Fission and radiative capture cross section of 25 for thermal neutrons

APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE UNCLASSIFIED , , LEASABLE FSS-16 Date: - CIC-14 Date:B / \ b b PU6UCLYRELG4S%JQ 1A REPORT 100 ‘~ Juric3 270 1.9? This document \ I FISSION AND RADIATIVE CAPTURE CROSS SECTION 0)?25 FOR THWIMAL NEUTRONS W’o.w DONE !3Yg REPOm WRITTEN 13Ys“ & * p- , / /“ p J+ , INCLASSIFIED APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE 2- ABSTRACT ● The fission cross ~ection of 25 has been compared with the activation crose seotiorisof AU19’7and Mn%e Tho deteotion of these radioactive moxi- itors was calibrated by the method of coiricide~e countiwzo The re~~lts ares i?romthese From this and the almorption values s * x 10”* cm29 da(25) = 645x 20”4cm2 and ~(f@) the i-atioof competing radiative capture to fission appears to be (dJ)M = 0.23 *0008 APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE —— IED —.— — + FISSION AIID RAJ)MTIVJZO@TURR CROSS SEC’JWXJOF 25 FOR TBHWL —— I?EVTRC)NS JWTRODUCTZON Precise maaurewents of the total cross motion of 25 f’or removing thermal neutrons from a beam have been made by Fermi 1, and byycDaniei et al 2) In the past it has been generally assumed that the only process available for removal was the [nJf) p~iCSrM30 DoubtB wore cast on this assumption by a) the sharp fission resonances observed by McDaniek 2, which indioate that tha oompound nUC~eU8 26* has a sufficiently long life to permit appreciable competition by (n8$); orom b)relative mea~urments by Fawelland othoes3) of.o%i~tion sections for neutron-induced radioaotivities oompared with (of)25~ which seemed to indicate consistently higher results than obtained by other methodse when the known absorption cross seotion of 25 was uaed~ It wm pointed out #a% thesd difficulties would be removed if one could demonstrate ‘&heexistence of a competing processO such ES (nO~) with a probability comparable with that of (nof)~ The ratio of these probabilities shall be called cyj The purpose of this investigation is to elucidate this point hymea6- Fermi, CE=1389 Anderson* Lavatelli, McDaniel and Sutton, Ji-9l0 LAMS-48 o “~~:” —— — .— — — UNCLASSIFIED APPROVED FOR PUBLIC RELEASE t ’ APPROVED FOR PUBLIC RELEASE “4-uring uf(25)/9’(Au) and oomparing it with the value of ~a(25)/&a(Au) obtain ed from transmission measurcmenta l!!lliminating &r(Au), which is of course assumed equal to one obtains I+tis ” (1) ii?the number of induoed proce6sea ob~erved per unit time, is the efficieno~ cf the instrument used for observing the process P, ie the cross section of a detector atom for this prccessj iB the number of atoms of deteotor material This equation may be solved for ~D & if the neutron flux nv ia known Xn or- der to put suoh measurements on an absolute baais ww must have at least one tab= solute standard for which &p is known from independent sneasuroments~ These latter usually consist in transmiaalon experiments, which determine the total crom section for removal of a neutron, o~ If there are processes P’ APPROVED FOR PUBLIC RELEASE alter- + ~!!!!mm! — _ -._ IJNCIASSIFIED I : APPROVED FOR PUBLIC RELEASE~ ~ -5’= is a mixture 6f i~otopw~ P P “ may, of course occu3:in a different isotope than Applying Eq (1) both to a standard A B~ whioh has no cGin- and an unknown (2) A ~uitable standard deteator must permit accurate determtiation of all relevant quantities in Eq (2) This mmns a) da >> ~ ~$ beiw the scattering oroas section, to permit aocurate transmission mea8urements0 AJao / da &]ould vary in a simplewaywith preferably as I/v b) & experiments ~) PA neutron velocity iu the t~ermal region, should be negligibly small or well known from other should be of suoh a nature that 6A cm be detamind acouratelye d) The quantity of deteotor material used must be suoh that it can be accurately wei@ed or otherwise determined — The process most commonly used as absolute stcmkrd is satisfies condition a) above very well, andp whqn used M F310(nS*) It BF~ gaa, also conditions u) and d) When it has to be used as foils, however, c) and d) require considerable care 4), Condition b) is generally assumed to be eatisfied, although ra=- diza%ivecflptx;re by either 13g or BIO oannot be entirely exoluded as a remote possibility A similar situation exists with respest to