Spins and parities of higher exited states in 188Os

I 1.E.1 : 3.B

Nuclear Physics 84 (1966) 577--585" ~

North-Holland Publishing Co., A m s t e r d a m

Not to be reproduced by photoprint or microfilm without written permission from the publisher

SPINS

AND

PARITIES

OF

HIGHER

EXITED

STATES

IN 188Os E. B A S H A N D Y and S. G. H A N N A Nuclear Physics Department, A t o m i c Energy Establishment, Cairo, U A R Received 7 February 1966 Abstract : The energy level spectrum of 188Os excited in the decay of 1SURewas investigated by means of a high resolution, double-focussing beta-ray spectrometer. The internal K-conversion coefficients of the 209, 288, 673, 828 and 931 keV levels were determined by the internal-external conversion method. The ratios o f conversion and. photon intensities were normalized with respect to the 633 keV E2 transition in lSSOs. F r o m the known internal conversion coefficient of this transition, the following K-conversion coefficients were obtained ~K(209) -- 0.263 -k0.032, 0~K(288) -- 0.027 ~0.004, CTK(673) = 0.0037±0.0004, 0~K(828) = 0.0025 d70.0003, ~K(931) = 0.0047i0.0005. The multipolarity assignments of these transitions were studied on the basis of the conversion coefficients. These measurements together with the previously k n o w n assigned levels establish the spins and parities 0 +, 3-, 2-, 2 + and 1+ for the 1086, 1306, 1461, 1750 and 1958 keV levels in ~SSOs.

E

]

measured E?, I v, lee, 188Os deduced levels ar, Jr, cc. Natural target.

1. introduction The even osmium nuclei are situated in an interesting transition region between deformed and spherical nuclei, where the moment of inertia and the energy of the K = 2 b a n d d e c r e a s e w i t h i n c r e a s i n g m a s s n u m b e r 1). P r e v i o u s i n v e s t i g a t i o n s 2 - 5 ) o f t h e l a S O s n u c l e u s h a v e e s t a b l i s h e d a g r o u n d - s t a t e r o t a t i o n a l b a n d w i t h s p i n s 0 +, 2 +, 4 +, a g a m m a v i b r a t i o n a l b a n d h e a d w i t h s p i n 2 ÷ a n d s e v e r a l h i g h e r e n e r g y s t a t e s . F o r s e v e r a l l e v e l s o f lSSOs, t h e s p i n a n d p a r i t y a s s i g n m e n t s a r e u n c e r t a i n . 5) T h e r e f o r e , t h e p r e s e n t s t u d y is a n a t t e m p t t o a s s i g n s p i n s a n d c h a r a c t e r t o o t h e r s t a t e s n o t y e t f u l l y i d e n t i f i e d . A s a r e s u l t o f d i r e c t i o n a l c o r r e l a t i o n m e a s u r e m e n t s 1, 3), s p i n s 2, 4, 2, 3, 0, 2, 0, a n d 2 h a v e b e e n a s s i g n e d t o t h e l e v e l s a t 155, 478, 633, 790, 1086, 1461, 1 7 6 5 a n d 1941 k e V , r e s p e c t i v e l y . In this paper we report measurements of several internal conversion coefficients f r o m w h i c h i n f o r m a t i o n o n t h e s t a t e s a t 1086, 1306, 1461, 1750 a n d 1958 k e V c a n b e 577

I

578

E. BASHANDY AND S. G . HANNA

ded u c e d . T h e i n t e r n a l - e x t e r n a l c o n v e r s i o n m e t h o d was used. This m e t h o d is very s ui t a b l e f o r c o m p l e x decays such as th at o f 188R, see fig. I. 188

Re

1=|°

~.~

19 58

1 .t,

1941

2 209

1765 17 50

0÷ 288 p.

1461 \

=,

2

--

3

--

1306 "

1086

1 ~s*l"

1673

1133

1308

I

1118

I

o,,

1

1

1,

479

3 +

i 1803 1783

16 I0 1595 1151

I 931

+

' 5~.

