New transitions and feeding of the $J^{\pi }=\left(8^{+}\right)$ isomer in ${}^{186}\mathrm{Re}$

The spallation neutron source at the Los Alamos Neutron Science Center Weapons Neutron Research facility was used to populate excited states in ${}^{186}\mathrm{Re}$ via $(n,2n\gamma )$ reactions on an enriched ${}^{187}\mathrm{Re}$ target. Gamma rays were detected with the GErmanium Array for Neutron Induced Excitations spectrometer, a Compton-suppressed array of 18 HPGe detectors. Incident neutron energies were determined by the time-of-flight technique and used to obtain $\gamma$-ray excitation functions for the purpose of identifying $\gamma$ rays by reaction channel. Analysis of the singles $\gamma$-ray spectrum gated on the neutron energy range $10\le E_n\le 25\mathrm{MeV}$ resulted in five transitions and one level added to the ${}^{186}\mathrm{Re}$ level scheme. The additions include the placement of three $\gamma$ rays at 266.7, 381.2, and 647.7 keV which have been identified as feeding the $2.0\times {10}^5\mathrm{yr}$, $J^{\pi }=\left(8^{+}\right)$ isomer and yield an improved value of $148.2\left(5\right)\mathrm{keV}$ for the isomer energy. These transitions may have astrophysical implications related to the use of the Re-Os cosmochronometer.

Figure 1

$s$ and $r$ processes, identified by black and gray arrows, involved in the production of ${}^{187}\mathrm{Re}$ and ${}^{187}\mathrm{Os}$. Long-lived nuclides are identified with bold boxes, with the dashed lines indicating weaker secondary processes. The outline around the isomer ${}^{186m}\mathrm{Re}$ and the associated $(\gamma ,{\gamma }^{\prime })$ pathways highlight the fact that their effects are not fully understood.

Figure 2

Summed $\gamma$-ray spectrum from the planar HPGe detectors, gated on $10\le E_n\le 25\mathrm{MeV}$. Prominent peaks from ${}^{186}\mathrm{Re}$ are labeled by energy in keV, up to 400 keV. Unlabeled peaks were identified as $\gamma$ rays from reactions other than ${}^{187}\mathrm{Re}(n,2n\gamma ){}^{186}\mathrm{Re}$. The inset shows the projection of the $E_{\gamma }$-vs-neutron-TOF matrix on the TOF axis and the channel locations corresponding to the neutron energy cuts of 10 and 25 MeV.

Figure 3

Experimental excitation functions for the 111.7-, 188.8-, and 266.7-keV $\gamma$-ray transitions in the ${}^{187}\mathrm{Re}(n,2n\gamma ){}^{186}\mathrm{Re}$ reaction channel, normalized for the purpose of comparing their shapes. Excitation functions for transitions that originate from higher-spin states show a slower rise after the reaction threshold energy than do those originating from low-spin states. The excitation function for the 266.7-keV transition is included in the plot according to its assignment in this work as deexciting a $\left(9^{+}\right)$ level to feed the $\left(8^{+}\right)$ isomer.

Figure 4

Partial level scheme for ${}^{186}\mathrm{Re}$ showing the transitions observed in this work, with energies labeled in keV and tentative assignments identified with parentheses. Levels or transitions with properties in bold italics indicate proposed level energies and spin-parity assignments or transitions placed in this work. All other level energies and spin-parity assignments are those in the adopted level scheme for ${}^{186}\mathrm{Re}$ [4].

Figure 5

Experimental and talys-modeled excitation functions for the 266.7-keV $\gamma$-ray transition in the ${}^{187}\mathrm{Re}(n,2n\gamma ){}^{186}\mathrm{Re}$ reaction channel, normalized for the purpose of comparing their shapes. talys input was modified to include several spin-parity assignments for the initial level to rule out low-spin states feeding the isomer. The shapes of the $6^{+}\to 8^{+}$, $7^{+}\to 8^{+}$, and $8^{+}\to 8^{+}$ excitation functions reveal contamination from the 267-keV transition in ${}^{185}\mathrm{Re}$ produced in the $(n,3n\gamma )$ reaction channel.

Figure 6

Excitation functions up to 25-MeV incident neutron energy for the (a) 266.7-, (b) 381.2-, and (c) 647.7-keV $\gamma$ rays observed in the GEANIE data. Plots are overlaid with simulated excitation functions from coh 3.4 and talys 1.6 for the assignments proposed in Table 2 and Fig. 4.