Evaluation of the beta spectral shapes

Following an increased demand from users for a precise knowledge of beta spectra coupled with well-established uncertainties, LNHB is leading a study of beta spectral shapes at the metrological level. The evaluation of these beta spectra, especially to reach metrological uncertainties, requires new measurements and new calculations with high precision. The first part of this contribution will describe a new experimental set up for the measurement of beta energy spectra based on a silicon detector with a very thin entrance window and operating at liquid nitrogen temperatures under high-vacuum. Monte Carlo simulations were performed to optimize the detection chamber and source holder. Some preliminary results are included. The second part summarizes the calculations already implemented. These complex calculations are compared to some measurements of beta spectra obtained by using metallic magnetic calorimeters, and highlight the major contribution of the atomic screening and exchange effects at low energy