Discovery of New Isotopes ${}^{160}\mathrm{Os}$ and ${}^{156}\mathrm{W}$: Revealing Enhanced Stability of the $N=82$ Shell Closure on the Neutron-Deficient Side

Using the fusion-evaporation reaction ${}^{106}\mathrm{Cd}({}^{58}\mathrm{Ni},4n){}^{160}\mathrm{Os}$ and the gas-filled recoil separator SHANS, two new isotopes ${}_{76}{}^{160}\mathrm{Os}$ and ${}_{74}{}^{156}\mathrm{W}$ have been identified. The $\alpha$ decay of ${}^{160}\mathrm{Os}$, measured with an $\alpha$-particle energy of 7080(26) keV and a half-life of ${201}_{-37}^{+58}\mathrm{\mu }\mathrm{s}$, is assigned to originate from the ground state. The daughter nucleus ${}^{156}\mathrm{W}$ is a ${\beta }^{+}$ emitter with a half-life of ${291}_{-61}^{+86}\mathrm{ms}$. The newly measured $\alpha$-decay data allow us to derive $\alpha$-decay reduced widths (${\delta }^2$) for the $N=84$ isotones up to osmium ($Z=76$), which are found to decrease with increasing atomic number above $Z=68$. The reduction of ${\delta }^2$ is interpreted as evidence for the strengthening of the $N=82$ shell closure toward the proton drip line, supported by the increase of the neutron-shell gaps predicted in theoretical models.

Figure 1

The decay chain of ${}^{160}\mathrm{Os}$. Half-lives, decay energies and branching ratios are taken from Ref. [20] and this Letter (red color). The expected $8^{+}$ isomeric state in ${}^{160}\mathrm{Os}$ is indicated by a thick horizontal dashed line.

Figure 2

(a) Energy spectrum of $\alpha$ particles occurring within $30–700\mathrm{\mu }\mathrm{s}$ of the implantation of an evaporation residue into the same PSSD pixel. Known $\alpha$-decay lines are labeled. (b) Alpha particles in (a) that were followed within 1.2 s by the proton decays of ${}^{156}\mathrm{Ta}$. The red arrow indicates the $\alpha$ peak attributed to the new isotope ${}^{160}\mathrm{Os}$. (c) The same as (b) except the energy gate selected for the second decay is below the proton peak.

Figure 3

The (a) $Q_{\alpha }$ and (b) $T_{1/2}^{\alpha }$ values of ground-state to ground-state transitions for neutron-deficient Er, Yb, Hf, W, Os, Pt, and Hg isotopes as a function of the neutron number $N$. Experimental data are taken from [25, 26, 27] and this Letter (open symbols). The corresponding theoretical values (dash lines) are taken from [28] or calculated using the Rasmussen method [24].

Figure 4

Reduced $\alpha$-decay widths ${\delta }^2$ of (a) even-$Z$ and (b) odd-$Z$ $N=84$, 85 isotones as a function of proton number. The datum of ${}^{160}\mathrm{Os}$ is marked by an open symbol. (c) Experimental neutron-shell gaps of the $N=82$ isotones from AME2020 [25] compared to the predictions of $\mathrm{WS}4+\mathrm{RBF}$ (open diamonds) [28, 35], HFB-32 (open triangles) [36], and LSSM (open squares) [2, 37, 38] models.