Absolute transition probabilities in 134Ba

Nuclear Physics A250 0975) 141 -- 148; ~ ) North-HollandPublishing Co., Amsterdam

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A B S O L U T E T R A N S I T I O N P R O B A B I L I T I E S I N 134Ba J. BURDE, S. ESHHAR, A. GINZBURG and A. MOLCHADZKI The Racah Institute of Physics, The Hebrew University, Jerusalem, Israel t Received 28 Aprd 1975 Abstract: Lifetime measurements of the excited states of ~34Ba were carried out by delayed coincidence measurements between the fl-spectrum of t3*Cs and the conversion lines and ?.transitions that de-excite the levels, using double-lens coincidence and fl-?. coincidence spectrometers. The mean lifetimes of the 1400 keV and 1643 keV levels were determined to be z = 12.5=k2.5 psec and 113+30 psec, respectively. The ratio of the reduced transition probabilities B(E2; 4 + --* 2+)/B(E2; 2 + ~ 0 +) = 0.154-0.03. This result does not agree with the prediction of the rotational or vibrational model, although the energy spacings, spin sequence and relative intensities of the de-exciting transitions for the first four excited states could be accounted for by the asymmemc rotational model. The results indicate that the reduction in collectivity as experienced by the 4 + member of the quasi-rotational band, due to an apparent change in the structure of the nucleus, is reflected also in the 3 + member of the ?'-vibrational band which is situated nearby in energy. E [

I

RADIOACTIVITY t3"Cs [from 133Cs(n, ?.)l; measured rice(t), fl?.(t), lS'Ba levels deduced Tt, B(E2). Natural target.

1. I n t r o d u c t i o n The nucleus la4Ba is situated in the t r a n s i t i o n region between spherical a n d deformed nuclei. The level structure o f this nucleus p o p u l a t e d b y the decay o f 134Cs a n d 134La has been investigated extensively ~- 5). A t a b o u t twice the energy o f the 2 + first excited state there are two levels - 1168 keV a n d 1400 keV - with spins a n d parities 2 + a n d 4 +, respectively. A 1774 keV level, which was p r o p o s e d by R e u l a n d 4), obviously c a n n o t be the third missing 0 + m e m b e r o f the t w o - p h o n o n triplet, as the a u t h o r observed a direct ~,-transition from this level to the g r o u n d state. The spins a n d parities o f the states which are p o p u l a t e d by the t34Cs decay, the energies, the relative intensities a n d the multipolarities o f the de-exciting transitions, were very well d e t e r m i n e d 2, 3, 5). Yet, the t r a n s i t i o n probabilities for all the transitions, except the one that de-excites the first 2 + level, are still u n k n o w n . I n order to gain more insight i n t o the n a t u r e o f the levels o f 134Ba, we carried out lifetime m e a s u r e m e n t s o f the 1400 keV 4 + a n d 1643 keV 3 + states. t Work supported in part by the Israel Academy of Sciences and HumaniUes. 141

