ANNUAL REPORTSON THEPROGRESS OF CHEMISTRY.RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.DTJEING the year no entirely new line of nuclear research has beenopened. The study of nuclear reactions has now reached the stagewhere it is important to collect a large quantity of precise data,and the energy changes involved have been particularly examined.The existence of a pebuliar selective absorption of very slow neutronsin atomic nuclei has led to the production of new theoretical ideasabout nuclear structure. Bohr has suggested that energy suppliedto a nucleus may be extensively distributed over its components(see p. 30), and the theory of Breit and Wigner involves similar ideasof the energy levels of the nucleus as a whole.A theoretical survey of a large part of nuclear physics has ap-peared.1 The problems of the behaviour of the cosmic radiation,though not of its origin, are gradually assuming more definite form.ISOTOPIC CONSTITUTION OF THE ELEMENTS.A good deal of work has been done during the year with massspectrographs.New instruments of this type have been describedby A. J. Dempster,2 K. T. Bainbridge and E. B. Jordan 3 and byM. B. Sampson and W. Bleakney.* New sources of ions are alsode~cribed.~*~*~ F. W. Aston has made. some improvements in hisdesign of mass spectrograph, and has applied it mainly to the precisedetermination of atomic masses. These instruments have boen usedin the discovery of new isotopes, the determination of isotopeabundance ratios, and the precise determination of nuclear masses.An isotope 5Li 7 was discovered in the emission from a, platinum1 H.A. Bethe and W. Bacher, Rev. Mod. Physics, 1936, 82, 8.3 Phyaical Rev., 1936, 50, 282.4 Ibid., p. 456.5 J. P. Blewett arid E. J. Jones, ibicl., p. 464; S. L. Ch'u, ibid., p. 212.6 Nature, 1936,137, 357.7 A. K. Brewer, Phg&aZ Rev., 1936, 49,636.Proc. Amer. Phil. Soc., 1935, 75, 76516 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.filament impregnated with lithium, but this was not coilfirmed byother worker^.^ A special search was made for 8Be,8 since thisnucleus appears as a product in the current interpretations of manynuclear reaction^,^ often recoiling intact with considerable energy.On the basis of nuclear mass considerations 8Be appears to be astrongly bound element.1° It was not found in the mass spectro-graph analysis, with a detection limit of 1 part in 104.Theappearance of induced radioactivity under neutron bombardmenthas led to a search for new isotopes of several elements, since theknown isotopes were not considered sufficient to explain the differenttypes of radioactivity observed. No third isotope was found for79p81Br,11 which shows three decay periods when activated with slowneutrons. The limits of detection varied from -& for 78, 82 upto much larger values. No third isotope was found for 1 1 5 9 1131n.12*4In the case of 59C0 an isotope 57C0 has been found.13 Bainbridgeand Jordan have applied their mass spectrograph to the problemof the existence of isobaric pairs of stable elements, differing by oneunit in atomic number.According to some theories of nuclearstability, such pairs should not exist.14 Previous mass-spectrographwork has often been complicated by the existence of hydrides, butin Bainbridge and Jordan's work hydrogen-free conditions wereobtained and la;In was found isobaric with 'iiCd, l:XIn with l;6,Sn,5XTe isobaric with l::Sb. The theoretical consequences of this arediscussed. Other isotopes found by the mass spectrograph during130*132Ba,18 136* 13*Ce.18 A number of determinations of abundanceratios have been made with the mass spectrograph and in the caseof oxygen20 some variation has been found between various sources.The new atomic masses observed by Aston6 are in good accordthe year include 22Na,15 58Fe,16,17 64NiJ17 84Sr,12, 4 102M0,lS 134Ba,4,198 W.Bleakney, J.P. Blewett, R. Sherr, and R. Smoluchowski, Physical Rev.,9 See this report, p. 20.10 M. L. Oliphant, Nature, 1936, 137, 396; cf. N. K. Saha, Proc. IndianI1 J. P. Blewett, Phpical Rev., 1936, 49, 900.12 J. P. Blewett and M. B. Sampson, ibid., p. 778.13 M. B. Sampson, L. N. Ridenour, and W. Bleakney, ibid., 50, 383.1 4 Cf. K. Sitte, 2. Physik, 1935, 96, 512.l6 A. K. Brewer, Physical Rev., 1936,49,866 ; but see ref. (4).1* J. de Gier and P. Zeeman, Proc. K . Akad. Wetenach. Amsterdam, 1935,1' A. J. Dempster, Physical €Lev., 1936, 50, 98.1936, 60, 646.Acad. Sci., 1936, 6, 110.38,969.J. de Gier and P. Zeeman, Proc. K . Akad. Wetensch. Amsterdam, 1936,39, 327.ID A.J. Dempster, Physical Rev., 1936, 49, 047.2o W. Bleakney and J. A. Hipple, ibid., 1935, 47, 800BRADDICK. 17with the values obtained from nuclear transformations.10 It isinteresting to note that mass differences of the heavy elements cannow be obtained sufficiently accurately to reveal the mass equivalentof the energy emitted in their radioactive transformations.21Spectroscopic Methods.-The method of hyperfine structure ofspectral lines has been used to check the abundances of the lead 22and the platinum 23 isotopes, and the cadmium isotopes have beenexamined in the band spectrum of CdH, CdD.24 The elaboratecorrections necessary to obtain isotopic mass ratios from bandspectra have been worked out by W. W. Watson 25 and applied to theratios :H/:H, the value obtained agreeing with that of Aston.The hyperfine structure data on nuclear spin have been usedz6 toformulate a set of rules for the building up of atomic nuclei, whichagree with the isotope abundance data.NUCLEAR TRANSMUTATIONS.The work in this field has been directed largely to the measure-ment of the energies involved in nuclear reactions and the excitationfunctions, i.e., the probability of reaction considered as a functionof the energy of the bombarding particle. The energies have beenlargely used in the determination of differences in nuclear mass, and aconsiderable amount of data, has been acquired on the energy levelsof lighter nuclei.a-Particles.