PARITY VIOLATION EFFECTS IN RESONANCE NEUTRON CROSS SECTIONS

HAL Id: jpa-00224033

https://hal.archives-ouvertes.fr/jpa-00224033

Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

PARITY VIOLATION EFFECTS IN RESONANCE NEUTRON CROSS SECTIONS

V. Alfimenkov, S. Borzakov, Vo van Thuan, Yu. Mareev, L. Pikelner, I.

Frank, A. Khrykin, E. Sharapov

To cite this version:

V. Alfimenkov, S. Borzakov, Vo van Thuan, Yu. Mareev, L. Pikelner, et al.. PARITY VIOLATION EFFECTS IN RESONANCE NEUTRON CROSS SECTIONS. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-93-C3-98. �10.1051/jphyscol:1984319�. �jpa-00224033�

Colloque C 3 , supplement au n ° 3 , Tome <i5, m a r s 1984 page C 3 - 93

PARITY VIOLATION EFFECTS IN RESONANCE NEUTRON CROSS SECTIONS

V.P. Alfimenkov, S.B. Borzakov, Vo Van Thuan, Yu.D. Mareev, L.B. Pikelner, I.M. Frank, A.S. Khrykin and E.I. Sharapov

Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, U.S.S.R.

Résumé.- On a observé la violation de la parité dans les réso- nances neutroniques des noyaux non fissiles. Les résultats de nos expériences sont en bon accord avec les données pour des neutrons thermiques et avec les prévisions théoriques.

Abstract.- Parity nonconservation in neutron resonances of non- fissible nuclei is observed. The experimental results are in agreement with the parity nonconserving effects in thermal neu- tron cross sections and with the theoretical predictions.

In 1980 the parity nonconserving (PNC) thermal neutron spin rotation and the helieity asymetry of total neutron cross section were obser- ved for 'Sn /1/. Both effects were claimed to be complementary 117 tools for the study of weak interaction in nuclei by more standard techniques, e.g. through the gamma-ray asymetry in neutron capture /2-4/. The nature of the effects found was unsertain: it was not pos- sible to make distinction between parity violation in the direct in- teraction neutron-target and parity nonconservation in nuclear exci- ted states. Stodolsky / 5 / gave a tentative explanation in terms of an unusual weak force and Sushkov and Plambaum explained the PNC effects quantitatively in the model of the mixed compound states.'The formula describing the resonance behaviour of the PNC effect was given even earlier by Lobov /!/. Other approaches, for example /8/,/S/ followed soon.

Since 1981 the systematical study of the helieity dependance of neut- ron cross sections was made with thermal neutrons by the Gatehina group /10,11/ and with resonance neutrons by the Dubna group /12,13/«

The full description of resonance neutron PNC effects is given in /14/.

They are discussed by Prank /15/ recently. Here we are going to report same new experiments which are in progress at Dubna and to compare the different PNC effects in neutron cross sections of nonfissible nuclei.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984319

C3-94 JOURNAL DE PHYSIQUE

I - SOME THEOFBTICAL RESULTS

The model of the mixed compound states /6/ in its so-called two-level approximation contains the following conclusions:

1. Parity violation in neutron resonances leads to the difference hdt~Z6-between cross sections with the 2 helicities. Near a p-wave

resonance it is described by

&(El-2 dP(;)g = 2 $ , ( E ) , ~ E 2% , ⁽¹⁾

s

were & ( 8 ) is the resonance part of the crosJpsection in the absen- ce of the neutron polarization P 6 ( E ) = (</k2) ( rn 1- ) / ( (E-E~)~+</~),

P np

Es(E ) is the energy of the s(p)-wave resonance,

rs( _ri)

r

P SP

weak Hamiltonian between s- and p-level under sdudy. The PNC effect for thermal neutrons nd(Eth) is connected with the resonance effect

n d ^(E,) by the formula (valid if E << I ~~l ) : P

A 6 (Eth)/&6 (Ep) = ( G / ~ E ~ ) ~

.

