Half-lives of two excited states of Gd156

Nuclear Physics 12 (1959) 413--417; ~ ) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

H A L F - L I V E S OF T W O E X C I T E D S T A T E S OF Gd ts6 R. E. B E L L * and M. H. J O R G E N S E N

Institute 1or Theoretical Physics, University o1 Copenhagen Received 27 April 1959 The half-lives of the 1507 keV a n d 89 k e y excited states of Gd xse h a v e been m e a s u r ed w i t h a fast t i m e - t o - a m p l i t u d e converter. The half-life of the 1507 keV 4 + state is (1.88:k0.10) × 10 -1° sec, in a g r e e m e n t w i t h estimates m a d e in t h e preceding p a p e r b y Gregers H a n s e n etal. 1). This result t h r o w s serious d o u b t on the s u p p o s e d m e a s u r e m e n t of the half-life of t h e 288 keV s t a t e b y Ofer 4). The half-life of the 89 keV first rotational s t a t e is (2.19-4-0.07)× 10 -s sec, differing slightly f r o m a previous m e a s u r e m e n t b y N a t h a n s). The n e w value leads to a reduced t r a n s i t i o n p r o b a b i l i t y for the reverse (Coul o m b excitation) process B ( E 2 ) : e s = (4.644-0.20)× 10 -4s cm 4, in excellent a g r e e m e n t w i t h t h e value directly m e a s u r e d b y Coulomb excitation b y Ram~ak etal. 7).

Abstract:

1. I n t r o d u c t i o n The electron-capture disintegration of T b 16eto excited states in Gd 15ehas rec e n t l y been studied with great care b y Gregers Hansen.et al. 1). The radiations in this d e c a y are such t h a t it is easy to measure the lifetimes of the 1507 keV excited state and the 89 keV first r o t a t i o n a l excited state, and this has now been done, T h e lifetime of the l a t t e r state was measured earlier b y N a t h a n 2) with a slightly different result. The m e a s u r e m e n t s were m a d e with a fast t i m e - t o - a m p l i t u d e c o n v e r t e r similar to t h a t described b y Green and Bell s), b u t with a n u m b e r of modifications and additions which we hope to describe at a later time. T h e a p p a r a t u s is capable of producing a time resolution curve for Co e° g a m m a rays having a full width at half m a x i m u m of 4 × 10 -1° see and sides dropping off with an i n s t r u m e n t a l half-life of 5 x 10 -11 sec. F o r 500 keV g a m m a rays, the l a t t e r figure increases to 6 or 7 x 10 -11 sec. Thus, g a m m a rays with half-lives above, say, 10 -1° sec can h a v e their half-lives measured directly from the logarithmic slope of the resolution curve, r a t h e r t h a n b y the less certain centroid-shift method.

2. E x p e r i m e n t s and R e s u l t s 2.1. H A L F - L I F E O F T H E 1507 keV S T A T E

A weak source of T b 15s ( ~ 0.5 pC) was placed between two g a m m a r a y counters consisting of diphenylacetylene crystals (2.0 cm d i a m e t e r × 1.5 cm t P e r m a n e n t address: R a d i a t i o n L a b o r a t o r y , D e p a r t m e n t of Physics, McGill University, Montreal, Canada. 413

414

R. E. BELL AND M. A. JORGENSEN

high) coupled to RCA-6342A photo-multipliers. Pulses from counter no. 1 lying between 210 keV and 320 keV electron energy were accepted by the pulse height selector. This counter was therefore sensitive to gamma rays above 335 keV, but most sensitive around 500 keV. In a similar way, counter no. 2 was set to be sensitive to gamma rays above 900 keV, but most sensitive around 1250 keV. Reference to the Tb ls6 disintegration scheme of Gregers Hansen et al. 1) and Ofer 4) shows that essentially all the coincidences between two such counters involve the 534 keV gamma ray leading to the 1507 keV state and one of several ~ 1 MeV gamma rays leading from it. Nevertheless a few coincidences, possibly 2 % of the total, may involve the 1620 keV state as well; unless the lifetime of this state were much longer than that of the 1507 keV state, these coincidences would not show up separately in the measurements, and an aUowance is made for them in assigning errors.

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1. T i m e d i s t r i b u t i o n of coincidences b e t w e e n t h e 534 k e V g a m m a r a y leading to t h e 1507 k e V state of Gd 15e, and a n y one of several m 1 MeV g a m m a r a y s leading f r o m it. A p r o m p t

Fig.

"coincidence curve m a d e w i t h Co 6° at t h e s a m e c o u n t e r biases is included for c o m p a r i s o n .

The result is given in fig. 1 in the form of a time spectrum of the delay of the counter 2 pulses relative to the counter 1 pulses. A prompt curve made with Co6° using the same counter biases is included for reference. We use this prompt curve only to demonstrate that the long decay observed for the

HALF-LIVES OF TWO EXCITED STATES OF Gd 158

415

Tb 15s source is real and not instrumental; exact details of the shape and position of the prompt curve are therefore unimportant. The delayed curve shows a clean straight line decay through over 3 decades ( > 10 half-lives) with a half-life of 1.88 × l0 -1° sec. The statistical error in this value is negligible, but we allow an error of 5 % to cover the time-to-amplitude calibration (3 %) and the possible presence in the curve of a few coincidences of different lifetime (3 %). The result m a y therefore be quoted as (1.884-0.10)× 10-1° sec.

