APPLICATIONS OF NUCLEAR DATA ON SHORT-LIVED FISSION ...

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from the decay data and abundances of these fission ... data: half-lives, neutron branching ratios, and ... stability. A common f e a t u r e f o r a l l t h e a p p l i c a -.
APPLICATIONS G. The

Rudstam, Studsvik

S-611

82

OF P.

NUCLEAR Aagaard,

Science

Nyköping,

DATA K.

ON S H O R T - L I V E D

Aleklett,

Research

and

E.

FISSION

Laboratory,

Sweden experiments the energy spectrum of the antin e u t r i n o s must be known.. The o r i g i n o f the antineutrinos i s the beta decay of the f i s s i o n p r o d u c t s i n the f u e l w h i c h means t h a t the energy spectrum can be e v a l u a t e d from the decay data and abundances of these f i s s i o n products. In particular, the data on those products with high Upvalues are of interest since i t i s these n u c l i d e s which are r e s p o n s i b l e f o r the important high-energy part of the a n t i neutrinos .

Abstract The s t u d y o f s h o r t - l i v e d f i s s i o n p r o ducts g i v e s i n f o r m a t i o n about the nuclear s t r u c t u r e on the n e u t r o n - r i c h side of s t a bility. The d a t a a r e a l s o o f i n t e r e s t for various a p p l i c a t i o n s both to basic science and to nuclear technology. Some o f t h e s e a p p l i c a t i o n s , t a k e n up by the O S I R I S group at Studsvik, are described in the present contribution. 1.

PRODUCTS

Lund

Another f i e l d of application of decay data of strongly neutron-rich nuclides i s the theory of nucleosynthesis. The r a p i d process of element b u i l d - u p , for example, follows a path displaced far out on the neutron-rich side of stability. The n u c l e i a c t u a l l y t a k i n g p a r t i n the process are almost entirely out of reach for experimental techniques of today. Nevertheless, a detailed knowledge about the decay p r o p e r t i e s of the most neutron-rich nuclides available for study forms an important b a s i s for e x t r a p o l a t i o n s t o r e g i o n s s t i l l f u r t h e r away f r o m stability.

Introduction

By now, many a s p e c t s o f t h e p r o p e r t i e s of n u c l e i far from s t a b i l i t y have been s t u d i e d i n g r e a t d e t a i l , and a m u l t i tude of experimental r e s u l t s have evolved. Apart from the inherent i n t e r e s t of these r e s u l t s for the understanding of nuclear s t r u c t u r e i n these r e g i o n s of the n u c l i d i c chart, the data c o l l e c t e d are also very v a l u a b l e f o r other branches of science and technology. The aim o f the p r e s e n t c o n t r i b u t i o n i s t o show how d a t a f o r short-lived f i s s i o n p r o d u c t s f i n d t h e i r way i n t o o t h e r f i e l d s , both fundamental and t e c h n i c a l ones.

A common f e a t u r e f o r a l l t h e a p p l i c a tions mentioned here i s that e f f o r t s are required which systematically explore wide ranges of n u c l i d e s .

Many o f the a p p l i c a t i o n s o f a t e c h n i c a l n a t u r e a r e c o n n e c t e d i n one way o r another to the problems a r i s i n g from the use of n u c l e a r power. Quite g e n e r a l l y one may s t a t e t h a t a n e x t e n s i v e k n o w l e d g e a b o u t the p r o p e r t i e s of a l l the f i s s i o n p r o d u c t s , the s h o r t - l i v e d ones included, i s indispensable from the simple fact that these r a d i o nuclei are produced in large quantities in the n u c l e a r power s t a t i o n s . An obvious r e q u i r e m e n t o u g h t t o be t h a t t h e p r o p e r t i e s o f t h i s waste from the power p r o d u c t i o n s h o u l d be known i n d e t a i l . Apart f r o m t h i s a s p e c t a few s p e c i f i c f i e l d s may be m e n t i o n e d where t h e O S I R I S g r o u p a t S t u d s v i k i s i n v o l v e d , namely

2.

