ANNUAL REPORTSON THEPROGRESS OF CHEMISTRY.RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.DURMG the year, very rapid progress has been made in the study ofartificial disintegration. The transformations among the lighterelements have been in many cases so inter-related that one canspeak of nuclear reactions and a nuclear chemistry. The energyrelations have led to a new scale of nuclear mass, and the detectionand measurement of excited levels in these atoms are the beginningof a stage in the ‘( nuclear quantum physics ” initiated by the relationof a- and 7-rays in former years. The use of neutrons as bombardingparticles and the development of the (( slow neutron ” process haveled to wide developments in transmutation among the heavierelements. Some progress has been made in the study of the p-raytransformation and of the penetrating radiation.1.ISOTOPIC CONSTITUTION OF THE ELEMENTS.In this field, work has continued with the mass spectrographand by spectroscopic methods. The technique of the formerconsists largely in obtaining ions from recalcitrant elements, andA. J. Dempster has introduced for this purpose a high-frequencyspark between solid metal electrodes in a high vacuum. Thissource yields abundant ions from many elements, including Pt,Au, W, and Sn. Platinum examined by this method gave isotopesat 192, 194, 195, 196, 198, the lightest being faint and 198 not sostrong as the other three; palladium gave isotopes at 102, 104,105,106, 108, 110. Gold gave no trace of Au 199 and it is suggestedthat its accepted atomic weight is too high.Iridium gave 191 and193, the latter being the more ab~ndant.~ Uranium gave oneisotope at 235 with intensity < 1% of that due to U238, and it is1 Nature, 1935,135, 542 ; A., 677.2 A. J. Dempster, ibid., p. 993; A., 909.3 Idem, ibid., 136, 909; cf. B. Venkatesachar and L. Sibaiya, ibid., p. 437 ;A , , 129516 RADIOACTIVITY AND SUB- ATOMIC PHENOMENA.suggested that this substance is the parent of the actinium ~ e r i e s . ~Further evidence on the actinium probleni is obtaiiied from achemical determination of the atomic weight of protoactiniumby A. von G r ~ s s e . ~ F. W. Aston has published new mass-spectrograph results on Hf, Thy Rh, Ti, Zr, Cay Ga, Ag, Ni, Cd, Fe,In, and the mass-spectrograph has also been used to obtain valuesfor the abundance ratio of isotopes of Li, K, and Rb.'X'ectroscopic Methods.-The study of hyperfine structure ofspectral lines which show an isotope splitting and the investigationof band spectra now provide a more or less definite contributionto our knowledge of isotopes. For instance, in platinum the ratioof the abundance of Ptlg2, Ptlg4, Ptlg5, Ptlg6 is given as - : 5 : 8 : 8 byB.Fuchs and H. Kopfermann and as 2 : 10 : 13 : 16 by B. Ven-katesachar and L. Sibaiya,Y both sets of workers using photometryof the hyperfine structure of PtI lines, while band spectra have beenused to investigate the isotopes of cadmium, zinc (hydride bands),lOand indium (iodide bands). l1Some additions have been made to the determinations of nuclearmechanical and magnetic moments by the methods discussed inthe Report for 1934.The new determinations include holmium,12hafnium,13 silver,l4 lanthanum,15 and potassium.16 The onlyimportant technical advance represented is a spectroscopic one,oix., the use of absorption-spectrum lines taken with an atomicbeam to avoid Doppler effect. Regularities in the distribution ofnuclear mechanical moments among atoms of odd and even atomicnumbers have been noticed by S. To1ansky.l'Among the lighter elements, the study of the nuclear transmut-A. J. Dempster, Nature, 1935, 135, 180; A . , 1048; A. von Grosse, J .J . Amer. C'hem. Soc., 1934, 56, 2501 ; see also these Reports, p, 146, andProc. Roy. SOC., 1935, [A], 149, 396; A., 802.Physical Chenz., 1934, 38, 487; A., 1934, 578.review by G.Elsen, Ghern. 'CZ;'eelcblad, 1935, 32, 343; A . , 910.' H. Rondy, G. Johannsen, and I<. Popper, 2. Physik, 1935, 95, 46; A . ,909; A. K. Brewer arid P. D. Kueck, Physical Bev., 1934, [ii], 46, 894; -4 .,1935, 140.ii-aturwiss., 1935, 23, 372; A., 909.Proc. Indian Acad. Sci., 1935, 2, A, 101; A., 1185.10 G. Stenvinkel and E. Svensson, Nature, 1935, 135, 955; A., 802.l1 M. Wehrli, Helv. Physica Acta, 1934,7,611; Chem. Zentr., 1934, ii, 2960 ;A . , 1935, 558; cf. A., 1934, 1286.H. Schuler and T. Schmidt, Naturwiss., 1935, 23, 69; A . , 424.la E. Rasmussen, ibid.; A., 424.H. Hill, Physical Rev., 1935, [ii], 48, 233; A., 1183.l5 ill. F. Crawford and N. S. Grace, ibid., 47, 536; A., 676.l G D.A. Jackson and H. Kuhn, Proc. Roy. Soc., 1935, [A], 148, 335; A.,l7 Nature, 198.5, 135, 620; A . , 676.555BRADDICK. 17atioiis which take place when these are bombarded with protons ordeuterons has led to an interesting revision of nuclear masses.The necessity for revision was pointed out independently byH. A. Bethel8 and by M. L. E. Oliphant, A. E. Kempton, and(Lord) R~therford.1~ In nuclear reactions there is strong evidencethat the law of conservation holds if mass and energy be regardedas equivalent. The energies involved in the transmutation of theH, He, and Li nuclei among themselves may be beautifully explainedon this basis,20 the masses for the nuclei being taken from themass-spectrograph work of Aston and of Bainbridge.If we regardthe nucleus Be9 as made up of two a-particles and a neutron, themass assigned by K. T. Bainbridge 21 (9.0155; 0l6 = 16.000)exceeds that of the components (8.0043 22 + 1.0080 23) by the equi-valent of about 3 M.E.V.* and the Be9 nucleus should therefore beunstable.No spontaneous disintegration had, however, been found bycareful experiment^.^^ Further, the energy of the a-particlesobtained when Be is bombarded with protonsis much less than that calculated from the mass data of Bainbridgeand of Aston.19 A separate experiment showed that the excessenergy was not radiated as a 7-ray. Similar difficulties arise inconnexion with the a-particles obtained by bombarding berylliumwith deuterons,Be9 + 1H2+ ,Li7 + ,He4and in the case of boron bombarded with protons.25Oliphant, Kempton, and Rutherford 19 point out that in the mass-spectrograph determinations the masses of HI, H2, Li6, and Li7were determined by comparison with He4, whereas those of Be9, B1*,and Bll were determined by comparison with 016 with differencesdetermined with reference to HI.A small error in the mass ratioHe4/O16 would give it cumulative error in the masses of the otherelements. By assuming a, small error in this ratio and applying aPhysical Rev., 1935, [GI, 47, 633.l9 Proc. Roy. Soc., 1935, [ A ] , 150, 241; A , , 910.2o Idem, ibid., 149, 406; A., 803.21 Physicdl Rev., 1933, [GI, 43, 367.22 F. W. Aston, Proc. Roy. SOC., 1927, [ A ] , 115, 487.23 J. Chadwick and M. Goldhaber, Nature, 1934, 134, 237.24 See, e.g., E.Friedlander, Compt. rend., 1935, 201, 337; A . , 1185; R. D.Evans and M. C. Henderson, Physical Rev., 1933, [ii], 44, 59; A., 1935, 141.25 D. M. Gans, W. D. Harkins, and H. W. Newson, ibid., p. 310; A., 1935,141 ; F. Kirchner, Physikal. Z., 1933, 34, 897 ; A., 1934, 128.