Excited states of 116Sn populated by the (α, 2n) reaction

1.E.4:

NuclearPhysws A134 (1969) 110--117, (~) North-HollandPubhshing Co., Amsterdam

3.A:3.B

Not to be reproduced by photoprmt or microfilm without written permissmn from the pubhsher

E X C I T E D S T A T E S O F 116511 POPULATED

B Y T H E (~t, 2n) R E A C T I O N

C.-h. C H A N G t, G. B. H A G E M A N N and T. Y A M A Z A K I tt The Niels Bohr Instltute, University of Copenhagen, Denmark Received 13 March 1969 Abstract: Spectra of prompt and delayed (~/~sec) gamma rays were observed from a target of i 14Cd bombarded with ~ 20 MeV or-particles At this energy, the (~t, 2n) reaction is dominant. Prompt internal conversion electrons were also observed. The angular &stributlon of prompt gamma rays with respect to the beam direction was measured. The multlpolarity of several transitions has been estabhshed, and the lifetime of an lsomerxc transition was measured to be 0.75/~sec. The isomeric transition itself is probably not observed, but several cascades showed this characteristic time distribution. The transition is probably connected with either the (h~) 2 10 ÷ or 8 + state proposed in the level scheme at ~ 3.3 MeV.

EI

N U C L E A R REACTION 114Cd(ct, 2n7), E~ ~ 20 MeV, measured 1¢e, E~,, 17, ct~(0), ccv-delay. ~6Sn deduced levels, Tt , cc, 7-multlpolanty.

1. Introduction Recently, Yamazaki, Ewan and Prussin

1,2) h a v e

s t u d i e d y-rays f r o m Cd(~t, x n ) S n

r e a c t i o n s e m p l o y i n g nsec t i m e analysis o f y-rays in n a t u r a l b e a m bursts o f a c y c l o t r o n 3). T h e r e b y i s o m e r i c states in e v e n Sn i s o t o p e s h a v e b e e n d i s c o v e r e d in a d d i t i o n to t h e a l r e a d y k n o w n 5 - a n d 7 - i s o m e r i c states o f lX6Sn a n d ~lSSn. T h e half-lives o f t h e s e i s o m e r i c states w e r e f o u n d to be m o r e t h a n 300 nsec. It was c o n j e c t u r e d t h a t these l o n g - l i v e d i s o m e r i c states a p p e a r i n g in 114,116,1 ! SSn m i g h t be d u e to t h e t w o - q u a s i p a r t i c l e c o n f i g u r a t i o n o f ( h ~ ) 2 l 0 +. T h e a i m o f t h e p r e s e n t s t u d y was to establish a m o r e definite level s c h e m e r e l e v a n t to the i s o m e r i c state o f

1165n. TO

this end, a psec p u l s e d b e a m f r o m t h e t a n d e m ac-

c e l e r a t o r o f t h e N i e l s B o h r I n s t i t u t e was utilized, a n d p r o m p t a n d d e l a y e d r a d i a t i o n s f r o m the 114Cd(~' 2 n ) l 1 6 S n r e a c t i o n were studied by m e a n s o f a G e ( L i ) d e t e c t o r a n d t h e o r a n g e - t y p e e l e c t r o n s p e c t r o m e t e r . A n g u l a r d i s t r i b u t i o n s o f y-rays w e r e also m e a s u r e d , a n d m u l t i p o l a r i t i e s o f m a n y t r a n s i t i o n s were d e t e r m i n e d b a s e d o n c o n v e r s i o n coefficients. T h e half-life was d e t e r m i n e d to be 0 . 7 5 _ 0 . 1 1 ~sec. H o w e v e r , since t h e i s o m e r i c t r a n s i t i o n ~tself still was n o t o b s e r v e d , its e n e r g y , m u l t i p o l a r i t y a n d location remained unknown. t Present address. Chung-Shan Umverslty, Canton, China. tt Present address. Department of Physics, Umverslty of Tokyo, Bunkyo-ku, Tokyo, Japan. 110

o

=o

<

10 z

10 3

10 ~"

10 s

---

100

,

l-

100

_~

l

157

i

n--

200

I

332

I

/,17

300

i

i--

,

'

-

I

~

I

500

I

CHANNEL NUMBER (Ey)

/.00

I

I

'

I

600

t

I

i

975

'

glg. I. The y-spectrum From 114Cd bombarded wltb 20 Me¥ =-partlcles.

,

'

?00

f

I

i

-~ . . . .

I

800

l

~

T--

r

1293

900

I

1~Cd(cl,2n)116Sn E==20 NeV

112

c.-h. CHANG et al.

