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Submitted on 1 Jan 1984

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THE β -DECAY ASYMMETRY OF THE FREE NEUTRON

P. Bopp, D. Dubbers, E. Klemt, J. Last, H. Schütze, W. Weibler, S.

Freedman, O. Schärpf

To cite this version:

P. Bopp, D. Dubbers, E. Klemt, J. Last, H. Schütze, et al.. THE β-DECAY ASYMMETRY OF THE FREE NEUTRON. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-21-C3-26.

�10.1051/jphyscol:1984305�. �jpa-00224019�

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JOURNAL DE PHYSIQUE

Colloque C 3 , supplement au n ° 3 , Tome 45, m a r s 1984 page C3-21

THE /3-DECAY ASYMMETRY O F THE FREE NEUTRON

P. Bopp, D. Dubbers, E. Klemt, J. Last, H. Schutze, W. Weibler, S.J. Freedman*

and O. Scharpf**

Physikalisches Institut der Universitat Heidelberg, D-6900 Heidelberg 1, F.R.G.

*Argonne National Laboratory, 9700 So Cass Av., Argonne,IL 60439, U.S.A.

**Institut Laue-Langevin, 156K, 38042 Grenoble Cedex, France

Résume - La désintégration g des neutrons polarisés a été étudiée à l'aide du nouveau spectromètre supraconducteur PERKEO à l'ILL. La dépendance en énergie de l'asymétrie g a été mesurée pour la première fois. De l'asymétrie g mesurée on obtient une nouvelle valeur pour le rapport g./g„ des coefficients du couplage

faible. V

Abstract - The g-decay of polarized neutrons has been studied with the new superconducting spectrometer PERKEO at the ILL. The energy dependence of the B- decay asymmetry has been measured for the first time. From the measured (5- asymmetry parameter we obtain a new value for the ratio of weak coupling constants g^/gv'

We report the first results obtained with the superconducting g-spectrometer PERKEO at ILL. PERKEO measures neutron decay g-spectra with a 4TT solid angle over a large volume of a cold neutron beam. The count rates from polarized neutron decay achieved with this instrument are several orders of magnitude higher than any previous experiment. In the present experiment, serious sources of systematic error, arising from B-backscattering and poorly defined source volume and detector solid angle, are largely eliminated. In the first experiment with PERKEO we have studied the size and g-energy dependence of the (3-decay asymmetry of the neutron.

The angular distribution of the electron emitted in polarized neutron decay is given by

W(E,Q) dE dfl = F(E) (1 + - PA cosO) dE dQ. (1) The Fermi function, F(E), is a function of B kinetic energy, approximately given

by the expression,

F(E) = (E - E )2 (E + mc2) A (E + 2 m c2) . (2) Here E is the maximum value of E, v/c is the electron speed in units of the

speed of light, 0 is the emission angle relative to the neutron polarization P, and A is the g-asymmetry parameter which is nonzero because of parity

violation. Except for small corrections from induced currents and radiative corrections, A is predicted in the V-A theory to depend solely on the ratio of the axial-vector and vector weak coupling constants, g./gy With X = |g^/gy|, we have

A = -2A X~l . (3)

1 + 3X

Since X turns out to be near unity measuring A is in general a good way to determine X relative to other neutron decay correlations or measurements of the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984305

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C3-22 JOURNAL DE PHYSIQUE

neutron l i f e t i m e . We n o t e t h a t s u b j e c t t o t h e assumption t h a t the v e c t o r c u r r e n t i s conserved gV i s r e l i a b l y determined from n u c l e a r B-decay d a t a but d i r e c t knowledge of gA comes s o l e l y from neutron decay. The p r e c i s e v a l u e s of gA and gV a r e important t o weak i n t e r a c t i o n phenomenology and should be determined a s w e l l a s p o s s i b l e . Recently t h e neutron weak form f a c t o r s have been recognized a s an important i n g r e d i e n t i n cosmological arguments-

PERKEO i s shown s c h e m a t i c a l l y i n Fig. 1. The p r i n c i p a l component i s a 1.7 m long 20 cm diameter superconducting s e l o n o i d which produces a

"

1.5 Tesla f i e l d a l o n g t h e d i r e c t i o n of t h e neutron beam. E l e c t r o n s from neutrons decaying i n s i d e t h e spectrometer move i n h e l i c a l p a t h s