828

790

.'1 °t'=

O0

0 4

+

O0

+

1306 633

478

1959

'~ ~L_

0

f

188

0s Fig. 1. Decay scheme of laSRe.

2. Experimental

Procedures

T h e i n t e r n a l - e x t e r n a l c o n v e r s i o n m e t h o d o f H u l t b e r g a n d S t o c k e n d a l 6, v) was used. T h e m e a s u r e m e n t s were p e r f o r m e d b y a 22.5 c m ra dius , i r o n - y o k e , d o u b l e - f o c u s s i n g b e t a - r a y s p e c t r o m e t e r *. T h e d e t e c t o r e m p l o y e d in the p r e s e n t studies was a G . M . c o u n t e r w i t h a 1.8 m g / c m 2 m i c a e n d - w i n d o w . 2.1.

SOURCE

PREPARATION

T h e 18aRe target was p r o d u c e d b y t h e r m a l n e u t r o n b o m b a r d m e n t o f r h e n i u m * Beta-ray

spectrometer

type

G.~-2(Moscow,

1957).

SPINS

AND

PARITIES

579

o x i d e o f n a t u r a l i s o t o p i c a b u n d a n c e o v e r a p e r i o d o f 2 d in t h e U A R r e a c t o r a t I n c h a s s . T h e flux w a s a b o u t 1013 n e u t r o n s / c m / . s. F o r t h e i n t e r n a l c o n v e r s i o n s t u d ies, r h e n i u m o x i d e w a s u n i f o r m l y s p u t t e r e d o n a l u m i n i u m foil o f t h i c k n e s s ~ I m g / c m 2. T h e s p u t t e r e d m a t e r i a l w a s d i s t r i b u t e d in a r e c t a n g u l a r f o r m o f d i m e n s i o n s 0.2 × 2 c m z. T h e t h i c k n e s s o f t h e m a t e r i a l d e p o s i t e d w a s e s t i m a t e d to be 100 k~g/cm a. F o r t h e e x t e r n a l c o n v e r s i o n s o u r c e , r h e n i u m o x i d e w a s e n c l o s e d in a s p e c t r o s c o p i c a l l y p u r e a l u m i n i u m c a p s u l e . T h e i n n e r d i m e n s i o n s o f the c a p s u l e w e r e 2 m m d i a m a n d 15 m m l e n g t h . T h e wall t h i c k n e s s w a s 0.5 m m . T h e a l u m i n i u m c a p s u l e w a s e n c l o s e d in a c o p p e r t u b e w i t h a w a l l t h i c k n e s s o f 0.5 m m t o s t o p t h e e l e c t r o n s e m i t t e d b y t h e s o u r c e . A u r a n i u m c o n v e r t e r 3 m g / c m z (5 × 30 m m 2) w a s u s e d . T h e c o n v e r t e r foil w a s o b t a i n e d f r o m t h e s a m e s u p p l y 8) t h a t w a s p r e p a r e d in S t o c k h o l m f o r s i m i l a r studies. 2.2.

MEASUREMENTS

The internal-external conversion method for the experimental determination of i n t e r n a l c o n v e r s i o n coefficients is b a s e d o n a s t r a i g h t f o r w a r d m e a s u r e m e n t o f t h e r a t e s o f e m i s s i o n o f i n t e r n a l c o n v e r s i o n e l e c t r o n s a n d g a m m a r a y s b e l o n g i n g to t h e s a m e t r a n s i t i o n . T h e r a t i o o f t h e s e r a t e s defines t h e i n t e r n a l c o n v e r s i o n coefficient.

13

u

I0

11

91-

17200

J

17100 17000 1/.000 Potentiometer reading

13900

13800

Fig. 2. The K - and L-conversion lines o f the 209 k e y transition.

