Beta⁺-delayed proton (or α) emission is a typical decay mode of very neutron-deficient nuclei. Valuable information for the ground state in the precursor, such as half-life, spin, and parity, can be obtained by studying the β⁺-p decay properties. The high efficiency and unique experimental signature for detecting protons allow one to study states in the β⁺-decay daughter that are not accessible through other means. By measuring the properties of protons emitted to a known state in the daughter, information on the structure of the proton-unbound state can be obtained.

Nuclei near the proton drip line with large Q values for β⁺ decay often β-decay to excited states that subsequently decay by the emission of a proton. This is known as beta-delayed proton emission (β⁺-p). It is a typical decay mode of very neutron-deficient nuclei. Valuable information for the ground state in the precursor, such as half-life, spin, and parity, can be obtained by studying the β⁺-p decay properties. The high efficiency and unique experimental signature for detecting protons allow one to study states in the β⁺-decay daughter that are not accessible through other means. By measuring the properties of protons emitted to a known state in the daughter, information on the structure of the proton-unbound state can be obtained.

Beta-delayed proton emission is a two step process in which a proton-rich precursor nucleus beta-decays into a state in the emitter that is proton unbound. Proton decay from this unbound state occurs rapidly, so the overall half-life is characteristic of the β decay half-life. This type of decay is energetically possible when the mass of the parent precursor (A, Z) is larger than the mass of the β⁺-p daughter (A-1, Z-2) plus the mass of the emitted proton. For nuclei that are across the proton drip-line (i.e. the proton separation energy S_p is negative), direct proton emission is a competing decay mode. When emission of a proton is energetically possible, the proton can tunnel through the Coulombic and centrifugal barriers. While the decay rate of β decay changes relatively slowly as the Q-value increases, emission of a proton from an unbound state changes rapidly as the proton energy increases. A similar process exists for β-delayed α emission (β-α). This is illustrated in figure 1 for a precusor nucleus that is slightly proton unbound.

Beta-delayed proton emission has been observed from every element from C (Z=6) to Lu (Z=71). For high Z nuclei above N=84 α emission becomes the dominant decay. Figures 2 and 3 graphically show the known β⁺-p as well as the deduced Q_εp values [2] for all known and predicted β⁺-p emitters.

Fig. 2 Known β⁺-p emitters and Q_εp values [2] for all known and predicted β⁺-p emitters from C to Sn.

The known nuclei that exhibit this decay mode are evaluated to give the recommended values for the nuclear properties of these nuclei. This includes branching ratios, and half-lives. In addition for those nuclei with known resolved proton transitions, proton energies, intensities, and the energies of the proton-emitting states are compiled. A list of experimental references for each β⁺-p precusor is also given in this evaluation. The last evaluations for β⁺ were published in 1989 [2] and 2008 [3], but these evaluations did not attempt to provide a complete list of emitted proton energies, intensities and populated levels as this evaluation does. There are a total of 200 known β⁺-delayed particle emitters, organized by their isospin projection are included in this evaluation. Roughly half of these nuclei have updated information that is not in current ENSDF evaluations.