EXPERIMENTAL STUDIES OF THE PARITY-VIOLATION EFFECTS IN HEAVY NUCLEI FISSION CAUSED BY POLARIZED NEUTRONS

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Submitted on 1 Jan 1984

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EXPERIMENTAL STUDIES OF THE

PARITY-VIOLATION EFFECTS IN HEAVY NUCLEI FISSION CAUSED BY POLARIZED NEUTRONS

A-Ya. Alexandrovich, G. Val’Skii, G. Petrov, A. Petukhov, Yu. Pleva, V.

Sokolov

To cite this version:

A-Ya. Alexandrovich, G. Val’Skii, G. Petrov, A. Petukhov, Yu. Pleva, et al.. EXPERIMENTAL STUDIES OF THE PARITY-VIOLATION EFFECTS IN HEAVY NUCLEI FISSION CAUSED BY POLARIZED NEUTRONS. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-85-C3-88.

�10.1051/jphyscol:1984317�. �jpa-00224031�

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EXPERIMENTAL STUDIES OF THE PARITY-VIOLATION EFFECTS IN HEAVY NUCLEI FISSION CAUSED BY POLARIZED NEUTRONS

A.Ya. Alexandrovich, G.V. Val'skii, G.A. P e t r o v , A.K. Petukhov, Yu.S. Pleva and V.E. Sokolov

Leningrad Nuclear Physics Institute, Gatchina, Leningrad 188350, U.S.S.R.

Résumé - Les résultats expérimentaux sur la vérification de la théorie de violation de la parité dans la réaction de fission sont discutés.

Abstract - Experimental r e s u l t s are discussed checking the theory of P - v i o l a t i o n i n f i s s i o n caused by slow polarized neutrons.

In 1977 the phenomenon of P - v i o l a t i n g asymmetry i n the emission of fragments from heavy n u c l e i f i s s i o n caused by the t r a n s v e r s a l - ly-polarized neutrons was discovered in USSR / 1 / . The e x i s t - ence of r a t h e r large (~10~4) e f f e c t i s not t r i v i a l since the number of e x i t channels in t h i s reaction i s quite large (^lO8).

F i r s t t h e o r e t i c a l explanations of t h i s phenomenon / 2 , 3 / are based on the hypothesis that the observed P - v i o l a t i o n i s caused by weak i n t e r a c t i o n mixing of compound nucleus resonances of opposite p a r i t i e s . I n order to explain the s u r v i v a l of t h i s e f f e c t a f t e r averaging over numerous exit reaction channels performed a u t o - matically i n each f i s s i o n experiment d i f f e r e n t p r o p e r t i e s of

the "cold" t r a n s i t i o n s t a t e s of f i s s i o n i n g nucleus were employed in / 2 , 3 / - In a d d i t i o n to the P - v i o l a t i n g asymnetry ( CLnf) i n t h i s r e a c t i o n one should also expect the P-conserving r i g h t - l e f t asymmetry ( CL% ) in f i s s i o n fragments emission which i s defined by the same properties of f i s s i o n i n g nucleus but a r i s e s due t o the i n t e r f e r e n c e of s- and p-resonances in f i s s i o n cross s e c t i o n . P - v i o l a t i n g asymmetry (P) i s also expected for the t o t a l cross s e c t i o n s ( &+ and &- ) of the longitudinally polarized neutrons with opposite h e l i c i t i e s . The energy dependences of these effects contain the phase f a c t o r s which are defined i n / 3 / through the usual p o t e n t i a l phase s h i f t s in e x i t and entrance channels giving

OL/

tf-[m ~sdV\mwj^'"rsr/CpI+pr/Ls]

cc

^RL

- m

^L

- £sf / sSt &H - A fy£-EJ-fr(E-5J (

²

)

n

f ~ dQ d$ / dQ.

+

dQ

D

'Q%f[p) +(ptf[s]

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984317

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C3-86 JOURNAL DE PHYSIQUE

Here A, B , C a r e t h e constants depending on compound resonanc- e s a n g u l a r momenta and n e u t r o n w i d t h s ;

cpf -

t h e n e u t r o n and f i s s i o n widths f o r t h e mixing s- and p-r&oneulces; [s3

,

[p] =

= (E

-

^Es ^{) 2}

+

1/4&.a ; &

,

and

Gap

a r e t h e e n e r s e s and t o t a l widghs o f thege resonances. Eqs. ( 1 ) - ( 3 ) show t h a t experimental s t u d i e s o f t h e s e c o e f f i c i e n t s a l l o w t o g e t v a l u a b l e i n f o r m a t i o n about t h e mechanism of t h e s e r e a c t i o n s , t h e enhancement e f f e c t s in P-conserving and P - v i o l a t i n g u a n t i - t i e s and about t h e matrix elements o f weak i n t e r a c t i o n

% .

