Actinide F i sc ion Rate K e a s u r e ~ e n t s i n Zebra If! D W Sb~eet

7 I n t r o d u c t i o n

F i s s i o n chafibers have beer, used t o measure t h e ~ 2 3 8 , h 2 4 0 , 31241, -242, A11241, Am243 and Cro2't4 f i s s i o n r a t e s r e l a t i v e t o f i s s i o n i n Pu239 a t t h s c e n t r e of Zebra Core 14/1, vhich had a neutron spectrum c l o s e l y resembling Ytat of PIX. The experimental technique, t h e r e s u l t s , and t h e i r comparison with E L 5 d a t a a r e r epor t ed below.

2 F i s s i o n Chazbers

2.1 General

The f i s s i c n chaxbers used 'qere o f the s tendard Zebra p a r a l l e l - p l a t e d e s i p (Ref 1). The plutonium and urmium chambers a l r e a d y e x i s t e d (Refs 1 ?:xi 2) . Ncv c h a r i x r s were obta ined con ta in ing d e p o s i t s o f t h e anericium an< cvrluiii i so topes ,prepared by Act in ides Group, Z w x e l l .

2.2 Plutonium and Uranium Chanbers

The p repa ra t ion of a c c u r a t e l y c a l i b r a t e d h2:9 f i s s i o n chamber d e p o s i t s of nominal 5 0 p g mass f o r Zebra has been desc r ibe2 i n i7ef 2. One o f t h e s e d e p o s i t s was used i n a f i s s i o n chamber (2-eb '45) f o r these measurements, while t h e second (Zeb 116) was used es an alpha-counting s tandard .

The h 2 b 0 , E'u241 and Pi12112 chasbers used vere those descr ibed i n Ref ?. I n previous Zeb-a co res , i e up t o I Z B , t hese chambers were counted t o - g e t h e r with t h e I31239 chambers descr ibed i n Ref 1, vtnich contained 2 rn.z o f Pu239. The quoted masses o f t h e P ~ 2 3 9 chambers have been shown t o be (2.8 2 1.0)% low by f i s s i o n comparisons with Zeb 45. It $as been found t h a t t h i s discrepancy coz2riseZ t:io c o n p n e n t s , v i z ( l .5 - 9.6):; a_risic,g from t h e a lpha c a l i b r a t i o n ( s e e Ref 3: a:? ( 7 - 5 'l 0.8j% duc et;o e r r o r s i!: t h e c o r r e c t i o n s (about %;> f o r i o s s of f rafyents when f i s s i o n c o u ~ t i n g t h e

. .- heavy depos i t s . The 3x240 chambers incur. s z r . l u r l a r g e c o r r e c t i o m , zr5 t h e i r nassea have consequently been increesed by (2.8 1 1.0)$5, s o t h a t t h e %2'iC//i>;12,9 f i s s i o n r a t i o s xeasurec! he re a r e d i r e c t l y co-parable ~ l t h previous values. The Fu241 and Fu242 chnnbers, however, con ta in cox- p a r a t i v e l y t h i n d c p o s i t s incur in8 c ~ r r e c t i o r . ~ f o r f i s s i o n f r a p e n f , i o s s of betwoer. 2 ar:i Yj4; t h e i r masses have t h e r e f o r e be92 incri-zscd by c n l j . . . 1 . 0 arA 1; IS necessary t o ir,cre.-se previous ly measured Fu24?/ - Pu239 and P~..242/Fu239 r a - t i c s by ( 1 .? - 0.8):; t o coapzre with the p resen t r e s u l t s ; _ i e previous C/E va lues f o r these two r a t i o s have t o be reduced by (1.5 - 9.8):;.

The ~ 2 3 8 f i s s i o n r a t e was aeasured .~s i r .g a n a t u r a l uraniun f i s s i o n chmbcr, Zeb 9. T h i s chamber had been c n l i b r a t i r i 'sy t h e r m l f i s s i o n a s s a y o f t h e U235 con ten t r e l a t i v e t o a s tandard U255 chaflber ( s e e Ref 2). 1 U235

f i . ss ion measurement was a l s o made h e r e t o c o r r e c t f o r U235 f i s s i o n i n the n a t u r a l uraniurfi. The a t o n i c content of Zeb Y is included i n . T a b l e 2.

