Nuclear spectroscopy of Ba133m

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Nuclear Physics 67 (1965) 625--630; (~) North-Holland Publishing Co., Amsterdam

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

N U C L E A R S P E C T R O S C O P Y OF Ba 133m J. E. THUN, S. T(3RNKVIST, F. FALK and H. SNELLMAN Institute of Physics, University of Uppsala, Uppsala, Sweden Received 7 December 1964 Abstract: The decay of the 38.9 h isomer of Batss has been investigated. The energy of the d~ ~ s½ transition was determined by conversion electron measurements to 12.294-0.04 keV and the L subshell ratios of the same transition were found to be 100/9.6/3.1. The half-life of the 12.3 keV level was measured to 8.1 4-2.0 ns. The K conversion coefficient and the K/(L-t-M) ratio of the 276 keV h~ --~ d~ transition were measured with the result: ctK(276) = 3.454-0.20 and K / ( L + M ) = 2.55:k0.10. The pairing plus quadrupole force model seems to explain the experimental data quite well. E[

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RADIOACTIVITY Ba 183 [from CsaS~(d,2n)]; measured ce, ce-~ coin, ce-ce delay, cc. Baxas deduced d(E2/M1), level T~..

1. Introduction T h e 38.9 h i s o m e r o f Ba 13a was s h o w n b y Hill et al. 1) to decay b y a two-step c a s c a d e to the g r o u n d state o f 7.5 y Ba 133. F r o m their m e a s u r e m e n t they suggested t h a t the levels c o u l d be described a c c o r d i n g to the shell m o d e l as h ~ 276 k e y d~ 12 key S½, which is in a c c o r d a n c e with the general systematics o f o d d - n e u t r o n i s o t o p e s in this region. F u r t h e r m o r e the f t values o f the E.C. t r a n s i t i o n s to Cs 133 s u p p o r t the s~ a s s i g n m e n t to the Ba 133 g r o u n d state. Selection rules for single-particle M1 t r a n s i t i o n s requires Al = 0. H o w e v e r , a n u m b e r o f M1 t r a n s i t i o n s involving AI = 2 a c c o r d i n g to the S P M c h a r a c t e r i z a t i o n o f the levels h a v e been o b s e r v e d with r e t a r d a t i o n factors r a n g i n g f r o m 10 to 3000. T h e d~ --, s , M 1 t r a n s i t i o n in Ba 133m is o f this type. V a r i o u s theoretical e x p l a n a t i o n s o f these t r a n s i t i o n s have been suggested, b u t the situation is far f r o m satisfactory. I n o r d e r to i m p r o v e the e x p e r i m e n t a l d a t a o n these transitions we have m e a s u r e d the lifetime o f the d~ level to which was earlier only assigned a n u p p e r limit 2). I n this c o n n e c t i o n we also r e m e a s u r e d the energy o f the 12 keV t r a n s i t i o n a n d determ i n e d the L subshell ratios in o r d e r to estimate the E2 admixture. O u r investigation also includes a d e t e r m i n a t i o n o f the K conversion coefficient a n d the K / ( L + M ) r a t i o o f the 276 k e V M 4 transition.

2. Source Production T h e Ba 133m activity was p r o d u c e d b y the r e a c t i o n Cs133(d, 2 n ) B a 13am using 20 M e V deuterons in the c y c l o t r o n o f the N o b e l Institute, S t o c k h o l m , Sweden. 625

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Barium was chemically separated from cesium by a procedure described in ref. 3). The activity was then evaporated onto the source backing of 1.2 mg/cm 2 aluminium. The thickness of the sources was less than 10/~g/cm 2. In the measurement of L subsheU ratios and energy of the 12 keV transition, extreme care was taken to get very thin sources. Several sources were used during the investigation. 3. Instrmnents