2. Experiments 2.1. T A R G E T A N D BEAM A 3 mg/cm 2 thick metallic target of t14Cd was used for both 7-ray and electron measurements.

It w a s b o m b a r d e d

w i t h a n a - b e a m o f a b o u t 20 M e V . A t t h i s e n e r g y ,

t h e p r e d o m i n a n t r e a c t i o n is (~, 2 n ) a n d a c t u a l l y a l m o s t all y - r a y s t u r n e d o u t t o b e l o n g t o x X6Sn. 2.2. G A M M A RAYS A 15 c m 3 G e ( L i ) d e t e c t o r was u s e d f o r o b s e r v a t i o n o f t h e 7-rays. A t y p i c a l s p e c t r u m a t E~ = 20 M e V is p r e s e n t e d i n fig. 1. T h e e n e r g i e s a n d i n t e n s i t i e s o b t a i n e d a r e l i s t e d TABLE 1

Experimentally determined gamma energies, relative gamma intensmes and conversion coefficients for transitions observed after the H 4 C d + ~ reaction E7 (keV)

I~, (arb. umts)

100 135 157 294 319 332 346 408 417 498 544 560 584 644 680 823 845 975 1075 1100 1293

8.3 6.0 17.3 28 9.4 1.1 1.1 17 9 8.9 3 05 14.2 8.8 6.2 7.8 6.8 8.1 14 45 9.3 14.2 100

I~,(21 MeV) I~,(19MeV)

1.32 1.61 0.32 2.29 1.66

1.52 1.32 1.75 1.35 1.90 1 51 2.25 1.17 1.19

1 30

Kconversion coefficient

Multipolarity from ~

Remarks

1.00 0.237

E2 ") M1

5- ~ 3- b) 7- -+ 6- b) (~, n)

0.0253 0 043 0 021

M1 El+M2 M1, E2

0.011 0.0098 0.012 0.0054 0 0019 0 0015 0.0080 0 00076

M1, E2 M1, E2 El + M 2 E2 a) E1 El M2 E1

0.0006

E2

(8 + --~ 7 - )

6- ~ 5- b) 4 + ..~ 2 + b) 7 - --~ 5 - b)

(735-4+ 2+

--~ 5 - ) b) ~ 2 + b) --* 2 + b) _+ 2 + b) _+ 0 + b)

Conversion coefficients have 10-20 % error. a) E2 transition used for internal normahzation of 7- and e-spectra. b) Ref. 6). i n t a b l e 1. T h e i n t e n s i t i e s w e r e a l s o m e a s u r e d a t 21 M e V a n d 19.5 M e V . T h e i n t e n s i t y r a t i o s p r e s e n t e d i n c o l u m n 3 o f t a b l e 1 a r e u s u a l l y b e t w e e n 1 a n d 2. A h i g h e r r a t i o i m p l i e s t h a t t h e 7 - r a y is h i g h l y i n g , a n d t h a t t h e e m i t t i n g s t a t e h a s a h i g h a n g u l a r momentum.

I n t h e e s t a b l i s h m e n t o f a level s c h e m e , t h e s e r a t i o s h a v e b e e n u t i l i z e d .

(=, 2n) REACTION

| 13

A ratio less than one (for instance, for the 157 keV ?-ray) indicates that the ?-ray belongs to the (c~, n) reaction. 2.3. CONVERSION ELECTRONS Conversion electrons were measured with an orange-type spectrometer 4). A spectrum is presented in fig. 2. Conversion coefficients were deduced by assuming the ere of the 100 keV and 544 keV transitions to be equal to the theoretical ctK(E2)

6000

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I

~

I

t

J

I

I

'

1

CONVERSION ELECTRONS ~ C d (o..2n)~6Sn

5000

E~, = 20 NeV

K t M 100 keV

L 319 keV o

4000

K L /-08 keV

x,(2s]

3000

K L~

keV I~ 6M, keV

z 2000

I K L 135 keV [--~K

L 1.17ke¢ P-q

K

K L 2gl,[~

t lsv key

II IIK L sso kov

KL~98k,vll F--~

I~ III ~

"~ ! [

1000

0

I o

50

100

150

0

i

I

50

100

=o

CHANNEL NUMBER (MOMENTUM) Fig. 2. Conversion electrons from 114Cd bombarded with 20 MeV s-particles. values. They are listed in column 4 of table 1. Multipolarities, thus determined, are presented in column 5. 2.4. DELAYED GAMMA RAYS The external beam was deflected electrostatically to produce a pulsed beam of about 1 iLsec width to measure delayed 7-rays. The details of this device are described in ref. 5). Prompt and delayed (about 1 ysec) ?-ray spectra are shown in fig 3. Time distributions of the delayed lines were taken by a 4096-channel pulse-height analyser for a two-dimensional analysis of 256 energy channels by 16 time channels. The time spectrum started in the centre of the bursts and subsequent time distributions (fig. 4) were followed for 1.5 ysec after the bursts. The beam stopped around the fourth time channel and the three ?-rays (135, 319 and 408 keV) decay with a half-life of 0.75_0.11 /~sec. These transitions are located above the known 5- isomeric state of 350 nsec half-life 6) and preceded by the new isomeric state of > 0.5 iLsec identified