In many rcwpeots the process 25 {nDf) Li (nap) seems to have desirable prop== erties as absolute cross seotion standard, beoause of tie relative ease of experimental procedure and beoause it lends itself readily to measurements at higher energiese A nuniberof activation cross seotions have been zneasuredby comparison with 250 H~ever, when the value (~~)~ = 6k5, found fromtranemicmionex- 4) Bailey, Blair and Russell, IA=$JO - — APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE I -6perknts 1), 2) wu used for ~fs the resultant values af a number of oross ;eotiom for ~nduced radioaativitiea were consi.uten%~yhigher than %hose found by other mthod6 3) o ~t W88 pOhtOd out by Far?tell and Segr#) be duo to ano~er proaes8 aompetl~ with the fission shoe been conpared with B (n, ii) by Bailey et al by ~~j,~ey$ inhmion, thllratio being It ~ dihOHi 1.21 (Fend J+) f~lld df md with -t ‘&on *is tit$t (&f)= has Li (n,p) by Ptmnl SMall*r t~n da f- b tr~s” ) and 101.6(Bailey) respeotivelyo sewmd desirable to measure &f by compari~ with a oaptureprocese leading’to a radioactive isotope, both beaause the possible diffioultiea of the’ other methods could be checked by an independent method and because the great convenience of the use of radioactive monitors led UB to hopo that the method might lend itself to absolute masurements~ Induced Radioaotivities an Absolute Standards ‘I’he neutran tnduaed activation of materials such as Xa, ~ and *b has been used for a long time for the relative rnea6urementof thermal neutron flux In order to extend the method to absolute meamrments able substances satisfying conditions a) to d) of Seotion 1, we must find suit!theeecondition limit us to substances having only one isotope (condition b), h large cross f3txtion (a) and for which the induced radioactivity shows a fairly simple and well understood disintegration soheme (c) and no isamric states (b) Unfortunately iridium,which has a very convenient cross section aqdhalf conditions b) and c) In fact the ody Zife ia ruled out by f3ub6tancesatisfying all conditions and Iv ~ section ie, at present not sufficiently well , whose oross section has been measured accurately by transmission seem to be Au We have thereforo used gold aa our absolute standard A fewmem.u%menta also made using l!n55,but ita crom APPROVED FOR PUBLIC RELEASE were APPROVED FOR PUBLIC RELEASE — — 0=4 known to @old &8 desired accuraoy~ If a substanoe yielding a radioactive product with decay oonatan~ is exposed to neutrons for a timo Y i8 n(t) ~ and the rate of production of actiiveatom then the rate of decay at the end of’bomlmrdment is T 111 = I h (m) f0 which may be solved for no if n itsconstant 11- whioh becomes, for very long exposure n s nQo m= This means that any error in T>}l/~ For very short exposuros, lt j.s (n’J\~c~l.~>T)”l A (x) enters direotly into nA Eq (?) unless which is impractical in the case reumlts and of a number Qf m~~uraents of x 10 -4 min“1 In the case of Kn5G we used -1 * confirmed by many experimenters The capture orosa esction of ~old haa been measuriedwi’khvelocity — selectors by Fermi et al 6) and by MoDanJel 7)0 The latter author also showed \ that it obeys tho I/v law in the thermal region .Thovalues for ity of 202 x 105 c~seo a:e 93 (Fermi) and ~ (McDaniel) x 20“a neutron veloc- cm2 tiehavo “ 6) Fermi andh!arshall, J., CP.1255 , 7) tideraonn tivatelli, McDaniel and Sutton, LA09~ — ,.— APPROVED FOR PUBLIC RELEASE — APPROVED FOR PUBLIC RELEASE — -8=9 used the value (~ & 2) x 10-a =P20 In the case of ltln8 no velooity selector meaauraaenta are available because of the emall value of the cross aeotion We have used the value 13 x 10”~ om2 Z 1% as the most probable value for 8) measurements (d)m from a number of ● There are~ at present, os80ntially three methods for the determination of the number of disintegrations taking ple.oein a soqrce of beta rays A Com plete detection, involving a ~~~.’am.mtern of some tyye It is applicable if there ia at least one eleotron per di~integration and no delayed radiations The use oi’exl.nvmelythin source~ makes it difficult to satisfy condition d)fi tSection1 and is generally somewhat inconvenient B Calibration by coincidence oounting This method is di60ussed in Seotion 30 It is applicable if th; dis- integration soheme is reasonably simplo and its relevant phases well understood rays are emitted C Calibration by natural sources This ref fers a standard ultimately to alpha ray oounting It i.aapplicable when there and if some i~ one and only one eleotron per disintegration amd if the beta rays are fairly penetrating, We used methods B and C for the8e measurements in the case of gold, attaohing more weight to the results of the more acourate method B which was the only one used in the ca8e of Mn # , I 8) Kubitsohek, CP-13890 —.