The mechanism of $\beta$-delayed two-proton emission in $^{31}$Ar

$\beta$-delayed two-proton decays, observed in several nuclei along the proton dripline, have only been studied in detail in very few cases. We have studied the decay of $^{31}$Ar, as it was suspected that this nucleus has several $\beta$-2p branches with quite large branching ratios. We report on two experiments done at ISOLDE-PSB (CERN) yielding to the most extensive investigation done so far on a $\beta$-delayed two-proton emitter. The analysis of the data have allow us to identify four $\beta$-2p branches connecting the IAS with ground and excited states in the two proton daughter $^{29}$P. About ten $\beta$-2p peaks have been assigned to decays from excited states in $^{31}$Cl at lower excitation energies than the IAS. In view of the energy and angular distribution of the individual protons the 2p-decay mechanisms are determined. (9 refs)