Chatterjee, S D

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ON THE SPONTANEOUS EMISSION OF NEUTRONS FROM THE URANIUM NUCLEUSBy S. D. CHAfrERJEE^ABSTRACT. U.sing a boron-lined ionisation I'Lauilor filled witfi l!!''u, llie spontnneoii.s emission of neutroms from (he urariiniu nucleus has been deterted. It is suggested that they are produced as a result of the spontaneous fission I'f uranium and that ,t neutrous lui' emitted per each such process.I N T R O D U C T O R YSoon after the discovery of the induced tission of the urauiinn nucleu.s due to neutron bonibardnient, it was realised tliiit in Avlialsoevcr way tlie salJ*'"' nucleus divided itself, there must be an excess of neutrons over that contained in normal nuclei of the same atomic number. 'I'here seemed to be two possibilities of getting rid of this neutron excess :(1) by the emission of a /?-ray, a neutron may be Iraiisformed into a proton, thus reducing the neutron cxce.ss by two units.(2) by the direct liberation of the neutrons taking place cither as a pail of the fission itself, or as an "evaporation" process from the resulting nuclei which would be formed in an excited slate.'Ihe evidence for the first process was obtained from the successive b’-trans- formation of many of the fission products. The direct emission of neutrons during the second process, was sought foi and obtained I>y several experimenters, employing various methods.Von Halban, Joliot and Kovvarski (ig.39)j nicusured the distribution of neutrons surrounding a neutron-source placed at the centre of a large container filled with a solution of uranyl nitrate. Another set of measurements was made in which a solution of ammonium nitrate of the same strengtii was used instead of uranyl nitrate. They found a 5% higher intensity in the former case and associated it with the production of secondary neutrons due to the induced fission process in uranium. The average number of neutrons produced per fission was calculated to be 3.510.7. Dod^ et al (1939) demonstrated that the neutrons produced during induced fission were fast neutrons.Further experimental evidence of neutron emission during induced fission was obtained by Anderson, Fermi and Ilanstein f 1939)1 tu^ d Haenny and Rosenberg {1939).The experimental investigations on the spontaneous fission of the uranium nucleus, carried out by the author, (Chatterjee, 1944) raised the interesting questi«ii whether the process pf spontaneous fission of U is likewise accompanied* Fellow of the Incliaii Pliysical Six-iety242 / 2 S, D, Chatterjeeby the emission of neutrons. The following experiment was therefore under­taken to verify the correctness of the above hypothesis and also to determine the half-life period of the corresponding process. A preliminary report of our findings w^ as published in a note in Science and Culture (1944).P R I N C 1 1 * h R o  r 'J' IT R M l i  T II 0  I) OF M K A S U R K M E N TA proportional counter, lined with a thin layer of l)oron powder and filled with BFa gas is used. Owing to tlic comparatively high gas-pressure atm), the countci is used as an ionisation chamber, in conjunction with a linear ampli­fier and a tiiyratron-opei nted mechanical recorder. The rate of counting of such an arrangement may lx represented byA + IM-C,where A represents the number of slow neutron-induced boron disintegrations, the resulting u-parlicles being counted,H is the number of recoil nuclei i)roduced by fast neutrons traversing the counter,C is the mmiber of other particles of high specific ioiii/ation passing through the counter.It is evident that A “  VNnfxrj,Vu *+• (NAp// )^RiiC‘ii.B - V N I i(v)(r(v)clv./ ‘‘ '3I(v )dvwhere V =  volume of the counter.N “ -the total number of nuclei per c,c. in the counter.N a =  Avogrado’s number.Nn =  lhe number of nuclei per c.c.P =  thc density of neutrons per c.c. with energies in the i/v  region,(/!,== the capture cross-section for for neutrons of velocity vn.Rij==the range of 0,-particles in boron./X—atomic weight of boron (B’ ®).i(iO(h' =  tlie current of fast neutrons of velocity between v and vH-dv.(r(x»)=the recoil cross-section of the nuclei of the gas for neutrons of velocityS =  the cross-sectional area of the counter. l(r)t?i  ^=  the ciuTeut per cm®, of particles of high specific ionization, of velocity between v and v + dv.The limits of integration, v, is the highest and Va the lowest neutron velocity producing measurable recoils, Va the the highest and d.  the lowest velocity of a particle traversing the counter and producing enough ions to be recorded as a count.It is evident that the counter is essentially a slow iieulion detector* 1>ccause ctd»  It can also be seen that tlie factor C is due practically entirely to the radio-active containination of the inner surface of the counter and that of the boron-layer; since llie numl)er of penetrating heavily ioni/ing particles in the