Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory
Title Spontaneous Fission
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Author Segre, Emilio
Publication Date 1950-11-22
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- 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. P, ~&er)which was ipited to ThO, and * deposited as a thicd layer using a small mount of collodion binder, The effective
mount was determined by @omparing the faat neutron fission of the ample with
that of a thin layer of Thoa in the same ohamber and source geometry, The thorium
did not undergo my speoial treatment and hence r~mnot radioactively oure. It
is clear, however, that the concentration of tIz015.u f roducts present in the
ample is exceedingly smdl, In each pair of discs used in the large chders
there were approximately 0.25 grams of thorium effective, In the Lo& Almos
experimimtr; 178 fissions in 1202 FELHZhours of observation were oounted. %ia C gives 4.1 x 10'' fissions per per secon4. $ram A Q There is a mall comic ray effect: even Far fast; neutron fiasions in
thorium, but thi8 i.8 practically negllig$ble, As a matter of Pact, we can estimate * that the cosmic ray ef+at will. be lsea than 10"' fissions per gram per second.
Since the thori2m spontaneous firsion is m smnll, the question of its r possible causation by impurities has to be considered, Of these only ordinary
uranium can be of importance and a special experbent was made on this point by
subjecting the thorim nemplee to a slow neutron bombardment. It shuwed an ap~arent slow neutron cross section of less than 104?7 m2, If this were all due to r natural uranium hpuriCy the corresponding qpontansous fission ~ouldstill be less than 0.1 of the effect observed in thorium. ". Maurer and Pose have reported ag~ilemaagclrements on neutron mission b- thorium, attributed te spmtaneous fissj.on. Be shall discuss them fn a subsequant
3-4, Protactinitxu 231
The material used for the samples was obtained from Dr. Agruss.
The samples'xrere eleetrolytlcally dsposited on platinum, adthe amount was 4 determined by alpha cowlking assuming a half life of 3,2 x 10 years.
Two stmoles kers uasdn one of 140 I 10'~grams, which was observed for 1119 hours and gave one fisdons and one of 490 grams, nhi L gave 10 f'isaions in 1129 hours.
From these data we conciude that protaotinim gives 5 x loo3 fissions per gram per second. f
Again, this is probably an upper Fimit. From data on thermal irradiations of this atme material, we haw that it contains less than 2 gereevlt uranium,
The effect of this impurity on the spontan&ue fission rate is negligible,
3-5. Uranium-232 '
This substance is formed by a (d-2n) reaction on Tn-232 followed b,'7 a beta decay. 'Be naterial used was prepared by the Berkeley group, It was empcrrated 6 on a platinum disc, and it had aa alpheactivity of 5 x 319 disintegrations per minute due to U-232, Assuming a half life of 30 yeara for U-232, tne smpla contains 5.3 x lom8 grams of U-232. The samela was dbservad for 950 hours, during which time, three fissions occurred. The sample msalso irradiated with slow neutrons md fission cmmLed in order to find its content of ordinary uranium.
'kis was found to be such that during the time of ~bsermtiononly 0.2 fissions weire to be expected. Is ~piteof this, ia view of the long period of observation during which a
decay constant
3-6. Uranium-233 +
m This substance is f'omed by a (dop) or as (n,'Q ) reaction on rn-232 0% ~everal folhwed by 2 beta emissions. Its half lifs ia 1.63 x year..
samlss were examined in the mall chambers. The material was obtained by (no Y)
on TI? in the Clinton pile and in Chicago by Seaborgfs group. It aras
eleotroplated on platinum loils. It waa observed so as to accmulai;e 1.35 gram
hours during which one fission occurred. This fissim may: be explained b~rthe
U-238 oontent of the sample. This resilt gives axeeoay constant smaller than
2 x loo4 fissions per gram per second. (lg)
3-7. Uranim-234 .
A sample of TJ-234 was bbtained on load from Dr. Latimer in aerkeley. The
material ma prepspd by extraction of UXI frnn irnimn sad subsequent decay of
this sub&ance.
It contains a little over grams sffectiw of U-234 as measured by its
alpha-activity. Re odssrved it for 3300 hours without observing any rpontaneoua
f is siona, from which we conclude that the spontgtneous decay. constant is smaller
t than 9 x 10~' fissions per gram per second.
