Coupling of the proton-hole and neutron-particle states in the neutron-rich ${}^{48}$K isotope

Excited states in the $Z=19$, $N=29$ neutron-rich ${}^{48}$K isotope have been studied using deep-inelastic transfer reactions with a thick target at Gammasphere and with a thin target at the PRISMA-CLARA spectrometer. The lowest excited states were located; they involve a proton hole in the $s_{1/2}$ or $d_{3/2}$ orbital coupled to a $p_{3/2}$ neutron. A new 7.1(5)-ns, $5^{+}$ isomer, the analog of the $7/2^{-}$ isomer in ${}^{47}$K, was identified. Based on the observed $\gamma$-decay pattern of the isomer a revised spin-parity assignment of $1^{-}$ is proposed for the ground state of ${}^{48}$K.

Figure 1

Single $\gamma$-ray spectrum measured with the PRISMA-CLARA spectrometer for the ${}^{48}$K fragments from the ${}^{238}$U $+$ 330-MeV ${}^{48}$Ca reaction. The selection of an $A=48$ mass window from the distribution of potassium isotopes is presented in inset (a). A high-energy portion of the spectrum, extending from 1300 to 1600 keV, showing weak 1409- and 1449-keV transitions is displayed in inset (b). See text for further discussion.

Figure 2

Selected $\gamma$-coincidence spectra measured in the thick-target ${}^{238}$U $+$ 330-MeV ${}^{48}$Ca experiment at Gammasphere. (a)–(c) Transitions identified in the ${}^{48}$K isotope with various gate conditions; (d) prompt $\gamma$-ray spectrum gated by delayed 449- and 1449-keV lines; (e) decay curve of the 7.1-ns isomer obtained with gates set on the 1409–1449 and 1409–449 keV transition pairs.

Figure 3

The ${}^{48}$K level scheme established in this study. See text for a discussion of the placement of the weak transitions and of the proposed spin-parity assignments.

Figure 4

Selected $\gamma$ coincidence spectra measured with Gammasphere. The data were sorted with a time condition of 8–45 ns after the beam pulse in order to enhance the detection sensitivity for weak $\gamma$ rays emitted in decays of nanosecond isomers. (a) and (b) Transitions identified in the 143–585–1449-keV cascade from triple $\gamma \gamma \gamma$ coincidences; (c) and (d) cascade of transitions at 143–2034 keV depopulating the ${}^{48}$K isomer at 2177 keV. The additional 209-keV line indicated in spectrum (c) gated by the 2034-keV transition originates from the decay of the 23-ns, $8^{+}$ isomer in ${}^{68}$Ni [22] which was also populated in this experiment and has a line of similar energy.

Figure 5

The Doppler-corrected $\gamma$-ray spectra showing the 143-, 279-, and 449-keV lines at various target-degrader distances as indicated in (a)–(d). These spectra are measured in detectors located at backward angles ($\theta \sim {154}^{\circ }$) in the array. Separation of components arising from $\gamma$ emission before (shifted to low energy) and behind the degrader (properly corrected) served to derive ratios $R=I_{\mathrm{after}}/(I_{\mathrm{before}}+I_{\mathrm{after}})$ used to determine lifetimes (see text and Fig. 6). The two components are indicated by dotted lines.

Figure 6

Experimental $R$ ratios for different target-degrader distances (see text) and fits used to determine the lifetimes of the states deexcited by the 143- (top) and 279- and 449-keV (bottom) $\gamma$ transitions. The feeding by the 449-keV transition is taken into account in the lifetime determination of the 279-keV state.