2.3 Americium and Curium Chambers

2.3.1 Deocs i t S n e c i f i c a t i o n

One d e p o s i t each of Am21t?, An243 and two o f Cm244 have been prepared. The genera l s p e c i f i c a t i o n of t h e d e p o s i t s w a s t h e same as t h a t o f t h e 2x239 s t andard d e p o s i t s (Ref 2); i e t h e y were 20 mm i n S a s e t e r on polished s t e e l t r a y s 28 mm i n d iameter and. prepared by vacuzm sub l ixa t ion . The nominal nasses f o r each nuc l ide ve re 10 ,q, 5 O p g and 1fig r e spcc t ivs l :~ . Tiiece -::ere s e l e c t e d t o provide adequate f a s t f i s s i o n r a t e s without i n c u r r i n g excess ive a lpha decay r a t e s .

2.3.2 I s o t o a e Stocks

The Am241 and Am243 d e p o s i t s ( l a b e l l e d Zeb 51 and Zeb 52 r e s p e c t i - ve lg ) were taken from s t o c k s used f o r t'ne r e c e n t c q t u r e cross- s e c t i o n nezsurexents i n Zebrs. They ha3 been t r e a t e d r e p e a t e d l y t o remove p lu tonius . I s o t o p i c aqalyses obtained at A'iXE have confirmed t h e absence o f s i g n i f i c a c t con tag ina t ion by o t h e r americium i s o t o p e s ( l e s s than C.35; bjr atoms, wit'n Am242 l e s s Ynan 0.01%) while f u r t h e r measurenents, descr ibed belcw, have confirmed t h e absence o f s i g n i f i c a n t contaxiination by o t h e r elements.

Alpha s p e c t r o n e t r y showed t h a t the f i r s t CnZ44 d e p o s i t (ZeS 53) was prepared from s t o c k con ta in ing a high l e v e l o f P u 2 k c o n t a i - n a t i o n ( s e e Sec t ion 2.3.3). A second d e p o s i t (Zeb 54; was p r e p a r d from t h e saxe s t o c k a f t e r chemical t r e ~ t m e n t t o r e s o w plutonium. I s o t o p i c aqalgses o f t h e curium obtained a t Harvel l rzveIao2 sis- n i f i c a t concent ra t ions o f Cn24j ( I . j$) Cm2Lt6 (5.g;) Cm247 (C . :ZL) and C m M (O.i?Z;) where t h e concent ra t ions a r e % by a t o n s o f t o t a l Cm coc ten t . No u n c e r t a i n t i e s were arovided with t h e ana lyses , b u t i t f-n t:-.cucht +.hz.t t hese ~ h m 1 5 07 3~2.12. ~omp.red wit?i t k c s e o f

t h e c ross - sec t iozs ,~ssuned when applying c o r r e c t i o n s f o r f a s t f i s s i o n . Tne analysts a l s o provided t h e i s o t o n i c content o f t h e lu coataxina- t i o n i n t h e f i r s t s t o c k , t h e lu25:/Fz2':3 azd Pd2:8/Fu24O r a t i o s be ing 0.019 and 0.C05. r e s s e c t i v e l y . 'inere +:as rio evidence on tile conccnt rc t ior , of plutonium i n t h e zepara ted s tock .

A l : h sy?tr-n--%c ana lyses of t h e & p o s i t s were -ad* at uarwell. Detectable c o n t a a i w t i o n o f both t h e a:fiericiun i so topes VJas in- si::nificant, i e m u l d c o n t r i b u t e l e s s than 0.?y of ?:?+ tc';-l i i b r a .The cs:iu:g a lpha ~nectr?.l I;:-.- f2sr.d t o ,- - \i.. i lo - 0.005)5:, Rl;?:iO a c t i v i t y , ir+l:;ina a !%t2!t3 a t a x content of I ^ ^ - .~.

