New nonlinearity calibration method for 3-D position-sensitive CdZnTe detectors

Nonlinearity in the output signals for the energy deposited by gamma-ray interactions can seriously degrade the energy resolution of a pixellated detector when the signals from more than one pixel are summed to get the total deposited energy. Such nonlinearity can be calibrated on a pixel-by-pixel basis by using multiple gamma-ray check sources of various known energy photopeaks for gamma-ray energy below 1.5 MeV. However such method becomes less practical above 1.5 MeV since only a few commercially available check sources provide a few prominent photopeaks between 1.5 MeV and 3.0 MeV and it takes a long time to get statistically enough counts in these high-energy peaks for each pixel. The nonlinearity in the response of a detector system consists of two parts - one from the nonlinearity of the readout electronics (pre-amp, shaping-amp, peak-hold, etc), the other from the nonlinearity of the detector (charge generation, charge transportation and charge collection). This paper will characterize the nonlinearity of 3-D position-sensitive CdZnTe detectors by using on-chip test pulse injection to separate the nonlinearity of the readout electronics and the detector. The nonlinearity of the detector can be much better modeled after deducting the nonlinearity of the electronics.