These isibtopes, which occur innatural uranium, co~~ldnotbc -
qum3i tatively, from each other and the observations were always performed 'a I on mix-bures containing all three of the natural uranium isotopas. However, the
cmpositiona of the mixtures could be changed by using matsrials em-iched by the (13 elsctromagnetic method. 1
In these ex-perinents largv chambers of the second type. were used,
If we call Ax, hy: br, the snontaneous fission decay comtants of
If-238, ti-235, U-234 in fissfons oer gram per second, we find the countiq rate
"5 in a ginen cmpls is8 ..here xi yg zi are the 81-ams of 17-238, 0-235, U-234 in that sanple. Practically.
%he tern zi tms out always to be negligible compared with the other tm
because, as stated above, is small and also ze is gonera'lly mall. By 2 obseming the counting rate in samples of different born isotopic composition,
we can solve tho equattons for and > X We ahall now describe in detail an example of these moasursmsnttz,
In this run three samples were used; 'me af ordinary umim and two of
enriched material. The lsotopic cmpositioa of these nztsriala is U-238% U-2858
U-234,= 141 t 1 t O,CK)?25 in mass and U-238: U-2358 U-234 = 0.334~ 1: 0.00583
in mass, respeci;ively.
I3e isotopic analysis was checked for the enriched material 3y Baas spectrograph
and by the Berkelay mekhod of analysis, (12)
The sanples were eleetropbted on platinum discs 13 centi:il.eters in d
and 0.01 centimeters thlok and ignited to UaOem The totdl mass of uranium was
determined by direct weighing; and the mass of 25, by measuring the fissions
occurring in a slow neutron flux. From these measurements we find Ulat the
ordinary uranium sample conLains (in two plates) 38.50 milligrams of' U and the
two enriched samples used contain 42.95 milligrams and 36,613 milligrams of
enriched material, respectively.
The rziormal sample gave 31.0 fissions in 381 bours, 'The enriched sam~lesgs=8e
101 fissions in 396 hours and 133 fissions in 558 hours. These raw data have
d to be corrected for the eficisncy of tlw cham'5er. This is done by taking a curve
of tho fission rate with a conktant neutron source versus Sias oPthe mplifisr
, and axtrapolating to zaro bias and then correcting further this result to take r into account, theoretically, that some ffssian fmgnents carmot egcaoe from the
lager because of the fhite thickness of the same, The firs% correction is 6
percent, the second is 3 percent. FJith these .corrections we find, s.gm, that 1 310 fissions normal material gives 13a39 hr 23.2 or 6.43 x 100~fissions per gram per
Introducing aeae numbershtb equation (1) and salving we obtain Ax 6.48
'fie errors its these vxJues come frm statistical error in counting for
8 which rn use the sTuare root of the, number of coun-kG error in the absolute mas 5 of the foil (2 percent) and in the counting sfficioncy (2 pertent).' and error * in the isotopic conpositi& (2 percent in the ratio of 25 to 281,
Calling R the ratio of U-238 to 0-235 in the enriched sample and wing
standard formulae of the theory of arron, one finds, naghating some saall tsrms:
In nfiich ml is the effective mass of the m&tple of normal material, a, is the
1 effective mass of the sample of enriched material, and cl and q, their counting .* '1 rates in counts per hour,
C In the run we srs considering we had:
m, 0,3350 g A ml * O.GOlO
% = 0..0350 g A m, rn 0.M)lO cx - 0.814 0h.r A a, - o,oas a, 0,240 chr A eZ 0~016
R 0.334 4 R O.X7 -10-
Bit21 these data we obtain, s~bstftutil?~:in (2) and (3),
and similarly for
The errors haw been expressed in fissions per gram per tiour, Expreesssd in more conventional units the results become:
Several series of measurements were raiade and the results are summarized in the Table 3-7
Dec. t& and Jan. '45 4.48 0.U Oe4C 0.23 8
Feb, - Xarch r45 7.55 a.42. a G.38 0.23
lo the find result we have to make anoWter correction to take into account a
residual. Pffect of cosmic rays on the apparent spontaneous fission of g235, and
our present best figures are
+ 10-3 f/g 'h = (6.90 - 0.24) x see
It would be possible to improve these nea'strrements far 8-235 using almost .ure
U-235 for the samples,
3-8 Uranium 236
This isatope of uranium is formed by an (n,~) reaction or?. U-235, The isototic
composition of the saqAs was determined by mass spectrograph(U) ad ,so by
irradiation data.( 14)
The samples were klated on platinum discs as were the other uranium isotopes
and were observed in the large chambers. The s~ontaneousfgssion attributable to
$36 was -. 3-9 He~tvniwn237
This isotope is formed by beta decajr of the 7-day TJ-237 which in turn is
obtained Qy an n-2n reaction bn 11-238. Its half life is 2.2 r lo6 years.