a c t i v i t y , though t h e l i m i t o f d e t e c t i o n was only 0.05% by a l p h a ' a c t i v i t y . The Ih21r0 content has been taken a s ze ro , bu t an e r r o r corresnonding t o t h e l i m i t on the a lpha s p e c t r o n e t r y h a s been included i n t h e r e s u l t s (I 11%). An u n c e r t a i n t y has a l s o been included t o al low f o r Am243 con tamina t ion up t o t h e l e v e l o f d e t e c t i o n on t h e first a lpha spect rometry measurement - v i z 0.01% by a l g h a - s i n c e no evidence is a v a i l a b l e as t o t h e l e v e l o f s e p a r a t i o n achieved ;hen t h e Cm244 s t o c k was o r i g i n a l l y p u r i f i e d . (Am243 is t h e previous long-l ived nuc l ide i n t h e capture chain l e a d i n g t o Cm244). Both Cm244 d e o o s i t s were r e t a i n e d and used i n f i s s i o n chambers.

Absolute c a l i b r a t i o n o f t h e d e p o s i t s was achieved by low-geonetry a l p h a assay a t Zebra, .using t h e same geometries as f o r t h e Fu239 d e p o s i t s (Ref 2). Each d e p o s i t w a s counted i n a t l e a s t two geo- me t r i e s , t h e e f f i c i e n c y o f which var ied by not l e s s than a f a c t o r of two. No s i g n i f i c a n t v c r i a t i o n i n t h e d:.sintegration r a t e s obta ined fror;, t h e d i f f e r e n t geometries was observed wi th in t h e experimental + accuracy o f - 0.3%.

.. . ~~

A s tandard R.1239 d e p o s i t (Zeb 46) was counted r epea ted ly through- o u t t h e measurements and no s i g n i f i c a n t v a r i a t i o n was observed from previous measurements wi th in a s t a t i s t i c a l accuracy o f about + - 0.3;. Absolute d i s i n t e g r a t i o n r a t e s were a l s o obta ined at Harwell, and t h e r e s u l t s a r e compared i n Table 3.

The depos i t masses were c a l c u l a t e d us ing t h e Zebra alpha-assay r e s u l t s and t n e a lpha s p e c t r o n e c t r i c analyses front Harwell. The h a l f - l i v e s used were obta ined from Ref 5 and a r e given i n Table 1. The atomic content o f t h e d e p o s i t s is given i n Table 2.

2.3.4 P r e l i l T n a r y F i s s i o n Keasurerfients

Following t h e a lpha a s say , t h e d e p o s i t s were sea led i n Zebra f i s s i o n chambers. Examination o f t h e pulse he igh t d i s t r i b u t i o n s obta ined from t h e chambers confirmed t h a t t h e q u a l i t y o f t h e d e p o s i t s was s a t i s f a c t o r y ; t h e c o r r e c t i o n s requi red f o r t h e l o s s o f f i s s i o n fragments helo~vr t h e d i s c r i x i n a t o r b i a s l e v e l were !3.4%, C.3: and 0.7y' f o r t h e >.m2itl, Am243 and Cm244 chaxbers r e spec t ive ly . The sonewhat lower r e s o l u t i o n exh ib i t ed by the C32Lili chambers was caused by a lpha build-u;, ( t h e alpha a c t i v i t y from a 1/Ug Cm244 d e p o s i t is equ iva len t t o t h a t from about 1 mz o f h 2 5 9 ) .

Tine spontaneous f i s s i o n r a t e i n t h e two curiur! chambers was recorded; t h e r e s u l t s o f t h i s measurement a r e cham i n Table 4 where t h e

. spontaneous f i s s i o n r a t e is compared with the a lpha decay r a t e and the r a t i o of t h e two decay codes comared w5.th published d a t a (Ref 6). Ths a g r 2 m m t is e>:celic:~t azixr n c c r r e c t i o n o f (5.1 2 0.419; f o r spontarieous fi.sc;$on i n o thnr C,z i::o?op3s. Tncre ie no si@fj .cnnt : l i f f r r ; -nc~? be":!^^.?. C:le t::c -1-;27;"r,.

