A coincidence study of the decay of 146Pm

1.E.I: 3.A

Nuclear Physics A122 (1968) 425--430; (~) North-Holland Publishing Co., Amsterdam Notlto be reproduced by photoprint or microfilmwithout written permission from the publisher

A C O I N C I D E N C E S T U D Y O F T H E D E C A Y O F l~6Pm H. W. TAYLOR and A. H. KUKOC t Physics Department, University of Toronto, Toronto, Canada Received 18 September 1968

Abstract: The gamma-ray spectrum accompanying the decay of 14ePm has been studied with a Ge (Li)-NaI(TI) coincidence spectrometer. Gamma rays with energies (intensities) of 146.2 ± 1.3 (0.33), 453.7±0.4 (100), 589.8~0.7 (0.55), 633.1 ±0.5 (3.6), 735.6~0.7 (36.4) and 747.0~0.8 (58.7) keV have been observed. The decay scheme of 146Pm was constructed with levels in 14eNd at 453.7±0.4, 1043.5±0.8 and 1189.5-4-0.9 keV and in 146Smat 747.0±0.8 and 1380.1 0.9 keV. E[

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RADIOACTIVITY :4sPm[from 146Nd(p, n)l; measured E~, Is, 7'y-coin; deduced log ft. 146Nd, 14eSm deduced levels. Ge(Li) detector.

1. Introduction Because o f the simplicity o f the decay o f 146pm, the literature on the isotope is not very extensive. The principal spectroscopic contributions are due to Buss, F u n k and Mihelich 1) and Pagden, Jakeways and Flack 2). The reader is referred to these papers for references to earlier works. M o r e recently, a paper by Bunney and Scadden 3) has appeared, in which a new low-energy g a m m a ray is reported for the first time. A new value of the half-life, which is some 35 ~o higher than the previous value, was deduced also. The experimental evidence collected to date indicates that 146pm decays by electron capture to 146Nd and by negaton emission to 1465m. In view of the new information supplied by Bunney and Scadden 3), we decided to try to place the new transition in the 146pm decay scheme by means of coincidence measurements and to re-determine the log f t values for the t - and capture transitions using the new value for the half-life. 2. Experimental procedure The sample o f 146pm was prepared by b o m b a r d m e n t of 146Nd with 10 MeV protons **. The active material was deposited onto a thin piece o f aluminium foil, which was subsequently formed into the shape of a right circular cylinder with length and diameter a b o u t 3 mm. The activity available during our measurements was a b o u t 5 × 10 - 3 /gC. The activation also p r o d u c e d a trace a m o u n t o f 144pm, but its effect was negligible at the time o f the present measurements. A period of 8 y elapsed between the time o f preparation and observation. t Permanent address: Institute of Nuclear Science B. Kidrich, Belgrade P.O. Box 522, Yugoslavia. tt We are deeply indebted to Dr. F. C. Flack, University of Exeter, for his generosity and cooperation in supplying the sample of 146Pm. 425

426

H.W.

TAYLOR AND A. H. KUKO(~

The Ge(Li) spectrometer used for energy measurements had an active volume of 10 cm 3 and a depletion depth of 9 ram. The electronics system consisted of an Ortec 118A preamplifier, a Canberra 1417 main amplifier and a Nuclear D a t a 4096-channel analyser. The resolution of the system at 1332 keV was about 3.0 keV.. For the coincidence measurements, a gain-stabilized 5 c m x 5 cm cylindrical NaI(T1) detector was combined with the above Ge(Li) detector and a conventional fast-slow coincidence system. The resolution of the N a I detector was about 8 % at 662 keV. The resolving time ~ of the coincidence system was 35 nsec.

3. Experimental results and discussion The features of a typical spectrum obtained with the Ge(Li) detector are shown in fig. 1. The data were observed with the detector surrounded by a 10 cm thick lead shield during a counting period of 100 h. There is no evidence of a 144Pm contaminant in the spectrum of fig. 1. Photopeaks due to g a m m a rays with energies 146.2, 453.7, 589.8, 633.1, 735.6 and 747.0 keV were clearly in evidence in the spectrum and are shown in fig. 1. The photopeaks at 583 and 609 keV are contributed by the naturally occurring thorium and radium activities in the laboratory background. We found no evidence for the 779 keV g a m m a ray reported by Buss et al. 1), since it would have been very difficult to observe with our background counting rate. In connection with this line, it is interesting to note that the summing of a 736 keV g a m m a ray with a K X-ray emitted during the electron capture process would produce a line at just about the energy 779 keV. The summing of a 453 keV g a m m a ray and a K X-ray would be impossible to observe when superposed on the Compton distributions associated with the strong 736 and 747 keV g a m m a rays. It is quite possible that the 779 keV line reported by Buss et al. 1) arises in this way. The results of our investigation are summarized in table 1 together with the data of refs. 1, 3) for ease of comparison. The intensities quoted are no better than + 15 % in accuracy. A perusal of the entries reveals that the sum 146.2+ 589.8 = 736.0_ 1.5 keV is in excellent agreement with the gamma-ray energy 735.6-t-0.7 keV, thus establishing a cascade-cross-over situation in the 1*6Pro decay scheme. To confirm the significance of the above energy agreement, a number of coincidence experiments were performed. In fig. 2, a typical coincidence spectrum is shown for which the gate of the N a I spectrometer was placed on the 453.7 keV photopeak. The appearance of 146.2, 589.8 and 735.6 keV photopeaks confirms that the 735.6 keV g a m m a ray "crosses over" the 146.2 + 589.8 keV cascade, and that all these radiations ultimately feed a level at 453.7 keV known to exist in 146Nd. (The 453.7 keV photopeak of fig. 2 occurs because of C o m p t o n pulses arising from detection of 735.6 keV g a m m a rays in the N a I detector.) Intensity estimates for the 146.2 and 589.8 keV g a m m a rays as revealed in fig. 2 indicate that the 146.2 keV transition occurs first in the cascade.