The seqles investigated were re pa ma in the pile and were suphlied to Los t Alznos by the Letallurgical Laboratory in Chicago. 5 An early investigation with a sample of S'x 10- grams grotracted so as to
accumulate 127 x 3.0~~gram hours of obsamatian gave only 1 fiusicu. grams Later with.3 stronger sam~les(about 8 x loU4/:/each) we accumulated 1,113
gram hours of observation with 6 fisoions recorded. This rauld give 1.4 x 10'3 fissions i;er gram per second.
The effect of a pdssible small contamination of plutonim or urenim in
the seinple is negligible as tested by slw neutron irradiation; however, it is
better to consider 1.4 z low3 fissions per gram per second rather as an upper
limit beaanse it is difficult to be absolutely sure of the genuineness of the
few fission pulses observed over U82 hours of counting,
4 3-10 .Neptunium 232 .. A sqle of this material was prepared by J. -kisksi from depleted uranium
irradiated en the water,boibr. The aa&le was purified from fission products,
uranium, and plutoniw and depiosited on a platinum disc by evaporation, Its
mass was determined' by the growth of tfie alpha activitr of plutonium 239 in an
aliquot. Apgrnximetev 0.45 x lom6 grams mere present initially and the sample
was observed for 146 hours dtming whi&.time no spontaneous fissions occurred.
Taking into account the decay of the Wp-239, we find that the gram houps aP
obeervatiozl arere approximately 2.5 x 10-5, From this we conclude t'nat I1 fissions per gram $er second is the upper limit for the spontaneous fission of Np-239,(15) 3-11 Plutbaiuui 238 C Tkis substance is prepared try' a (d,2n) reaction on U-238 (I6) and it has a half life of about 60 years.
The sample used was kindly sup~liedby G. T, Seaborg, and had been acci-
dentally contaminated with h-239 in such-a way that the ratio af the alpha
activity of fi-z-5238 Lo that of fu-239 was O,@5. This was Getermined with differen-
tial alpha rage app=atus.
me Fu-238 was mounted by evaporation on platinum discs and the effective amount &resent was measured by observing the slo~neutron fission of the con- tamintnt plutonium 239, and using the ratio of the al$a activities quoted above, -13- From &is last number
Hence the effective amount of Fu-238 present ia 0,0022 times the effective amount
of Fu-239 present,
In each of our three snm$as we had a~mxht- paw of Iu-239 and
2 r 16' paw of 1x1-238 effective. The sqlas were abserpgd for $30 hours
total, ccrrespmding to le.9 z loe6 gram hours for Xu-238 md 3.6 x 1m3 gram
hours for Fn-239. L!& fissions were oozzz~bd,
From *is we deduce a spontaneous fission decay constant of 2.1 x ld U fissions per gront 2ar second, The possible ~mtributio~fof Q~~IES~Pu isoeops to
spontaneous fissi~nis negligible, being at the mat 02 the order of I ger cent
3-I2 E 239 and..Pu 240 .--- Plutonbm 239 was invewtigabd for sgantaneous fissian soon &er its discovery and no fissions were deteeted during about 5 x grm hours of
became ~lvailableat successive dates,
The samples were deposited on glatinu~discs by evaporation, electro-
painting.
thin standad, in a eonstat slow neufirm flux, The thin standard was in turn alpha eounted. A half life for Fu-239 of 24,300 gears was used,
haterial. produced in %he ~erkelegqclotroa' gave on crbsemat ions extending over about 10,000 hours, 3.2 fissions, a& this correqmnds to G.010 fissions
fission of hG0waa dso measured, deduced fmm the alpha activity assuming a half life of 40 pears. The samples wers obsemd for a total of 2700 hours corresponding to 1,g x 10'5 gram hours, Three fissions vere registered,
For reasons stated several times we consider the resulting nwber, 46
fissions per gram per second, an upksr limit for the spontenew~fission conatsplt
of this material.
