Two-proton radioactivity: the status and perspectives

Experimental nuclear structure data from various reactions and decays are compiled and evaluated for all known nuclides with mass number A=48 (S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni). For each nuclide, detailed evaluated nuclear structure information is presented for each individual reaction and decay, and the best values combining all available data are recommended for spectroscopic properties, such as level energies, half-lives, γ-ray energies and intensities, decay radiations. No excited states have been identified in ⁴⁸S, ⁴⁸Cl, ⁴⁸Co and ⁴⁸Ni, among which the first three even have no measured ground-state half-lives and decay modes. For ⁴⁸Fe, a level scheme with a sequence of excited states was established for the first time from a recent one-neutron removal measurement by 2021Ya33. Data for excited states in ⁴⁸Ar, ⁴⁸K and ⁴⁸Mn remain limited; no decay scheme has been measured for ⁴⁸Ar and ⁴⁸K yet, while the decay scheme for ⁴⁸Ni is incomplete due to unobserved levels. ⁴⁸Ti is the most studied nuclide through various reactions and decays, followed by ⁴⁸V, ⁴⁰Ca, ⁴⁸Sc, and ⁴⁸Cr, among which no decay event to ⁴⁸Sc has been observed and the decay schemes for ⁴⁸V and ⁴⁸Cr are considered as incomplete. This work supersedes the previous full evaluations of A=48: 2006Bu08, 1993Bu04, 1985Al14, 1978Be01.

This article aims at covering various low energy nuclear physics themes that can benefit from taking advantage of active targets and time projection chambers. They are naturally oriented towards the study of short-lived radioactive nuclei, for which high efficiency and thick targets are necessary to boost the luminosity of the experiments due to the weak intensity of the available beams. The use of active targets is particularly crucial when the recoil energy of the kinematically important particle is small and looses too much energy or does not emerge from a solid target.

The experimental data are evaluated for known nuclides of mass number A = 43 (Al,Si,P,S,Cl,Ar,K,Ca,Sc,Ti,V,Cr). Detailed evaluated level properties and related information are presented, including adopted values of level and γ–ray energies, decay data (energies, intensities and placement of radiations), and other spectroscopic data. This work supersedes earlier full evaluations of A = 43 published by 2001Ca24 and 1990En08 (also 1978En04, and 1998En04 update).

No excited states are known in ⁴³Al, ⁴³Si and ⁴³Cr. Only one excited state is known in ⁴³V which is the probable the Isobaric Analog State (IAS) of ⁴³Cr ground state. Information for ⁴³P, ⁴³S, ⁴³Cl, ⁴³Ar and ⁴³Ti is limited; there is either no decay data available or the decay schemes are incomplete in view of large Q values and known excitations much lower than allowed by Q values. The ⁴³K, ⁴³Ca and ⁴³Sc nuclides remain the most extensively studied from many different reactions and decays.

Nuclear spectroscopic information for experimentally investigated nuclides of mass 34 (Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca) has been evaluated. The principal sources of the Adopted Levels presented for nuclides close to the stability line are Endtʼs evaluations (1990En08, 1978En02). The data sets for reactions and decays, including all available gamma–ray data, are based mostly on the original literature. There are no data available for the excited states in ³⁴Ne and ³⁴Na. The existence of the ³⁴K and ³⁴Ca nuclides has been searched and reported in a secondary publication but no evidence was found for their detection. Both nuclides are possible candidates for one or two–proton emission. Only upper limits of half–lives have been proposed based on expected cross sections. Decay schemes for the β decays of ³⁴Ne, ³⁴Na, ³⁴Mg, and ³⁴Si are not known. Very little information is available for β–delayed neutron decays of ³⁵Na, ³⁵Mg and ³⁵Si which lead to population of levels in A = 34 nuclides. Neutron–rich nuclides in this mass region are relevant to “island of inversion”.

The 1995 Interim Evaluation for A = 48 (literature cutoff date of June 16, 1995) has been revised using experimental decay and reaction data received by April 14, 2006.

The 1991 evaluation of A = 67 (1991Bh06) has been revised using the experimental decay and reaction data received by the cutoff date noted below.