HFS-interaction of the Gd155 nucleus in the 87 keV excited state

Volume 11, number 1

PHYSICS LETTERS

1 July 1964

References 1) D.M. Brmk and G.F.Nash, Nuel Phys 40 (1963) 608 2) G. F Nash, Nuovo Clmento 32 (1964) 727 3) D.M Brink, Nucl Phys 40 (1963) 593 4) G. F Nash, Nuovo Clmento 31 (1964) 992 5) H.Horie and T.Yokozawa, Physms Letters 7 (1963) 145 6) D. Blum, P Barreau and J Belheard, Physms Letters 4 (1963) 109 7) N.W Tanner G C.Thomas and E D.Earle, Nuel Phys 52 (1964) 29 $ $ $ $ $

HFS-INTERACTION O F T H E Gd 1 5 5 N U C L E U S 87 k e V E X C I T E D STATE

IN THE

E. BOZEK, A. Z HRYNKIEWICZ, S O G A Z A and J. S T Y C Z E I q Instztute of Nuclear Physics, Cracow Received 2 June 1964

T he m t e r a c t l o n of the g a d o l m m m Gd 154 and Gd 156 n u c l e i with the e l e c t r o n i c s h e l l of Gd 3+ions w a s i n v e s t i g a t e d by S t le n m g and D e u ts c h 1) and B a u e r and D e u t s c h 2). Then" e x p e r n n e n t s showed that the attenuation of the 7 - 7 an~_~l_~r c o r r e l a t i o n m the c a s e of g a d o l m m m is due to the h y p e r f m e s t r u c t u r e i n t e r a c t i o n and that the a m s o t r o p y can be r e s t o r t e d , when a longitudinal m a g n e t i c fiel d i s applied. Fr~om the m e a s u r e m e n t s of the a n g u l a r c o r r e l a t i o n a n l s o t r o p y in the s t r o n g t r a n s v e r s e m a g n e t i c f i e l d , the h y p e r f m e s t r u c t u r e c o n s t an t s w e r e d e t e r m i n e d and the g - f a c t o r s f o r the f i r s t e x c i t e d s t a t e s f o r both g a d o l m m m i s o t o p e s w e r e obtained. It s e e m e d wo r t h while to p e r f o r m s n n d a r e x p e r n n e n t s f o r the 87 keV e x c i t e d s t a t e m C-d155. F o r t h e s e i n v e s t i g a t i o n s the r - r c a s c a d e (182 + 180) - 87 keV was used. The p a r e n t 5.6 days Tb 155 i sot ope was s e p a r a t e d f r o m the p r o d u c t s of the s p a l l a t l o n r e a c t i o n m ta n t a l u m . The s o u r c e was u s e d m the f o r m of TbC13 d l s o l v e d m w a t e r . The l i f e - t i m e of the 87 keV state m Gd 155 3) was r e m e a s u r e d by m e a n s of r - r c o i n c i d e n c e s using a t i m e - t o - p u l s e - h e i g h t c o n v e r t e r . The d e c a y c u r v e of the 87 keV state is shown m fig. 1. The l e a s t s q u a r e fi t of the slop~ g i v e s the v a l u e T_~ = (6.68 ± 0.12) n s e c 2

F r o m the m e a s u r e d t i m e - s p e c t r a at 90 ° and 180 ° the t i m e d e p e n d e n c e of the p r o d u c t G2(t)A 2 m the a n g u l a r c o r r e l a t i o n function

W(O,t) = 1 + G 2 t A 2 P 2 ( c o s was obtained.

6)

(1)

) osec

t)

6

~

2b

3b

~b

5'o

6'0 ....

