Observation of the second proton alignment in ${}^{160}\mathrm{Tm}$

High-spin states in the doubly odd nucleus ${}^{160}\mathrm{Tm}$ have been investigated using the ${}^{130}\mathrm{Te}$(${}^{35}\mathrm{Cl}$,5n) reaction and the Euroball γ-ray detector array. The previously established rotational structures in ${}^{160}\mathrm{Tm}$ were extended, leading to the first observation of the second $(h_{11/2}){}^2$ proton band crossing in this nucleus. This crossing is found to occur at a higher rotational frequency in ${}^{160}\mathrm{Tm}$ than in lighter Tm isotopes. This trend resembles that of the first $(h_{11/2}){}^2$ proton band crossing found in neighboring Er nuclei and is associated with a predicted increase in deformation as a function of neutron number in these isotopes.

Figure 1

Level scheme of ${}^{160}\mathrm{Tm}$ as deduced from the present work. Spin and parity assignments are based on those of previous works [6, 7, 8], and new γ-ray transitions placed on top of the bands are assumed to have stretched electric quadrupole (E2) character. The excitation energies of the bands relative to the ground state and to each other are unknown. The 1178-keV transition and the ordering of the 1048- and 986-keV transitions in band 1 are tentative.

Figure 2

(a) A background subtracted spectrum of γ rays detected in coincidence with a pair of γ rays with energies 982 and 1042 keV, corresponding to transitions placed in band 1 of ${}^{160}\mathrm{Tm}$ (see Fig. 1). (b) A spectrum created by requiring simultaneous coincidences with γ rays corresponding to the 982-keV transition in band 1 and any transition placed below it in the same band. (c) A similar spectrum of γ rays in coincidence with the 993-keV transition and any transition placed below it in band 4. In panels (a) and (b) the energies of coincident γ rays in band 1 are indicated, and filled circles show the location of γ rays placed in band 2. For panel (c) the energy labels correspond to transitions within band 4 and filled circles indicate transitions in band 3.

Figure 3

Aligned angular momenta ($i_x$) as a function of rotational frequency ($\hslash \omega$) for the bands in ${}^{160}\mathrm{Tm}$. The numbering of the bands is that of Fig. 1, and the parity, signature, and configuration at low excitation energies are given for each band. The spin of a reference rotor based on the Harris parameters [9] $J_0=30\hslash {}^2$/MeV, $J_1=32\hslash {}^4$/MeV${}^3$ has been subtracted. The arrows indicate the AB and BC neutron alignments and the $B_p{C}_p$ proton alignment. The inset shows the frequency at which the $B_p{C}_p$ proton crossing occurs for ${}^{158}\mathrm{Tm}$ [12], ${}^{159}\mathrm{Tm}$ [13], and ${}^{160}\mathrm{Tm}$, as well as the $A_p{B}_p$ proton-crossing frequency for the yrast bands in their isotones ${}^{157-159}\mathrm{Er}$ [10].