An optimized design of the cryogenic detector for the spectrum-shape method to study the 99Tc forbidden β decay.

The spectrum-shape method has been proposed to determine the effective value of weak coupling constants ratio of vector coupling constant (gV) and axial-vector coupling constant (gA) in the forbidden β decays. We use 99Tc which has a second non-unique forbidden β decay. The shape function strongly depends on the gV/gA in the variant nuclear-model frameworks and the spectrum. The end point energy (Q value) of 99Tc β decay is 293.5 keV, making it suitable for deposition in a thin gold foil absorber. We use cryogenic detector, metallic magnetic calorimeter is used to achieve good energy resolution. The detector's signal amplitude depends on the heat capacity, which directly influences the energy resolution. To enhance the detection efficiency, a perfect solid angle absorber (4π-absorber) is used to contain the radioactive source and full deposition of the beta particles in the absorber. This absorber is manufactured by dropping and drying 99Tc, which is dissolved in a nitric acid solution, onto a thin gold foil. After drying, it is covered with another gold foil as a sandwich-like structure and each layers are welded together. The simulation is conducted to achieve the total absorption of energy of particles prepared with various sizes (1 mm2, 4 mm2) and concentrations (2.5 Bq, 5 Bq). We will present manufacturing details of the source, the setup of detectors, and the preliminary results of the measurements. [ABSTRACT FROM AUTHOR]