TIME

Fig 1 Decay curve of the 87 keV state of Gd 155. The r z s u l t i s shown m fig. 2. Th e slope of the attenuation c u r v e c o r r e s p o n d s to a v a l u e of the r e l a x a t i o n constant ~2 = (1.88 ±0.53) x 107 s e c -1 Using this v al u e and the v al u e of the U f e - t l m e the i n t e g r a l attenuation c o e f f i c i e n t G2 was c a l c u l a t e d . G2 = 0 845 • 0.040 T h i s r e s u l t i s m good a g r e e m e n t with the v a l u e s G2 = 0 8 2 ± 0 0 2 a n d G2 = 0 84 + 0 0 2 f o r Gd 154 and Gall56 r e s p e c t i v e l y 1,2). An analogous e x p e r n n e n t was p e r f o r m e d with a m a g n e t i c f i el d of about 1500 G a p p h e d along the p r o p a g a t i o n d i r e c t i o n of one of the g a m m a - r a y s . The v al u e k2 = (4 4 ± 3.6) × 106 s e c -1 c o r r e s p o n d 63

Volume 11, number 1

PHYSICS LETTERS

1 July 1964

that a s a t u r a t i o n of the d eco u p h n g effect was r e a c h e d The a v e r a g e s a t u r a t i o n v al u e f o r G2 is G2 = 0.123 ~:0.025 o~

,

The o b s e r v e d s a t u r a t i o n effect m the s t r o n g t r a n s v e r s e m a g n e t i c field can be explained by a c o m p l e t e d eco u p l m g of the h f s - m t e r a c t i o n ( P a s c h e n - B a n k effect) and by such an i n c r e a s e of the e l e c t r o n i c r e l a x a t i o n t i m e that du~mg the l i f e t i m e of the ex ci t ed n u c l e a r state t h e r e a r e no t r a n s i t i o n s between d i f f e r e n t m j - s u b s t a t e s of the e l e c t r o n i c sh el l A c c o r d i n g to this i n t e r p r e t a t i o n the m a g n e t i c m o m e n t s of gadolinium n u c l e i , m the c a s e of a s t r o n g e x t e r n a l m a g n e t i c f i e l d , int e r a c t with 8 v a l u e s of a l h g n e d m t r a - a t o m l c m a g n et i c f i e l d s , c o r r e s p o n d i n g to the 8 m j - v a l u e s of the 8SZ state of Gd3+-lons. Th u s, the s a t u r a t i o n value ~f the attenuation co ef f i ci en t G2 can be ex p r e s s e d by the f o r m u l a 3 1 G2 = ~ ~ l + [ ( 2 v + l ) a T ) 2 , (3) v=0

155Gd

005

0 03

A~ - 0 1 0 4

"-o oo5

G2- 0 8 4 5 "-oo4o

T/tIE

Fzg. 2 Time depends of the attenuation m g to the i n t e g r a l c o e f f i c i e n t s G2 = 0.96 ± 0.04 obtained m this c a s e shows that the attenuation of the angular c o r r e l a t i o n is r e m o v e d a l m o s t c o m pletely The i n t e g r a l m e a s u r e m e n t s of the p r o d u c t G2A 2 w e r e p e r f o r m e d m t r a n s v e r s e m a g n e t i c f i e l d s of i n t e n s i t i e s 0, 8, 15, 21 and 28 kG F o l lowing the notation of Stlenmg and D e u ts c h 1) we use m the f o r m an g u l a r c o r r e l a t i o n function m the convenient f o r m W(0)= 1 + G 2 B 2 c o s 20

,

(2)

where 3A 2

B2 = 4+A----2 and G 2 -

4G 2 + G2A 2

4+G2A2

To obtain G2 f r o m the r e s u l t of our m e a s u r e m e n t s we need, b e s i d e s the m e a s u r e d p r o d u c t s G2A2, the value of G2 It can be c a l c u l a t e d f r o m the following f o r m u l a (G2A2)H G2 - (62A2) 0 (G2)0

