Alignments, additivity, and signature inversion in odd-odd ${}^{170}\mathrm{Ta}$: A comprehensive high-spin study

High-spin states ($I\lesssim 50\hslash$) of the odd-odd nucleus ${}^{170}\mathrm{Ta}$ have been investigated with the ${}^{124}\mathrm{Sn}$(${}^{51}\mathrm{V},5n$) reaction. The resolving power of Gammasphere has allowed for the observation of eleven rotational bands (eight of which are new) and over 430 transitions ($~$$350$ of which are new) in this nucleus. Many interband transitions have been observed such that the relative spins and excitation energies of the $11$ bands have been established. This is an unusual circumstance in an odd-odd study. Configurations have been assigned to most of these bands based upon features such as alignment properties, band crossings, $B(M1)/B(E2)$ ratios, and the additivity of Routhians. A systematic study of the frequency at which normal signature ordering occurs in the $\pi h_{9/2}\nu i_{13/2}$ band has been performed and it is found that its trend is opposite to that observed in the $\pi h_{11/2}\nu i_{13/2}$ bands. A possible interpretation of these trends is discussed based on a proton-neutron interaction.

Figure 1

Partial level scheme of ${}^{170}\mathrm{Ta}$ displaying negative-parity sequences. Tentatively placed $\gamma$ rays are denoted with a dashed line.

Figure 2

Partial level scheme of ${}^{170}\mathrm{Ta}$ displaying the positive-parity bands 6, 7, 8, 8a, and 9. Tentatively placed $\gamma$ rays are denoted with a dashed line.

Figure 3

Partial level scheme of ${}^{170}\mathrm{Ta}$ displaying the positive-parity bands 10, 10a, 10b, 11, 11a, and 11b. Tentatively placed $\gamma$ rays are denoted with a dashed line.

Figure 4

Coincidence spectra for (a) both signatures of band 1, (b) the $\alpha =0$ signature of band 2, and (c) the $\alpha =1$ signature of band 2. All spectra were produced by summing as many clean triple coincidence gates of the inband transitions as possible from a hypercube. The insets in each panel display the highest possible energy regions.

Figure 5

Excitation energy minus a rigid-rotor reference (where $A=0.0071$ MeV) for (a) all negative-parity bands in ${}^{170}\mathrm{Ta}$, (b) bands 6, 7, 8, 8a, and 9, and (c) bands 10, 10a, 10b, 11, 11a, and 11b. Filled (empty) symbols denote the $\alpha =0$ (1) signatures.

Figure 6

Representative spectra of bands (a) 3, (b) 4, and (c) 5 obtained as a result of summing all triple-gated coincidence spectra using inband transitions. In panels (a) and (b), the open left triangles denote peaks from band 2. The plus signs in panel (b) represent peaks assigned to band 3, and the open squares and circles in panel (c) are transitions in bands 1 and 6, respectively.

Figure 7

Coincidence spectra for bands (a) 6 and (b) 7. These spectra were generated in the same format as that described in Figs. 4 and 6. Insets (1) and (2) in panel (a) display the highest spin portions for the $\alpha =0$ and 1 signatures of band 6, respectively.

Figure 8

Representative spectra obtained from triple coincidence gate analysis of (a) the $\alpha =0$ signature of band 10, (b) the $\alpha =1$ signature of band 10, (c) the $\alpha =0$ signature of band 11, and (d) the $\alpha =1$ signature of band 11.

Figure 9

Alignments for the bands in ${}^{170}\mathrm{Ta}$; Harris parameters [24] of ${\mathcal{J}}_0=28\hspace{0.5em}\hslash {}^2$/MeV and ${\mathcal{J}}_1=58$ $\hslash {}^4$/MeV${}^3$ were used. The ${}^{168}\mathrm{Hf}$ yrast band is shown to illustrate the well-known $\mathit{AB}$ neutron crossing ($\hslash \omega ~$ 0.27 MeV) and the $E_p{F}_p$ proton crossing ($\hslash \omega ~$ 0.55 MeV). Filled (empty) symbols denote the $\alpha =0$ (1) signature.

Figure 10

Cranked shell-model calculations for quasiprotons in ${}^{170}\mathrm{Ta}$. Deformation parameters ${\beta }_2=0.24$, ${\beta }_4=0$, and $\gamma =0^{°}$ were used [15, 16]. The pairing energy was determined from experimental masses using the five-point fit procedure described in Ref. [26]. The lowest crossing is denoted. Solid lines correspond to configurations having $(\pi ,\alpha )=(+,+1/2)$, dotted lines to those having $(+,-1/2)$, dashed-dotted lines to those having $(-,+1/2)$, and dashed lines to those having $(-,-1/2)$.

Figure 11

Cranked shell-model calculations for quasineutrons in ${}^{170}\mathrm{Ta}$. Deformation parameters $\beta$${}_2=0.24$, $\beta$${}_4=0$, and $\gamma =0^{°}$ were used [15, 16]. The pairing energy was determined from experimental masses using the five-point fit procedure described in Ref. [26]. The lowest four crossings are denoted. Lines are labeled in the same fashion as described in the previous figure.

Figure 12

Measured $B(M1)/B(E2)$ ratios for bands 1, 6, and 7 in ${}^{170}\mathrm{Ta}$. Filled (empty) symbols represent the experimental values for the $\alpha =0$ (1) sequence. Theoretical calculations for bands 1, 6, and 7 are represented as dashed lines with the following configurations: 1: $\pi h_{11/2}$$\nu i_{13/2}$; 2: $\pi d_{5/2}$$\nu i_{13/2}$; 3: $\pi d_{5/2}$$\nu i_{13/2}\mathit{BC}$; 4: $\pi g_{7/2}$$\nu$i${}_{13/2}$.

Figure 13

Measured $B(M1)/B(E2)$ ratios for bands 2 and 5 in ${}^{170}\mathrm{Ta}$. Filled (empty) symbols represent the experimental values for the $\alpha =0$ (1) sequence. (a) The theoretical calculation for band 2 is represented as a dashed line and is interpreted as the $\pi h_{9/2}$$\nu i_{13/2}$ configuration. (b) The theoretical calculation for band 5 is shown as a dashed line and is the $\pi (h_{11/2}$,$d_{5/2}$,$g_{7/2}$)$\nu i_{13/2}$ configuration.

Figure 14

Signature splitting function, $S(I)$ (as defined in text), vs spin for the (a) $\pi h_{11/2}\nu i_{13/2}$ and (b) $\pi h_{9/2}\nu i_{13/2}$ bands in ${}^{170}\mathrm{Ta}$. The $\alpha =0$ (1) signature is displayed by solid (empty) symbols.

Figure 15

Reversion frequency, as defined in the text, for the $\pi h_{9/2}\nu i_{13/2}$ (left panel) and the $\pi h_{11/2}\nu i_{13/2}$ bands (right panel) in the $A\approx 170$ region.