PARITY NON-CONSERVING NEUTRON SPIN ROTATION

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PARITY NON-CONSERVING NEUTRON SPIN ROTATION

B. Heckel, M. Forte, N. Ramsey, G. Greene, K. Green, J. Byrne, J. Pendlebury

To cite this version:

B. Heckel, M. Forte, N. Ramsey, G. Greene, K. Green, et al.. PARITY NON-CONSERVING NEUTRON SPIN ROTATION. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-89-C3-92.

�10.1051/jphyscol:1984318�. �jpa-00224032�

JOURNAL DE PHYSIQUE

Colloque C 3 , supplement au n ° 3 , Tome 4 5 , m a r s 1984 page C3-89

PARITY NON-CONSERVING NEUTRON SPIN ROTATION

B. Heckel, M. Forte*, N . F . Ramsey**, G.L. Greene***. K. Green**** J. Byrne*****

and J.M. Pendlebury*****

Physics Dept., University of Washington, Seattle, WA 98195, U.S.A.

*Physics Division, Joint Research Center, 1-21020 Jspra, Italy

**Harvard University, Cambridge, MA 02138, U.S.A.

**

National Bureau of Standards, W ashington, D .C. 20234, U.S.A.

****Rutherford-Appleton Laboratories, Chilton, Didcot, Oxon 0X11 OQX, U.K.

*****Sussex University, Brighton, BN1 9QH Sussex, U.K.

Résumé - L'interaction faible entre les neutrons et les noyaux est étudiée par la mesure de la non-conservation de la parité dans la rotation des spins des neutrons. Une revue des techniques expérimentales et des résultats sera présentée.

Abstract - The neutron-nucleus weak interaction strength can be studied by measurements of the parity non-conserving neutron spin rotation. A review of the experimental techniques and results obtained will be presented.

When a neutron beam interacts with ordinary matter, the weak interaction gives rise to a variety of parity-violating observables. For the case of a polarized cold neutron beam propagating through an unpolarized target, the weak interaction gives the neutron coherent forward^scatteri/ig amplitude a parity non^conserving (pnc) component proportional t<£ s-p where s is the neutron spin and $ its linear momentum.

Just as the interaction s-B causes the spin to rotate about a magnetic field t, a s-p interaction will cause the neutron spin to rotate about its momentum vector.

This effect has been labeled pnc neutron spin rotation, and is characterized by a neutron spin rotation angle in the plane perpendicular to p, <J>

, per unit target length, I , given by /1,2/:

where p is the atomic number density of the target material, and G' is a weak amplitude linear in the universal Fermi constant, Gp, that contains contributions from neutron-nucleus and neutron-electron scattering.

Distorted wave Born approximation estimates for <j>

/£ range from 10" to 10"

rad/cm for targets consisting of light nuclei (H^He)/^,"?/, to 10" rad/cm for heavy nuclei targets (Pb, Bi) /2,5/. It was first pointed out by M. Forte that in the vicinity of a p-wave neutron resonance, enhancements of <t>

/£ may be expected /6/. The enhancement comes from the observation that <f> „ is due to a mixing of s

T

pnc

and p-wave components of the neutron scattering amplitude. Near a p-wave resonance, the p-wave amplitude (which is otherwise 1 0

times smaller than the s-wave

amplitude for cold neutrons) can be increased by several orders of magnitude, causing an increase in <f>

/& by a similar amount. Forte's observation coincided with advances made in cold neutron polarization techniques that made it feasible to measure neutron spin rotation angles to a precision of 10"

rad, and led to a program to measure <l>

,_/£ in various elements. This paper describes the neutron polarimeter developed by us at the Institut Laue-Langevin to measure <f>

> and

reviews the results that have been obtained. P The neutron polarimeter consists of a magnetically shielded target region that is

sandwiched between two neutron polarizers (a polarizer-analyser pair), and is shown