-Detailed investigations have been made of thetransmutations of several nuclei under a- particle bombardment.The reactions are of the (a ; p ) type 27 first investigated by Ruther-ford, in which the product nucleus is stable.The maximum energy of the protons may be calculated from theconservation of energy and momentum, a quantity of energyequivalent to the mass difference between initial and product nucleiappearing in the reaction.The proton may, however, carry awayless than the full energy of the transformation, the product nucleusbeing left in an excited state, which subsequently emits aThe investigation of the energies of the proton groups gives,therefore, mass differences between nuclei and the energies of nuclear21 A. J. Dempster, Nature, 1936, 138, 120, 201.22 J. L. Rose and R. K. Stranathan, Physical Rev., 1936, 4g, 916.23 S.Tohnsky and E. Lee, Nature, 1936, 137, 908.24 A. Heiner amd E. Hulthen, Naturwiss., 1936, 24, 377.46 Physical Rev., 1936, 49, 70.2s H. Schiiler and H. Korsching, 2. Yhysik, 1936, 102, 373.27 Thie notation is used to denote a change in which an a-particle is capturedA similar notation is used for proton, deuteron, neutron and a protonernitted.reactions; cf. Ann. Reports, 1935.38 H. J. von Baeyer, 2. Physik, 1935, 95, 41718 RhDlOAUTIVITY AND SW-ATOMC PHENOMENA.exoited state^. 0. Haxel 29 found that the reactions Mg (a; p ) Al,Si ( a ; p) P, S ( a ; p ) C1 gave very simiIar proton spectra consistingin each ewe of three groups. The excited atates are given in Table I.A. N. &y and R. Vaidymathan 30 find that the reactions 3' (a; p )Ne, Na (cc ; p ) Mg, P (a ; p) S each give four proton groups correspond-ing with three excited levels differing by about 1 M.E.V.C. J.Brasefield and E, Pollard 31 find for S (a ; p ) C1 three groups generallysimilar t o those of Haxel, while for K (a ; p ) Ca, C1 ( a ; p ) A, P ( E ; p ) Sthey find excited levels differing by about 1-5 M.E.V. Prom thefull energy proton group of S ( a ; p ) C1 they calculated the ma~s of325 from Bainbridge's 36Cl value. The evidence, then, points to theexistence of homologous series of nuclei differing by four units ininase and two in atamic number. These may be correlated withthe existence of a-particles as such in the nuolei (Hrtxel) or withthe formation of successive shells of neutrons and protons.Thehomologous series 27Al, 31P, 36Cl does not appear to extend to 1lBand "N, which have different level systems.32TABLE I.E'nepgy of a-particle r e c d o m : excitation levels of lzuclei fromtransitions. A (a ; p ) + B (+ Q).Reaction.Energy change,Q(M.E.V.). - 1 4 - -- 2-28 - 2.4 + 1-4 + 1.94- 2.1+ 0-0 + 0.1 - 1.0IEnergy of ex-Product. cited levelsnucleus. (M.E.V.).1.40.8, 1.7A = 31p 0.8, 1.6 i" 0-6, 1.254n + 3 8KC1 0.65, 1.62ike 1.5, 3.5, 4.62.3, 4.0, 5.039S 1.2, 2.6, 4.63sA -, 2.6, 4-342Ca -, 1-3, 2-641% $. 2The transmutation function in the case of S (K ; p ) C1 appears toconsist of an increase of transmutation probability with a-particleenergy corresponding to the Gamow theory €or penetration of apotential barrier.May and Vaidyanathan point out that the exist-ence of preferred velocities for a-particle entry to the nucleus29 Physileal. Z., 1936, 38, 804.30 Proc. Roy. SOC., 1936, A , 155, 519; cf. R. F. Paton, 2. Physik, 1934, 90,3l Physical Rev., 1936, 50, 296, 890.Y2 J. D. Cockroft and W. B. Lewis, f r o c . Roy. Soc., 1936, A , 154, 246, 261.33 G. Stetter, 2. Physik, 1936,100, 052.34 W. E. Duncanson and H. Miller, Pmc. Boy. SOC., 1934,146, 396.586BRADDICE. 19(resonance penetration) may give rise to groups in the protonspectrum when an inhomogeneous soiirce of or-particles or a thicktarget is used. An extra group in the reaction P (a ; p ) S is probablyaccounted for in this way.Protom.-The transmutation of some of the light elements byproton bombardment has been studied in detail. The excitationfunction for the reaction7Li + :H + 2*Hehas been determined from 23 k ~ ., 3 ~ 40-225 k ~ . , ~ ~ and up to 110038 The curve rose monotonically with energy ; no speciallypreferred energy values (resonance) were detected. A new wave-mechanical treatment of the problem of a nucleus with a potential wellunder proton bombardment 3g has been given. The simpler treatmentof the problem as penetration through a potential barrier is not ade-quate; the depth and width of the well have a marked effect on theefficiency curve. The calculation gives the position of the stationaryproton levels and the mass of the 8Be nucleus, and the potential wellhas to be adjusted to give consistent values for these data as well asfitting the excitation curve. A further calculation using '' exchangeforces" which vary with the velocity of the proton gives betterresults.4°The angular distribution of the a-particles produced in this re-action has been shown to be r&ndom,dl and the variation of rangewith angle is in agreement with the conservation of energy andmomentum.42 If 8Be is formed as an intermediate step, the factthat it does not lose e3ergy by collision shows that its life is lessthan 3 x 10-14 sec.The y-ray emission from lithium, showing resonance at about 450kv. and a rise at higher voltage^,^' has been confirmed and shown toarise from 7Li.43 The y-ray emission process is independent of thatwhich produces the x-rays; it may consist of the excitation of *Beto a state which is debarred from disintegration by a selection rule,or to the production of an excited and a normal a-particle.A y-ray emission from fluorine bombarded with protons 37 showsa6 H.D. Doolittle, Physical Rev., 1936, 49, 779.36 N. P. Heydenburg, C. T. Zahn, andL. D. P. Khg, a i d . , p. 100.37 L. R. Hafstad, N. P. Heydenburg, and M. A. Tuve, ibid., 50, 504;38 L. R. Hdstad and M. A. Tuve, ibid., p. 306.38 M. Ostrofsky, G. Breit, and D. P. Johnson, ibid., 1926, 49, 22.40 M. Ostrofaky, W. 33. Bleick, and G. Breit, ibid., p. 362.4 1 J. Giarratana and C . G. Brennecke, ibid., p. 35.42 A. Roberts, T. Zrcndstra, R. Cortell, and F. E. Myers, ibid., p. 783.43 L. H. Rumbaugh and L.R. Hefstad, {bid., 50, 681.R. G. Herb, D. B. Parkinson, and D. W. Kent, ibid., 1835,48,11820 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.resonances at 328, 892, and 942 kv. and probably some weak mul-tiple structure between 500 and 700 kv., but