A discussion of the lifetime of the 1507 keV state and of the radiations proceeding from it is given in the paper of Gregers Hansen et aI. 1) immediately preceding this one. The measured lifetime agrees with expectation for the disintegration scheme proposed there. Ofer 4), working at Brookhaven, has recently attempted to estimate the halflife of the 288 keV second excited state of Gd 156 by observing coincidences between conversion electrons of the 199 keV gamma ray and gamma rays of energy 534 keV and higher. The necessary assumption of this method that all the higher excited states have negligibly short half-lives - - is now seen to be incorrect. Ofer's result was 1.5 × 10-1° sec. This result describes some mixture of the half-life of the 1507 keV state and other, presumably shorter, half-lives. We conclude that the half-life of the 288 keV state is probably shorter than 2 × 10-l° sec, but still unknown. 2.2. H A L F - L I F E

OF

THE

89 k e V S T A T E

The arrangements for this measurement were very similar to those used previously for the same measurement by Nathan *), and the main justification for the repetition is the improved time resolution now available. A weak source ( ~ 0.25/~C) of Tb 15e was deposited on a plate of plastic phosphor, 1.5 mm thick, and the deposit was covered with a second plate. This 4z electron counter showed a large peak in the region of 75 keV, with a width of 30 keV, due to the L, M . . . . conversion electrons of the 89 keV gamma ray. A broad pulse height window was set on this peak, but this counter also necessarily detected some of the background under the peak, caused by higher energy gamma rays. A gamma-ray counter (one of those mentioned previously) was brought close to the electron counter, and biased to accept gamma rays above 335 keV. Under these conditions, most of the coincidences are due to the 89 keV gamma ray, but the background pulses already mentioned will give rise to some prompt coincidences and some others showing the half-life of 1.88 x 10-l° seconds of the 1507 keV state. On the long time scale used for measuring the 89 keV lifetime, these will appear as a "prompt b u m p " at the beginning of the resolution curve. The results are shown in fig. 2. The prompt bump appears as expected, followed by the long decay due to the 89 keV lifetime. This decay was not

416

R . E . BELL AND M. H. JORGENSEN

followed as far out as it could have been, because it was not at the moment convenient to lengthen the time scale of the time-to-amplitude converter sufficiently. (At the right-hand end of the curve of fig. 2 the coincidence rate is still more than two decades above the chance coincidence rate.) The prompt resolution curve of fig. 1 has been repeated in fig. 2 in order to give an impression of the instrumental resolution, but neither its shape nor its position is exactly right for this measurement. The half-life read from fig. 2 is (2.194-0.07) × 10-9 sec, the assigned error of ~ 3 % being mainly an allowance for calibration errors. The same prompt bump effect would be present in Nathan's work, and in fact his published curve indicates its presence. He ignored it in evaluating the slope of his curve, however, and thus produced a result for the half-life about 13 % smaller than ours.

GdlS6:DECAYOF FIRST EXCITED STATE

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Fig. 2. Time d i s t r i b u t i o n of coincidences b e t w e e n t h e L, M . . . conversion electrons of the 89 keV g a m m a r a y of Gd ls6 a n d g a m m a r a y s above 335 keV. T h e p r o m p t curve is repeated f r o m fig. 1, and is intended only to give a n impression of the t i m e resolution w i t h w h i c h this m e a s u r e m e n t w a s made.

The radiative mean life of the 89 keV state is ~:r = T½(l+~)/ln 2, where is the total conversion coefficient. To evaluate ~, we use the average of the K and L conversion coefficients from the tables of Rose s) and of Sliv and Band e). (The worst disagreement between the two tables is 5 % for the Lil shell.) In addition, we know experimentally x) that the M, N . . . conversion for this transition is 0.334-0.03 of the L conversion. We finally get

HALF-LIVES OF TWO EXCITED STATES OF Gd1~$

417

c¢ ~ 3.93, with an estimated standard error of 3 %. This gives ~7 = (1.564-0.08) × 10-8 sec. Following the procedure of Nathan ~), this value of z~ leads to a reduced transition probability for the reverse (Coulomb excitation) process B(E2; I o -~ If) : e~ -~ (4.644-0.20) ×

10 ~ 8

cm 4.

The value of B(E2) measured directly from Coulomb excitation by Ram~ak et al. ~) is (4.50-/-0.25) × 10-as cm 4, in excellent agreement with the present result. One of us (R. E. B.) is grateful for the hospitality of the Institute for Theoretical Physics, and we thank Professor Niels Bohr for the excellent working conditions in the Institute. Thanks are also due mag. scient. B. Elbek for urging that the 89 keV half-life be re-measured. References 1) P. Gregers Hansen, N. O. Lassen, O. B. Nielsen and R. K. Sheline, Nuclear Physics 12 (1959) 389 2) O. Nathan, Nuclear Physics 5 (1958) 401 3) R. E. Green and R. E. Bell, Nuclear Instruments 3 (1958) 127 4) S. Ofer, Phys. Rev. (1959) to be published 5) M. E. Rose, Internal Conversion Coefficients (North-Holland Publishing Company, Amsterdam, 1958) 6) L. A. Sliv and I. M. Band, Internal Conversion Coefficients, tables circulated by the University of Illinois, Reports 57 ICC K1 (1957) and 58 ICC L1 (1958) 7) V. Ram~ak, M. C. Olesen and ]3. Elbek, Nuclear Physics 6 (1958) 451