Search for virtual electron capture in the investigation of the internal bremsstrahlung from the decay of 59Ni

The internal bremsstrahlung spectrum from the non-unique second-forbidden pure electron capture decay of ⁵⁹Ni was studied in a limestone low-background chamber using a 20 cm³ Ge(Li) detector, searching for the effects of the virtual electron capture in the spectrum shape of internal bremsstrahlung. Comparing the experimental data with the Cutkosky, Zon-Rapoport and virtual capture theories, the best fit was found for the Zon-Rapoport theory and thus the virtual capture theory is not indispensable to the interpretation of the experiment. Some additional results are also obtained in the present study. Thus, the combination of the nuclear matrix elements λ has been determined, λ = 0.028 ± 0.006, as well as the total disintegration energy $\text{Q(}{}^{59}\text{Ni}\text{-}{}^{59}\text{Co}\text{) = 1075.1 ±1.3}\text{keV}$. The existence of a positive β-branch was proved with an estimated value of 1.05 × 10^?7 for the branching.