— - APPROVED FOR PUBLIC RELEASE I APPROVED FOR PUBLIC RELEASE — — -9” BY COINCIDENCES B .——— DETERMINATION OF COUN’IW?EFFICIENCY ~ Method The principlo of the method has been disoueaad by Dunwrth 9) siderO first, the 85mple disintegration 6uheme shown in Fig Us beta ray speotrum accompanied by a single genmm ray co?l- with a simple If such a source is placed between a beta and a gamma ray aounter in an arrangement suoh as shown Fig which allows us to count pulses in each oountcw a8 well ax coincident oounta, then we have for these counting rates ‘P ‘r =noc n Coinc = If (b) ‘O%tf (4C) where no is the rate at whioh disintegrations take place and the the net efficiencies of the two counter6 Those equations am efficiencies and for no 6$’s are be 8olved for the If the disintegration scheme is more complicated we can write more general formulas, providod none of the radiating are delayed If the various modes or “pdh8n from the initial to a final state have relative probabilities fk we have 9) Dunworth, Rev Sc Inst., lJ, 167 (l$A!IO) APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE -MiExperiment leading to its establi@ment and references to other papers are given by these authors 56 90 Calibration of——.—— Gamma Counter — - forMn -Writing Eq6 (5) for the disintegration scheme of Mn!% we obtain (?a) (7b) ,“ + ‘“ y“%, [ +“ %2) Acxsordingto Fig lC, f~ = 0.15, caee of.gold, the seoond term in (7C) fa = 0.25, and in (7c) (Ta] fz = 0,60 As in the is duo to the effect of gaimna ray8 on the beta counter and can be corrected for by observation in which mfficient absorber is used to raove all of the beta rays The three Qfios win depend on the amount of absorber and on the source thioknesa When a very thin source and no absorber is used the three efl’icienciesbecome equal In this ease we may write, dropping the gamma ray termaO which can be oorrected for ‘1 = ‘o n coinc = C@ Pti = ‘o )7~ or n~pj — ~— — ——— -— —— .— -——.— —— - APPROVED FOR PUBLIC RELEASE y{= I APPROVED FOR PUBLIC RELEASE -17Fig shows the value Of ~fyi~l for varfous absorbers The value for zero thickness i~ obtained by extrapolating The value obtained is 2.4fjx 10”3~~ The 8ouroe consisted of a thin (about mg/om2) film of b!nelectroplated on mil silver foil Tho mall ~cttvi%y of the Soil was uorreoted for in all measurements There is a alight oohwmtion to the value of ~~ becw.eof the reflection of the beta rays by the baalcing This correction was estimated as follows It was found that a mil silver foil absorbs about 20 percent of all tho beta ray8 from the source We assume that the absorption for the three groups varies E-1033 and that HO peroent of the absorption of the ~1 11) ~8 is due to back backscatter- ing, and 60 percent in the aase of the two soft spectra The result is not very wnsitive to these assumptions From the curve Fig it appears that the gamma rays accompanying the soft speota% are counted about three times as efficiently as ~ alone Thus we oalculate that the correction due to backscattering is ‘3 3A L percent Thus we get The beta ray oounter was not calibrated because the mxroes which were used for the cross seotion determination were strong enough to be counted on the gemma ray counter They consisted of rollod manganese foil 100 q#cm2 thick LO Calibration by means of lJX4 Before the calibration by means of the coinoidenoe method was attempted, the oounter referred to above as counter B was calibrated by counting a weighed 11) Evan8, R D, - Introduction to the Atomio Nuoleus - Ml?ZLecture Notes APPROVED FOR PUBLIC RELEASE I APPROVED FOR PUBLIC RELEASE — -M amount (about 20 mg) of uranium We assume that the uranium io in equilibrium only ones counted Uf3in~the value 12) of which 1,.9percent is due to of 250Z0 Q/seo gm givozaby Kovarik and Adsm8 with UX2 and that the UX2 