decay

heat

problem

The i m p o r t a n c e o f k n o w i n g t h e h e a t developed i n n u c l e a r f u e l elements by d e caying fission products is very great. The r e a s o n f o r t h i s i s o b v i o u s . It is p o s s i b l e to stop the f i s s i o n process in a reactor rapidly, but i t i s impossible to stop the decay of the f i s s i o n products. The power d e v e l o p e d i n t h i s d e c a y i s considerable amounting to about 7 % of the t o t a l r e a c t o r power at e q u i l i b r i u m . This r e p r e s e n t s a v e r y l a r g e power o u t p u t , and in case of a loss-of-coolant accident it might well melt the fuel with a d i s a s t rous evolution of r a d i o a c t i v i t y as a r e s u l t u n l e s s emergency c o o l i n g i s quickly put into force. The knowledge a b o u t t h e decay power i s i n d i s p e n s a b l e b o t h f o r judging the r e s u l t s of s e r i o u s accidents and f o r d e s i g n i n g emergency c o o l i n g systems. The economic impact i s a l s o great because the heat generated w i l l , at the e n d , d e t e r m i n e t h e maximum r u n n i n g power permissible for a given reactor type.

the e v a l u a t i o n o f the power d e v e l o p e d i n n u c l e a r f u e l by decaying fission products ( e s s e n t i a l data: decay p r o perties, especially half-lives and a v e r a g e b e t a a n d gamma e n e r g i e s , fission yields); the e f f e c t i v e delayed-neutron energy spectrum in nuclear fuel (essential data: h a l f - l i v e s , neutron branching r a t i o s , and neutron energy spectra of individual delayed-neutron precursors, fission yields); the independent-yield pattern of f i s s i o n products (essential data: halfl i v e s , presence of isomers, branching r a t i o s of gamma-rays used i n the measurements of abundances).

The decay power can be d e t e r m i n e d i n t e g r a l l y , but such determinations have the weakness that the r e s u l t s are strictly v a l i d only for the experimental conditions used during their determination. A more g e n e r a l method i s the summation method where the c o n t r i b u t i o n s from the i n d i v i dual f i s s i o n products are added together, weighted according to the abundances of the f i s s i o n products i n the f u e l . It is here where the knowledge about n u c l e a r

There are also f i e l d s within basic s c i e n c e where the knowledge about the p r o perties of short-lived fission products is of great value. One s u c h f i e l d i s related to experiments u s i n g a n t i n e u t r i n o s from reactors. For the i n t e r p r e t a t i o n of these

-

The

696

-

properties we n e e d t o i)

comes know

in.

the h a l f - l i v e s of cerned (including

For

these

all nuclides conlong-lived isomers),

ii)

the branching to d i f f e r e n t isomeric s t a t e s i n the daughter, and branching to neutrons for delayed neutron p r e cursors ,

iii)

the

average

beta

U s i n g t h e gamma m e a s u r e m e n t s o f typ i c a l peaks to determine the r e l a t i v e abundances of the d i f f e r e n t i s o b a r s i n the samples, the set of beta s p e c t r a , correct e d f o r gamma e f f e c t a n d b a c k g r o u n d , could be d e c o m p o s e d i n t o i s o b a r i c components. These i n t u r n , were t r a n s f o r m e d into electron energy spectra using the response function of the beta spectrometer and, f i n a l l y , the average beta e n e r g i e s were deduced.

summations

and

gamma

energies.

I n a d d i t i o n , the f i s s i o n y i e l d s and neutron capture c r o s s s e c t i o n s are needed for c a l c u l a t i n g the abundances. A s s o o n a s we have t h i s set of data at hand, however, t h e d e c a y h e a t c a n be e v a l u a t e d f o r a n y i r r a d i a t i o n c o n d i t i o n s and f u e l c o m p o s i tion. I n the e v a l u a t i o n of the decay heat the s h o r t - l i v e d f i s s i o n products have a special importance. They have u s u a l l y a large total disintegration energy, and many o f them a r e s i t u a t e d c l o s e t o t h e peak of the charge d i s p e r s i o n c u r v e s . Cons e q u e n t l y , t h e y w i l l be r e s p o n s i b l e f o r a l a r g e c o n t r i b u t i o n to the decay heat d u r i n g a l i m i t e d time a f t e r the shutting down o f a r e a c t o r . 2.1