the reaction involvedis uncertain.The reactionIiB + iH --+ 3iHehas been confirmed and e~arnined.4~ The products of disintegrationhave been studied by the Wilson chamber.45 The distribution ofenergy among the a-particles is continuous, with a small homo-geneous group superposed. The symmetrical emission of threea-particles, formerly regarded as a preferred process, is now shownto be rare, and the mechanism suggested is the formation of an ex-cited 8Be nucleus ( p ; a change) and its subsequent splitting into twoa-particles. The life of the excited 8Be lies between 1O-l' andsec., and it is suggested that according to the uncertainty principlethe excitation energy is rather indefinite and the particles emittedin the ( p ; a) reaction have a velocity spread.On these assumptionsthe continuous distribution may be explained, the total energyreleased in the transmutation being 8-7 M.E.V. in accordance with therevised nuclear masses.l0 The small homogeneous group may thencorrespond to the reactionl;B + ;tH ,j :Be + $He (& = + 8.7 M.E.V.)with the mass of SBe very nearly equal to that of two a-particles. Asimilar argument to that above may be used to explain the deuteronreaction46lgOB + ;H-+ 3$HeThe excitation function for the two proton-boron processes showsa marked diff eren~e,~' the continuous distribution rising steadilywith increase of bombarding voltage while the longer range groupshows evidence of resonance.W. H.Wells and E. L. Hill 48 have attempted to avoid the neces-sity for the nucleus !Be in this and similar reactions by postulatingthat some of the nuclear particles are coupled in sub-groups.Deuterons.-Several new nuclear transmutations have been ob-tained by deuteron bombardment, and some of the formerly-knownreactions have been studied in more detail. A number of excitationfunctions have been determined, and the energy of the productparticles has been the subject of special attention. By intercompari-44 J. D. Cockroft and W. B. Lewis, Proc. Roy. Soc., 1936, A, 154, 246.46 P.I. Dee and C. W. Gilbert, ibid., p. 279.46 Cf. M. L. E. Oliphant, A. E. Kempton, and (Lord) Rutherford, ibid.,4 7 J. H. Williams and W. H. Wells, Physical Rev., 1936, 50, 186.da Ibid.? 49, 858.1935, A, 150, 24121 BRADDICK.son of the energy liberated in the reactions with the atomic massscale, the latter has been corrected and further various proposednuclear reactions have been checked. This check is important, sincethe reactions produced by deuteron bombardment of the lighternuclei are in general complicated.The excitation function for the reaction;H + ;H-+ :He + inhas been investigated by T. W. Bonner and W. M. Br~batker.~~The probability of transmutation rises rather slowly with the energyof the deuterons, while the yield of neutrons from the reaction:Be + ;H 4 I;B + inwhich was also studied, rises much more steeply.The neutronsproduced are nearly homogeneous in energy,50 2.55 M.E.V., andthe energy released in the reaction is 3.2 M.E.V. No yradiationcould be dete~ted.~lThe reactionl;B + :H+ l;C + ingives two groups of neutrons, the lower one corresponding with theproduction of an excited I2C nucleus. The excited level of 12C atabout 4 M.E.V. has also been detected by J. D. Cockroft andW. B. Lewis52 in the reactionl$N + :I3 --+ l;C + :Heand by Chadwick, Bothe, and others in the reaction:Be + ;He 4 l;C + tnAn investigation 53 of the neutrons from the bombardment of beryl-lium, boron, and carbon, which are believed to be produced by thereactions:Be + fH --+ IiB + inl;B + ?H -+ liC + inIiB + ;H -+ 3;He + in + ;H + + inl;C + TH ---+ $N + tnl;C + fH ---+ l;N + inhas led to energy values for these reactions and to a revised atomic-mass scale.In addition to the neutrons of maximum energy,neutron groups of lower energy were emitted, the nuclei being left inan excited state. No satisfactory correlation has been made with the49 Physical Rev., 1936, 49, 19.50 Cf. P. I. Dee and C. W. Gilbert, Pro(:. Roy. Soe., 1935, A , 149, 200.51 K. D. Alexopoulous, Naturwiss., 1935, 23, 817.52 Proc. Roy. SOC., 1936, A, 154,246, 261.53 T. W. Bonner and W. M. Brubaker, Physical Rev., 1936, 50, 30822 RADIOAC!MVITY AND SUB-ATOMIC PHENOMENA.y-rays which accompany the bombardment .a The transmutationof boron, carbon, nitrogen, and oxygen by deuterons has also beenstudied by J.D. Cockroft and W. B. Lewis.5% In this work theranges of the a- and H-particles were measured. With boron,homogeneous groups of a-particles were attributed to'iB + ;H+ !Be + $He':B + :H _I, :Be + :Hewhile proton groups were attributed toand a continuous distribution of a-particles toloB + 2H --+ 34Heand with less certainty tollB + 2H + 34He + inThe energy values of several reactions with carbon, nitrogen, andoxygen were determined and used to amend the mass scale.A new group of a-particles from carbon probably arises from13C + 2H +- 11B + "e, though the energy balance with otherreactions is not satisfactory.During the experiments on boron, an attempt was made to detectthe recoil of SBe, a nucleus which rather frequently appears in pro-posed transmutation reactions.The energy data lead to the con-clusion that the mass of sBe should be slight'lygreater than that oftwo a-particles :8Be + Z4Me + 0.3 M.E.V. (cf. p. 20)The production of P-radioactive elements of short life fromlithium, boron, and fluorine 55 has been further in~estigated.~~The radio-elements are now believed to be ?Li, 12B, and 2oF, formedby ( d ; p ) processes. The corresponding proton emission has beenfound for lithium and fluorine and the energy balances calculated.The protons from boron have not been definitely identified, buttheir energy is < 2.5 M.E.V. These radioactive elements are allbelieved to have atomic weights given by 2 2 + 2, containing twomore neutrons than protons. The positron-emitting elements 13N[12C ( d ; 12)13N], 150 [14N ( d ; 12) 150], 16N [15N ( d ; p ) 16N], 11C[1OB ( d ; n) W] were also examined.No evidence was obtained forpositron emission from l0Be or 14C-these nuclei are either stableor are not formed by 9Be ( d ; p ) and I3C ( d ; p ) . For all these radio-64 H. I&. Crane, L. A. Delsasso, W. A. Fowler, and C. C. Lauritsen, PhysicalRev., 1935, 47, 782.56 Crane, Delsasso, Fowler, and Lamitsen, ibid., pp. 971, 887 ; 48, 848.86. Idem, ibid,, 1936, 49, 501.'EB + !H -> 'kB + :BRADDICK . 