I 1

cm i n diameter with a x i s along

magnetic f i e l d l i n e s . Four t r i m c o i l s a t e i t h e r end d i s t o r t f i e l d l i n e s c a u s i n g e l e c t r o n t r a j e c t o r i e s t o bend away from t h e neutron beam. The e l e c t r o n

t r a j e c t o r i e s a r e f i n a l l y i n t e r c e p t e d by p l a s t i c s c i n t i l l a t i o n c o u n t e r s l o c a t e d a t each end of t h e spectrometer. The magnetic f i e l d s t r e n g t h d e c r e a s e s continuously towards both ends of t h e s p e c t r o m e t e r , s o no e l e c t r o n s can be m a g n e t i c a l l y trapped. The e l e c t r o n f l i g h t times between t h e d e t e c t o r s i s e f f e c t i v e l y w i t h i n t h e range 7 ns

I

t

I

300 ns.

NEUTRON BEAM

SCINTILLATION COUNTER

SUPERCO~~DUCTING SOLENOID

Fig. 1. Schematic l a y o u t of PERKEO.

Each s c i n t i l l a t o r i s coupled t o two p h o t o m u l t i p l i e r s and coincidence s i g n a l s with t h r e s h o l d s below t h e s i n g l e p h o t o e l e c t r o n l e v e l a r e r e q u i r e d t o reduce n o i s e . The d e t e c t o r s a r e c a l i b r a t e d with v a r i o u s conversion l i n e s o u r c e s . Fig. 2 shows t h e t y p i c a l response. The observed r e s o l u t i o n corresponds t o = 160

photoelectrons/MeV.

The s c i n t i l l a t o r s a r e l a r g e enough s o t h a t t h e t o t a l d e t e c t i o n s o l i d a n g l e i s 4 ~ - s t r w i t h i n t h e spectrometer. An e l e c t r o n back- s c a t t e r e d from a d e t e c t o r i s e i t h e r r e f l e c t e d back by t h e magnetic m i r r o r e f f e c t o r it moves along a

f i e l d l i n e t o t h e o t h e r d e t e c t o r . Since both d e t e c t o r s i g n a l s a r e summed t h e t o t a l e l e c t r o n energy i s recorded. About 1% of t h e neutron decay e v e n t s have t h e measured e l e c t r o n energy shared i n the two d e t e c t o r s . The d e t e c t o r h i t f i r s t and t h u s t h e i n i t i a l e l e c t r o n v e l o c i t y component along t h e spec- trometer a x i s i s i d e n t i f i e d by t h e timing of t h e detected s i g n a l s .

PERKEO i s l o c a t e d a t t h e p o l a r i z e d neutron experimental s t a t i o n PN7 a t ILL. The t o t a l c a p t u r e f l u x i n t o t h e spectrometer

F i g . 2. D e t e c t o r response t o v a r i o u s conversion l i n e s o u r c e s .

(4)

from t h e PN7 s u p e r m i r r o r p o l a r i z e r i s ( 8

*

3) x 10' s e c - l . The beam i s c o l l i m a t e d both h o r i z o n t a l l y and v e r t i c l l y by a 1.8 m long 3 x 3 channel

c o l l i m a t o r made e n t i r e l y from s i n t e r e d 'LiF. The beam c r o s s s e c t i o n t h u s d e f i n e d i s 3.8 x 5.6 cm 2 a t t h e e x i t of t h e s p e c t r o m e t e r . No evidence f o r a h a l o on t h e n e u t r o n beam i s observed a t t h e l e v e l . During measurements of t h e 8- asymmetry n e u t r o n s a r e l o n g i t u d i n a l l y p o l a r i z e d and t h e p o l a r i z a t i o n i s s w i t c h e d e v e r y 5 s e c w i t h a c u r r e n t s h e e t s p i n f l i p p e r .

I I I I I

F i g u r e 3 i s t h e background s u b t r a c t e d u n p o l a r i z e d 8-spectrum o b t a i n e d by summing t h e s p e c t r a t a k e n f o r e q u a l times from each d e t e c t o r and both n e u t r o n p o l a r i - z a t i o n s . The s o l i d c u r v e through t h e d a t a i s a f i t t o t h e expected shape i n c l u d i n g d e t e c t o r r e s o l u t i o n ; o n l y d a t a above 250 keV i s used i n t h e f i t . The u n s u b t r a c t e d background a t low e n e r g i e s , e v i d e n t i n Fig. 3 , i s beam r e l a t e d . Improvements planned f o r f u t u r e runs a r e expected t o e s s e n t i a l l y e l i m i n a t e t h i s background.

