A short-lived isomer 194mIr

I 1.E.I: I

Nuclear Physics AI06 (1968) 417---420; (~) North-Holland Publishing Co., Amsterdam

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I

A SHORT-LIVED ISOMER 194mlr A. LUNDA.N and A. SIIVOLA Reactor Laboratory, Otaniemi, Finland Received 21 September 1967

Abstract: An isomeric state ~"mlr with a half-life of 30~2 msec has been studied with a rapid pneumatic transport and a Ge(Li) spectrometer. Both natural iridium and enriched isotopes were irradiated with reactor neutrons and their gamma spectra measured quickly after irradiation. The K X-rays and two gamma rays with energies 84 ! 2 and 112+2 keV were observed in the decay of 194mlr.

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RADIOACTIVITY ~94mIr[from t'Sir(n,7)]; measured E.t,y7 coin, T$. 1941r deduced levels. Enriched targets, Ge(Li) detector.

1. Introduction A short-lived isomeric activity in iridium was first reported by Alexander and Brinkmann t). They used a chopped neutron beam with a frequency of approximately 200 pulses/min for irradiation of natural iridium targets, whose activity they measured with a NaI scintillation counter. They found an activity with a half-life of 3 2 + 2 msec associated with gamma rays of energies 6 7 + 5 keV (probably the iridium K X-ray) and 115__+10 keV, the latter being much weaker. Another study of this activity was carried out by Fettweis and Campbell 2) with the aid of a rapid pneumatic transport system. They observed a 90 keV gamma ray with a half-life of 50 msec and assigned this activity to t 9'~'lr by irradiating enriched 191lr and 19air. We have attempted to study this activity with a Ge(Li) spectrometer and a rapid pneumatic transport system.

2. Experimental procedure Pieces of metallic Ir wire (99.99 % pure) weighing from 1 to 50 mg and 2 mg samples of enriched 191Ir (94.7 %) and tO3Ir (97.3 %) were inserted into nylon balls that are used as rabbits in our pneumatic transport 3). The system operates by pressurized air, and samples can be shot back and forth between reactor core and measuring station in quick succession. Minimum transport time from the core is approximately 50 msec. An important feature of the system is that it can be synchronized with the power pulse of the reactor (a Triga Mk II), which produces short-lived sources with very high specific activity. Gamma spectra were measured by using a 3 cm 3 Ge(Li) detector and a TMC 4096 417

418

A. LUND,~N AND A. SIIVOLA

channel two-parameter analyser. The latter was modified in such a way that several spectra can be taken as a function of time. The analyser was started by the arriving sample through a simple counting-rate meter. Some half-life measurements were performed by using another analyser in a multi-scaler mode. Coincidence spectra were taken by using a 7.6 cm x 7.6 cm diam. NaI(Tl) detector and the germanium detector as well as by using two identical Nal detectors. 3. Results A singles spectrum is shown in fig. 1 as a function of time. It is a sum of a total of 200 one sec irradiations of three separate samples of natural iridium. Two shortlived gamma rays 84.3+2.0 and 112.4+__2.0 keV are present with a large amount of GAMMA 60

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Fig. I. A series o f gamma spectra showing the short-lived 112 and 84 keV transitions. The measurement began 80 msec after the end o f the irradiation, and each spectrum was measured for 17.5 msec followed immediately by the next one. In order to increase clarity, the curves have been displaced by h a l f a decade f r o m each other.

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419

Ir K X-rays. The relative intensities of the short-lived K X-rays and 84 and 112 keV g a m m a rays are (6.7+0.2) : (0.58+0.06) : l, respectively. The half-life of this activity was measured with a multi-scaler from a sample produced with the reactor power pulse. The result 30 + 2 msec agrees with that of Alexander and Brinkmann J). No other gamma rays were found between 25 keV and 1 MeV that have the same half-life. This activity is produced by reactor neutrons from 193Ir only. This was confirmed by measurements with enriched isotopes. Activation of 191|r produced 19,mlr whose g a m m a energy and half-life were measured as 129.6_+0.5 keV and 4.96+_0.2 sec, respectively. The 43.5 keV 4) isomeric transition was not seen. In agreement with Fettweis and Campbell 2), we assign these g a m m a rays to the decay of an isomeric state in 1941r. It is produced from 193Ir by neutron irradiation. Low-lying states in 1931r are known well 4), and it seems unreasonable to postulate new levels to accommodate the new g a m m a transitions. Therefore the (n, n') reaction is probably excluded. The possibility that the short-lived activity belongs to the nucleide 193Os that could be produced by fast neutrons through an (n, p) reaction is ruled out by the absence of short-lived gamma rays in Os irradiations, and by the fact that the observed X-rays belong to iridium. (5 +)