,oo

114

C.-h. CHANG et

al.

10~ 1~Cd (a.2n)1~6Sn E a = 20 HeY 13s

15 cc

319

~j 10~

Ge(LI)

~ol

l,,o L+

Z z ,¢ -r (O nLU

a

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z

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102 •

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°, DELAYED

101 0

r

1

I

50

100

150 CHANNEL

Fig.

3. Prompt

and

10 0 0 0

~ ,

NUMBER

1 i+sec d e l a y e d ,

I

I

250

300

J--

200

~

+

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+

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t

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•

£rom ,

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~ ~4Cd-F20 ,

,

400

MeV

0~-particles.

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;~Cd (,.,, 2n) -~S i i

I

I

"~'''''--i~

II 000 000

101 keV J

~,.,= 0 74± 0 11 ~s

~-~-.

I ~V2~V2 " 075+- 011 pS

5 keY

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-r

1 000

319 k+v

r~ LLI 0(J) t,,2= 0 7 6 + - 0 11 ps

=

0

c~

100

i

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2

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I

3 4 5 6 7 8 9 10 11 CHANNEL NUNBER (1CH=0191p.s)

%7

1

350

(Ey)

7-spectra ,

k~

~i" ' ~

I

12 13

4. T i m e distribution o f delayed 7-rays from the ~ 4 C d ( ~ , 2n) ~~6Sn reaction.

450

(~, 2n) REACTION

I 15

by Y a m a z a k i , E w a n a n d P r u s s m 1,2). The half-life o f 0.75 /~sec is thus assigned to this isomeric state. T h e 100 keV 7-ray exhibits a different time distribution. The g r o w t h decay p a r t a r o u n d the fifth channel is a t t r i b u t e d to the fact t h a t this t r a n s i t i o n is p r e c e d e d by the two xsomeric states in series. 2 5 ANGULAR DISTRIBUTION OF GAMMA RAYS A n g u l a r d i s t r l b u t m n s of),-rays were m e a s u r e d with the same G e ( L i ) detector, which was r o t a t e d a r o u n d the target from 20 ° to 90 ° . C o u n t i n g rates at each angle were n o r m a l i z e d by the integrated current. Legendre coefficients A2 a n d A , o b t a i n e d f r o m TABLE 2 Experimental values of the angular distribution coefficients and o f the alignment parameter :~2 for some g a m m a transitions In 1lOSn populated in the (ct, 2n) reaction

E~, (keV)

A2

135 319 408 417 544 1293

--0.1710.02 --0.07±0.03 --0.324-0.02 0.24=t=0.03 0 24±0 03 0.13±0.025

Aa

0.05 ±0.03 --0.03 ±0.03 --0.04 ~0.03 0.025±0.025

Ahgnment ~2

0.54:t:0.06 (7- --* 6-, M1) 0.1 (8 + -'~ 7-, 0.5 El+0.5 M2) 0 99=t_0.06 (6- "-~ 5-, MI) 0.54±0.07 0.18±0.03

(7- -+ 5-, E2) (2 + ~ 0 +, E2)

least-squares fits are listed in table 2. Because o f the u n k n o w n p o p u l a t i o n o f m a g n e t i c substates, these d a t a c a n n o t be c o m p a r e d dwectly with theoretical values. The a c t u a l coefficients A2 a n d A4 for a given transition jl(L1,L2)J2 are a t t e n u a t e d by factors o f a z a n d c¢4, respectively, c o m p a r e d with theoretical coefficients (B2F2, B4F4) for the c o m p l e t e l y aligned state (m = 0) as follows: A2 = c~2BEF2, A4 = c%B4F4. J

