Terminating states in the positive-parity structures of ${}^{67}\mathrm{As}$

The energy levels and $\gamma$-ray decay scheme of the positive-parity states in the $T_z=\frac{1}{2}$ nucleus ${}^{67}\mathrm{As}$ have been studied by using the ${}^{40}\mathrm{Ca}({}^{36}\mathrm{Ar},2\alpha p){}^{67}\mathrm{As}$ reaction at a beam energy of 145 MeV. Two new band structures have been identified which can be connected to the previously known levels. The results for these bands are compared with configuration-dependent cranked Nilsson–Strutinsky calculations. The good level of agreement between theory and experiment suggests that these structures can be interpreted in terms of configurations that involve three ${\mathrm{g}}_{\frac{9}{2}}$ particles and that both possess noncollective terminating states.

Figure 1

Energy-level and $\gamma$-ray decay scheme for ${}^{67}\mathrm{As}$ determined from the present work. The width of the transitions is proportional to the intensity of the $\gamma$ rays. Transitions shown in red are new.

Figure 2

$\gamma$-ray spectrum projected from the radware cube discussed in the text. The figure shows the double-gated prompt-coincidence spectrum with gates on the 983 and 1604 keV transitions. The inset provides the same spectrum expanded in the region between approximately 620 and 880 keV for clarity.

Figure 3

Spectra showing the sum of prompt-coincidence double $\gamma$-ray gates taken from the radware cube discussed in the text. (a) The spectrum is the sum of many double-gate combinations between all $\gamma$ rays shown in Fig. 1, except the 69 keV transition, up to the 1604 keV line in band 1 and the 2218, 2434, and 2669 keV transitions. The upper part of this spectrum shows the transitions in bands 1a and 1b (see Fig. 1). The inset provides the upper portion of the spectrum from the sum of double gates between all $\gamma$ rays in band 1 up to the 1604 keV transition, except the 69 keV line and the 2434 keV transition. This illustrates the upper part of band 1a. (b) Spectrum showing the sum of double-gate combinations between all transitions in the lower portion of band 1, except 69 keV, plus transitions in band 2 up to 2019 keV and the 2640 and 2785 keV transitions. The inset shows the upper portion of the spectrum containing the sum of double gates between the 775, 1078, 1345, 1250, 1641, and 2019 keV $\gamma$ rays and the 2085 keV transition. This illustrates one of the two decay pathways seen in the upper part of band 2 in Fig. 1.

Figure 4

(a) Experimental energies relative to a rotational liquid drop energy for bands 1a, 1b, 2a, 2b and the ${\frac{5}{2}}^{-}$ ground state. (b) Calculated energies for the configurations shown relative to the rotational liquid drop energy. Predicted noncollective terminating states ($\gamma ={60}^{\circ }$) are enclosed in large open circles. (c) Difference in energy between the calculated and experimental energies shown in panels (b) and (a).

Figure 5

Tilted Fermi-surface plots for protons (upper panel) and neutrons (lower panel) which illustrate how the favored ${\frac{39}{2}}^{+}$ state is built. Note that the $f_{\frac{5}{2}}{,}_{\frac{3}{2}}$ state is not occupied for either protons or neutrons. The states are labeled according to the dominant $j$ shell in their wave functions, where solid lines are used for positive-parity states and dotted lines for negative-parity states.