r.~-~ iif:I i.+b ~ .t ·~ ~tm 18 + r:+, e-l-,..,_,; t-~--~ ~ + 16 :µs 14 + + . . ._ 12 -l-!. ;.,...-,-~'a:~tjj:.; Rtt..;. !:L-:r i+lt+ + -~"+ • -~•ti:1±'±:- Eb ~ -I.I ...1...1.l..- ' -,.....-1.. ' ''-,-,- '1:;.:-~~~.-~._.:±::+ :::.:p..;.. ±-<- ~M -j-o•· L..L.1.._._. . ;. -+-!--- - ~ ::::tp: j ±t J_,......, ~ . .,.., + e, ~ __ ,1- I 6 ~ -+1- 4 2 Jf. 0 F Ne Al s C/ 0 3 + - H+I--HN ' ~'\ ' t ttTI & + - a , I 0 0.4 t:J9 Pet~ctor I I .So~rte I .loo( I I 3S-C111t ...I I , HtitJ~E /, 3 .~ e. I /, 07 /, I /. 0 /, I e. hhffllJl»>A 0 e. 0 /,3 /, 3 rlti.>1. 1,3 .Vefectcr '77tick .\ Silver 46sorher, o. 90 ~z 7- EFFEcr OF PErc:cro,e 711/cKIVESS, Ere. (see Ce~t) /. 38 2./ /,Jt, /, 3 /,S ..s. so .S:8 ..S:5 :l. I s.7 .S.S' /,2 /, 4 /, 3 d t<<<<««e«c 0 ~./ s. 7 .s.7 S.33 »>>>»mm 0 /,4 S.S" s. 72. """8'" r /. 3 /, 3 /. 4 /,S /,3 .S.7 S: 2 s:2(, \ ,.o Thin "T7tlc~ .S. 3 /,.O .J / Pe-lee.tor Tlt/ck 77,/ n S:4 .s;3 \ i»»~,,,, • MEASURED ACTIVITY ""9 .s. I S:S \ 2.Jetec.l'or Tltick. Tlrirt ZJetec. tor 6. <<<C(C<((c, 0 b. 1 CONZJIT/0/VS OF ACTIVAT/0/V c. r d C0/111)/T/ONS OF MEAStJREMEIVT T c. r d Thln I '"'"'""" 0 6. Tlticlc .Petector (l. »»>>m,,,, ..Petector ~I ~ndow Eledroscqt,e a. rrAtffiT;»rf" O.sokrce "i · :t)eted:or a. - ·''' . '.'' ,_j i i=r ,-tt;-, - 11+l+l+I+ ~ J -t , fflll . ol" "t:. 't,f' ,~ ttl ~ , rr I ' t,j'I "' :iol[J 0 0 o.4 o.8 /.Z ± 40 80 /20 - /{,O u.-~~ ' 2()0 - f. l, ., ,,,,, _,,,,,,,,,, ,,,,,,,,,, ,,,,,,,,,, ,,,,,,,,,, ,,,,,,-,.,.. , . ~ ~ ~ ~ ~ -•-~·~·'£,· ' ' z. 0 ·"'+f 2 ,4 ■TTTTTTTTT=m=fl'f. . .~ 2.B r.,' -1:.1 - ii J-f r~ £~ 1-' ~ . ...L + 240 -· ' Hi 280 ~~ 320 J B; I..~!'i,J .:5,o a:- 400 440 E ft-:r_;.:cc:_ ill+'~ .,.... :rt.::; ;:.~ , -i-~ f - ~ ~ ~ ,- :l.,7j ;la; ~t,'f~~~ __.-4- .,.µ.;. ' ::;, : . +1 .t ii :;:P,:± 7b:: ::1'::.: trfiti'- ·r'' +1·~-- fI. ~l+';:t~ ~ l 1$,c °t' T .-, :tj: j rn:i .i m..i ...1.:t:l. 1. ' * r±;:p m:: =hi ti$ ~£:F : : ~ t· t .+tt +-+-+ .... ' '30 f. r u:):i ~t'tl ti . ·I J(l f.' "+ + t ......1--,·~ n + I IH .....,.t=::::t:C :.r.:::; ~ ~-- ·.ttt!:tt:tt:.i ....... ,4- J:;:t;:. ~ ~'t;:::; "Tt-+ .,.........,._..~-H , -H ,;-t't - 1-1:: ~ -v.--__...i-1 't-i.n. -4 +. ri rz rm: . ....... /0 :..;++ ~• •~ :t .., r ::U q,;1 · r cl -+: .:r: le. I 2 4 APPENDIX A LAW OF ABSORPTION OF NEUTRONS FROM A NON-PARALLEL BEA1J The assumption is explicitly made in the following treatment that the absorp- dQ tion of neutrons is exponentia l when measured in a strictly parallel beam. a. Neutrons are incident at all angles on one side of a slab of absorbing material of thickness d, whose cross-section for neutron capture, defined in the ordinary way, is rr . Then the number of neutrons passing in unit time through an area dS, normal to the surface and at a depth x, having come from the cone of opening d.Q and angle normal, is - a-Nx.. z;;;p. #adJ2 dS ~Je i from the u, where N = nuclei/unit volume in the absorber, and N0 = neutrons/steradian/second incident on unit area. dtfl = ~ Jl.tl-Pdf, Writing we have 'TN>< ;¼ d$ ~ J~ ~ t:/Jdf e - (,;r.