Estimating the γ-summing detector HECTOR efficiency using Geant4*

The High Efficiency Total Absorption Spectrometer (HECTOR) is a NaI(Tl) 4π summing detector designed to measure radiative capture cross sections relevant for astrophysical processes. In a radiative capture, an excited compound nucleus de-excites via a series of gamma rays which are absorbed by the detector's scintillating array. In order to extract the cross section, the summing efficiency of the detector needs to be well understood. To determine efficiency, experimental yields are typically compared to Geant4 simulations if the level scheme and gamma-branching ratios are known. However, this technique proves to be particularly challenging for heavy nuclei, for which these quantities are not well known.

 We present a statistical approach in which we correlate the efficiency with the average number of segments fired in an event “multiplicity” and the total energy of the gamma cascade.  Gamma cascades of 100 branching ratios are  generated from a uniform distribution and are then simulated using Geant4 to determine the efficiency. As a benchmark of the procedure, the spectroscopic factors of resonances in ²⁷Al(p,γ)²⁸Si measured recently with HECTOR will also be presented.