Artificial Radioactivity Citation Crane, Horace Richard (1934) Artificial Radioactivity. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/PKRZ-6N82. https://resolver.caltech.edu/CaltechETD:etd-06032004-132356 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 Awards: Caltech Distinguished Alumni Award, 1968 Thesis Availability: Public (worldwide access) Research Advisor(s): Unknown, Unknown Group: Caltech Distinguished Alumni Award Thesis Committee: Unknown, Unknown Defense Date: 19 May 1934 Record Number: CaltechETD:etd-06032004-132356 Persistent URL: https://resolver.caltech.edu/CaltechETD:etd-06032004-132356 DOI: 10.7907/PKRZ-6N82 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 2415 Collection: CaltechTHESIS Deposited By: Imported from ETD-db Deposited On: 03 Jun 2004 Last Modified: 22 Jul 2021 20:52 Thesis Files Preview PDF (Crane_hr_1934.pdf) - Final Version See Usage Policy. 1MB Repository Staff Only: item control page

ARTIFICIAL RADIOACTIVITY Thesis In partial fulfillment of the requiremente for the degree of Doctor of Philosophy Submitted by Horace Richard Crane California Institute of Technology Pasadena, California, May 19, 1934. The following thesis is a presentation and discussion of the work done up to date with the artificially produced radioactive substances, in this and in other laboratories. In many instances the product of disintegration is positrons, and this has mede possible several experiments concerning the annihilation of positrons and the conversion of their rest mass into radiant enerry. The recent work of Fermi on the production of f#-ray bodies by neutron bombardment is also described. Introduction At the time of this writing, the phenomenon of artificial radioactivity has been known for a little less than three monthse It is therefore possible now, as it will undoubtedly not be possible within a short time, to include in a brief paper a reasonably detailed account of the whole of the field. Although attempts to induce radioactivity, and especially to influence the rates of decay of the natural radioactive substances have been made since the discovery of the radioactive substances themselves, not one of the many successes reported has stood the test of closer examination, so it can be said with a great deal of certainty that the history of artificial radio- activity began with the work of Curie and Joliot, published in January ,1934.* % had been clear for some time, in the light of experiments on the arti- ficial disintegration of the light elements, that bringing about a change in nuclei as heavy as those of the radioactive seriea by penetrating them with the projectiles we have available would have a probability so low that it would be hopeless.» The only method of attack therefore lay in the trans- mutation of the Licht elements, and even this did not seem highly promising, in view of the fact that no natural radioactive substances were known to exist in the region of the low atomic numbers, indicating that perhaps oly the very complex nuclei were capable of such behavioure If we arrange the elements according to mass number and atomic number, as shown in Fige 1, we see that they fall along a well defined line, and this we may think of as the region of the stable nucleic 24 20 I6 ym MASS NUMBER Bec tetrad aaen Ea aun anne EES be Sh iY frome peeae GOMES Upees es! tee 0123456078 9 10 12 13 46 IS m WH Heli BeB C N O F Ne Nag Al Si: P Fige 1. The nuclei arranged according to mass number and atomic number. 8) stable nuclei <@® artificially produced radioactive nucleie The total absence of nuclei outside this region is consistent with the idea that the "stable region" is a trough toward which all nuclei rapidly move by means of spontaneous transformationse In the processes going on in nature, especially in parts of the universe where temperatures and pressures are extreme, nuclei of many descriptions must be produced continually, and if there were not a strong tendency for those not in the trough to move into the trough, we should expect to find a detectable number of them in existence. It is not unreasonable to think, however, that in the artificial trang- mutation of elements, under some conditions products might be formed which would not lie in the stable region of the diagram, but which would nevertheless have a lifetime long enough to make possible their identificatione In order to move into the stable region, a nucleus on the right hand side would have to throw off a positive charge, and a nucleus on the lef hand would have to throw off a negative charge, assuming that it would not undergo the more serious change of breaking up into two or more new nucleic The fact that 8 -ray bodies in the radioactive series emit only negative electrons is consistent with bees beens seems Soba