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In order to determine whether the two g a m m a rays are in cascade, coincidence experiments were carried out. In the first one, a NaI detector sought gamma rays between 30 and 150 keV, and coincident events in the Ge(Li) detector were recorded. The resolving time was 50 nsec, and the spectrum was measured as a function of time. A strong K X-ray peak was observed with the proper half-life ( 3 5 + 5 msec), but no other peaks could be seen above the background from random events. Another measurement was performe~,wJth two NaI detectors in a conventional energy versus energy two-parameter mode. An iridium sample was irradiated for I sec and measured

420

A. LUNDAN AND A. SIIVOLA

for 1 sec repeating the cycle I00 times. A K X-ray plus K X-ray peak was observed, but weak maxima were also present at positions corresponding to an 8 4 + 112 keV coincidence. This was slightly more intense than a coincidence between the K X-rays and 112 keV. These results can be explained if the two g a m m a transitions are in coincidence, and both have fairly large K conversion coefficients. If either one of the observed g a m m a rays belongs to the isomeric transition, it should be an E3. The Weisskopfestimates [taking interval conversion into account 5)] are 30 msec for a 112 keV E3 and 40 msec for an 84 keV E3 transition. On the other hand, the probable existence of 84+ 112 keV coincidences indicates that the other transition is fairly fast (El, E2 or MI). Assuming that we have a cascade of 112 and 84 keV transitions, it is possible to fit the relative intensities of K X-rays and g a m m a rays by using tables of internal conversion coefficients. This can give more information about the multipolarities. From the experimental intensities, we get a relationship 0.94 (ctzt 12 +0.58 (XK84) = 6.7, where 0.94 is the K fluorescent yield in iridium. By using the tables of Sliv and Band 5), we found that the best agreement with this formula is achieved by the combination l l2 keV E3 (~tt = 1.2)+ 84 keV M l (ctK = 7.6) which yields a value of 5.6. The next to the best value is 3.9 given by the combination I 12 M l (ctK = 3.5) + 84 E3 (ctK = I. 1). All other multipolarities give values which differ from the experimental one by a factor of three or more. Although the value 5.6 is in better agreement with the experimental intensities, the result that the 112+84 keV coincidence is more intense than the l l 2 + K X-ray coincidence seems to favour the other possibility. We adopt tentatively the latter order and place the 84 keV transition above the 112 keV transition. The energy levels of 19*It have been studied by Williams and Naumann 6) and by Johnson and Bishop 7) through beta decay of 5.8 y 194Os. Both report two levels with energies 42.8 (43.0) keY and 82.3(78) keV and assign spins ( 0 - , l - ) and ( l - ) (fig. 2.). The ground state configuration of t94Ir most probably consists of a d~ proton and a Pi neutron coupled to 1-. Our proposal for the decay of the isomeric state is also shown in fig. 2. It is not possible to make any firm conclusions about the structure of the isomer. One possibility is that it is due to coupling of the d~ proton with a neutron in an i v state, which can be seen at low energies in odd-mass platinum isotopes,

References

I) 2) 3) 4) 5)

K. F. Alexander and H. F. Brinkmann, Z. Naturf. 16a (1961) 210 P. F. Fettweis and E. C. Campbell, Nuclear Physics 33 (1962) 272 K. E. Anttila, diploma thesis, Reactor Laboratory, Otaniemi, Finland, unpublished Nuclear Data Sheets S-13-17 (May 1963), 61-3-97 L. A. Sliv and I. M. Band, in Alpha-, beta- and gamma-ray spectroscopy, ed. by K. Siegbahn (North-Holland Publishing Co., Amsterdam, 1965) p. 1639 6) D. C. Williams and R. A. Naumann, Phys. Rev. 135B (1964)249 7) N. R. Johnson and W. N. Bishop, Nuclear Physics 69 (1965)q31