Systematics of plutonium fission isomers

Volume 3113, number 8

SYSTEMATICS

PHYSICS L E T T E R S

OF

PLUTONIUM

13 April 1970

FISSION

ISOMERS*

S. C. B U R N E T T $, H. C. B R I T T , B. H. E R K K I L A and W. E. S T E I N Los Alamos Scientific Laboratory. Uniz,e~'sily of California Los A l a ~ o s . New Mexico 87544. USA Received

6 March

1970

Half-lives, thresholds and isomer ratios of 235,237-240pure fission isomers are given. Systematics indicate that 236,238Pu m are anomalous. The measured thresholds of 239. 240pure are larger than those deduced from (n, f) data.

F i s s i o n i s o m e r s of p l u t o n i u m i s o t o p e s h a v e b e e n s t u d i e d by m e a n s of s - p a r t i c l e b o m b a r d m e n t of u r a n i u m t a r g e t s . R e s u l t s i n c l u d e h a l f l i v e s , r a t i o s of the n u m b e r of i s o m e r s to p r o m p t f i s s i o n s as a f u n c t i o n of o r - p a r t i c l e e n e r g y , and t h r e s h o l d s f o r p r o d u c t i o n of 2 3 5 p u r e , 23~7pu m, 23Spurn, 2 3 9 p u m and 2 4 0 p u m i s o m e r s . An u p p e r l i m i t f o r p r o d u c t i o n of the 2 3 6 p u m i s o m e r is a l s o obtained. S y s t e m a t i c s of t h e s e data t o g e t h e r w i t h the p u b l i s h e d [1] h a l f - l i f e and t h r e s h o l d of 2 3 6 p u m i n d i c a t e that 2 3 6 p u m and 2 3 8 p u m a r e a n o m a l o u s . M e a s u r e d t h r e s h o l d s of 2 3 9 p u m and 2 4 0 p u m a r e , r e s p e c t i v e l y , 0.9 MeV and 0.6 MeV l a r g e r than the i s o m e r e x c i t a t i o n s d e d u c e d f r o m (n, f) r e s o n a n c e s s t r u c t u r e by L y n n [2]. T h e b e a m f r o m the v a r i a b l e - e n e r g y c y c l o t r o n t r a v e r s e d t h r e e t a r g e t s in t a n d e m e a c h v i e w e d by two s e m i c o n d u c t o r d e t e c t o r s . F i s s i o n e v e n t s w e r e i d e n t i f i e d by a c o i n c i d e n c e b e t w e e n t h o s e f r a g m e n t p u l s e s w h i c h e x c e e d e d a l o w e r bound s e t to d i s c r i m i n a t e a g a i n s t s c a t t e r e d b e a m p a r t i c l e s . P r o m p t and d e l a y e d i s o m e r i c f i s s i o n s w e r e i d e n t i f i e d by the t i m e r e l a t i o n s h i p b e t w e e n t h e c o i n c i d e n t f r a g m e n t p u l s e s and a s i g n a l f r o m t h e c y c l o t r o n o s c i l l a t o r . A 232Th s a m p l e in one of t h e t a r g e t p o s i t i o n s p r o v i d e d s i m u l t a n e o u s m o n i t o r i n g of the p r o m p t - f i s s i o n t i m e s p e c t r u m . Negligible background from neutron-induced fiss i o n was a t t a i n e d by c a r e f u l b e a m c o l l i m a t i o n and a d e q u a t e n e u t r o n s h i e l d i n g . E x a m p l e s of o b s e r v e d d e c a y c u r v e s c o r r e c t e d f o r the c o n t r i b u t i o n f r o m p r o m p t f i s s i o n s w h i c h o c c u r r e d in the t a i l of t h e b e a m p u l s e a r e shown * Work performed under the auspices of the U.S. Atomic Energy Commission. :~ Present address: Savannah River Laboratory, Aiken, South Carolina.