Average

beta

energies

I t i s p o s s i b l e to evaluate the a v e r a g e b e t a e n e r g y from the decay scheme o f a nuclide provided that a l l beta branches, and shapes of these branches, are known. This situation i s not very frequent. A more s t r a i g h t f o r w a r d method i s to measure the g r o s s beta spectrum from as low an e n e r g y as p o s s i b l e up to the e n d - p o i n t , and then to evaluate the average energy o f t h i s s p e c t r u m w i t h due a c c o u n t taken of the p a r t f a l l i n g below the low-energy experimental limit. T h i s method i s independent of any assumptions c o n c e r n i n g , for i n s t a n c e , the k i n d of beta transitions taking place. T h i s has been done a t O S I R I S I ) u s i n g the f o l l o w i n g computerc o n t r o l l e d sequence of steps a)

c o l l e c t i o n o f the beam c o r r e s p o n ding to the d e s i r e d mass f o r a predetermined time,

b)

movement o f the s a m p l e suring position,

c)

simultaneous recording of beta a n d gamma s p e c t r a f o r a p r e d e t e r mined time with a Si(Li)-detector system for the beta p a r t i c l e s and a G e ( L i ) - d e t e c t o r f o r t h e gammarays , for

to

a

a predetermined

mea-

d)

waiting

time,

e)

repetition of steps (c) a n d (d) a g i v e n number o f t i m e s , normally with d i f f e r e n t measuring and waiting periods,

f)

r e p e t i t i o n o f (a) - (e) a g i v e n number o f t i m e s t o a c q u i r e c o u n ting statistics,

g)

r e p e t i t i o n o f (a) - ( f ) w i t h a n aluminium absorber placed between the sample and the beta detector t o m e a s u r e t h e gamma e f f e c t in this spectrometer.

The method d e s c r i b e d h a s been u s e d t o d e t e r m i n e t h e a v e r a g e b e t a e n e r g y o f 36 fission products. This is only a small p a r t of the a c t u a l l y known r a d i o a c t i v e f i s s i o n products, which i s about 400, but a s many o f t h e m e a s u r e d o n e s b e l o n g t o t h e most important f i s s i o n p r o d u c t s their combined c o n t r i b u t i o n to the t o t a l beta e f f e c t i s a b o u t 30 % a t c o o l i n g times between 0 and 100 s e c o n d s a f t e r stopping a reactor. F o r a f u r t h e r s e t o f 39 n u c l i d e s r e s u l t s from an e a r l i e r beta s t r e n g t h s t u d y c a r r i e d o u t a t O S I R I S.'2' ) have been used to e v a l u a t e the average b e ta energy. Apart from these f i s s i o n p r o ducts and those w i t h known decay schemes from w h i c h the a v e r a g e b e t a e n e r g y c a n be d e t e r m i n e d , t h e r e s t i l l e x i s t a b o u t 130 n u c l i d e s known t o be f i s s i o n p r o d u c t s but f o r w h i c h no measurements e x i s t . Approximate v a l u e s o f a v e r a g e b e t a e n e r g i e s for these n u c l i d e s have been obtained u s i n g the assumption t h a t the beta s t r e n g t h is p r o p o r t i o n a l to the l e v e l density ). 2

Putting a l l data from the four groups of f i s s i o n products - those with directly measured average e n e r g i e s , those with energies evaluated from decay schemes, those with energies deduced from beta s t r e n g t h measurements, and those w i t h energies determined by the extrapolation procedure - t o g e t h e r and c o m b i n i n g them with other kinds of decay data and fission y i e l d s , we a r e n o w i n t h e p o s i t i o n t o d e termine the beta part of the decay heat for any i r r a d i a t i o n h i s t o r y and f u e l composition. An example i s g i v e n i n F i g . 1 where the d a t a can be compared w i t h r e s u l t s from an i n t e g r a l m e a s u r e m e n t ) . Apparently, the agreement i s excellent. A l s o t h e u n c e r t a i n t i e s o f t h e two t y p e s of determinations are very similar (in t h e c a s e o f t h e O S I R I S r e s u l t s we h a v e evaluated the u n c e r t a i n t i e s from the e r r o r s of the decay data and the fission yields). The f i g u r e a l s o shows results from a summation c a l c u l a t i o n u s i n g the ENDF/B-IV data l i b r a r y ) which are in less good agreement w i t h experiments. 3

4

I t may be o f i n t e r e s t t o n o t e that for long i r r a d i a t i o n s the average beta e n e r g y , a v e r a g e gamma e n e r g y , d e c a y c o n s t a n t s , and f i s s i o n y i e l d s g i v e similar c o n t r i b u t i o n s to the u n c e r t a i n t y of the decay heat v a l u e s . Of much l e s s importance are the e r r o r s of the d e l a y e d neutron branching r a t i o s and neutron capture cross sections^).