In t h e f o l l o w i n g p a r a g r a p h s we b r i e f l y d e s c r i b e t h e e x p e r i - ments checking t h e main t h e o r e t i c a l c o n c l u s i o n s o f /2,3/

which were c a r r i e d OD in Leningrad Nuclear P h y s i c s I n s t i t u t e on the VVR-M r e a c t o r .

. RL

I

-

s ~ l m r s s OF POSSIBLE D E P E ~ ~ C E S

m

TAE

dn

~ D ~ ~ C O E : ~ I C I E ~ S ON TEE PRBWd6MT MASS AND TOTAL KINETIC

kWOY

The e x i s t e n c e of P - v i o l a t i n g asymmetry h a s been r e g a b l y

e s t a b l i s h e d i n / 1 , 4 , 5 , 6 / , where t h e d n ) and coef f i c i - e n t s i n t e over a l l l i g h t fragments Y ~ t e g r a l C P

e f f i c i e n t s $ a i Z e been measured ( Z ? H J ~ U , 2 3 5 ~ , 2 9 ~ u and 2 4 1 ~ ~ . In o u r experiments /4,6/ we employed t h e method o f measuring t h e two fragments k i n e t i c e n e r g i e s E ,2 f r o m t h e thin t a r g e t s ubich allows t o f i n d t h e d l v and

d$

dependence on fragments masses and t o t a l k i n e t i c e n e r g i e s , S i (Au)- - d e t e c t o r s and i o n i z a t i o n chambers were used as fragment d e t e c t o r s . The i n s t a l l a t i o n syrnnetry was checked i n s p e c i a l experiments w i t h d e p o l a r i z e d n e u t r o n beam. In a d d i t i o n t o this we c o n t i n u o u s l y con t r o l l e d t h e pos si b l e e quipment a s y m- metry d u r i n g t h e experiments comparing t h e d i f f e r e n t combina-

t i o n s o f experimental d a t a f o r o p p o s i t e d i r e c t i o n s o f t h e l e a d i n g magnetic f i e l d s o n t h e t a r g e t /7/. The two-dimensional m a t r i c e s of t h e experimental asymmetry c o e f f i c i e n t s were f i t - t e d b y t h e expansion:

dnf

⁼

do +dl

( M a o ) + d 2 ( E - E o ) * (4 Here M,E = (E1+E2) a r e t h e c u r r e n t values of fragment mass and energy, & and Eo a r e t h e i r most p r o b a b l e values. The c o r r e c - t i o n due t o t h e f i n i t e M and B r e s o l u t i o n ( 20%) was defined as a r a t i o of t h e corresponding " t h e o r e t i c a l g t and e x p e r i m e n t a l d i s p e r s i o n s . Ecperimental r e s u l t s a f t e r i n t r o d u c i n g o f a l l t h e n e c e s s a r y c o r r e c t i o n s f o r on 23% a r e g i v e n in Table 1 , As seen from Table 1 t h e E and I dependences of and

were not observed w i t h i n t h e experimental accuracy. / 4/

4 P o l a r i z e d

D e t e c t o r s a n f * 1 0 beam D ep o l a r i z ed Equipment

b earn asynnnetry

S i (h)- d o 4.66Q.15

-

^0.239.15

d e t e c t o r s d r 0.022b.030

-

0.006% .030

d2 -0 .OW% .0 16

-

0.006% .016

I o n i z a t i o n d o 4.21W.14 0.2 1*0.16 -0.13fO. 14

chamber 0.045% .023 0.002~0.028 0 . 0 0 4 b ,023

a'

0.023%.015 -0.02&0.017 0.022b.015

--

^a²

-

Mean v a l u e d o 4.43b.10 0.2 1f 0.16 0.050b.10

0.030%,021 0.002~0.028 0.004b.021

0 . 0 l 0 ~ . 0 l l -0.02&0.017 0.014%.011

-'

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2 3 9 ~ ~ A h 9 2 3 3 ~ /8,9/

The i n t e g r a l c o e f f i c i e n t s

d

nf and

8

^were ^measured i n t h e v i c i n i t y of t h e w e l l known s-resonance (0.29 eV) in2 3 9 h and weak 0.17 eV resonance in 2 3 3 ~ . !&e monochromatic p o l a r i z e d neutrons were obtained with t h e a i d of c r y s t a l monocbromator

(Cu2MnAl), magnetized up t o s a t u r a t i o n . Xe used f a s t gas

s c i n t i l l a t o r c o u n t e r s t o d e t e c t f i s s i o n fragments. d x p e r i m e n t a l r e s u l t s a r e shown in Figs. 1 and 2.

