Structure of ${}^{90,91}\mathrm{Kr}$ nuclei: Solving the puzzle of their population in fission

Excited states of ${}^{90,91}\mathrm{Kr}$ nuclei have been populated following the cold-neutron-induced fission of a ${}^{235}\mathrm{U}$ target. The $\gamma$ rays emitted following fission reactions were measured using the highly efficient array of high-purity Ge detectors, EXILL, at the Institute-Laue-Langevin, Grenoble. The surprisingly low population of ${}^{91}\mathrm{Kr}$ reported in the spontaneous fission of ${}^{252}\mathrm{Cf}$ measurement has been explained and new level schemes of ${}^{90,91}\mathrm{Kr}$ nuclei were established. Moderate $\gamma$ collectivity is observed in both nuclei. Large-scale shell-model calculations support the experimental picture of the $Z=36$, Kr isotopes forming a border line between lower-$Z$ nuclei showing moderate $\gamma$ collectivity and the heavier-$Z$ nuclei, where distinct shape changes are observed.

Figure 1

Coincidence $\gamma$ spectrum doubly gated on the 199.3- (${}^{143}\mathrm{Ba}$) and 707.3-keV lines. We have used a nonlinear energy calibration to keep constant width of lines. $E=0.95$ channel + 0.000232443 (channel)${}^2$.

Figure 2

$\gamma$ spectra doubly gated on the 707.3-keV line and the 655.5-keV lines (a), 707.3-keV and the .4-keV lines (b), and 707.7 keV and 799.3 keV s (c).

Figure 3

Mass correlation for Kr isotopes as obtained in this work. See text for further explanation.

Figure 4

Level scheme of ${}^{90}\mathrm{Kr}$, as obtained in the present work.

Figure 5

Level scheme of ${}^{91}\mathrm{Kr}$, as obtained in the present work.

Figure 6

Angular correlation analysis for the 401.7–1362.7-keV cascade in ${}^{90}\mathrm{Kr}$, assuming the $3\to 2\to 0$ spin hypothesis. For the detailed description see the text.

Figure 7

Calculated and experimental schemes of excited states in ${}^{90}\mathrm{Kr}$. The 1240.3-keV level and its decay are taken from Ref. [22].

Figure 8

Systematics of positive-parity, yrare excitations in the (a) $N=52$ and (b) $N=54$, Ge-Mo even-even nuclei. The experimental data are taken from the present work and from the nuclear-data base [34]. Points in parentheses have a tentative spin assignment.

Figure 9

Calculated and experimental energies of excited states in ${}^{91}\mathrm{Kr}$. The lines represent observed or predicted $\gamma$ transitions. The 144.3-keV level is taken from Ref. [25].