cosmic rays at the sea-level is extremely lo;\, likewise, tlie coniribulion of factor B (due to cosmic ray fast neutrons at seaduvel) is negligible. In fact, the back-ground of such a counter is practically solely determined by(i) the radio-active contamination of the internal surface layer of the counter and Ui) the presence of slow ueulroiis in tlie atmosphere.Owing to the high-value of the half-life period for the vSi)ontaneous fission of the uranium nucleus (as obtained in a separate experiment and described in details in {ho Tuxns. Bose Rcs\ Insl.j (1945) the expected nunil)er of neutrons emitted by one gram of uranium per minute is extremely small in comparison with the back-ground count. The sensitivity of our arvangeineni has, Ihercfovc, been sought to be enhanced by(1) employing a large quantity of TaOtj (coircsi)onding to i.::: I<g. of metallic uranium),(j) surrounding the I'jOH-conlainer with a thick paraflin cylinder in order to slow’ down the neutrons, and(3) increasing tlie efficiency of the slow-ncutron counter by using an opti­mum thickness of the boron layer on tlie inner surface and filling it with Bb\-gas at a pressure of 50 cins. of Pig.!• X P R I M P: N T a h A K R A N C li I\I Iv N 'VThe experimental arrangement is shown diagrammatically in I'ig. i. 'fhe neutron counter consisted of a brass*tube 5 cnis. in diameter and 6^ cnis. inSpontaneous Emission of Neutrons from the Uranium Nucleus 213CYLtfJCL^ile n g th . T h e  tw o  open en d s w ere d o s e d  w itli ebonite discs fitted w ith  am ber  p lu g s  and earth  g u a rd  rin g s. In  ord er to p re ve n t the o ccu rren ce of a n y  p a ra ly sis  of th e  su b th resh o ld  cou n ter (M o n tg o m e ry  and M o n tg o m e ry , 10 4 0 ), du e to .an  electro n  b a c k -g ro u n d  o f h ig h  in te n sity , th e field -stren gth  in  the n eigh b o u rh oo d  o f th e  c e n tra l w ire  w a s  red u ced  b y  re p la cin g  it w ith  a th ick  brass-rod  of abo u t2t 4 5, /). Chatterjee2 mins, diameter. The iiinei surface of the outer brass-tube was lined with a thin layer of amorphous boron powder (^ o .i  mm. thick) so that the range of the cx-particles produced by the 58^ *^  (ii^  cx) ^Li  ^ reaction was somewhat greater than the stopping-power of the layer. J^'he counter tube was filled with BF^ gas at a pressure of 50 cms. of Hg. The surface of the ebonite end-pieces within the counter was protected from the action of BF., gas by a thin layer of paraffin, while only piecein was used foi making the counter gas-tight. A  potential of + 1200 volts was given to the body of the counter, while the inner electrode was comicclcd directly to the grid of the first tube of the proportional amplifier, 'rhe counter was operated in the ionisation chamber region. The pulses were recorded by a tliyratron operated mechanical counter.The UflOn-container consisted of a hollow double-cylinder made of galvanized iron sheet, whose inner diameter was just large enough to accommodate the neutroii-counter tube within i1. Thus we had a layer of UsOtt of about 1 cm, thick surrounding the counter tube. The n\ctal container was earthed. The UaOg was prepared by igniting uranium-nitrate crystals,'Die uranium container was further surrounded by a bigger double-cylinder of galvanised iron sheet containing paraffin of about 6 cms. thickness. ^Ilie two open ends of tlie paraffin cover were also closed with paraffin bricks of suitable size.i‘lie necessary measiirenieiit comprises of the following steps :(1) ne-terninaiiou of the back-ground of the counter, when it is surrounded by the paraffin cover.(2) Determination of the effect of slow neutrons by interposing the inanium container between ihc neutron-counter and the paraffin cover.(3) Dctenniiiation of tlie effect of fast neutrons, any other penetrating ionising particle 01 y-rays emitted by U by keeping the uranium container in si in and removing the paraffin cover.(4) calibration of the over-all efficiency of the neutron couiiler under working conditions by means of a weak standard (Rn-t-Be) source.The calibration of the detector was i>erfornicd by a method similar to that adoi>ted by Booth, Dimning and vSlack (1939) in their investigation 011 the delayed neutron emission from I h'anium after the removal oL.the source of irradiation. For this purpose, the container was replaced by a weak(0,11 me.) Rn + B e  source. In order to maintain the symmetry of the arrange­ments, as far as practicable, the (Rn-fBe) source was contained in a narrow glass lube of alK)Ut the same length as tjiat of the uranium cylinder and inserted into a nariow groove in tlic inner surface of the paraffin cover as shown in Fig. i .  