Half-lives of excited states in 77As, 97Tc and 192Os

Nuclear Physics A209 (1973) 461 --469; (~) 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 O F E X C I T E D STATES I N 77As, 97Tc AND 192Os R. C. C H O P R A , P. N. T A N D O N , S. H. D E V A R E and H. G. D E V A R E

Tata Institute of Fundamental Research, Bombay-5, India

Received 24 April 1973 Abstract: The half-lives of the excited states in 77As (264 keV, ~-), 97Tc (216 keV, {-+ and 325 keV,

~+) and 192Os (206 keV, 2 +) have been measured to be 0.35-4-0.02, ~ 0.15, 0.374-0.02 and 0.30 ± 0.02 ns, respectively, using the delayed coincidence technique. The M 1 retardation and E2 enhancement factors have been deduced and are compared with systematics of the neighbouring nuclei. I E

RADIOACTIVITY 77Ge (from *6Ge(n, y), natural target), 97Ru (from 96Ru (n,~,), enriched target), J92Ir (from J9*lr(n,y), natural target); measured fl-7, y-?,, X-y delay. Levels in 77As, 97Tc, 192Os; deduced T.}, B(M1), B(E2).

1. Introduction

There has been considerable theoretical interest *-4) in the odd-A isotopes of As whose energy levels cannot be described by simple single particle or collective models. Recently Scholz and Malik 3) and Imanishi et al. 4) using Coriolis coupling, have obtained a reasonably good agreement with the experimental level spectra and transition probabilities. Simple phenomenological calculations 5) using core excitation model have explained most of the transition probabilities in 75As" The decay of 11.3 h 77Ge to levels in 77As has been studied by several workers and its level scheme is well established 6-8). In spite of the complexity of the level scheme, one can see very striking similarities in the low lying levels of 75As and 77As" The ground state spin of both these nuclei is ~:-. Recently 9) a 1 - isomeric level (T, = 7.4___0.3 ns) has been established to be the first excited state in 77As at 194 keV, identical to the first excited state at 199 keV (½-) in 75As. The states at 215 keV (~--) and 264 keV ( ~ - ) in 77As similarly correspond respectively to 265 keV (~:-) and 280 keV ( ~ - ) states in 75As" The lifetime of the 280 keV level is known lo) to be 0.417___0.014 ns and one would expect the 264 keV level to have a measurable half-life. An upper limit of 0.3 ns for the half-life of this level has been suggested by Drost and Weyer 9). We have measured the half-life of this level using fl-~ and 7-~' delayed coincidence techniques. The levels in 97Tc populated in the decay of 2.9 d 97Ru have been recently investigated in detail by Phelps and Sarantites 11). Here again one finds great similarity in the low energy region to the levels in 99Tc. Both these nuclei have a ground state spin ~+ 2 and very closely spaced levels having spins ~+ and ~+. There have been many suggestions about the nature of these states 12-14). The !z+ state has been suggested 461

462

R.C. CHOPRA

et al.

to be an "intruder state". The ~+ 2 state at 181 keV in 99Tc is known to have T~ = 3.61___0.07 ns [ref. 15)]. One would expect the 325 keV state in 97Tc, which appears to have a similar character, to have a measurable half-life. We report here the measurement of the half-life of this level. Recent results of Sioshansi et al. 16) on the measurement of nuclear g-factors of the first excited 2 + states in even Os isotopes show a variation of g-factors with the mass number A. While the results on lS6Os and lSSOs have been very well explained by the K u m a r and Baranger theory 17), the results of 19o, 192Os ' however, are inconsistent with all other theories l s - 2 o ) except Z/A. In the case of 1s6, lSSOs ' the half-lives of the 2 + states have been directly measured, but in the case of 19o, 192Os no direct lifetime measurements exist and the lifetimes have been derived from the measured B(E2) values using Coulomb excitation. Sioshansi et al. 16) have pointed out inconsistency to the extent of 15 ~o in the case of lSSOs where both the direct lifetime measurement and the B(E2) values are available. It is therefore desirable to have a direct lifetime measurement of the 2 + state in 192Os which can be measured using the delayed coincidence technique. We have done this measurement using X-y delayed coincidences in the decay of 192ir"

2. Experimental procedure The half-lives of the excited states were measured using the usual time-to-amplitude converter (TAC) technique. Fast plastic scintillators of various dimensions depending on the requirements of the experiments, were mounted on R C A 8575 photomultiplier tubes. The fast pulses for timing of the T A C were obtained from fast discriminators of the constant fraction timing type. Standard commercial modular electronics was used for slow coincidence and gating the TAC spectra which were recorded on a 512-channel analyzer. The half-life of the excited state was obtained by a least-squares fit of the delayed part of the TAC spectrum. The time Calibration was done using standard cables of accurately known length. In all the cases, the y-spectrum of the activity used was taken using a 20 cm 3 Ge(Li) detector to check for the purity of the sources used.

