Pulse shape discrimination techniques for correcting the effects of radiation damage on germanium detectors

Abstract

For germanium detectors (GeDs), which provide the highest energy resolution for studying gamma ray line features from astrophysical sources, exposure to energetic particles in space leads to radiation damage. Trapping centers created in the GeDs, preferentially hole traps, reduce the efficiency of the transport of charge carriers and thereby degrade the energy resolution and line efficiency. In addition, the trapping may affect the performance of Pulse Shape Discrimination (PSD) techniques used in background reduction. We present here computer simulations of photon interactions and charge transport in a reverse-electrode, closed-end coaxial GeD. These simulations show that radiation damage does not significantly alter the shape of the current pulses, rendering the effect an PSD performance negligible. Furthermore, the simulations show that with PSD, significant improvements in the energy resolution of radiation damaged detectors can be obtained by applying a hole trapping correction to the energy measured by the detector PSD provides the sizes and locations of the two largest energy depositions for photons that stop within the GeDs. For moderately damaged detectors (mean hole trapping length /spl lambda//sub h/=200 cm, which is equivalent to cosmic ray irradiation of /spl sim/2 years), correcting for the trapping suffered by these two depositions provides almost complete recovery of the line shape and sensitivity: undamaged resolution and relative sensitivity (1.58 keV FWHM, 1.0), damaged (2.10 keV, 0.69), corrected (1.75 keV, 0.93). Even for severely damaged detectors (/spl lambda//sub h/=50 cm), a marked improvement is obtained. These improvements translate directly into an increase in sensitivity for the detection of weak lines.