Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory Title Spontaneous Fission Permalink https://escholarship.org/uc/item/8v41m1pb Author Segre, Emilio Publication Date 1950-11-22 eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Division, Ext. 5545 BERKELEY, CALIFORNIA UNIVERSITY OF CALIFORNIA Radiation Laboratory Cover Sheet INDEX NO. UC"~,!A-\O'L\ Do not remove This document contains 24 This is copy of Series ued to a.d Each person who receives this document must sign the cover sheet in the space below Route to Noted by Date Route to Noted by Date - Department of Physi cs, Radiation Laboratory University of California, Berkeley, California il 1. iii storical The first attempt to discover spontaneous fission in urmium'was made by Libby, 0 who, however, failed to detect it on acoomt of the smallness of effect. In 1940. Petrshak end Flero~.(~)using more sensitive methods, discavered' spontaneous fission in uranium and gave some rough estimates of the spontaneoue fission decay constant; of this substance, Subsequently, extensive experimental-work on the subject has been performed by ssvzral investigators md will be quoted in the various seotions, Bohr and i~heelar'~'have given a theory of the effect based on the usual ideas of penetration of potential barriers, On this project spontaneous fission has been studied for the past several years in an effort to obtain a complete picture of the enon, For this purpose the spontmeoua fission decrty constants h have been measured for separwlted isotopes of the heavy elements wherever possible. Eiloreover, the number u of neutrons emitted per fi asion has been measured wherever feasible, and other characteristics of the s~ontaneousfission precess have besn studied, 'Phis report summarizes the spontaneous fission work done at beFJamos up to January 1. 1916. A chronological record of the mrk is contained in the Los Alamos monthly 1 reports. (41 # 2, Experimental Techniques * The experiments directed to the measurement of consisted in principle of 4 -----------putting a certain amount of the material to be inmatigated in ionization ohambers 1 * This is a partial reproduction of a re~ortwritten in Los Uamos in 1945, / connected to linear amplifiers, and oounting the fission pulses. The material (5) was deposited on platinum discs as a thin layer. In all these experlm~ntsme of' the main difficulties is offered by the alpha activity or" the smmles. As a maCter of fact, this often lhig amount of a substance that can be studied at one time in an ionization chamber. The reason for this is that the fissicns are recopfzed from the large size pulses that they give in the ionfzation chambers. Now the nulsea generated by single alphas are from 'LO to 20 times smaller: however, if the alpha emission is very strong, fluctuations in the alpha activity background may simulate large pulses and cause spurious fission counts to be recorded, Qualitatively, these fluctuations will be roughly proportional to the gquare root of the number of alphas emitted during %he "rresolving time" of the apparatus. Attempts have beex made to obtain a more qua& picture of *bw ? effect by detrsl.sping a suitable theory, but the phenomen&tha-t give rise to snurious pulses are too complex to be analyzed in a really satisfactory my and we shall limit ourselves to the statement above and to some experimental results to be given later, It is clear, however, that it is d.esirable to have a high resolving powsr in the apparatus. me limitations to this may come frm the collection time of electrons in the chamber and from the frequency response or rise time of the amplifier. For the chamber, electron collection with its high velocity (6 1 is inperative. The chambers were filled with tank argon, and special pre- A 1 cautions had to be taken to avoid the presence of traces or" organic vanors with their poisoning effects on the electron collaotion. Two models of chmbers were used. 