Decay Scheme ofFm255

Abstract

The alpha decay scheme of ${\mathrm{Fm}}^{255}$ has been investigated by alpha particle, gamma ray, and electron spectroscopy. Nine alpha groups were observed with alpha-particle energies and abundances of: 7.122, (0.09±0.01)%; 7.098, (0.10±0.01)%; 7.076, (0.43±0.05)%; 7.019, (93)%; 6.977, (0.11±0.02)%; 6.960, (5.3±0.1)%; 6.887, (0.60±0.03)%; 6.803, (0.12±0.02)%; and 6.58 MeV, (4.5±0.9)×${10}^{-2\mathrm{}}$%. The ground state of ${\mathrm{Cf}}^{251}$ was assigned a spin of ½ and is probably the $\frac{1}{2}+\left[620\mathrm{}\uparrow \right]$ Nilsson state. Five members of the rotational band based on the ground state were observed. The value of the decoupling parameter $a$, is 0.24±0.03, and $\frac{{\hslash }^2}{2\mathfrak{F}}$ has a value of 6.4±0.3. The ground state of ${\mathrm{Fm}}^{255}$ decays by an unhindered alpha transition to a 106-keV state in ${\mathrm{Cf}}^{251}$. Both of these states are assigned a spin of $\frac{7}{2}$ and are presumably the $\frac{7}{2}+\left[613\mathrm{}\uparrow \right]$ Nilsson state. Four members of the rotational band based on the 106-keV state in ${\mathrm{Cf}}^{251}$ were observed, and their energies correspond to a value of $\frac{{\hslash }^2}{2\mathfrak{F}}$ of 6.69±0.03. The 106-keV state with the $\frac{7}{2}+\left[613\mathrm{}\uparrow \right]$ assignment has a half-life of (3.7±0.2)×${10}^{-8\mathrm{}}$ sec. The gamma rays which de-excite this state to the ground-state band are thought to originate through a multiple Coriolis interaction between two states 0.56 keV apart. Two other intrinsic levels in ${\mathrm{Cf}}^{251}$ were detected. One at 546 keV was given the assignment of $\frac{11}{2}-\left[725\mathrm{}\uparrow \right]$ and the other, tentatively at 425 keV, was given the assignment $\frac{9}{2}+\left[615\mathrm{}\downarrow \right]$. The appropriate alpha transition probabilities for the decay to the various states agree well with the expectations for unhindered and hindered alpha decay.