  In the present work, we report extensive GEANT4 simulations in order to study
the dependence of sensitivity of GAGG:Ce scintillation crystal based detector
on thickness of the crystal. All the simulations are made considering a
thermalised Am-Be neutron source. The simulations are validated, qualitatively
and quantitatively, by comparing the simulated energy spectra and sensitivity
values with those obtained from experimental measurements carried out using two
different thicknesses of the crystal from our own experiment (0.5mm and 3mm)
and validated with three other thicknesses (0.01mm, 0.1 mm and 1 mm) from
literature. In this study, we define sensitivity of GAGG:Ce as the ratio of
area under 77 keV sum peak to 45 keV peak. The present studies clearly confirm
that, while it requires about 0.1 mm thickness for the GAGG:Ce crystal to fully
absorb thermal neutrons, it requires about 3 mm to fully absorb the thermal
neutron induced events. Further, we propose an equation, that can be used to
estimate the thickness of the GAGG:Ce crystal directly from the observed
sensitivity of the GAGG:Ce crystal. This equation could be very useful for the
neutron imaging community for medical and space applications, as well as for
manufactures of cameras meant for nuclear security purposes.
