Study of the β decays of ³⁷Ca and ³⁶Ca

The β-delayed proton and γ-ray emission of ³⁷Ca and ³⁶Ca was studied at the projectile fragment separator at GSI Darmstadt. In the ³⁷Ca study, the β-delayed γ decay was observed for the first time, and the half-life was redetermined, under essentially background-free conditions, as 181.1 ± 1.0 ms. The surprisingly high Γ_γ/Γ_p values found for proton-unbound states in ³⁷K drastically reduce former discrepancies between the Gamow-Teller strength values B(GT) measured in the ³⁷Ca β decay and those deduced from the ³⁷Cl(p,n)³⁷Ar mirror reaction. Our data provide evidence, on a level of 1.6 standard deviations, for isospin-mixing of the lowest T = 3/2 level in ³⁷K. In the ³⁶Ca study, a complete β decay scheme, including the β-delayed proton emission into the ground and first excited states of ³⁵Ar as well as the β-delayed γ decay, was measured for the first time. The ³⁶Ca half-life was remeasured with improved accuracy to be 102 ± 2 ms. Our data exclude a 1⁺ assignment for two near-threshold states in ³⁶K that could be important resonances in ³⁵Ar(p,γ) reactions of astrophysical interest. The B(GT) distributions in ^37,36Ca decays are compared to results obtained from large-scale sd shell-model calculations.