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

Precise β-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-Moskawa 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 β-particles, and a HPGe detector for γ-rays. Both β singles and β-γ 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 β-detector efficiency. We report here on a study of our β-detector response function, which used Monte Carlo calculations performed by the Geant4 toolkit. Results of the simulations are compared to measured β-spectra from several standard β-sources.