s t o c k before producing

/

t h e second chamber. The a lpha spectrometry r e s u l t s f o r t h e f i r s t s t o c k , taken t o g e t k c r with t h e mss s p c c t r o n c t r y r e s u l t , i n d i c a t e a concentra- t i o n of O.& o f 211239 i n good agreenent; t h e balance o f the con tan ina t ion is equ iva len t t o (1.5 ' 0.1)$ by atoms of Cm21t5, which is a l s o i n e x c e l l e n t agreement with t h e concent ra t ion obtz.in& by i s o t o p i c ana lys i s . The Cm245 would no t have been cietected. by a lpha a s say ( c o n t r i b u t i n g 3.1 x 10-3; .of t h e a c t i v i t y or, t he low energy s i d e o f the Cm244 peak). The Cm245/h2:9 thermal f i s s i o n c r o s s - s e c t i m r a t i o o f 2.7 was obta ined from Ref 7.

The Cm24j and 3.1239 atom concent ra t ions deduced f a r t h e Cm chambers are included i n Table 2.

To summarise, t h e anericium chambers con ta in adequate ly pure d e p o s i t s bu t t h e Cm244 chanbers a r e s i j p i f i c a n t l y contaminated as follows:-

1 h 2 k 0 c o n t r i b u t e s (25 + 1.11% of fyt f i s s i o n s i n Zeb 53. co r res - ponding f i g w e f o r Zeb 5 4 is (0.0 - I?)?;.

4.

2 Cm21t5 and Cr.247 toge the r con t r ibu te (6.9 - 2.4)% and (9.6 2 3.2)% of fas t f i s s i o n s i n Zeb 53 and Zeb 54 re spec t ive ly . Cm245 c o n t r i b u t e s (2 ,6 - 1.31% a d (3.6 - 1.8)7L.

3 Am243 contamination may con t r ibu te up t o about 3% o f f a s t f i s s i o n s i n t h e chambers.

. .

3 Measurenent i n Zebra

3.1 Exoerimental Procedure

- F i s s i o n chambers were l o c a t e d i n t u r n i n a half-element a t p o s i t i o n 50-50, t h e c e n t r e o f Core 14/1. The half-element was t e r ~ h a t e d a t t h e c e n t r e l i n c aqd contained 5 core c c l l s with a sodiun p l a t e a t t h e top. F i s s i o n chanbers were i n s e r t e d - on a r i g i d extens ion v i a a b r a s s guide tube zr.3 were loca ted (1.0 - 0.5) mm above the sodium p l a t e . The arrangement is i l l u s t r a t e d i n Figure 1.

During t h e neasurenents t h e r e a c t o r power was monitored by t'ne four r e a c t o r pulse channels. Var i a t ions i n poc:e7 were m a l l ( l e s s than

3 . 0.&); t h e no:r.ir.al c e n t r a l f l u x was 12 x :O neat rons cs-2 S-I. Two chamhe:-s ve re osod t o neasure the h 2 b , Pu241, h 2 k Z and Cn2l t& f k s i o n r a t e s . The :%233, h 2 b l an.d An243 f i s s i o n r a t e s were measured with a s i n g l e chanber. but t h e ceascrenents were re-eated. One rr,ezsurcment o f t h e U238 fi:cr;ion r a t e w a s naac. The d a t a f o r the IU39 c h x b e r was . . checked by countin% r e l a t i v e t o a second I'J.239 cit4ncl~rCI ckznt'er, Ze!: 32; no c i g n i f i c m t c h a g e i n hie o f t h e r e s p o m s o f t h e b.ic chamb$r; compared with prsviovs measurement.; was observed 'n'i:hin t h e experimental accuracy o f - 0.3%.

Th,- r e s u l t s a r e s u s c a r i r e d i n Table 5. Associated u n c e r t a i n t i e s a r e l i s t e d i n Table 6 .

Tiie f i iZ i+ l ci!:::i,;,Jr. ?;rasr;es used 'nore ';!:3!? correct*::!. f o r fiecay of t h e is ' - - + .

