Properties of phonon-coupled levels: The levels of 135Ba populated in the decay of 135La

l

Nuclear Physics A164 (1971) 552--564; (~) North-Holland Publishiny Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

PROPERTIES OF P H O N O N - C O U P L E D LEVELS: THE LEVELS OF *SSBa P O P U L A T E D IN THE DECAY OF XSSLa F. BAZAN and R. A. MEYER

Lawrence Radiation Laboratory, University of California, Livermgre, California 94550 t Received 18 March 1970 (Revised 26 January 1971) Abstract: The radioactive decay o f 13SLa has boon studied with the aid of ordinary Ge(Li)

spectrometers and Compton suppression Ge(Li) spectrometers. The levels in the deduced decay scheme are discussed in terms of the odd neutron coupled to the core vibration. Some evidence is given for the preferential decay of the levels to other phonon-coupled configurations rather than by the expected E2 phonon de-excitation mechanism.

E

RADIOACTIVITY X35La [from aaSBa(p, n), enriched targetsl; measured Er, Is; deduced Q, log ft. Isotopically separated activity; Ge(Li) detector. Compton suppression spectrometer, 135Ba deduced levels, cc, J, zr, odd-particle phonon coupled level properties.

1. Introduction Kisslinger and Sorensen 1) have described the occurrence and relative ordering of levels in nuclei that are near closed shells. Their model for nuclei with simple residual forces has been shown to provide a good description for these spherical nuclei. In particular, the works of Waiters 2, 3) and co-workers have shown that the phononcoupled configurations predicted by Kisslinger and Sorensen are correct for these nuclei. They have demonstrated that these phonon-coupled levels are populated in /~-decay, albeit weakly. However, little work has been done on the nature of the decay of these levels. A more stringent test than the relative ordering of levels is how well the model predicts transition moments among these levels. If transitions are to be detected from these levels, a nucleus must be chosen in which the quantity Q is sufficiently high to populate these levels but below the pairing energy. The decay of 135 La to the levels of 135Ba meets these requirements. The study of this system is aided by the fact that the odd nucleon is limited to three shell-model states for low-energy excitations. We have been able to demonstrate that many more levels occur at an energy less than 2A in this N -- 79 nucleus than in neighbouring nuclei with N = 81, such as those studied by Bruge et aL 4) and Waiters et al. z, 3). Moreover, we present some evidence for the existence of a vibration-coupled g~ configuration much lower than the gk single-particle state. We propose that the decay of these coupled levels can be t Work performed under the auspices of the US Atomic Energy Commission. 552

13SBa LEVELS

553

explained by considering a phonon jumping de-excitation in competition with the expected phonon dumping de-excitation. 2. Experimental method Sources of the 19.5 h 135La activity were produced by the (p, n) reaction on isotopically enriched 135Ba. The 135La was chemically separated from the ~35Ba target material by bubbling ammonia gas into a 2 M HC1 solution containing ~35Ba. The La(OH)3 precipitate was subsequently ignited to the oxide and made ready for an isotope separator. The average yield for these separations of 135La was 30 % of the original activity. (An alternate procedure could be used for separating ~35La from 135Ba ' based on the selective elution of La with ~-hydroxyisobutyric acid from 50 x 8 Dowex cation resin.) The final oxide form of La was isotopically separated with an isotope yield of 40 %. A separate experiment involving the column chemistry was performed to look for population of the ~35~Ba in the decay of ~35La. In this experiment the Ba was recovered and mounted to be used as 135mBa source. A double Ba-La separation was performed to ensure separation of any 135tuBa from the target material in the ~35La activity. A separation factor of approximately 10-1 o for Ba from La was obtained using this double-pass technique. These sources of ~35La were counted on several Ge(Li) systems. Two separate measurements were made using the Compton suppression spectrometer and sources prepared from two different irradiations. Duplicate measurements were made using a high-resolution X-ray detector as well as precision calibrated 8 cm 3 Ge(Li) diodes. Sources were counted initially at several source-to-detector distances. The 220.91 and 480.52 keV y-rays were found to produce sum peaks. Consequently, all subsequent measurements were made at a source-to-detector distance of 15 cm or greater. 3. Results Fig. 1 shows a 7-ray spectrum of 135La taken with the Compton suppression device. The spectra were analysed using the Gunnink-Niday code 5) on a CDC-6600 computer. These calculations are described elsewhere 5). The y-ray energies and intensities are given in table 1. The energies of the more intense peaks were determined by counting the 135La simultaneously with several known standards. Several interlevel transitions could not be detected. In table 2, upper limits are presented for the intensity of these apparently forbidden transitions in 135Ba" Table 3 presents our energy measurements of the -~-- isomeric level in 135Ba. Morinobu et al. 6) have obtained conversion-electron spectra from the decay of 135La" We have used their conversion-electron intensities and our y-ray intensities to calculate the K conversion coefficient for nine of the transitions. We were able to obtain a more precise determination of ctK due to the increased precision with which the y-ray intensities were measured.

