J . R A D I O A N A L . N U C L . C H E M . , L E T T E R S 166 (3) 187-194 (1992)
ISOMERIC YIELDS OF 130Sb, 132Sb, 134I, AND 136I IN THE T H E R M A L N E U T R O N F I S S I O N OF 235U H.N. Erten D e p a r t m e n t of Chemistry, B i l k e n t University~ Bilkent, Ankara, T u r k e y R e c e i v e d 13 J u l y 1992 A c c e p t e d 27 July 1992
Isomer y i e l d ratios of 130Sb, 132Sb, 134I and 136I isomers formed in the t h e r m a l n e u t r o n fission of 235U have been c a l c u l a t e d from our previous expe- rimental studies that led to the i d e n t i f i c a t i o n of these species. In those studies the iodine and an- timony fractions formed in fission w e r e r a p i d l y sep- a r a t e d and the decay of y - r a y s b e l o n g i n g to each isomer pair were followed using Ge(Li) detectors and a m u l t i c h a n n e l analyzer. The isomer ratios w e r e c a l c u l a t e d from g r o w t h and decay c o n s i d e r a t i o n s of these y-rays. The results are c o m p a r e d w i t h the re- cently p u b l i s h e d values o b t a i n e d w i t h an on-line isotope separator, those from LOHENGRIN, and those from model calculations. A n g u l a r m o m e n t a of fission fragments c o r r e s p o n d i n g to the m e a s u r e d isomer yields have also been calculated.
I N T R O D U C T I O N
Isomer y i e l d ratio m e a s u r e m e n t s in fission provide in- formation about the intrinsic a n g u l a r m o m e n t u m and scis- sion point d e f o r m a t i o n of fission fragments. Several iso- mer r a t i o m e a s u r e m e n t s w e r e c a r r i e d out for m e d i u m and
ERTEN: ISOMERIC YIELDS OF 13~ 132Sb, 134I AND 136I
h i g h e n e r g y f i s s i o n I-II and t h e r m a l n e u t r o n fission 12-16. F u r t h e r m o r e , an e x t e n s i v e study of f i s s i o n p r o d u c t
y i e l d s in the t h e r m a l n e u t r o n fission of 235U u s i n g an o n - l i n e isotope s e p a r a t o r has b e e n r e c e n t l y p u b l i s h e d by R u d s t a m et al. 17. M a d l a n d and E n g l a n d 18 u s e d a simple s t a t i s t i c a l m o d e l for c a l c u l a t i n g isomer y i e l d ratios of p r o d u c t s formed in n e u t r o n i n d u c e d fission.
In this w o r k we r e p o r t the isomer y i e l d s of 40-min and 6.5-min 130Sb, 2.8-min and 4.2-min 132Sb, 3.5-min and 5 2 . 6 - m i n 134I and 46-sec and 83-sec 136I isomers, e x t r a c t e d from our p r e v i o u s e x p e r i m e n t a l studies, w h i c h led to the i d e n t i f i c a t i o n and c h a r a c t e r i z a t i o n of these
19-22
species . The r e s u l t s are c o m p a r e d w i t h r e c e n t ex- p e r i m e n t a l r e s u l t s and those from m o d e l c a l c u l a t i o n s .
E X P E R I M E N T A L
In our a f o r e m e n t i o n e d studies, iodine and a n t i m o n y species w e r e r a p i d l y s e p a r a t e d from the t h e r m a l neu- tron fission p r o d u c t s u s i n g r a d i o c h e m i c a l techniques. T i p i c a l l y 2.0 mg of 93.5% e n r i c h e d 235U samples w e r e i r r a d i a t e d for 15 sec in a flux of 2x1013 n . c m -2.
-I
sec . C o u n t i n g was s t a r t e d about 1-2 min after ir- radiation.
The T-ray s p e c t r a of iodine and a n t i m o n y samples w e r e o b s e r v e d w i t h 18-45 cm 3 Ge(Li) d e t e c t o r s whose F W H M v a l u e s for the 6 6 2 - k e V T-ray of 137Cs w e r e ~2.8 keY. They w e r e u s e d in c o n j u n c t i o n w i t h a 4 0 9 6 - c h a n n e l analyzer.
The ratio of the i n d e p e n d e n t y i e l d s of the 130Sb, 132Sb, 134I and 136I isomers w e r e d e t e r m i n e d by follow- ing the d e c a y of the 793-keV, 697.4-keY, 8 4 7 - k e V and the 1313.3 k e Y y-rays a r i s i n g from the decay of b o t h isomers in e a c h case, r e s p e c t i v e l y , and by u s i n g ap-
ERTEN: ISOMERIC YIELDS OF 13~ 132Sb, 134I AND 136I
TABLE 1
Independent vields (IY) of antimony and iodine species in 235U(nth,f) d e t e r m i n e d in this work
17 Nuclide IY (%) (This work) R u d s t a m et al.
130gSb 0.19• 0.204• 130mSb 0.29• 132gSb 0.68• 0.34• 132mSb 1.40• 1.32• 134gi 0.36• 1.2 • 134mi 0.50• 0.28• 136gi 1.72• 0.29• 136mi 2.25• 1.09•
propriate growth and decay equations and the known decay 19-22
schemes of the isomers The isomer ratios as well as individual independent yields were calculated.
RESULTS AND D I S C U S S I O N
The results of the independent yield m e a s u r e m e n t s of antimony and iodine species in the thermal neutron fis- sion of 235U are given in Table i.
