The Mössbauer effect in 239Pu

Volume 41B, n u m b e r 1

PHYSICS LETTERS

4 September 1972

T H E M O S S B A U E R E F F E C T IN 239pu J. GAL and Z. HADARI Nuclear Research Center, Negev, Israel

and E.R. BAUMINGER and S. OFER The Racah Institute o f Physics, The Hebrew University, Jerusalem, Israel Received 19 June 1972 The recoilless resonance absorption o f the 57.3 keV "r-ray of 239pll has been observed. Thehalf-life o f the 57.3 keV level was found to be 101 -+ 5 ps.

We report here the first successful M6ssbauer effect measurement of a plutonium 7-ray transition. The M6ssbauer effect has been observed in a number of actinide elements. Efforts to observe the effect in a Pu "r-ray transition have not been successful until now. In the present work, the M6ssbauer effect has been observed with the 57.3 keV "r-ray of 239pu. The source was 2.33 d 239Np which was produced by neutron irradiation of 400 mg of uranium metal. Neutron capture in 238U produces 239U, which decays by ~3emission to 239Np, with a half-life of 23.5 rain. The Np activity was separated from the U bulk by extraction from an aqueous nitric acid reducing solution, into tri-iso-octylamine dissolved in xylene [ 1]. The Np was then stripped from the organic phase with dilute HC1; 400 mg of Th(NO3) 4 and 200 mg of oxalic acid were added and the precipitated Np and Th oxalates were fired at 750°C to give a highly diluted solid solution of NpO 2 in a matrix of ThO 2. The area of the NpO2(ThO2) source used in the Mt~ssbauer experiments, which contained 80 mg of Th, was 1 cm 2. The absorber used was PuO2 with a thickness of 100 mg/cm 2. The measurements were carried out at 4.2°K. The 57.3 keV 7-ray emitted from the 239Np source is relatively weak and good energy resolution is needed in order to detect it, resolved from the 61.5 keV 239pu 7-ray and the 59.4 keV ")'-ray emitted by the 241Am contaminant, present in the Pu absorber. For this purpose a 500 eV resolution Ge-Li detector (I cm 2 × 2 mm) was used. The above resolution could be maintained only with a maximum counting rate of 150 s-1 in the energy

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channel used. The spectrum of the "),-rays emitted from the source is shown in fig. 1. The M6ssbauer spectrum obtained is shown in fig. 2. Thorium oxide and plutonium oxide have the cubic CaF 2 structure. Thorium oxide is diamagnetic and the ground crystal field level of Pu 4+ in PuO 2 is a non-magnetic singlet [2]. It is expected therefore that the M6ssbauer spectrum is not broadened by hyperfine interactions. The FWHM of the absorption line, determined by a least-squares computer fit to the experimental spectrum (fig. 2) assuming a Lorentzian line shape, is 5.13 -+ 0.10 cm/s. In order to deduce the lifetime of the 57.3 keV level, the broadening of the 53

Volume 41B, number 1

PHYSICS LETTERS

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Fig. 2. Recoilless absorption spectrum of the 59.3 keV q,-ray of 239pu in PuO2 at 4.2 K. line due to finite absorber thickness has to be estimated. The Debye temperature of PuO 2 is not known, but relying on available data concerning the Debye temperatures of various oxides, it can be estimated to be 230 + 50 K. The recoil free fraction " f " corresponding to such a Debye temperature is 0.4 -+ 0.1. The value of F/F~ - the ratio of the total width of

54

4 September 1972

the resonance level to the partial width for the 57.3 keV "T-ray transition - is estimated to be 300. This estimate is based on the theoretical value of the conversion coefficient of the 57.3 keV E2 transition and the experimental value of the branching ratio of the 57.3 keV and the 49.4 keV transitions. Taking into account these estimates, it is found that the effective absorber thickness (nfo o t) is 0.65 -+ 0.15. Such an effective thickness is expected to broaden the absorption line by (8.8 -+ 2.2)%. The value determined therefore for the natural line width (2Po) is 4.73 +- 0.25 cm/s, corresponding to a half-life of 101 -+ 5 ps for the 57.3 keV level. This value is consistent with the systematics of half-lives of rotational levels decaying by E2 transitions. We would like to thank J. Lapid for carrying out the chemical preparation o f the source and M. Asolai for technical assistance.

References [1] R.A. Schneider, Anal. Chem. 34 (1968) 522. G. Raphael and R. Lallement, Solid State Commun. (1968) 383.

[2]

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