* M.E.V. signifies one million electron-volts here and throughout thisReport18 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.corresponding correction to the mass-spectrograph data, it ispossible to obtain a set of masses which agree much better withtransmutation data. F. Kirchner and H. Neuert 26 had previouslysuggested a revision of the mass of Be9 to fit in with their transmut-ation data. Bethe 1* pointed out that the excited CI2 nucleuswith 4-8 M.E.V.extra energy was still stable against disintegrationinto three or-particles, while the masses given by Aston wouldnecessitate instability a t a much lower excitation energy. Hedrew up a new scheme of masses for the elements up to ol', andsuggested a revision of the He4/O16 ratio. The new atomic massesare given in Table I. F. W. Aston 27 has redetermined the massesof some of the light nuclei and has found values in better accordwith the masses deduced from energy changes.TABLE I .Revised Nuclear Masses based on Transmutation Data.Bethe. 18 Oliphant et al. l9 Aston, etc.lH1 1.0081 1.0081 1.0081 * onl ..................... 1.0085 1.0083lH2 ..................... 2.0142 2.0142 2.0148 *1H3 ..................... 3.0161 3.0161 3.01512Hp4 ..................4.0034 4.0034 4.0041 *,L16 ..................... 6.0161 6-0163 6.0175,Li7 ..................... 7.0169 7.0170 7-0176,Bes .................. 9-0135 9.0138 9.01646B10 ..................... 10.0146 10.0143 10.01355B11 .................. 11.0111 11.0110 11.0121&I2 ..................... 12.0037 12.0027 12.0048,C13 ..................... 13.0069 13.0061,N14 .................. 14.0076 14.0042,N15 .................. 15.0053 15,00328 0 1 6 .................. 16*0000 16.0000 16.0000.....................8 0 1 7 .................. 17.0040Values marked * are from Aston.27The study of artificial disintegrations has led to new precise valuesof a few other nuclear masses.28 C. D. Ellis and W. J. Henderson 29have shown that the upper limit of the pray or positron spectrummay be used to calculate the energy differences between the initialand final nuclei in a p-transition, and this reveals the possibility ofdetermining a large number of nuclear masses, since the greatmajority of the artificial radioactive transformations %re of thistype.It should be pointed out that the exact nuclear masses arequantities of great importance, since they have a very directconnexion with the binding forces in the nucleus.2 6 Physikal. Z., 1935, 36, 54; A., 277.a? Nature, 1935, 135, 541 ; A., 677.28 E. 0. Lawrence, Phy8icaZ Rev., 1935, [GI, 47, 17; A., 277.29 Proc. Roy. SOC., 1935, [ A ] , 152, 714BRADDICK. 19The mass of the neutron which appears in this table rests uponseveral independent experimental results.30 The transformationwith the new Aston masses 27 for H1 and H2 and the energy of the7-ray determined by Fleischmann 31 gives a value 1.0083.P. I. Deeand C. W. Gilbert’s investigation 32 of the reaction 1H2 + 1H2 4,He3 + on1 enabled them to deduce the mass 1.0080 & 0.0004.The neutron may therefore give a proton and an electron by anexothermic reactionon1 = lH1 + e + 0.2 M.E.V.332. NUCLEAR TR~SMUTATION.Work on the production of new atomic species by bombardmentof elements with a-particles, protons, deuterons, and neutronshas continued during the year, so there is now a regular ‘‘ nuclearchemistry.” Many of the nuclei obtained are radioactive withhalf-life periods varying from 0.02 sec. to several months. Inseveral cases, a given new nucleus can be obtained by several routes,and in many cases the new elements have been identified by chemicalmethods, e.g., precipitation with appropriate reagents after theaddition of an isotopic or homologous ion.Following Fleischmann and Bothe,31 we shall denote, e.g., thereaction ,W4 + ,He4 -+ 8017 + lH1 as one of type (a; p).Theexpulsion of neutrons by a-particles then becomes a reaction oftype (a; n). Protons may give reactions of types ( p ; a), ( p ; n), or( p ; -) (capture). Deuterons give ( d ; a), ( d ; n), and, rather fre-quently, ( d ; p ) , while neutrons give ( m ; a), (n; p ) , and (n; -).cc-ParticZes.-No new ( a ; p ) reactions giving rise t o stable nucleihave been discovered, but several processes of this type, givingradioactive nuclei (Curie-Joliot type) have been reported.Siliconbombarded with a-rays gives a product of half-life 17-18 days,which is probably 15P32 and gives electrons and not positrons ondisintegration. The half-life and the absorption coefficient of thep-rays agree with those of 15P32 obtained by bombarding 16Si32 withneutron^.^^^ 35 Magnesium bombarded with a-particles gives30 Cf. ref. (23).31 2. Physik, in the press; see also R. Fleischmann and W. Bothe, Ergeb.exakt. Naturwiss., 1935, 14, 1.32 Proc. Roy. Xoc., 1935, [A], 149, 200; A,, 678.33 G. Wentzel, Naturwiss., 1935, 23, 35; A., 278.34 H. Fahlenbrach, ibid., p. 288; A., 803.35 Idem, 2. Physik, 1935, 96, 503; A., 80320 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.several active elements, the most probable process being 349 36* 3712Mg25 + ,He4+ 13A128 + lH113A128-+ 14Si2B + e- (7 == 2.1 m.)but the following reactions probably also take place :,,Mg26 + 2He4---+ 13A129 + lH113A129 + 14Si29 + e-12Mga + 2He4 + 14Si27 + on114Si27 ---+ 13A127 + e+0.R. Xri~ch,~* on bombarding sodium fluoride or lithium fluoridewith cc-particles, found an activity which he ascribed to Na22formed by an ( a ; p ) process. The half life was > 6 months, and themean energy of the positrons was 0.2 M.E.V. Calcium gave apositron-emitting element, probably Sc43, with a half period ofabout 44 hours. This is the heaviest nucleus for which there isconvincing evidence of disintegration of this type.A few furtherinvestigations have been made on previously known reactions.New determinations 39 of the halving period of radio-nitrogen,7N13, give 11.0 and 10.73 minutes, which is identical with thatof 7N13 obtained from &P2 by ( p ; -) and ( d ; n) processes.Several new processes of the type ( a ; n) have been discovered.Silicon and phosphorus were found to give neutrons when bom-barded with a-particles,40 ahd 0. Haxe141 finds that nitrogengives under fast a-bombardment a radioactive nucleus of half life1.2 minutes in addition to the long-known (cc ; p ) transformationinto stable O17. The new nucleus is probably PI7 produced by thereaction 7N14 + 2He4 --+ + on1 (cf. p. 23). K. Schnetzler 42has interpreted his results on the excitation function (see below) oflithium as indicating an excitation process without capture of thea-particle or emission of charged particles.P. Save143 has foundevidence of a similar process in Li, N, Al, IF. In the (a ; p ) process5B10 + ,He4-+ &Y3 + lH1the C13 nucleus may often be left in an excited state which emitsa 7-ray, the proton carrying away less than the full energy of thetransformation. This interpretation has been confirmed by a36 C. D. Ellis and W. Henderson, Nature, 1935, 136, 755.37 A. Eckhardt, Naturwiss., 1935, 23, 527; A., 1049.38 Nature, 1935, 136, 220; A., 1186.39 C. D. Ellis and W. Henderson, ibid., 135, 429 ; A., 559 ; H. Fahlenbrach,2. Physik, 1934, 94, 607.40 E. Amaldi, 0. d'Agostino, E. Fermi, B. Pontecorvo, F. Rasetti, andE.Segr6, Ric. Sci., 1935, 0, [i], 11/12.dl Z. Physik, 1935, 93, 400; A., 426. 42 Ibid., 95, 302; A., 1049.43 Ann. Physique, 1935, [xi], 4, 88; A . , 1049BRADDICK. 21coincidence counter method by W. Bothe and H. J. von B a e ~ e r . ~ ~A group of full-energy protons is also emitted, and these show nosimultaneity with y-ray emission.Protons.