Nuclear Levels in a Number of Even-Even Rare Earths (150≤A≤184)

To obtain more data on the system of levels in even-even nuclei, a number of such nuclei ($150\le A\le 184$) were studied with electron-capturing sources in permanent magnet spectrogaphs. Some measurements were made with scintillation counters. Data confirming recently reported results on the decay of ${\mathrm{Tb}}^{152}$, ${\mathrm{Tb}}^{156}$, and ${\mathrm{Ho}}^{162}$ have been obtained. It was found that ${\mathrm{Eu}}^{150}$ has two isomeric states ($T_{\frac{1}{2}}=14\mathrm{}$ hr and > 5 yr). Levels at 740.7 (0+) and 773.3 (4+) kev in ${\mathrm{Sm}}^{150}$ are proposed. A study of the two isomeric activities of ${\mathrm{Tb}}^{154}$ indicated the existence of levels in ${\mathrm{Gd}}^{154}$ which may be described as a gamma-vibrational band (at 997.3 kev) and a beta-vibrational band (at 680.6 kev). The new data for ${\mathrm{Tm}}^{166}$ (7.7 hr) are consistent with levels in ${\mathrm{Er}}^{166}$ at 2137.3 and 2164.6 kev, both of which are probably 3 states and which exhibit considerably different branching ratios of the de-exciting transitions. The decay of ${\mathrm{Lu}}^{172}$ appears to populate a large number of even-parity levels in ${\mathrm{Yb}}^{172}$ which may be arranged in rotational bands corresponding to primary or base states at 1174.0 kev ($I=3+$), 1467.5 kev ($I=2+$), 1664.3 kev ($I=3+$), 1702.1 kev ($I=3+$), 2075.0 kev ($I=4+$), and 2287.3 kev ($I=4+$). The very complex decay of the two isomers of ${\mathrm{Re}}^{182}$ excite many odd-parity levels which may be arranged in seven or more bands. In addition, even-parity beta- and gamma-vibrational bands may be populated. Electron-capture decay of ${\mathrm{Re}}^{184}$ populates a gamma-vibrational band in ${\mathrm{W}}^{184}$ of spins 2, 3, and 4. Data relevant to the rotational energy parameters and ratios of gamma-ray transition probabilities from the various states are presented. As a corollary, data on the decay of ${\mathrm{Eu}}^{149}$ are presented since this activity was present in some of our composite sources.