Cueed Gubet Eat the diagram, as we shall see if we gE 7 a extend it upward to the high atomic aa Feat Wass cazecazecsteoed eee pages) A atest numberse For the low atomic numbers 3 =| /. | (Fig. 1) the slope of the curve is : = : such that the mass number changes te 3 just twice as fast as the atomic r ; number, but in the region of the 2 ied high atomic numbers the curve be~ ys x = : 3 comes considerably steeper (Fige 2). = : : : . : Te = : : : 7 3 d- particle transformations BBEEBEEY = ate necessarily cause a shift along a Pas F 45 degree line, since the ratio of . ue Tap “4 zh mass number to atomic number of the i “aa se : Be : = re o- particle is 2. Therefore after ATOMIC NUMBER Fig. 2. one or more d= transformations an element always finds itself off the curve to the left -=- never to the right -- and the emission of a negative electron is required to shift it back onto the curve. Experiments Curie and JoLiot made the striking observation that certain of the light elements, namely boron, aluminum and magnesium, continued to eject ionizing particles in large numbers for a considerable length of time after being bombarded with the od-particles from poloniome They studied the disintegration particles by means of a cloud chamber and also by means of the magnetic analyser used by Thibaud* y and found that they were in all cases positrona. The rates of decay of the active substances, and also the ranges of the positrons in absorbing foils were studied, using a Geiger-Muller counter, and the following results were obtained. Decay per iod** Upper limit of energy of positrons B 14 min. 0.7 x 10° e.v. Al 34m ine 2.2 x 10° ew. Me obmine Oe7 x 10° e.ve H, Li, C, Be, N, 0, F, Na, Ca, Ni, and Ag were tried but gave no observable effect. This negative result is not at all conclusive, because if an active substance were produced whose period of decay was either very long or very short, it would escape detectione It may indeed seem. fortunate, at first glance, that the decay periods of three or these substances happen to be of such convenient length, when one remembers that the decay periods of the natural radioactive substances range between 1010 years and 107} secondss Upon closer examination, however, it is seen that, while the Qeray bodies are distributed apparently at random over this wide range, the B-ray bodies fall within a very much narrower rangee Of the 17 Bray bodies in the radioactive * Described on page ** Time required for unit change in loge intensity. -~4o -. series (this does not include potassium, of which little is known) the greatest half life is 25 yeara and the smallest 1 minute, and A- ray bodies A= ray bodies UX, 2h days ‘Ac 8=6.:13 yrae MsTh1 7 yree tT 1010 yrs, - UXo 1 min. . AcB 36 mine MsTh2 6 yrse mo! 107! ‘gee. Uy 24 bree AcC 2 mine ThB 10 hrs. RaB 26 mine Acct! 5 min. Tho 1 hr. others at random Rac 19 mine tnott 5 mine between these Ractl = 1 min. limits RaD 25 yrae RaE 5 days 9 of the 17 fall between the narrow limits of 1 minute to 1 hour. In order to account for the production of radioactive substances from the three elements bombarded, Curie and Joliot suggested the following nuclear reactions: (1) Bl Het mem Ne nt mp lent ge? 24 a 2 1 2 1 (2) Me + He wy sivtan aay nts et (5) artnet > Pend —» 91a nd 4 Bt the yi? , sic! and p being the radioactive iostopes. These are indicated as «@ in the diagram (Fig. 1), the small arrows indicating the direction in which they shift as a result of the positron emissions In suggesting the above reactions, Curie and Joliot pointed out that, if the radioactive substance in the first case were really nid, it should be possible to produce it by bombarding carbon with deutonse The reaction would be Ol, He me tt nb em cl% nb y Bt In order to investigate carbon and other elements under deuton bombard- ment, Prof. Ce. C. Lauritsen, Mre W. W. Harper“and the author constructed a multiple target disk to be used in conjunction with the 1,000,000 volt positive ion tube already described’, on which was fixed targets of a number . of different substancese The disk could be rotated inside the vacuum by means of a shaft so that it was possible to bring any one of the targets first into the ion beam for bombardment, and then into view of a Geiger counter having a thin window for recording the delayed emission of particles or Y-rayse Targets of LiF, Be, Hy BOs » C, Mg and Al were subjected to bombardment, each for about 15 minutes with 5 microamperes deuton current, at 900,000 volts, and immediately rotated into view of the Geiger counter. In the case of boron and carbon, a large number of coumts was recorded during the first few minutes after bombardment. Carbon gave the largest effect: several hundred counts per second” (calculated for the total solid angle) and decreased at a rate