beta rays ure We 25 13) ~ we find that there are 12.220 ~/seo gm of uranium was found that ocnanter B was 0.260 efficient for U% Using this value it beta partioles Si~e these are quite enex-getioand both source and counter window are thin, it was assumed that absorption oorreotions were negligible in this case,, In the case of AU1* corrections were made both for absorption in the window aud in the source These corrections wercimade from thickness vs cmunt~ng rate curves obtained in the geometry “actuallyused &W In this way an efficiency of 00231 wa6 found for beta rays To this we must add the number of conversion eleotrons, taken to be percent of the beta rays (Section 5) Thus we find 7p = 0.250 in excellent ugremont with the value obtained by coincidence measurements (Section 6)0 Co FISSION COUNTING -,—— -— M Apparatua — — Fig shows a diagram of the ionization chamber used ‘Thischamber was placed in the carbon oolumn about feet from the cyclotron The cadmium ratio both for iridiumand for fission deteotors was several thousand The chain= ber was placed so that the active deposit raced away from the noutro.~~ource, the radioactive monitor foils being closer to the source It is known that the neutron 12) Kovarik and Adams, 13) Frisch, O %, Jo Apple physO, _, 12 296 (lg+l) Private communication APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE —.—— —- 019 flux is abou% twice aa strong outward from the source as inward One run waq taken in which a gold foil was plaaed on the baok side of the steel high-voltage eleotrodo, between it and the Pt toil supporting the U deposit, in order to , deteot any possible effeot due to absorption by the steel oup There was no s’ignif’iaarat difference between the result of this run and others in which the foil was plaoed on the other side of the oup The ohamber was filled with nitrogon at a pressure of 90 cm Hg A curve of counting rate vs gain is shown in Fig 5; the operating gain was 12 mVO The plateau appears quite flat and we assume that virtually every fiesion particle which emerges from the source is oountedo lZIJranium -— .— Samples — Most of the measurements were taken with the enriched sample Mg17B8 whoao 25 content was determined by O Chamberlain to be (~~ 2) x 10°6 ~, by comparing the number of slow neutron induced fissions with the number induced in a sample of EIO material C)nerun eaoh was taken with the highly enriched (7WO) sample R5D, analyzed by I& Chamberlain to contain (7.60 *0.15) a sample of normal alloy, alpha oount to be 4080 ml x 10-4 w of 25, and with whose 25 content was deduoed from the total x 10-6 gm The superficial density of all three films was very nearly the same, namely about 0.15 x 10“3 gm of U30~CIn20 For a film of this thiokness we oan make a correction for the fraction of the fission fragmen’tswhich fail to e8cape from the deposit This fraction will be t is the thickness (in &cm2) of the deposit and R APPROVED FOR PUBLIC RELEASE t/2R where is the range of the frag- I APPROVED FOR PUBLIC RELEASE -=eoments in the eamo units Usi~ the measurcanent a by Segre and Wiegand 14) we find that the thickness correction of our deposits is (1.8 -+ c)e~)% D RESULTS 13 Measurement of (&f/o&)~ - The prooedure in comparing the two cross 8i30tiOnS-s a8 fOlhWs or two gold foils were plaood in the chamber (Fig h), closeto the25 two runs the monitor foils were shielded by other, hmvier, I Ona foil In gold foils to elimi- nate any possible effect of resonance neutrons, as indicated in!Cable men that no significant ohange was observed In the ease of run ~ It is the ehield- ing was so heavy that it may conceivably have disturbed the neutron flux Run #6wasmade inanaluminum chamber, kindly loaned tousby R R.1’iilson The platsau of this chamber was not investigated as oarefu~ly as that of our steel chamber The ohember was then exposed in the carbon column for about two hours, during which time sample Mgl?B8 gave about 106 x 105 fissions The semple EM \ of normal alloy gave only 4.000counts, Two thicknesses of gold leaf were used One was pure gold leaf of about 10 mg/cm20 tie other waa 25 karat, about ~ m~cm The impurity in the latter was determined spectroscopically to be oopper and silver The short life of the silver aotivity and the small cross soation of Cu allowed us to negle~t any - effect due to these on the awtivity but they had to be taken into account in determining the mass of gold used The leaves were weighed and counted on one or both of the calibrated oounters and with or without glass backing The prob== 14) Segre and~iegand, u-6$ ~“ ._ .