In-beam ${\hat{I}}^3$-ray spectroscopy at the proton dripline: ${}^{40}$Sc

Abstract

We report on the first in-beam ${\hat{I}}^3$-ray spectroscopy of the proton-dripline nucleus ${}^{40}$Sc using two-nucleon pickup onto an intermediate-energy rare-isotope beam of ${}^{38}$Ca. The ${}^9$Be(${}^{38}$Ca,${}^{40}$Sc$+{\hat{I}}^3$)X reaction at 60.9 MeV/nucleon mid-target energy selectively populates states in ${}^{40}$Sc for which the transferred proton and neutron couple to high orbital angular momentum. In turn, due to angular-momentum selection rules in proton emission and the nuclear structure and energetics of ${}^{39}$Ca, such states in ${}^{40}$Sc then exhibit ${\hat{I}}^3$-decay branches although they are well above the proton separation energy. This work uniquely complements results from particle spectroscopy following charge-exchange reactions on ${}^{40}$Ca as well as ${}^{40}$Ti EC/$β^+$ decay which both display very different selectivities. The population and ${\hat{I}}^3$-ray decay of the previously known first $(5^{-})$ state at 892 keV and the observation of a new level at 2744 keV are discussed in comparison to the mirror nucleus and shell-model calculations. On the experimental side, this work shows that high-resolution in-beam ${\hat{I}}^3$-ray spectroscopy is possible with new generation Ge arrays for reactions induced by rare-isotope beams on the level of a few $\hat{I}¼$b of cross section.

Accepted for publication in Phys. Lett. B