}-;,,. The number of these neutrons absorbed in a thickness dx will n fl /l be Md$4U('V't.6?vd--i7dt.fe - er:#* U'3 <T"NdK , UKJ-P- • it( so that the total number absorbed in the slab of thickness d Nd a-NK will be ;Vt:1 d$ df ~ J~;. (T' _ / e--;;;:;,: d ~ ~11'1% 1 I) 0 " ?o:, Performing the integrations with respect to x and J:f _ <rNXlx.::e< -~tr~ dS () ~ t~--P L~ e c-c,~ x=o J i = ; ll ,;;rNd = ~11'~ d,$ {l-e-~) ~ ~C,u.J~d,P. 0 Let cos .,J ~ () then sin ,P d J = - d9 . f , we have of base unity and height d: [1- e-~Ke -~ ~ E/{-KJj 11;Yo d-5 f-.1Ffd}j = tr!V'odS where k = d'" Nd. 2 The number transmitted through unit area of -..1Ftd)f} an absorber of thickness d is '1l'!Yo/;-[1 .=: ..l?r/Yo J="°(d) where 2F(d} = e-k_ ke-k - k 2Ei(-k), and k = ~ Nd . The percentage transmission is then .,?g/V:,(d } =- 2 F(d) ff. () Vvhile it is easy to see that 2F(d) decreases more rapidly for small absorber thickness (i.e. small d and smatt k) than does e -k ' nevertheless this function does not describe the experimental results. See Table IV and Figure 11. Since edge effects are negligible in the experiments of Ridenour and Yost, it may be that this discrepancy is to be explained as being due to the absorption of the neutrons according to a nonexponential law, even in a parallel beam. TABLE IV VALUES OF e-k AND OF 2F(d) FOR ROUND VALUES OF k k 2 Ei(-k) 2F(d) log 2F(d) 0 1 0 0 0.01 0.91 0.09 -1.82 -0.018 0.83 9.920 9.957 0.2 0.04 0.82 0.16 -1.22 -0.049 0.70 9.848 9.913 0.3 0.09 0.74 0.22 -0.91 -0.082 0.60 9.779 9.870 0.4 0.16 0.67 0.27 -0.70 -0.112 0.51 9.711 9.826 0.5 0.25 0.61 0.30 -0.56 -0.140 0.44 9 .646 9.783 1.0 1.00 0.37 0.37 -0.22 -0.219 0.22 9 .340 9.566 2.0 4.00 0.14 0.27 -0.05 -0 .19 6 0.06 8. '785 9.131 3.0 9.00 0.05 0.15 -0.013 -0 .117 0.017 8.230 8.697 4.0 16 -0.061 0.006 7.778 8.263 k k2 e-k ke-k E i (-k) 0 0 1 0 - 00 .018 .073 -0.004 log e-k H- T -r- ~~; -tj_-t-t . ;. . ,. r::..--• .......-:-,- ----E ,t~ -+~ ~~- ~· !q..... ,-r-1 + F r?---rr:-ti m:r ::::1r :-·-n- . . . . . . ;. :;:tr:· ...::tt: ~ ..1.J..:..... '±l' 8o 1- ~-: : r-~ ~~;_, ~ ~~ ~.~. ~ Tc~-~- u,. --1 . j+ ~ ffl:+_.ltt:i.... ...:-. ';~ ~ ~ b: ~ · S.,Ij±I r.WJ= ! ir - 6o ~:.; :: :..,;_ - 7 ±d1fIHl- t:: - ::::t± t , -. t---+.........,.... -.