NUCLEAR SPIN POLARIZATION OF BETA-RADIOACTIVE NUCLEI CREATED BY MEANS OF TILTED FOIL TECHNIQUE

HAL Id: jpa-00230940

https://hal.archives-ouvertes.fr/jpa-00230940

Submitted on 1 Jan 1990

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.

NUCLEAR SPIN POLARIZATION OF

BETA-RADIOACTIVE NUCLEI CREATED BY MEANS OF TILTED FOIL TECHNIQUE

Y. Nojiri, S. Momota, A. Ozawa, A. Kitagawa, M. Fukuda, K. Matsuta, T.

Minamisono

To cite this version:

Y. Nojiri, S. Momota, A. Ozawa, A. Kitagawa, M. Fukuda, et al.. NUCLEAR SPIN POLARIZATION OF BETA-RADIOACTIVE NUCLEI CREATED BY MEANS OF TILTED FOIL TECHNIQUE.

Journal de Physique Colloques, 1990, 51 (C6), pp.C6-549-C6-552. �10.1051/jphyscol:1990670�. �jpa-

00230940�

NUCLEAR SPIN POLARIZATION OF BETA-RADIOACTIVE NUCLEI CREATED BY MEANS OF TILTED FOIL TECHNIQUE

Y. NOJIRI, S. MOMOTA, A. OZAWA, A. KITAGAWA, M. FUKUDA, K. MATSUTA and T. MINAMISONO

Department of Physics, Osaka University Toyonaka, Osaka 560, Japan

Le mdcanisme fondamental de la technique des feuilles inclinees a BtB Btudid expdrimentalement pour ses applications aux 'Bmetteurs P de courte vie. Le gain de polarisation nuclBaire du " B par des feuilles multiples a 6t6 mesurB en fonction du nombre et de l'espacement des feuilles. La technique a ensuite dt6 appliqude B des Bmetteurs 6 plus lourds 'OF et 4 1 ~ c .

ustract

-

The fundamental mechanism of the tilted foil technique has been experimentally studied for its effective application to short-lived beta- emitters. The multi-foil enhancement in 12B nuclear polarization was measured as a function of the foil number and the foil spacing. The technique was further applied to heavier beta-emitters, 2 0 ~ and 4 1 ~ c .

Since the first discovery of atomic polarization produced through beam-tilted foil interactions/l/, the tilted foil technique has been applied successfully in a series of various measurements on nuclear physics/2/. In fact, the technique has proved its effectiveness for experimental studies on short-lived radioactive nuclei, particularly in NMR measurements on beta-radioactive nuclei,

2 7 ~ i (~1/2=4.2 sec) /3/ and 3 3 ~ 1 (~112'2.51 sec) /4/. In these experiments, it has played a very crucial role to create nuclear polarizations of the beta emitters produced through nuclear reactions. In order to expand its applicability to a wide range of beta-emitters, the technique was recently applied to the creation of polarized projectile fragments, 3 9 ~ a (T1i2=0. 86 sec) and 4 3 ~ i (T1,2=0. 51 sec) produced in high-energy heavy-ion collisions/5,6/.

Essential mechanisms constituting the technique are 1) atomic polarization creation through beam-tilted foil interactions and 2) the atomic polarization transfer to nuclei through possible hyperfine interactions. The mechanism 1) has been well studied experimentally through measurement of atomic polarization, and it is concluded from the studies that the polarization creation is resulted from asymmetric surface collisions between outgoing atoms and and lattice ones.

For the mechanism 2), quantum oscillation of atomic polarization was confirmed.

Direct evidence of the creation of the nuclear polarization by the mechanisfi has been shown in the measurement of the nuclear polarization in short lived beta emitter 12~/7/. Enhancement in the nuclear polarization by use of multi-foil stack was also confirmed in Ref. 7.

Although the mechanism of the polarization transfer and the multi-foil enhancement have been proposed theoretically, it should be confirmed

experimentally, in order to use the technique effectively for the wide range of application. In particular, we should know more about the optimum condition for the inter-foil spacing of the multi-foil stack, which is defined as the distance between foils in the direction of beam path, and the foil number, in addition to the tilt angle.