23element& the positron or electron spectra were determined, Thereis some evidence 57 for a radioactive substance I0Be with a, very longlife (> 10 years) and a I4C with a, half-life of about 3 months.The deuteron bombardment of heavier elements has led to severalinteresting transmutations.Magnesium gives a composite p-radio-activity attributed to the processes 58The transmutation function for the latter process follows a Gamowrelation ; that for the former reaction agrees with the theory of J. R.Oppenheimer and M. Phillips 59 based on the idea that the deuteronsplits into a proton and a neutron, only the neutron penetrating thenuclear potential barrier. S. N. Van Voorhis 6* finds two radioactivesubstances from copper, which he aacribes to ( d ; p) reactions withthe two stable isotopes. He considers that B4Cu may decay either toWZn with emission of an electron or to HNi with emission of a posi-tron, since the half-life of positron and electron activities was thesame (12.8 hrs.).gives an electron-emitting radioactive substance of half-life 110 minutes.This is almost certainly *lA formed by a ( d ; p )reaction. It behaves chemically like argon. The excitation functionagrees with the theory of Oppenheimer and Phillips. A weak activityof the same type was excited in argon by intense neutron bombard-ment. The ps2- and emission from 41A has been investigated.The former seems to consist of two groups with upper energy limitsa t about 1.5 M.E.V. and 5 M.E.V., and the latter consists of a singleline with energy 1.4 M.E.V. Copper, zinc, antimony, ruthenium,bismuth, and tin give radioactive products when bombarded withdeuterons of 6 M.E.V.64 Copper gives 64Cu, as observed by E.Permiby neutron bombardment,65 and some other activities of obscureorigin. Zinc, antimoiiy, and ruthenium give complex activitieswhioh cannot yet be assigned with certainty to particular nuclei.Bismuth gives radium-E (210Bi) by it (d ; p ) (Oppenheimer-Phillips)Argon6 7 E. McMillan, Physical Rev., 1936, 49, 876.5 8 M. C. Henderson, ibid., 1935, 48, 855.59 Ibid., p. 500; see Ann. Reports, 1935, 32, 23.60 Physical Rev., 1936, 49, 876; 50, 895; cf. E. 0. Lawrence, E. McMillan,and R. L. Thornton, ibid., 1935,48,493.6f A. H. Snell, ibid., 1936, 49, 555.62 F. N. D. Kurie, J. R. Richardson, and H. C. Paxton, ibid., p. 368.63 J. R. Richardson, ibid., p. 203.434 J.J. Livingood, ibid., 50, 428; J. J. LiVingood and. G. Seaborg, ibitl.,8 6 Ann. Reports, 1936. p. 43524 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.reaction. This represents the first artificial production of a natur-ally occurring radio-element. Radium-E decays to the a-emitterXa-P (polonium) with a half-life of 5 days. The cc-particle emissionfrom this substance was det'ected, and the identity of the range of theparticles with polonium a-particles was verified. The complexactivities observed from zinc, antimony, and ruthenium indicatethat the Oppenheimer-Phillips process is not the only method ofattack of deuterons on the heavy nucleus. The alternative is pre-sumably a resonance process, but excitation functions have not yetbeen determined.Tin also shows a complex activity, and a chemicalseparation showed the presence of active isotopes of indium, tin,and antimony. The antimony fraction emits both positrons andelectrons, and there is some evidence that this is a branched trans-formation and not due merely to the presence of two different radio-elements. Platinum 66 bombarded with 5 M.E.V. deuterons gives acomposite activity with emission of both positrons and electrons.A chemical separation reveals isotopes of platinum and iridium.The reactions involved possibly include'iiPt + :H -> ':iPt + iH 193Pt + 1931r + e+ (49 mins.)1;6gPt + :H --+ l:;Pt + ;H lg7Pt I_, 19'Au + e- (14.5 hrs.)'P;Pt + :H + lP;Ir + :He 194Ir --+ 194Pt + e-The transmutation functions in this case definitely show the maximaexpected for a resonance process, and there is evidence that theresonance is involved in the production of iridium isotopes, theplatinum isotopes being formed by an Oppenheimer-Phillips processrising monotonically with the energy of bombardment.Neutrons.-The reaction of slow neutrons with boron18B + iLi + $Hehas been investigated by D.R ~ a f , ~ ~ using boron trifluoride in anexpansion chamber, and the energy released in the reaction is foundto be 0-61 M.E.V. A new value for the mass of loB was obtained.The helium obtained by bombarding boron (methyl borate) withslow neutrons has been separated and observed spectroscopically.This is the first time a product of artificial transmutation has beenobserved by such methods.6BThe disintegration of nitrogen by neutrons has again been investi-gated in the expansion chamber.69 The reactions observed weresupposed to be6 6 J.M. Cork and E. 0. Lawrence, Physical Rev., 1936, 49, 788.67 Proc. Roy. SOC., 1936, A , 153, 568.6 8 F. A. Paneth and H. Loleit, Nature, 1935, 136, 950.69 T. W. Bonner and W. M. Briibalcer, Physical Rev., 1936, 49,223, 778; 50,781BRADDICK. 25';N + in --+ l;B + :He'$N' + in ~ _ f 23€e + iLiand the first of these reactions was supposed to be exothermic and totake place with slow neutrons. A closer consideration of the energybalance as compared with other known reactions of the nuclei in-volved showed that this conclusion was wrong, the reaction beingendothermic (& = - 0.3 M.E.V.).The reaction which does occurwith slow neutrons is the (n ; p ) change leading to 14C. The radiationof energy during the capture of fast neutrons 7o by nitrogen is notsupported by these experiments.Some additions have been made to the results on the productionof radioactive nuclei by neutron bombardment; these are given inTable III.Beryllium bombarded with neutrons and examined after a veryshort interval showed a strong activity of period 0.9 sec.71 Theradioactive substaiice was identified as a helium isotope, probably 726He formed by a (12 : a) process. It is not produced by very slowneutrons, but is formed by neutrons of 1.5 M.E.V.73 It emits @-rayswith maximum energy about 3-7 M.E.V.Lithium 74 bombarded with slow neutrons gives a @-emittingsubstance, probably sLi, with half-period 0.7 sec., and identical withthe substance produced from lithium by a (a; p ) reaction.75 Thediscrepancies in the results obtained on the rare earths, due to thedifficulties of separation, have led to a re-examination of theseelements by G.Hevesy and H. L e ~ i . 