-Considerable data have been accumulated on thedetails of the disintegration of lithium, beryllium, boron, carbon, andfluorine by artificially accelerated protons ; carbon alone gives anunstable product-nucleus, apparently by the simple capture reaction6Cl2 lH1-+ 7W3 hv,N13 + 6C13 + e+ 45946The radioactive substance behaved physically like nitrogen 45 andgave a positron emission with an energy distribution extending upto about 1.1 M.E.V., which is in reasonable agreement with thevalues for N13 obtained by bombarding boron with a-particles (foridentity of periods, see ref.39). The ‘‘ capture y ” radiation wasdetected : 46 it was ( 6 hard ” but was too weak to be examined indetail. The y-rays produced in the bombardment of boron4‘ andlithium 48 by protons have been examined by measuring the energyof recoil electrons in a cloud chamber. The energies in the case ofboron go up to about 14.5 M.E.V. and are ascribed to the reactionBll + H1+ C12. The lower energies present may be due to analternative process Bll + HI+ 3He4 or to a step-wise relaxationof a C12 nucleus excited to 14.5 M.E.V. The possible existenceof a 14.5-M.E.V. excited level in C12 raises interesting theoreticaldifficulties. I n the case of lithium the capture y radiation is foundto show a complicated structure of discrete lines with energies upho 16 M.E.V.The reaction is supposed 49 to be Li7 + H1 + He4 + He4 and the y-rays are due to excitation of the a-particles, whichmust have a series of energy levels between 0 and 16 M.E.V.50The transmutation functions of CI and p processes, i.e., thevariation of the probability of a transmutation with the energy ofthe bombarding particles, have been the object of special study.The bombarding particle may get inside the nucleus by passingover a potential barrier representing the repulsive forces of the44 Nach. Ges. Wiss. Qbttingen., 1935, 1, 195; von Baeyer, 2.Physik, 1935,85, 417.45 J. D. Cockcroft, C. W. Gilbert, and E. T. S. Walton, Proc. Roy. SOC.,1935, [ A ] , 148,225; A., 276.4 6 L. R. Hafstad and M. A. Tuve, Physical Rev., 1935, [ii], 48, 306; H.Neuert, Physikal. Z., 1935, 36, 629; A., 1297.47 H. R. Crane, L. A. Delsasso, W. A. Fowler, and C. C. Lauritsen, PhysicalRev., 1935, [ii], 48, 102.48 Idem, ibid., p. 125; A., 1186.49 M. L. Oliphant, E. S. Shire, and B. M. Crowther, Proc. Roy. SOC., 1934,60 C. C. Lrturitsen and H. R. Crane, PhysicaZ Rev., 1934, [ii], 46, 637, 1109.[A], 146, 922; A., 1936, 722 RADIOACTIVITY AND SUB -ATOMIC PHENOMENA.nucleus or by a ‘‘ resonance ’’ process in which a particle with adefinite special amount of energy is transmitted through the barrier.In the former case the transmission is calculated by Gamow’s theory,which gives a monotonic, approximately exponential increase withenergy of the incident particle; in the case of resonance, there are’peaks in the transmission curve and therefore in tho transmutation.function.The resizlts available up to October, 1934, have been.used in a paper by E. C. BollardJsl in which the values deduced for.the height of the potential barriers are tabulated, together withresonance levels. The barrier heights and resonance levels lie on asmooth curve when plotted against atomic number. The barrierheight is not the same for a-particles and for protons. Resonanceoccurs both for a-particles and for proton transmutations; it isparticularly well marked for proton-capture processes of the type&12 + lH1--+ ,W3 + y.This reaction shows two sharp reson-ances at 480 and 400 kv. The 7-ray emission from lithium bom-barded by protons shows resonance at 450 and 860 lw.? but theserays from beryllium similarly bombarded do not appear to showresonance.52 A theoretical analysis has been made by G . Breitand F. L. Yost,53 who find that the simple model of the nucleus as apotential well surrounded by a region of inverse-square field must bemodified to account for the experimentally observed probability ofcapture in the case of carbon.Deuterons.-The use of the H2 nucleus (deuteron) as a bom-barding particle has led to very interesting results.The transmutations of nitrogen ,El4 + 1H2 --+ &12 + ,Ho4[(d ; a) process] and 7N14 + 1 H 2 t + lH1 [(d ; p ) process] havebeen studied by E.0. Lawrence, E. McMillan, and M. C. Hender~on.~~The existence of excited levels in the C12 and W5 nuclei gives riseto a complicated set of ranges among the products, since differentamounts of energy may be taken up by the nuclei and radiated asy-rays. The y-rays emitted in these processes have been studied bythe cloud-chamber recoil method,65 and a set of y-ray lines observed.These data may also be compared with the energies of the 7-raysobserved in the reactions 561 53B1l + H2-+ C12 + ,,nland,Be9 + ,He4+ &12 +51 Physical Rev., 1936, 47, 611; A,, 804.51 L. R, Hafstad and M. A. Tuve, ibid., p. 506; 48, 306; A., 1297.53 Ibid., p. 203; A., 1186.55 H. R. Crane, L.A. DeIsiZsso, W. A. Fowler, and C. C. Lcturitsen, ibid.,56 Idem, ibid., 1934, [ii], 46, 1110.5 7 H. Becker and W. Bothc, 2. Physik, 1932, 76, 421.54 Ibid., 47, 273; A., 659.4,100BRADDIOK. 23There is therefore good evidence of the existence of series of excitedstates in the light nuclei; in some cases there is doubt as to whichproduct of a reaction gives rise to a y-ray. T. W. Boniier andW. M. Brubaker 58 find, by studying the energy distribution of theneutrons, that the reaction 3Li7 + 1H2-+ 2,He* + on1 may takeplace either as shown or with 33e* as an inkermediate nucleus.Li, Be, B, C, N, 0, F, Na, Si, Al, Cu, and Pt all yield radioactiveproducts under deuteron bombardment. The reactions with lithium,boron, and fluorine are probably of ( d ; p ) type : 59 they givep-emitting substances of short period.Nitrogen gives 6o radioactive 015 and oxygen gives 61 radioactiveF17 by ( d ; n) processes, and the identity of the new radio-elementshas been established by chemical tests.The case of Na24, obtainedby a ( d ; p ) process, on bombarding Na23, has been studied exhaust-ively by E. 0. Lawrence.62 This substance may have extensiveapplication, for it has a half life o€ 15.5 hours, and already in 1934Lawrence prepared sources equivalent to about 1 mg. of radium.It gives electrons up to 1.2 M.E.V., and homogeneous y-rays of5.5 M.E.V.The transmutation functions for deuteron bombardment have beenstudied by E. 0. Lawrence, E. McMillan, and R. I;. Thornton 63with very important results.They find that the efficiency of thebombardment of sodium , silicon, and aluminium does not increasewith the energy of the deuterons as rapidly as would be expectedon the Gamow theory of a charged particle penetrating a potentialbarrier. The production of radioactivity in copper by deuteronsaccelerated to less than 3.6 M.E.V. was observed, and it is veryremarkable, since the comparatively heavy nucleus is protectedby a, formidable potential barrier. The processes investigated areall of (d ; p ) type, and a theory worked out by J. R. Oppenheimerand M. Phillips 64 explains the results on the view that the deuteronconsisting of proton and neutron is deformed when it approaches apotential barrier, and the neutron may be captured by the nucleuswithout the proton penetrating the barrier.The theory rests onthe relatively small binding energy, E, of proton and neutron in thedeuteron, and this energy appears as a parameter in the formulaobtained. The formula fits the experiments with E equal to about2.2 M.E.V., which is in agreement with other estimates. It appearsthat quite heavy nuclei may be transformed by deuteron impact,58 Physical Rev., 1935, [ii], 48, 742.59 Crane, Delsasso, Fowler, and Lamitsen, ibid., 47, 887, 971.60 E. McMillan and M. S . Livingston, ibid., p. 452; A . , 559.61 H. W. Newson, ibid., 48, 790.82 Ibid., 47, 17 ; A., 277. 