In the present experiment, the fundamental mechanism in the polarization enhancement achieved by use of multi-tilted foils was studied. As a proof of the proposed polarization mechanism, the multi-foil enhancement effect of nuclear polarization in short-lived beta-emitter l2B(1& l+, T1/2= 20 msec) was measured as a function of distances between two foils and also of numbers of multi-foils. In the following, the technique was further applied to heavier

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990670

C6-550 COLLOQUE DE PHYSIQUE

s h o r t - l i v e d beta e m i t t e r s , 2 0 ~ (I& 2+, T1/2= 11.03 ( 6 ) s e c ) and 4 1 ~ c (I*= 7/2-, T1/2= 596.3 ( 1 7 ) msec)

,

t o prove t h e e f f e c t i v e n e s s of t h e technique.

The e s s e n t i a l p a r t of t h e present measurement i s s i m i l a r t o t h e previous one/8/. The experimental s e t u p i s shown i n F i g . 1. Beta e m i t t e r s , 1 2 ~ , 2 0 ~ 1 and 4 1 ~ ~ , were produced through t h e nuclear r e a c t i o n s 1 1 ~ (d, p ) 1 2 ~ , 19F (d, p ) 2 0 ~ 1 and 4 0 ~ a (d, n) 4 1 ~ c , r e s p e c t i v e l y . Deuteron beams (Ed- 4 MeV) were e x t r a c t e d from t h e Van de Graaff a c c e l e r a t o r a t Osaka University. Recoil n u c l e i emerged from t h e t a r g e t i n t o t h e r e c o i l angle 30' were s e l e c t e d by a c o l l i m a t o r . Nuclear p o l a r i z a t i o n was induced i n t h e n u c l e i by passing through a m u l t i - f o i l s t a c k . The s t a c k c o n s i s t s of 1-6 carbon f o i l s of 5 pg/cm2 t h i c k t i l t e d 60' r e l a t i v e t o t h e beam d i r e c t i o n . The p o l a r i z e d n u c l e i were focussed by use of an

e l e c t r o s t a t i c l e n s and implanted i n t o a c a t c h e r . A week magnetic f i e l d (- 1 0 Oe) was used t o maintain t h e atomic p o l a r i z a t i o n during t h e f l i g h t of t h e r e c o i l n u c l e i while a s t r o n g magnetic f i e l d (- 2 kOe) was a p p l i e d t o preserve t h e nuclear spin p o l a r i z a t i o n a f t e r t h e implantation. The stopping m a t e r i a l s were a m e t a l l i c Pt f o i l , a s i n g l e c r y s t a l CaF2, and f i n e g r a n u l a r c r y s t a l s T i c f o r 1 2 ~ , 2 0 ~ , and 4 1 ~ c , r e s p e c t i v e l y . The nuclear p o l a r i z a t i o n was determined through measurements of asymmetries i n beta-ray emissions from t h e n u c l e i by means of beta-NMR technique.

Beam (d)

f

^{Fig. 1.} Schematic view of experimental s e t u p .

The nuclear spin p o l a r i z a t i o n of 1 2 ~ produced with two f o i l s was measured a s a function of i n t e r - f o i l spacing. The induced nuclear p o l a r i z a t i o n showed an exponential i n c r e a s e a s t h e i n t e r - f o i l spacing increased a s shown i n Fig. 2.

The p o l a r i z a t i o n P (X) was well described by a form,

where X i s t h e i n t e r - f o i l spacing, PO i s a s a t u r a t e d value of induced

p o l a r i z a t i o n , PO-P1 i s t h e p o l a r i z a t i o n c r e a t e d by s i n g l e f o i l , and X0 i s a f r e e d i s t a n c e parameter. From t h e l e a s t

x2

a n a l y s i s , t h e d i s t a n c e parameter was

F i g . 2. Enhancement of n u c l e a r F i g . 3. M u l t i - f o i l enhancement p o l a r i z a t i o n of 12B a s a f u n c t i o n of l2B n u c l e a r p o l a r i z a t i o n . of i n t e r - f o i l s p a c i n g .

deduced t o be Xo= 1.26(46) mm. The parameter X0 i s r e l a t e d t o t h e t r a n s f e r time

z

of t h e p o l a r i z a t i o n from atomic s p i n t o t h e n u c l e a r s p i n , a s Xo= vt, where v i s t h e v e l o c i t y of 1 2 ~ a f t e r p a s s i n g through a f o i l . Using t h e a v e r a g e v e l o c i t y v= 3 . 1 mm/nsec o f 1 2 ~ , t h e t r a n s f e r t i m e was deduced t o be %= 0.41(15) nsec.

