  The spectral shape of the electrons from the two first-forbidden unique beta-
decays of Ar-39 and Ar-42 were calculated for the first time to the
next-to-leading order. Especially the spectral shape of the Ar-39 decay can be
used to characterise this background component for dark matter searches based
on argon. Alternatively, due to the low thresholds of these experiments, the
spectral shape can be investigated over a wide energy range with high
statistics and thus allow a sensitive comparison with the theoretical
predictions. This might lead to interesting results for the ratio of the weak
vector and axial-vector constants in nuclei.

Nuclear Theory;High Energy Physics - Phenomenology;Nuclear Experiment