2* The prediction for the /B -values taking into account the dyna- mica1 enhan~ement of weak interaction in compound nucleus is SP

tsl%if',

8

.

sP

mi-

It follows from the estimate of the single particle matrix element of weak interaction < I Hwl> = G & ~ J with We1 MeV and G=IO-?~;

4 5

as the F e m i weak constant. N (10 -10 ) is the number of the compound states in the interval W

.

3. The neutron spin-rotation angle A(P (in rad/cm) is described by formula

nV(E)

=(4

where n is the number of target nuclei per crn2 and 1 is thesample length

4. The coefficient ax of the asymetry of gamma-ray following neut- ron capture is connected with the same matrix element 4 as

SP

study in the-mixed s(p)- resonances , A(I,Ji,Jf- is known spin-coef- ficient.

I1

-

The resonance behaviour of the PNC effect in neutron cross sections

fligth method at the Dubna IBR-30 pulsed reactor /16/. The resonance neutrons were polarized by transmitting through a polarized proton target as it has been proposed in /17/. The transmission of the longi- tudinal~ polarized neutrons through the samples under study was measu- red. This gave the so-called transmission effect E ⁼ (N+-N-)/ (N++N-)~, where N f are the numbers of the detector couts for the 2 neutron heli-

cities and fn (=0.6) is the beam polarisation. The transmission effect is connected with the PNC effect 9 ^by

Having the @-value one can obtain the module of matrix element

Bso

from the eq. (I ) using the known neutron resonance parameters. The big- gest effect found is for the 0.75 eV resonance of I3%a. It is shown on Fig.1. It demonstrates the resonance behaviour of the effect under

ello ^4, study. Up to date the dependance

of total crags sections on neutron

2 , 0 E n helicitywasstudiedfor 14reso-

-20 nances in the nuclei 8 1 ~ r , 9 3 ~ b , 111Cd,117Sn,1271,139La,145xd,238U,

-4 0

Pig. 1

-

fable 1. The PNC effects in reso- nances.

-

For the most part the results were presented in /14/. The cases where the PNC effect was found are given Table 1.

Note that the result for lllCd as compared with the given in f14/

is remeasured and represents clear evidence for the negative sign of the 9 -value. Other results of our new measurements of the 9-va-

C3-96 JOURNAL DE _PHYSIQUE

lues are: for 93Nb 2.0*1.7(Ep=36.9 eV) and 0.322.4 (Ep=42.3), for 1 4 5 ~ d -0.120.4 (Ep=2.05 eY), for l1lcd 1.821.8 (Ep=6.94 eY)

-

in units. The reason for the absence of the experimental effects is either the! effect is weaker than the experimental accuracy, or the resonance has the spin J=I *3/2 not suitable for mixing with a s-reso- nance. The latter does not hold for two Nb-resonances with J=I 1/2.

The spin assignement for the p-resonances under study become an im- portant problem.

The #sp values for the resonances,where the PNC effect was observed, were calculated under assumption that the p-resonance is mixing with the nearest most strong s-resonance, often at negative energy. Compa- ring the +sp -values from Table 1 with the theoretical estimate ( 3 ) one can conclude that they are in agreement. This can be regarded as an argument to the dynamical enhancement concept of the theory, though one should bear in mind that there are other successful explanations.

I11

-

-

P P

late the ratio R = )/nd(Eth) and compare it with theoreti-

* ^{exp P}

cal value R = (2Ep/

rp12

Table 2. The comparison of the PNC effects at different energies.

- -

One can note that the experimental effects n d ( ~ ) and a&(Eth) dif- fer by nearly three order of magnitude, nevertherless the agreement P with teory is more than satisfactory. That means that the Breit-Wig- ner formula for the isolated level is valid at the far-away wings of resonance

.

