$B(E2;2_1^{+}\to 0_1^{+})$ value in ${}^{90}\mathrm{Kr}$

A smooth onset of collectivity in ${}^{88,92,94,96}\mathrm{Kr}$ has been determined from reported $B(E2;2_1^{+}\to 0_1^{+})$ and $E\left(2_1^{+}\right)$ values. This is in contrast to the sudden onset in even-even Zr, Mo, and Sr isotopes. Our objective was to complete the systematics by determining the $B(E2;2_1^{+}\to 0_1^{+})$ value in ${}^{90}\mathrm{Kr}$, which was produced by cold-neutron-induced fission of ${}^{235}\mathrm{U}$. The lifetime of the $2_1^{+}$ state in ${}^{90}\mathrm{Kr}$ was measured via the electronic $\gamma -\gamma$ timing technique using the EXILL and FATIMA spectrometers. Based on the measured mean lifetime of $\tau$ = 15(10) ps, the $B(E2;2_1^{+}\to 0_1^{+})$ value of $13{}_{-5}^{+26}$ W.u. in ${}^{90}\mathrm{Kr}$ is determined for the first time and the smooth onset of deformation in the even-even Kr isotopes beyond neutron number $N=50$ is confirmed.

Figure 1

A portion of the EXILL and FATIMA spectrometer, which consisted of a ring of eight BGO Compton-suppressed EXOGAM Clovers (one removed for the photo), each made of four Ge crystals [18] and 16 ${\mathrm{LaBr}}_3$(Ce) scintillator detectors.

Figure 2

Double-gated triple coincidence spectra showing the cleanliness of the data in the region of the 1123-keV peak. The results are out of 8 TB of data digitally acquired trigger less from a 12 d measurement on cold-neutron induced fission of ${}^{235}\mathrm{U}$. For visibility, the EXILL coincidence spectrum is reduced by factor 3. The spectra are not background subtracted, as the Compton background is needed to derive its contribution to the time spectra presented in Fig. 3.

Figure 3

The two EXILL-gated FATIMA time spectra of the 1123–707 keV $\gamma -\gamma$ cascade in ${}^{90}\mathrm{Kr}$. The result consists of the sum of spectra obtained using two different EXILL gates (199 keV and 331 keV in ${}^{144}\mathrm{Ba}$). The time spectra include large contributions of $\gamma$-Compton events, as can be deduced from Fig. 2.

Figure 4

The centroid-difference diagram for the reference energy of 707 keV corresponding to the common gate of all time spectra. The presented PRD curve (time response of prompt full-energy $\gamma -\gamma$ events) is adjusted by a parallel shift of the calibrated PRD curve to cross the energy axis at the reference energy. The result at 1123 keV, as obtained from Fig. 3, includes $\gamma$ (707 keV) vs. Compton events. By fitting the data obtained by setting gates in the Compton background, the timing (time response) of the $\gamma$-Compton events is derived. The values needed for lifetime determination are inter- or extrapolated at 1123 keV, as explained in the text.