- 697 -

o

Fig.

1.

L_I_LJ

1

10°

'

I»I—I

10

i

i n — I

10

1

i

1

III

10

2

i

i ,

III

10*

A

COOLING TIME IN SECONDS

i

IM

The q u a n t i t y c o o l i n g time m u l t i p l i e d by b e t a power ( i n M e V / f i s s i o n x s) versus cooling time. Open c i r c l e s w i t h e r r o r s t a p l e s (± o n e s t a n d a r d d e v i a t i o n ) : OSIRIS work*) . Closed circles: Experimental r e s u l t s from 2 3 5 Ref. ). S o l i d c u r v e : Summation c a l c u l a t i o n u s i n g E N D F / B - I V ' ) . F u e l : U. 3

2.2

Average

gamma

3

4

energies

F o r t h e a v e r a g e gamma e n e r g y t h e s i t u a t i o n i s somewhat e a s i e r than f o r the c o r r e s p o n d i n g b e t a e n e r g y s i n c e we a r e n o t concerned with spectral shapes. In fact, w h a t i s n e e d e d i s a s e t o f gamma e n e r g i e s and t h e i r a b s o l u t e b r a n c h i n g r a t i o s . If one wants to c o u n t the e f f e c t o f convers i o n e l e c t r o n s and X - r a y s apart, convers i o n c o e f f i c i e n t s m u s t a l s o be k n o w n . One s o u r c e o f e r r o r may b e , h o w e v e r , t h a t o u r t a b l e o f gamma e n e r g i e s may n o t b e c o m p l e t e . R e f e r r i n g to the "Pandemonium" discussion ) we n o t e t h a t a s u b s t a n t i a l p a r t o f t h e g a m m a - r a y s may be m i s s i n g . I f t h e s e gammar a y s s h o u l d be g r o u p e d i n c e r t a i n e n e r g y r e g i o n s t h i s w i l l l e a d to an e r r o r of the a v e r a g e gamma e n e r g y , w h i c h i s h a r d t o estimate. Therefore, d i r e c t measurements o f t h e g r o s s gamma s p e c t r u m u s i n g a l o w r e s o l u t i o n spectrometer such as Nal should b e w o r t h w h i l e , a n d we a r e p l a n n i n g s u c h a project at Studsvik. The t e c h n i q u e w o u l d t h e n be s i m i l a r t o t h a t f o r t h e a v e r a g e beta energy.

science. N e v e r t h e l e s s s , we w o u l d l i k e to p o i n t o u t t w o f i e l d s w h i c h may p r o f i t from these measurements. In the first place, the average beta energy i s an e x c e l l e n t c h e c k - p o i n t f o r l e v e l scheme c o n s t r u c t o r s who h a v e t o r e a c h a r e s u l t which y i e l d s an average beta energy in agreement w i t h the d i r e c t l y measured one. Secondly, the average energies are useful for dynamic theories of nucleo-synthesis where the energy developed by r a d i o a c t i v e decay of the n u c l i d e s i n the "r-process" must be t a k e n i n t o a c c o u n t ) . In this c a s e e x t r a p o l a t i o n s have t o be u s e d a s few o f the v e r y n e u t r o n - r i c h nuclides t a k i n g p a r t i n the process are a v a i l a b l e for experiments. D i r e c t measurements of cases as f a r out on the n e u t r o n - r i c h side of s t a b i l i t y as possible w i l l provide valuable check-points for the e x t r a p o l a t i o n procedures or the t h e o r e t i c a l calculat i o n s which one has to r e l y upon.

Another d i f f i c u l t y with the average gamma e n e r g y i s t h a t t h e a b s o l u t e b r a n c h i n g r a t i o s a r e o f t e n much l e s s precise than the r e l a t i v e ones. T h e r e f o r e , an attempt to determine a c c u r a t e l y and on the absolute scale the branching r a t i o of one o f the gamma-rays i s o f t e n c a l l e d for.

3.1

6

8

3.