Fig. 1 . . The dependence

a

^{n f (}^Bhnj ^{f o r}

23 ~ P U .

1

-

f i t t i n g t o t h e bypo t h e s i s d & = c o n s t . 2

-

f i s s i o n c r o s s s e c t i o n O f ( J n ) . 3

-

t h e c o n t r i b u t i o n of 0.29 eV resonance t o t h e 239Pu f i s s i o n c r o s s s e c t i o n .

$,* - ^d&

v i ~ w s f o r polarized and de- p o l a r i z e d b e a m .

Fig.2. The d e end- e n g d7 ^(En)( @?and

&,-(En f o r 233U.

1

-

f i s s i n c r o s s s e c t i o n a f ( % ) . 2

-

(n- )-cross s e c - t i o n

$(%'.

5

cont%olling p o u l t s f o r the de- p o l a r i z e d beam.

One can see from Bigs. 1 and 2 t h e approximate constancy o f d n f ( x n f

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C3-88 JOURNAL DE PHYSIQUE

f o r 2 3 9 ~ u , t h e i n c r e a s e of

d E $ ( E ~ )

^Y

JFn

and t h e change of s i g n f o r d n f ( % ) in v i c i n i t y o f 0.17 eV resonance f o r 2 3 5 ~ . This t y p e of energy dependence f o r t h e c o e f f i c i e n t s d a d d @ is expected from t h e o r e t i c a l e x p r e s s i o n s ( 1 ) and ( 2 y

3

-

P-VIOLATncJG ASYBliMETRY I N THE 2 3 3 ~ TOTAL CROSS SETION

The t y p i c a l behaviour ~ f d ~observed i n 23%J c a s e ( ~ ~ ) is t h e o r b t i c a l l y p r e d i c t e d in t h e v i c i n l t y of p-resonance /2,3/. According t o ( 3 ) one can e x p e c t i n t h i s r e g i o n t h e resonance enhancement o f t h e P - v i o l a t i n g asymmetry c o e f f i c i e n t P i n t o t a l c r o s s s e c t i o n . This c o e f f i c i e n t w a s measured e a r l i e r f o r

kz

0.01 eV / l o / and En =

= 0.025 eV /11/

P = - ( 0 . 4 ~ 0 . 7 ) * 1 0 - ~ / l o / , P = ( 1 . 1 7 k . 6 9 ) * 1 0 - ~ /11/.

Vie have made t h e f i r s t a t t e m p t t o measure P f o r En = 0.17 eV/12/.

P = -(1.52)' 1.26)*10-~.

Thus, we have n o t found y e t t h e c o n s i d e r a b l e resonance enhzncement e f f e c t .

4

-

CONCLUSION

It seems t o us t h a t t h e above experimental s t u d i e s a l l o w t o confirm t h e b a s i c t h e o r e t i c a l statemerrts f o r P - v i o l a t i n g e f f e c t s in heavy n u c l e i f i s s i o n . Our a t t e m p t s t o compare t h e d a t a obtained w i t h t h e s i m p l e s t t h e o r e t i c a l two l e v e l approximations a l r e a d y a l l o w u s t o e s t i m a t e t h e weak i n t e r a c t i o n nrztrix elements ( v w ( 1 - 5 ) - 10-4 eV)and m k e some p r e l i m i n a r y c o n c l u s i o n s on t h e c h a m c t e r i s t i c s o f mixing resonances / 8 , 9 / . One should p o i n t t h a t t h e c o n s i d e r a b l e p a r t o f t h i s illformation can n o t be obtained otherwise.

References 1. DANILYAN

7. ALEXANDROVICII A.Ya., PETROT G.A. e t a l . , P r e p r i n t I&TPI-804(1982J 8. VAL'SKII G.V., P&TROV G.A. e t a l . , Yad.Mz. 3 6 (1982) 44.

9. VAL'SKII G.V., PhTROV G.A. e t a l . , P r e p r i n t UJPI-796 (1982).

10. WSNA V. A.

,

UBASHLV V.N. e t a l .

,

Z T P P i s 'ma 37 (1983) 410.

11. BONDARENKO L.N., DANILYAN G.V. e t a l . , JhTP PiFm 28 (1983) 14 1.

12. A L ~ ~ D R O V I C H A.Ya., PETROT G. A. e t a l . , P r e p r i n t LNPI-890 (1 983)

EXPERIMENTAL STUDIES OF THE PARITY-VIOLATION EFFECTS IN HEAVY NUCLEI FISSION CAUSED BY POLARIZED NEUTRONS

HAL Id: jpa-00224031

https://hal.archives-ouvertes.fr/jpa-00224031