$\gamma$-ray linear polarization measurements and ${\left(g_{9/2}\right)}^{-3}$ neutron alignment in ${}^{91}$Ru

Linear polarization measurements have been performed for $\gamma$ rays in ${}^{91}$Ru produced with the ${}^{58}$Ni(${}^{36}$Ar, $2p1n$$\gamma$)${}^{91}$Ru reaction at a beam energy of 111 MeV. The EXOGAM Ge clover array has been used to measure the $\gamma$-$\gamma$ coincidences, $\gamma$-ray linear polarization, and $\gamma$-ray angular distributions. The polarization sensitivity of the EXOGAM clover detectors acting as Compton polarimeters has been determined in the energy range 0.3–1.3 MeV. Several transitions have been observed for the first time. Measurements of linear polarization and angular distribution have led to the firm assignments of spin differences and parity of high-spin states in ${}^{91}$Ru. More specifically, calculations using a semiempirical shell model were performed to understand the structures of the first and second (21/2${}^{+}$) and (17/2${}^{+}$) levels. The results are in good agreement with the experimental data, supporting the interpretation of the nonyrast (21/2${}^{+}$) and (17/2${}^{+}$) states in terms of the $J_{\max }$ and $J_{\max }-2$ members of the seniority-three $\nu {\left(g_{9/2}\right)}^{-3}$ multiplet.

Figure 1

(a) The total projection of a $\gamma$-$\gamma$ matrix obtained in coincidence with two protons and one neutron; the $\gamma$-ray peaks are transitions in ${}^{91}$Ru and ${}^{49}$Cr. (b) A background-subtracted spectrum of $\gamma$ rays in coincidence with the 974-keV $\gamma$ ray, corresponding to the transition that depopulates the (13/2${}^{+}$) state in ${}^{91}$Ru. (c) A spectrum gated on the 436-keV transition of ${}^{91}$Ru observed in the present work.

Figure 2

(a) The projection of the DCO matrix on the 135${}^{\circ }$ axis in coincidence with the 497-keV transition of ${}^{91}$Ru at 90${}^{\circ }$ and (b) the projection on the 90${}^{\circ }$ axis in coincidence with the same transition at 135${}^{\circ }$.

Figure 3

Experimental DCO ratios for the transitions belonging to ${}^{91}$Ru ($\blacksquare$) and ${}^{91}$Tc (•). The lines correspond to the values obtained for known dipole and quadrupole transitions using gates on stretched quadrupoles and have been drawn to guide the eye.

Figure 4

Normalization factor $a$ in the linear polarization measurements as a function of $\gamma$-ray energy $\left(E_{\gamma }\right)$ for the EXOGAM array.

Figure 5

Polarization sensitivity of the EXOGAM detector. The solid line is the fit to our data.

Figure 6

Two-dimensional plot of the asymmetry ratio $A$ as a function of the DCO ratio ($R_{\mathrm{DCO}}$) of the $\gamma$ rays belonging to ${}^{91}$Ru. Stretched $E1$, $E2,$ and $M1$ transitions and nonstretched $E1$ transitions are indicated in the plot. The dashed lines parallel to the $y$ axis correspond to the value obtained for known pure stretched dipole and quadrupole transitions. These lines have been drawn to guide the eye. The $R_{\mathrm{DCO}}$ values have been obtained after gating on a quadrupole transition.

Figure 7

Level scheme of ${}^{91}$Ru proposed in the present work. The new transitions are indicated by asterisks. Those indicated with red asterisks will be discussed in a forthcoming paper.

Figure 8

Comparison between the calculated (thin lines) and experimentally observed (solid bars) (17/2${}^{+}$) and (21/2${}^{+}$) states in ${}^{91}$Ru. See text for the configuration notation.