Shell-model study of the 4th- and 6th-forbidden β -decay branches of Ca 48

The highly forbidden β- decay of Ca48 is reexamined by performing shell-model calculations with the GXPF1A effective interaction. We examine the three available decay branches to the lowest 6+, 5+, and 4+ states of 48Sc, and extract a theoretical half-life of T1/2β=5.2-1.3+1.7×1020gA-2 yr for the β- decay, where gA is the value of the axial-vector coupling constant. The current half-life estimate suggests stronger competition between the single-β-decay and double-β-decay branches of Ca48 than previously expected on theoretical grounds.