Radioactive in-flight decays and continuum spectroscopy by particle emission

The existence of nuclei is not restricted to the stable and β-decaying nuclei only. The limiting line of bound nuclei is the so-called drip-line and it is one of the goals to explore the drip-line for as many elements as possible. However the drip-line is not the end of the existence of nuclei and nuclei beyond the proton and neutron drip-lines show interesting phenomena and may have half-lives exceeding the characteristic time of nucleon orbital motion in nuclei (10-21 s).

These nuclei are called resonances and their lifetimes are determined by the centrifugal and Coulomb barriers and also strongly affected by nucleon correlations. Nuclear resonances can be studied by exclusive reactions or invariant-mass methods. Alternatively, they can be studied by their decays (emission of proton(s) or neutron(s), so called proton radioactivity and neutron radioactivity, respectively). Because of their special properties and the fine balance of nuclear forces, these nuclei are subject of high theoretical interests. Outside the proton drip-line,
proton radioactivity prevails and some nuclides with two-proton decays have been observed.
They allow studying two-proton correlations in nuclei. Four-proton decay is also expected in some cases of proton-rich nuclei. Neutron radioactivity has not been observed yet, mainly due to the fact that the drip line is reached only for light elements where only orbitals of low angular momentum are involved, and thus the centrifugal barrier is not high enough to retard the decay. Because of the pairing interactions of neutrons in nuclei, it is expected that two-neutron decays have longer half-lives.
It is interesting to see whether such long-lived neutron radioactivity exists; if so, two-neutron correlations in nuclei could be studied in detail. Such decays and angular correlations can be studied ideally at high kinetic energy and directly at the Super- FRS, where highest transmission is obtained and where the most exotic species can be reached.
The in-flight decay technique with relativistic exotic nuclei was pioneered at the FRS. Also the Optical-TPC (O-TPC) was used effectively in a recent experiment, where the beta-delayed 3p-decay of the very exotic nucleus 31Ar was studied.
The Super-FRS will provide even more exotic nuclei and dripline nuclei for heavier elements. For the study of proton and neutron decays near and beyond the driplines, it is necessary to produce the tertiary nucleus of interest in a reaction from a nearby secondary beam (relatively large cross section, simple identification, high transmission) and provide low background conditions (for example, the two-proton emitter 26S can be produced by neutron removal reaction of 27S, a possible four-neutron decay nucleus 28O could be produced by one-proton removal from 29F, etc.).
Therefore, the Super-FRS facility is essential and most suitable for the study of tertiary nuclei. The detection schemes employed cover half-life ranges from ~1ps to 100ns (in-flight decay technique) and ~100ns to 1s (O-TPC). Based on angular correlations and with more inclusive measurements, these experiments will provide important information and be complementary to other NUSTAR activities.

The discussed physics cases will be addressed in future experiments by the setup EXPERT (EXotic Particle Emission and Radioactivity by Tracking), see details here (http://aculina.jinr.ru/EXPERT.php/)