T h e i n t e r n a l c o n v e r s i o n s p e c t r u m f r o m ~ 125 k e V to ~ 870 k e V w a s r e c o r d e d . T h i s r e g i o n w o u l d i n c l u d e K - c o n v e r s i o n lines o f t r a n s i t i o n s o f e n e r g i e s b e t w e e n ~ 200 k e V a n d ~ 950 k e V . A n e x a m p l e o f t h e c o n v e r s i o n lines m e a s u r e d is s h o w n in fig. 2. E a c h c o n v e r s i o n line w a s m e a s u r e d a t l e a s t twice. T h e d a t a w e r e half-life c o r r e c t e d

580

E. BASHANDY A N D S. (3. H A N N A

using a half-life o f 18 h for JSSRe. The intensities o f the electron lines were determ i n e d by integrating the areas o f p e a k s on an Nip versus p plot (p is the m o m e n t u m ) .

21000 20900 Potentiometer reading Fig.

3.

The external K-conversion line of the

209 keV

transition.

3 70X10

633 K

65

°60

0

¢n

g o u

55

673 K ..-

%', °

7750

1

7500

Potentiometer F i g . 4.

7250

reoding

The external K-conversion lines of the 633 and

673 keV

transitions.

The standard error o f these intensities is believed to be ~ 4 ~ for the intense lines and m o r e than 10 ~o for the w e a k e r lines. T h e c o n v e r s i o n electron intensity results were corrected for the a b s o r p t i o n in the m i c a counter w i n d o w .

SPINS

AND

PARITIES

581

T h e external c o n v e r s i o n spectra for g a m m a - e n e r g y range 209-931 keV were m e a s u r e d in o r d e r to d e t e r m i n e the relative g a m m a intensity for the t r a n s i t i o n s in question. Figs. 3-5 show p a r t s o f the s p e c t r u m t a k e n . T h e emission o f p h o t o e l e c t r o n s f r o m the c o n v e r t e r is n o t isotropic, a n d in o r d e r to r e d u c e scattering effects the ext e r n a l c o n v e r s i o n process m u s t t h e n t a k e place in very thin layers o f m a t e r i a l . T h e p h o t o n i n t e n s i t y f r o m a c o n v e r t e r o f u n i f o r m density is p r o p o r t i o n a l to the t o t a l cross section z~, (for the p a r t i c u l a r a t o m i c shell a n d t r a n s i t i o n energy) a n d to a f a c t o r f , which d e p e n d s on the c h a r a c t e r o f the a p p r o p r i a t e p h o t o e l e c t r i c a n g u l a r d i s t r i b u t i o n a n d on the details o f the e x p e r i m e n t a l a r r a n g e m e n t of the g a m m a - r a y source 931 K

828K

30

I i

~o31

0 29 0 rq 4..., c

g 28_

,< 27 6250

i

6000

j

I

5750 5500 Potentiometer reoding

Fig. 5. T h e e x t e r n a l K - c o n v e r s i o n lines o f t h e 828 a n d 931 k e V t r a n s i t i o n s .

a n d the p h o t o e l e c t r i c c o n v e r t e r inside the s p e c t r o m e t e r . A c c o r d i n g to H u l t b e r g 7) t h e intensity of a g a m m a r a y can be expressed as I~, ~

C

A~

f~ w h e r e c is a c o n s t a n t d e p e n d i n g on the c o n v e r t e r thickness, source s t r e n g t h a n d the i n s t r u m e n t a l t r a n s m i s s i o n f a c t o r a n d A~ the m e a s u r e d intensity of a p h o t o l i n e t a k e n as the a r e a u n d e r the p h o t o c o n v e r s i o n p e a k after n o r m a l i z i n g to the unit m o m e n t u m interval. 3. Internal Conversion Coefficients

I f we let Ain a n d Aex d e n o t e the intensities of c o n v e r s i o n electrons a n d p h o t o e l e c t r o n s , respectively, it can easily be s h o w n t h a t the internal c o n v e r s i o n coefficient ~i

582

E. B A S H A N D Y A N D S, (3, H A N N A

is given b y ~i -- (Ai.)~ z ifjKdbq.