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I100 il20 1140 CHANNEL NUMBER Fig. 1. Spectral features of a typical 146Pm gamma-ray spectrum observed with a 10 cm ~ Ge(Li) detector. A l l energies are in keV. T o t a l c o u n t i n g t i m e was 100 h. TABLE 1 E x p e r i m e n t a l intensities Present w o r k en erg y (keV)

intensity

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100 0.9 3.4 <0.3 35.4 53.1 <0.61

Bunney et al. 3) energy (keV) 146 453 589 634 736 747 1189

428

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Fig. 3 shows the spectrum o f g a m m a rays in coincidence with pulses entering a gate centred on a 7 3 6 - 7 4 7 keV composite p h o t o p e a k in the N a I spectrum. The 453.7 k e V peak arises from the 735.6-453.7 keV cascade m e n t i o n e d above. The 633.1 keV line arises from a 633.1-747.0 k e V cascade in 1468m, thus confirming the results o f Buss et al. 1). 3OO 735.6 146.2

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DECAY OF t4Opm

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The half-life a) of 20204-18 d and our intensities were used to compute new values for l o g f t for the fl- and capture branchings of 146pm. The results are shown in table 2. Our t46pm decay data are summarized in the decay scheme of fig. 4. We have

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% 23 0.14 40 2.3 34

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not shown the 779 keV transition reported by Buss et al. ~) or the 1189 keV crossover transition reported by Bunney et al. a), although the latter probably represents a genuine E3 transition from the 3 - 1189 keV state to the ground state in ~46Nd. A comparison of the intensities of the 146.2 and 589.8 keV transitions indicates that the 1043.5 keV level in 146Nd is fed directly by electron capture; the branching

430

n.w.

TAYLOR AND A. H. KUKO(~

is estimated at 0.14 ~ with an uncertainty of about 20 ~ . In previous decay schemes, this was considered to be the only feed for this level since the position of the 146.2 keV transition in the level scheme was not known. The spins shown in the scheme were deduced by Buss et al. 1) using angular correlation measurements and intensity considerations. The 3 - assignments in both 146Nd and 146Sm are supported by nuclear systematics in this mass number region 1). However, the log f t values are rather difficult to understand. Both 3- ~ 3- and 4 - ~ 3 - capture and fl- transitions would be classified as allowed, which means that the observed l o g f t values are anomalously high. In fact, the results tend to be in the range of values for unique first-forbidden or/-forbidden beta transitions. Although the l o g f t values cannot be used to determine the spin of 146pm directly, the similarity between values for corresponding branchings to 146Nd and 146Sm suggests that J~ = 3- for 146pm rather than 4 - . Such arguments are purely qualitative, however, and can be misleading. Nevertheless, such qualitative considerations make it likely that the spin of the 1380 keV state in 146Sm is 3- since the log f t value for the branching to this state is essentially the same as that to the 1189 keV state in 146Nd. To support this contention, one needs only to note the similarity of the l o g f t values for the first excited states of the two daughter nucleides. Buss et al. 1) have suggested on the basis of angular correlation studies with 146pm and 146Eu that the level at about 1380 keV in 146Sm is actually a closely-spaced pair of levels with spins 3- and 4 + . The question naturally arises as to whether or not both of these states are populated by decay of 146pm. The equivalent states in 146Nd (at 1189 and 1043 keV, respectively), are populated in the ratio of 36.4/0.3 = 120/1. Allowing for energy effects, it is likely that the population of the corresponding states in 146Sm would follow a similar pattern. Since only 2.3 ~ of all the decays populate the level(s) at 1380 keV in 146Sm, it is doubtful if the two transitions of nearly the same energy (i.e. approx. 633 keV) proposed by Buss et al. 1) would be individually observable in the 146pm decay. Of course, this conclusion presupposes a very small energy separation between the lines. The value suggested by Buss et al. 1), namely 1.3 keV, is so small that the point seems to be well taken. The authors wish to express their thanks to the National Research Council of Canada for financial support of this work. References

1) D.J. Buss, E. G. Funk and J. W. Mihelich, Phys. Rev. 141 (1966) 1193 2) I. M. H. Pagden, R. Jakeways and F. C. Flack, Nucl. Phys. 48 (1963) 555 3) L. R. Bunney and E. M. Scadden, J. lnorg. Nucl. Chem. 29 (1967) 2497