554

F. B A Z A N

AND

R. A. M E Y E R

Several i t e m s s h o u l d be m e n t i o n e d in c o n n e c t i o n w i t h these values. First, we chos( the 220.94 k e V transition as our fiducial. This is in c o n t r a s t to p r e v i o u s ~K d e t e r m i n a tions in w h i c h the 220.94 k e V 7-ray c o u l d not be m e a s u r e d w i t h any degree o f p r e c i

480.51 (overflow)~I ~

220.94

~.. 4 --

_

:;

Jl

!,107.32 '~"~-~ ~ ,,.,.....~L..-- - - - - - - - ~

J

:I .~,

3-

511.oo ~ '

:

2 2--

]

1-

t

0 6

5

1

i

128

384

r

I

,

640

1

i

l

896

I

1152

i

:

I

1408

I

1792

I 2048

t

m

874.51 4--

1634.05 ,i

c

g u

3

979.98 :

~ o

2

":

.

!7-

1

0

i

i 2304

r

I 2560

i

T 2816

I 3200 Channel

I

[ 3456

I

I 3712

r

I 3840

i 4000

number

Fig. 1. Gamma-ray spectrum of laSLa taken with a Compton suppression device•

sion. T h e 220.94 k e V transition is superior to others as a fiducial not o n l y b e c a u s e its M 1 / E 2 ratio has b e e n d e t e r m i n e d f r o m a m e a s u r e m e n t o f its K - t o L - c o n v e r s i o n electron intensity ratio but b e c a u s e the c~K values for M1 and E2 are nearly equal. T h e 394.04 k e V transition has n o t b e e n detected before. M o r i n o b u et aL, in their table o f

TABLE 1 G a m m a - r a y energies a n d intensities for laSLa Energy (keV)

107.32 220.94 259.58 267.17 268.238 366.84 374.46 394.04 480.51 511.00 587.83 634.05 758.94 787.9 855.00 874.51 979.98 1008.4

Error a, b)

(K x-ray) (0.09) (0.015) (0.04) (0.09) (0.010) (0.01) (0.01) (0.04) (0.01) (0.01) (0.01) (0.09) (0.5) (0.02) (0.02) (0.02) (0.5)

Intensity

Error a)

50, 100 0.65 35.96 2.98 0.19 0.0003 21.1 12.4 2.96 1000 9.28 73.3 14.3 0.49 0.08 11.9 107.2 3.36 0.011

(1500) (0.03) (0.19) (0.13) (0.04) (0.0002) (0.3) (0.1) (0.26) (1) ~) (0.31) (0.4) (0.1) (0.14) (0.03) (0.4) (0.3) (0.05) (0.006)