'hey are drawn in Figures 1 and 2. The large chambers (Figure 2) were used for material of low specific activity (U-235, U-738, Th) for which it is posslible to use many milligrams of a substanas withouk troubles due to the alpha activity, This requires large surfaces for the samples in order to preserve -their thinness. The small chambers were used for the more active substances (Figure 1). The eunplifiers used aust have high resolving power snd good stability in . They must be absolutaly ;'reg from disturbaaces such as high tansion sparks, surges in the power supplies, atcg for this reason we have used battery operated units shisldac? in large metal boxes. The wirkng diagram of one of L%ese amplifiers is given in Figure 3. Its layout can bs seen. in Figure 4. The pulses oft the amplifier wBre 'registered on aur inpulse meter md could also be fad through a pulse lengthener to an ~starlixm-%lgusrecorder (Figure ti), I The recording affords a usexu1 check on the bshaviour of the anoaratus and was I made 3eriodically on all units. Figure .6 shows a pic-tilre of ans of the, comple%s units, including the amplifier, ionization chamber, B batteries, high tnnsion supply, md Esterlina- Angus, T'no chamber an the right is covered by a sheet netal can containing B20a for cosmic ray neutron shislding purposes, The Sstarline-Angus recording was also used to check that the Pission pulses obey the Poisson distribution law. EIow~wellthis occurs is shown in b Figure 7 *ere we have reported the di stribution of 141 uranium fissions . wocial precmtionskave to be taken also to shield substances that undergo neutron fission from rteutrms due to cosmi.c raye. IIott- important this ei'feot may be, is shown by the following numbers concerning U-235, ~t'sea level (~erkele~) this substance, observed in a wooden building, showed (l0.3f 3 -6 )XXO'~ fissions per gram per second. Xear Los Alamos (1900 meters above sea level), in a light wood building, we found (15.522~2)x10'~ fissiom par gram per second, but shielding with 1.3 grams per square centimeter of cadmium reduced the counting rate to (l,lf@,d)xl~~3fissians per gram per second. Starting from this last datum, it is possible to plan adequate boron shielding, III the Los Alamos experinients, a shield of 17,gS 7.4 grams per square eent;imetsr thick was used. This means 2-7 grams per square centimeter of boron, which should eut dorm all the effecf s due .to slow neutrons. originating in cosmic rays, by 5 . -4- uca-1021 more than a factor af I@. It may be added that such a shield, cuts off substantially all neutrons below 200 electron volts of energy, In order to determine the spontaneous fission decay constat for the variouer su5stances, it is esserttial to know the amount &X'ectciwly counted in each sample. This is done for aoat substaces by suhjeo%iag the chamber containing the sample to be calibrated to a constant neu-tron flux produced. by a Ra+Be source and counting the fissions obtained, Wthout changing the sourcs or the gerrroetry, we then replace the sample to be calibrated with a standard sample containing a known aiount of substace: and deposited in such a wZY as to be sure that it is thin for fission fragments. The mount of' substance in the standard multiplisc! by the ratio of the fissibn rates of the .unknown to the standard t'len gives the effercltive amount contained in the sample, After this calibration a curve giving 1. the observed fission rats .versus the gain of the amplifier be taken in order to estimate the size of the errara %hat may be introduced by small gain changete. Fi ere 8 shows one of these plateau ourves, hring operation the gain of each unit was checked every one or kivo days wiich a pulse generator (wiring diagram Figure 9). Also, for long periods polonium samples having alpha aotivities larger than the samples investigated, were substituted for the latter in the ionization chambers in order to check that no spurious counts would be registered. 'ghe samples were also periodically rotated among the units available, the Puelei 4 3. Spontanecus Fission of Yar$ous The single substmces invr-tstiga.tad will now be discussed. This substance was investigated by D. ~est,"l'. He found an upper liztit of 0.6 fissions per gram per seoond for its spontaneous fission decay constant. 4 A sample of ionium extracted by k, Fontana @om uranium ores, was kindly put at our disposal by Dr. Bamilton. In thiacsamle the &/1o ra?;io is 3.4. The -5- 'Jca-ic21 plates were prepared as thin layers by evaporo;tion on platinum discs and the mount af ioni& was calculated from the alpha actipiity arssuing a half life 4 of ionium of 8.3 x LO years. ?he plates had a diameter of 4 eentirneter~and - contained approximately l' milligram of ioniraa each. Tnsy were observed for about I 1300 hours total, corresponding to 1-45 gram h&s of observetion. Two fissions r, occurred in %hat timeg however, they may we12 be 6ue' to the thorium in the Indeed, one muld expect (see belaw) in 5 grm hours of observation oil - sample, We conclude that 3.8 x 1ga4 ff srtions per gram per sebond is the upper limit for the sponfuaneous flssian of ionim, The large chmbers were used fer the investigstion of' this substance, 'rhe % material used was thor nitrate (C.