• . ~j:i............. ~ 'I-W ~:17.· Ho ➔ ...... -H ··m;- . ➔ h tr t+ + 0 0 az 0.6 IO /.2 /.4- /.6 ERRATA Page 3, line 4 For detain read detail. Page 10 Insert page number at bottom of page. line 14 Omit parentheses around 5. !?age 14 , reaction I In equation, read 1 eT for le. omit and. line 19 l?age 15, line 8 Insert comma after reaction. :Page 15, line 9 Insert at least between known in and one case. reaction III In equation, read -le - for e - • In equation, read e,f, for e+ • reaction I V In equation, read r eaction II 1 -1 - e - for e - • Page 16, re a ction V Rea d 1 e+ for e .. • Add+ hv reaction VI to end of equation for reaction v. Read _1 e- fore-. Page 17, rea ct. VII Read _ e fore • 1 reaction VIII Read _1 e- fore-. Page 18, line 8 At end of line, references are 4 3 , 44, not 42 , 4 4 • Page 21, 5th line under Nitrogen: for Wettenstein read Wertenstein. 30 Page 22, last line in Na VII(?): Reference should be 20 , not • Page 23, lirie 6 Omit~ preceding~~ function. 6th line under Silicon: For lifes read lives. last line: For Fermk. read Fermi; also for - read i. Page 24, 2nd line under Pot a ssium: 4th line under Potassium: 3rd line under Calcium: For and read has. For indiced read induced. Insert comma after Fermi. Page 25, 2nd line under Cobalt: For VII( ? ) read VIII( ? ). 8th line under Cob a lt: For t wo~ read too. - Page 27, 1st line under Yttrium: - For VII( ? ) read VIII( ? ) MORE ERRATA l?age 8, 5th line from bottom: for electro:aco:pe read electroscope. l?age 10, line 4: for incluside read inclusive. Page 16, reaction VI, in equation: for ziz read Z-2. Page 30, 3rd line under Samarium: for An read In. Acknowledgements, line 8: for discouragement read encouragement. INSERTION OF FIGURES Insert Figure Agter page 1 3 2 14 3 20 456 35 7 41 8 42 9, 10 43 Appendix A 44 CORRI GEUDA I should like to have what now stands under Fluorine I(?) replaced by the following: Frisch103 has recently reported that a positron emission first observed by Meitner47 from fluorine bombarded with alpha-particles corresponds to the formation of Na22, whose half-life is extremely long: of the order of 6 months. The half-value thickness for the positrons is 0.03 g/cm2 of Al. A chemical seuaration of the active Na has been made. Fahlenbrach85 mistakenly attributed Iv1ei tner' s result to the format ion of F1 '1 from atmospheric nitrogen and its collection by recoil on the target. Under Calcium one should also add a new paragraph, as follows: I Frisch 10 3 has reported that Sc 43 , a positron emitter of 4.4 hour half-life, is formed by bombarding Ca with alpha-particles. Half-value thickness, 0.06 g/cm2 Al.