A s s i g n m e n t b) from EC 581 221 480 855 268 587 855 874 480 /3 + decay 587 855 979 1008 855 874 979 1008

to

480 g.s. d) 221 587 g.s. 221 480 480 g.s. g.s. 22 l 221 221 g.s. g.s. g.s. g.s.

a) T h e error for the particular value is given in parenthesis. b) T h e error for the m o r e intense peaks represents the fitting error observed in the energy calibration experiments. T h e internal s t a n d a r d s were fit to an average deviation o f 20 eV. ~) Represents c o u n t i n g statistics. d) g.s. = g r o u n d state. TASTE 2 U p p e r limits for 7-ray intensity o f " f o r b i d d e n " transitions between k n o w n levels in 135 Ba Energy (keY) 125.0 235.6 242.6 c) 286.70 351.61 c) 380.8 ¢) 392.13 420.75 499.46 606.31 (607.6) c) 653.60 (706.81) ~) 909

Transition

Intensity a. b)

from

to

979 717

854 480

874

587

979 1008 979 874

587 587 480 268

_< 0.016 ~ 0.01 0.02 4-0.01 < 0.004 0.09 4-0.05 . 0.16 4-0.05 < 0.1 0.15 ± 0 . 1 5 =< 0.009 < 0.007

874

220

< 0.015

909

g.s.

< 0.01

a) Intensity relative to 480 keV 7-ray with an intensity o f 1000. b) T h e categories o f limits are: (i) If no 7-ray was detected, we use the limit sign ( < ) . (ii) If there was s o m e evidence for a 7-ray, we use the limit sign ( ~ ) . (iii) If we detected a 7-ray in m o r e t h a n two spectra, we list the intensity with error. c) T h e s e are 7-rays we were able to detect by C o m p t o n s u p p r e s s i o n techniques. T h e y do n o t belong to t h e only k n o w n possible i m p u r i t y (la2La); however, they c a n n o t be unequivocally assigned to 135La decay.

556

F. BAZAN AND R. A. MEYER

electron intensities, note that their L367 + M367 line is too intense. They ascribe the extra intensity to the presence of a K394 line. We have used the theoretical L/K and M / K ratios to calculate the contribution of this complex line. In table 4 we present the conversion coefficients we obtained using the electron data of Morinobu et aL, comparing them with the theoretical values of Hager and Seltzer 7) in fig. 2. TABLE 3 Energy measurement of the a~ - level (value obtained: 268.238:fi0.018 keV)

Fiducial ),-rays Nucleide

accepted value

value found by code

57Co 57Co 2o3Hg 137Cs

122.046 4- 0.020 136.465 4-0.020 279.179 i 0 . 0 1 0 661.615±0.030

122.047 4- 0.003 136.493 !0.013 279.208 ± 0.002 661.609i0.0006

deviation from accepted value a) + 0.001 +0.028 + 0.029 --0.006

a) Average deviation from accepted value is 0.016. TABLE 4 Conversion coefficients for transitions a) in 135Ba Transitions 220.94 366.84 374.46 394.04 c) 480.51 587.83 634.05 855.00 874.51

~K(A~K) b) 0.0953 0.0257 0.0215 0.012 0.00954 0.00667 0.00580 0.0044 0.00185

(26) d) (15) (6) (5) c) (11) (8) (67) (12) (33)

Multipolarity ¢) (M1 +0.1 E2) a) MI MI E2(+MI?) 0.75E2+0.25 M1 0.50E2+0.50 MI MI ( + E 2 ) M1 E2

a) These transitions are calculated from the published conversion electron intensities of Morinobu e t aL 6).

b) Error is to last significant figures; hence, 0.091 (13) is 0.091+0.013. c) The values of Hager and Seltzer 7) were used. ~) Value calculated from K/L ratios by Morinobu et aL 6). e) Morinobu e t aL assumed this line to be due entirely to the L + M 367. They did note that a 390 could exist and would lower their value for L367+M367. We used the theoretical L2M/K ratios to calculate the intensity of the K394 line, from the K394+L367+M367 intensity reported by Morinobu et aL