2

Applications within basic

of average science

energies

ratios

A check of the completeness of our d e l a y e d - n e u t r o n d a t a can be o b t a i n e d by comparing the neutron y i e l d expressed as number o f d e l a y e d n e u t r o n s o b t a i n e d p e r 10 f i s s i o n s w i t h recommended v a l u e s (from a number o f i n t e g r a l measurements). For

I t may seem s t r a n g e t h a t q u a n t i t i e s such as average v a l u e s of the beta and gamma e n e r g i e s e m i t t e d i n t h e d e c a y o f n u c l e i can have any importance for basic -

Branching

neutrons

I t i s w e l l known t h a t the d e l a y e d neutrons are very important for running a reactor. T h e i r number i s l e s s t h a n 1 % o f t h e a v e r a g e number o f n e u t r o n s emitted per f i s s i o n , but t h e i r time distribution i s e s s e n t i a l for c o n t r o l l i n g the reactor. C o n s e q u e n t l y , p r a c t i c a l l y a l l t h e d a t a we c o l l e c t on delayed neutrons from f i s s i o n products are of value for nuclear technology. At the present time there are 6 7 p r e c u r s o r s w i t h known h a l f - l i v e s and d e layed-neutron branching ratios. For about h a l f o f them, w i t h o n l y few e x c e p t i o n s inc l u d i n g a l l the important cases, i . e . those w i t h h i g h f i s s i o n y i e l d s and l a r g e branching r a t i o s , we a l s o k n o w t h e s h a p e o f t h e neutron energy spectrum.

I t may be n o t e d t h a t t h e r e s u l t s of the experimental beta s t r e n g t h s t u d y ) can be u s e d t o deduce v a l u e s o f the a v e r a g e gamma e n e r g y . F u r t h e r m o r e , t h e extrapolation procedure to unmeasured cases, used to evaluate beta energies, i s also applicable t o t h e gamma e n e r g i e s . Results from such determinations are found in a laboratory report?). 2.3

Delayed

4

698

-

235 U t h e r m a l f i s s i o n summing the n e u t r o n s u s i n g P - v a l u e s f r o m R e f . 9 ) a n d t h e new f i s s i o n - y i e l d evaluation ENDF/B-V ) gives a n e u t r o n y i e l d of 173±6 n/10 f. The r e commended v a l u e i s 1 6 2 . 1 ± 0 . 5 . For P u thermal f i s s i o n the corresponding values a r e 69±4 and 6 2 . 8 ± 3 . 8 . I n both cases the v a l u e s o b t a i n e d by summation of p r e c u r s o r s d a t a a r e a b o u t two s t a n d a r d d e v i a t i o n s h i g h e r t h a n the recommended v a l u e s . This discrepancy i s not very grave, still i t i n d i c a t e s t h a t some P - v a l u e s may be too h i g h . F o r most p r e c u r s o r s where P n values have been determined independently at s e v e r a l l a b o r a t o r i e s , the agreement i s now v e r y g o o d , b u t t h e r e a r e a f e w i m p o r t a n t c a s e s w i t h o n l y one p u b l i s h e d value. T h e s e c a s e s s h o u l d be c h e c k e d by independent measurements.

the missing spectra only correspond to a minor part of the delayed-neutron effect a t e q u i l i b r i u m , , e . g . 13 % i n t h e c a s e o f 235y t h e r m a l f i s s i o n . T h e r e f o r e , we can construct the composite spectrum, and i t s time v a r i a t i o n , from the known s p e c t r a , e x p e c t i n g i t to a p p r o x i mate w e l l the t r u e energy spectrum except perhaps at v e r y low and v e r y h i g h energy when the e x p e r i m e n t a l d a t a a r e l e s s complete.

n

1 0

4

2

3

9

From the b e g i n n i n g o f the s t u d i e s of d e l a y e d neutrons i t h a s b e e n c u s t o m a r y in reactor physics to d i v i d e the p r e c u r s o r s into six half-live groups, with halfl i v e s a r o u n d 55, 22, 6, 2, 0 . 6 , and 0.2 s. A l t h o u g h we k n o w t h a t a l l t h e s e g r o u p s except the f i r s t one ( 8 7 R r ) are complex the r e a c t o r p h y s i c i s t s s t i l l w i s h to keep t h i s six-group d i v i s i o n , probably because t h e i r computer programmes are w r i t t e n to take only s i x groups into account and because t h i s l i m i t a t i o n i s s u f f i c i e n t for an adequate d e s c r i p t i o n of the d e l a y e d neutron effect. We h a v e a t Studsvik,as p a r t o f our a p p l i e d programme c o n s t r u c t e d energy spectra corresponding to the six neutron groups. The r e s u l t s c a n be c o m pared with i n t e g r a l measurements for groups 2-4 i n F i g s . 2 - 4 . The c o m p a r i s o n c o n c e r n s t h e r m a l n e u t r o n f i s s i o n o f 235u. The agreement i s , on the whole, q u i t e good. This indicates that similar spectra can c o n f i d e n t l y be c o n s t r u c t e d f o r a n y o t h e r fissile material. In fact, it turns out t h a t these s p e c t r a depend v e r y l i t t l e on the actual f i s s i l e nuclide.