7 ~ Their results will be found inTable 111. They investigated in addition the absorption of slowneutrons by the rare-earth elements and found very large absorptioncross-sections for europium, dysprosium, gadolinium, and samarium.In the cases of the last two and possibly some other elements, theinduced radioactivity is very small compared with the absorptioncross-section, and the product nucleus is presumably stable. Thecomplicated system of reactions observed when uranium is bom-barded with neutrons has been further investigated.7' In additionto previously-known activities with half -lives 10 sees., 40 sees.,13 mins., 100 mins., and 3 days, an activity of 12 hours' half-life has70 F.N. D. Kurie, Physical Rev., 1935, 47, 97; Ann. Reports, 1935, 32, 24.71 T. Bjerge, Nature, 1936, 137, 865.72 Idem, ibid., p. 400.73 M. L. Olipliant, quoted in ref. 72.74 K. S. Knol and J. Veldkamp, Physica, 1936, 3, 145.7 5 Crane, Delsasso, Fowler, and Lamitsen, PhysicaE Rev., 1935, 47, 971;76 Nature, 1936, 137, 185.77 (Frl.) L. Meitner and 0. Hahn, Naturwiss., 1936, $24, 158.',4N + :?z -> 'tC +see, however, ref. 5626 RADIOACTWITY AND SUB-ATOMIC PHENOMENA.been assigned to eka-oamium(as'eka-0s).Evidence is obtained forthe following reactions :(1) ":U + n + ';6,Th + o! + 235Pa --+ 235U --+235eka-Re --+The reaction goes with slow neutrons and 239U may be formed by a(12; -) reaction and behave as an a-emitter of very short life.B B BB I::eka-Os a Q,9ieka-IrThe reaction requires fast neutrons, and may be of a new type inwhich a neutron is absorbed and two neutrons are emitted :The reaction of neutrons with thorium has been investigated ;78 thethorium salt used had been chemically kept free from its isotopicradioactive products of moderate life. The processes suggested are2!iTh + n .+ =%Ra + a ; ";Ra + "8",4Ac __+ 2i:Th --+ B B BB ":Pa a:iAc _I, YgThThe two isotopes of actinium have half-lives of 3.5 and 42 hoiirs, andan isotope of thorium a half-life of 25 mins.y-Rap.-The disintegration of nuclei by y-rays (nuclearphiliceffect 79) has been further investigated in beryllium and deuterium.80No other reactions of this type have been discovered, and from thepresent nuclear masses it appears that no other reactions among thelight elements are energetically possible using 2-3 M.E.V.y-rays.*lIn the case of 2H the threshold value for the reactionZH + h v + lH + nis found to be 2.26 M.E.V.82 The mass of the neutron may be cal-culated from this result to be 1.009.The threshold for 9Be + hv --+ *Be + n is found to be 1.6&LE.V.83 The efficiency of the process (yield per quantum) falls78 E. Rona and E. Neuninger, Naturwiss., 1936,24, 491.70 Nuclear photo-effect.80 D.P. Mitchell, F. Rasetti, G. A. Fink, and G. B. Pegram, PIiysicoZ Rev.,1936, 50,189; S. Nishida, Japan. J . Physics, 1936,11, 9.81 N. Feather, '' Nuclear Physics," Cambridge, 1936.82 J. Chadwick, N. Feather, and Bretscher, quoted in ref. 81.83 J. Chadwick and M. Goldhaber, Pvoc. Roy. Soc., 1935, A , 151, 479; cf.G. Iairig and M. Helde, Nature, 1936, 137, 273BRADDICK. 27TABLE 11.Energies of some nuclear reactiom (in lo6 electron-volts) .(Note : 1 M.E.V. corresponds with 0-00106 unit of atomic mass.)For the calculation of masses from data of this type, see, e.g., refs. 51, 53.with increasing y-ray energy rather like that of the photoelectriceffect in the extra-nuclear54 R. Fleischmann and W. Gentner, 2. Physik, 1936, 100, 440.85 P.I. Dee and C. W. Gilbert, Proc. Roy. SOC., 1935, A , 149,200.86 J. Chedwick and M. Goldhaber, Proc. Camb. Phil. SOC., 1935, 31, 612 ;87 P. I. Dee, Proc. Roy. SOC., 1935, A, 148, 623.88 M. L. Oliphant, B. B. ICinsey, and (Lord) Rutherford, ibid., 1936, A ,see also ref. 81.149,40628 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.82ow Neutrons.-A great deal of work has been done on the mech-anism by which neutrons are slowed down by passage through matterand on the capture of slow neutrons by nuclei. Experiments showthat the efficiency of neutrons in causing transmutation and thecross-section for absorption of slow neutrons were in some casesincreased by cooling the paraffin or other hydrogen-rich materialin which the neutrons were slowed down.5 The interpretation of the4 Be9 F11 Na15 P17 C127 Co28 Ni35 Br39 Y46 Pd50 SnTABLE 111.New activities produced by neutrons.12 5.15 d.96 60 Nd 1 h.80 d. 97 62 Sm 40m.,long.72 h. 98 63 Eu 9.2 h.IY 1 yr. 1 3 64 Gd 8 h.3 h. 9s 65 Tb 3.9 h.24 h. 66 Dy 2.5 h.70 h. 76 67 Ho 35 h.12 h., 3 m., 60 h. 68 Er 12h., 7 m.0-8 m. 1, 18 m. ? Og* 70 Yb 3.5 h.71 Lu 6 d., 4h.80 Hg 205Hg 40 h.81 T1 T197 m., 4m.g6 :g :h )seep. 26.0-9s 'It95 57 La 1.9 d. 7658 Ce - 95195 59 Pr 19h., 5 m .experiments is complicated by geometrical considerations and bybut it is clear that some of the neutrons are slowed down89 J. D. Cockcroft and E. T. S. Walton, Proc. Roy. Soc., 1934, A, 144, 704.90 M. L. Oliphant, A. E. Kempton, and (Lord) Rutherford, ibid., 1935, A,91 T.W. Bonner and W. M. Brubaker, Physical Rev., 1935,4$, 742.92 Oliphant, Kempton, and Rutheriord, Proc. Roy. b'oc., 1935, A, 150, 240.93 H. Miller, W. E. Duncanson, and A. N. May, Proc. Comb. Phil. SOC., 1934,94 J. Chadwick, N. Feather, and W. T. Davies, ibid., 1935, 31, 357; see95 31. E. Nahmias and R. J. Walen, Compt. rend., 1936, 203, 71.96 P. Preiswerk and H. von Halban, ibid., 1935, 201, 722.97 E. 13. Andorsen, 2. physikal. Chem., 1936, B, 32, 237.9 8 Idem, Nature, 1936, 138, 76,99 R. Naidu, ibid., 137, 578.1 C. H. Johnson and F. T. Hamblin, ibid., 138, 504.2 I. V. Kurtschatov, G. D. Lutischev, L. M. Nemenov, and I. P. Selinov,3 M. E. Nahmias, Compt. rend., 1936, 202, 1050.4 E. B. Andersen, Nature, 1936, 137, 457.5 Proc.Roy. Soc., 1936, A, 153, 476.6 P. B. Moon, Proc. Physical SOC., 193G, 48, 648; J. R. Tillman, ibid., p.642 ; P. Lukirsky and T. Zarewa, Nature, 1936, 136, 681 ; W. F. Libby andE. A. Long, Physical Rev., 1936, 50, 575.149, 406.30, 549.also ref. 81.Physilcal. Z. Sovietunion, 1936, 8, 589BRADDICK. 