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984318

JOURNAL DE PHYSIQUE

ounler

P

FIG.^l. Schematic view of exper~mental appnatus. The neutron beam polarization is along Z and momentum along

i n F i g u r e 1. The p o l a r i z e r s are o f t h e curved S o l l e r type; a stack o f magnetic m i r r o r f o i l s separated by 1 mm and curved along t h e neutron beam a x i s t o ensure t h a t each neutron undergoes a t l e a s t one r e f l e c t i o n . The b e s t r e s u l t s were achieved w i t h super-mirror f o i l s developed by 0. Schaerpf 1 7 1 . When placed i n a 1 KG magnetic f i e l d , t h e super-mirror p o l a r i z e r s t r a n s m i t t e d 30% o f t h e i n c i d e n t neutron f l u x w i t h a p o l a r i z a t i o n o f 97% over a neutron beam c r o s s - s e c t i o n a l area o f 3 cm x 3 cm. As w i l l be discussed f u r t h e r l a t e r , t o c o n t r o l systematic e r r o r s i n t h e measurement o f

i t i s imperative t h a t t h e p o l a r i z a t i o n product o f t h e p o l a r i z e r - a n a l v s e r p a i r

~ ~ ~ f i ~ i f o r m over t h e neutron beam c r o s s - s e c t i o n a l area.

R e f e r r i n g t o Fig. 1, t h e neutrons e x i t t h e p o l a r i z e r through a c o l l i m a t o r w i t h t h e i r spins a l i g n e d along t h e z a x i s . The magnetic f i e l d o f t h e p o l a r i z e r ( 1 kG) i s matched t o t h a t o f a r e c t a n g u l a r " e n t r y s o l e n o i d " whose windings a r e s p l i t a t t h e neutron e n t r y side. The purpose o f t h e e n t r y s o l e n o i d i s t o a d i a b a t i c a l l y guide t h e neutron spins i n t o a u n i f o r m 10G f i e l d w i t h i n the solenoid, a t which p o i n t they pass n o n - a d i a b a t i c a l l y through the windings o f t h e s o l e n o i d i n t o t h e t a r g e t region.

I n t h i s way the neutron beam enters t h e low f i e l d t a r g e t r e g i o n w i t h a u n i f o r m p o l a r i z a t i o n along t h e z a x i s . I g n o r i n g t h e t a r g e t s f o r a moment, t h e neutrons n e x t encounter t h e c e n t r a l r e c t a n g u l a r s o l e n o i d ( p i - c o i l ) , whose a x i s i s along t h e z a x i s and whose magnetic f i e l d s t r e n g t h i s adjusted t o cause t h e neutron spins t o Larmor precess by p i rad. d u r i n g passage through the c o i l . Any p r o j e c t i o n o f t h e neutron s p i n along t h e y a x i s i s thus reversed upon passage through t h e p i - c o i l . The e x i t solenoid i s i d e n t i c a l t o the e n t r y solenoid, b u t i s o r i e n t e d w i t h i t s magnetic f i e l d along t h e +y axis. Tapered mu-metal shims extend t h e analysing p o l a r i z e r ' s z a x i s d i r e c t e d magnetic f i e l d i n t o the e x i t solenoid, r e s u l t i n g i n a t o t a l magnetic f i e l d t h a t t u r n s a d i a b a t i c a l l y from t h e y d i r e c t i o n a t t h e c u r r e n t sheet s i d e o f t h e e x i t s o l e n o i d t o the z d i r e c t i o n a t t h e analyser side. Reversing t h e c u r r e n t i n t h e e x i t s o l e n o i d t u r n s t h e magnetic f i e l d from the -y d i r e c t i o n t o t h e z a x i s , e f f e c t i v e l y f l i p p i n g t h e analyser by p i radians w i t h respect t o t h e p o l a r i z e r . By f l i p p i n g t h e c u r r e n t i n t h e e x i t solenoid, any p r o j e c t i o n o f t h e neutron s p i n along t h e y a x i s

( i . e . any r o t a t i o n o f t h e neutron s p i n i n t h e y z plane) appears as a change i n neutron count r a t e a t t h e d e t e c t o r . I f N+ represent t h e d e t e c t o r counts f o r +

c u r r e n t i n t h e e x i t solenoid, then:

where $ i s t h e neutron r o t a t i o n angle i n t h e y z plane, and PP i s t h e p o l a r i z a t i o n product o f t h e p o l a r i z e r - a n a l y s e r p a i r . By u s i n g t h e t r i m c o i l (Fig. I ) , i t i s p o s s i b l e t o make 4 = ~ r / 2 rad. t o measure PP.