On t h e o t h e r hand, t h e t r a n s f e r t i m e Z can be c a l c u l a t e d from t h e known h y p e r f i n e i n t e r a c t i o n s t r e n g t h . Here, we assume t h a t t h e atomic p o l a r i z a t i o n can o n l y be induced i n atomic s t a t e s with non-zero o r b i t a l - a n g u l a r momentum.

Because t h e boron atoms a r e populated i n a l a r g e p a r t i n t h e ground s t a t e a f t e r p a s s i n g through f o i l s , t h e ground s t a t e of n e u t r a l boron, 2 ~ 1 / 2 i s n a t u r a l l y assumed t o t a k e l a r g e p a r t i n producing t h e n u c l e a r p o l a r i z a t i o n . From t h e known h y p e r f i n e i n t e r a c t i o n s t r e n g t h %f- 1.24 X 109 ( l / s e c ) / 9 / , t h e c a l c u l a t e d v a l u e f o r t h e t r a n s f e r time was deduced t o be 2- 0.8 n s e c . A good agreement between t h e p r e s e n t o b s e r v a t i o n and t h e c a l c u l a t i o n shows above assumption t o be p l a u s i b l e from t h e view p o i n t of fundamental mechanisms of t h e p o l a r i z a t i o n p r o d u c t i o n i n t h e beam-tilted f o i l i n t e r a c t i o n .

P o l a r i z a t i o n enhancement of t h e 12B by u s e of m u l t i - t i l t e d f o i l s was observed by changing t h e f o i l number from 1 t o 6. I n t h i s o b s e r v a t i o n , t h e i n t e r - f o i l s p a c i n g was kept t o be 1 mm. The r e s u l t showed a n e x p o n e n t i a l r i s e a s shown i n F i g . 3 . According t o t h e d i s c u s s i o n i n Ref. 10, t h e d a t a was analyzed u s i n g t h e r e l a t i o n ,

where P(N) i s induced n u c l e a r p o l a r i z a t i o n by use of N f o i l s , and P(-) i s t h e s a t u r a t e d v a l u e of t h e p o l a r i z a t i o n . From t h e l e a s t

x2

a n a l y s i s , t h e optimum v a l u e f o r t h e parameter k was deduced t o be k= 0 . 4 2 ( 7 ) . The parameter k i s r e l a t e d t o t h e atomic s p i n J and n u c l e a r s p i n I a s ,

k= P ( I , J ) ( P J + (1- P j ) J / I ) ,

p ( I , J) = ( 1 1 4 1 2 ) {21+ (h2- 1) l n ( I + h) / (l-

1)

1 ,

h= ~ I J / ( I ~ + J ~ )

.

( 3 )

where PJ i s t h e atomic p o l a r i z a t i o n c r e a t e d a t t h e e x i t s u r f a c e of t h e f o i l s . So, t h e k i s determined by t h e r a t i o J / I . Considering t h e n u c l e a r s p i n 1=1 of 12B, t h e average atomic s p i n <J> which e f f e c t i v e l y p a r t i c i p a t e s i n t h e

p o l a r i z a t i o n c r e a t i o n p r o c e s s was deduced t o be <J>= 0 . 8 8 ( 2 0 ) from t h e experimental v a l u e of k . This i n d i c a t e s t h a t t h e e x c i t e d s t a t e 2 ~ 3 / 2 has

a n o t h e r c o n t r i b u t i o n t o t h e p o l a r i z a t i o n c r e a t i o n t o g e t h e r w i t h t h e ground s t a t e 2 ~ 1 / 2 of t h e n e u t r a l boron atom.