The precise value for the mass difference of 13 SLa and 135Ba has been in doubt, because annihilation ~-ray have not been detected in the 135La ~-ray spectra. We were able to measure a positon component in the ~-ray spectra from isotopically separated laSLa sources. Using this value, the relative Ba X-ray intensity, and theoretical fl+/EC ratios, we have been able to determine a value of QEc = 1220+20 keV. The

lSSBa LEVELS

557

main contribution to the error is imprecision in the measurement of the amount of electron capture to the ground state. We used a value of 97.6 %, which is the average of our value and that determined by Morinobu et al. 200

400

600 [

J

]

r

i

10-1 --

14

10 -2

10- 3

I

I

r

r

I

6(30

I

800 E -- keV

,

r

I

1000

Fig. 2. Conversion coefficients for transitions in ~35Ba. The fiducial used is the 220.94 keV transition. The electron intensities are those o f M o r i n o b u et al. 6) and the solid curve represents the theoretical values calculated by Hager and Seltzer 7).

4.

Decay

scheme

The decay scheme deduced from our experiments and those of previous workers is presented 6, 8.9) in fig. 3. The energy of the levels is calculated by the least-squares routine LEVEN-B. These values are given in table 5. In table 6, we present the l o g f t values calculated by the detailed balance technique. This latter method incorporates the conversion electron intensity values of Morinobu et aL When the precision of their measurements was insufficient, we used the theoretical values of Hager and Seltzer. In fig. 4 we present intensity limits for some of the undetected transitions.

558

F. B A Z A N

AND

R. A. M E Y E R

"[he level at 220.95 keV is known to have 9) a J= value of ½+. Morinobu et al. measured the K/L ratio for the transition to the ~+ ground state and found an E2 contribution of 0.1. The l o g f t value of < 11.6 was obtained by a detailed balance technique. If statistical averaging is also employed, a l o g f t value of 12.1 is obtained 135 , 5/2 +

QEC= 1220 =-

57La78

19.5h

20

(1008.6) 979.98

2 + ( 5 / 2 +) ol

~

7/2+

~;

~

,,ci,

874.52 854.99

3/2 + &-~

2+

o~ +

o~

r-..w

~

c;

o.

co

I 587.85

co

I 480.55

d

D '

2 -+O

o

o.

os c~l

co

oo

,,o

~"

268.24 220.95

2+

q-

o

o. v

c~

co

di

O,

135. 56 Da 79

Fig. 3. D e c a y scheme of ~3SLa. In the right-hand portion of the diagram are the limits set by the authors on the intensity of undetected v-rays.

for the electron capture transition to this level. Von Ehrenstein et al. 10) have studied the (d, p) reaction populating the levels of 135Ba and find a spectroscopic factor of 0.21 for this level. Cork et al. i t ) identified an I2~Lisomeric level with a 28.7 h half-life. We find the energy of this level to be 268.238 keV. The relative intensity of 0.003 for this y-ray

13SBa LEVELS

559

represents a population of one out of every 10 s La decays. If this occurred solely from electron capture decay, the value would be three orders of magnitude too large. The major part of the population of this level presumably arises from the decay of higher energy levels. The two levels at 480.52 and 587.81 keV are known to have a J~ value o f ~ + or -~+. We assign a 3 + value to the level at 587.81 keV. This is based on a l o g f t value of 7.9 1008 979 1,/2+

909 879

854 717 v

v

o

0

o.

0 V

co o

',.0

587

c~

480 c; o

co o

268 220

E

?

