Energy Levels ofPu239Populated by the Beta Decay ofNp239

The conversion electron spectrum arising from the beta decay of 2.3-day ${\mathrm{Np}}^{239}$ has been studied with three 180° permanent magnet photographic spectrographs. The following transitions, with multipolarities where known, were observed (energy in kev): 44.64 ($M1+E2\mathrm{}$), 49.40 ($M1+E2\mathrm{}$), 57.25 ($E2\mathrm{}$), 61.4 ($E1\mathrm{}$), 67.82 ($E2\mathrm{}$), 106.1 ($E1\mathrm{}$), 106.4, 181.8, 209.9 ($M1\mathrm{}$), 226.4, 228.4 ($M1\mathrm{}$), 254.6, 273.1, 277.7 ($M1\mathrm{}$), 285.6, 316.1, and 334.5. Gamma-gamma coincidence experiments have also been performed. It is necessary in order to account for the present data to modify the previously accepted level scheme for ${\mathrm{Pu}}^{239}$; a revised scheme is presented, with spin and relative parity assignments. The energy and intensity data are discussed in terms of the Bohr-Mottelson unified nuclear model.