142

J. BURDE et

aL

2. Experimental arrangement The 134Cs source was produced by evaporating a 3 mm circular spot of CsCI on a thin foil of aluminum which was subsequently irradiated by a flux of 2x 1014 neutrons/era2 • see for about a week. One lifetime measurement was carried out with the aid of a double-lens coincidence fl-spectrometer 6). The transmission and momentum resolution of the spectrometer were adjusted to about 3 ~. The other lifetime was measured by performing fl-7 delayed coincidences. In this measurement the fl-spectrometer was converted into a fl-y coincidence spectrometer by using only one half of the instrument and by placing a cylindrical NE111 plastic scintillator about 5 cm away from the back of the source, to detect the y-rays. The focused electrons were detected by N E l l l plastic scintillators coupled to XP1020 photomultipliers. The pulses from the collectors were limited and fed into an ORTEC delay-to-pulse-height converter. The pulses were channelled within 60 ~o to 70 ~o of the peak voltage. The lifetime measurements were carried out by self-comparison and external comparison methods. The shifts between the centroids which is correlated to the mean lifetime of the excited state under investigation, in both methods, was calibrated by introducing a General Electric air line 874L30 1.0036_ 0.0016 nsec delay between one of the limiters and the time-to-amplitude converter. To eliminate time-dependent drifts, the two current settings of the spectrometer in the self-comparison method or the two sources in the external comparison method, were interchanged automatically every 5 rain and the respective counting rates were stored in two halves of the split memory of the multichannel pulse-height analyzer. The lifetimes were deduced from many such sets of measurements, with and without the delay line. The performance of the experimental setup was described elsewhere 7), and the reliability was tested by carrying out the lifetime measurement of the 412 keV state of 19SHg by utilizing an 19aAu source. The half-life thus obtained was T½ = 22 + 1 psec in very good agreement with the previous result 7).

3. Results 3.1. LIFETIME DETERMINATION OF THE 1400keV STATE Fig. 1 shows the higher energy part of the electron spectrum observed by one of the spectrometers. A similar spectrum was observed by the other half of the instrument. In the first part of the experiment, the lifetime sum measurement of the 604 keV and 1400 keV levels was carried out by the self-comparison method using 80000 coincidences between the ]/-particles that populate the 1400 keV level and the K 604 keV line. The 1400 keV level is de-excited via the 604 keV state. About 8 ~o of the K 604 keV line and tails due to the 563 keV and 569 keV lines, which contributed about 1 ~ and 5 ~ to the total coincidence counting rate, respectively, were accepted by the

134Ba TRANSITION

PROBABILITIES

143

xlO 3

\

100 o o =

-...

80

563

604.7

569

o N to IZ

0 o

•

60

g

V

:

4o •

604.7

796 802

L

K

20

1 0

I 100

I

,,,,

_ _t ~

200

CHANNEL

I 300

I

NUMBER

F i g . 1. S i n g l e s e l e c t r o n s p e c t r u m o f 134Cs. T h e c h a n n e l n u m b e r is p r o p o r t i o n a l of the electrons.

to the momentum

spectrometer at the lower off-line position. The tails of the K, L 604 keY line which were accepted at the higher off line setting, contributed about 20 ~ to the coincidence counting rate. There were also very small contributions to the coincidence counts due to tails of the 604 keV lines in the off-line positions and the tails of the 563 keV and 569 keV lines which were accepted in the on-line settings. The direct population of the 1168 and 604 keV levels by p-decay is negligible (0.045 ~o and 0.008 ~o, respectively). A careful analysis furnished us with the following equation for the half shift between the centroids, 0.75 [,(1400) + ,(604)] +0.0033,(604) = 15.1+2 psec,

(I)

where z(E) denotes the mean life of the level whose energy (in keV) is written in the brackets. The lifetime of the 604 keV state was measured previously s). Hence, the result for the sum of the mean lifetime was ~(604)+~(1400) = 20.1 + 4 psee. The specified error included also the uncertainty in ascertaining the various contributions to the coincidence counting rate (mainly due to the tails of K and L, M 604 keV lines in the off-line positions). In the second part, one spectrometer focused the K 796 keV line in 134Ba and the second was set on the K 412 keY line in 19SHg. The 134Cs and ~98Au sources were interchanged automatically every 5 rain with the aid of a pneumatic arrangement and the respective time spectra coincidences were stored in two halves of the split memory of the multiehannel pulse-height analyzer. Special precautions were taken to ensure that both sources would occupy exactly the same position inside the spectrom-