Fig. 4. Intensity limits for undetected transitions.

and the assignment of M 1 multipolarity to the transition between the 587.81 keV and the ½+ 220.94 keV level. We assign the 480.52 keV level of J~ as 3 +. Both of these levels are observed in the (d, p) studies of von Ehrenstein et al., albeit weakly. We assign the 854.99 keV level as ~2+ ; this level was at the threshold of detection ir~ the (d, p) reaction studies. The level at 874.52 keV has been observed by previous investigators 6, 9). Morinobu et al. assigned the spin of this level as ~2+ or 3 + on the basis of a 655 keV transition to the 220.94 keV ½+ level. We find no such transition. We place an upper limit of 0.015 on such a transition. Using the conversion electron data of Morinobu et al., we assign the 874.51 keV transition as pure E2. We have also detected a 394.04 keV y-ray. This represents a transition from the 874.52 keV level

560

F. BAZAN AND R. A. MEYER

to the 480.52 k eV level. It should be n o t e d that this level was n o t observed by v o n E h r e n s t e i n e t aL in their (d, p) studies. O n the basis o f the f o r e g o i n g evidence, we assign it a J " value o f ~ +. T h e 979.98 k eV level has n o t been previously observed in decay scheme studies. H o w e v e r , recent (d, p) e x p e r i m e n ts 1 o) have s h o w n this state to exhibit an angular d i s t r i b u t i o n c o r r e s p o n d i n g to l = 2 and a ( 2 J + 1) weighted spectroscopic factor ot 0.35. A n a s s i g n m e n t o f ~+ is preferred, due to the relatively intense decay to the i + level; however, a 5+ assignment is n o t entirely precluded. TABLE 5 Least-squares energy of levels in a35Ba Level (keV) 221 481 588 855 874 979

Best energy (keV) 220.952 480.552 587.850 854.991 874.522 979.980

ere 0.024 0.023 0.024 0.023 0.051 0.055

TABLE 6 Electron-capture log f t values from intensity balance technique Level (keV)

EC(~)

log f t

g.s. a)

97.7 b) ~ 7 x l 0 - S c) i> 3x10 -8 1.54 0.15 ~ 4 × 1 0 -s 0.060 0.17 < 2 × 1 0 -5 0.0060 ( ~ 3 x 10 -~)

5.7 ;:~ 11.6 (ILl) c) ~ 15 7.0 7.9 ~ 13 7.8 7.4 ~> 11 8.4 ( > 10.0)

221 268 480 587 717 855 874 909 979 (1008)

a) g.s. = ground state. ~) We averaged our results and those of Morinobu et al. 6) to obtain the 97.7 ~ figure. c) 12.1 = logft for this level if we use a statistical approach to the intensity balance of this level and include the theoretical M + N + . . . to L ratios for calculating the conversion electron intensities associated with the transitions into and out of this level.

A level at 1008.6 keV is tentatively p r o p o s e d on the basis o f the very low intensity 1008.4 a n d 787.9 keV 7-rays which a p p e a r in the spectra. T w o levels b el o w 2201 keV n o t observed in this study b u t r e p o r t e d in the (d, p) r e a c t i o n studies are the 717 a n d 909 keV levels. T h e 909 keV level was assigned as ½+ by v o n Ehrenstein et al. on the basis o f an 1 = 0 a n g u l a r distribution: this level was

xasBa LEVELS

561

determined to have a spectroscopic factor of 0.024. Presumably, the 717 keV level possesses a spin of ~2 or greater. We can set an upper limit of 0.01 for the relative intensity of a 235.6 keV v-ray. This would represent the transition from a 717 keV level to the ~2+ 480.52 keV level. 5. Discussion

The shell model predicts three low-lying states at an energy less than the pairing energy (2A) in 135Ba" However, it has been demonstrated that in nucleides as light as s 7Co the single-particle states couple with the 2 + collective vibration of the core 12). Kisslinger and Sorensen's calculations well describe the nucleide levels near the closed shells n = 50 and v = 82. (See the works of Walters and co-workers in this regard.) The nucleus 135Ba is on the borderline for applicability by Kisslinger and Sorensen's calculations. Although Kisslinger and Sorensen ~"13) do not give all possible levels, they do give the configuration of the ~+ ground state (g.s.) and lowest lying ½+ and -1--11levels in 135Ba: 2 g.s.:

0.76(d ~ 0 0 ) + 0 . 1 1 ( d !z 12)+0.05(½ 12)+0.04(g ~ 12)

"

0.31(½ 00)+0.55(d ~ 12)+0.05(d ~ 12)

'"

0.72(h ~ - 0 0 ) + 0 . 2 5 ( h '2' 12).