The

- E

o t a l n e u t r o n decay count r a t e i s 165 s e c

.

We d e f i n e t h e e x p e r i m e n t a l asymmetry a s

where t h e N's a r e energy s p e c t r a f o r e q u a l times: 1 and 2 r e f e r t o t h e d e t e c t o r .

4 and

+

t o t h e p o l a r i z a t i o n . F i g . 3 . The 6-decay energy spectrum from one r u n . The s o l i d c u r v e From (1) we have i s a f i t t o t h e r e s o l u t i o n c o r -

1 r e c t e d Fermi shape. The f i t

A =

2.

( 1

+

f ) PSA [ l

+

a(E)

1.

( 5 )

8 i n c l u d e s o n l y t h o s e d a t a above

250 keV.

The f a c t o r of 112 i s from a v e r a g i n g over t h e d e t e c t o r s o l i d angle; f i s t h e s p i n f l i p r e v e r s a l e f f i c i e n c y ; and S c o r r e c t s f o r magnetic m i r r o r r e f l e c t i o n s i n t h e inhomogeneous f i e l d s a t t h e end of t h e s p e c t r o m e t e r . S i s r e l i a b l y

c a l c u l a t e d from t h e known f i e l d d i s t r i b u t i o n , we o b t a i n S = 0.890(15). The energy dependence of A8 i s dominated by t h e f a c t o r v / c but t h e a d d i t i o n a l f a c t o r 1

+

a(E) a c c o u n t s f o r a s m a l l but n o t n e g l i g i b l e e f f e c t from weak magnetism and r a d i a t i v e c o r r e c t i o n s ( s e e f o r example r e f e r e n c e 1 ) .

The u n s u b t r a c t e d background e v i d e n t i n Fig. 3 o b v i o u s l y g i s t o r s A8 a t low e n e r g i e s . For t h e p r e s e n t a n a l y s i s we f i t t h e s p e c t r a f o r Ni

+

Nit t o t h e r e s o l u t i o n c o r r e c t e d Fermi shape u s i n g o n l y t h e h i g h e r energy d a t a and u s e t h e determined curve i n s t e a d of t h e d a t a a s t h e denominators i n Eq. 4 . Presumably t h i s procedure w i l l not be n e c e s s a r y i n f u t u r e experiments. F i g u r e 4 is t h e r e s u l t i n g graph of A from one run determined by t h e above procedure. The d a t a a r e i n accord w i t h tt!e energy dependence expected from t h e v / c f a c t o r .

The n e u t r o n p o l a r i z a t i o n and s p i n f l i p e f f i c i e n c y a r e determined t o be P = 96.7(7)% and f = 99.8(3)% from measurements w i t h a second s u p e r m i r r o r and s p i n f l i p p e r and s t u d i e s of t h e time of f l i g h t p o l a r i z a t i o n spectrum of t h e n e u t r o n beam.

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C3-24 J O U R N A L DE PHYSIQUE

To d e t e r m i n e A from A we use o n l y t h e d a t a above 250 keV. We o b t a i n t h e p r e l i m i n a r y v a l u e A = -0.188(3) which i s c o r r e c t e d f o r weak magnetism and r a d i a t i v e e f f e c t s . The i n d i v i d u a l c o r r e c t i o n s and s o u r c e s of e r r o r s a r e l i s t e d i n Table 2. From Eq. 3 we o b t a i n t h e p r e l i m i n a r y r e s u l t X = 1.27(1). The s o l i d c u r v e i n F i g . 4 i s t h e p r e d i c t i o n f o r t h e e x p e r i m e n t a l A6 w i t h X = 1.27; t h e dashed c u r v e i s t h e c o r r e s p o n d i n g p r e d i c t i o n f o r X = 1.24 which i s t h e v a l u e s u g g e s t e d i n Ref. 9.