LOirO:,L- sillCC: csE;ag t4ss.,::;inc; a k i a l : ].ifi. of ,iL.c - 3.5 p i r - s . Current r'c- ass::scysztr;, ho:;e,,cr (2ef 5): i r .djcate a vn?*E of 14.9 years . 1' t h i s Cigar* wc-e u%d, the ir.easurcd Mt241 f i s s i o n ri?-te would be reduced by

- < '

It h a s been noted above (Sect ion 2.7) t h a t although curium s tock

t r e a t e d t o remove plutonium was used t o produce the depos i t f o r Zeb 54, no eviclence is yc t ava i l ab le t o prove t h a t t h e nI.uloni.um concentrat ion is i n s i g n i f i c a n t ; the modest discrepancy of 1.7%bctween the two r e s u l t s is, however, encouraging.

4 Coaparison with E L 5 Data

Data f o r a l l the i s o t o ~ e s under d iscuss ion is avai1aV.e irk the E L 5 l i b r a r y . For 7233, Fu239, Fd40 and h 2 4 1 the d a t a is i n f i n e (2000) group form; f o r Fu242, Am241, An243 and Cm244 i t is i n 37 groups.

AWRAL c a l c u l a t i o n was made f o r t h e C13-1A c e l l us ing FGL5 d a t a i n which t h e sodium p l a t e adjacent t o t h e f i s s i o n chamber dur ing the measure~en t s , an5 t h e cores of the plutonium metal and uranium oxide p l a t e s were i s o l a t e d . The remaining regions \:ere horogenised. Since approxinately h a l f t h e neutrons inc iden t on the cha-ber d e ~ o s i t cone from the ad.dj'acent sodium p l a t e and t h e remainder o r i g i n a t e above the chamber i n the var ious regions of the neigh- b o u r i n ~ ; elements, it is thou&tthat the mean of a volunie average r e a c t i o n r a t e acd the valxe o f the r e a c t i o n r a t e i n the sodium p l a t e provides the b e s t es t imate of the value i n t h e chmber (Ref 10). A valuc f o r t h e f i s s i o n r a t e of each isotope of i n t e r e s t i n the f i s s i o n chamber r e l a t i v e t o h 2 3 9 f i s s i o n was obtained i n t h i s vay. The d i f fe rence between the two r a t i o s making up the mean was i n no case g r e a t e r t h i n 5% (see Table 5). The 37-group spectrum i n t h e sodiu? p l a t s is given i n Table 8; the standard sgec t rcn use< t o ob ta in the m e - g r o ~ p EOZrj5(h) d a t a s e t (3cf 8) f o ~ CX? 2r.d Fi3 I s a l s o ~ i - ~ i . n and the one-erou3 cross-sect ions obtained here f o r the sodium p l - t - e =e coa- pared wi th the FCGD5cA) values i n Table 9. The o r i g i n of the FGL5 d a t a is ind ica ted i n t h i s Table.

I n Table 5, the ca lcula ted and exper inenta l f i s s i o n r a t i o s a r e conparer?, and C/Z values obtainsd. Estimated cor rec t ions of (0.0 1 0 .1 )$ , fo r Pu2!+l/~u2jg f i s s ion , + (1.5 1 .O)% f o r U238/%2l:9 f i s s i o n and +(0.6 f 0.4'1% f o r each of t he o the r r a t i o s have been applied 'to ehe chamber neasurenents t o account f o r h e l a s t i c s c a t t e r i n g i n the f i s s i o n chamber wzlls an& the empty s h a t h . These c o r r e c t i o n s are based on c a l c u l a t i o n s repor ted i n Refs 1 a ~ d 11.

Addit ional u n c e r t a i n t i e s a r i s e w'nen the c a l c u l a t i o n s a r e coxpared with exper i - ment because of the chanber geo-etry i n Zebra. These a r e estimated t o be + + - 1% f o r F:/F9 and - 47: f o r the r e m ~ i n i n g r a t i o s (Xef 13). It is thoueht - t h a t f u r t h e r work. could reduce t h e - 4% e r r o r , perhaps by a f a c t o r of two.

It should be noteci t h a t f w t h e r work t o e s t a 5 l i s h more accura te ly t h e mami- tude of .4m21:3 and plutonium contamination i n the curium s tock could reduce tnc uncer ta in ty on t h e r e s v l t repor ted her* from over 6% t o about 43.