The expression (d ~ 12) is the dk single-particle state coupled to the first 2 + vibration (phonon). Thus (d 1z 00) refers to the d~ state alone. Alternatively we note this as (~ 12) and (~ 00), respectively. Of immediate interest are that the ½+ level is predicted to have a 55 ~ (d ~: 12) character and that the ground state has a 11% character of the d~ coupled to the first collective state of the ~34Ba core. Next, we consider what extra levels are created by coupling the core vibration to the available shell-model states. Ikegami and Sano 14) have calculated the effect of the strength of this coupling on the relative spacing of the multiplet that arises from j~b. This has been reviewed by Peker 15). In general, he finds that the j - 2 and j - 1 members are significantly lowered in energy relative to the other members of the multiplet. The j member [vizj(j 12)] also is lowered but not as significantly. If the coupling strength is sufficiently strong, the j - 1 member of (j 12) can occur at a lower energy than the (j 00) parent. The levels observed in 135Ba from 300 keV to 1220 keV can be ascribed to such phonon coupling. These phonon-coupled states are expected to decay by E2 emission to their parent configuration. Presumably, the 874.52 k e y level is an example of such a (j 12) ~ ( j 00) transition. Further evidence that the levels above 300 keV populated in 135La decay have ( j 12) components as their major configuration is the hindered nature of the electron capture transitions into them 2,16). The average increase of 2.0 units in the l o g f t value is approximately that found when a comparison is made of the decay of 134La to the 134Ba ground state and first 2 + level.

562

F. B A Z A N A N D R. A. M E Y E R

One serious incongruity presents itself: most of the levels in x35Ba above 300 keV decay by M1 emission rather than E2 emission as would be required by simple (j 1 2 ) ~ ( j 0 0 ) transition rules. The following is offered as an explanation. The 220.94 keV transition from the ½+ level to the ~+ ground state has a 10 ~ E2 character. This can be accounted for by the (~:, 1, 2) to the (~, 0, 0). We term a transition " p h o n o n dumping" when the nucleus is de-excited by the core going from the n + 1 collective vibration to the nth collective vibration. The M1 part of the transition can be described by the single-particle transition (½, 00) --* (~, 00). However, we suggest as an alternative mode of decay "particle jumping" where the core remains in the same TABLE 7 Transition probabilities f r o m the levels in 13SBa To level F r o m level

g.s. ") 220 480 587

480

587

854

874

B(E2)

B(M1)

B(MI)

B(E2)

1.00 0.087

0.42 1.00 ~ 1.23

0.081 ~ 0.24 1.00 ~ 0.042

> 0.67 1.00

979

1.00 0.3 q--~-0.02 < 0.4

") g.s. -- ground state.