corresponding to a half life of about 10 minutes. The -effect from boron was somewhat less intense, and the half life was about 20 minutese Other substances bombarded gave effects which were appreciable, but which might have been caused by carbon contamination on the surface of the targetse In the case of these small effects a further investigation is to be made, and a knowledge of the rates of decay, especially, will help to decide whether or not they are to be attributed to carbon contaminatione To determine the nature of this delayed activity Dr. Carl D. Anderson and Mre Seth He Neddermeyer placed a piece of freshly bombarded carbon in their Wilson cloud chamber and took a series of photographs, extending over a period of about two hours after bombardment. During the first hour each expansion revealed a number of electron tracks, all of which were of positive signe In addition, a few short negative electron tracks appeared, which originated in the gas, indicating the presence of some gamma rayse Examples of the tracks observed are shown in the following photographse * Subsequent measurements with an ionization chamber showed that the total number of positrons emitted, including the very slow ones, was much greater than this -=- about 10° per seconde I Garbon target, a few minutes after bombardment with deutona of 900,000 eave energy. Magnetic field,760 gauss. II Same as I, slightly longer time after bombardment. -7- TTI Same target as shown in TIT and IT, more than one hour after bombardment, showing the decrease in activity. Two branched tracks appear, which probably represent negatrons set in motion by the positrons near the end of their range. It is interesting that in these close collisicns annihilation of the positron does not cecurs = & = TV and V Two photographs taken without the uagnetic field, te give an idea ag to the amount of scattering suffered by positrons in thia energy range and therefore an idea as to the dependability of the measurementa of curvature of the tracks obtained with a magnetic field. Note the uniform curvature of one of the tracks in the upper picture, which is due to a succession of deflections in the same direction. VI ByOz target after bombardment with deutons of 900,000 e.v. energy, with a 19,000 gauss field, showing a number of tracks which originate in the gas in the chamber. This indicates that the positron-radioactive substance produced in the Bo0 is gaseous and diffuses out into the chambers Experiments which Give Evidence of the Annihilation of the Positron and the Conversion of its Mass into Radiant Energy Due to the large amount of radioactive material which it is possible to produce by bombarding carbon and boron with deutons, Profe Lauritsen and the author’ found it possible to perform a series of experiments in which _ the total ionization produced by the positrons and by the Y -rays associated with them was directly made use of, instead of counting the individual particles. This eliminated, for all practical purpooses » statistical fluctuations, and the resulta obtained were very closely reproducible. The experimental procedure was as follows: A target of the substance to be investigated was first bombarded for a suitable length of time, generally fifteen minutes, with an ion current of 10 microamperes, consisting principally of H, at 069 x 10° GeVe energy. The target was then removed -# 10 ~ from the tube and placed in the bottom of an ionization chamber, and the rate of production of ionization as a fimction of time was measured. The ionization observed was attributed to particles which were ejected from the target and traversed the chamber, and also to@erays, in case such were presente In order to separate the effect. contributed é CHAMBER L ACTIVE. TARGET “AM * ABSORBER ELECTROSCOPE Z-CHAMBER IL Fige 3. Arrangement of the ionization chambers and lead absorber used for measuring positron emission and for measuring the absorption of the -rays resulting from the annihil- ation of the positronse by Y-rays alone, a second ionization chamber was placed directly below the first. The walls and linings of the chambers were sufficiently thick to prevent charged particles from entering the lower chamber and giving a direct effect. Therefore any ionization recorded in the lower chamber was attributed to Yerayse In Fig. 4 are plotted the log ° intensities for the two chambers ol against time after bombardment when PN y hs ™ a carbon target was placed inside H nS by 2 LaGns 9 the upper chamber. Curve I refers A ALA N H é \ IN > to the upper chamber and curve II to 2 \ S la oe 2 X E the lower chamber. It is seen from ¢ VX a a at these plots that the half period ~ m | A (10.3 minutes) is the same within aN = ° I the experimental error, whether he oO determined from the rate of emission