The use of Geant4 for simulations of a plastic ββ-detector and its application to efficiency calibration

Precise ββ-branching-ratio measurements are required in order to determine ft  -values as part of our program to test the Electroweak Standard Model via unitarity of the Cabibbo–Kobayashi–Maskawa matrix. For the measurements to be useful in this test, their precision must be close to 0.1%. In a branching-ratio measurement, we position the radioactive sample between a thin plastic scintillator used to detect ββ-particles, and a HPGe detector for γγ-rays. Both ββ singles and ββ–γγ coincidences are recorded. Although the branching ratio depends most strongly on the HPGe detector efficiency, it has some sensitivity to the energy dependence of the ββ-detector efficiency. We report here on a study of our ββ-detector response function, which used Monte Carlo calculations performed by the Geant4 toolkit. Results of the simulations are compared to measured ββ-spectra from several standard ββ-sources.