Half-lives of the first excited states in 182W and 170Yb

Nuclear Physics 75 (1966) 109--112; (~) North-Holland Publishino Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

HALF-LIVES OF THE FIRST EXCITED STATES IN

182W A N D X~°Yb

B. V. NARASIMHA RAO and SWAMI JNANANANDA Laboratories for Nuclear Research, Andhra University, Waltair, India

Received 11 May 1965 Abstract: The half-lives of the first excited states of la2W and a~°Yb are measured with a fast timeto-amplitude converter. The half-life of the 100 keV level of xa~W is 1.434-0.05 ns and that of the 84 keV level of 17°Yb is 1.64-0.05 ns. Experimental internal conversion coefficients are calculated using the present values of the lifetime measurements and the published data of the Coulomb-excitation B(E2) values. Comparison with the theoretical internal conversion coefficients show an agreement within 1.5 %. E]

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RADIOACTIVITY la~Ta, l~°Tm; measured ~ce-delay; deduced co. i82W, 178yb levels, deduced T½.

1. Introduction Recent results on the internal conversion coefficients in the deformed region o f nuclei, are at variance. It has been reported 1) that the K-shell conversion coefficients for a n u m b e r o f pure E2 transitions are appreciably larger than the theoretical values. The total internal conversion coefficients determined 2) f r o m the measured half-lives of the first excited 2 + states o f pure E2 transitions and the C o u l o m b excitation B(E2) values were in disagreement with the theoretical values. A similar conclusion was drawn by Fossan and Herskind a). Some direct precision measurements 4) o f K conversion coefficients exist which indicate a fair agreement with the theoretical values. It can be seen that the validity o f the conclusion drawn by Fossan and Herskind 3) depends mainly on the accuracy of the lifetime measurements and results taken f r o m the C o u l o m b excitation. The existing 3, 5-9) values o f the first excited states o f 182W and i 7 ° y b differed by large magnitudes. I n the present w o r k lifetimes o f the first excited states o f these two nuclei are remeasured with the presently developed arrangement.

2. Experimental Arrangement The lifetime measurements are made by the delayed coincidence m e t h o d using a 6BN6 time-to-amplitude converter. The associated circuitry is almost similar to the one described by Green and Bell 1o). Plastic scintillators with a conical well arrangement are used. The effective thicknesses o f the scintillators used for the detection of the conversion electrons, beta rays and g a m m a photons, respectively, are 1 ram, 2 m m and 2.54 cm. The time spectra are recorded on a 10-channel pulse-height 109

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analyser. The chance rate is subtracted and the resultant observations are plotted. The full width at half m a x i m u m of the prompt time spectrum, obtained with 6°Co

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Fig. 1. Lifetime of the 100 keV level of ls2W.

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Fig. 2. Lifetime

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is 780 psec and the sides o f the prompt curve decreases by a factor of two in 81.2 psee.

HALF-LIVES

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3. Results and Discussion The isotopes ~S2Ta and 17°Tm are supplied by the A E E T India. The lifetime of the 100 keV level of ~82W is determined by observing coincidences between the highenergy g a m m a rays and L conversion electrons of 100 keV g a m m a rays. The gammaray channel is set in such a manner that all the g a m m a rays above 1 MeV are accepted. The other channel is set to accept the L conversion electrons of 100 keV g a m m a transition. Though the setting of the electron channel includes electrons corresponding to the 113 keV transition they will not be in coincidence with the hard g a m m a rays. The selection of the hard g a m m a rays beyond 1 MeV eliminates the error due to the lifetime of the 329 keV level. Moreover, the intensity, of the transitions feeding the 329 keV level is weak. The lifetime of the 84 keV level of 17°Yb is determined by observing coincidences between the beta rays and the conversion electrons of the 84 keV transition. The two time spectra are recorded. Four such measurements are made and one distribution of IszW and one of m ° Y b are shown in figs. 1 and 2 respectively. TABLE 1 Comparison of the experimental conversion coefficients with the theoretical values

Nu- Energy of the cleide transition (keV)

Half-life (ns)

B(E2) Coulomb excitation × (e~× 10-4s cm4)