(Aex)j

The internal conversion takes place in the ith shell or subshell and the external conversion in the j t h shell or subshell. The q u a n t i t y K is the relative source strength if different radioactive sources are e m p l o y e d for internal c o n v e r s i o n a n d external conversion, d the thickness o f the converter (usually expressed in mg/cm2), b a dimensional f a c t o r a n d q the relative i n s t r u m e n t a l transmission if the latter is n o t the same for internal conversion and external conversion. The basic q u a n t i t y for the determination of the internal conversion coefficient is the integrated photoelectric cross section z. It is therefore essential to have access to accurate tables o f v. A t present the m o s t accurate calculations are those for TK by Hultberg, NageI a n d Olsson 9). T h e y are corrected to any order in ~z but neglect the effect o f screening. The correction for the latter effect is rather small for the K-shell and can partialiy be corrected for. A rectangular source a n d converter is the most useful g e o m e t r y for m e a s u r e m e n t s with a magnetic spectrometer o f a double-focussing type; correction factors fK including all the above m e n t i o n e d effects are c o m p u t e d by means o f the electronic computer B E S K , S t o c k h o l m , for 13 g a m m a energies f r o m 159 to 5000 keV for the specific g e o m e t r y used in the present experiment. In the present w o r k the m e t h o d was e m p l o y e d w i t h o u t a n y numerical calculation o f K, d, b and q by c o m p a r i n g the equation above for a transition in the same source with k n o w n ~: with the transition o f u n k n o w n 7K to be measured. The c o m p a r i s o n was m a d e using the k n o w n K-conversion coefficient o f the 633 k e V transition. The 633 keV g a m m a ray was a s s u m e d to be a pure E2 t r a n s i t i o n ao), de-exciting 3) the second 2 + excited state in 188Os. The K-conversion coefficient ~K(x) of a n y transition was thus determined as ~i,:(x) = ~xK(633)

(Ai,) x

(Aex) 633 zK(x )

(~z~in)6 3 3 (Aex) x

f(x)

zK(633) J(633)

This c o m p a r a t i v e m e t h o d of d e t e r m i n i n g the internal conversion coefficient is straightforward. It does n o t depend u p o n any knowledge o f the decay scheme and it shows great advantages over other m e t h o d s in cases where the decay is complicated.

4. Results and Discussion

The conversion coefficients are calculated by the use of the theoretical value, which agrees with experiments 10), for the 633 keV E2 transition. In this way the K-conversion coefficients o f the 209, 288, 673, 828 and 931 keV transitions were determined. The results o f the calculations are given in table 1. The largest u n c e r t a i n t y in these m e a s u r e m e n t s is in the d e t e r m i n a t i o n o f the area of the K-external conversion line. A n u n c e r t a i n t y of 12 ~ was estimated for the weakest line.

SPINS AND PARITIES 4.1. M U L T I P O L A R I T Y

583

ASSIGNMENT

O w i n g to the lack o f a high-precision c o n v e r s i o n electron singles s p e c t r u m the multip o l a r i t y a s s i g n m e n t s c o u l d n o t be m a d e u n a m b i g u o u s l y f r o m K / L a n d LI + Ln/Lm ratios, except in the case of the 209 k e V t r a n s i t i o n , w h e r e the K- a n d L - c o n v e r s i o n coefficients agreed with a m i x t u r e o f 25 ~ M1 a n d 75 ~o E2. This is in fair a g r e e m e n t with the c o r r e s p o n d i n g value d e d u c e d f r o m the i n t e r n a l - e x t e r n a l c o n v e r s i o n rate. T a b l e 2 shows the c o n v e r s i o n ratios o f t h e 209 k e V t r a n s i t i o n t o g e t h e r with t h e theoretical values calculated by Sliv a n d B a n d 11). T h e a b s o l u t e K - c o n v e r s i o n coefficients o f the o t h e r t r a n s i t i o n s are used to d e t e r m i n e m u l t i p o l a r i t i e s of t h e t r a n s i t i o n b y c o m p a r i s o n with the theoretical la) values. T h e results show t h a t the 288, 673 a n d 828 keV t r a n s i t i o n s have an electric dipole c h a r a c t e r while the 931 keV t r a n s i t i o n has a n electric q u a d r u p o l e character, see table 1. TABLE 1 D e d u c t i o n o f m u l t i p o l a r i t i e s o f t h e 209, 288, 673, 828 a n d 931 k e V t r a n s i t i o n s in a88Os Transition energy (keV) 209 288 673 828 931