5 Sunnary

Measureen t s of f i s s i o n i n seven a c t i n i d e i s o b p ? s r e l a t i v e t o Pu239 f i s s i o : ~ have been m d e a-d t h r e z i t s compared w i t ? X i5 data . C/E values obtained were a s follo+!s: - ~ 2 3 8 1.043 1 4.4% ( c f 1.032 ?- 4.,4~?6 i n NEB) P;&C 7 .Cq - 4.67; (cf 1.000 i - 4.4" i n ?23)

ilm241 1.260 4,$% * corrected value (sce Sec t ion 2.2) An243 0.882 4.1::

It is hoped t o improve t h e accuracy of t h e Cm r e s u l t by about Z% by determining t h e l e v e l s o f Pu2'tO and Am24j contamination more p rec i se ly .

AEE Winfri th

References

1 J M Stevenson, A M Sroonficld. Measurements and Calcula t ions o f Ratios of E f f e c t i v e F i s s ion Cross-Sections i n the Zero-Power Fas t Reactor, Zebra. AEEd - R 526. .-

2 D W Sweet. Ca l ib ra t ion of F i s s ion Chanbers f o r Fas t Reactor Experiments. AEZ4 - R 936.

3 D W Sweet. The Present Pos i t ion on the Absolute Ca l ib ra t ion of Zebra h239 F i s s i o n Chambers. Winfri th I n t e r n a l Document.

0 4 X I, Pope, J S Story. Minigal Output from UK Muclear Data Library - NGL 1 (1973) Thermal Cross-Sections Resonance I n t e g r a l s and F i s s ion Spectrum Averages. AE-<:I - .I-M 1191.

5 N F James. An Inter im S e t of Decay Data f o r the Actinides. U i n f r i t h I n t e r n a l Documezt.

6 M-F James. Notes on the Decay o f Cm244. Winfr i th I n t e r n a l Document.

7 BNL 325. Vol 1. :rd Edi t ion .

8 J L Roi~lands. Recomcended one-group Cross-Sections f o r PFR and CFR Calcu la t ions (The FCGDS(A) S e t of Constants) . V i n f r i t h I n t e s m l i)ocur;iect.

9 C J Desn. P r i v a t e Communication.

10 G Ingram. I n t e g r a l Xeasuren:ents i n a S e r i e s of Zero Leakage Systems and t h e i r Comparison with KX4 Calcula t ions . h ' infr i th I n t e r c a l Document.

J l,l Stevenson. Calcula t ions of the % a c t i v i t y and Centra l Reaction Rate 2 a t i o s f o r MZB with Fi15 Gaia, a d Conparison with i q e r i n e n t a l Lasul ts . Yin. I n t . Docurer,t.

12 J P LtHer i teau, P Ribon. (To be published)

15 UT L D a ~ l a n d s e t a l . The X L j an3 FD5 Cross-Section Se t s . li:i.cfrith Internal . Docucent.

15 M Lou-sburg, R ';I Durham. Proceedings of BiYES C o n f e r e ~ c e cn Chentical Nuclear Data, .. u n i v e r s i t y of Kent, ~ 1 2 ? 7 < 3977). -. 76 4. 5 J a i i e y e t a l . rnys 2ev 2, '1889 (1971)

Table 1 - Aloha Half-Lives Used t o Calculate Denosit Kasses

Isotope

U234 U235

Pu239 Pu240 h 2 4 1 Pu242 . Am241 Am243 Crn244

. .

I f a l f - l i f e (years)

0.2442 x lo6 0.704 109 2.435 x lo4 2 0.2%

6.600 x lo3 t 0.6%

Reference -7

Note: U231t and U235 ha l f l i v e s quoted were used t o ca l ib ra te the UZ35 chamber sgainst which 2 9 natwal uranium chamber was cal ibrated by thermal f i s s ion assay.

Table 2 - h'umher o f Atoms x loq8 of F i s s i l e I s o t o p e s P resen t i n S i g n i f i c a n t Q u a n t i t i e s i n Chamber Deposi t s --

Kotea: 1 The e r r c - r s ns roc ia t ed u i t h t h e atom numbers are given i n Table 5 . See a l s o t e x t , S e c t i o n 2.3.

