Higher order effects accompanying forbidden beta decay

We have investigated internal bremsstrahlung (IB) accompanying the 1u electron capture in 41Ca. In this case there is a 100% ground-state -to-ground-state transition with the decay energy Q = 421.3 keV and the half-life T1/2 = 105 y. By measuring the IB photons in coincidence with the potassium KX-rays we have selected the 1s component of the IB, i.e. the radiation emitted during K-capture. Then we studied the 2nu decay of 59Ni. It was pure ground-state-to-ground-state transition with the decay energy Q = 1073.6 keV and the half-life T1/2 = 7.5 x 104 y. We have measured the 1s component of the IB by means of coincidences with the cobalt KX-rays. The shape factor for this component extracted from experimental data is shown. In this order of forbiddenness the theoretical shape factors depend on the parameter connected with nuclear matrix elements. This parameter was adjusted to reproduce the shape of the spectrum and the resulting shape factors. (author)