29to thermal values. This is confirmed by direct experi~nents.~ Evid-ence has been obtained for the diffraction of slow neutrons at crystals,in accordance with the de Broglie wave-length of the neutrons.8 Anenormous mass of experimental material has been obtained on thescattering and absorption of the slow neutron^.^ The scatteringof neutrons by protons has been studied in order to check a theor-etical formula of Wigner, but the results are not entirely consistent.1°TABLE IV.Properties of slow neutrons.( a ) Groups of neutrons according to Fermi.laStrongly Stronglyabsorbed by Activates absorbed by ActivatesC Cd B Ag (moderately) AgD Rh, In Rh, InA Ag Ag, Ir I I I(b) Resonance energies for slow neutrons (from Rasetti, " Elements of NuclearElement and Resonance Element and ResonancePhysics," 1936, Blackie, London).period. energy (volts).period. energy (volts).Rh 44s. 1.1 In 54m. 1.3Rh 4 m . - 1 Ir 19 h. * 1.6Ag 22 8. 2.5, 4-6 Au 2.7 d. 2.5In 16s. - 2The fist theory of the capture of neutrons by nuclei 11 shows thatin the absence of resonance the capture cross-section varies inverselywith the neutron velocity. The experiments of P.B. Moon andG. A. Fink, J. R. Dunning, G. B. Pegram, and D. P. Mitchell, PhysicalRev., 1936,49, 103.D. P. Mitchell and P. N. Powers, Physical Rev., ibid., 50, 486; H. vonHalban and P. Preiswerk, Compt. rend., 1936, 203, 73.@ C. H. Collie, Nature, 1936, 137, 614; C. H. Collie and J. H. E. GrifEths,Proc. Roy. SOC., 1936, A , 155, 434; C. T. Zahn, E. L. Harrington, and S.Goudsmit, Physical Rev., 1936, 50, 570; D. P. Mitchell, ibid., 49, 453; G. A.Fink, J. R. Dunning, and G. B. Pegram, ibid., 49,340; F. Rasetti, E. SegrB,G. A. Fink, J. R. Dunning, and G. B. Pegram, ibid., p. 104; C. Y . Chao andC. Y. Fu, Sci. Rep. Not. Tsing Hua Univ., 1936, 3, 451; I. Kara, L. Rosen-kevitsch, C. Sinebrikov, and A. Walther, PhysikaL 2. Sovietunion, 1935, 8,219; V.Fomin, I?. G. Houtermans, J. V. Kurtschatov, A. I. Leipunski, L.Schubnikov, and G. Schtschepkin, Nature, 1936, 138, 326; H. von Halbanand P. Preiswerk, ibid., 137, 905; N. Dobrotin, Compt. rend. Acad. Sci.U.R.S.S., 1936,2, 235; V. Fomin, F. G. Houtermans, A. I. Leipunski, L. B.Rusinov, and L. V. Schubnikov, Nature, 1936, 138, 505; 0. R. Frisch, G.Hevesy, and H. A. C. McKay, ibid., 13'7, 149; A. C. G. Mitchell, E. J. Murphy,and M. D. Whitaker, Physical Rev., 1936,50,133,10 M. A. Tuve and L. R. Hafstad, ibid., p. 490 ; M. Goldhaber, NGture, 1936,127, 824.11 H. A. Bethe, Physical Rev., 1936, 48, 265; for other references, seeAnn. Reports, 1936,82,2'730 RADIOACTIVITY AND SUB-ATOMIC PHENOMENA.J, R. Tillmans and of E, Fermi and E. Ama1dil2 show a seleotiveeffect, and Fermi divides neutrons into groups as in Table IV.The neutrons of the C-group seem to have “thermal” ve10cities~~J~with energies of the order 0-03-1.0 volt.The energies correepondingto the other bands have been determined 13* 14* 15 by assuming, ontheoretical and experimental grounds,14 that the capture of neutronsin boron and lithium (12; a transformation) follows the l / v law. Itbeing assumed that the capture process in cadmium takes placewith maximum probability at about 0.03 volt, the energy for anyother band can be determined from the absorption in boron orlithium. It appears that the resonances of rhodium, silver (224.period), indium, iridium, and gold lie in the range 1-5 volts, whilemanganese, copper, arsenic, bromine, silver (2.3 m.), iodine andrhenium are activated by higher-energy neutrons of 30-85 volts.These phenomena have led to very interesting theoretical con-clusions.N. Bohr l7 has given reasons for the existence of closelyspaced energy levels in excited heavy nuclei, and he and G. Breit andE. Wiper 1* have shown that the assumption of such levels leads to a,satisfactory explanation of slow neutron capture. The neutronentering the nucleus forms an excited compound nucleus, whichbreaks up with re-emission of a neutron scattering, or of anotherparticle (n; p or n ; cc process), or goes into a lower state (capture).The existence of a number of closely spaced levels gives a possibilityof resonance capture of a neutron at low energies. H.A. Bethe l9calculates by it statistical argument that the spacing of the levelsis of the order 50-500 V. for elements of medium atomic weight,while the excitation energy of the compound nucleus is of the order10 M.E.V. The theory accounts for the fact 2o that good absorbersfor slow neutrons are not always strong scatterers, the procesaesof capture and scattering being alternatives after the absorption hasonce taken place. Bohr l7 produces interesting arguments of ageneral kind to explain various features of the process. He suggeststhat the energy of capture is diffused inside the compound nucleusla Ric. Sci., 1935, 2, 344, 443; 1936, 1, 66, 233, 310; L. Szilard, Nature,1s H. H. Goldsmith and F. Resetti, PhylskE Rev., 1936, 60, 328.14 0.R. Frisch and G. Placzek, Nature, 1936, 187, 357.15 D. F. Weekea, M. 8. Livingston, and H. A. Bethe, Phpicul Rev., 1936,16 F. Resetti, D. P. Mitchell, G. A. Fink, and Q. B. Pegram, aid., p. 777.l7 Nature, 1936, 137, 344.18 Phyeicd Rev., 1936,49, 519.lo Ibid., 50, 332.20 A. C, Gt. Mitohell and E. J. Murphy, ibid., 1935,47,881; 48, 653; 1936,1935, 136, 950.49,471.49,400,401 ; 60,133 ; see also ref. 76BRADDICK. 31and finally becomes concentrated on an escaping particle. G.Gamow 2l has predicted resonances for the oapture of fast neutronsby lighter nuclei to which the Bohr and the BreitFWigner theoryare inapplicable, but these have not yet been observed.The production of y-radiation attending the capture of slow neu-trons has been studied by various workers,22 and represents amethod of studying nuclear excitation energies. S.Kikuchi, K.Husimi, and H. Hoki 23 find that the maximum y-energy for a, givenelement is connected with the atomic number by one of two smoothcurves.The p-Disintegration.