2 The atom numbers have been reduced t o a l low f o r s e l f - a b s o r p t i o n o f f i s s i o n fragments i n t h e depos i t s . The c o r r e c t i o n s were no t s i g n i f i c a n t ( i e l e s s than 0.2%) except f o r chambers 4 and 7 ( 4 . 9 0 , 22 and

3 2 3 and 24 an*. 25 (1 .@). No c o r r e c t i o n was n e c e a ~ a r y f o r t h e n a t u r a l uranium chamber s i n c e it wes c a l i b r n t c d by t h e r c a l f i s s i o n assay. -;;,

0 7 3 The content6 o f t h e h241 and Pu242 f i s s i o n chambers have been co r rec ted t o a l low f o r t h e Pu241 decay. A h a l f - l i f e o f (14.0 0.6) y e a r s hni; been used. d

-\

Table 3 - Comparison of Alpha Assay R e s u l t s Obtained a t Harwell and Zebra

I Chamber No Obtained a t Zebre

6 -1 I Alpha d i s i n t e g r a t i o n r a t e ( t o t a l ) x 10 G

k ]obta ined a t Iiarwell

Table 4 - Cm244 Alpha and' Spontaneous F i s s i o n Decax

Zebra r c s e 1iarv:ell r e m l t

Chamber Alpha F i s s i o n Experimental Xean d i s i n t e g r a t i o n Alnha 6 experimental

Expected Exnected r a t i o i:o

r a t e 5 -' F i s s i o n Rat io

Ra t io Experimental r a t i o

Note t o Table 4: The random e r r o r s on t h e experimental numbers a r i s e from count ing s t s t i s t i c s and from c o r r e c t i o n s f o r b iased o f f low energy events. The sys temat ic e r r o r on t h e spontaneous f i s s i o n ~ e a s u r e n e n t s a r i s f s f rom t h e u n c e r t a i ~ t i o s i n t h e Cm246 and Cm2Jt8 spontaneous f i ~ s i o n h a l f l i v e s (i 6% and +- 11%) and i n t h e i s o t o p i c analyses of t h e s t o c k s ( 2 %), c o r r e c t i o n s f o r spontaneous f i s s i o n i n t h e s e i so topes being (Jt.8 2 0.41% and (0.2 + O.02)% respec t ive ly .

l 'able 5 - F i s s i o n Hntes i n Chambers Exnressed Re ln t ive t o Pu2B F i s s i o n

E A p r i m e n t a l f i s s i o n r a t e

r a t i o ( 7 )

Xcnn cxparimcntal . . l s s i o n r a t e r a t i o ' f o r i so tope

+ - .&kr 0.02706 i - 1.y6s

Expcr ixen te l ' i s s i o n r a t e r a t i o

corrected f o r n e l a s t i c s c a t t c r i n p

+ - I . 1%- 0.02747 + 1 .ljSls

Calcula ted f i s s i o n r a t e

r a t i o ( 2 )

C a l c u l a t i o n Exper izent

1.043 : 4.4%

i

Notes t o Table 5 - Fitision Ra tes i n Chambers Expressed R e l a t i v e t o Pu2j9 F i s s i o n

1 r and s s i g n i f y random and sys t ema t i c e r r o r s r e s p e c t i v e l y .

2 The c a l c u l a t e d r a t i o s a r e means of va lues i n t h e sodium p l a t e a d j a c e n t t o t h e chamber and t h e volume average ( s e e S e c t i o n 4). The d i f f e r e n c e between t h e 2 c a l c u l a t e d va lues were as follows:-

Table 6 - U n c e r t a i n t i e s Associated with Experimental F i s s i o n Ra t ios i n F i s s i o n Chambers -

