POLARIZABILITY OF NEUTRON FROM PRECISE NEUTRON MEASUREMENTS

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POLARIZABILITY OF NEUTRON FROM PRECISE NEUTRON MEASUREMENTS

G. Samosvat

To cite this version:

G. Samosvat. POLARIZABILITY OF NEUTRON FROM PRECISE NEUTRON MEASURE- MENTS. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-51-C3-53. �10.1051/jphyscol:1984311�.

�jpa-00224025�

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JOURNAL DE PHYSIQUE

Colloque C 3 , supplement au n ° 3 , Tome 45, m a r s 1984 page C3-51

POLARIZABILITY OF NEUTRON FROM PRECISE NEUTRON MEASUREMENTS

G.S. Samosvat

Joint institute for Nuclear Research, Head Post Office Box 79, Moscow, Dubna 10100, U.S.S.R.

Résumé - Une brève revue des estimations expérimentales du coefficient de polarisabilité électrique du neutron, basées sur les mesures de l'asymétrie avant-arrière dans la diffusion de neutrons de quelques keV par des noyaux lourds est donnée. On montre que la sensibilité la plus forte peut être obtenue avec une expérience précise, non encore réalisée, utilisant des neutrons de 1 à 300 eV.

Abstract - A short survey is given of the experimental evaluations of neutron-polarizability coefficient based on the measurements of keV forward-backward asymmetry of neutrons scattered by heavy nuclei. It is shown that the most sensibility can be provided by a precise experiment with the ~1-300 eV not yet rea- lized.

Almost thirty years passed since the idea of electrical polarizability of the neutron had been introduced and its experimental searches begun /1 / . During this time the polarizability of the proton'2/ and pion/3/ was observed while the polarizability of the neutron has not been observed up to now. History and results of the neutron polarizability investigations are described in detail in

the book'V. The present report contains no new results and has a purpose to draw attention to one more experimental possibility omitted in /4/.

The polarizability of a particle in the electrical field 8 results in the induced electrical moment p= o(B* that leads to a further in- teraction H'=-o<82/2. The coefficient of , which is a measure of polarizability, is very small (~10~42 Cm3) that causes difficulties of its experimental observation. It is found from the experiments on scattering of Y -rays and pions that for the proton O(TJ = (1.1 ± 0,2) 10-42^{c m}3/2/ and for the pion o(,=(0,68 ± 0,14)10-42

cm3 /3/. n

The best estimates of an upper limit of the neutron - electrical - polarizability coefficient « n were obtained from the analysis of the angular dependence of neutrons scattered by nuclei with a large charge Ze at keV energies. The anisotropy of this scattering is reduced essentially to the forward-backward scattering asymmetry which is characterized by the coefficient «o^ in the expression for the differential cross section

ds/dQ ~ 1 +co4f>(co$9) + cOzP^a(tos&)+...

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984311

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C3-52 JOURNAL DE PHYSIQUE

where Q i s t h e s c a t t e r i n g angle i n t h e c e n t r e of mass system. A s i t i s shown i n t h e work /5/ t h e scat-tering asymmetry 0 1 has a simple energy dependence :

CA)? = aE

-

b ~ 1 / 2 , (1 1

where t h e f i r s t term orr responds t o t h e pure n u c l e a r s c a t t e r i n g and the s e c ~ n d one t o t h e i n t e r f e r e n c e of n u c l e a r s c a t t e r i n g w i t h a n a d d i t i o n a l s c a t t e r i n g due t o p o l a r i z a b i l i t y of t h e neutron. For heavy n u c l e i a= 2 M ~ V - 1 , and b i s defined by t h e q u a n t i t i e s d n , Z and n u c l e a r s c a t t e r i n g r a d i u s R' :

b = 3 , 5x10-6+ r t ~ e v - ~ / ~ ( 2 1

( d n i s i n units 10-42 cm3, R' i s i n fm).

L ^URA

The f i g u r e demonstrates t h r e e s t a g e s of d , e v a l u a t i o n s using the W1 d a t a and formulas (1) and (2). The energy dependence of Cc)? i n . the absence of p o l a r i z a b i l i t y i s shown by s o l i d l i n e s , dot-dashed curves correspond t o d n e s t i n a t e s from previous experiments and dashed curves t o d n e s t i m a t e s which have heen o r can be obtained a t the given s t a g e .

~t t h e f i r s t s-Lage ( p a r t a of t h e f i g u r e ) the e s t i m a t e dn-.l~-40cn?

w a s known from t h e experiments w i t h f a s t neutrons s c a t t e r e d a t s m a l l a n g l e s /4/. The d a t a on s c a t t e r i n g asymmetry of t h e neutrons up t o 300 keV f o r uranium nu e i allowed t h a t e s t i m a t e t o decrease down t o the l i m i t dn<2rIO-Btcm3 /5,6/. The second s t a g e c o n s i s t e d i n employing more slow neutrons (up t o 26 keV) and i n measuring _&Q1 on t h e l e a d n u c l e i . It w a s done i n t h e wsrk/7 ( s e e p a r t b of the

$

f i g u r e ) where t h e new l i m i t d n < 6 , 1 x 1 Q - 4 2 cm had been obtained.

Por a f u r t h e r s p e c i f i c a t i o n of o(, i t i s a d v i s a b l e t o come t o s t i l l bower e n e r g i e s of s c a t t e r e d neutroaas a t which the v a l u e s of both t e r n s s f t h e e x p r e s s i o n (1) a r e comparable, t h e curve

e s s e n t i a l l y n o n l i n e a r and crogses t h e a b s c i s s a a x i s .

f i g u r e i l l u s t r a t e s t h e s i t u a t i o n w i t h LA), f o r t h e l e a d n u c l e i a t e n e r g i e s up t o 3Q0 eV. One may expect the measurements of i n

t h i s energy r e g i o n w i l l g i v e a t l a s t a d e f i n i t e value of d n

-

10-42 cm3. However, the corresponding experiment i s h i g h l y d i f f i c u l t because t h e t b s o l u t e e r r o r s f t h e W1 measurements should n o t

exceed -10- Besides a h i g h s t a t i s t i c a l accuracy i t i n necessary

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t o e n s u r e a n a c c u r a t e t a k i n g i n t o account of t h e background, e s p e c i - a l l y of t h o s e n e u t r o n s which e n t e r t h e d e t e c t o r a f t e r s c a t t e r i n g on t h e sample a n d r e s c a t t e r i n g on s u r r o u n d i n g m a t e r i a l s and air.

R e f e r e n c e s

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2. GQLIDANSKI V . I . e t a l . , Zh.Eksp. Theor. Fix. 28 (1960) 1965;

BARANOV P.S. e t a l . , Yad. F i z . 21 (1975) 689.

3. ANTIPOV Yu.M. e t a l . , Phys. L e t t . 121 B (1983) 445.

4. AIJ3KSRNDRQV Yu. A. llbundamental n i e s v o i s t v a n e u t r o n a " , Energoizda-t, Moscow, 1982.

5. THALER B.M., Phys. Rev. 114 (1959) 827.

6. LANE R.Q. e t a l . , Ann. Phys. 12 (1961) 135.

7. ALEKSANDROV Yu. A. e t a l . , P i s l m a Zh. Eksp. Thesr. F i z . 5 (1966) 196.