3. Results 3.1. HALF-LIFE OF THE 264 keV LEVEL IN 77As

The levels in 77As are populated in the decay of 77Ge (11.3 h). The activity was obtained by irradiating high purity finely powdered natural Ge metal with neutrons, for a period of 30 m in the C I R U S reactor at Trombay. The sources used had activities of 75Ge and 77As, besides 77Ge, but they did not affect the measurements. The halflife of the 264 keV level was measured using fl-y and y-y delayed coincidences. A simplified decay scheme is shown in fig. 1. For fl-y coincidences, a 2.5 cm dia x 2.5 cm thick plastic scintillator (KL 236) was used for detecting fl-particles and the y-rays

77As, 97Tc, 192Os H A L F - L I V E S

463

were detected on a 2.5 cm dia x 1 cm thick NE 111 plastic scintillator. The fl-energy gate was selected to be greater than 1.5 MeV to avoid contribution from higher levels. The y-gate was fixed using the 279 keV y-ray from 2°aHg and the gate was selected so as to minimise considerably the contribution from 215 keV y-ray. Several runs were given for fl-y delayed coincidences. Fig. 1 shows the time spectrum which consists of a prompt and a delayed spectrum. The prompt contribution comes essentially from the 632 keV level. The delayed contribution, as expected, is from the 264 keV level.

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CHANNEL NUMBER Fig. 1. Delayed coincidence s p e c t r u m taken between fl-rays o f energy 1.5 MeV and y-rays o f energy 264 keV. T h e full curve was obtained w h e n the y-gate was shifted to higher energy. T h e inset s h o w s the relevant part o f the decay s c h e m e o f 77Ge.

When the y-gate was put higher than 264 keV so that essentially one has contribution from the 632 keV level, we obtained a p r o m p t spectrum (fig. 1). G a m m a - g a m m a delayed coincidences were also done, using the same system. The fl-particles were absorbed and the y-gate was fixed to detect the 1290 keV y-ray. The time spectrum using the 77Ge source was very much identical to that obtained using fl-y coincidences. The half-life of the 264 keV level thus obtained is T, = 0.35___0.02 ns. The data also gave an upper limit of 85 ps for the half-life of the 632 keV level which is in agreement with the value T~ = 75___15 ps reported by Tucker and Meeker 21).

464

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et al.

CHOPRA

3.2. HALF-LIFE OF THE 216 AND 325 keV LEVELS IN

97Tc

The nucleus 97Ru decays by electron capture to levels in 97Tc. Enriched 96RH, obtained from O R N L , USA was irradiated with neutrons in the C I R U S reactor for 30 min. Besides 97Ru, the activity contained 103Ru and 1o ~Ru. The source was allowed to cool for 4 d to reduce the l°SRh (36 h) activity produced from the decay of o 5Ru" The 7-spectrum taken with the Ge(Li) detector revealed no other impurities. The ~°3Ru activity was very small and did not affect the measurements. The isotope 97Rtl decays essentially to levels at 216 keV ( ~ 90 ~o) and 325 keV ( ~ 10 ~ ) . The decay to higher levels is very small ( ~ 1 ~o). We have used X-7 delayed coincidences to measure the half-lives of the 216 and 325 keV levels. The fl-rays from I°3Ru and o 5Rh impurities were absorbed using 3 m m of perspex. The X-ray was detected by a 2.5 cm dia x0.3 cm thick, and the 7-rays by a 2.5 cm d i a x 1 cm thick N E 111 plastic scintillator. The K X-ray from the decay of 97Ru (~ 18 keV) was selected using ~5~Sm and 57Co radioactive sources which emit 21.6 and 14.4 keV ~-rays respectively. The y-gate was selected by taking the spectrum of 97Ru; the 216 keV y-ray, being very intense, is very easily seen. The time spectrum obtained using the X-216 keV 7-ray coincidence is shown in fig. 2a. The spectrum consists of a huge p r o m p t