144

J. B U R D E et ai.

eter. The shift between the centroids which was obtained by using 80000 and 40000 coincidences from ta*Cs and 19SAu sources, respectively, corresponded in this method to the sum of the mean lifetimes of the 1400 keV state in 134Ba and the 412 keV level in t 9SHg" After subtracting the mean lifetime of the 412 keV level (obtained in the present work) the mean lifetime for the 1400 keY states was ¢(1400) = 12.5_+3 psec. Kerns et al. s) determined the reduced transition probability B(E2;0+--* 2 +) for the 604 keV level which corresponded to a mean lifetime of z(604) = 7.5+0.2 psec. Subtracting this value from the result of the first part of the experiment, the figure for the mean lifetime of the 1400 keV state was z(1400) = 12.6_+4 psec, in very good agreement with the result of the second part. The combined result was ~(1400) = 12.5+2.5 psec. 3.2. L I F E T I M E

MEASUREMENT

O F T H E 1643 keV S T A T E

The lifetime of the 1643 keV state was carried out by using coincidences between the//-particles that populate this level and the 1039 keV and 1168 keV y-rays through which the state is de-excited. The intensity of the oource which was used in the fl-eexperiments was reduced by about a factor of ten. A lead absorber 1 cm thick sandwiched between 2 mm sheets of tin, copper and aluminum was placed between the back of the source and the gamma plastic detector, to suppress the intensity of the ~,-ra~s which were lower in energy. The//-y coincidence counting rate with the 796 keV y-rays was much higher than the coincidence rate due to the 1039 keV and 1168 keV y-'ransitions. This factor and the poor energy resolution of the plastic scintillator made the measurement very difficult. The//-spectrometer focused 200 keV electrons, and two regions from the upper part of the y-spectrum were chosen to minimize the coincidence contribution due to the 796 keVy-rays. Two separate coincidence measurements were made with the aid of a slow-fast coincidence arrangement. A thin source of 6°Co was interchanged with the J3*Cs source every 5 rain in an arrangement similar to that described in the previous section. The shift between the centroids corresponded to the difference between the//-y time delays due to the two sources. A careful analysis, using the data from the decay scheme furnished us with the following equations that connected the shifts between the centroids with the lifetimes of a number of states weighted in proportion to their contributions to the coincidence counting rate for the two sections of the y-spectrum, 0.845~(1400)+0.155~(1643)+0.037~(1168)-z(6°Co) = 26.1+4 psec,

(2)

0.291T(1400)+0.709T(1643)+0.325¢(l168)--T(6°Co) = 83.3+ 14 psec,

(3)

where ~(6°Co) denoted the fl-y time delay for the 6°Co source and the channelled regions of the ?-spectra in eqs. (2) and (3) were 780-1110 and 1170-1500 keV, respectively. As is seen from the equations, the lifetime of the 1643 keV state is mainly affected by the lifetime of the 1400 keV state, which has been measured in the present work.

la4"Ba TRANSITION PROBABILITIES

145

The lifetime of the 1168 keV level was calculated roughly on the assumption that the 563 keV transition has the same B(E2) value as the 604 keV transition. In the same way the lifetime of the 4 + state in 6°Ni was estimated, using the known lifetime of the 2 + level 9) in the same nucleus. The time delay z(6°Co) was thus estimated as 2.5 psec and 3 psec for the lower and higher channelled regions, respectively. The results for the mean lifetime of the 1643 keV state as deduced from eqs. 2 and 3 were 114 + 26 psec and I 12 + 20 psec, respectively. The combined result was T(1 643) = I 13 + 30 psec, where the error was increased to take into account the uncertainty in determining the various contributions to the coincidence counting rate and the lack of knowledge of the lifetime for the 1168 keV state.