E o = 2 4 . 6 MeV 4

30 ~ - ' - ~ ' - - % ~ % F - ~ . $ - ~ [

~

z35 u

--

120

6.5,¢o

,-u 20

40 60 T I M E DELAY (n$)

80

Fig. 1. Representative corrected decay curves for the indicated targets. Corrections for the tail of the beam pulse were obtained from the 232Th results. Solid lines are least-square fits to the corrected data with the indicated hlaf-livcs. in fig. 1. C o r r e c t i o n s w e r e d e r i v e d f r o m the 232Th data t a k e n c o n c u r r e n t l y . R e s u l t s f o r 233,234,235 U t a r g e t s a r e w e l l - r e p r e s e n t e d by a s i n g l e c o m p o n e n t , w h e r e a s the d e c a y f o r 236,238L1 t a r g e t s is c o m p l e x . H a l f - l i v e s d e t e r m i n e d by l e a s t - s q u a r e s f i t s to one o r two c o m p o n e n t s a r e g i v e n in t a b l e 1. T e s t s of t h e 238U d e c a y c u r v e s f o r a p o s s i b l e 29 ns c o m p o n e n t [3] g a v e an u p p e r l i m i t of 2% of t h e 3.8 ns yield. H a l f - l i v e s f r o m t h i s w o r k a r e in r e a s o n a b l e a g r e e m e n t with p r e v i o u s l y p u b l i s h e d d a t a [1, 4, 5], R a t i o s of t h e n u m b e r of i s o m e r - t o - p r o m p t 523

PHYSICS

Volume 31B, n u m b e r 8

LETTERS

13 April 1970

s t a t e c r o s s s e c t i o n s v i a F n / F f s y s t e m a t i c s [8]. For 240pum a corresponding value was obtained f r o m t h e ((~, 2n) c r o s s s e c t i o n of W i n g et al. [9] and the fission cross section measured by Huiz e n g a et al. [10]. In t h e f o l l o w i n g d i s c u s s i o n it i s a s s u m e d t h a t 2 4 0 P u m a n d t h e o d d - m a s s i s o m e r s of p l u t o n i u m are typical and represent the lowest state in the s e c o n d m i n i m u m of t h e f i s s i o n b a r r i e r . A s o b s e r v e d in t h e a m e r i c i u m i s o t o p e s , o d d - N p l u t o n i u m i s o m e r s a r e e x p e c t e d to h a v e h a l f - l i v e s a b o u t t h r e e o r d e r s of m a g n i t u d e l o n g e r t h a n t h o s e f o r t h e e v e n - N i s o m e r s b e c a u s e of t h e i n creased fission barrier associated with the spec i a l i z a t i o n e n e r g y of t h e u n p a i r e d n e u t r o n . C o m p a r i s o n of t h e t a b u l a t e d v a l u e s i n d i c a t e s t h e f o l l o w i n g a n o m a l i e s of 2 3 6 p u m a n d 2 3 8 p u r e : 1) h a l f - l i v e s a r e l o n g e r t h a n e x p e c t e d f o r t h e s e e v e n - e v e n i s o m e r s ; 2) t h r e s h o l d s a r e a p p r o x i mately one MeV higher than neighbouring iso-