In the course of the investigation three different instruments were used; (i) an electron-gamma coincidence spectrometer consisting of a medium thick magnetic lens and 6.8 em diam. × 5.1 em thick NaI(T1) gamma detector (the electron detector of this instrument is an anthracene crystal coupled to an EMI 6260 photomultiplier tube), (ii) a new 50 cm radius double-focussing iron-free spectrometer recently completed at this institute 4), (the detector used in this experiment was a G M tube having a Formvar window with a cut-off energy of about 2.5 keV), (iii) an electronelectron coincidence spectrometer especially equipped for measurements of short lifetimes using a time-to-pulse-height converter 5). 4. Measurements

4.1. THE ENERGY OF THE d~ -+ s~r TRANSITION The position of the I.a conversion peak was determined during several runs in the iron-free spectrometer. Calibration was accomplished by a measurement of the energy difference between the In and M l conversion lines, using the known 6) binding energies. Using these binding energies we obtain the result Edt ~ sj = 12.2904-0.036 keV. 4.2. THE L SUBSHELL RATIO OF THE 12.3 keV TRANSITION The electrons spectrum in the region of L electrons was recorded several times in the 50 cm radius iron free spectrometer with a resolution of 0.8 °/oo. The L electron spectrum is shown in fig. 1. The analysis of this spectrum gives ~/Ln/Iai I

= 100/9.6+2.0/3.1 4-1.5.

The data were corrected for decay and window absorption in the G M tube. We were also able to record the L electron spectrum in the lens spectrometer, fig. 2. This means that it was possible to detect 6 keV electrons in the electron detector, a good figure for an instrument of this kind. The resolution obtained was 2 ~ . This measurement gives h / L l d h , l = 100/11.6/5.2

with somewhat greater errors than the abovementioned, which is due to the more ditficult unfolding procedure of the composite L line.

NUCLEAR SPECTROSCOPY OF Balaam

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4.3. THE HALF-LIFE OF THE 12.3 keV LEVEL T h e h a l f life o f the 12.3 k e V level was d e t e r m i n e d b y m e a s u r e m e n t s b o t h with the e l e c t r o n - g a m m a s p e c t r o m e t e r a n d with the electron-electron spectrometer. T h e 2500- Ne/'Smin 12.3LI

2000-

1500-

f

1000-

500-

3.~0

12.3 L

3.60

' potentiorneter 3.80 reading

3.70

Fig. 1. The L conversion lines of the 12.3 keV transition recorded with the iron-free double focussing spectrometer.

5KI1%'/30

12.3 L

30 K Ne~30 276 K

4K-

20 K

3K-

2K o.~o

~

o.is

276LoM

10K

o.~

'

0

80

9'0

~

pote~tiomet~ readtnO

Fig. 2. The 12.3 keV L lines and 276 keV K and Lq-M lines recorded with the lens spectrometer e - - 7 results were o b t a i n e d b y d e l a y e d coincidences a n d a n a l y s e d b y the e e n t r o i d shift m e t h o d , while in the e - - e - case a time-to-pulse-height converter was used a n d

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the lifetime inferred from the slope. In the latter experiment a H.V. of 25 kV was put on the source in order to accelerate the electrons. The result of these measurements is t½ = 8.1_2.0 ns. 4.4. THE CONVERSION COEFFICIENTS OF THE 276 keV TRANSITION

The K conversion coefficient of the 276 keV transition was determined by recording the 276 keV K line and 276 y-single rates is the magnetic lens spectrometer and the gamma detector respectively. Utilizing the known 7) conversion coefficient of the 279 keV transition in TI 2°3, the spectrometer transmission and gamma detector efficiency plus solid angle were determined. Recordings of the e - and 7 rates were made for different resolution settings and different gamma detector distances. The conversion coefficient is calculated from N276Kco276y8276y N276y CO276K$276K

Ba 133,

(1)

0~279K = N279Kco279~'82797 N2797 CO279K$279K

TI2°3,

(2)

0~276K =

where N denotes the counting rate and co the solid angle transmission and efficiency. Assuming co~e~ (276) = co~8r (279) and coK~K (276) = %¢coIC(279), eqs. (1) and (2) give Ba N276K N279~, TI 0~276K -~279K" N276y N279K