o © 79 Me IN Minutes 8? of positrons or from the Yerays Fic. 4. Intensity of ionization as a function of time after bombardment due to: I, positrons from carbon target; II, . . . y-rays from carbon; III, y-rays from carbon after 7.1 cm associated with the processe This lead filtration; IV, positrons (?) from BO, target. would seem to indicate that the Fig. 4 same radioactive process is responsible for both the positrons and the {- raya, and the most attractive assumption is that the Y=rays have their origin in the annihilation of the positrons together with electrons. To determine the absorption coefficient of the ¥-rays, we made provision for interposing a sheet of lead 7-1 mm. thick between the two chamberse For the first half hour after bombardment, readings were taken every minute on the lover chamber and the lead was put in and removed at intervals of five minutes. We thus obtained alternately four one-minute readings with lead and four one-minute readings without lead. These are plotted on a log scale in Fig. 4, curves II and III, each set of four one-minute readings being averaged. The log difference between the positrons of the two curves is 1.23, corresponding to a linear absorption coefficient of 1.58 per cme Since the source was very close to the lead = 12 = sheet, a large part of the radiation would necessarily pass through the lead at an angle, tending to compensate the effect of scattering. A . calculation from the Gray formula and the Klein Nishina scattering give ag the absorption coefficient for 24 x.sue (mc) radiation 1267 per cme, go it seems that the quantum energy of the radiation here observed is, to within our experimental error, equal to’me2, the energy equivalent of the rest mass of the electron. In order to ascertain further that the Y-rays originate in the annihilation of the positrons, we performed COVERY, c the following experimente A ACTIVE piece of freshly bombarded carbon . TARGET was placed, active side up, directly above one of the chambers, as shown in Fige 5. Supposedly in this case half the positrons are ejected in the downward direction and are annihilated in the carbon, while the Fig.5 other half escape and are annihilated in the air at a considerable distance away from the target and the ionization chamber. However, by covering the target with a sheet of some dense material, the positrons can be prevented from escaping, and they will all be annihilated in, or very near the carbone Alternate readings were made on the chamber with the carbon covered with aluminum and with it uncovered. The result was that the rate of production of ionization in the chamber was about twice as great with the carbon covered ag with it uncovered. -136 a In Fig. 5, curve I refers to the carbon covered with aluminum; curve II to the carbon without the aluminum cover. 2 As an additional check, we wished to compare the number of LOG. INTENSITY | Y -ray. quanta emitted with the ° 4 number of positrons. By on f+ _ calibrating the ionization chamber oN . . fo} against a standard radium source, we co) 5 10 15 20. «25 ~=©<30.~C*«S TIME IN MINUTES determined that immediately after Fic. 2. Intensity of ionization in the chamber: I, with the carbon covered; and II, with the carbon uncovered. bombardment about 6 x 104 photons per second are emitted from the carbon targets Making appropriate allowance for solid angle and considering that half of the positrons die in the target and the other half on the walls of the upper chamber, it turns out that there are 500 ion pairs to be associated with each positron passing through the upper chamber. The mean length of path in the chamber is approximately 5 cm; therefore the positrons would be producing, on the average, 100 ions per cm. of path, which is a reasonable figure and conatitutes evidence that the number of photons is very nearly twice the number of positronae . : 6 By comparing this with out previous work on carbon’, it appears that carbon can be transformed by deutons in the following two ways: of. y2ey Gly Diny 12 2 13 1 13 1 + C + H —, N' + n —> C4 n + EF 14 - § From our estimate of the number of photons obtained in each case, we are led to believe that the firat process takes place about 10 times as frequently as the seconds This is on the assumption that the annihilation ef one positron produces two photonse To accoumt for the radioactivity from boron, we may suggest a completely analogous reaction, BIO, 72mm cll s ni —» sll, nis Et In this case, also, there are alternative reactions which undoubtedly take place more frequently than the one which producea the radioactive substance. These are plo, He —> 3 He 4 Bog we me Blt eg | The rate of decay of the activity of a Bo Oz target, as measured by placing it inside chamber I is show by curve IV in Fig. 5. Je Thibaud! has performed an experiment, using a technique quite different from that just described, but which also demonstrates clearly that radiation results from the annihilation of positronse A sketch of his apparatus is shown in Fige 7e .