-The following table summarizes a11 the data accunmLated uk to the present
pl tine on sponfansaus fission deca~constutts, .,
Ia coluhnn 1 the cha&ad symbol af the element is given; fa colarim 2, its
P atomic wass A; in mlum 3, th.e total number of fissions observed in a31 c. sm~les;in colum 4, the total number of hours over which the obsemationa
have extended. This infohation is impartant because it is clear that the
possibility of qurious fissions is pmprtional to the duration of the obererva-
tions. Oolumn 5 gives the gram bows of obsemation su-d &wr all sanples.
Column 6 gives the sgcut~n~ousfission Constant h, in fissions per g~8mger I second, whenwer it is known, with its probable error,
The expression 4t means that if one fission had been observed instead of
none, that would be the calculated spontaeaus fission constant; Chis mans that
one has the probability 14~that the sponttmeous fzssion deaay cona.tant is lager
than L. Bore generally it caz be shorn that if Lhem bave been no spont;aneous
fis~iansin a time t the probability that the decw oaastant is smeller than l/r ia it/;* -1.5- UGRL-1021
Somtims if is Gonvenisnt to use the nhnlf life for spontaneous fissionR, i.e. the half life of a nuclear species that 'i"1ou1d obtain if the only decay possibilitg were spontaneous fission. This quantity is oorrnected nith the fissions Z1 per gran per second by the relation: e
* where 'P is given in years, ,J\ in fissions $er gram per soomd, and 2, is the
The tr~bdilitg, that a given aton: undergoes spontaneous fission in one second is
Table 3-34
FZ saons Hours af G & of A in Elenelat Obaamd Observation Obwmat ion f/g ssc
not observed (6.90 + 0.24) x A most intesestisg que'stion is tc find the number V of neutrons emitted, on the average, per fission. .This number is well horn for slow ;neutron inauced
fission in U-235 and Pu-239 and it is clearly desirable to lmoa it also for spn-
tarmms fission. As a matter of fact the detm of priw Sntereat to the project
was not so moh Lhe qmntaneaus fission decw constant, as &fie number of neutrons
c3pontsnecasly a~3.ttodper p5t time, her unit by the substace studled.
Once ~1 pile qith an appreciable aultipLication %as set u~,it was observed
that even with a11 sources removed, the neutron density kt the pile was quite
e.pfreciab2.a. This density was -attsibn%ed"E We s~o23.trneousemission of neutrons
by -Wle urani-am and by ~easurj;ngi-b with indium detectors and by comparing it with
the density gsrxtuced by souroes emitting a known ~UmBer of neutrons, it was gossi-
b ble to deternine the number of neutrons emittad spontaneously lqr uraniufi:, In this I.
The spontaneous1;. emitted neutrons were ~BOdeteated by G. 3charfS Gold-
haber rrnd G. S. Kfaiber, These authors obtained bout 1.75 x loo2 neutrons
per gram per second of an. energg &ave 800 Key.
0 0. Pose ) measured the neutron emission of ordinaq uranium and of
thorium. Ria results have to be corrected because be etss~~1esthat I rnillicurie
Rn + Be e&%s 15,060 neutrons per secoad. If me uses the figure 11,000 which
is the best estimate available, based upon 13,WC neutrons per second for 1
aillinrria Ra + Be, one obtains for uranium an emission of 1.54 r lom2 neutrons I per gram pr second and for thorium 0.24 x neutrons her gram per seoon6. The
latter figme is in alf probability i~ error because combined ~iththe knom -L?- UCIIL-1023
spontaneous fission constant of thorium, it would give V (Th) * 5.7, which seems
higher tha is likeLgr.
~oasonf~)has also measured the neutmn emission from a uranium sphere with a long bomn counter and found (1.o 2 0.1) x loe2 neutrons par gram per second.
Other memurements made s.-b tha Clinton pile gave a value of 1,5 x low2 and
think tlat the >resent best value for the spontaneous neutron emission of urmim
is 1.5 x neutrons per gram per second. It is believed that this value is
accurate to about 10 per cent not including psszbfe errors in the calibration
or" primary neutron standards. Since the calibration of such standards, howaver,
enters in ~racticallyall neutron aeasurments in the sane m.7, errors in the
calibrstion cancel in relative masurements,
Fmn! this value of the neutron emission and from the spontaneous fission
eonskant given pevi~usly,we find V (28) = 2.2 2 O.3.