4. Discussion

Fig. 2 shows the level scheme of 134Ba populated by the beta decay of lagCs. The present mean lifetime results were also included. In table 1 are given the transition probabilities in units o f s.p. estimate 1o)for the transitions that de-excite the 1643 keV and the 1400 keV states. The result for the 604 keV transition is also included. It turns out that whereas the 604 keV transition probability is enhanced by a factor of 32 withrespect to the s.p. estimate, the 796 keV transition is only enhanced by about a factor of 5. The ratio of the reduced transition probabilities B(E2; 4 + --, 2+)/B(E2; 2 + --* 0 +) is equal to 0.15__.0.03. This result is very unexpected. According to the rotational model, this ratio should be equal to 1.43 and the vibrational model predicts that it should be equal to 2. Such a big discrepancy between theory and experiment makes it very questionable whether the 4 + level can be considered as a rotational key 4*

1 ~1~.1

4 ÷,

g

2÷

2 4.

1969.8

psec

1643.3

113 ± 30

1400.5

12.5:1:2.5

1168.1

....

604.7

7.5*0.2

Qp= 2062 keV

O*

0

Fig. 2. Decay scheme of 134Cs. The present mean lifetime results

are also

included.

146

J. B U R D E et aL TABLE 1 Transition probabilities in units of the s.p. estimate

Transition

Iilr

h ~r

energy (keY) 604.7 795.8 232.5 475.4 242.7 1038.6

2t + 4+ 4+ 3+ 3+ 3+

0+ 2x+ 2~+ 2~+ 4+ 2x+

T(E2)c~,

T(M1)®xp

T(E2),.p.

T(M1),.p.

32.5-4-0.1 a) 4.94 4-1 < 0.27 b) 4.23-t-1.1 1.62-4-0.9 c) 0.0444:E0.012

~ 5.1 × 10-6 <: 3.3 × 10-4 d) (2.4-t-0.8) × 10- s

") The result was taken from ref. s). b) Deduced from the upper limit on the intensity of this transition, given in ref. 2) and the present result for the lifetime measurement. c) Deduced on the assumption that the transition is pure E2. a) The limit was deduced on the assumption that the transition is pure MI.

or vibrational state. On the other hand, the mode of de-excitation of the other levels in la4Ba exhibit collective properties. The transitions that de-excite the 2 + and 3 + states have predominantly E2 multipolarities with exceedingly small M1 admixtures. In fact the energy spacings of the first four excited states can be reproduced reasonably well by the asymmetric rotational model 11) with the non-adiabaticity parameter # = 0.45 and non-axiality parameter y = 28 ° which gives tz) also the right value for the relative intensities of the transitions that de-excite the 2 + 1168 keV state. In the first row of table 2 are given the experimental reduced transition probabilities in units of the B(E2; 2 + ~ 0 +) and in the second row the corresponding theoretical 12) values. As is evident, the theoretical ratios exceed the experimental ratios by at least one order of magnitude. If, however, a similar comparison is made (in rows 3 and 4) when the reduced transition probabilities are expressed in units of the B(E2; 4 + ~ 2+), the correspondence between the experimental and theoretical lesults is reasonable. These results indicate that the reduction in collectivity as experienced by the higher angular m o m e n t u m state of the quasi-rotational band is reflected also in the level of the y-vibrational band, which is situated closeby in the excitation energy. In rows 5 and 6 are compared the relative reduced transition probabilities B(E2; 3 + ~ Ie) deduced from experiment and theory, respectively. The agreement is remarkable despite the apparent change in the structure of the nucleus at the higher excitation energy as reflected by the reduction in collectivity. It is interesting to note that the low energy level structure and mode of de-excitation of 192pt shows a striking resemblance to the 134Ba states. In fact, the energy spacings of the first four excited states in 192pt can be reproduced also reasonably well by the asymmetric rotational model with # = 0.40 and ~ = 28 ° parameters. In rows 7 and 8 of table 2 are compared the experimental and theoretical relative reduced transition probabilities B(E2; 3 + ~ If). The B(E2; 4 + ~ 2+)[B(E2;2 + --, 0 +) ratio

TABLE 2 Experimentally reduced transition probabilities a n d c o m p a r i s o n with t h e a s y m m e t r i c rotational m o d e l predictions Initial a n d final states ua

