In-beam Spectroscopy

Ground states of odd-Z isotopes with neutron numbers, N, being less than their proton number, Z, are weakly if at all bound against the emission of protons in the region between N=Z=28 56Ni and N=Z=50 100Sn. An explanation for the lack of experimental information on those nuclei thus lies in their intertwined challenge and interest: weak nuclear binding requires comprehensive experiments including dedicated detection systems to be sensitive to fast proton emission in direct competition to γ-ray emission of quasibound nuclear quantum states beyond the proton dripline.

The nuclei of interest are formed in fusion-evaporation reactions such as, for example, 40Ca+24Mg → 64Ge*. Following rare cases of evaporating one proton and two neutrons from a highly-excited 64Ge* compound nucleus, excited states in 61Ga can be populated, which is an exotic N<Z isotope of gallium (Z=31). 

The γ rays are detected in multi-germanium detector arrays such as GAMMASPHERE, illustrated to the right. To associate γ rays with a certain isotope, one can either measure the evaporated particles (see right) or the recoiling nuclei, or both. To conduct proton-γ peak-peak coincidence spectroscopy, the charged particles must be detected in a dedicated detector system, in our case comprising two CD-shaped double-sided Si-strip detectors (DSSD). They provide 2x2048 detector pixels to allow for high angular and thus energy resolution measurements of proton energies as well as beam-position tracking, which is a novel feature in this context.

Owing to the setup's unprecedented in-beam proton spectroscopy and tracking capabilities, a coincidence between a 957.6-keV γ ray and a 1.876-MeV proton line was observed, which identified the quasi-bound proton g9/2 single-particle state in 61Ga. This coincidence is shown in the decay scheme of 61Ga to the right, together with the γ-ray and proton energy spectra. Both spectra are conditioned with a final residue of mass number A=60, because the emission of a proton from states in 61Ga lead to states in N=Z 60Zn, a minimum of three detected γ rays as well as a maximum of two detected protons, and certain sum-energies of γ rays and particles. 

In terms of physics, this observation probes isospin-symmetry at the limit of nuclear binding by providing a unique challenge for the shell-model interpretation of mirror nuclei beyond doubly-magic 56Ni. In terms of instrumentation, the set-up serves as template for future explorations of nuclei along and beyond the proton dripline. 

For more details, see the respective publication in Physical Review Letters or Yuliia Hrabar's PhD thesis.