- APPROVED FOR PUBLIC RELEASE —-—-—’~ !, , APPROVED FOR PUBLIC RELEASE able error assigned to the varibus measurmenta in Table I depends on the number of foils and the number of independent counts taken Beoause the exposure time was only about peroent of thome)an life of Au198, we can use Rq (~) in conjunction with Eq (2) Wewrite during the exposure time T Nf s nfT Thus, using %5 for the total number of andMAu f to denote &&emass counts of 25 and.gold used reapectively~ we have (8) we use ~ s {1.78 t 0002) x 10”4 min”l (Seotion 2), tion 12) The error in Nf The oounlxingerror in n’, 105~ and ~ runs in the six &f == 0.98 ~ 0~005 is negligible except in run ~ where it was (Seo~%* the deoay rate after bombardment, varies between MAU ~E weighed to about ~ ~% me average error in was taken to be ~M (Seotion7) although it varied somewhat between me % k3everalruns, depending on whether one or more foils were counted and on which counter was usede Inoalculating the probable error of the average (~/~u} Table I account wa8 taken of the fact that the errors in the first four runs) ~~ ~ Gf, in VP and (for not average out in the 8everal runs but rmain con- stant The value of the average is 40 Measuraent of (~/~n)& Measurements with manganese were perfomed The foils were 100 m~om2 exactly as those with gold and were counted on the gamma ray counter 130cau8eof the shorter moan life ofl!n~, Eq (2) must be used with Eq (3b) Thue (9) ——_ APPROVED FOR PUBLIC RELEASE I - I APPROVED FOR PUBLIC RELEASE — -22The error in ~ — is negligible in this aase, as is the error of the greater weight involved The probable error in , The crbher errors are the same as in Seotion 130 the oaae of @d, in is *7A W because MD (Section 9)0 The average, calculated a~ in 18 ~o~ ~ ~o* W%.)m 15 Calculation of (d”)m and radiative oapture by 25 In order to calculate the value of &f for neutrons of 2200 m/seo Volooity we must take into account the deviation of by MoDaniel et al 3) From hi.sresults we find &f from the l/v law found ‘ (~~)~ s 1.025~v ~ 2jZfor “ a Maxwellian dida-ibution and room temperature of the neutrons Thus = 5.51 & 0.35 (@#qu)~ We find and, using = (* ~ 2) x ~&’~ n2 (Se~tiOn 2]* (&f)M = (518’* 35) x 10-~ cm2 Similarly from the value of (qn)M (O&]M z 13 x 10“~ om2 ~ 1~ (&f/~n) given above we oalculate, using (section 2), or, as an average Here we weight the two values according to the accuracy of the absorption cross sections (iOeo 1s5) rather than the total probable errors since much of the latter, e.g the error inld 25D ia common to both determinations !fhiacompares with the value (a5 ~ 16) x 1(1”~ CXR2for the absorption oross section (Section 1) — —— APPROVED FOR PUBLIC RELEASE -.— .— –.— i- APPROVED FOR PUBLIC RELEASE 239= The ratio of the two values is (1+ &)m = 1.23 *0.W The excess is prmwmably due to a competing process, probably radiative capture by 25, or conomivably$ by ~ TABLE I —— Run sample’ M25 /’/% Position of foil (~f/@&)~ — 99 1$ 5027~0.3~ Directly under 25 semple 5.39=0.35 In steel cup d 5.4320.35 pOmg/om2 gold shield tl 5e71to.42 1~0 mg/cm2 5.17to.50 In cup 5.2020.50 In Al ohambor —— 760 Average 5.37~0.32 - -._ — - — - APPROVED FOR PUBLIC RELEASE - APPROVED FOR PUBLIC RELEASE -a” TABLE 32 TABLE 111 SUMMARY OF PRCVIABLEERROl& IN INDIVIDUAL RUMS AND IN CALCULATIONS Quantity ———.- MR8S Of 25 Maas of gold Thickness correction for f foil f hount 005 negligible Gold decay (sonstant Beta count 105’ Counter efficiency Gold cross section 25 total cross seution — 2.5 —— APPROVED FOR PUBLIC RELEASE -—— -— :-:;:i:’’-a~ -~ APPROVED FOR PUBLIC RELEASE —4 - ., -, - ~- ,.- — I ‘/ , I “.{ I f ~“ ‘i,P,‘ ,, , ,“ , , ’ .“ >., r - - “ ! -d , - -7 -.” ’ , : -4 - /% , .-j, \ , , ‘ I “” ._ , .’ ! ,“ -’ -.-: ;, 1’ I - 19$ — Au - ~-— $ ~ s “.’,.’ - : L ‘ $ .- - - - >,_ -,’” p, O& m;’ - + < , ): :.,,: y , k , ,.,, P‘“ ‘ - ,~~: ” ,, , , i I 1f , “ ““ , Ww “ yfj[z “ $ - I /, ;” + i _ - -, - —— — 4r g ,3 cl>+ ,“ ,” i & - ’ —.4 .,.4 -, ”” ‘l I !) ”., ,! 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