The high si;ontaneous fission rats of I;\1-2$0 makes possible the investigation
of the euergy spectrum of fission fragmmls for spontaneous fission.
Azr experiment to this effect was ,performed by ~eg&and Niegand. Figure 10 shows a schematic drawing of the ahmber used. The chamber was filled with
argon and electrons were collec'ked,' The electrons collected gave to the grid of
the preamplifier a pulse proportional to the ioaizztion, because of the gresence of the screen gri& The srsf~lplewas at -1700 v cKld the screen grid at -900 v aith
respect to the collecting electrode. The positive ions did not contribute to the pulse beaause tine decw time @onstant (5 lnricroszccndsj of the amplifier
short, Tfie tine af collection of the electrons was about ~'Hlicroseccndend the
time of rise of the amplifier was 0.2 dcrosecond, The pulse Bas passed froro the linear amplifier to oscilloscope and recorded pbotograb"aicdly.
d histogram of the kuises reooxdsd is given in Figure 13.. For comparison
%e rscordeu also the guises pvluced on the sanw simple, in the saxe cpi;aratus, It is dear that tae t~whistograms are veqi siailar, the one of spon-
tarteouo fission pulses being slightly shifted towards lower energies,
%is experiment is &ateresting because one m6y suspect Ulat in tihe spsn- 5 tmerjus fission of fu-240, which- is certaialy a rare grocess, only veq few possibilities of fragmensntoCion exist. The slow neutron fission of Fu-239 gives
m exciM h;r-ZL+O with a masl lifs for fission estimated to ba df the order of
mwmds, a huge factor ahortor than that of the fjndamentd state. In
qite a% *is, one does not see a very great change in the nodes of fragmentation.
It is perhaps possible that 43nis nay be due to some re-shuffling of the
nnclsar roatter occurring aftar-the barrier for fission has been passed, but before
the two fragments coma co~pleteJ$apart,
experiment could be improved br cher;;ical investigation of the ;.ields of the various fission chains in spantpneous Pission of Fu-~@ and comparison with
the yields of the same chains in slow neutron fission of Fu-239.
6. TEEQi?3! OF SPOMTj&EGUS FX&310N b
Swerel attenrpts h& been made to explain spontaneovs fission by a mechanism sirnile~~.~tothat invoked bg Garnow in t&e tl~eoqyof alpha disintegration.
Buhr and Yheeler (23) in Weir fundamental paper on fission give a calculation of We s~ont~ecusfission yrobstbility assuming that a nucleus comes about I 1s times pjer second ilito the optima configuration for fisaion and that the transgareney of the bisrrisre is given by
in which Ef is the ~hotofission threshold for the nueleus in question, E: the nass of the nucleus, and ct a le&th of the order of magnitude of the nuclear read ias . t They em;hasize, however, titst this estimate can give only the order of ongnltude of the transparency, i.e. of the expkent. in the eqresaion of the 1 lifetl%,
Atteqts have besnOmade'b S. and L. ~u~er(ato make more precise evaluations of the spmtaneoua fiasicn constant using the fcrmla of Eohr end &eeTer and an expression for Ef given Zjg the same authors; but they have not been suecossfnl, as shorn by the folloning table in ski& we report the spntaneous fission probabilities in sedl calculated by these authors, and the experimental
In the case of the rmrabers given by Turner the probabilities have been so normalized as to give the correct value for U-238.
ess of the expression.(4) g?lven above foi!' a precise oalculation of the spontaneous fission constant is borne out even more by the experirnentd. values for the photo fission threshold reparted by Back, ii,cZllinney, ~astei~er(~')and 4 given in column 4, of the pame table.
How much the nuclear s~inpresent in the odd isotopes aay affect the trans- C parevlcy of the barrier is an open question.
An attempt to measure directly the trmspwency of the fisjion barrier, a little belaw the top, has been made along the following lines: 14~237and Ea-231 have a mtron fission threshold of about 400 Bev. A slow neutron capture hence fureites the compouncT nuclms to about 400 Kev below the threshold. k~elirxlinary experiments try C;. Farwell and E. Eahn give 0.1 barns as an upper limit for the fission cross section at thema1 energy for both substances, Photo Fission -Turner Threshold
=- 1 the ratio of the fission cross sectiw to the capture ?/ .; a ,
cross section, can be expressed as 1
in which ) is the number' of times per second in which the nuoleus comes into a configuration most favorabae tc fission, ,J is the tranqirency of the barrier, and Tr is the probability per unit time thak the coml;ound nucleus lose its excitation by gawa say emission.