-The conclusion that the upper limit of thecontinuous energy spectrum of P-particles repesents the energyliberated in the nuclear reaction24 has been strengthened by ob-servations on the artificial p-ray emitters lzB 56 and 13N.25 In boththese cases the energy emitted in the @-ray change can be calculatedindependently, the energies of other reactions being used.The @-rayand positronenergy distributions from the radio-elements13N, 17F, 24Na, "Si, 32P, C1, 4lA, d2K26 and from indium, silver,rhodium, manganese, and dysprosium activated by neutrons2' havebeen determined.The Konopinski-UHenbeck theory of the disinte-gration has been used in these experiments t o provide an extrapoh-tion formula for the observed energy distribution curves. The justi-fication for this procedure is empirical. The relation between themaximum @-ray energy and the decay period of these lighter radio-active nuclei does not, apparently, show the regularity associatedwith the natural radio-elements (Sargcnt rules). The continuousP-spectra of some of the heavy radio-elements have been re-examined,28 and the end-points determined with the aid of the Kono-pinski-Uhlenbeck formula. The values obtained for radium-E amnot very concordant, but lie rather above the accepted value bmedon simple examination of distribution curves.The cornpaxison of81 Physical Rev., 1936, 50, 946.2' R. Fleischmmn, 2. Phyeik, 1935, 97, 242, 266; F. Rasetti, ibid., p. 64;C. Rammy, Compt. rend., 1936,203, 173; H. Hersxfhkiel and L. Wortenstein,Naure, 1936,137, 106.23 Ibid., pg. 186, 745, 992; see also ibid., pp. 30, 398; cf. R. Fleischmann,Natumoias., 1936, 24, 77.Ann, Repork, 1935, 32, 32.86 W. A. Fowler, L. A. Delsasso, and C. C. Lamitsen, Phy8kd Rev., 1936,26 F. N. D. Kurie, J. R. Richardson, and H. C. Paxton, ;bid., p. 369.27 E. R. Gaerthner, 3. J. Turin, and H. R. Crane, ibid., p. 793.28 F. A. Soott, Wd., 1936, 48, 391; F. C. Champion and N. 5. Alexander,Nature, 1936, 137, 744; P. C. Ho and H. Wang, C h k e J.P h y k , 1936,2, 1; M. Lecoin, Compt. rend., 1936, 202, 1067; L. Goldstein and M. Leooin,ibid., p. 1169.49, 66132 RADIOACTIVITY AXD SUB-ATOMIC PHENOMENA.the form of the continuous p-spectrum of radium-E with that of3OP29 shows a considerable difference in the shapes of the curves,which is presumably connected with the difference between heavyand light nuclei.There has been little development in the theory of the p-raychange. The current ideas of the continuous spectrum involve anon-ionising particle (neutrino) to carry away the energy differencebetween the energy of the nuclear reaction and that of the @-raysctually emitted from a particular nucleus. An attempt has beenmade to detect the neutrino by measuring the energy distributionof the nuclei recoiling after a 8-ray change.3O A light element, llC,was used so that the recoil velocity might be as high as possible.The experiment is difficult and the results rather indefinite, but theydo favour the neutrino hypothesis.0. Gamow and E. Teller31have modified the Fermi theory as it refers to selection rules for thep-disintegration by taking into account the spin of the heavy particlesin the nucleus. Their new selection rule is consistent with the ob-served lives of the different atoms of the thorium series. C. Mdler 32considers the possibility of the simultaneous creation of an electron-positron pair, together with the proton-electron pair of the Fermitheory. He uses this process to explain the positron emission ob-served by Alichanow, Alichanian, and Kosodaew from Th-C + llC.A theoretical calculation on the basis of the Fermi theory= showsthat a weak continuous y-ray spectrum should accompany the (3-decay, especially in light elements.There is as yet no experimentalevidence for this.THE PASSAGE OF ENERGETIC 8- AND ?-RAYS THROUGHMATTER.The scattering of fast @-particles by nitrogen nuclei has beenexamined by F, C. Champion,= who finds general agreement withN. F. Mott’s theoretical treatment, no losses of energy by radiationbeing found. D. Skobeltzyn and E. Stephanowa 35 find with p-rays ofenergy between 1.5 and 3 M.E.V. that there are sudden losses ofenergy when 8-particles pass through nitrogen, which they ascribet o a special intranuclear effect.Similar collisions have been observed2* A. I. Alichanow, A. I. Alichanian, and B. Z. Dgelepov, Nature, 1936,137, 314.8O A. I. Leipunski, Proc. Camb. Phil. Soc., 1936, 32, 301.31 Phy8iwl Reu., 1936, 49, 895.32 Nature, 1936, 137, 314.33 Ibid., 1935, 136,475, 719; F. Bloch, Physicd Rev., 1936, 50, 272; J. K.sc Proc. Roy. SOC., 1936, A, lS3, 353.8 5 Nature, 1936, 137, 234, 466.Knipp and G. E. Uhlenbeck, Phy8iCa, 1936,3,42533 BRADDICK.in argon.36 Skobeltzyn and Stepanowa have also found evidenceof electron-positron pair production by @-rays.37 These develop-ments must still be regarded as under investigation. Theoreticalinvestigations of pair production by @-rays and protons have beenmade, which give a much lower order of magnitude for this effect.3*The production of pairs by y-rays has been investigated theor-etically 39 and experimentally.40THE PENETRATING RADIATION.The origin of the cosmic radiation remains obscure.Some furtherevidence has appeared against the reported increase in the radiationdue to super-nova stars.41 A. Ehmert 42 has reported that there is adiurnal change in the cosmic ray intensity. On account of the com-plicated paths followed by the particles in the earth’s magneticfield, barometric changes which cause the rays to be filtered by moreor less air have an effect on the phase of the diurnal variations whichobscures the latter over long periods unless allowed for. Thisconclusion must for the present be accepted rather tentatively. Theproblems attacked by cosmic ray workers have been the nature of theprimary particles and their behaviour in the earth’s atmosphere andin other absorbers. The showers are especially interesting, bothfrom the point of view of sub-atomic physics and in that they