In nuclear physics, determining precise values of extremely small decay branches (in the order of 10-6) or crystal γ-ray spectrometry with very high resolution can provide crucial experimental input for understanding aspects of nuclear structure. The focus will be on β-decay and β-delayed particle-decay studies over other decays (e.g., two-proton decay). In particular, systematic measurements of β-delayed neutron probabilities Pn are of special interest in nuclear astro-physics to model the nuclear paths of the rapid-neutron process, as well as in nuclear applications as nuclear-reactor control and non-destructive characterization of nuclear waste. The required equipment will consist of a passive or active stopper, or a trap catching the radioactive ion to be investigated, and detectors for β, γ, charged particles and neutrons. In order to obtain the desired statistics with such rare-event decays or inherent limited detection efficiencies of crystal spectrometers, not only high-intensity beams, but also the availability of long beam times can be of crucial importance.
Gamow-Teller strength distributions
The Gamow-Teller strength B(GT) distribution in a nucleus provides an important test of structure calculations for that nucleus. A major part of the B(GT) strength, however, is situated in the Gamow-Teller Giant Resonance (GTGR). Although β transitions to the GTGR have a large B(GT) strength, the corresponding β-branching ratios are small as a consequence of the small phase-space factor f.
Theoretical B(GT) distribution (left) and measured B(GT) distribution (right) in β-delayed particle emission: example of 14Be. (picture from )
Strength distributions can be directly measured with particle detectors. However, it can also be indirectly determined by measuring the line shape of gamma transitions after β-delayed particle decay. Moreover, additional information can be deduced like the half-life of the excited state from which the γ transition arises and other resonance parameters. Especially in the light nuclei, the broadening effects are large. A recent example is the β-delayed neutron decay study of 11
Li measured with the 8π
spectrometer, an array of 20 Compton-suppressed HPGe detectors , see also picture. A much more precise determination of the line shape can be obtained, however, by using a γ-ray crystal spectrometer, which allows γ-ray energy resolutions down to the part per million precision in the energy domain 0.1-6 MeV. Its inherent low detection efficiency has to be compensated by the intense ISOL@MYRRHA RIBs and long beam times.
Measured gamma line shapes in β-delayed particle emission: example of 11Li. (picture from )
 H. Jeppesen et al., Nucl. Phys. A709
 F. Sarazin et al., Phys. Rev. C 70
Beta-delayed multi-particle emission
When multi-particle decays occur (e.g., βxn, βxp, βpn), one of the key questions concerns the particle-emission mechanism that is involved. This can be investigated by measuring the correlation between the decaying particles. Although the β-delayed multiparticle process is energetically allowed in many nuclei and should occur in many more than the presently known emitters (see figure), information on the particle-emission mechanisms is very scarce.
Picture from B. Jonson and K. Riisager, Nucl. Phys. A693 (2001) 77
The three most studied halo nuclei are 6He, 11Li, and 11Be. However, a few others, such as 14Be, 14B, 15C and 19C are waiting for a more thorough experimental and theoretical study. These systems can typically decay through different β-delayed (multi-) particle channels at high excitation energies in the daughter and, thus, (extremely) low branching ratios are involved. If high-precision data can be obtained, these decays provide, however, a detailed test of halo wave functions, for which microscopic ab-initio calculations with realistic nucleon-nucleon and even nucleon-nucleon-nucleon interactions can be performed.