For 810% neutron fissioners like U-235 ur Eu-239 we kno~that l/a 2s a fe.crr units. On the other. hand for these nuclei 1 is suppas& to be abaut one.
He conclude from tkis that -'/ \T is cf the order of a few units, say 5.
For N1--237, since the capture cross section for themal neutrons is about
100 barns, we fi~uthat l/a is at least 1000, hence where ae have
actinium, with a capture cross sectirr; of' abaut 300 bans, gfves a similar result.
It mst be remembered that -this value of the tran~pereot=;~is ueT crude and pobezbb represents upper Xinit because all, erperirsentd. errors ir, "che deterlilimtion of the sLoa neutron fission cross sections of Kg-237 and Ta-231 tend to make them apFear too large, Libby, R. E. , khys. Rev. jJ, 1269 (1939) Fetrzhdc, K. A, and Flerov, G,N., C,R, dcad. Sci, U.S,S.R, 28, 500 (3.940); Journal General Ihysies 2, 275 f 1949)
Wost of the samples were prepared by D, EuSford, Xav klUerd J. 3ske1, and 3. Potter,
Linde Co., incandescent hip grade. l;WTity of this argon is 99.5 per cent or better, the reminder being nitrogen, Special precautions are taken to remove all oqgen,
:Jest, D. , BU-1028, also rcports observations of the spnta~leousf f ssion of U-233 which are, however, less grotracted than ours,
Ttle first work of this type was done by J, 3. Kennedy and E. %grb, 4 159. Ses klso 0. Chamberlain, C. Farwell, E. %gr%, LA-86; and II, Ckmbsrlah, LA- 517.
Kennedy, J. E,, and seg&, E,, UC%-9, * Hilllams , Q, , and Yuster , k ., WLS-286 and LA- 512 Group R-4, wS-256
See e.g,, E. Fermi, €2-26.
if. Scharff Coldhaber and C. S. Elaiber. CE-3.463, REFERERCES ( Continued)
(23) N. Bohr and A. Jheeler, Ws, Rev. $6, 426, (1939).
(24) S. Flkge, ZSf. Ihrs. 121, 294 (1943).
(25) L. Riiwer, Kev. Lod. ihys. &7, 292 (1945).
(26) H. 1. Kcck, 2. LWllinnsy, E. L. Gasteiger, 6F-3292. 7E.7". k---- 6 .O CM. -1
Figure 1 Argon Chamber
1. Brass Cones 2. kounting Sample 3. Sam,-le Holder and Iilgl? Tension Electrode 4. Collecting Electrode ' 5. Guard Ring 6. Polystyrene Insuletlng Supports 7. Rubber Gaskets 8. Threaded Collar Fastens Chamber to flniplifier Chassis -9. High 'Eensian Lead (platinum Glass Seal ?axed in lace) 10. Grid Lead 11. Gas Outlet UCRL- 1021
Figure 2 Double Desk Nitrogen Chamber A. Collecting Electrode B. Brass Cover C. Saqle Folders and High Tension Electrodes D. hounted Sali~ples E. Brass Supports F. Brass Base Plate G. Grid Lead H. Threaded Collar Fastens Chamber to Amplifier Chassis I, High Tension Lead (?latinurn Glass See1 -'axed in place) J. Gas Outlet' I<. Rubber Hose and Clamp L. Polystyrene Insulating Sup~~orts 1,. Rubber Gasket Figure 3
6~~5 6AL5 BOTTOM OF SOCKET MU 1006
NUMBER OF COUNTS (N) MU I007 I c. Figure 7 Time distribution of Uranium Spontaneous Fissions BIAS (MILLIVOLTS) i MU 1008
Figure 8 Fission Bias Curve NEQATIVE OUTPUT
0 VOLT STORABL 0-1 M A c OUTPUT AND OUTPUT BATTERY ewe CABLE ADJUSTED TO OWE I MV PER OHM WHEN CATHODE CURRENT tS 4 MA MU 100s
MEV MU 1011
Energy of Fission Fragments