The isospin dependence of the nuclear interaction was probed using energy differences between analogue excited states across the A=74 isobaric triplet. The first spectroscopy of transitions between states in the proton-dripline nuclide, 74Sr was performed using the highly-selective recoil-beta tagging technique with the JUROGAM II + RITU + GREAT setup at the University of Jyvaskyla. Considerable experimental development was required to acheive the requisite level of sensitivity. This included the development of a phosphor-sandwich detector, a finely segmented doublesided silicon-strip detector (DSSD) and a charged-particle veto array (UoYtube).
The 2+ to 0+ and 4+ to 2+ transitions were identified through a combination of time and energy gates, and the implementation of a charged-particle veto. A beta-decay half-life was also extracted, in good agreement with a complementary measurement performed at RIKEN. Comparison with Skyrme Hartree-Fock calculations indicates that 74Sr is approximately 1-MeV less bound than previously supposed, implying that both observed excited states are two-proton unbound. Through comparison with published data from the A=74 isobar, triplet-energy differences (TEDs) were determined, indicating a continued need for an additional isotensor, J=0, 100-keV isospin-nonconserving (INC) interaction component, consistent with that found to be required at lower masses. The implication of this result is that such an INC component is mandated across the nuclear landscape, regardless of local nuclear structure effects.
Additionally, candidates for the beta-delayed proton decay of 73Sr via 73Rb to 72Kr were observed using the same experimental setup. The lowest energy proton detected is consistent with that which might be expected for the decay of the ground state in 73Rb, allowing for the first measurement of the proton separation energy, Sp. This value is of considerable astrophysical interest since 73Rb is considered to be a waiting point in the rp process.
