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

Abstract

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.Key words: Precise β-branching-ratio measurements; the Electroweak Standard Model; unitarity of the Cabibbo-Kobayashi-Moskawa matrix; Monte Carlo simulations; Geant4 efficiency calculation for plastic β-detector.

A GEANT4 Monte-Carlo simulation code for precision spectroscopy

“…Good agreement with experimental data at the level of a few percent was obtained in all cases. Recently, GEANT4 simulations were also performed for scintillation detectors, leading to similar conclusions (25).…”

The β Decay of 38Ca: Sensitive Test of Isospin Symmetry-Breaking Corrections from Mirror Superallowed 0+ → 0+ Transitions

“…For the 1568-keV γ ray, the peak efficiency is ǫ γi = 0.1777(4)%. The relative efficiency of the plastic scintillator has been determined as a function of β energy by Monte Carlo calculations, which have been checked by comparison with measurements on sources that emit both betas and conversion electrons [12]. For the β-transition feeding the 1568-keV γ ray, the ratio is ǫ β /ǫ βi = 1.0038 (4).…”

Kinematic sensitivity to the Fierz term ofβ-decay differential spectra

“…For example, atomic effects due to screening are known to be large for low energy β particles from transitions in medium and heavy mass nuclei [39] and require therefore particular attention in precision measurements. On the instrumental side, the effects of scattering of electrons from matter in the setup and their back-scattering from detector surfaces also increases toward lower energies and the description of the processes with current simulation tools [40,41] is not yet sufficiently accurate for competitive measurements of the β spectrum shape. These two criteria tend also to disfavor transitions with small endpoint energies in the selection of candidates.…”