Spins and mixing ratios in 138Ba

Nuclear Physics A212 (1973) 382--386; (~) North-Holland Publishin9 Co., Anlsterdam Not to be reproduced by photoprint or microfilmwithout writtenpermissionfrom the publisher

S P I N S A N D MIXING RATIOS IN 13SBa B. SINGH. and M. W. JOH.NS Department of Physics, McMaster University, Hanlilton, Ontario Received 14 May 1973 (Revised 6 July 1973)

Abstract: The spin assignments to the 1899 (4+), 2308 (3 + or 4 +) and 2446 (3 +) keV levels in ~38Ba have been confirmed by ~-~/ directional correlation measurements. In addition, the multipolarity and E2/M1 mixing parameters for a number of transitions have been established as follows: 409 keV (MI +E2, --0.75 < ~ -< --0.45 or --0.85 < 6 < --0.05 depending on the choice ofJ ~r -- 3 + or 4 + for the initial state), 463 keV (E2, 0 < 6 < 0.15 for M3/E2 admixture), 547 keV (MI+E2, --0.06 < b < --0.015), 872 keV (MI'-E2, ¢~ undefined) and 1010 keV (MI+E2, 0.015 < ,3 -~_ +0.020).

E

RADIOACTIVITY a3aCs [from 233U (fission)]; measured 72:(0). 13SBa deduced levels, J, ~, 6. Comparison with theory. Ge(Li) and NaI(T1) detectors.

1. Introduction A l t h o u g h a large n u m b e r of decay I-3) and reaction 4, 5) studies have been m a d e of the levels of ~38Ba, m a n y ambiguities in J ~ assignments still exist a n d the knowledge o f E2/M1 mixing ratios is very fragmentary. These a n g u l a r correlation studies were u n d e r t a k e n to provide J ~ assignments for the three strongly populated states which are a m e n a b l e to the a n g u l a r correlation technique a n d to o b t a i n f-values for the transitions de-exciting them. D u r i n g the course of the measurements, Achterberg et al. 6) reported conversion coefficients for m a n y of the transitions. The present results are consistent with their spin assignments, and yield the m a g n i t u d e of the m i x i n g ratios for a n u m b e r of the transitions.

2. Experimental details The techniques of gas sweeping and gas c h r o m a t o g r a p h y 7) were used to isolate 138Xe from the gaseous products produced in the fission of 233U. Sources of ~38Xe, a d s o r b e d o n q u a r t z wool at liquid nitrogen temperature were allowed to decay for 15 m i n to form 30 m i n ~38Cs. The quartzo wool with its ~38Cs deposit in the form of a m o n o a t o m i c layer was then transferred to a hollow cylinder 0.2 cm in diameter and 1.0 cm in length and m o u n t e d on the rotating vertical axis of a correlation 382

~3SBa SPINS A N D

MIXING RATIOS

383

spectrometer. The spectrometer employed a 7.5 cm x 7.5 cm N a I ( T I ) detector and a true coaxial cylindrical 42 cm 3 Ge(Li) detector placed at distances of 7 cm and 5 cm respectively from the rotating source. The measurements were carried out with a conventional coincidence circuit of resolving time 50 nsec, using a time-to-amplitude converter and a Nuclear Data Model 161 multichannel analyser. The Ge(Li) spectrum in coincidence with the 1436 keV peak in the NaI(T1) detector was recorded at seven angles, using five sources at each angle. The contribution to the coincidence spectra from the Compton background in the NaI(TI) window was completely negligible. The chance corrected intensities of the photopeaks associated with the 409, 463, 547, 872 and 1010 keV 7-rays were obtained from the Ge(Li) spectra as a function of angle. The angular distributions were analysed by least squares to obtain values of the correlation coefficients. Solid angle corrections were made using the tabulated o

463

1436

[ 5

b

4 0 9 ~ ,[4~3~

436

I

12

{ {

....

,~(e}

i I -

/

t

W(81

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1

.

°i1

. 90

.

.

c

.

.

b2O

. 150

e

160

e

10~0-[436

d

547

(463)-L436

r

I

ro

---

'°!* 0

l i

!

O90

i

9o~ 90

120

t

i it

o as,

I

150

L9'5

180

.

.

.

.

.

,2o

e

,8o

LSO e

44

!o4 ?

.... -~..

!

J,

--'.