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77As, 97Tc, t92Os HALF-LIVES

465

and a small delayed contribution. When the y-gate was shifted higher to accept only the 325 keV contribution, the time spectrum shown in fig. 2b was obtained. This establishes that the delayed spectrum originates from the 325 keV level. When the X-rays were completely absorbed using an aluminium absorber, the time spectrum showed only a prompt spectrum. The half-life of the 325 keV level obtained from a least-squares fit is T½ = 0.37+0.02 ns. From the analysis of fig. 2a, an upper limit of T÷ ~ 0.15 ns has been obtained for the 216 keV level. 3.3. HALF-LIFE OF THE 206 keV LEVEL IN 192Os The 206 keV level in 192Os is fed in the electron capture decay of 74.2 d 192ir" The activity was obtained by irradiating spectroscopically pure iridium oxide with neu1921r

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466

R . C . C H O P R A et al.

trons in the C I R U S reactor, for a period of 30 min. The X-~ delayed coincidences were used to measure the half-life of this state. To have a higher efficiency for X-ray detection, a 5 ~ Pb loaded plastic scintillator 2.5 cm dia x 2.5 cm thick was used. The ?-rays were detected on a 2.5 cm dia x 2.5 cm thick K L 236 plastic scintillator. The energy calibration of the X-ray detector was done by observing the full energy peaks of the 46.5 and 122 keV ~-rays from R a D - E and 57Co radioactive sources respectively. The ?-ray energy gate was fixed using the 216 keV y-ray from 97Ru. The time spectrum obtained using X-? coincidences is shown in fig. 3. The spectrum shows a very large prompt along with a definite delayed spectrum. The large prompt spectrum is expected as 192ir decays mostly by fl-decay to levels in 192pt which has intense ?-ray cascades. When the ?-energy gate was shifted to higher energy (280 keV), no delayed part of the spectrum was obtained. When the X-ray was absorbed using lead absorbers, the intensity of the delayed spectrum was reduced considerably. The half-life of the 206 keV level thus obtained is T~r = 0.30___0.02 ns.

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467

77As, 97Tc, 19208 HALF-LIVES

4. Discussion The low lying levels in 75, ~7As and 97, 99Tc are shown in fig. 4 for comparison. The similarity in the levels is very obvious from this figure. In table 1, the B(E2) and B ( M 1 ) values are calculated from the present measurements and are compared with those of corresponding transitions in the neighbouring nuclei. The low energy levels of the 75As and 77As nuclei have been reasonably well explained using the Coriolis coupling model. While Scholz and Malik 3) have used the statically deformed collective model with the inclusion of the Coriolis coupling and TABLE 1

Comparison of transition probabilities in 77As and 97Tc with neighbouring nuclei

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a residual interaction of the pairing type, Imanishi et al. 4) have considered the motion o f an unpaired quasiparticle moving in a Nilsson orbit to be coupled to the rotational motion by a Coriolis force. Both these calculations lead to good agreement with the experimental level spectrum; however, the agreement with the measured transition probabilities is not very good. Recently, the static and transition moments of the levels in 75As have been successfully explained using the core excitation model s). Since there is a marked similarity in the level structure of 7 SAs and 77As, the same model is expected to account for the levels in 77A5 also. In 75As, the low lying ½- and ~ -

