Development of a 4π detection system for the measurement of the shape of β spectra

The need of precise measurements of the energy distribution of emitted β particles has been addressed developing a specific detection system. An ultra-thin radioactive source, prepared on purpose, is enclosed between two silicon detectors in a compact geometry with a quasi-4π sr solid angle. The remaining distortions have been corrected with a dedicated deconvolution process based on mono-energetic Monte Carlo simulations that are used to build the response matrix. The β spectra from 14C and 204Tl decays have been measured and analyzed. Extracted maximum energies are fully consistent with the AME2020 recommended Q-values. A linear slope of a = −0.430 (37) MeV−1 that comes from the weak magnetism term in the 14C beta spectrum has been determined, in excellent agreement with the theoretical predictions. The measured range of the 204Tl beta spectrum has been extended down to 60 keV and a shape factor (dq2+λ2p2) has been extracted with d=1.075 (13).