C6-552 COLLOQUE DE PHYSIQUE

I n t h e s u c c e e d i n g e x p e r i m e n t s , t h e t i l t e d f o i l t e c h n i q u e h a s been f u r t h e r a p p l i e d t o h e a v i e r b e t a - e m i t t e r s , 2 0 ~ and 4 1 ~ ~ . Atomic c h a r g e s t a t e s o f b o t h n u c l e i were m a i n l y n e u t r a l and p l u s one o u t of t h e f o i l s i n t h e p r e s e n t e x p e r i m e n t a l c o n d i t i o n s . The r e s u l t s a r e shown i n F i g . 4. S i x c a r b o n f o i l s w i t h 1 mm i n t e r - f o i l s p a c i n g were u s e d f o r 20F. The o b s e r v e d p o l a r i z a t i o n f o r 2 0 ~ i n d u c e d w i t h t i l t e d f o i l s were 0.7f0.2 %. I n t h e 4 1 c a s e , t h e r e s u l t ~ ~ i s f o r s i n g l e f o i l . The o b s e r v e d p o l a r i z a t i o n o f 4 1 ~ c was 0 . 8 f 0 . 3 % . The m u l t i - f o i l enhancement c o u l d n o t b e a p p l i e d i n t h e 4 1 c a s e due t o s m a l l k i n e t i c ~ ~ e n e r g y .

As a c o n c l u s i o n , t h e mechanism of p o l a r i z a t i o n t r a n s f e r became r a t h e r c l e a r from p r e s e n t e x p e r i m e n t . F u r t h e r a p p l i c a t i o n s o f t h e t i l t e d f o i l t e c h n i q u e t o w i d e r v a r i e t y o f r a d i o a c t i v e n u c l e i are e x p e c t e d t o a c h i e v e new developments i n n u c l e a r p h y s i c s .

b)

1.5

-

_Average

1.0

0.5

-

0.0

'

^T

1 2 3

Runs Runs

F i g . 4 . N u c l e a r s p i n p o l a r i z a t i o n o f 2 0 ~ and 4 1 ~ c c r e a t e d b y t h e t i l t e d f o i l t e c h n i q u e .

Open c i r c l e s 0 and a n open c i r c l e w i t h d o t O d e n o t e t h e d a t a w i t h c a r b o n f o i l s o f 5 pg/cm2 t h i c k and 1 0 pg/cm2 t h i c k , r e s p e c t i v e l y . C l o s e d c i r c l e s d e n o t e

a v e r a g e s o f t h e s e p a r a t e r u n s . S i x c a r b o n f o i l s w e r e u s e d f o r 2 0 ~ . I n t h e 4 1 ~ ~ c a s e , t h e d a t a a r e f o r s i n g l e c a r b o n f o i l .

REFERENCES

/ l / B e r r y , H.G. e t a l . , Phys. Rev. L e t t .

2

(1974) 751.

/ 2 / B e r r y , H.G. and M. Hass, Ann. Rev. Nucl. P a r t . S c i .

32

(1982) 1.

N o j i r i , Y. e t a l . , J . Phys. Soc. J a p a n (1986) S u p p l . 391.

/ 3 / N o j i r i , Y . e t a l . , J. Phys. Soc. Japan (1986) S u p p l . 1086.

/ 4 / Rogers, W.F. e t a l . , Phys. L e t t . JJB (1986) 293.

/ 5 / N o j i r i , Y. e t a l . , Nucl. I n s t . Meth. Phys. R e s . ,

m

(1988) 193.

/ 6 / M a t s u t a , K. e t a l . , H y p e r f i n e I n t e r a c t i o n s , 1990.

/ 7 / N o j i r i , Y . and Deutch, B . I . , Phys. Rev. L e t t .

a

(1983) 180.

/ 8 / Ozawa, A. e t a l . , H y p e r f i n e I n t e r a c t i o n s , 1990.

/ 9 / F r a g a , S . e t a l . , Hand book o f Atomic Data, E l s e v i e r S c i e n t i f i c Pub., 1976.

/10/ G o l d r i n g , G . a n d Niv, Y . , H y p e r f i n e I n t e r a c t i o n s , 21 (1985) 209.