,

w h e r e (G2A2) H and (G2A2)0 a r e the value m e a s u r e d m the s a m e g e o m e t r y f o r the t r a n s v e r s e and z e r o m a g n e t i c f i e l d , and (G2) 0 i s the value obtained f r o m the d i f f e r e n t i a l m e a s u r e m e n t d e s c r i b e d above. We found that the G 2 - v a l u e s obtained m this way d e c r e a s e with an m c r e a s m g m a g n e t i c f i e l d , but f o r the two h i g h es t f i e l d s used m t h e s e e x p e r i m e n t s , 21 and 28 kG, no change of G2 w a s obs e r v e d within the hm~ts of e r r o r . T h i s m e a n s 64

w h e r e a is the h f s - c o n s t a n t and r is the m e a n hfe. t i m e of the excited state. The g r a p h i c solution of (3) g i v e s f o r av the v al u e aT = 1.26 +0.22 - 0 17 r a d l a n s

,

hence the h y p e r f m e s t r u c t u r e constant f o r the 87 keV ex ci t ed state of Gd 155 a = 2 0 6 _ 2+

8 MHz

3 6

which c o m p a r e s with a = 11 9 + 0.4 Mttz and a = 15 9 ± 0.3 MHz f o r Gd 155 and Gd 157 ground s t a t e s 4) and with a = 26.4 ± 5 MHz and a = 23 9 ± 4.3 MHz f o r the 2+ fLvst ex ci t ed s t a t e s m Gd 154 and Gd 156 1,2) The g - f a c t o r of the 87 keV ex ci t ed st at e m Gd 155 can be calculated f r o m the obtained h f s constant a and the known a and Iz-values f o r the ground s t a t e s of C,d155 and Gd 157. Th e g - v a l u e s a r e 0.242 ± 0.002 nm and 0 3225 ±0.0010 nm r e s p e c t i v e l y 5) In this way we obtain f o r the 87 keV state g = 0.28 +0.05 - 0.04 A s the s,gn of this g - f a c t o r is not d e t e r m i n e d and the spin of the 87 keV ex ci t ed state is not c e r t a i n (3+ or §+) it i s difficult to d r a w any concluslon on the s t r u c t u r e of t h i s state. One can expect f o r Gd 155 a s t r o n g m i x i n g of the Nllson s t a t e s [402]~ and [65113 and [642]~ The t h e o r e t i c a l g - v a l u e s f o r t h e s e s t a t e s with y = 6 and

Volume 11, number 1

PHYSICS LETTERS

1) R.Stlenlng and M.Deutsch, Phys Rev 121 (1961) 1484 2) R W. Bauer and M.Deutsch, Phys Rev 128 (1962) 751 3) M.Vergnes, Ann Phys 5 (1960) 11 4) W. Low and D Shaltlel, J. Phys Chem Sohds 6 (1958) 315 5) I. Lmdgren, Table of nuclear moments, Perturbed angular correlatmns (North-Holland Pubhshmg Co , Amsterdam, 1964) 6) J.De Boer and J H Rogers, Physics Letters 3 (1963) 304

gR = 0.31 + 0.04 (as for the ground state band 6)) a r e +1.18, -0 24 and -0.26 r e s p e c t i v e l y , thus, the e x p e r n n e n t a l g - v a l u e i n d i c a t e s a mixing. The a u t h o r s wish to e x p r e s s t h e i r gratitude to P r o f e s s o r H Nmwodnlczgnskl for his constant i n t e r e s t m t h i s work. They a r e also indebted to Dr. I Strofiskl and Dr J Mfl~ulskl for the c h r o m a t l c g r a p h l c s e p a r a t i o n of t e r b i u m and p r e p a r a t i o n of s o u r c e s

DELAYED

PROTON

EMISSION

1July1964

FOLLOWING

THE

DECAY

OF

N e 17

R McPHERSON, J C HARDY and R E BELL Rad~atzon Laboratory, McGzll Umverszty, Montreal

