In-beam $\gamma$-ray spectroscopy of the $N=85$ isotones ${}^{159}$W and ${}^{160}$Re

Excited states have been identified in the isotones ${}_{74}^{159}$W and ${}_{75}^{160}$Re, which were produced in fusion-evaporation reactions of a beam of 310-MeV ${}^{58}$Ni ions with an isotopically enriched ${}^{106}$Cd target. The strongest $\gamma$-ray transitions in ${}^{159}$W and ${}^{160}$Re are interpreted as continuing the regular sequence of states identified in lighter $N=$ 85 isotones that are built upon 9/2${}^{-}$ and 10${}^{+}$ states, respectively. The half-life of ${}^{155}$Hf, the $\beta$-decaying daughter of the $\alpha$ decay of ${}^{159}$W, was measured with improved precision to be 840 $\pm$ 30 ms.

Figure 1

Schematic decay chains of (a) ${}^{159}$W and (b) the $\pi h_{11/2}$ isomer in ${}^{161}$Re. Note that the $\beta$ decays of ${}^{155}$Hf may populate the 1/2${}^{+}$ and 11/2${}^{-}$ $\alpha$-decaying isomers in ${}^{155}$Lu via intermediate states. Half-lives (in ms) and $\alpha$-decay energies (in keV) are taken from [14] except for the $\beta$-decay half-life of ${}^{155}$Hf, which is from the present work.

Figure 2

(a) Energy spectrum of $\alpha$ decays observed within 40 ms of the implantation of an ion into the same DSSD pixel. The peak between 6.2 and 6.3 MeV is an unresolved mixture of ${}^{159}$W and ${}^{161}$Re $\alpha$ decays, with a contribution from the high-energy tail of the ${}^{157}$Ta $\alpha$-decay peak. The energy spectrum of these decays that were not followed within a further 20 ms by any other signal in the same DSSD pixel is shown in red (gray). This requirement suppresses ${}^{161}$Re $\alpha$ decays. (b) Energy spectrum of $\alpha$ decays that follow the ${}^{159}$W $\alpha$ decays in the same DSSD pixel between 20 ms and 4 s later. Both of the low-energy $\alpha$-decay lines of ${}^{155}$Lu [14] are visible, as well as ${}^{157}$Ta $\alpha$ decays that occur more than 20 ms after a ${}^{161}$Re $\alpha$ decay.

Figure 3

(a) Energy spectrum of $\gamma$ rays obtained by tagging on ${}^{159}$W $\alpha$ decays occurring within 40 ms of an ion implantation and followed within 4 s by an $\alpha$ decay from either decay peak of ${}^{155}$Lu shown in Fig. 2b. (b) Energy spectrum of $\gamma$ rays obtained by tagging on ${}^{159}$W $\alpha$ decays that are not followed within 20 ms by any other decay in the same DSSD pixel. Transitions previously assigned to ${}^{161}$Re and ${}^{157}$Ta are indicated by the solid triangles and diamonds, respectively. (c) Energy spectrum of prompt $\gamma$ rays observed in coincidence with the 885-keV $\gamma$-ray transition in ${}^{159}$W. (d) Energy spectrum of prompt $\gamma$ rays obtained when tagging on the proton- and $\alpha$-decay branches of ${}^{160}$Re. The ${}^{160}$Re decays were required to occur within 4 ms of an ion being implanted into the same DSSD pixel, while the proton decays were required to be followed within a further 40 ms by a ${}^{159}$W $\alpha$ decay. The strongest $\gamma$-ray transitions assigned to ${}^{159}$W and ${}^{160}$Re are labeled with their energies in keV.

Figure 4

(a) The low-energy part of the energy spectrum of ${}^{159}$W $\gamma$ rays measured using JUROGAM and shown in Fig. 3a. (b) Energy spectrum of $\gamma$ rays emitted in the decay of the microsecond isomer in ${}^{160}$Re [2] measured in the planar Ge strip detector of GREAT. (c) Energy spectrum of $\gamma$ rays from (b) observed in coincidence with the 177-, 626-, or 837-keV prompt $\gamma$-ray transitions measured in JUROGAM.

Figure 5

Systematics of energy levels built upon 9/2${}^{-}$ and 10${}^{+}$ states in $N=85$ isotones. The energies of the 7/2${}^{-}$ and 9${}^{+}$ states below these states, respectively, are also shown. The 7/2${}^{-}$ level in ${}^{159}$W is shown by the dashed line at 81 keV, but this should be regarded as a lower limit; see text for details. Data are taken from Refs. [4, 19, 23, 24, 25, 26, 27] and the present work.