The coefficients B2F2 a n d B4F4 are t a b u l a t e d in ref. 7). The m o s t plausible spin a n d m u l t i p o l a r l t y assignments p, esented in c o l u m n 4 o f ~ b l e 2 have been used to d e d u c e the a l i g n m e n t factors c~u as given in the same column. The 100 keV 7-ray showed an i s o t r o p i c d l s m b u t i o n . This means t h a t the extranuclear field at the Sn nuclei p r o d u c e d in a metallic Cd target is such t h a t the alignm e n t vanishes a l m o s t completely in a p e r i o d o f the half-life o f 350 nsec. T h e C d metal has a hexagonal lattice, a n d an l n h o m o g e n e o u s electric field is s u p p o s e d to exert on the p r o d u c e d Sn nuclei. T h e a n g u l a r distributions o f the 135, 408 a n d 544 keV 7-rays are c o m p a t i b l e with the 7 - --* 6 - ~ 5 - sequence in the level scheme as shown in fig. 5. The c¢2 values for the 135 keV a n d 544 keV transitions are f o u n d to be the same, which is consistent with the fact t h a t b o t h transitions p r o c e e d from the same level. The u n u s u a l l y large a 2

1 16

C.-h. CHANG et al.

value for the 408 keV transition was deduced by assuming a pure M1 multipolarlty, This might indicate an admixture of E2 causing such a large angular distribution.

~/-ZM!SS~N-G---

(lO+) ( 6 ~)

7-

mN

2913

.2777

6-

4+ 63-

075MS 3232

'

-

p

2393 2368 2268

350 nsec

I

2+

1293

0+

116Sn Fig. 5. L e v e l s c h e m e o f 116Sn.

The 319 keV y-ray showed an almost isotropic distribution. This could not be explained by the E l - M 2 admixture obtained from % without assuming an attenuation factor ~2 as small as 0.1. The 319 keV y-ray involves a prompt component z) but is mostly preceded by the 0.75 /~sec isomeric state, in which the alignment is smeared out, as well as in the 350 nsec 5- state. 3. Discussion

The multipolarity of the 319 keV delayed transition is assigned as E1 + M2. This transition is placed up on the 7 - level, thus postulating a level at 3232 keV. The delayed intensities seem to be balanced at the 7- level. Because of the presence of a prompt

(~) 2II) R E A C T I O N

1 17

c o m p o n e n t , the 319 keV t r a n s i t i o n 2) c a n n o t be the isomeric transition. The spin a n d p a r i t y o f the 3232 keV level are m o s t p r o b a b l y 8 +. W i t h i n the present experiment, no isomeric transition o f the 0.75/zsec half-life was identified Therefore the location, the spin a n d p a r i t y a n d the transition c h a r a c t e r of the isomeric state are open to further investigation. However, two possibilities as to the missing isomeric t r a n s i t i o n are (i) the transition energy is t o o low to be observed; (ii) the 319 keV p e a k a p p e a r i n g 2) at E~ = 28 MeV m a y be composite. While a p r o m p t c o m p o n e n t is present in the nsec time analysis 2) at 28 MeV, no p r o m p t c o m p o n e n t seems present in our m e a s u r e m e n t (see fig. 4). A c c o r d i n g to the present m u l t l p o l a r l t y assignment, the 319 keV transition is 50/° o E1 + 50 % M2. C o n s i d e r i n g the W e i s s k o p f estimate for M2 (T~ ~ 70 nsec), we should expect this transition in itself to be isomeric, because an M2 transition is n o t expected to be enhanced. This case implies that the p o s t u l a t e d 8 + state is isomeric a n d t h a t the E1 t r a n s i t i o n to the 7 - state is h i n d e r e d by 108, which m i g h t be reasonable in view o f the possible shell-model structure o f the two states (h~k) 2 and ( h ~ ,

T h e a u t h o r s are grateful to Dr. S. B j o r n h o l m for helpful discussions a n d encouragement. T w o o f the a u t h o r s (C.-L.C. and T.Y.) w o u l d like to t h a n k Professor A. B o h r for the h o s p i t a l i t y at the Niels B o h r Institute.

References 1) 2) 3) 4) 5) 6) 7)

T. Yamazaki, G. T. Ewan and S. G Prussm, Phys Rev. Lett. 20 (1968) 1376 T. Yamazakl and G. T. Ewan, Nucl. Phys. A134 (1969) 81 T. Yamazakl and G. T. Ewan, Phys. Lett. 24]3 (1967) 278 G. B. Hansen, B. Elbek, K. A. Hagemann and W. F. Hornyak, Nucl. Phys. 47 (1963) 529 S Blornholm, J. Borggreen, H. J. Frahm and N J S. Hansen, Nucl Phys. 73 (1965) 593 E. Bodenstedt et al., Nucl. Phys 89 (1966) 305 T. Yamazakl, Nucl. Data, Sect. A, 3 (1967) 1