C%xp

cqheor Disagreement (~o)

aT°Yb

84.26

1.474-0.04 5.35-I-0.25

7.20010.4

6.4

12.5

ls2W

100.07

1.264-0.04 4.004-0.20

4.5 4-0.2

3.99

12.8

17°yb

84.23

1.604-0.05 5.35!0.25

6.5154-0.3

6.4

xB~W

100.09

1.434-0.05 4.004-0.20

3.9194-0.144 3.99

1.80 1.78

Author

Fossan and Herskind Fossan and Herskind present results present results

The lifetimes are determined from the slopes of the time spectra. The lifetime on the negative delay time in fig. 1 is due to the coincidences between the high-energy g a m m a rays and beta rays, feeding the high-energy levels included in the selection o f the conversion electrons of 100 keV transitions. However, the slope on the right hand side of the time spectrum is unaffected. A least-squares fit analysis made over the entire wing of the exponential slope of the time spectra gave a value of 1.43 ___0.05 ns and 1.60___0.05 ns for the lifetimes of the 100 and 84 keV levels, repectively. The errors quoted are mainly the result of uncertainties allowed for the time calibration and other systematic errors, such as instrumental and statistical. The present value of the lifetime of the 100 keV level is in good agreement with a recently published value ~1). Our 17°yb value is in agreement with the results of ref. 9).

112

B. V. N. RAO A N D S. J N A N A N A N D A

The r e d u c e d t r a n s i t i o n p r o b a b i l i t y B ( E 2 ) for the E2 t r a n s i t i o n within one r o t a t i o n a l b a n d is related to the half-life a n d energy in the following m a n n e r : B(E2; 0 + ~ 2 +) = 282[T½(1 + CttotErS)]-1, where E~ is the energy o f the transition in keV, ~total is the t o t a l internal conversion coefficient, a n d B(E2) is in units o f e 2 x 10 - 4 s c m 4. U s i n g this relation, the t o t a l internal conversion coefficients are calculated f r o m the p r e s e n t values o f the lifetime m e a s u r e m e n t s a n d p u b l i s h e d d a t a 12) o f B ( E 2 ) values a n d energies 13). These results are listed in table 1 t o g e t h e r with the theoretical values. The values o b t a i n e d b y F o s s a n a n d H e r s k i n d 3) are also included in the same table for c o m p a r i s o n . It can b e inferred f r o m the present results t h a t there is g o o d a g r e e m e n t between the experim e n t a l c o n v e r s i o n coefficients a n d the theoretical values. One o f the a u t h o r s ( B . V . N . R . ) is grateful to Dr. V. L a k s h m i N a r a y a n a for his helpful suggestions d u r i n g the course o f this w o r k a n d t h a n k f u l to the C o u n c i l o f Scientific a n d I n d u s t r i a l Research, G o v e r n m e n t o f I n d i a f o r a w a r d i n g h i m a R e s e a r c h Fellowship.

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

F. K. McGowan and P. M. Stelson, Phys. Rev. 107 0957) 1674 E. M. Bernstein, Phys. Rev. Lett. 8 (1962) 100 D. B. Fossan and. B. Herskind, Nuclear Physics 40 (1963) 24 J. F. W. Jansen, S. Hultberg, P. F. A. Goudsmit and A. H. Wapstra, Nuclear Physics 38 0962) 121 E. Bashandy, M. S. E1 Nesr and S. Pancholi, Nuclear Physics 41 (1963) 346 H. J. Korner et al., Z. Phys. 173 (1963) 203 R. Graetzer and E. N. Bernstein, Phys. Rev. 129 (1963) 1772 F. E. Wagner, F. W . Stanek, P. Kienle and H. Eicher, Z. Phys. 166 (1963) 1 M. S. E1 Nesr and. E. Bashandy, Z. Phys. 168 (1962) 349 R. E. Green and R. E. Bell, Nucl. Instr. 3 (1958) 127 M. Dorikens, O. Segaert, J. Demuynckand.L. Dorikens-Vanpraet, Nuclear Physics 61 0965) 33 B. Elbek, M. C. Olesen and O. Skilbreid., Nuclear Physics 19 (1960) 523; O. Hansen, M. C. Olesen, O. Skilbreid and B. Elbrek, Nuclear Physics 25 (1961) 634 Nuclear Data Sheets, National Academy of Sciences, National Research Council