Evolution of collectivity and evidence of octupole correlations in ${}^{73}\mathrm{Br}$

High-spin states in the ${}^{73}\mathrm{Br}$ nucleus have been populated via the ${}^{50}\mathrm{Cr}({}^{28}\mathrm{Si},\alpha p){}^{73}\mathrm{Br}$ fusion evaporation reaction with a beam energy of 90 MeV. The deexciting gamma rays were detected using the Indian National Gamma Array (INGA). Using the $\gamma -\gamma$ coincidence technique, two new bands and eight new interconnecting transitions have been added. The strong interconnecting $E1$ transitions, between positive and negative parity bands, ensure the existence of octupole correlations at low spin. Line shapes have been observed for 17 transitions, which were analyzed by the Doppler-shift attenuation method to determine the lifetime of excited states of the yrast negative parity band and its signature partner band along with the positive parity band. The deduced transitional quadrupole moments $Q_t$ for the ground state band decrease with increasing spin, with their values ranging from 2.88 to 1.00 $e\mathrm{b}$. A similar trend in the quadrupole moment has also been observed for the signature partner as well as for the positive parity band. This decrease in $Q_t$ with increasing spin for these bands is interpreted in terms of the cranked Nilsson-Strutinsky model and total Routhian surface calculations, which indicate possible band termination at higher spin.

Figure 1

Partial level scheme of ${}^{73}\mathrm{Br}$ based on the present work and the previous study [13]. The newly observed transitions in the present work are marked by asterisks.

Figure 2

Double-coincidence gated spectrum of 187 and 525 keV transitions, indicating the quadrupole transitions of bands D and E.

Figure 3

Double-coincidence gated spectrum of 187 and 884 keV transitions indicating 782 and 934 keV interconnecting transitions marked by asterisks.

Figure 4

Plot of polarization asymmetry for different $\gamma$ transitions.

Figure 5

Least-squares fit of line shapes of some representative transitions (994 and 1066 keV) for band A of ${}^{73}\mathrm{Br}$. The left-hand frames show the data for the ${148}^{\circ }$ detectors and right-hand frames show the data for ${32}^{\circ }$ detectors. The contaminant peaks are shown by blue color.

Figure 6

Least-squares fits of line shapes of some representative transitions (884 and 1035 keV) for band B of ${}^{73}\mathrm{Br}$. The left-hand frames show the data for the ${148}^{\circ }$ detectors and right-hand frames show the data for ${32}^{\circ }$ detectors. The contaminant peaks are shown by blue color.

Figure 7

The observed transitional quadrupole moments compared with those calculated in the CNS (labeled with configuration) and TRS formalisms.

Figure 8

Calculated low-lying configurations in ${}^{73}\mathrm{Br}$; energy is relative to the rotating liquid drop energy. The close-to-prolate minimum moving towards $\gamma ={60}^{\circ }$ has been followed. Positive parity bands are drawn with full lines and negative parity with dashed lines. Closed symbols are used for signature $\alpha =1/2$ and open symbols for $\alpha =-1/2$. The same convention is used in Figs. 9 and 11.

Figure 9

Comparison between experiment and calculations. The calculated energies are obtained as discussed in the text for Fig. 8.

Figure 10

Total energy surfaces for the $[{43}_{+},64]$ configuration of ${}^{73}\mathrm{Br}$. The contour line separation is 0.25 MeV.

Figure 11

Comparison between observed bands and configurations assigned to them in $I$ vs $E_{\gamma }$ plots. At configuration crossings, the configurations are followed beyond the crossings, while the large open circles shows the $E_{\gamma }$'s if the lowest energies are followed through the crossings. A feature of this comparison is that it is not sensitive to excitation energies and thus is more independent of parameters. This is so because the main result of a parameter change is to change the relative energies of the different configurations.

Figure 12

Contour plots having 0.250 MeV energy difference between two consecutive contours of the TRS calculations for the positive parity quadrupole band A in ${}^{73}\mathrm{Br}$. The rotational frequencies ($\hslash \omega$) for the calculations are (a) 0.200 MeV, (b) 0.400 MeV, (c) 0.800 MeV, and (d) 1.100 MeV.

Figure 13

Contour plots having 0.250 MeV energy difference between two consecutive contours of the TRS calculations for the negative parity quadrupole bands B [(a) and (b)] and C [(c) and (d)] in ${}^{73}\mathrm{Br}$. The rotational frequencies ($\hslash \omega$) for the calculations are (a) 0.400 MeV, (b) 0.800 MeV, (c) 0.400 MeV, and (d) 1.100 MeV.

Figure 14

Calculated quasiproton energy levels for $Z=35$ corresponding to the ${}^{73}\mathrm{Br}$ nucleus. Positive parity, positive and negative signature, and negative parity positive and negative signature orbitals are denoted by green, blue, red, and magenta colors, respectively.

Figure 15

Calculated single-particle energy for the positive parity positive and negative signature bands as a function of the triaxiality parameter $\gamma$ for the fixed quadrupole and hexadecapole deformation parameters $({\beta }_2,{\beta }_4)=(0.37,-0.01)$ with two different frequencies, 0.05 and 0.6 MeV respectively.