— ELECTRON PATH ,- ABSORBER yj YW, PHOTOGRAPHIC PLATE SOURCE Fige 7. Thibaud's apparatus for deflecting electrons from a source onto absorbing foils over a photographic plate. The magnetic field is inhomogeneous and perpendicular to the plane of the papere The large block of lead shields the plate from the sourcee ~15- ‘ Positrons are produced by allowing polonium od=-particles to bombard continuously a target of aluminum. The positrons are then bent in a magnetic field so that they will fall on absorbing foils placed directly over e photographic plates In addition to the bremsstrahlung, or continuous X-radiation which is produced when the positrons are stopped in the foils, there appears a strong monochromatic component of radiation of quantum ene ray about 0.5 x 10° @eVe, Which is attributed to the conversion of the positron rest mass into radiation. By replacing the positron source with a radioactive @~ray emitter and reversing the magnetic field, the same experiment was performed with negative electrons. Here the monochromatic component Was absent, and only the bremsstrahlung appeared. The curves obtained, showing the intensity of radiation recorded by the photographic plate as a function of the thickness of absorbing foil, for positive and for negative electrons, are shown in Fige 6 It should be noticed that, for large thicknesses 10°' INTENSITY “3 yo? 10 -4+ 10 NEG. ELECTRONS 3 ro i i it 2 4 6 8 0 _6M/cm* ABSORBER a of absorbing foil” the total amount of radiation resulting from the positrons striking the foil is about 40 times as great as that resulting from an equal number of negative electrons of the same velocity. Thibaud has calculated that this excess is about two quanta per electron, as. would be expected on the assumption that the annihilation of a positive electron, together with a negative produces two quantae Chemical Tests of the Artifically Produced Radioactive Substances Due to the rapid disintegration of the positron radioactive substances, extremely gmall amounts of these materials can be detected and followed through chemical proceasese By using 100 millicuries of polonium, Curie and Joliot® were able to produce, in the most efficient of the reactions, a total of about 10° radioactive atoms, with which to perform chemical experiments, and in this way obtained confirming evidence that the active substances were those which they had at first suggested (see page 5)e An example of their chemical experiments will be briefly outlined below. Boron Boron nitride powder was bombarded with the particles from the polonium source until approximate equilibrium was reached between the production of the radibactive substance (supposedly yn) and its decay. The powder wag then mixed with soda and heated. This converted the nitrogen into ammonia, which was driven off by the heat. After this operation no activity could be detected in the residue. The ammonia gas Wag collected in a thin walled glaas tube, and found to contain all the active material. The gas was then exposed to a piece of blotting * The foils were in all cases thick enough to prevent an appreciable number of primary electrons striking the photographic plates ~17= paper soaked with hydrochloric acid, which very quickly absorbed all the ammonia, converting it into NHACL in the blotter. It was found, after “this operation that all the active substance had been collected by the blotter. Although these experiments do not constitute a rigorous chemical analysis, they confirm the belief that the active substance is in this case an isotope of nitrogen, and that, in the experiment just described, it existed in the form of N ‘Hb, mixed with the n™ Hy, which was Imown to be present in a large amount, due to the nitrogen in the boron nitride powder used. Similar chemical experiments were performed with the radioactive substances produced by bombarding aluminum and magnesium with d=particles, and evidence was found indicating that the active substanceea were isotopes of phosphorous and silicon, respectively. Radioactive Substances Produced by Deuton Bombardment In observing the activity of a BS 05 target in a cloud chamber after deuton bombardment, Dr. Anderson and Mr. Neddermeyer noticed that after a short time a large number of tracks appeared which originated in the gas (see photograph VI on page 10). Although it had been assumed that the radioactive substance in this cage was git, the above observation indicated that the active material was a gas which rapidly diffused out of the Bo Oz As further evidence of this, Prof. Lauritsen