M3

E3

Experimental K-conversion coefficient

9.3 2.8 0. t 5 0.82 0.056

0.42 0.18 0.0218 0.0135 0.0102

0.26 0.027 0.0037 0.0025 0.0047

T h e o r e t i c a l K - c o n v e r s i o n coefficient M1 0.59 0.24 0.026 0.0152 0.011

E1

M2

E2

0.05 0.023 0.0035 0.00235 0.0019

2.7 0.93 0.070 0.039 0.028

0.,149 0,066 0.0092 0.006 0.0048

-~0.03 ±0.004 dc 0.0004 ~0.0003 ±0.0005

M ultipolarity

MI @E2 El--M2 E1 E1 E2

TABLE 2 I n t e r n a l c o n v e r s i o n r a t i o s o f t h e 209 k e V t r a n s i t i o n in ~SsOs Conversion shell K/L LIq-LII/LtI I

Theoretical conversion ratios El 6.09 6.51

MI 6.34 111.08

E2 1.38 2.07

M2 3.96 10.74

E3 3.05 2.27

M3

Exper imental conversion ratio

1.953 2.305

2.53=L0.15 3.12~-0.21

T h e spin a n d p a r i t y a s s i g n m e n t s o f the higher levels in 18SOs have b e e n d e t e r m i n e d o n the basis o f the k n o w n 0 spin a n d positive p a r i t y o f the laSOs g r o u n d state, 2 + a s s i g n m e n t o f the 155 k e V first excited state a n d the 633 keV first state f r o m the s e c o n d b a n d a n d on the m u l t i p o l a r i t i e s o b t a i n e d in the p r e s e n t study. The 1086 k e V level. F o r the 1086 keV level, a 0 + a s s i g n m e n t is given. This conclusion is b a s e d on the m e a s u r e d values o f t h e c o n v e r s i o n coefficient o f the 931 k e V g a m m a r a y a n d on a d d i t i o n a l i n f o r m a t i o n a) o b t a i n e d f r o m the 931-155 keV g a m m a g a m m a directional c o r r e l a t i o n results w h i c h are in g o o d a g r e e m e n t with the t h e o r e t i cal coefficients for a 0 ( Q ) 2 ( Q ) 0 sequence, see fig. 1. The 1306 k e V level. This level is m a i n l y de-excited to the g r o u n d state b y the casc a d i n g 673 k e V - 6 3 3 k e V a n d by the 1151 k e V - 1 5 5 keV transitions. F r o m the K - c o n -

584

E.

BASHANDY

AND

S.

G.