,'

2446

3%4"

2~OB

l 3-2-0

-

~

'

",

2"

0°

I

I

I

~

1899 t i

1436

I 56B082

Fig. 1. Angular correlation functions for 7"Y cascades in 13SBa ' T h e lower left hand section s h o w s a theoretical plot o f A4 versus A2 for a n u m b e r o f cascades together with the experimental (A4, Az ) values f r o m the present work. The lower right hand section presents a partial level scheme.

384

B. SINGH AND M. W. JOHNS TABLE 1 Correlation coefficients and ~'-~' cascade (keY)

A2

.44

463 ~ 1436

0.14 --0.03

0.003~0.05

409 -+ (463) -~ 1436

0.27 _L0.05

--0.01 ~0.08

872-+1436

0.05 ~0.10

-0.12 ±0.18

1010 -> 1436 547 ~ (463) --->1 4 3 6

--0.065~0.014 --0.107~0.024

--0.002±0.040 --0.002:~0.040

Spin sequence

->2 -~0 -~2-->0 -> 2 --->0 2-->4>-2-->0 3 -+ 4 -+ 2 -+ 0 zl --> 4 --*-2 ~- 0 (2 -->2--~-0

{i

/3-o-2-->0 f k4 -~- 2 -~-0 3 -~ 2 --> 0 3 -*- 4 -~- 2 --~ 0

a) Derived from the ~K measurements of Achterberg et al. 6). b) Ratio of M3 reduced matrix element to E2 reduced matrix element. d a t a 8, 9)

for

these detectors. The values appropriate to this experiment were

Q2 = 0.83 a n d Q4 = 0.53.

3. Experimental results and discussion The correlation functions for four of the five cascades studied are shown in fig. 1. The f u n c t i o n for the 872-1436 keV cascade which was essentially isotropic suffered from p o o r statistics a n d is n o t presented. Since the intermediate state in all of these cascades is the 2 + level at 1436 keV, the theoretical parametric plots of A 4 versus Ae are functions o f the mixing ratio, 6, in the first transition. The lower section of the figure presents these plots together with the experimental (A4, A2) points. Table 1 presents the experimental values of Az a n d .44 for each of the five cascades, together with the spin sequences a n d associated 6-values which are consistent with this data. F o r comparison, the values of [6l deduced from the i n t e r n a l conversion d a t a 6) are also shown. The values of ~ : are consistent only with M I , E2 type transitions. Finally, the last three c o l u m n s tabulate the acceptable choices o f multipole order for the initial t r a n s i t i o n of the 7-7 cascade a n d the energies a n d acceptable J ~ choices for the initial states themselves. The 7-~ correlation of the 463 -~ 1436 keV cascade would allow J ~ = 2 +, 3 + or 4 + for the 1899 keV state. However, the ~ a n d ]6[~ values are consistent only with the 4 + choice which limits the M3 admixture in the E2 t r a n s i t i o n to < 2 %. The p r o t o n transfer experiments 4) limit J ~ for this state to 4 + or 6 ÷, consistent again with the 4 + choice.

X3SBa SPINS A N D M I X I N G R A T I O S

multipole mixing ratios in

'

38Ba

Properties of initial transition

6,,:7 0.25 0.10 0.0 --0.25 --0.75 --0.85

<2 6 <2 0.40 < 6 < 0.20 <26<2 0.15 b) <2 6 <2 --0.05 b) <26<2 --0.45 < 6 < --0.05

--0.01 0.05 2.0 --0.25 1.5 --0.015 --0.06

<2 < < <2 <2 <2 <2

6<2 0.04 d <2 0.35 6 < 5.0 6 <2 0.10 b) 6 <2 3.0 6 < 0.020 b < --0.015

385

'i6[~: ~)

1.1 < tO[
0 < ]~1 <2 0.3 b) 0 <2 161 <2 0.9 0 <2 ]6] < 0.2

Properties of initial state multipole order

E21 ( < 2 2 ~ M 3 )

energy (keV)

j~r

1899

4+ 4+

MI ~-E2

2308

3 + or

MI i E2

2308

(2),3,4

or E2 MI ~ ( < 0.04~/o E2) M I - - ( < 0.4~/0 E2)

