  Precise beta-branching-ratio measurements are required in order to determine
ft-values as a 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 beta-particles, and a HPGe detector for
gamma-rays. Both beta singles and beta-gamma 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
beta-detector efficiency. We report here on a study of our beta-detector
response function, which used Monte Carlo calculations performed by the Geant4
toolkit. Results of the simulations are compared to measured beta-spectra from
several standard beta-sources.