fissions were determined for each half-life by d i v i s i o n of t h e i n t e g r a l of t h e d e c a y c u r v e b y t h e n u m b e r of p r o m p t f i s s i o n s . R e s u l t a n t r a t i o s a s a f u n c t i o n of i n c i d e n t a - p a r t i c l e e n e r g y a r e s h o w n in figs. 2 a n d 3. I s o t o p i c a s s i g n m e n t s of the isomers were determined from comparison w i t h t y p i c a l ( a , x n ) r e a c t i o n c r o s s s e c t i o n s [6] a n d e n e r g e t i c s . Low y i e l d s a n d t h e s i m i l a r i t y of t h e h a l f - l i v e s f o r (~, n) a n d ( a , 2 n ) r e a c t i o n s p r e c l u d e d t h e a s s i g n m e n t of i s o m e r s f r o m t h e 234U target. Calculated excitation curves were obt a i n e d f r o m t h e n e u t r o n e v a p o r a t i o n m o d e l of J a c k s o n [7]. B e s t f i t s w e r e o b t a i n e d w i t h a t e m p e r a t u r e ~ = 0.6 M e V , a l t h o u g h a d e q u a t e f i t s to t h e l o w - e n e r g y s i d e of t h e e x c i t a t i o n f u n c t i o n s w e r e o b t a i n e d w i t h 0 = 1 MeV. T h r e s h o l d u n c e r t a i n t i e s g i v e n in t a b l e 1 e n c o m p a s s t h e t e m p e r a t u r e r a n g e 0.4 to 1 MeV. I s o m e r - t o - p r o m p t y i e l d s at t h e p e a k of t h e e x c i t a t i o n f u n c t i o n s h a v e b e e n c o n v e r t e d to r a t i o s of i s o m e r to g r o u n d

l

l

1

I

Z3~U (e, 2hi 237mPu

z33U( a, 2n) z3s"l=. I01

,

I

T- 120~5Ons-

T- 30±Sns i0 i

--

/.-s.4..v-

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\.

.,,, ,oo

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l/

0

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I

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I0"

I

I

I

l

I

,

[

,

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i

ZNUll,

T, 6.5~1.5ns

2nlz40"Pv

T • 3.8±0.3ns

o

-- I0'

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I ~

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20 (MeV)

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28

Fig. 2. I s o m e r - t o - p r o m p t excitation functions for (Ot, 2n) reactions. Dashed and solid lines are, respectively, the calculated ground state and i s o m e r excitation functions for 8 = 0.6 MeV. 524

Volume 31B. number 8

PHYSICS

LETTERS

13 April 1970

Table 1 Experimental r e s u l t s . Isomer

Half-life (ns)

240pure

Threshold (MeV)

Peak isomer/prompt (× 10 -6)

Crisomer (~gr ound state (×10 -4 )

3.8 ± 0.3

3.0 ± 0.2

135 ± 40

3.6 ~: 1 3 . 8 ~ 1 a)

20 ~ 2

1.7 J:O.2

239pum

> 100

3.0 ~ 0.2

238pure

6.5 ± 1.5

4.4 ± 0.4

6 ± 2

0.5 ± 0.2

237pum

120 ± 50

3.4 ± 0.2

19 ± 2

2.5 ± 0.3

236pure

34 ± 8 [1]

4.1 ± 0.2 [1]

235pure 234um

30 -L 5 __

<2

< 0.5

3,0 ± 0.2

4.8 ± 0.5

2.1 ± 0.2

--

< 0.2 b)

< 0.004 b)

a) Value obtained from peak i s o m e r / p r o m p t . Crground state [8], and (~prompt [9]. b) Values obtained with the assumption that T1/2 > 20 ns. m e r s ; 3) r a t i o s of i s o m e r to g r o u n d s t a t e c r o s s s e c t i o n s a r e a b o u t an o r d e r of m a g n i t u d e l o w e r t h a n t y p i c a l v a l u e s . T h e a n o m a l y of 2 3 6 p u m w a s r e c o g n i z e d b y V a n d e n b o s c h a n d W o l f [4] a n d w a s a t t r i b u t e d to t h e e x i s t e n c e of a t w o - q u a s i p a r t i c l e s t a t e in t h e s e c o n d p o t e n t i a l w e l l . O u r n e w d a t a

"

!

234U(a'xn)

I T • I00 n|

0.4

~

4.0

,

~0

'

'

"

'

'

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~~'~L~",

' •

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•

/ Z3SU(m,nl 23te,p~

1.0 0.4

I

I

I

I

I

I

I

I

l

~20.0 I0.0-5.0

T..