n

I t may be n o t e d t h a t t h e u n c e r t a i n t y o f t h e summed y i e l d i s t h e same a s t h a t o f the recommended v a l u e f o r 2 3 9 p This s i t u a t i o n i s q u i t e g e n e r a l a s shown i n Ref.9). T h u s we c a n w i t h c o n f i d e n c e u s e the set of data for individual precursors for evaluating macroscopic properties in any nuclear fuel. u #

3.2

Delayed

neutron

energy

spectra

The shape o f the d e l a y e d - n e u t r o n energy spectrum, and i t s time v a r i a t i o n , i s of great importance for reactor kinetics, especially for fast reactorsl2). B y n o w , we k n o w t h e s h a p e o f t h e n e u t r o n s p e c t r a f o r 29 p r e c u r s o r s . Since most o f t h e i m p o r t a n t p r e c u r s o r s a r e among t h o s e w i t h known s p e c t r a ,

600 N e u t r o n e n e r g y in keV

Fig.

2.

Delayed-neutron Group 2. S o l i d curve: Energy spectrum, normalized to u n i t y , from O S I R I S w o r k ^ ) . The u n c e r t a i n t y (± o n e s t a n d a r d d e v i a t i o n ) h a s b e e n i n d i c a t e d a t 50 k e V i n t e r v a l s . Dashed curve: Energy spectrum, normalized to u n i t y beteeen 100 and 1200 keV, from R e f . 1 3 ) . D a s h - d o t c u r v e : E n e r g y s p e c t r u m , n o r m a l i z e d t o u n i t y between 100 and 1200 keV, from R e f . l * * ; .

-

699

-

J 0

200

400

600

800

1

1000

1

I

1200

I

1400

Neutron energy in keV

Fig.

3.

D e l a y e d n e u t r o n Group 3. S o l i d curve: Energy spectrum normalized to unity, from OSIRIS work ). The u n c e r t a i n t y (Í one s t a n d a r d d e v i a t i o n ) h a s b e e n i n d i c a t e d a t 50 k e V i n t e r v a l s . Dashed curve: Energy spectrum, normalized to unity between 100 and 1200 keV, from R e f . ). Dash-dot curve: Energy spectrum, normalized to unity between 100 and 1200 keV, from R e f . ). 9

i

1

5

-

4

-

1

1

1

1

1

1

1

1

1

3

1

4

1

1

i

i

i

3 -

Neutron energy in keV

Fig.

4

D e l a y e d - n e u t r o n Group 4. S o l i d curve: Enerqy spectrum, normalized to unity, from O S I R I S w o r k ' . The u n c e r t a i n t y (Í one standard d e v i a t i o n ) h a s b e e n i n d i c a t e d a t 50 k e V i n t e r v a l s . Dashed curve: Energy spectrum, normalized to u n i t y between 100 and 1200 keV, from R e f . ). Dash-dot curve: Energy spectrum, normalized to u n i t y between 100 and 1200 keV, from R e f . ). 9

1

3

1

-

700

-

4

4.

Fission

yields

Table

The f i s s i o n y i e l d i s not a n u c l e a r p r o p e r t y , but i s an i n d i s p e n s a b l e q u a n t i ty for a l l applications of nuclear propert i e s to nuclear technology. It is quite strange that the f i s s i o n y i e l d pattern a f t e r 4 0 y e a r s of s t u d y s t i l l i s not known w i t h an acceptable accuracy even for 235y thermal f i s s i o n . At the l i g h t peak measurements begin to assemble thanks to r e s u l t s from f a c i l i t i e s such as LOHENGRIN^5) and HIAWATHA16), but a t the heavy peak and at the wings independent y i e l d measurements are scarce. F o r t h i s r e a s o n we h a v e started a project at OSIRIS aiming at determ i n i n g y i e l d s o f a s many a s p o s s i b l e of the f i s s i o n products obtained there. Since these f i s s i o n products correspond to about 98 % o f t h e t o t a l f i s s i o n y i e l d , a v e r y complete mapping of the y i e l d surface is within reach. 4.1

