Prompt and delayed $\gamma$ spectroscopy of neutron-rich ${}^{94}\mathrm{Kr}$ and observation of a new isomer

Prompt and delayed $\gamma$-ray spectroscopy of the neutron-rich ${}^{94}\mathrm{Kr}$ was performed, as part of the fission campaign at the ALTO facility of the IPN Orsay, using the fast-neutron-induced fission reaction ${}^{238}\mathrm{U}(n,f)$ in combination with the $\nu$-Ball array, a novel hybrid $\gamma$ spectrometer for energy and lifetime measurements. Several new yrast and nonyrast transitions were observed for the first time, extending the previously known level scheme. Additionally, we report on the observation of a new short-lived isomer at 3444 keV with a half-life of 32(3) ns. The analysis of the Nilsson orbitals obtained from Gogny cranked Hartree-Fock-Bogoliubov calculations suggests a ($9^{-}$) spin and an oblate deformation for this isomer corresponding to a two-quasineutron state, indicating an isomeric structure very similar to that of the neighboring isotones ${}^{96}\mathrm{Sr}$ and ${}^{92}\mathrm{Se}$.

Figure 1

Schematic drawing of the $\nu$-Ball array, coupled to the LICORNE fast-neutron source. The $p({}^7\mathrm{Li},{}^7\mathrm{Be})n$ reaction produced a focused fast-neutron beam that induced fission in the ${}^{238}\mathrm{U}$ material. The $\gamma$ rays stemming from the excited fission products were measured with the $\nu$-Ball array, consisting of 10 phase I HPGe, 24 Clover HPGe, and 20 ${\mathrm{LaBr}}_3$ detectors. The solid angle coverage was approximately 70%.

Figure 2

(a) Background-subtracted energy spectrum coincident to the gate on the 853.8-keV $4_1^{+}$ to $2_1^{+}$ transition in the prompt time window. The efficiency-corrected intensities normalized to the strongest transition at 665.5 keV (green) are shown next to the peaks. Only events where at least three different HPGe detector hits were recorded are considered. (b) The energy spectrum after a second gate on 1001.8 keV is applied.

Figure 3

Background-subtracted energy spectrum coincident with the gate on the $4_1^{+}$ to $2_1^{+}$ 853.8-keV transition in the delayed 35–180 ns time window. The efficiency-corrected intensities normalized to the strongest transition at 665.5 keV (green/gray) are shown next to the peaks.

Figure 4

Double-gated energy spectra in the delayed 35–180 ns time window. (a) Gated on the 853.8- and 1001.8-keV transitions. (b) Gated on the 853.8- and 1267.1-keV transitions. (c) Gated on the 186.8- and 665.5-keV transitions.

Figure 5

Level scheme constructed for ${}^{94}\mathrm{Kr}$. Energies are given in keV. The widths of the transitions arrows correspond to their efficiency-corrected intensities. New transitions and levels are shown in red (gray). Transitions denoted by an asterisk are only observed in the prompt time window.

Figure 6

Time spectrum and fitted decay curve used to determine the half-life of the 3444-keV isomer. For this spectrum, only events that have a delayed multiplicity of $\ge 2$ and $\le 6$ in the delayed time region (beyond the dashed line) were considered. The decay curve is fitted with an exponential decay function (thick red/gray curve) together with a time-dependent background (black curve). To maximize statistics the time distributions of different $\gamma$-ray coincidences below the isomer were added up to one decay curve (thin blue/gray curve).

Figure 7

Background-subtracted prompt energy spectrum generated by double gating on coincidences of delayed $\gamma$ transitions below the 3444-keV isomer in the 35–180 ns time window.

Figure 8

Prompt-delayed $\mathrm{\Delta }T$ spectrum of prompt transitions populating and delayed transitions depopulating the 3444-keV isomer. The decay curve is fitted with an exponential decay function (thick red curve) together with a time-dependent background (thick black curve). To gain statistics, several prompt-delayed transition combinations were added up to one decay curve (thin blue/gray).

Figure 9

Partial Nilsson diagram for neutrons. The Fermi surface for $N=58$ is denoted with black dots. Adapted from Delaroche et al. [31], obtained with Gogny CHFB calculations.

Figure 10

Comparison of partial level schemes of ${}^{92}\mathrm{Se}$, ${}^{94}\mathrm{Kr}$, and ${}^{96}\mathrm{Sr}$. Data are from this work and Refs. [18, 33, 35, 36].