By means of photographic magnetic spectrometers located close to the heavy water moderated reactor, the following radioactive nuclei have been examined: ${\mathrm{Se}}^{77m}$ (17.5 sec), ${\mathrm{Se}}^{79m}$ (3.5 min), ${\mathrm{Se}}^{81m}$ (58 min), ${\mathrm{Se}}^{81}$ (17 min), ${\mathrm{Se}}^{83}$ (69 sec), ${\mathrm{Se}}^{83}$ (26 min), ${\mathrm{Nd}}^{147}$ (11.9 days), ${\mathrm{Nd}}^{149}$ (1.8 hr), ${\mathrm{Nd}}^{151}$ (12 min), ${\mathrm{Pm}}^{149}$ (50 hr), ${\mathrm{Pm}}^{151}$ (27.5 hr), ${\mathrm{Sm}}^{145}$ (410 days), ${\mathrm{Sm}}^{151}$ (>20 yr), ${\mathrm{Sm}}^{153}$ (46.5 hr), ${\mathrm{Sm}}^{155}$ (23.5 min), ${\mathrm{Eu}}^{155}$ (1.7 yr), ${\mathrm{Mo}}^{101}$ (14 min), ${\mathrm{Tc}}^{101}$ (15 min), and ${\mathrm{Th}}^{233}$ (23.6 min). The 27.5-hour activity assigned to ${\mathrm{Pm}}^{151}$ was discovered in this work. A new vacuum gate for the camera of the spectrometer permits the study of activities with half-lives as short as ten seconds. Energy level schemes for the product nuclei are proposed on the basis of a spectrographic analysis of the gamma-rays and also of coincidence and absorption measurements. Changes in spin and parity between many of the energy levels are determined from measurements of the $\frac{K}{L}$ ratios of the corresponding gamma-rays.

• Received 4 February 1952

DOI:https://doi.org/10.1103/PhysRev.86.775