contri-bute to the radiation observed in the lower atmosphere.G. Pfotzer 43has made balloon flights nearly to the top of the atmosphere withself -registering triple-coincidence sets of Geiger-Muller counters, andvery recently R. A. Milliban, H. V. Neher, and S. K. Haynes 44 havesent electroscopes to comparable heights. Pfotzer’s curve has as itsmain features a rise in the vertical intensity of the rays with altitude,a maximum a t a barometric pressure of about 70 mm. of mercury,and a rapid fall at the top of the atmosphere.There is also a, smallsubsidiary hump at about 300 mm. Pfotzer45 has attempted arather complete analysis of this curve, using the idea due to W. P. G.36 L. Leprince-Ringuet, Compt. rend., 1935,201, 712.37 Nature, 1936, 13’9, 272.38 H. J. Bhabha, Proc. Roy. SOC., 1935, A , 152, 559; Y . Nishina, S. Tomo-naga, and M. Kobayasi, Sci. Papers Inst. Phys. Chern. Res. Tokyo, 1935,27, 137.39 J. C. Jaeger and H. R. Hulme, Proc. Roy. Soc., 1936, A , 153,443.40 M. N. S. Immelman, Naturwiss., 1936, 24, 61; W. Bothe and H. Klar-mann, 2. Phyeik, 1936, 101, 489.4 1 J. Barn6thy and M . Forrb, Nature, 1936, 138, 544.4 2 2. Physik, 1936,101, 260.43 Ibid., 102, 23.44 Physical Rev., 1936,50, 992 ; cf. E. Regener and G. Pfotzer, Phyailcal. Z.,45 2.Physib, 1936, 102, 41.1934, 35, 782.REP.-VOL. XXXIII. 34 IEADIOACTLVITY AND SUB-ATOMIC PHENOMENA.Swann46 that primary particles come through a large part of theatmosphere, producing secondaries by the shower process with anefficiency increasing with the primary energy. The majority of theparticles in the lower part of the atmosphere are secondary. Thehump is due to the fact that below a certain altitude primary par-ticles are being progressively rernovcd as they come to the end oftheir range, while particles which would end above tho criticalaltitude have been filtered out by the action of the earth’s magneticfield. Yfotzer finds it necessary to postulate, in addition, a hardcomponent which is absorbed exponentially and which he considersmay be pr0tons.4~ W.F. G . Swam 48 has produced independently amore complete and general but essentially similar theory. He findsit possible that the ‘‘ hard component ” is not affected by the earth’smagnetic field, and may be due to photons. An unpublished argu-ment by P. M. S. Blackett, based on a balance-sheet for the cosmicray intensity entering the atmosphere, leads to the view that thenumber of particles entering the atmosphere is small compared withthe number of secondaries formed and leaves place for an appreciablephoton component. The existence of protons in the cosmic rays hasbeen the subject of several investigation^,^^ and it has been foundthat a few per cent. of the particles a t sea level may be protons.Ifthe hard component of the cosmic rays consists of protons, they mustact as producers of secondaries.I?. M. S. Rlackett and R. B. Brode 50 have obtained an energydistribution for the cosmic ray particles at sea, level by measuring thecurvature of their Wilson chamber tracks in the field of a largeelcctro-magnet. The spectrum is approximately of the formg(E) = lIE2for energies between 2 x 109 and 2 x 1010 E. V. There isan irregularity in the spectrum at about 2-5 x lo9 E.V., which isprobably significant and indicates a selective absorption action onparticles of this energy, or possibly a singularity in the primarydistributic~n.~~Several absorption measurements of the primary rays have been4 6 Physical Rev., 1936, 48, 641 ; i l n i z .Reports, 1935, 32, 37; cf. B. Cross,* 7 Cf. A. H. Compton, Guthrie Lecture, PTOC. Physical SOL, 1936, 47,4 8 Phy8ical Rev., 1936, 50, 1103.49 C. G. Montgomery, D. D. Montgomery, W. E. Ramsey, and W. F. G.Swrtnn, ibid., p. 403; R. B. Brode, H. G. MacPherson, and M. A. Starr, ibid.,p. 383; W. F. G. Swarm, ibid., 49,478; q’. 1%. Wilkins and H. St. Helens, ibicl.,p. 403 ; L. H. Kurnbaugh ant1 G. L. Locher, iDitE., p. 885.50 Proc. Roy. rSoc., 1936, A, 154, 564, 673 ; cf. L. Lepsirice-Ringuet, Cyompt.rend., 1935, 201, 1184.5 1 I?. M. S. Blackett, unpublished.l’hysikal. Z., 1936, 37, 12.747BRADDICK. 35made, showing hard and soft components.b2 The cosmic rays havebeen detected in a deep mine under the equivalent of 700 m.of water.53 Evidence was found for the existence of showers atthis depth. At a depth of 30 m. under London clay (68 m. waterequivalent), D. H. Pollett and J. D. Urawshaw 54 found thatthe proportion of showers to vertical rays was much the same asat sea level. The production of “ showers” and “ bursts” hasbeen investigated with counter arrangements and with ionisationchambers. The experiments of H. CarmicEiael 55 show that evenlarge bursts contain thinly ionising particles and are essentiallyidentical with showers observed in the Wilson chamber or withcounters. Thc producfion of very large bursts may occasionallybe observed with a comparatively thin-walled steel chamber, so it isprobabIe that showers start either in a single act or as a result of acascade process with an enormous efficiency. The production ofshowers by eIectrons has been observed in the cloud chamber.56 Theevidence available shows that a ratlicr large proportion of showersis initiated in this way.56* 57 The efficiency of shower production,measured in thin layers of different elements, varies with thc squareof the atomic number.5s This relation holds also a t an altitude of4000 m.59 The average size o€ showers aiid bursts, as well as theirfrequency , increases with a1 t i t ude .Go13. J. J. RRADDICK.52 J. Clay, Pkys;cn, 1936,3, 332 ; P. Bugor and A. Itosenberg, Compt. rmd.,53 J. Barn6thy and M. Forri), Nature, 1936, 138, 325, 399.64 Proc. Roy. SOC., 1936, A , 155, 546.b 5 Ibid., 154, 224; cf. T.V. Ehrenberg, ibid., 155, 532.5 6 E. C. Stevenson and J. C. Street, Plzysical Kev., 1926, 49, 425.6 7 J. Clay and A. Van Gemert, Physica, 193G, 8, 763.68 C. G. Montgomery and D. D. Montgomery, Physical Rev., 1936, 50, 490;59 Hu Chim Shan, unpublished.6o C. D. Anderson and S. Neddermeyer, Physical Eev., 1936, 50, 263;1936, 202, 1923.J. E. Morgan and W. M. Nielsen, ibid., p. 852.1L. T. Young, ibicl., p. 638