collective vibrational state and the odd particle "jumps" from one shell-model state to another. (This is more probable when one uses a model such as that of Ikegama and Sano.) This mechanism may contribute to the M1 part of the 220.94 keV transition, particularly if the Kisslinger-Sorensen description of the ½ level is correct. We next consider the "particle jumping" transitions. The level at 854.99 keV is apparently a phonon-coupled level. This level has been shown to be g)+ but with a high l o g f t value for the electron capture transition into it. Also, von Ehrenstein et aL could only set a limit on the detection of this state in their (d, p) reaction studies. These facts suggest little single-particle character for this level; however, the level decays primarily by M1 emission. These apparently contradictory observations can be reconciled if one takes into account "particle jumping" in the de-excitation of this level. The g.~ shell-model state is expected to be 2 MeV. If the g~_ phonon coupling were of sufficient strength the ~+ and z2+, members of the multiplet might be brought down quite low in energy. The (~- 12) level then would be "trapped" below its (½ 00) parent, and the only possible mode of allowed decay would be "particle jumping" to other (j 12) configurations. In table 7, we present some of the relative B ( M I ) and B(E2) transitional probabilities; we note that the 854.99 keV level decays preferentially to the 480.52 keV level rather than to the ground state. The two levels at 480.52(~z +) and 587.81 (~+) keV may provide some information about the relative probability of particle jumping versus phonon dumping. Both these levels exhibit phonon-coupling properties. They have very small cross sections when

~aSBa LEVELS

563

observed in (d, p) reaction studies, and they have high logft values for allowed transitions. We note that the relatively fast EC to the 480.52 keV (~+) state could be accounted for by either a (d ~ 00) component in the Ba or a phonon admixture in the La ground state. As seen in table 7, the 480.52 keV (~+) level decays predominately to the ground state by an E2 transition. Some (d ~- 00) character could produce the M1 component. Alternatively, this phenomenon could be due to particle jumping between the (d -~ 12) and (d 3 12) components of the 480.52 keV level and ground state, respectively. The multiplet that arises from the coupling of the -~- single-particle state to the core vibration should occur at approximately 800 keV. The ~ - ( ~ - 12) member has been predicted by Kisslinger-Sorensen to account for 25 % of the 268.24 keV level. The ½- member of the (J~- 12) multiplet should be the lowest-lying member, occurring at an energy below 1 MeV. The level at 717 keV observed by yon Ehrenstein et al. may be this level. Presumably the major mode of decay of a ~-(1@- 12) level is an E2 transition to the 268.24 keV isomeric level. (One should not rule out the possibility, however, of a fast 3 - ( h ~ ~- 12) ~ -~(g ~ 12) particle jumping E1 transitions.) If we use the results of our experiments in which the 135Ba was milked from the 13 SEa ' we get a l o g f t value of approximately 14 for electron-capture decay to a 3 - level at 717 keV. If we include the possibility of a phonon jumping El, the l o g f t value is reduced to 13 or greater. However, such a large l o g f t value is not inconsistent with the assumed character of the 717 keV level. The second 2 + level of 134Ba occurs 17) at 1167.7 keV. Thus, states of the type J ( j 22) can be expected to influence the configuration of levels above 800 keV. The strong E2 branch from the 874.55 keV level might represent the de-excitation 7 3 5 3 ~-(~22) -~ ~(z 12). 6. Conclusions

We have discussed the levels of 13 SBa in terms of the available shell-model states coupled to the 2 + collective vibrations of the core. The properties of the 13 SLa decay and 135Ba levels are consistent with this description except for one serious discrepancy: A number of levels decay primarily by MI y-ray emission instead of the expected E2 transitions associated with phonon decay. We propose that this discrepancy be reconciled by taking into account the particle jumping mechanism whereby the core vibration remains unchanged and the single-particle j-state changes. Some evidence has been presented to substantiate the lowering of the ~+ and ~+ (g 3 12) level below the 3(g ~ 00) level in ~35Ba and the occurrence of the J ( h lz~l-12) levels below 1200 keV. We acknowledge the help of P. Johnson who produced the isotope-separated 13 SLa" Thanks are due to H. Perdue for her suggestions concerning the column separation technique. We also acknowledge the assistance of H. B. Levy, who supplied us 18)

564

F. B A Z A N A N D R. A. MEYER

with his routine LEVEN-B.

The encouragement

a n d s t i m u l a t i o n p r o v i d e d b y W . E.

Nervik are acknowledged.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18)

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