1 3 ~ Ibid., 48,493. 6* Ibid., p. 50124 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.and an artificial radioactivity produced in this way has been reportedfor platinum.neutrons.-A few further experiments have been done directlyon the atomic disintegrations produced by neutron bombardment,using the cloud chamber.W. D. Harkins, D. M. Gans, and W.W. Newson e5 have studied reactions which they consider to be10Ne20 4- onl + *017 + ,He49F19 + on1 --+ 7N1 + ,He4The nucleus $"6 appears to be radioactive. F. N. D. Kurie 66used nitrogen in a cloud chamber and interpreted his results asshowing capture of the neutron and emission of an a-particle. Hesuggests that W5 is formed by capture and disintegrates with a half lifeof about 10-20 sec. He finds that the kinetic energy of the productsof disintegration is independent of the energy of the incident neutron,and suggests that in the capture of the neutron the excess energy isradiated away.This is different from the cases of disintegration pro-duced by a-particles studied by Blackett in which energy is conserved,and Kurie suggests that it is characteristic of neutron disintegration.The reactions in which slow neutrons are captured by boron andlithium have been studied by J. Chadwick and M. G~ldhaber,~~ andby H. J. Taylor and Goldhaber,68 using a new method in which thetracks left by the particles in a photographic emulsion are examinedwith a microscope. The capture of neutrons by a number ofelements with emission of y-rays has also been observed. Thesereactions will be considered below.An enormous mass of results has been accumulated on the pro-duction of radioactive nuclei by neutron bombardment : these areincluded in Table 11.At the end of 1934, Fermi and his co-workers 69discovered that the activation of a large number of elements wasgreatly increased by interposing a body of water or other hydrogen-rich substance around the source and the material t o be irradiated.They correctly attributed the effect to the slowing down of theneutrons by collision with the hydrogen nuclei, and a great deal of65 Physical Rev., 1933, [ii], 44, 945; A., 1933, 1225; 1935, [ii], 47, 52;66 Ibid., p. 97.67 Nature, 1935,135, 65; A., 277; see also B. Kurtschatov, I. Kurtschatov,and G. Latichev, Compt. rend., 1935, 208, 1199.Nature, 1935, 135, 341 ; A., 426; H. J. Taylor, Proc. Physical SOC., 1935,47, 873; A., 1297.6B E.Amaldi, 0. d'Agostino, E. Fermi, B. Pontecorvo, F. Rasetti, andE. Segr6, Proc. Roy. SOC., 1935, [ A ] , 149, 522; A., 910; preliminary reportsin Ric. Sci., 1934, 2, 280, 380, 381, 467; 1935, 1, 123; 0. d'Agostino, Gazzetta,1934, 64,835; A,, 1935,276.A . , 277BRADDICR. 25work has been done on the mechanism of this action and theproperties of slow neutrons.In many cases the radio-elements formed by neutron bombardmenthave been identified by chemical tests.69 It will be seen fromTable I1 that by far the mostl common reaction is a simplecapture of the neutron, giving an isotopic nucleus one unit heavier.This seems always to be the case when the activation is enhanced bywater, and is therefore due to slow neutrons. The capture isoften accompanied by y-emission, which may include y-rays of veryhigh energy.70 In some cases the capture can be demonstrated bythe absorption of the slow neutrons, using a disintegrable substanceas a detector, though the product of capture shows no radioactivityand is apparently a stable nucleus.This is the case with Cd and Y.In the cases of lithium and boron, the bombardment with slowneutrons gives rise to capture 7's and the emission of an ac-particle.These reactions may be used to detect slow neutrons.Some of the products of neutron bombardment are of specialinberest. Fermi and his associates 71 found that the productsof bombardment of uranium with neutrons included radio-elementsnot isotopic with any known element, and which they ascribed toelements with Z>92.A. von Grosse and M. S. Agruss 72 examinedthis view and concluded that it W;LS not valid, but that the elementswere isotopic with Pag1. New evidence has been brought forward fromthe chemical side by 0. Hahn, L. Meitner, and F. Stra~smann,~~ whoshow that the substance of half life 13 minutes is eka-rhenium with2 = 93, and the later paper of Ferrni 69 not only produces new chemicaltests but shows, by the identical water effects on the various activeproducts, that they are probably products of successive transformationThe phenomenon of successive transformation is shown 74 by theseries C137 + n -+ C138 ; A38 -+ S34 + ~ 4 ,and by the elements formed by the irradiation of thorium withne~trons.'~ The series is probably to be represented asgZU + n+ 9ZU+ + P- + PC138 -+ A3* + e- ;'0 F.Joliot and L. Kovarski, Compt. rend., 1935, 200, 824.71 Proc. Roy. SOC., 1934, 146, 483; J4., 1934, 1284.72 Physical Rev., 1934, [ii], 46, 241 ; J . Amer. C'hem. SOC., 1935, 57, 438.73 Naturwiss., 1935, 23, 544; A., 1050.74 W. F. Libby, M. D. Peterson, and W. 111. Latimer, Physical Rev., 1935,[ii], 48, 571.7 5 0. Hahn and L. Meitner, Naturwiss., 1935, 23, 320; A., 910; (Mme.)I. Curie, H. von Halban, end P. Preiswerk, Compt. rend., 1935, 200, 1841,2079; A., 911, 1050; cf. ref. (69)26 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.The chemical behaviour of the activity is consistent with this schemeand the elements belong to the hitherto-unknown radioactiveseries with W = 4n + 1. The rare earths have been investigatedby S.Sugden,76 G. von Hevesy and H. Levi,T7 and J. C. M~Lennan.~~The activations of europium, dysprosium, and holmium are amongthe strongest which have been observed. There are some discrep-ancies which are probably due to the difficulties in separating theseelements.The Transmutation Function for Neutrons and the Speed of SlowNeutrons.-T. Bjerge and C. H. Westcott 79 examined various casesof artificial disintegration, using neutrons from different sources.They found that the reaction 15P31 + on1 + 13A128 + 2He4could not be observed with neutrons of about 2 M.E.V. from thebombardment of deuterium with deuterons, while the reaction15P31 + 14Si31 + lH1 was only reduced to one-third ascompared with an equal number of neutrons from a Ra-Be source(up to 14 M.E.V.).Similar researches,8° using as alternativesources the neutrons from Ra-Be and Ra-8 (up to 4.5 M.E.V.),showed that the (n; a) activation of silicon and phosphorus did notoccur with the slower neutrons, while the reactions 13A127 + on1 --+12Mg27 + lH1, 15P31 + ,,nl --+ 14S131 + lH1 did. P. Preiswerk 81made similar investigations, using the neutrons from Ra + B(up to 6 M.E.V.), and found that the (n; p ) reactions with silicon,aluminium, and iron did not take place, while that with magnesiumdid. For a large number of transmutations, however, the yieldis greatly increased by slowing the neutrons down in a hydrogen-rich material, and Fermi suggested that they were slowed down bymultiple elastic collisions with hydrogen nuclei till they attained‘‘ thermal ” velocities.The correctness of this view has been shownby P. B. Moon and J. R. Tillman’s experiments,82 in which theblock of para& surrounding the test material was cooled withiiquid air. The efficiency of activation in some cases was increasedconsiderably, but the increase was not the same for all elements.The interpretation of the results is complicated by an increasedabsorption of the very slow neutrons in the paraffi itself. Similarresults have been found by Fermi and others.83 A direct proof7 6 Nature, 1935, 135, 469; A., 559; J. K. Marsh and S. Sugden, ibid.,7 7 Ibid., p. 103; A., 1050. 