High-spin structures of ${}_{36}^{88}$Kr${}_{52}$ and ${}_{37}^{89}$Rb${}_{52}$: Evolution from collective to single-particle behavior

The high-spin states of the two neutron-rich nuclei ${}^{88}$Kr${}_{52}$ and ${}^{89}$Rb${}_{52}$ have been studied from the ${}^{18}$O+${}^{208}$Pb fusion-fission reaction. Their level schemes were built from triple $\gamma$-ray coincidence data, and $\gamma -\gamma$ angular correlations were analyzed in order to assign spin and parity values to most of the observed states. The two level schemes evolve from collective structures to single-particle excitations as a function of the excitation energy. Comparison with results of shell-model calculations gives the specific proton and neutron configurations which are involved to generate the angular momentum along the yrast lines.

Figure 1

Level scheme of ${}^{88}$Kr established in this work. Transitions that are new or are placed differently as compared to Ref. [12] are marked with a star. The width of the arrows is proportional to the $\gamma$-ray intensity. The color code is the same as that of Fig. 7.

Figure 2

Spectrum of $\gamma$ rays in coincidence with two transitions of ${}^{88}$Kr, the 1272- and the 868- or 775-keV transitions; (a) low-energy part and (b) high-energy part. Transitions emitted by ${}^{132–134}$Xe, the complementary fragments of ${}^{88}$Kr, are labeled.

Figure 3

Spectrum of $\gamma$ rays in coincidence with two transitions of ${}^{88}$Kr, the 1630- and the 868- or 775-keV transitions, showing the transitions belonging to the right part of the level scheme. The peaks marked with a star are contaminants.

Figure 4

Spectrum of $\gamma$ rays in coincidence with the 460- and 868-keV transitions. (a) The red arrows show the broadening of the 440-keV $\gamma$ line. (b) The red arrows show the broadening of the 751-keV $\gamma$ line.

Figure 5

Spectrum of $\gamma$ rays in coincidence with two transitions of ${}^{89}$Rb, the 1193- and the 836- or 809-keV transitions; (a) low-energy part, (b) high-energy part. The $\gamma$-rays involved in the decay of the 1195-keV state are written in red. The transition emitted by ${}^{132}$I, the main complementary fragment of ${}^{89}$Rb in the ${}^{18}$O+${}^{208}$Pb reaction, is labeled.

Figure 6

Level scheme of ${}^{89}$Rb established in this work. The width of the arrows is proportional to the $\gamma$-ray intensity. Transitions, which are new or are placed differently as compared to Ref. [20], are marked with a star. The 1195-keV level is isomeric, $T_{1/2}=15.2\left(28\right)$ ns [19] or 8(2) ns [20]. The color code is the same as that of Fig. 8.

Figure 7

(a) Results of the shell-model calculations for ${}^{88}$Kr (see text). The states having the same main configuration are linked by a solid line. (b) Experimental states of ${}^{88}$Kr drawn with the same symbols as in (a). (c)–(t) For each state of ${}^{88}$Kr drawn in (a), decomposition of its total angular momentum into the $I_p\otimes I_n$ components. Each percentage is written inside a square, drawn with an area proportional to it. Low percentages are not displayed. (c)–(k) Positive-parity states, (l)–(t) negative-parity states.

Figure 8

(a) Results of the shell-model calculations for ${}^{89}$Rb (see text). The states having the same main configuration are linked by a solid line. (b) Experimental states of ${}^{89}$Rb drawn with the same symbols as in (a). (c)–(t) For most of the states of ${}^{89}$Rb drawn in (a), decomposition of their total angular momentum into the $I_p\otimes I_n$ components. Each percentage is written inside a square, drawn with an area proportional to it. Low percentages are not displayed. (c)–(k) Negative-parity states, (l)–(t) positive-parity states.

Figure 9

Levels of ${}^{87}$Br predicted by the SM calculations, the color code is the same as the one of Fig. 8a. The experimental states identified from the $\beta$ decay of ${}^{87}$Se [26] are shown in the right part (the 5/2${}^{-}$ value of the ground-state spin is from Ref. [17]).