and the author” found that warming the active By Oz to about 200° © in air completely drove off the radioactive constituent. It therefore seemed desirable to collect, and if possible, determine the nature of the active gas. To do this we warmed a small amount of the activated Bo Oz in a flask containing air, and ‘tranaferred the air into an ionization chamber, through a liquid air trap. As long as the chamber was protected by the liquid air, no activity was _recordede However, upon remov ing the liquid air and allowing the trap to rise to a temperature considerably above that of liquid air, a large amount of the active material made its appearance in the chamber. The Bo Os, which was left in the flask was afterwards tested and found to have no activity. From this we concluded that after the By 0 3 Was bombarded, nearly all the gil existed in the target as 00 or COo, and very little, if any, as carbone A graphite target after bombardment with deutons, which supposedly conta ined wld, was also heated in an effort to drive off the active constituent, but after heating to a red heat, about half the activity remained in the target. This result does not necessarily cast doubt upon the belief that the active substance is nl), because we know that nitrogen, as well as other gases, is very strongly adsorbed on carbone Prodi tion of Radioactive Substances by Proton Bombardment Very shortly after the first production of radioactive materials by means of deutons, it was found, independently by Cockroft and Walton’, by Lawrence and his associatea-!, and by Prof. Lauritsen and the author”, that protons directed against some targets, particularly carbon and boron, produced a considerable amount of positron activity, and that the rate of decay was in each case the same as that of the activity produced by deutonse The intensity of the effect produced by protons, as compared to that produced by deutons under similar conditions was, in the case of carbon, about 10 percent, and in the case of boron oxide (Bo Oz), about 20 percent. - 19 = The Pact ‘that the decay periods of the active substances produced by protons and by deutons are quite exactly the same suggest strongly that the same radioactive products are formede In the case of deuton bombardment the radioactive substances are supposedly w!3 ana git respectively, and hence for the case of proton bombardment we may suggest the following alternative reactions which would involve the same radioactive products: ty pt ee We oy Bt a) 1 Bro, yt ee cht» ply BP where the excess energy calculated from the change in mass in the overall reaction is given, and oh, Ht >» we nt —>cl?y nb y Et pity pt up otte nt > alte ante ct 2) where in accordance with the presumable mass of the neutron the excess energy from the change in masa is nearly negligible and might be either 13 and pit at the beginning positive or negative. If we assume that the C and at the end of the process are identical, then the overall process is just equivalent to changing a proton into a neutron and a positrone One hesitates to accept the first type of proposed reaction, mainly because the probability of a particle being added te a nucleus without the ejection of some other particle to carry away the excess energy is extremely smalle So far no example of it haa been established. The second type of reaction seems more attractive, since any excess energy could easily be carried gway by the neutron. If we take the masses of the proton, neutron and positron as 1.0072, 1.0067 and 0,0005, respectively, then the only kinetic energy available for the neutron and the positron is the kinetic energy contributed by the incident proton - 2 - “(Cin this case 0.9 x 10° eev.). Nevertheless, we may not be justified in expecting to find the energies of the positrons limited to 0.09 x 10° CeVes -since the known phenomena of the continuous -<-ray spectrum have already indicated that the etrict law of the conservation of energy may not hold in the case of electron emission. It might be reasonable to expect, how- ever, that the energies of the positrons plus those of the neutrons would satisfy the conservation of energy statistically, and therefore have 009 x 10° GeV. a9 & mean energye Two exper imenta suggest themselves, the results of which might help in deciding which of the two types of reaction is the correct one. If the latter process is the correct one, it would seem reasonable tmt a change in the total energy available, or in other words, a change in the energy of the bombarding protons, should “have some effect in addition to the usual change in the probability of penetration of the nuclear barrier. The first thing to look for would be some minimum energy below which the reaction could not take place, regardless of the penetration of the proton