Cm2Llt Chamber 54

I so tope and Kagnitude o f Uncer ta in ty

Or ic in o f Uncer tn in ty I 1 U238 Cm244

Chamber 53: - ) -4:: s 0. y;r

0. :?Ls 0 -m; . ,/.S

0.47Lr

0.3%r

0 .O%r

0.1%

3.8%~ -

0.2%

.6%r 3.8%

F~23.9 f i s s i o n measurement . . 0. 4%sO. 3

Assay of C e ~ o n i t .. . . . .. 0.9;s

l i e .. . . . . 0.6%

F i x i o n co11r.-tin&, s t a t i s t i c s . . O.&r

Corr. f o r biased o f f fi:=:ionevenk 0.4%r

Correction f o r f i s s i o n coun t inc (i~..:.d t i n e ~ C S S ~ S . . -. . . 0 -0%

C o ~ r e c t i o n f o r se l f - absorp t ion o f f i u s i o : ~ frsgments . . -. I - Correc t ion f o r f i s s i o n i n impurity

i s o t o n c s . . . . 0.3%

Correc t io- f o r decay o f Fu241

Correc t ion f 3 r Cm244 spon. fiss. -

T o t a l e r r o r on chcrnbx- cwmt r a t e

. - ,7 r o t a l c r r o r on p r i n c i p a l i s o t o p e

9 & r I * y $ ~ ---/- W'f- Note: I r ax3 s s i g n i f y random and sys temat i c e r r o r s r e s p e c t i v e l y . C!-i

I .

Chazbrr No and Principal Isotopes

C- !

Zeb 3, 3238

Zeb 45, Pu239

Zeb 4 and 7 , Pu2b

Zeb 22 and 23, R12~tl

Zeb 24 and 25, 1'11242

Zeb 51, Am241

Zeb 52, Am243

Zeh 53, CmNt

Zeb 54, 'Cn2'tlt -

Table 7 - Contributions from Fission in Imourity Isotopes

-.. Impurity Isotopes and ?$ Contributions to Total Fission Rate -

otal contribd

27.4 10.3

f 0.2 - 0.1

4.1 f 0.4

+ 15.1 - 0.2

30.6 +- 0.6

0.0 2 0.0 +

0.0 .- 0.0

37.5 1 3.8%

13.9 2 1%

~'Jotcs: 1 Insignificant contributions, ie less than 0.1%, are ignored.

I

A 2 Where possible, the cwrections were obtained from the fission ratios measured here. Otherwise calculated

9 1.

cross-cections were used. The Pu2:8 and Cm2Jt3 cross-sections were obtained from Ref 8, while the Cn245 I cross-section was calculated from the neutron spectrunf and dcta from Ref 9. The cross-section for Cm246, C!

-3 relative to Cm244, was obtained from Ref 14.

-2%

II- 3 Cm21t7 has been included with Cm245 in the Cm deposits. The concentration of Cm247 is small (0.13% of the C-; Cn atoms). J

r:*j

Table 8 - Spectrum i n Sodium P l a t e Adjacent t o F i s s i o n Chamber

Group Lower energy

10.0 Mev 6.065 3.679 2.231 1.353

820.9 Kev 497.9

Spectrum i n sodium # l a t e a d j a c e n t t o chamber

0.012 0.233 1.147 3 . 2 b

Standcard s p e c t f u ~ from 3 e f 8

0.012 0.229 1.115 3.149 4.366

Table 9 - One-Groun Cross-Sections

Notes: 1 The ,apparently anornolous v a r i a t i o n o f t h e Am241 c r o s s - s e c t i o n a r i s e s because o f t h e high propor t ion o f f i s s i o n even t s below t h e ' t h resho ld ' at about 0.3 b!eV.

FCGD5 ( A ) Cross-Section

-- 0.047 1.82

0.376

2.64

0.295

0.404

0.188

0.548

2 The FGL5 c ross - sec t ion s e t is described i n d e t a i l i3 Ref 13.

EL5 d a t a d e t a i l s (2) h U L

Dy?q (34:

272 269

402

403

955

956

957

958

Ref and Notes I I

Adjusted ( '71) I Adjusted ( '71)

Adjusted ( '71)

Ref 12

EhQFI32

ENDFB2

ENDF52

ENDFa2

I I

Bras " . . ~ m

-. _ -._A . --, Chamber stem

Deposit Disc

(Dram approximataly Lo sca l e )