78 Ibid., p. 831. 70 Ibid., 1934,134,177,286.I. Kurtschatov, L. Missovski, M.Erernejev, and G. Schtschepkin, Physikal.Z. Sovietunion, 1936, 7, 257.Compt. rend., 1935, 200, 827; A., 558.82 Nature, 1935, 135, 904; 136, 66; A., 802, 1049; Proc. Roy. Soc., 1936,[A], 163, 476.8a Ric. Sci., 1935,6, 1, 11/12; see also J. R. Dunning, G. B. Pegram, G. A.Fink, and D. P. Mitchell, Physical Rev., 1935, [ii], 47, 796; 4, 266; A,, 1186.136, 102; A., 1050BRADDICK. 27of the velocities of the neutrons has been achieved by Fer~ni,8~by 0. R. Frisch, and by E. T. Spensen,86 who fastened source andtest piece to a rotating wheel surrounded by paraffin and observed akind of aberration effect, and by J. R. Dunning, G. B. Pegram,G. A. Fink, D. P. Mitchell, and E. Segr6,*6 who used a toothed-wheel velocity selector with absorbing sectors of cadmium.The mechanism of the slowing down of neutrons, the cross sectionof the hydrogen nucleus, etc., have been investigated by a number ofworkers.87 It seems that the greater part of the slowing in hydrogenis due to elastic collisions.The efficiency of the slowing process isreduced with heavy water.88 There is, however, another process bywhich neutrons can be slowed down, vix., the excitation of at nucleuswithout capture of the nucleus.89~90~91 It is found that screens ofheavy metals, i.e., gold or lead 90 or silver,g1 do increase the intensityof '' water sensitive " reactions by this process.The cross section for interaction of these slow neutrons withnuclei, as determined by the absorption method, is in many casesvery large (3 x 10-20 cm.2 for gadolinium, 3 x 10-21 cm.2 forcadmium). It varies enormously from element to element, whereasthe collision cross sections for fast neutrons lie on at fairly smoothcurve when plotted against atomic numbers.92 The cross sectionhas been dealt with theoretically by several authors,93 and Bethehas attempted a rather complete theory of the interaction ofneutrons and nuclei, making a straightforward application of wavemechanics.The probability of capture may be large for slowneutrons on account of the long time they spend in the nucleus, andmay be further increased by a resonance factor which cannot bepredicted and varies from nucleus to nucleus. The theory shows(a) that a nucleus may have a large cross section for the elastic134 Ric.Sci., 1935, 6, No. 1, 11-12. 8 5 Nature, 1935, 136, 258; A., 1186.8B Physical Rev., 1935, [ii], 48, 704.87 R. Fleischmann, Naturwiss., 1934, 22, 839; A., 1935, 41; T. Bjergeand C. H. Westcott, Proc. Roy. SOC., 1935, [ A ] , 150, 709; A., 1186; Proc.C a d . Phil. SOC., 1935, 31, 145; A., 426.88 H. Herszfinkiel, J. Rotblat, and M. Zyw, Nature, 1935, 135, 653; A . ,678; C. H. Collie, J. H. E. Grifliths, and L. Szilard, ibid., p. 903.89 D. E. Lea, ibid., 1934, 133, 24; P ~ o c . Roy. SOC., 1935, [A], 150, 637;A., 1186; J. R. Dunning, G. B. Pegram, and G. A. Fink, Physical Rev.,1935, [ii], 47, 325 ; R. Fleischmann, see ref. (87); P. Auger, Compt. rend., 1934,198, 365.90 L. Wertenstein, Nature, 1934, 134, 970.9 1 W. Ehrenberg, Nature, 1935,136, 870 ; B.Kurtschatov, I. Kurtschatov,** Dunning, Pegram, Fink, and Mitchell, Physical Rev., 1935, [ii], 48, 265.03 Amaldi et al., see ref. (69); H. A. Bethe, Physical Rev., 1935, [ii], 47,747; F. Perrin and W. Elsasser, Cornpt. rend., 1935, 200, 460; G. Beck andL. H. Horsley, Phy8icaZ Rev., 1935, [ii], 47, 510.L. Missovski, and I. Roussinov, Cornpt. rend., 1935, 200, 120128 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.scattering of slow neutrons; ( b ) that capture with the emission ofy-rays may take place with large probability if there is a suitablevacant neutron level in the nucleus-in the absence of resonancethe probability of capture decreases inversely as the velocity ;(c) that disintegration with emission of a-particles may take placewith high probability if the process is exothermic and the potentialbarrier opposing the exit of the a-particle is not too high-for thisreason it is confined to light elements-(%; p ) disintegration isalways endothermic and does not take place with slow neutrons;(d) that excitation without capture may also occur.It is, how-ever, incomplete, since evidence is appearing that the crosssection of different nuclei can vary in different ways with theneutron velocity.This appears strongly from the experiments ofMoon and Tillmans2 and from those of Fermi and Amaldi,94 inwhich an absorber is shown to absorb selectively (a) the neutronswhich activate it and ( b ) some other bands of neutron energy.y-Rays (NucEeur Photo-eflect).-The reactions 1H2 + hv -+lH1 + ,,nl, ,Be9 + hv -+ ,Be8 + ,,nl have been confirmed andfurther st~died.~5 Arzimovitch and Palibin find that the latterreaction does not proceed with y-rays of energy less than 1.3 M.E.V.Electrons.-Some reports have been made of an artificialradioactivity produced in aluminium by fast electron bombard-ment.96 No activity was found by Livingood and Snell after850-kv. bombardment, and this disintegration by electrons doesnot seem very likely.TABLE 11.Nuclear Reactions and Radioactive Transformations of LightNuclei (References in parentheses).This theory collates many of the known facts.lH1 + on1 _3 1H2 + y (89)1H2 + y 1H2 + lHa --+ ,He3 + onl (33)1H2 + 1H2 -+ 1R3 + IH1 (33),Li6 + on1 --+ $3' + 2He4!61V)-> lH1 + on1 (95),Lis + --+ ,He3 + ,He (,Lie + 1H2 _j 2,He4 + y (49),Li6 + ,He4 -+ 5BQ + onl (98, 99).e+B9 i--z BeQ94 Ric.Sci., 1935, 6, ii, 9/10.Q5 J. Chadwick and M. Goldhaber, Proc. Roy. SOC., 1935, [A], 151, 479;A., 1293; cf. A., 1934, 1053; W. Gentner, Cornpt. rend., 1935, 200, 310;L. Arzimovitch and P. Palibin, Physikal. 2. Xovietunion, 1935, 7, 245.96 M. Tenaka, Physical Rev., 1935, [ii], 48, 916 ; J. J. Livingood and H. H.Snell, ibid., p. 851.97 Rutherford, Chadwick, and Ellis, " Radiations from Radioactive Sub-stances," Cambridge, 1930.98 Ann. Reports, 1934. 99 Ibid., 1933BRADDTCK. 29TABLE IT. cont.,Li7 4- lH1 --+ 2,He4 + y (49, 48),Li7 + 1H2 _j 2,He4 + (49),Li7 + 1H2 --+ ,Be8 -t- +Z,He4 + o ~ ~ l (58),Li7 + 1H2 + ,Li8 + lH1 (49, 59).&!7 + ,He4 + , B I O + on1 (98, 99),L17 + ,He4 (excitation) (42), T i 7 + ,He4 _j Belo + lH1 (95).,Be9 + y,Be0 -1- lH1 4 ,Belo + y (98),Be9 + 1111 _j ,Lis + ,He4 (98),Be0 + lH6 --+ 5B.10 + on1 + Y (57),Be9 + 1H2 _3 ,L17 + ,He4 (98),Be0 + 1H2 + ,Belo +,Be9 + ,He4+ 6C12 + on1 (98, 99, 57)BLi8 -+ Be8+2He40.5 sec.PBelo --+ Bl0 ?-j- ,Be8 + on1 _j 2He4 + on1 (95)B(98). Belo -+ BIO ?+ onl + ,Li7 + ,Hc4 (67)e+5Bl0 {- 1H2 6C1l f 0,' (98).6C1l aomz 5Bl16131" + 1H2 --+ 5Bl1 + lH1 (98)5B10 + 1H2 + 3,He4 (98)5B10 + ,He4-> 7N13 (98). 7N13 &l'*,B10 + ,He4-+ + y + lH1 (97)-Bll -b lH1 _3 3,He4 (98)iB11 + lH1 __t ,Be8 + ,He4 --+ 3,He4 (98)5B11 f- 1H2 4 GC12 + ,nl f y (56),B1l + 1H2 --+ 3,He4 + (98)5B11 + 1H2 + ,Be9 + ,He4 (98)efB0.02 sec.$11 + H2 + 5B12 + lH1 (59). 5B12 - 3 c12ef7N13 ~ ~ 2 . 6C1'*6c12 + 1H2 --+ 7N13 + lH1 (98) do.:C12 + 1H2 -+- bB1o + ,He4 (98)7N1* + on1 -+ 5B11 + ,He (98, 99)6C12 + lH1 + 7N1' + y (45, 46)..C12 + 1H2 -> GC1s + y (98)7N14 f lH17N14 + --+ *015 + on1 (GO). ---.-+ N15_3 6C1l + ,He4 (98)e+126 sec.7N14 + 6Ha7N14 + 6H2C j 7 W 6 + lH1 (54, 55)_t 6c1' f ,He* (54, 55)e+,N14 + ,He4 _t sF17 + o ~ ~ l (41).7N14 + ,He4 -+ 8017 + lH1 (97)8 0 1 6 + lH1 + sF17 + onl (61). 