25-Minute Isomer ofSe83

Abstract

${\mathrm{Se}}^{83}$ was produced by irradiation of a selenium sample, enriched to 75% in The ${\mathrm{Se}}^{82}$ isotope, in the Pennsylvania State University research reactor. The gamma-ray spectrum of the 25-minute isomer was investigated by means of a 3-in. thick×3-in. diameter NaI(Tl) crystal scintillation spectrometer. Gamma rays of energy 2.294±0.030 Mev, 1.880±0.015 Mev, 1.309±0.005 Mev, 1.058±0.005 Mev, 0.833±0.005 Mev, 0.712±0.010 Mev, 0.524±0.012 Mev, 0.358±0.005 Mev, and 0.225±0.005 Mev were found. The beta-ray spectrum was investigated by means of a plastic scintillator. In order to eliminate a competing activity from ${\mathrm{Se}}^{81}$ it was found essential to restrict the observation of beta rays to those in coincidence with the ${\mathrm{Se}}^{83}$ gamma-ray spectrum. A coincidence circuit of 0.25-microsecond resolving time was developed for this purpose. The beta-ray spectrum was found to be complex with end-point energies of approximately 0.45 Mev, 1.0 Mev, and 1.7 Mev. From these data, together with gamma-gamma coincidence measurements, a decay scheme is proposed.