T a b l e 3 l i s t s g i v e n v a l u e s of h determined by o t h e r r e c e n t e x p e r i m e n t s . Our experiment i s t h e f i r s t t o measure t h e 6-asymmetry w i t h o u t c o u n t i n g p r o t o n r e c o i l s i n c o i n c i d e n c e and it i s t h e f i r s t t o v e r i f y t h e v / c dependence of t h e e x p e r i m e n t a l asymmetry.

We thank t h e I L L management and s t a f f f o r t h e i r h o s p i t a l i t y . T h i s work h a s been s u p p o r t e d by t h e Bundesministerium f i i r Forschung und Technologie and t h e United S t a t e s Department of Energy.

F i g . 4. Experimental 6-ray asymmetry from one r u n a s a f u n c t i o n of 6- e n e r g y . The c u r v e s a r e e x p e c t a t i o n s f o r two v a l u e s of A .

0 . 0 2

- 0 0.2 0.4 0.6 0.8

ENERGY (MeV)

TABLE 1. Count r a t e s and p o l a r i z a t i o n s i n v a r i o u s p o l a r i z e d n e u t r o n decay e x p e r i m e n t s .

--- ---

Year Ref. B-counts/hour n - p o l a r i z a t i o n R e l a t i v e Qua i t y

N P F a c t o r Q P

4

N

- --

1970 2 100 0.870(30) 1

1971 3 200 0.770(20) 1.6

1975 4 420 0.790(15) 3.5

1979 5 420 0.733(20) 3.0

1982 T h i s work 600,000 0.967(7) 7,500

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TABLE 2. C o r r e c t i o n s a p p l i e d i n d e t e r m i n i n g A.

...

C o r r e c t i o n due t o : C o r r e c t i o n E r r o r

n - p o l a r i z a t i o n P n - f l i p e f f i c i e n c y f e l e c t r o n magn. r e f l . S

& d e t e c t o r :

e n e r g y c a l i b r a t i o n e n e r g y r e s o l u t i o n r e s i d u a l background r a d i a t i v e c o r r e c t i o n s weak magnetism (CVC)

t o t a l c o r r e c t i o n s t a t i s t i c a l e r r o r

t o t a l e r r o r

*

0.0031

p r e l i m i n a r y r e s u l t -0.1178 f 0.003

...

...

TABLE 3. Summary of r e c e n t A--measurements

.

...

Year Ref. Q u a n t i t y Measured A

T112 = 636.6(9.6) T112 = 607.8(5.4) T112 = 649(12) PDG recomm. v a l u e T112 = 641(8)

1983 This work A = -0.118(3)

( p r e l i m i n a r y )

1983 11 Various hyperon decay 1.233(16)

p a r a m e t e r s

+

Cabbibo h y p o t h e s i s

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JOURNAL DE PHYSIQUE

R e f e r e n c e s

1. D. H. Wilkinson, Nuclear P h y s i c s

m,

424 (1982).

2. C. J. C h r i s t e n s e n , V. E. Krohn, G. R. Ringo, Phys. Rev. C

1,

1693, (1970).

3. B. F. E r o z o l i m s k i i , L. N. Bondarenko, Yu. A. % s t o v o i , B. A. Obinyakov, V.

I. Fedunin, A. I. Frank, JETP L e t t . 13, 252 (1971).

4. V. E. Krohn, G. R. Ringo. Phys. ~ e t t F x , 175 (1975).

5. B. G. E r o z o l i m s k i i , A. I. Frank, Yu. A. Mostovoi, S. S. Arzumanov, L. R.

V o i t z i k , Sov. J. Nucl. Phys.

30,

356 (1979).

6. C. J. C h r i s t e n s e n , A. N i e l s e n , A. Bahnsen, W. K. Brown, B. M. Rustad, Phys.

Rev. D

2,

1628 (1972).

7. L. N. Bondarenko, V. V. Kurguzov, Yu. A. P r o k o f i e v , E. V. Rogov, P. E.

Spivak, JETP L e t t .

28,

303 (1978).

8 . J . Byrne, J. b r s e , K. F. Smith, F. Shaikh, K. Green, G. L. Greene, Phys.

L e t t .

z,

274 (1980).

9. P a r t i c l e Data Group, Phys. L e t t .

m,

91 (1982).

10. Chr. S t r a t o w a , R. Dobrozemsky, P. W e i n z i e r l , Phys. Rev. D

18,

3970 (1978).

11. M. Bourguin e t a l . CERN p r e p r i n t (1983).

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