Received 8 June 1964

This r e p o r t d e s c r i b e s the decay of Ne 17, the h e a v i e s t neon lsotop~ that can be a p r e c u r s o r of ~ - d e l a y e d u r o t o n s . The existence (1 e p a r h c l e stability) and ~ - d e c a y e n e r g y of Ne 17 have been p r e d i c t e d by s e v e r a l a u t h o r s 1 - 3 ) u s i n g i s o b a r i c m v a r m n c e p r m c l p l e s and known m a s s e s of n e i g h b o u r m g or m i r r o r n u c l e i to p r e d i c t the m a s s e s of p o s s i b l e light nuclei. E x p e r i m e n t a l l y , Ne 17 was f i r s t identified by Barton et al. 4) when p r o ton p e a k s at a p p r o x i m a t e l y 3.8 and 5.0 MeV ( c e n t r e - o f - m a s s ) w e r e detected following i t s d e cay. T h e s e p r o t o n s were p r o b a b l y the s o u r c e of the otherwise unidentified delayed proton events f i r s t o b s e r v e d by Karnaukhov et al. 5) following b o m b a r d m e n t of N1 with 130 MeV Ne 20 tons. A l a t e r work by the s a m e group 6) shows a broad proton group at r o u g h l y 5 MeV with an o b s e r v e d half life of 85 ± 15 m s , a t t r i b u t e d by the a u t h o r s to a n u c h d e not differing g r e a t l y m m a s s f r o m Ne 20 (for example Ne 17 o r Mg 20) The m o r e detailed r e s u l t s to follow definitely a s s i g n an a c t i v ity to Ne 17 that would explain the R u s s i a n r e s u l t s . The a p p a r a t u s c o n s i s t e d of a s u r f a c e b a r r i e r s i l i c o n detector of 200 m m 2 a r e a and 300~ dep l e h o n depth (6 MeV for p r o t o n s ) , mounted 6 cm f r o m a thin t a r g e t foil. This whole a s s e m b l y was mounted on a p r o b e that could be i n s e r t e d into the c n ' c u l a t m g proton beam of the McGlll s y n c h r o c y c l o t r o n Depending on the cyclotron r a d i u s chosen for b o m b a r d m e n t , the proton beam t r a v e r s e d the t a r g e t foil f r o m 10 to 50 t n n e s , and hence the countmg r a t e s were such a~ would be obtained

with a beam of some tens of m l c r o a m p e r e s . Counting was p e r f o r m e d between r e p e t i t i v e 40 m s b u r s t s of cyclotron operatlon A dlgltally cont r o l l e d counting p e r l o d of d e m r e d length was m l h a t e d 100 m s after each b u r s t . Thls delay was chosen long enough to allow d l s s l p a t l o n of cyclot r o n beam storage effects and obtain qmet spect r a . The counting p e r l o d could be dlvlded mto four equal p e r l o d s , enabling the s p e c t r u m to be s e q u e n t i a l l y stored m the four q u a d r a n t s of a 256 channel a n a l y z e r . F o u r pomt decay c u r v e s were thus obtained f r o m the spectra. Flg 1 shows the delayed proton s p e c t r a f r o m two types of thm t a r g e t containing f l u o r i n e (monoisotopic F l 9 ) . The upper c u r v e shows the spect r u m obtained following b o m b a r d m e n t of 2 5 m g / cm 2 of LIF deposited as a s l u r r y on a 2.6 m g / c m 2 m y l a r film backing. The t h r e e proton p e a k s a r e a t t r i b u t e d to the decay of Ne 17. The lower c u r v e is the s p e c t r u m from 2.0 m g / c m 2 of L1F vacuum evaporated onto A1 foil of 2.4 m g / c m 2, m which c a s e t h e r e appear also the delayed proton peaks f r o m S125 produced m the a l u m m l u m 4). The e n e r gies shown have been c o r r e c t e d to c e n t r e - o f - m a s s and have u n c e r t a i n t i e s of about +0 05 MeV T a r gets of teflon (CF2) n gave a s p e c t r u m s u m l a r to that f r o m L1F on m y l a r , w h e r e the background contmumn i s believed due to some activity reduced m carbon. The a p p a r e n t yield of Ne 17 i s lower m teflon than in LIF, an effect a t t r i b u t e d to the high gas p e r m e a b i l i t y of teflon, allowing diffusion of neon into the cyclotron vacuum. 65