Care was taken to start the standardisation measurenients about 4 hours after admitting radon gas into the glass tube containing Be, when the calculated rise of RaC from the pure emanation attained the maximum values. The effective mimbei of neutrons per see. per me. from the (Rn-f Be) source was taken to be 15,000 as recommended by Dunning id aLr e s u l t sThe experiineutaj results are presented in the following t a b l e •Sponiahecuis Emission of Neutrons from the Uranium Nucleus 2 J3TABUi IICxiitT)Disposition of the expermi^ntal arrangementCounter+paraftiu coverCounter+UaOg cylinder ^+parallin coverCounter+ U3O8 cylinderCounter +  (Rn-Hhe) source (u 11 me.)Counts per minute0.83 ±0.02±O.U,|0.85 ± u.l ?a )6Renun kshack-ground effectCounts of slow neutrons per nn'initr =- h -A -2.74Kliocl of iiciitions finiltrd (lirirll.v )iy lu-HMiiini (Aitlm iit king kIovmm'I down by p.namiii C -A  = o o'Kiliciiiny of the coiititcr I» Determined from the ratio of the aetnal mm,her of counts recorded hv the neutronP - ' i -It Win bfc recognised from a comparative study of (C-A) with (B-A) that uranium emits essentially fast neutrons. When these are slowed down l,y scatteiing in the paralTm cover, they produce an cfTcct wliidi is aiiproximately three times llic back-ground effect. Inddentally, it is also clear that the 7-ray emitted by U has practically no influence on the number of counts per minute. Thai the observed increase in the number of counts is really due to the detection of slow neutrons was further verified liy covering tlic counter with a cadmium foil of 0.5 111.m. thickness, when the number of counts per niiii. i.ractically dropped down to the hack-ground level.Now, since the efficiency of our detector is only 1 in 4S0, the actual miiiilier of neutrons emitted by i.g  Kg of uranium is 1027 per mimilc. Assuming, as before, that U®’ *, the most abundant isotope (99.2%) of uranium, undergoes spontaneous fission, and that a single neutron is emitted jier fission process, the corresponding half-life period comes out to he 3.S x 10' '’ yeans.DISCUSSION OF TIIF RESULTWe have used two different methods to measure the half-life period o f  the products of the spontaneous fission of the uranium nucleus. The first one des­cribed in the Transactions of the Bose Research Institute (1945}, is based upon the actual counting of the frequency of fission by measuring the large impulses produced by them in an ionisation chamber or a proportional comiter. Tlic second melliod described in the present paper counts the number of neutrons emitted per given mass of uranium nucleus. Now, in order to calculate the mean fialf-life of the spontaneously disintegrating nucleus, we have to make some assumption as to \i'hich of the tliree isotopes of U, is undergoing fission.210 S. D. ChaltcrjeeFor the sake of (kfiuitcness, \vc assume that the most abuiulaiit of the isotope, viz., is undergoing fission, Hased on this assumption, it is found thatand— J.3  X 1 0 ' years by method (i) T/.^  =  3.M X Kj ’ years by method (2).We find that there is a discrepancy of the order of 3.4 m the latio of the values (jf the mean life of spunlaneons fission of I ■ nucleus given by the methods ( t l andf j ) .  The second value is ba.sed on the assumption that associated udlh each spontaneous fission oidy (me neutron is emitted. This discrepancy is removed if the assumption is made that on an average about 3.4 neutrons are emitted instead of one. This value is of the same order as that found by Dode' {he. ( it.) for the emission of fast neutrons associated with the induced fission of U nucleus. 'I lie value found will be independent of any particular assump­tion ns to which particular isotope undergoes fission. It is obvious that the absolute value of '1'/ will change if it is found that some other isotope of manium is responsible for the spontaneous fission ; but the ratio of the two IV , which gives the number of neutrons emitted ])er unclear^ fission is independent of any such particular assumption. More precise experiment is m progre.ss.The author is indebted to I’rof. I). M. Bose, Director, Bose Re.search Insti- lute, for his keen interest, heljiful suggestions and di.scussion.I’nvSICU. r^ .MiuKAIOkV,R h s k a k c h  i N . s T i i r i i ' . .Cau'ctta.R p: p' iv r p: n c sAniler.'ion, I '^rlIli .ind llun.stciii, lo.vi, I'liys. Re.g, 66, 7^7.llootli, t>nuning iinil Slack, nj.yj, /'/(v.s K’ lO., 66, (;Si.Cliiilterji'C, S. D. ami 1‘. R Siirkar, u).i4, .Sn’nni- and Caltiin', 9, slin.ClintlcTjee, S. I) , iijpi, .Si/t’ iui’ unit i'liUiin', 10, 263.Cluitterjce, S. IP, 7'miii.s, Hose Rcscaicli histHuti'.Doric' i'( al, i()3g, Rend., 208, 995.ITaeiiny ami RosenPeig, 1939, Coni[>. Rend., 208, S(>S.MonlKoincn' aiul Montgomen, 194", R..S.I., 11, 137..Ssiilard and Zinn, 1939, Phy.\. Res., 66, 799.Von Helban, Jcjliot and Kowarski, 1939, Nature, US, 680.

On the Spontaneous Emission of Neutrons from the Uranium Nucleus

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On the Spontaneous Emission of Neutrons from the Uranium Nucleus

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