The Ba 133m and Pb 2° 3 sources used were of the same diameter. A small correction for the contribution of the 400 keV ~ peak in T1203 was applied in this case. By inserting our data corrected for decay and scaler dead-time, we obtain u,:(276) --- 3.45+0.20. The K/(L + M) ratio was determined from the recorded e - spectrum which is shown in fig. 2. The result is K / ( L + M ) = 2.55+0.10. 5. Discussion The decay scheme with the new data inserted is shown in fig. 3. Our results do not contradict the earlier accepted shell-model spin and parity level assignments h~ --, d~ ---, s~r. The conversion data for the 276 keV transition show that the transition is of pure M4 character. Taking our error limits into account we get an upper limit offi 2 < 0.09 for the E5 admixture.

NUCLEAR SPECTROSCOPY OF Ba138m

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The energy of the dt ~ s~ transition was determined to be 12.290+_0.036 keV, which differs from the earlier 1) accepted value 11.7+_0.2 keV. Our measured L subshell ratios of this transition indicate that it is of pure M1 character with a maximum possible E2 admixture of c32 = 1.6" 10 -4. With the d t ~ si level assignment, the M1 component of this transition must be interpreted as/-forbidden. Our value of 8.1 + 2.0 ns for the d~ half life gives with ~L+rd + N = 110 a retardation factor of 65___20 relative to the SP estimate s) (r ° = 1.2 fm). This is in agreement with general systematics of/-forbidden M1 transitions in odd-neutron nuclei. h11/2

39h

0.2880

d 3,/2

8.1 ns

0.0123

s 1/2

~2.v

M1

0

56Ba133 Fig. 3. D e c a y s c h e m e of Ba zssm.

Several explanations of the non-zero rate of these transitions have been advanced9-11). The most recent is given by Sorensen 12), who shows that by using wave functions resulting from shell model plus residual pairing plus quadrupole force, the one-phonon component of these wave functions allows /-forbidden M1 transitions to proceed. The result of Sorensen is expressed as a theoretical retardation factor. In the case of Ba 133m the agreement between the predicted value of Sorensen (inferred from ref. 12), fig. 2) and our experimental value is quite good. We therefore conclude that the non-zero transition rate of the /-forbidden d~ ~ s~r M1 transition in B a 133m can be fully explained by the effect of quadrupole vibrations. It is interesting to observe that this model also predicts an M4 (h¥ ~ d~) transition rate which is in agreement with the experimental value 13). It thus seems that the dynamical electromagnetic properties of Ba la3m so far measured are well explained by the pairing plus quadrupole force model 13). We are indebted to Professor Kai Siegbahn for the experimental resources put at our disposal, to the members of the iron free crew for lending us the spectrometer and for their assistance and to the e -e spectrometer group for the loan of the spectrometer. We are also indebted to Mr. C. Bergman for performing the chemical separation and to lie. F. Kropff for taking part in the recording of data. This investigation has been sponsored, in part by EOAR, U.S.A.F.

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References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

R. D. Hill, G. Scharff-Goldhaber and M. McKeown, Phys. Rev. 84 (1951) 382 M. Langevin, Compt. Rend. 236 (1953) 89 R. W. Fink, NAS-NS 3010, p. 49 K. Siegbahn et aL, Nucl. Instr. 27 (1964) 173 T. R. Gerholm and J. Lindskog, Ark. Fys. 24 (1963) 171 I. Andersson and S. HagstrOm, Ark. Fys. 27 (1964) 161 W. Croft, B. -G. Pettersson and J. H. Hamilton, Nuclear Physics 48 (1963) 267 J. M. Blatt and V. F. Weisskopf, Theoretical nuclear physics (Wiley & Sons, New York, 1952) p. 627 R. G. Sachs and M. Ross, Phys. Rev. 84 (1951) 379 A. B. Volkov, Phys. Rev. 94 (1954) 166 A. Arima, H. Horie and M. Sano, Progr. Theor. Phys. 17 (1957) 567 R. A. Sorensen, Phys. Rev. 132 (1963) 2270 L. S. Kisslinger and R. A. Sorensen, Revs. Mod. Phys. 35 (1963) 854