The t a r g e t i s a l t e r n a t e d between p o s i t i o n s 1 and 2 . I f a pnc r o t a t i o n , @ nc, occurs when the t a r g e t i s i n p o s i t i o n 1, then t h e p i - c o i l reverses t h e r o t a t i o n Po -mpnc.

which i s then measured. With t h e t a r g e t i n p o s i t i o n 2, the r o t a t i o n t h a t i s

measured i s +$pnc,.making t h e d i f f e r e n c e between t h e two t a r g e t p o s i t i o n s equal t o 24pnc. Due t o r e s i d u a l magnetic f i e l d s , t h e r e w i l l i n general be a p a r i t y

conserving (pc) r o t a t i o n , 4 ,, as w e l l , t h a t can be 100 t o 1000 times l a r g e r than 4 . To t h e e x t e n t t h a t t h & t a r g e t i s non-magnetic and tha t t h e neutron beam p K F i l e does n o t change when t h e t a r g e t i s moved, ( w i l l be the same f o r b o t h t a r g e t p o s i t i o n s . The main challenge i n these expe!~ments i s t o prove t h a t apparent pnc r o t a t i o n s are n o t pc r o t a t i o n s t h a t change when t h e t a r g e t i s moved. To t e s t f o r t h i s p o s s i b l e systematic e r r o r , one h a l f o f a l l data i s taken w i t h t h e p i - c o i l t u r n e d o f f . I n t h i s case, i n t h e absence o f systematic e r r o r s , t h e measured neutron s p i n r o t a t i o n should be the same f o r each t a r g e t p o s i t i o n . For t h e r e s u l t s presented i n t h i s paper, no systematic e r r o r s were observed f o r the p i - c o i l o f f data. A f t e r t h e measurement w i t h t h e p i - c o i l o f f , t h e p i - c o i l i s turned on f o r a measurement o f

C#I

,,. By t u r n i n g t h e c o i l on, however, e x t r a s t r a y magnetic f i e l d s are induced i n tRe t a r g e t region, making a comparison w i t h the p i - c o i l o f f data l e s s than i d e a l . To e l i m i n a t e e f f e c t s associated w i t h t h e leakage magnetic f i e l d s from t h e p i - c o i l , t h e leakage f i e l d s are reversed by r e v e r s i n g t h e c u r r e n t i n t h e p i - c o i l . Systematic e r r o r s q u a d r a t i c i n t h e p i - c o i l magnetic f i e l d a r e t e s t e d f o r by doubling t h e c u r r e n t i n t h e p i - c o i l (causing a Larmor precession by k211 rad.).

One measurement c y c l e ( 5 days) includes t h e various p i - c o i l c u r r e n t s described above.

A f t e r such a c y c l e , t h e t a r g e t i s r o t a t e d i n o r i e n t a t i o n w i t h respect t o t h e neutron beam and a new c y c l e begun. 4 must, o f course, be independent o f t a r g e t

o r i e n t a t i o n . When possible, aPPFnal systematic check can be made by e x p l o i t i n g t h e f a c t t h a t +pnc increases l i n e a r l y w i t h t h e t a r g e t length. The experiment i s t h e r e f o r e repeated w i t h t a r g e t s o f d i f f e r e n t lengths.