HANNA

version coefficient o f the 673 k e V transition, it was c o n f i r m e d that the transition has a pure electric dipole character. C o n s e q u e n t l y the spin and parity 1 - , 2 - or 3 - assignm e n t s are p o s s i b l e for the 1306 k e V level. The directional correlation results 3) exc l u d e d spin 1 and was consistent with spin 3 or 2. The log f t > 9 value o f the beta branch feeding the 1306 k e V level has been calculated f r o m the observed g a m m a - r a y branching, m a k i n g 3) use o f the k n o w n g r o u n d - s t a t e beta intensity and c o n v e r s i o n coefficients for the 155 k e V g a m m a ray. This l o g f t v a l u e is consistent with a s e c o n d 288K 3 10

12.4

INFERNAL

CONVERSION

g

m 123 0 0

S 12.2_ L3

12.1 I

1

13300

~ 3z5

I

L

I

13200 13100 Potentiometer reading

13000

288 K i

3 10

O O

TERNAL

CONVERSION

in Z = 9 2

£9

34.6

33.5

isooo

i~9'oo Pote n t i o m e t e r

Ikdoo reoding

Fig. 6. T h e K - i n t e r n a l c o n v e r s i o n line a n d K - e x t e r n a l c o n v e r s i o n line o f the 288 k e V t r a n s i t i o n in asSOs.

f o r b i d d e n transition. I f we a s s u m e that the spin and parity o f the aSSRe g r o u n d state is 1 - , then 3 - s h o u l d be the m o s t likely a s s i g n m e n t for the I306 k e V Ievel in 188Os. The 1461 k e V level. The m e a s u r e d K-internal c o n v e r s i o n line intensity and g a m m a - r a y intensity o f the 828 k e V transition gave the c o n v e r s i o n coefficient aK = 0 . 0 0 2 5 4 - 0 . 0 0 0 4 . This value agrees with the theoretical coefficient for E l . Since the 633 k e V level has 2 + assignment, the spin and parity 1 - , 2 - or 3 - are p r o p o s e d

SPINS AND PARITIES

585

for the 1461 keV level. A r n s et al. 3) gave a s pin v a l u e 2 w h i c h fit their o b s e r v e d d i r e c t i o n a l c o r r e l a t i o n e x p a n s i o n coefficients a n d e s t i m a t e d a q u a d r u p o l e c o n t e n t n o t m o r e t h a n 0.001 in the 828 k e V g a m m a ray. T h e r e f o r e a 2 - a s s i g n m e n t to t h e 1461 keV level w o u l d be the m o s t p r o b a b l e . The 1750 k e V level. T h e 288 k eV g a m m a ray is a t r a n s i t i o n b e t w e e n the 1750 keV level a n d the 1461 keV state. T h e c o r r e s p o n d i n g c o n v e r s i o n p e a k s are s h o w n in fig. 6. T h e 288 keV t r a n s i t i o n was also o b s e r v e d in a s p e c t r u m of g a m m a rays c o i n c i d e n t w i t h the 633 keV p h o t o p e a k . F r o m th e m u l t i p o l a r i t y o f the 288 keV t r a n s i t i o n determ i n e d in o u r i n v e s t i g a t i o n a n d f r o m p r e v i o u s 3) s pin a s s i g n m e n t , s pin a n d p a r i t y 2 + are e s t i m a t e d for the 1750 k e V level. The 1958 k e V level. T h e 209 keV t r a n s i t i o n was i n t e r p r e t e d as a t r a n s i t i o n f r o m the 1958 k e V level to the 1750 keV level. T h e s p i n a n d p a r i t y o f the 1958 ke V level c o u l d be e s t i m a t e d , since it is de-excited b y an M1 + E2 t r a n s i t i o n (209 keV g a m m a r a y ) to the 2 + 1750 k e V level. T h e r e s u l t suggests 1 +, 2 + or 3 + a s s i g n m e n t s , b u t 3 ÷ is e x c l u d e d since the level is fed by a b e t a - b r a n c h w ith logJ~ = 7.17. T h i s b e t a b r a n c h c o u l d be c o n s i d e r e d as a f i r s t - f o r b i d d e n b e t a t r a n s i t i o n w ith A J = 0 or ___1 a n d the p a r i t y o f the successive level s h o u l d b e c h a n g e d . C o n s e q u e n t l y the 1958 ke V level is a s s i g n e d 1 ~, h o w e v e r 2 + c a n n o t be r u l e d out.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)

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