9F17 ---+ Oi79 ~ l w + o ~ lSFIS + on1 + sN16 + ,He4 (65). 7N16 8 0 l 6BF1° + lH1 -+ loNe20 + y (98)oF19 + lH1 4 * 0 l 6 + ,He4 (98)9F1e + 1H2 + 8 0 1 7 -f- 2 ~ ~ e 4 (98)9F17 lym-. 8 0 l 7e f1.2 min.B + 8 0 1 9 + lH1 (59).8010 --f gF1O 40 sec.BgF1s + 1H2 + 1oNe2O f Y (9830 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.TABLE 11. cont.e+llNa22 -+ loNe22 > 6 months DF19 + ,He4 + 11Na22 + on1 (38).,FIB + ,He4 --+ loNe22 + lH1 (97)loNe20 + onl + + ,He4 (65, 99)BllNa24 F~? I z M ~ ~ ~ 11Na23 + on1 + 11Na24 (69).11Nae3 + on1 4 + lH1 (69). 10Ne23 csz llFa23,,Naa3 + lH1 + loNe20 + 2He4 (27)'B11N~23 + 1H2 --+ 11Naa4 + lH1 (62). $a2' ljd 12MgZ4llNrt23 + 1H2 --+ lzMga4 + on1 (62)11Naa3 + 1H2 -+ loNe21 + ,He4 (62),,Nae3 + ,He4+ 12Mg26 + lH1 (97, 98)11Na23 + ,He4+ 13A126 + on1 (98, 99)12Mg24 + on1BB -+ 11Na24 + lH1 (69). llNa24 --+ 15h.lzMg94 + 1H2 4 11Na2z + ,He4 (2)12Mg24 + ,Me4 -j 14Si27 + on1 (36). 1pSi27 T+12Mg24 + ,He4 + 13MZ7 + lH1 (97)B 12Mg25 f ,He4+ 1 3 u 2 ' + 1H' (36).l ~ x ~ ~ a 14s12812Mgas + on1 -+ 12Mg27 (69). 12Mg27 <+ 1sM2',,Mg26 + onl + 10Ne23 + ,He4 (69). --f llNa2340 sec.12Mga6 + lH1 -+ 11Na23 + 4He4 (2)B12Mg26 + ,He4+ ,3A129 + lH1 (36). 1 3 x 2 ' -313A127 + ,nl -+- 13M2' (69). 2ym-s 14Si281 3 ~ 1 2 7 + on1 + 12Mg2' + lH1 (69). 12Mg27 cmi;;. 13AJ27e+BBBBllNa24 ---+ 1zMg2415 min. laA127 + ,nl --+ llNa24 + ,He4 (69).1 3 u 2 ' + O H 113A127 + 1 H 2 + 1H2+ izMgrP 4- ,He4 (2)+ 13A.I" + lH1 (1). + l2MgZa + ,He4 (1)1 3 d 2 ' + rH2 + 14Si2' + om1 (1)B 13A128 -9 Si2'ef2-1 min.I&'' + ,He4 + + on1 (98). I5P3O a1 3 ~ i 2 7 + ,He4 -+ 14Si30 + IH1 (97, 98)For elements of 2 > 13, see Table 111, but also :14Si27 + ,HeO4 -+ 15P32 + lH1 (34)20Ca40 + ,He4 --+ Sc43 + IH1.21Sc43 r-6 (38)ef1 E. McMillan and E. 0. Lawrence, PhysimZ Rev., 1935, [ii], 47, 343.M. S. Livingston, M. C. Henderson, a-d E, 0. Lawrence, ibid., 1933,[ii], 44, 316BRADDICK .TABLE 111.Artijicial Radioactivity produced by Neutrons.References not marked are to Fermi et al. (40, 69). The carrier of theThe mark * indicates that the activity activity and the half life are given.is enhanced by water and is probably due t o (n; -) transformation.1 H2 He3 Li4 Be5 B6 C7 N8 09 F10 No11 Na12 Mg13 Al14 Si15 P16 S17 C118 A19 K20 Ca21 sc22 Ti23 V24 Cr25 l\ln26 Fe27 Co28 Ni29 Cu30 Zn31 Ga32 Ge33 As34 se35 Br36 Kr37 Rb38 Sr39 Y40 Zr41 Nb42 Mo43 Ma44 Ru45 Rh46 PdOle 40 s., N16 9 s.N@ ~ O S ., Mgz7 lorn.,40 s., NaZ4 15 h.NaZ4 15 h.A 1 2 8 2-3 m., Mga7 10 m.,Na24 15 h. (cf. 97).Si31 2.4 h., A128 2.3 m.Alas 2.3 m., Si31 2.4 h.P32 14 d.C135 m., P32 14 d.K42 16 h.Ca 4 h., K42 16 h. (5)K42 16 h.VS2 3.75 m.V52 3.75 m.Mn56 2-5 h., V52 3-75 m.34x156 2.5 h.Co60 20 m., MnS6 2.5 h. (4)Ni ( ? hr.), C060 20 m. (4)Cu 5 m., Cu 10 h.Cu B m., Cu 10 h., Zn*Ga 20 m. ? 23 h.30m. ?As76 26 h.Se 35 m.Br 18 m.. Br 4.2 h.100 m. ( 5 )Zr 40 h. (5)30 in. 36 h.40 s., 100 s., 11 h., 75 h. (8)Rh 44 s., Rh 3-9 m.Pd, Fyh, ? 15 m.3147 Ag Ag 22 s.*, Ag 2.3 m.48 Cd49 In In 54 m., In 3 h., ? 13 s.50 Sn51 Sb Sb 2.5 d.52 Te Te* 45m.53 I I 2 5 m .54 Xe55 Cs Cs* 75 m.(7, 6)56 Ba Basom., ? 3m.57 La 1.9 d. (10)58 Ce59 Pr Pr* 19 h. (cf. 10)60 Nd61 -62 Sm 40 m. (cf. 10)63 EU 9.2 h. (10)64 Gd 8 h. none (10)65 Th 3.9 h.66 Dy 2.5 h. (10, 11)67 HO 2-6 h. (lo), 35 h. (11)68 Er 7 m., 1.6 d., 12 h. (11)69 Tu70 Yb 3-5 h. (10, 11)71 LU 4.0 h.72 Hf Hf (months) (5)73 Ta74 W W Id.75 Re Re 20 h.76 0 s77 Ir Ir 19 h., 3 d. 50 m. (12)78 Pt ? (13)79 Au AU 2.7 d.80 Hg81 T1 T1* 97m. (6)82 Pb83 Bi Bi (6)84858687888990 Th 1 m., 24 m. (cf. p. 25).9192 U 15 s., 40 s., 13 m., 100 m.93(Cf. 9).(cf. p. 25).3 I. Kurtschatov, B. Kurtschatov, L. Missovski, G. Schtschepkin, and A.Vibe, Compt. rend. Acad. Sci., U.R.S.S., 1934, 3, 422.4 J. Rotblat, Nature, 1935, 136, 515; A , , 1297.6 J.C. McLennan, L. G. Grimmett, and J. Read, Nature, 1935, 135, 505;G. von Hevesy and H. Levi, ibicl., 135,580; A , , 678.A., 67832 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.3. THE RAY DISINTEGRATION.Not much definite progress seems to have been made in testing thetheories of @-decay which were described in last year’s Report.15The Fermi theory has been criticised theoretically by R. L. Dolecek,16who develops a method allowing the calculation of the energydistribution of the p-electrons for any appropriate assumptionsabout the transfer of angular momentum from the nucleus to theemitted particles. He was unable to account for the shape of thep-ray energy distribution in the cases of the relatively heavy nucleiof potassium and rubidium, and supports the necessity for additionalassumptions in the theory.E. J. Konopinski and G. E. Uhlenbeck17introduce a new weight factor into Fermi’s treatment and find thatthe new distribution, which should hold strictly only for lightnuclei, fits more satisfactorily to the experimental curves for thepositrons from F30, N13, and the electrons from A128. The experi-mental data were provided by the work of C. D. Ellis and W. J.Henderson l8 and of A. J. Alichanov, A. J. Alichanian, and B. S.Dielepov,lg and the agreement between the results of the two setsof workers was not very good, though both show the asymmetrywhich is a characteristic of the modified theory.Much attention has been coiicentrated on the high-energylimit of the @-ray continua.The view was put forward by C. D.Ellis and N. F. Mott 2o that the upper limit of the continuumrepresents the energy available for the p-ray in every case of agiven nuclear transition. When a p-ray of lower energy is emittedthe surplus energy is carried off by a (‘ neutrino.” This view hasbeen supported by the experiments of W. J. Henderson,21 who usedW. M. Latimer, D. E. Hull, and W. 3’. Libby, J . Arner. Chew. Soc., 1935,I . Kurtschatov, L. Nemenu, and I. Selinov, Compt. rend., 1935, 200,L. Szilard and T. A. Chalmers, Nature, 1935, 135, 98; A., 271.57, 781; A., 678.2162.lo S. Bugden, ibid., p. 469; A., 559; J. K. Marsh and S. Sugden, ibid.,l1 G. von Hevesy and H. Levi, ibid., p. 103; A., 1050.l2 L. Sosnowski, Compt.rend., 1935, 200, 922; A., 678.l3 Idem, ibid., p. 446; A., 426.Ann. Reports, 1934, 31, 394.Physical Rev., 1935, [ii], 48, 13 ; A., 1048.l7 Ibid., pp. 7, 107; A,, 1048.Proc, Roy. SOC., 1934, [A], 146, 206.l9 2. Physik, 1935, 93, 350; A., 426.2o Proc. Roy. SOC., 1933, [A], 141, 502; A., 1933, 1100; see also Ann,21 Proc. Roy. SOC., 1934, [ A ] , 147, 572; A . , 276; cf. P.C. Ho, Proc. Carnb.136, 102; A., 1049.l4 Idem, ibid., p. 1027 ; A., 559.Reports, 1934, 31, 394.Phil. Soc., 1935, 35, 119BRADDICK. 