into the nucleuse An experimental curve which we have obtained, giving the rate of formation of the radioactive substance ina thick carbon target aa a function of the voltage used to accelerate the protons is shown in Fig. 9, curve Ie The curve obtained by differentiating this curve is also shown (curve II) and this should represent the effect for a thin target, or the true probability of the reaction taking place as a function of voltages It is seen that the overall probability rises very rapidly between 550 and 650 kv. and is thereafter nearly constant. A decision as to whether this rapid rise reveals anything more than the probability of the penetration of the proton into the nucleus must await more accurate measurements and an investigation of analogous cases. 3 RELATIVE INTENSITY 500 600 709 B00 900 KILOVOLTS Fige 9. Efficiency of production of radioactive substance in carbon bombarded with protons, as a function of voltage. ay 20 T NUMBER OF POSITRONS 8 1] 16 r T ¥ 4 Ww 2 4 6 8 Lo (2 1.4 1.6 1.8 ELECTRON VOLTS x \0® Figel0. Energy spectrum of positrons from a carbon target bombarded with 700 kv. protons (open circles) and with 900 kve protons (filled circles). A second implication of the type of reaction under discussion is that, since the kinetic ene rey of the positrons must be derived in a large part from the energy of the bombarding protons, a change in the energy of the protons might be expected to influence the energy spectrum of the positrons. This would not be true, however, if the only requirement were that the energy supplied be greater than some minimum value, the excess over this being carried away by the neutrone To test this possibility Dr. Anderson and Mre Neddermeyer ' took a large number of cloud chamber photographs of tracks from targets bombarded with protons at 700 and at 900 kve The results are shown in Figs 10. No difference in the energy spectrum of the resulting positrons could be established with certainty, and it will be necessary to have a great deal more data to decide the point definitely. Production of B-active Substances by Neutron Bombardment Within the last few weeks Fermil2 hag reported the production of a number of radioactive substances by neutron bombardment. In each case he assumes that the neutron is captured and an a particle emitted, as in the typical reaction plo 1 1 + nt pion Het the ni6 then emitting a negative electron, becoming ot 6, The activity which can be produced by neutron bombardment is very small (of the order of 1 electron per second) compared to the positron activity “produced by deuton bombardment, which is of the order of 104 positrons per seconde This is due in part to the very small absorption of neutrons in the target, and in part to the relatively small intensity of neutrona - 23 = “Which can be produced. - Neutrons have the advantage, however, that they penetrate the very heavy nuclei apparently as easily as they penetrate -the light nuclei, and Fermi has already been able to disintegrate barium (at. no. 56) 6 A list of the elements which Fermi hag been able to trans- form into radioactive substances by neutron bombardment is given below, with asterisks to indicate the relative intensities. Element Decay period Fluorine* Silicon *** 3 mine Phosphorous ** 3 hre Chlorine Vanadium 5 mine Chromium** 6 mine Iron 2 hre Alum inum* Arsenic 2 days Silver ** 2 mine Tellurium 1 hr. Iodine** 30 min. Barium 2 mine Some activity was also obtained with Na, Mg, Ti, Zn, Zr, Se, Sb, Br and Lae The results of the experiments on artifically stimulated electron emitters which have been described in this paper, and especially those stimulated by protons are intimately tied up with the problems of the continuous beta ray emission and the question of the validity of the principle of the conservation of energy, both as applied statistically and to the elementary processe After more experimental data has been made available something of the previous history of the electron emitters will be knowm as well as the history of their decay, and hence they promise to shed a great deal of light on the general problem of electron disintegration and the apparent failure of the principle of the conservation of energye ~ oh & REFERENCES le Curie and Joliot, Comptes Rendus 198 254 (1934) 2. Lauritsen,Crane and Harper, Science, 79 234, (1934) 3e Crane, Lauritsen and Soltan,PhyseReve 45 507 (1934) 4, Neddermeyer and Anderson, Phys.Rev. 45 498 (1934) 5e Crane and Lauritsen, Phys.Reve 45 430 (1934) 6- Lauritsen and Crane, PhyseReve 45 345 (1954) 7e Je Thibaud, Comptes Rendus 197 1629 (1933) 8. Curie and Joliot, Comptes Rendus 198 Phy (1934) 9- Crane and Lauritsen, PhyseRev. 45 497 (1934) 10. Cockroft,Gilbert and Walton, Nature 133 328, (1934) li. 12. Fermi, La Ricerea Scientifica Vole I, N6 Anno V