Delayed backbending in the $\pi h_{9∕2}$ band of ${}^{187}\mathrm{Ir}$

High-spin states of ${}^{187}\mathrm{Ir}$ have been populated in the ${}^{176}\mathrm{Yb}\left({}^{15}\mathrm{N},4n\right)$ reaction and measured with the YRAST Ball spectrometer. The $\pi h_{9∕2}$ rotational band has been extended beyond the first alignment crossing, which was found at rotational frequency $\hslash {\omega }_c\approx 0.39\text{MeV}$. Two different scenarios for describing this crossing are considered: the alignment of an $h_{9∕2}$ proton or $i_{13∕2}$ neutron pair and it is concluded that a proton band crossing is more likely. A systematic study of the rotational alignment crossings in the $\pi h_{9∕2}$ bands in the $N=104,106,108$ isotopes of ${}_{73}\mathrm{Ta},{}_{75}\mathrm{Re},{}_{77}\mathrm{Ir},$ and ${}_{79}\mathrm{Au}$ is presented.

Figure 1

A spectrum displaying the transitions belonging to the $\pi h_{9∕2}$ band in ${}^{187}\mathrm{Ir}$. It results from the summing of all double-gated coincidence spectra, obtained by gating on the 165-, 325-, 464-, 583-, 680-, 751-, and $782\text{−}\text{keV}$ transitions. Contaminated peaks labeled with + belong to ${}^{186}\mathrm{Ir}$ and those labeled with $\ast$ belong to ${}^{186}\mathrm{Os}$. The energies of the uncertain transitions are put in brackets.

Figure 2

A partial level scheme showing the $\pi h_{9∕2}$ band in ${}^{187}\mathrm{Ir}$ as deduced from the present study. The arrow widths are proportional to the intensities of the corresponding transitions. Tentative transitions are shown with dashed lines and uncertain spin and parity assignments are shown in parentheses.

Figure 3

Aligned angular momenta for $\pi h_{9∕2}$ bands in ${}^{181–187}\mathrm{Ir}$. The reference parameters and data used for the plot are ${}^{181}\mathrm{Ir}$ ${\mathfrak{I}}_0=22$, ${\mathfrak{I}}_1=90$ [9]; ${}^{183}\mathrm{Ir}$ ${\mathfrak{I}}_0=25$, ${\mathfrak{I}}_1=55$ [4]; ${}^{185}\mathrm{Ir}$ ${\mathfrak{I}}_0=27$, ${\mathfrak{I}}_1=55$ [5, 18]; ${}^{187}\mathrm{Ir}$ ${\mathfrak{I}}_0=22\text{MeV}{}^{-1}{\hslash }^2$, ${\mathfrak{I}}_1=55\text{MeV}{}^{-3}{\hslash }^4$.

Figure 4

Single-particle neutron levels calculated with a Woods-Saxon potential as a function of deformation parameter ${\beta }_2$ and ${\beta }_4=\gamma =0$.

Figure 5

Aligned angular momenta (left-hand side) and second moments of inertia (right-hand side) for $\pi h_{9∕2}$ bands in the Au, Ir, Re, and Ta isotones. Data are shown relative to a reference configuration parametrized by different sets of Harris parameters, which provide constant aligned angular momentum before and after a band crossing. Experimental data are taken from Ref. 25. The graphs on the top refer to the $N=104$ isotones, middle 106, and bottom 108.