Energy Calibration of MVT Digitizers in All-Digital Gamma Cameras

The multiple voltage threshold (MVT) digitizer implements data acquisition for the gamma camera by sampling pulses with all-digital electronics and recovering them with the prior pulse model. It has been employed in several nuclear science applications, such as positron emission tomography detectors, personal dosimeters, survey meters, and wastewater activity probes. In these applications, a wide dynamic range of energy is usually required to measure multiple energies accurately. However, the energy nonlinearity of the MVT digitizer constrains the dynamic range of energy. To calibrate the nonlinearity of digitizers in gamma cameras, we propose a novel method to acquire the mapping from digitizer energy to pulse energy by obtaining their scatter plot with a high-speed oscilloscope. To demonstrate our method, we use a gamma camera based on the silicon photomultiplier (SiPM) and an inorganic scintillator to detect gamma photons from a Na-22 radioactive source. We show that the mapping can be empirically modeled as the quadratic function for the MVT digitizer. We further perform the calibration on the MVT digitizer, showing that the average deviation between digitizer energy and pulse energy is reduced from 245 to 28 keV at 1274 keV. Moreover, we show the energy spectra obtained with different sequences of digitizer calibration and photoelectric converter calibration. The energy spectrum obtained with digitizer calibration and then photoelectric converter calibration shows the most minor distortion. This suggests that digitizer calibration should be performed before photoelectric converter calibration. It is reasonable to extend our method to other gamma cameras by simulating sampling patterns of corresponding digitizers. This provides a way to realize gamma spectrometry with higher accuracy and a wider dynamic range.