468

R.C. CHOPRA et aL

levels arise due to the coupling of a p~ proton to the 2 + state of the core, while the ~ - level at 280 keV is identified as the single particle level. The MI transition from this level to the ground state being/-forbidden is retarded; however, the enhancement of the E2 component can be explained due to the admixture of the core coupled ~ state. The B(E2) and B(M1) values of the 264 keV transition in 77As are given in table 1 along with the equivalent 280 keV transition in 75As" The agreement for B(E2) and B(M 1) of the two transitions is remarkable and supports the validity of the core excitation picture in 77As" The ~+ ground state and ½- excited state in 97, 99Tc can be interpreted to be the g~ and p~ shell model states. The presence of low lying ½-, ~z- and ~+, ~+ states cannot be accounted for in this model. The ~- and ~- states have been described by Kisslinger and Sorensen 1) in terms of the p½ quasiparticle coupling to the 2 + phonon state of the core. This is supported by the work of Black et al. 22) in l°3Rh where also one obtains similar levels. One can then expect the low lying positive parity levels to be formed due to the coupling of the ~+ particle to the 2 + state of the core. Recent calculations of Goswami et al. 13, 14) on the extended quasiparticle phonon coupling model have accounted for these positive parity states. McDonald et aL 15) have measured the transition probabilities in 99Tc and their measurement suggest that this model does not correctly describe the observed behaviour of nuclei in this region. They have proposed an oblate deformation for this nucleus and have tried to explain the level structure of 99Tc using the Nilsson model and a mixing of these states due to rotational states formed on the intrinsic states. The proposed structure of 99Tc can be directly tested using Coulomb excitations as the high spin positive parity states, which one expects would be easily excited in Coulomb excitation of 99Tc. The Coulomb excitation of 99Tc using a 35C1 beam has been recently studied by Bond et aL [ref. 23)]. Their measurements of B(E2) values indicate that the 99Tc nucleus is not deformed. It is quite well established now that none of the existing theoretical calculations can account for the properties of low lying positive parity states in 99Tc" A similar situation is supposed to hold true in 97Tc which has similar levels. This is further supported by the half-life measurement of the 325 keV level in 97Tc. The B(E2) value agrees very well with the B(E2) of the corresponding transition at 181 keV in 99Tc (see table 1). The half-life of the 206 keV level in 192Os (T½ = 0.30___0.02 ns) agrees well with that obtained from the B(E2) using Coulomb excitation (T~r = 0.284___0.014 ns) [ref. 15)]. The measurement does not change any of the conclusions of Sioshansi et al. [ref. 16)]. The authors wish to thank Professor B. V. Thosar for his interest in this work. They also thank the Reactor Operation Division CIRUS, Trombay, for the irradiation of the samples.

WAs, 97Tc, 192Os HALF-LIVES

469

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25)

L. S. Kisslinger and R. A. Sorensen, Rev. Mod. Phys. 35 (1963) 853 L. S. Kisslinger and K. Kumar, Phys. Rev. Lett. 19 (1967) 1239 W. Scholz and F. B. Malik, Phys. Rev. 176 (1968) 1355 N. Imanishi, M. Sakisaka and F. Fukuzawa, Nucl. Phys. A125 (1969) 626 R. C. Chopra and P. N. Tandon, Nucl. and Solid State Phys. (India) 15B (1972) 427 D. P. Donnelly, J. J. Reidy and M. L. Wiedenbeck, Nucl. Phys. A l l 2 (1968) 145 A. Ng, R. E. Wood, J. M. Palms, P. Venugopal Rao and R. W. Fink, Phys. Rev. 176 (1968) 1329 N. Imanishi and T. Nishi, Nucl. Phys. A154 (1970) 321 I-I. Drost and G. Weyer, Nucl. Phys. A159 (1970) 540 U. Baverstam and M. Hojeberg, Nucl. Instr. 95 (1971) 611 M. E. Phelps and D. G. Sarantites, Nucl. Phys. A171 (1971) 44 L. S. Kisslinger, Nucl. Phys. 78 (1966) 341 A. I. Sherwood and A. Goswami, Nucl. Phys. 89 (1966) 465; Phys. Rev. 161 (1967) 1232 A. Goswami and O. Nalcioglu, Phys. Lett. 26B (1968) 353 J. MacDonald, A. Backlin and S. G. Malmskog, Nucl. Phys. A162 (1971) 365 P. Sioshansi, D. A. Garber, Z. W. Grabowski, R. P. Scharenberg, R. M. Steffen and R. M. Wheeler, Phys. Rev. C6 (1972) 2245 K. Kumar and M. Baranger, Nucl. Phys. A l l 0 (1968) 529, A122 (1968) 273 O. Prior, F. Boehm and S. G. Nilsson, Nucl. Phys. A l l 0 (1968) 257 W. Greiner, Nucl. Phys. 80 (1966) 417 R. J. Lombard, Nucl. Phys. A U 4 (1968) 449 A. B. Tucker and P. D. Meeker, Nucl. Phys. A145 (1970) 362 J. L. Black, W. J. Caelli and R. B. Watson, Nucl. Phys. A125 (1969) 545 P. D. Bond, E. C. May and S. Jha, Nucl. Phys. A179 (1972) 389 A. J. Becker and R. M. Steffen, Phys. Rev. 180 (1969) 1043 M. B. Martin and M. L. Wiedenbeck, Nucl. Phys. 48 (1963) 65