Experimental

" H a l f - l i v e s " f o r escape from the t a r g e t - i o n source arrangement

Element

Zinc Gallium Bromine Krypton Rubidium Silver Cadmium

-

Element

"Halflife", s 7.7±3.1 6.0±1.9 190±20 120±10 390±10 21.2±1.4 140±20

OSIRIS

Indium Tin Antimony Tellurium Iodine Xenon Cesium

"Halflife" s 3.2±0.3 210±10 190±20 900±50 53±7 58±3 470±40

Using the delay parameters the fission y i e l d s a r e now b e i n g d e t e r m i n e d f o r a l a r g e r a n g e o f f i s s i o n p r o d u c t s . I t s h o u l d be borne i n mind that there i s s t i l l another f a c t o r coming i n t o p l a y , namely the o v e r a l l separator e f f i c i e n c y , defined as the p r o b a b i l i t y t h a t an atom i n the g a s e o u s phase of the d i s c h a r g e chamber w i l l be c o l l e c t e d i n the measuring p o s i t i o n . Apart from a small mass e f f e c t ) the efficiency s h o u l d be t h e same f o r a l l i s o t o p e s o f t h e same e l e m e n t , b u t l a r g e v a r i a t i o n s may occur from element to element. Therefore, r e l a t i v e isotopic y i e l d s are e a s i l y obtained whereas absolute y i e l d s require a s p e c i a l determination of the a b s o l u t e yield of one of the i s o t o p e s f o r each element, f o r i n s t a n c e by r a d i o c h e m i c a l techniques.

technique

One i n h e r e n t d i f f i c u l t y i n y i e l d s t u d i e s u s i n g ISOL-systems i s the c o r r e c t i o n for the decay between p r o d u c t i o n and measurement. Such a correction requires knowledge about the delay i n the system. F o l l o w i n g W i n s b e r g l ? ) we h a v e c h o s e n to r e p r e s e n t the delay by a f u n c t i o n g i ving the p r o b a b i l i t y p ( t ) d t for a delay between t and t+dt a f t e r the p r o d u c t i o n . I t c a n be shownl8/19) t h a t the d e l a y can be d e s c r i b e d by two p a r a m e t e r s , o n e f o r the d i f f u s i o n through the target material or the d e s o r p t i o n from the t a r g e t surface, w h i c h e v e r happens to be t i m e - d e c i s i v e , and the other for the delay i n the gaseous volume of the ion source. -In the O S I R I S i n t e g r a t e d t a r g e t - i o n source the i n fluence of the l a t t e r parameter i s negligible for nuclides with half-lives above 1 s e c o n d , b u t has to be t a k e n i n t o account for shorter half-lives. T h i s l e a v e s one p a r a m e t e r t o be d e t e r m i n e d f o r e a c h e l e ment. Some r e s u l t s a r e g i v e n i n T a b l e 1 in terms of a " h a l f - l i f e " corresponding to the time i t takes to reduce the content of i s o t o p e s of a g i v e n element to h a l f i t s o r i g i n a l value (assuming d e s o r p t i o n to be t i m e - c o n t r o l l i n g ) for the part i c u l a r conditions used at OSIRIS (1500°C, uranium on graphite c l o t h ) . I t i s seen that this release h a l f - l i f e varies widely from element to element. I t i s i n the o r der o f a few seconds f o r z i n c , g a l l i u m , s i l v e r and i n d i u m , 1-2 m i n u t e s f o r b r o m i ne, k r y p t o n , t i n , antimony, i o d i n e , and xenon, about 7 minutes for rubidium and c e s i u m a n d 15 m i n u t e s f o r tellurium.