Also given are the experimental results of Rudstam 17
et al. obtained with an on-line isotope separator. For the antimony species the radiochemical and the instru- m e n t a l methods g a v e comparable results, whereas for the
ERTEN: ISOMERIC YIELDS OF 13~ 132Sb, 134I AND 1~6I '~ I E c "6 e- 9 ~6 0 10-I - t 0 1 ' Z - Z p Fig. i. E x p e r i m e n t a l f r a c t i o n a l i n d e p e n d e n t y i e l d s from t h i s w o r k (full, c i r c l e s ) , t h o s e f r o m R u d s t a m et al. 17 (open circles) a n d the n o r m a l c h a r g e d i s t r i b u t i o n c u r v e f r o m s y s t e m a t i c s u s i n g the e x t e n d e d Zp m o d e l of W a h l 2 3 w i t h ~ = 0 . 5 3 1 a n d e v e n - o d d n e u t r o n and p r o t o n f a c t o r z set as I
T h e e x p e r i m e n t a l f r a c t i o n a l i n d e p e n d e n t y i e l d d a t a are p l o t t e d in Fig. 1 as a f u n c t i o n of Z-Zp w h e r e the m o s t p r o b a b l e c h a r g e , Zp, v a l u e s w e r e t a k e n from the r e s u l t s of the e x t e n d e d Zp m o d e l of W a h l 23. The c u r v e s h o w n r e p r e s e n t s a n o r m a l G a u s s i a n c h a r g e d i s p e r s i o n w i t h w i d t h p a r a m e t e r ~Z = 0.531, w i t h o u t o d d - e v e n pro- ton a n d n e u t r o n e f f e c t m o d u l a t i o n . T h e n e g a t i v e d e v i a t i o n s of the e x p e r i m e n t a l p o i n t s of t h i s w o r k f r o m the n o r m a l c u r v e r e s u l t from the odd-
ERTEN: ISOMERIC YIELDS OF 13~ 132Sb, '34I AND 136I
o d d n a t u r e of the species studied. The o d d - o d d f a c t o r F(A) for the n u c l i d e s of i n t e r e s t is thus e s t i m a t e d as 0.73.
The e x p e r i m e n t a l isomer y i e l d ratios from this work, 17
from R u d s t a m et al. as w e l l as those c a l c u l a t e d u s i n g the M a d l a n d and E n g l a n d 18 m o d e l are given in Table 2. A l s o given are the v a l u e s o b t a i n e d by the m a s s s e p a r a t o r L O H E N G R I N at the m e a n k i n e t i c e n e r g y of the fragments 24
G e n e r a l l y w i t h i n e x p e r i m e n t a l errors, our r e s u l t s are c o n s i s t e n t w i t h those o b t a i n e d from m o d e l c a l c u l a t i o n s . The r e s u l t s u s i n g L O H E N G R I N w i t h the p o s s i b l e e x c e p t i o n of 134I isomers, are also in a c c o r d a n c e w i t h our r a d i o - c h e m i c a l m e a s u r e m e n t s . No a g r e e m e n t seems to e x i s t how- ever w i t h the v a l u e s of R u d s t a m et a117. Their q u i t e large u n c e r t a i n t i e s p r o b a b l y arise from the e x p e r i m e n t a l t e c h n i q u e employed.
The i s o m e r i c y i e l d ratios given in Table 2 w e r e con- v e r t e d into f r a g m e n t a n g u l a r m o m e n t a (Jrms) u s i n g a g a i n the M a d l a n d and E n g l a n d m o d e l 18. In this m o d e l the ini- tial a n g u l a r m o m e n t u m d i s t r i b u t i o n is a s s u m e d to be of the form
P(J) ~ (2J + l)exp J (J+l) B 2
Here B ~ Jrms" The m o d e l is simple, w i t h no c o r r e c t i o n s for p r o m p t n e u t r o n e m i s s i o n or for c a s c a d e y-emission. The r e s u l t s o b t a i n e d are given in Table 3.
The a v e r a g e Jrms o b t a i n e d from v a l u e s g i v e n in T a b l e 3 is 7.6. This a n g u l a r m o m e n t u m is in v e r y g o o d a g r e e - m e n t w i t h the value of 7.5 a s s u m e d for all t h e r m a l neu- tron fission p r o d u c t s in 235U fission by M a d l a n d and E n g l a n d !8. F u r t h e r m o r e , the r a t h e r h i g h v a l u e s are an i n d i c a t i o n of a p p r e c i a b l e d e f o r m a t i o n of fragments at scission.
E R T E N : I S O M E R I C Y I E L D S O F 13~ ~32Sb, 1341 A N D 1361 0 E~ 0~ ..c, -,.-I " 0 O ~ u,4c~ c~ ~ 0 O 0 u,~ .,.4 O ~ o 0J " 0 " o ~0 " 0 " 0 ,-I ,-I H L~E Z 0 4 a , - m a ~ a m ~ 9 ,-i ~. ..el ( I~ -,-I 0 i~1 H ,,~ ~o ,,~ o D.~ o~ i "~ co o r ~-q tl)
d
g
g
+1 +1 +1~ d
d
O~ O O t'~ +1 +1 +1 I r O'~ r 00 i 9 t/'l +1 +1 +F +1 kO ',.D r-.q '.~ '~P oo ' ~ r co ,,~ ~ kD ,.Q ,-Q r.,q I--t H o , c"q ,,~ k~ERTEN: ISOMERIC YIELDS OF la~ 132Sb, Ia4I AND 1~I
T A B L E 3
Isomeric yield ratios and c o r r e s p o n d i n g angular m o m e n t a from this w o r k
Isomer pair Spin values
i i (~) m g Ym/Yg Jrms 130Sb 8 4 0.66• 7.5• 132Sb 4 8 2.1 • 6.9• 134I 8 4 1.4 • 9.5• 136I 6 2 1.3 • 6.6• REFERENCES
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ERTEN: ISOMERIC YIELDS
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