Fig. (2) Spectrum of electrons and photons recorded in the GERDA experiment [56] with germanium detectors enriched in 76Ge (red spectrum with higher intensity) and non-enriched Ge detectors (blue spectrum with lower intensity). The continuous spectra correspond to the sum of energy of two electrons released in the 2νββ decay of 76Ge with the half-life of 1.5·1021 y. The low energy part of the spectrum is made mostly by β particles from the 39Ar decay and photon bremsstrahlung of these particles. Pure β emitters 39Ar (T½ = 269 y, Qβ = 565 keV) and 42Ar (T½ = 33 y, Qβ = 600 keV with daughter 42K) are produced via cosmogenic activation of atmospheric argon. The specific activity of 39Ar is 1 Bq kg-1. Spectral lines with higher energy are due to γ quanta emitted in a decay of natural radioisotopes. Vertical band at Qββ = 2039 keV with the bandwidth of ±20 keV denotes the region where the line from the 0νββ decay is expected. The background in this region is about 0.02 counts keV-1·kg-1· y-1.