33a coincidence Geiger counter in conjunction with a magneticspectrograph to study the high-energy end of the thorium-C and-C”’ spectra. He found that the energy changes in the branchedprocesseswere equal if the upper limit were taken as the energy of theP-transformation and the y-ray energy properly allowed for.Ellisand Henderson 22 studied the positrons from the artificial radio-active nucleus P30, using an absorption method, and they assumethat the upper limit of the spectrum gives the difference in energybetween the ground states of P30 and its disintegration productSi30, since no y-rays were emitted in the disintegration. Thisassumption can be checked by equating the energies in the two setsof processes : 2313A127 + zHe4 = 14Si30 + lH113A127 + 2He4 = 15P30 + on113P30 = 14Si30 + E+It is therefore probably true that the Ellis-Mott rule holds for bothheavy and light nuclei, electron and positron emission. IE. R.Crane, L. A. Delsasso, W. A. Fowler, and C.C. Lauritsen24 drew thesame conclusions by studying the electrons produced by bom-barding boron with deuterons :Bll + H2 -+ B12 + H1-+ C12 + E- 4- H1The masses are here determinable by other nuclear reactions, andthe upper limit lies at about 11 M.E.V.The upper limiting energies have also been studied from thepoint of view of the empirical Sargent rule,25 which connects thelogarithm of the limiting energy with that of the decay period byone of two nearly linear relations. One of these is supposed to holdfor “ allowed ” and the other for “ forbidden ” nuclear transitions.The theory was also tested by F. N. D. Kurie, J. R. Richard-son, and H. C. Paxton26 with the electrons from radio-sodium,Naw, and radio-silicon, and satisfactory agreements were obtained.The distributions obtained by Crane, Delsasso, Fowler, andLauritsen 27 for the electrons obtained on bombarding lithium and22 Proc.Roy. SOC., 1936, [ A ] , 152, 714.23 L. Meitner and R. Jaeckel, 2. P h y ~ i k , 1934, 91, 493.3% Physical Rev., 1935, [GI, 47, 887.26 For a recent modification, see G. J. Sizoo, Nature, 1935,136,142 ; A., 1048.0 6 Physical Rev., 1935, [ii], 48, 168. 27 lbid., 4’9, 971.REP.-VOL. XXXII. 34 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.fluorine with deuterons also fit the relation for the energydistribution. The experimental data on the artificially activeelements are conflicting. Alichanov, Alichanian,22 and D%elepov,28who have examined a number of elements, conclude that their resultsdo not fit on the Sargent lines. An absorption estimate byLawrence 29 of the maximum energy of the @-particles from radio-sodium, Na24 (1.2 M.E.V.), fits on the curve, and the results of Craneet aL2' for the short-lived products of bombarding lithium (probablyBes) and fluorine seem to fit fairly well.Kurie, Richardson, andPaxton 26 use a theoretical energy-distribution curve to estimate theend-points for C, Si, and Na. Na (2-1 M.E.V.) is in rather markeddisagreement with Lawrence, and does not fit on the curve; siliconseems to fit tolerably well. The unpublished results of R. Naidu andR. E. Siday for rhodium, silver, and fluorine agree with the rule, butsilicon, dysprosium, and europium do not; the probable errors arelarge, however. It is clear that the experiments are still inadequate.The low-energy end of the @-ray spectrum of radium-E hasbeen studied by H.0. W. Richardson,30 who finds an unexpectedlylarge number of slow particles. It is uncertain how far these areof secondary origin. It has been pointed out that many modernviews on the continuous F-spectrum require a neutrino to carryoff unobserved a part of the energy, and a few further attempts 31have been made to detect this particle. H. A. Bethe 32 has made acalculation of the ionising power of a neutrino possessed of magneticmoment, and in conjunction with the negative results of the experi-ments this shows that the magnetic moment cannot exceed 2 x 1o-PBohr magnet on.4. THE POSITRON.The positron was treated rather fully in last year's Rep0rt.3~Positrons have been found in the cosmic radiation; positron-electron pairs are created by the transformation of the energy of hard7-rays and perhaps fast electrons in the neighbourhood of an atomicnucleus, and they are annihilated by combination with an electron,with emission of radiation.During 1935, considerable attentionwas devoted to calculating the probability of production of a pairby interml conversion of a 7-ray emitted by a nucleus in its own28 Nature, 1935,136, 257; 1935, 135, 393; A., 426, 1186; see also ref. (19).29 LOC. cit., ref. (62).30 Proc. Roy. Soc., 1934, [ A ] , 147, 442; A , , 1935, 6.31 J. Chadwick and D. E. Lea, Proc. Camb. Phil. SOC., 1934, 80, 59; A.,1935, 276; M. E. Nahmias, ibid., 1935, 31, 99; A., 426; M.Wolfke, Bull.Acnd. Polonaise, 1935, [ A ] , 107; A., 911.32 Proc. Carnb. Phil. SOC., 1935, 31, 108; A., 426.33 Ann. Reports, 1934, 31, 397BRBDDICK. 35atom.= Jaeger and Hulme calculate the internal conversioncoefficient for element 84 for both dipole and quadruple transitions.There is in this case a marked difference between the averageenergies of the positrons and electrons emitted, the former havinggreater energies, while for light elements there should be a symmetricdistribution of energies between the particles. The number ofconversions predicted is in agreement with the results of A. J.Alichanov and M. S. Kosodaev 35 on the emission of positrons fromradioactive sources. H. J. Bhabha, 36 has calculated the probabilityof pair production by collision of charged particles.The annihilation radiation due to the combination of an electronand a positron has been studied experimentally by 0.Klem~erer,~’who used the positrons from artificially radioactive elements anddetected the radiation with two Geiger counters in a coincidencecircuit. He found that in the normal annihilation process twoy-ray quanta are emitted in opposite directions. The energyavailable for each quantum is then wix2 = 0.5 M.E.V., and absorptionmeasurements on the radiation were consistent with this. Theannihilation has been dealt with theoretically by H. A. Bethe,38who calculates that a fast positron has rather a high probability ofbeing annihilated during its motion, the energy being usuallyemitted as a pair of quanta (cf.ref. 37). There is, however, anappreciable probability of a single-quantum annihilation in theneighbourhood of a heavy nucleus. The annihilation of slowlymoving positrons takes place almost entirely with emission oftwo quanta. (This process is the one which would have beenobserved in Klemperer’s experiments.)The part played by the annihilation process in the anomaloushard scattering of 7-rays is discussed by Bethe 38 and by E. J.Williams39 and K. Tsu T~ng.~o The present view appears tobe that the hard scattered radiation contains a broad band due toannihilation, since the annihilation radiation carries off the energyof motion of the positron as well as the annihilation energy itself.There is also present an important component due t o the radiationemitted in the slowing down of Compton and pair electrons.34 M.E. Rose and G. E. Uhlenbeck, Physical Rev., 1935, [ii], 48, 211; A,,1187; J. R. Oppenheimer, ibid., 47, 144; A., 278; J. C. Jaeger and H. R.Hulme, Proc. Roy. SOC., 1935, [ A ] , 148, 708; A., 557.36 2. Physilc, 1934, 90, 249; A., 1934, 1150.36 PTOC. Roy. SOC., 1935, LA], 152, 559; cf. L. Nordheim, J . Phys. Radium,1935, [vii], 6, 135; A., 677; L. Landau and E. Lifschitz, Physiknl. Z. Soviet-union, 1934, 8, 244; A., 677.37 Proc. Garnb. Phil. SOC., 1934, 30, 347; A., 279.38 Proc. Roy. SOC., 1935, [ A ] , 150, 129.40 Sci. Rep. Nat. Tsing Hua Univ., 1935, 3, 85.39 Nature, 1935, 135, 26636 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.5. THE RADIOACTIVITY OF POTASSIUM.The discovery of widespread neutron-induced radioactivity hasenabled the problem of the natural activity of potassium andrubidium to be approached from a new angle.The activity of theseelements is extremely weak, and it is now generally ascribed torare isotopic constituents. Several papers 41 have appeared inwhich the activities are ascribed to K40 and Rbs5. These substancescould have been formed in pre-terrestrial times by neutron bom-bardment of K39 and Rb85 and persist only in traces