1

2 u

A t t h e p r e s e n t t i m e we h a v e a n a l y z e d our data for the i s o t o p e s of the e l e ments i o d i n e , xenon, and cesium. For these elements absolute y i e l d s determined radiochemically are a v a i l a b l e for normalization21-24). The r e m a i n i n g elements mentioned i n Table 1 are being analyzed. I n a d d i t i o n , we p l a n t o c a r r y o u t complementary s t u d i e s of those elements w h i c h can be o b t a i n e d by t h e C F ^ m e t h o d ^ ) , i.e. strontium, yttrium, zirconium, barium, lanthanum, cesium, and p r a s e o d y mium. A f t e r t h i s w o r k we e x p e c t t o h a v e a very complete picture of the yield d i s t r i b u t i o n f o r 235u thermal f i s s i o n . It would t h e n be v e r y i n t e r e s t i n g t o c a r r y out a similar study for thermal fission of U and 2 3 9 p j for fast f i s s i o n of 238rj. 2

2

3

3

u

701 -

a n c

5.

Antineutrino

energy

spectra

therefore a s p e c i a l study of the decay of f i s s i o n products with very high Q^-values and yet a r e s o n a b l y h i g h y i e l d i s recommended .

Reactors are useful as sources of a n t i n e u t r i n o s formed i n the decay of the fission products. For the interpretation of the experiments c a r r i e d out with t h i s k i n d o f s o u r c e i t i s i m p o r t a n t t o know the energy spectrum of the a n t i n e u t r i n o s . I t i s therefore tempting to evaluate this energy spectrum from our improved knowledge about the p r o p e r t i e s of the fission products. H e r e a g a i n s p e c i a l e m p h a s i s may be p u t on n u c l i d e s f a r f r o m s t a b i l i t y which have high f i s s i o n y i e l d s and, moreover, p o s s e s s h i g h Qg-values and t h e r e f o r e are responsible for the high-energy part of the antineutrino spectrum.

6.

Other

applications

I t i s probable that the I S O L - f a c i l i t i e s , when the needs f o r p h y s i c a l investigations are f i l l e d , w i l l pass into a phase of i n c r e a s i n g a p p l i c a t i o n s to other branches of sciences and technology. As far as nuclear technology i s concerned more i n v e s t i g a t i o n s of the p r o p e r t i e s o f f i s s i o n p r o d u c t s may be c a l l e d f o r i n c o n n e c t i o n w i t h the development o f more a d v a n c e d s y s t e m s . A b o v e all,however, the value of the ISOL-technique for the production of radioactive sources of various kinds and f o r v a r i o u s purposes i s expected to i n crease. I t s g r e a t advantage i s t h a t the c o l l e c t i o n of s t r o n g l y active samples can be done a l m o s t f u l l y a u t o m a t i c and w i t h l i t t l e radiation hazards. Furthermore, t h e c o l l e c t i o n c a n be made o n a n y b a c k i n g and w i t h s m a l l and v a r i a b l e penetration and almost p o i n t - l i k e samples of high p u r i t y can be made.

A t O S I R I S we h a v e c a r r i e d o u t s u c h a n evaluation26) g i v i n g r e s u l t s such as the spectrum shown i n F i g . 5 a s a band c o r r e sponding to a c o n f i d e n c e r a n g e of - one standard deviation. T h i s work was completed in 1979. S i n c e t h e n two o t h e r a n a l y s e s have a p p e a r e d ^ , 2 8 ) They are a l s o p l o t t e d i n F i g . 5. They e s s e n t i a l l y fall w i t h i n the l i m i t s of e r r o r of the O S I R I S r e s u l t i n d i c a t i n g a good agreement between these three analyses. Nevertheless, the h i g h energy end of the spectrum i s still very p o o r l y known. I t is important because the a n t i n e u t r i n o induced r e a c t i o n s o f t e n have a s t r o n g energy dependence, and t

7

#

10

Z

10

H

1

1

0

2

1

1

1

4

1

1

6

ANTINEUTRINO

Fig.

5.

1

1

1——i

8 ENERGY

10 IN

1

12

1

1

'

14

MEV

Number o f a n t i n e u t r i n o s p e r MeV p e r f i s s i o n for U irradiated for 10 s . The r e s u l t s from the O S I R I S work26) are shown by the s o l i d c u r v e s w h i c h c o r r e s p o n d to ± one s t a n d a r d d e v a i t i o n from the mean v a l u e s . Spectra from r e f s . ) and 8 ) w i t h e r r o r l i m i t s i n d i c a t e d at odd o r even mass number, are shown as d o t t e d and dashed l i n e s , respectively. 2

3

5

7

2

7

2

-

702

-

References 1)

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