The conversion electron spectrum of ${\mathrm{Np}}^{239}$ has been studied at resolution settings of 0.1% and 0.05% with a 100-cm radius air-cored $\pi \sqrt{2}$ $\beta$-ray spectrometer, using a proportional counter as detector. One hundred and forty-three lines were identified corresponding to Auger transitions or the internal conversion of $\gamma$ rays of energies 44.65, 49.41, 57.26, 61.46, 67.86, 88.06, 106.14, 106.47, 181.7, 209.8, 226.4, 228.2, 254.4, 272.9, 277.6, 285.5, 315.9, and 334.3 kev. The results require the addition of a level at 163.75 kev to the basic level scheme for ${\mathrm{Pu}}^{239}$ proposed by Hollander et al. in order to account for three of the weak gamma transitions observed in this work. The relative conversion line intensities are used to determine the transition multipolarities, mixing ratios and expected quantum intensities assuming the $K$ and $L$ shell theoretical conversion coefficients of Sliv. The precisely determined level spacings and the relative transition probabilities are compared in some detail with those expected from an interpretation of the ${\mathrm{Pu}}^{239}$ level scheme in terms of the "Unified Model."

New and more accurate $K$, $L$, $M$, and $N$ electron subshell binding energies for Pu have also been deduced from the experimental data of this work. These values are 20 to 100 ev higher than the values tabulated by Hill, Church, and Mihelich, which were estimated by extrapolation from data at lower $Z$.

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