First observation of high-spin states in ${}^{83}\mathrm{Se}$

The level structure of ${}^{83}\mathrm{Se}$ was studied via prompt γ-ray spectroscopy of fragments following the fission of the compound nucleus ${}^{226}\mathrm{Th}$ formed in the ${}^{18}\mathrm{O}$ (91 MeV) + ${}^{208}\mathrm{Pb}$ fusion-evaporation reaction. The first four high-spin states above the ${9/2}^{+}$ ground state were established. The coupling of the neutron hole in the $g_{9/2}$ orbital to the yrast states in the ${}^{84}\mathrm{Se}$ core can account for these states. The experimentally observed high-spin states are compared with predictions of shell-model calculations.

Figure 1

Level scheme assigned to ${}^{83}\mathrm{Sr}$ in the present work. Transition and excitation energies are given in keV. The width of the arrows is representative of the intensity of the transitions.

Figure 2

Spectra gated on known transitions of ${}^{138,139,140}\mathrm{Ba}$ [8]. Specifically, the spectra are double gates between the following transitions: (a) the ${}^{138}\mathrm{Ba}$ spectrum, between the 1435.8- and 462.8-keV transitions; (b) the ${}^{139}\mathrm{Ba}$ spectrum, between the 1308.1- and 520.6-keV transitions; and (c) the ${}^{140}\mathrm{Ba}$ spectrum, between the 602.4- and 528.3-keV transitions. The 1808.5-keV transition assigned to ${}^{83}\mathrm{Se}$ in the present work can be clearly seen. The peaks of the other complementary Se isotopes [8] are indicated by “+” (1409.9-keV transition of ${}^{82}\mathrm{Se}$) and “*” (1454.6-keV transition of ${}^{84}\mathrm{Se}$).

Figure 3

Ratios of intensities of transitions in Se isotopes observed in spectra gated on ${}^{139}\mathrm{Ba}$ [Fig. 2b] and on ${}^{138}\mathrm{Ba}$ [Fig. 2a]. The known transitions in ${}^{81}\mathrm{Se}$ (467.7-keV, circle), ${}^{82}\mathrm{Se}$ (1409.9-keV, diamond), and ${}^{84}\mathrm{Se}$ (1454.6-keV, triangle) were used, as well as the 1808.5-keV transition assigned to ${}^{83}\mathrm{Se}$ (square). The ratio for the 1808.5-keV transition, intermediate between the values for ${}^{82}\mathrm{Se}$ and ${}^{84}\mathrm{Se}$, supports this assignment.

Figure 4

Example of typical spectra of transitions assigned to ${}^{83}\mathrm{Se}$ in the present work. (a) The sum of double gates on the 179.7- and 1808.5-keV transitions and on the 409.8- and 1808.5-keV transitions is displayed. (b) The double gate on the 179.7- and 1297.1-keV transitions is displayed. (c) The double gate on the 179.7- and 490.8-keV transitions is displayed. The energies of the transitions are in keV and the peaks of the complementary Ba isotopes [8] are indicated by “+” (${}^{138}\mathrm{Ba}$) and “*” (${}^{139}\mathrm{Ba}$).

Figure 5

Comparison of the high-spin states assigned to ${}^{83}\mathrm{Se}$ in the present work with known high-spin states in ${}^{85}\mathrm{Kr}$ [2], ${}^{87}\mathrm{Rb}$ [3, 4], and ${}^{89}\mathrm{Y}$ [5]. The excitation energy of the ${9/2}^{+}$ isomers in ${}^{87}\mathrm{Rb}$ (1578 keV) and ${}^{89}\mathrm{Y}$ (909 keV) has been normalized to zero.

Figure 6

Comparison of the high-spin states assigned to ${}^{83}\mathrm{Se}$ in the present work with the predictions of the shell-model calculations described in the text. The first two excited states in the $N=50$ core of ${}^{84}\mathrm{Se}$ [3] are shown together with the shell-model prediction for these states.