The systematic checks described above a r e c r u c i a l . A f t e r our i n i t i a l measurements o f 4 nc i n isotopes o f Sn ( R = 5 cm), a l a r g e r p o l a r i m e t e r was b u i l t t o handle t a r g & t s o f 50 cm length. With the l a r g e r p o l a r i m e t e r , c r i p p l i n g spurious r o t a t i o n s were observed which we a t t r i b u t e d t o changes i n t h e neutron f l i g h t path (upon t a r g e t r e v e r s a l ) coupled t o inhomogeneous leakage f i e l d s from the solenoids. The c u r e t o t h i s problem was twofold. F i r s t , a neutron v e l o c i t y s e l e c t o r was i n s t a l l e d t o ensure t h a t no neutrons w i t h wavelength s h o r t e r than t h e Bragg c u t - o f f o f t h e t a r g e t would e n t e r t h e p o l a r i m e t e r ( e l i m i n a t i n g mu1 t i p l e Bragg s c a t t e r i n g ) . Second, the solenoids were redesigned as m o d i f i e d t o r i , and were wound i n such a way t h a t t h e magnetic f i e l d 1 ines were t o t a l l y confined i n s i d e t h e solenoids (leakage f i e l d s l o + "

t o l o t 5 times smaller than t h e i n t e r n a l f i e l d ) . With t h i s m o d i f i e d p o l a r i m e t e r , we were a b l e t o measure (pnc i n t a r g e t s o f Sn and Pb up t o 30 cm i n length.

The r e s u l t s o f our i n v e s t i g a t i o n s a r e given i n Table 1.

Target A(cm2) 4pnc/R (rad/cm) Ref.

'l7Sn 6 1 -37.022.5 x ~ O - ~ /8/

Natural Sn 10 , 20 3-4 -3.19+0.40~10-'j /9/

Natural Pb 10, 20, 30 3-4 +2.24+0.33~1 0-6 /9/

Table 1 Experimental Results

R = t a r g e t length, A = neutron beam cross-sectional area Suoted e r r o r s are one standard d e v i a t i o n

We quote a 2 standard d e v i a t i o n l i m i t f o r "'Sn due t o

u n c e r t a i n t i e s i n t h e i s o t o p i c p u r i t y o f t h e t a r g e t .

C3-92 JOURNAL DE PHYSIQUE

The l a r g e pnc r o t a t i o n s i n '17Sn and 1 3 ' ~ a a r e c o n s i s t e n t w i t h t h e h e l i c i t y - dependent n e u t r o n t r a n s m i s s i o n asymmetries measured i n t h e s e i s o t o p e s i n Gatchina and Dubna /10,11,12/, and a r e a t t r i b u t e d t o t h e presence o f narrow n e u t r o n p-wave resonances /13,14/. However, '"Sn has a broad p-wave resonance a t 62 eV, y e t no enhanced r o t a t i o n was measured (an enhancement o f r o u g h l y 200 i s expected f r o m a p o t e n t i a l s c a t t e r i n g model / 6 / ) . More knowledge about t h e i n t e r n a l s t r u c t u r e o f t h i s resonance i s needed t o understand t h e s m a l l pnc r o t a t i o n . The l a r g e r o t a t i o n i n Pb i s s u r p r i s i n g because near a doubly magic nucleus, minimal p a r i t y m i x i n g i s expected due t o a l a r g e energy s e p a r a t i o n between o p p o s i t e p a r i t y energy l e v e l s . T h i s g e n e r a l i z a t i o n may n o t h o l d f o r '07Pb (20% n a t u r a l abundance) which l o o k s l i k e a

P 1 / 2

n e u t r o n h o l e i n an o t h e r w i s e doubly magic nucleus.

The main r e s u l t o f t h e above measurements has been t o emphasize t h e importance o f p-wave resonances i n t h e u n d e r s t a n d i n g o f pnc e f f e c t s . I n t h e s e medium t o heavy n u c l e i , t h e c o m p l e x i t y o f understanding t h e m i c r o s c o p i c p-wave s t r e n g t h makes f o r m i d a b l e t h e t a s k o f e x t r a c t i n g t h e neutron-nucleon weak i n t e r a c t i o n s t r e n g t h f r o m t h e pnc measurements /15/. Toward t h i s end, i t i s c l e a r t h a t experiments s h o u l d be conducted i n l i g h t n u c l e a r t a r g e t s , most n o t a b l y H, D, 3He, and 'He. Although pnc r o t a t i o n s o f o r d e r lo-' rad/cm a r e t o be expected, such s e n s i t i v i t y i s w i t h i n reach e x p e r i m e n t a l 1 y.

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