2446 2446

3+ 3+

The spin and parity of the 2446 keV state are uniquely established as 3 + by either the 1010-1436 or the 547-(463)-1436 keV cascade. The negative anisotropy of the former cascade has been observed by Bunker et al. ') but no A2, A4 values were determined. Both the 547 and 1010 keV transitions are seen to be almost pure Ml. The limits on [~5[ set by the correlation data are much smaller than those from the internal conversion measurements; the 3 + assignment to the 2446 keV state is consistent with either set of results. Neither the angular correlation nor the internal conversion data are able to uniquely determine J ~ for the 2308 keV state. The 2 + choice would demand an unreasonably large M3 component in the 409 keV transition while the 5 + choice is ruled out by the presence of the 872 keV transition. Either a 3 + or 4 + choice fits the conversion data and the 409-(463)-1436 keV correlation data equally well. The ambiguities are not resolved by the properties of the 872 keV transition or the 872-1436 keV cascade and one must regard both the 3 + and 4 + assignments as open options. Since the 1899 keV 4 + state has a measured lifetime of 3.13_+0.11 nsec [ref. ,o)], one cannot ignore the possibility of attenuation effects for cascades in which this level is an intermediate state. The attenuation must be small in this case since the observed A 2 coefficient of 0.27_+0.05 for the 409-(463)-1436 keV cascade is comparable to the maximum unattenuated theoretical A2 coefficients of 0.36 and 0.26 for the 3 ~ 4 --+ 2 --* 0 and 4 --, 4 --, 2 --* 0 cascades respectively. The only effect of assuming that there is zero attenuation is to broaden slightly the range of acceptable &values for the 409 keV transition. A number of attempts have recently been made to calculate the properties of the

386

B. S I N G H A N D

M. W. J O H N S

levels in ~38 ~. 56~a82, with rather limited success. The most extensive of these is that of Waroquier and Heyde ~t) who calculate the positions of the two-quasiparticle states above the energy gap. These authors and Wildenthal ~2) predict the positions of the 1899, 2308, and 2446 keV states within ~ 150 keV and assign them spins of 4 +, 4_,* and 3+ respectively. The former authors also calculate the reduced transition rates for the 3~+ --, 4+ and 3~- --, 2~+ transitions studied in this work. Table 2 compares TABLE 2

A c o m p a r i s o n o f experime~ltal a n d theoretical mixing ratios

E,: (keV)

6,p

16o.L,.I ")

409 547 1010

- 0.85 -< ~3 < --0.05 --0.06 < d < --0.015 --0.015 < ~ < 0.020

0.076 0.335

!,sw~,") 0.018 0.024 0.041

a) F r o m W a r o q u i e r et al. 11) using a two-quasiparticle calculation a n d a residual interaction with a G a u s s i a n radial dependence. b) F r o m the W e i s s k o p f estimate using a nuclear radius c o n s t a n t o f 1.2 fin.

the mixing ratios predicted by these authors with those observed. The table also includes the values of 6 deduced from the Weisskopf single particle estimates. The agreement between these model calculations and experiment is not very convincing. The authors are thankful to the National Research Council of Canada for the financial support of this work.

References 1) M. E. Bunker, R. B. Duffield, J. P. Mize a n d J. W. Starner, Phys. Rev. 103 (1956) 1417 2) T. N a g a h a r a , N. Miyaji, H. K u r i h a r a , Y. Mizuro artd Y. lshizuka, J. Phys. Soc. Jap. 28 (1970) 283 3) J. C. Hill a n d D. F. Fuller, Phys. Rev. C5 (1972) 532 4) W. P. Jones, L. W. B o r g m a n , K. T. Hecht, J. Bardwick aad W. C. Parkinson, Phys. Re~. C4 (1971) 580 5) J. H. Barker a n d J. C. H.iebert, Phys. Rev. C6 (1972) 1795 6) E. Achterberg, F. C. lglesias, A. E. Jech, J. A. M o r a g u e s , D. Otero, M. L. Perez, A. N. Prato, J. J. Rossi, W. Scheuer a n d J. F. Suarez, Phys. Rev. C5 (1972) 1759 7) N. P. Archer a n d G. L. Keech, Can. J. Phys. 44 (1966) 1823 8) M. J. L. Yates, Perturbed a n g u l a r correlations, ed. E. Karlsson, E. Matthias a n d K. Siegbaha ( N o r t h - H o l l a n d , A m s t e r d a m , 1964) 9) D. C. C a m p a n d A. L. v a n L e h n , Nucl. Instr. 76 (1969) 192 10) W. M. Currie, Nucl. Phys. 48 (1963) 561 11) M. W a r o q u i e r and K. Heyde, Nucl. Phys. A164 (1971) 113 12) B. H. Wildenthal, Phys. Rev. Lett. 22 (1969) 1118