There are also the spin-rotation results for thermal neutronst for Il7sn bvexp= -3.67f0.27 /I/, for I3'Ii3 = -22*2 and for Pb = 0.22'0.03 /B. R. Heckel( 1982,1983)/, all in units rad/cm. One can

equation (4). One can also calculate the ay(Eth) using the resonan- ce result for the h 6 fEp) and equations (21, (4). The corresponding figures are given in Table 3.

Table 3. The calculated A Y -values (in units rad/cm)

There is quantitative agreement between all figures and the perfect agreement in the sign of the calculated and the experimental values.

The Pb-result does not obtained explanation so far in view of the ab- sence of the low energy neutron resonances.

Such a comparison can be made in case of 1 1 7 ~ n using the %-results

ad ⁼ (8.9k1.5) 10-4 /3/ or Cby = (4.420.6). 10-' /4/ and parti- al gamma-widths of allowed and forbidden transitions in I8sn /18/.

We measured the intensities of the gamma-rays afterneutron capture in the p-resonances 1.33 eV (Il75n) and 0.75 eV (139~a) as well as in the thermal energy region, that is, in the range of parity-admixed s-resonance located at negative energy. The Ge(Li)-detector of 60 cm 3 volume and N~I(IE~)-crystal of 200x200 mm2 were used in measurements made by time-of-fligth method. For I7Sn the ratio of the partial widths

r p rc

= 0,42 2 0.07 meV according to the above mentioned ad-results.The later agrees well with the value of the matrix element obtained from the total cross section measurements, Table I.

10 a'L. ^lo d i -

In the case of 1 3 9 ~ the ratio of integrated quantities Zrp/z

rS

= 0.07 2 0.02 was obtained for 10 gamma-transitions with energy 4600

-

5200 lceV.!Chis means that the kinematical enhancement of the PNC effect for the strongest primary transitions in Lantanwn is absent.

C3-98 JOURNAL DE PHYSIQUE

Concluding we would like to state that all the known PNC effects in neutron cross sections for nonfissible nuclei are in the mutual agre- ement and testify the weak interaction in nuclei compound states.

The further experiments of this type are necessary to clarify the pe- culiarities of weak interaction in nuclei.

REFERENCES

1. FORTE L e t al. ,Phys,Rev.Lett. fi (1980) 2088, 2. ABOV Yu.G.et al.,Phys.Lett. 12 (1964) 25.

3. DANILYAN G.V.et al. ,Pis1ma ZhETP 2 (1976) 380.

4. BENKOULA H.et al. ,Phys.Lett., (1977) 287.

5. STODOLSKY L. ,Phys. Lett

.

6. SUSHKOV 0,P.and PIAMBAUM V,V. ,Pis 'ma ZhETP 32 (1980) 377, 7. LOBOV G. A.

,

8. BUNAKOV V.E. and GUDKOV V. P. ,Z.Phys. (1 981 ) 285.

9. ZARETSKY D.F. and SIROTKIN V.K. ,Yad.Fis. 37 (1983) 607.

10. KOLOb5NSKY E.A,et al. ,Phys.Lett. (1981) 272.

11. VESNA V.A.et al.,Pislma ZhETP 2 (1982) 351.

12. ALFUVfENKOV V. P. e t' al. ,Pis 'ma ZMTP 2 (1 981 ) 308.

13. ALFIMENKOV V.P.et al. ,Pis 'ma ZhETP 2 (1982) 42.

14. ALFIMENKOV V.P.et al.,Nucl.Phys. A398 (1983) 93.

15. FRANK I.M.,In: Neutron Induced Reactions. Proc.Europ.Topica1 Conf.

Bratislava. Ed. by P. Oblozinsky ( 1982) 16.

16. FRUK I.M. ,Particles and Nuclei2-part4 (1972) 805.

17. TARAN Yu.V.and SKAPIRO F.L. ,ZhETP fi (1963) 2185.

18, ALFUIIIENXOV V. P. e t al. JINE-Preprint P3-83-634 Dubna ( 1 983).