Low-background gamma-ray detector system for radiative capture reaction experiments with RI beams

A low-background gamma-ray detector of NaI(Tl) comprising a core surrounded by an annulus was developed for nuclear astrophysics experiments with radioactive ion beams, mostly for detailed studies of low-energy radiative capture reactions that involve proton and alpha particles. Monte Carlo simulations using EGS4 were performed to calculate the full-energy detection efficiency. The background was satisfactorily reduced as designed. The best S/N ratio for very low-event rate was obtained using the annulus in anticoincidence with the core. A consideration of an experiment of astrophysical interest, the 19Ne(p,γ)20Na reaction, is also briefly discussed.