4 + ..-> 21 +

4 + ..-> 2z +

3 + .-~ 2~ +

3 + .-> 21 +

3 + ..-> 4 +

0.15+0.03 1.66

< 0.00082 0.0172

0.13±0.043 2.43

0.0014±0.00046 0.0262

0.050±0.028 1.33

> Z

< 0.055

0.86 ± 0 . 2 8

0.0090 -t-o.oo3 o.o158

o. 327 ± o. 18 0.80

Z

38.3±20

g

B(E2; Ii ~ It)/B(E2; 2t + ~ 0 +) exp. theor. B(E2; Ii -~ If)/B(E2; 4 + -~- 2t +) exp. theor.

0.01

1.47

aB(E2; Ii ~ If)134Ba exp.

(100)

bB(E2; It -+ If)

(100)

1.08

55

cB(E2; I, -+ [f)t92pt exp.

000)

0.535

36.9

dB(E2; 11 --> If) theor. (~ ---- 0.40; y = 28 °)

(100)

I.I

55

theor.

1.o5±o.14

= 0 . 4 5 ; y = 28 °)

a, b, c, d are multiplying factors to equalize t h e B(E2; 3 + ~ 2z +) value in the last four rows o f t h e table.

N ,-t

N

148

J. B U R D E et al.

in 192pt was found 13) to be equal to 0.45-t-0.1 with no obvious explanation for the discrepanc~ between experiment and theory. The c o m m o n denominator for 134Ba and 192pt is that the former has 78 neutrons and the latter 78 protons. In a recent work in our laboratory 7) it was found that th*. ratio [B(E2; 6 + --, 4 + ) + B(E2; 4 + ~ 2+)]/B(E2; 2 + --, 0 +) in 13°Xe (76 neutrons) agreed within the experimental errors with the prediction of the asymmetric rotational model. On the other hand, Dehnhmdt et al. 14) had found that the ratio B(E2; 4 + --, 2+)/B(E2; 2 + --. 0 +) in 134Ce(76 neutrons) was equal to 0.60 and that the reduced tlansition probability B(E2; 4 + --, 2 +) in 136Ce (78 neutrons) was only enhanced by a factor o f 8.1 with respect to the s.p. estimate. It turns out that in the transition region near a closed shell very strong anomalies can be observed in tbe lifetimes of the 4 + and 3 + states, even though the level structure and relative intensities o f the de-exciting transitions comply with the asymmetric rotational model predictions. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)

C. M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes (Wiley, New York, 1968) D. E. Raeside, J. J. Reldy and M. L. Wiedenbeck, Nucl. Phys. A98 (1967) 54 A. Abdul-Malek and R. A. Naumann, Nucl. Phys. A106 (1967) 225 R. D. Reuland, Nucl. Phys. A176 (1971) 657 M. Behar, R. M. Steffen and C. Telesco, Nucl. Phys. A192 (1972) 218 T. R. Gerhoim, Rev. Scl. Instr. 26 (1955) 1069 J. Burde, S. Eshhar, A. Ginzburg and E. Navon, Nucl. Phys. A229 (1974) 387 J. R. Kerns and J. X. Saladin, Phys. Rev. C6 (1972) 1016 J. Lindskog, T. SundstriSm and P. Sparrman, Alpha-, beta- and gamma-ray spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1965) p. 1599 S.A. Moszkowski, Alpha-, beta- and gamma-ray spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1965) p. 863 A. Davydov and A. A. Chaban, Nucl. Phys. 20 (1960) 499 A. Davydov and J. Ovcharenko, Soy. J. Nucl. Phys. 3 (1966) 740 A. Schwarzchild, Phys. Rev. 141 (1966) 1206 W. Dehnhardt, S. Mdls, M. Muller-Veggian, U. Neumann, D. Pelte, G. Poggl, B. Povh and P. Taras, Nuci. Phys. A225 (1974) I