undetectableby the mass spectrograph. Von Hevesy finds that bombardmentof scandium with neutrons gives K42 by the transition 21Sc45 +on1 -+ 19K42 + 2cx4; K42 decays with a 16-hour period and isexcluded as the naturally occurring radioactive constituent.K.Sitte 42 oonsiders, on mass-energy grounds, that K40 shouldshow a positron activity as well as the @-activity, and since hedetects no positrons, he ascribes the potassium activity to K43.Von Hevesy, however, finds that the assumption of Ka fits hisdata on the activity of the fractionated isotopes of potassium.Evidence for the existence of K40 has recently been found with themass-spectrograph by A. 0. Nier,43 who finds K40: K39 = 1 : 8600,and by A. K. Brewer,M who finds K40: K39 = 1 : 8300. Evenassuming that the activity is due to rare isotope, the persistenceof the activity over very long periods cannot be connected withthe energy of the p-rays by the Sargent relation.25 Klempererconsiders that K40 probably has a large nuclear spin, and that thetransition to Ca40 is a kind of ‘‘ super-forbidden ” one for which alow probability is to be expected.456.THE PENETRATING RADIATION.Nothing has been discovered during the year to disturb theconclusion that the primary cosmic radiation consists mainly,and perhaps entirely, of charged particles. W. F. G. Swann46says that the variation of intensity with latitude, due to deviationin the earth’s magnetic field, indicates that at least 25% ofthe incident radiation is of charged corpuscular type, and that theobservations of the east-west asymmetry of the radiation in the4l 0. Klemperer, Proc. Roy. Soc., 1935, [ A ] , 148, 638; G. von Hevesy,Nature, 1935, 135, 96; A., 276; A. Ruark and K. H. Fussler, Phy8icaZ Rev.,1935, [ii], 48, 151; A ., 1185; F. H. Newrnan and H. J. Walke, Nature, 1935,135, 98, 508; A., 677; Phil. Mag., 1935, [vii], 19, 767; G. von Hevesy,Nctturwiss., 1935, 34, 583.42 Nature, 1935, 136, 334.41 Ibid., p. 640.4 6 Physical h’ev., 1935, [ii], 48, 641.43 Physical Rev., 1936, [ii] 48, 283.45 Cf. C. Hurst, Nature, 1935, 135, 905BRADDICR. 37equatorial region brings this value up to 31%. A. H. &mpton,47relying largely on a balloon measurement of the cosmic rays inequatorial considers that at least 97% of the rays arecharged, and an extrapolation of the data to the top of the atmo-sphere indicates that 99% are charged. Analyses of the generalnature of the rays have been made by Compton49 and by Swann.Compton attempted an analysis of the rays, starting with theintensity-height data obtained on balloon flights.When allowancehas been made for the isotropic incidence of the rays on the top ofthe atmosphere, these curves show kinks which are interpreted asdue to the successive removal of groups of particles of finite range.An application of the theory of the magnetic deviation of the particlesin the earth’s field gives the minimum energy which the particlesmust possess to reach the earth’s atmosphere a t all. Combiningthese data, and assuming (rather uncertain) relations between therange and energy of different kinds of fast particles, Comptonascribes one of his range groups to a-particles, one to electrons, andone to protons. The showers of particles detected with counterarrangements and in the Wilson chamber are regarded as producedby a secondary radiation. A difficulty in the theory lies in the factthat protons have not been distinguished in Wilson chamberphotographs a t sea level, though they should theoretically be present.Swann considers that the charged particles which constitute theprimary rays come right through the atmosphere; possibly notionising directly, but producing long-range secondaries throughouttheir path.These secondaries are those observed in Geiger-countermeasurements. The theory in this form gives an exponential lawfor the absorption of the measured radiation if we assume that thenumber of secondaries produced per unit length of primary path isproportional to the energy of the primary.The comparativelysmall departures from the exponential law may be explained by adistribution of the primary energy. The fact, discussed below,that showers and bursts increase more rapidly with altitude than thegeneral radiation, is explained on the assumption that, althoughsecondary production in air increases linearly with the energy of therays, yet secondary production in lead increases more rapidly thanthis.Extensive experimental work has been done on the cosmicradiation. The assumption usually made, that the coincidences ofa set of Geiger-Muller counters in line represent the passage of47 Guthrie Lecture, Proc. Physical SOC., 1935, 47, 747.48 J. Clay, Physica, 1934,1, 363.49 A. H. Compton and H. A. Bethe, Nature, 1934, 134, 134; see also ref.(47)38 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.a single ionising particle through the system, has been checked.60Absorption data for various materials have been obtained.51Auger and his co-workers interpret their results as indicating thepresence of a hard and a soft component which may perhaps beidentified with protons and electrons respectively.An analysis by C. G. and D. D. Montgomery 52 shows that there isno sharp distinction between the (‘ showers ” which discharge Geiger-counter systems and the large “ Stosse ” observed in ionisationchambers. Unpublished work by W. Ehrenberg and by I€. Car-michael also tends to this conclusion. Several sets of experimentsshow that showers and bursts increase more rapidly than the generalcosmic radiation with increase in 53 and unpublishedwork in an aeroplane by H. J. J. Braddick and C. W. Gilbert showsthis very markedly. These experiments either demand a special“ shower-producing component ” of the radiations, or they are tobe explained slow the lines indicated by Swann. A very recentpaper by C. G. and D. D. Montgomery 54 indicates that the rate ofburst production varies differently with altitude for differentmaterials, and this produces severe difficulties for the “ showerproducing radiation ” hypothesis. New data on the variation ofshower production with thickness of material have appeared. 55E. C. Stevenson and J. C. Street 56 have published photographsshowing showers apparently produced in a lead plate by incidentelectrons. This is a new type of shower production, previouswork having led to the conclusion that showers were always producedby a non-ionising radiation. T. H. Johnson 57 has continuedwork on the directional distribution of the cosmic rays, findingan unbalanced positively charged component of the incomingradiation.H. J. J. BILADDICK.50 J. C. Street, It. H. Woodward, and E. C. Stevenson, PhysicaZ Rev.,1935, [ii], 47, 891 ; A., 1050.51 J. Clay, Phyeica, 1935, 2, 645; A., 1050; H. Tielsch, 2. Phyeik, 1934,92, 589; A., 143; G. Alocco, Nature, 1935, 135, 96; A., 278; P. Auger,Compt. rend., 1935, 200, 739; A., 560; P. Auger, A. Rosenborg, and F.Bertein, ibid., p. 1022; A., 660; P. Auger, L. Leprince-Ringuet, and P.Ehrenfest, ibid., p. 1747; A., 1050.62 Physical Rev., 1935, [ii], 48, 786.63 B. Rossi,and S. de Benedetti, Ric. Sci., 1934, 2, 5, 95, 119; A., 804;E. C. Stevenson and T. H. Johnson, Physical Rev., 1935, [ii], 47, 678; A.,803; R. D. Bennett, G. S. Brown, and H. A. Rahmel, ibid., p. 437; A . , 560;C. G. Montgomery and D. D. Montgomery, ibid., p. 429; A., 560.Ibid., 48, 969.65 J. E. Morgan and W. M. Nielsen, ibid., p. 773.66 Ibid., p, 464. 6 7 Ibid., p. 287