Induced radioactive continuum background in the integral spectrometer (SPI) germanium detectors

This paper presents a detailed study of the β decay continuum background in a spaceborn Ge spectrometer (SPI) and methods for its reduction. In the nuclear γ-ray line energy range (10 keV, 20 MeV), much of the continuum background in the Ge spectrometers is due to the cosmic-ray generated protons and neutrons interacting in the detectors themselves. We have performed Monte Carlo simulations to calculate the neutron spectrum inside the BGO shield of the spectrometer SPI, thus allowing the calculation of the continuum background induced by the β− and β+ decays of the Ge residuals nuclides. It is shown that if we use the Pulse Shape Discrimination (PSD) and energetic signatures in energy ranges well chosen to reduce the background, the main effect is an improvement of the sensitivity of SPI.