$\gamma$-ray spectroscopy of low-lying yrast and non-yrast states in neutron-rich ${}^{94,95,96}\mathrm{Kr}$

We report on $\gamma$-ray spectroscopy of low-lying excited states in the neutron-rich ${}^{94,95,96}\mathrm{Kr}$ isotopes measured as part of the “Shell Evolution And Search for Two-plus energies At RIBF” (SEASTAR) campaign at the RIKEN Radioactive Isotope Beam Factory. Excited yrast and non-yrast states were observed, and half-lives extracted via geant4 simulations. In ${}^{94,96}\mathrm{Kr}$ candidates for the $3_1^{-}$ state were identified. For ${}^{95}\mathrm{Kr}$, the prompt SEASTAR data were combined with delayed spectroscopic data measured with the EURICA array to observe transitions on top of the known ${(7/2)}^{+}$ isomer at a level energy of 195.5(3) keV. The comparison of the new experimental results with five-dimensional collective Hamiltonian (5DCH) and mapped interacting boson model (IBM) calculations, both using the Gogny D1M interaction, could suggest oblate-prolate shape coexistence already in ${}^{96}\mathrm{Kr}$.

Figure 1

Schematic of the experimental setup including the BigRIPS fragment separator, MINOS target system, DALI2 array, ZeroDegree spectrometer, and EURICA array (see text for details). Quadrupole magnets are shown in red, dipole magnets in blue. At F1 and F5, degraders are depicted in black. Plastic scintillators and parallel plate avalanche counters were placed at several focal planes. Figure adapted from [23].

Figure 2

Particle identification plots for (a) BigRIPS and (b) ZeroDegree spectrometer. The secondary beam in front of the reaction target and reaction residues are labeled and clearly separated. See text for details.

Figure 3

Doppler-corrected $\gamma$-ray energy spectra of DALI2 in prompt coincidence with the ${}^{95}\mathrm{Kr}(p,pn){}^{94}\mathrm{Kr}$ reaction channel. The spectra were fitted with simulated response functions (red) and a two-component exponential background (blue dashed curve). (a) Full spectrum with $\gamma$ multiplicity $M_{\gamma }<6$. (b) Spectrum gated on the $2_1^{+}$ to $0_1^{+}$ 665.5 keV transition (marked in green) without background subtraction.

Figure 4

Doppler corrected $\gamma$-ray spectrum measured with DALI2 for the ${}^{94}\mathrm{Kr}(p,p^{\prime }){}^{94}\mathrm{Kr}$ reaction channel. The spectrum was fitted with simulated response functions (red) and a two-component exponential background (blue dashed curve). The inset shows part of the spectrum gated on the newly observed 1350(38) keV transition with background subtraction.

Figure 5

Level scheme constructed for ${}^{94}\mathrm{Kr}$ based on observed transitions in this work. Energies are given in keV. The widths of the transition arrows correspond to their observed efficiency-corrected intensities normalized to the strongest transition. The newly placed 1350(38) keV transition is shown in red (see Fig. 4). In addition, transitions with energies 428(17), ${880}_{-17}^{+22}$, and ${1083}_{-27}^{+29}$ keV were observed, showing increased intensities when gating on the $2_1^{+}\to 0_1^{+}$ transition, but could not be placed in the level scheme.

Figure 6

Doppler corrected $\gamma$-ray energy spectra measured with DALI2 for the ${}^{97}\mathrm{Rb}(p,2p){}^{96}\mathrm{Kr}$ reaction channel. The spectra were fitted with simulated response functions (red) and a two-component exponential background (blue dashed curve). (a) Full spectrum with $\gamma$ multiplicity $M_{\gamma }<6$. The inset shows the comparison between the experimental shape of the 334 keV transition, assuming a half-life of 0 ps (black) and 134 ps (red) of the proposed 888(16) keV state. (b) Spectrum with $M_{\gamma }=1$. (c) Gated on the $\left(2_1^{+}\right)$ to $0_1^{+}$ transition (green peak) without background subtraction. The dashed vertical lines in the three spectra denote the 819 and 887 keV $\gamma$ transitions. While the former is enhanced by a gate on the $\left(2_1^{+}\right)$ to $0_1^{+}$ transition [see (c)], the latter is stronger in the $M_{\gamma }=1$ spectrum.

Figure 7

Doppler corrected $\gamma$-ray energy spectrum measured with DALI2 for the ${}^{96}\mathrm{Kr}(p,p^{\prime }){}^{96}\mathrm{Kr}$ reaction channel. The spectrum was fitted with simulated response functions (red) and a two-component exponential background (blue dashed curve). The inset shows part of the spectrum gated on the newly observed 1390(36) keV transition with background subtraction.

Figure 8

Level scheme constructed for ${}^{96}\mathrm{Kr}$. Energies are given in keV. The widths of the transitions arrows correspond to their observed efficiency-corrected intensities normalized to the strongest transition. Newly placed transitions are shown in red (see Fig. 7). In addition, the 515 keV transition was observed in coincidence with the $2_1^{+}\to 0_1^{+}$ transition, but could not be placed in the level scheme. (see text for details)

Figure 9

Background-subtracted EURICA energy spectrum with a gate on ${}^{95}\mathrm{Kr}$ in the ZeroDegree PID. The two peaks correspond to the two delayed transitions in ${}^{95}\mathrm{Kr}$ at 81.7(2) and 113.8(2) keV depopulating the known ${(7/2)}^{+}$ isomer [29, 40] as shown in the inset of the figure (see text for details).

Figure 10

Doppler-corrected $\gamma$-ray energy spectra measured with DALI2 for the two-nucleon removal and one-neutron knockout reaction channels ${}^{97}\mathrm{Rb}(p,2pn){}^{95}\mathrm{Kr}$ and ${}^{96}\mathrm{Kr}(p,pn){}^{95}\mathrm{Kr}$. The spectra were fitted with simulated response functions (red) and a two-component exponential background (blue dashed curve).

Figure 11

Doppler-corrected $\gamma$-ray energy spectra measured with DALI2 for different reaction channels with an isomer gate on the delayed transitions measured with EURICA. The spectra were fitted with simulated response functions (red) and a two-component exponential background (blue dashed curve).

Figure 12

Theoretical low-energy excitation levels of ${}^{94}\mathrm{Kr}$ compared with experimental values [14, 33]. Levels in black (left) were calculated using a 5DCH, in blue (right) using the mapped IBM [18]. $E2$ transition strengths in W.u. are shown along the arrows. See text for more details.

Figure 13

Theoretical low-energy excitation levels of ${}^{96}\mathrm{Kr}$ compared with experimental values. Levels in black (left) were calculated using a 5DCH, in blue (right) using the mapped IBM [18]. Levels in red were measured for the first time in this work. $E2$ transition strengths in W.u. are shown along the arrows [14]. See text for more details.

Figure 14

SCMF ($\beta ,\gamma$)-deformation energy surfaces for ${}^{94,96}\mathrm{Kr}$ obtained with the Gogny-D1M EDF [18, 41]. The energy difference between neighboring contours is 100 keV. See text for more details.

Figure 15

SCMF (${\beta }_2,{\beta }_3$) PESs for ${}^{94,96}\mathrm{Kr}$, calculated using the relativistic EDF. Contours join points on the surface with the same energy, and the difference between neighboring contours is 1 MeV [19].

Figure 16

Low-lying positive-parity excited states for ${}^{95}\mathrm{Kr}$ on the left, predicted by the IBFM [20] compared with experimental data (on the right) from this work and Refs. [29, 40]. The newly suggested excited levels decaying into the (${7/2}^{+}$) isomer are shown in red.