1

'3sff(e,3n) ' ~ l " Pu\ ~

/'

. -. Z3,U(,,n ) . . . . Pul

j r~-3.0 MeV~

r

1.0 0.5

/ i

I i

i

ALPHA PARTICLE ENERGY (MeV)

F i g . 3. I s o m e r - t o - p r o m p t e x c i t a t i o n f u n c t i o n s f o r the indicated reactions. Circles and squares are observed d a t a . T r i a n g l e s a r e t h e (ot, 3n) d a t a c o r r e c t e d f o r the (or, n) c o n t r i b u t i o n . C a l c u l a t e d {or, 3n) e x c i t a t i o n f u n c t i o n s f o r 0 = 0.6 M e V a r c s h o w n . S t r a i g h t l i n e s w e r e determined from a least-squares f i t to t h e 2 3 8 U ( o t , n) 2 4 1 p u m d a t a a n d a d j u s t e d to r e p r e s e n t t h e s a m e i n i t i a l compound nucleus excitation energy.

show that 238pum is s i m i l a r l y a n o m a l o u s and i n d i c a t e a p a r i n g gap of 1 to 1.4 M e V , w h i c h i s s i m i l a r to t h e v a l u e 2A ~ 1.2 M e V c h a r a c t e r i s t i c of t h e f i r s t p o t e n t i a l w e l l . F o r 239pu m and 240pum, this work p r o v i d e s measured thresholds for comparison with those d e d u c e d f r o m (n, f) r e s o n a n c e s t r u c t u r e . T h e s e (n, f) c a l c u l a t i o n s [2] w e r e b a s e d on a F e r m i - g a s l e v e l d e n s i t y f u n c t i o n c o r r e c t e d f o r p a i r i n g and a s s u m e d t h a t t h e s a m e p a r a m e t e r s p e r t a i n to b o t h p o t e n t i a l w e l l s . In t h e m o s t r e l i a b l e , s u b b a r r i e r c a s e of 2 3 8 p u ( n , f), t h e c a l c u l a t e d i s o m e r e x c i t a t i o n i s a b o u t 0.9 M e V b e l o w t h e m e a s u r e d value. F o r 239pu(n,f), the c a l c u l a t e d t h r e s h o l d i s a l s o l o w e r by a b o u t 0.6 MeV. T h e s e discrepancies indicate that different parameters a r e p r o b a b l y r e q u i r e d f o r t h e two p o t e n t i a l w e l l s in o r d e r to d e d u c e p r o p e r i s o m e r e x c i t a t i o n e n e r g i e s f r o m (n, f) data. 1. N. Lark, G. Sletten, J. P e d e r s e n and S. Bjc¢rnholm, Nucl. Phys. A139 (1969) 481. 2. E. Lynn, 2nd Symp. on Physics and c h e m i s t r y of fission, Vienna, 1969 (IAEA,Vienna, 1969)p. 249. 3. A . J . Elwyn and A. T. G. Ferguson, ibid. p. 457. 4. R. Vandenbosch and K. L. Wolf, ibid. p. 439; K. L. Wolf, T h e s i s , U. of Washington, 1969 (unpublished). 5. V. Metag,R. Repnow, P. yon Brentano and J. D. Fox, 2nd Syrup. on Physics and c h e m i s t r y of fission, Vienna, 1969 (IAEA, Vienna, 1969) p. 449; and Z. Phys. 226 (1969) 1. 6. R. Vandenboseh et al., Phys. Rev. 111 (1958) 1358 7. J. D. Jackson, Can. J. Phys. 34 (1956) 767. 8. J . R . Huizenga and R. Vandenbosch, Nuclear R e actions, eds. P. M. Endt and P. B. Smith (NorthHolland, A m s t e r d a m , 1962) Vol. II, p. 42. 9. J. Wing, W. L. R a m l e r , A. L. Harkness and J. R. Huizenga, Phys. Rev. 114 (1959) 163. 10. J . R . Huizenga, R. Vandenbosch and H. Warhanek, Phys. Rev. 124 (1961) 1964. 525