Energy calibration method for an all-digital high resolution gamma-ray spectroscopy

Digitizing scintillation pulses with the Multi-voltage threshold (MVT) method shows great potentials for building all-digital positron emission tomography (PET). In this paper, we introduce the MVT method into gamma-ray spectroscopy. An accurate scintillation pulse model used in pulse fitting is especially important to get precise energy when implementing the MVT in gamma-ray spectroscopy, since a wide-range energy needs to be measured with limited MVT samples. However, accurately model the scintillation pulse is a not trivial work, as the physical model of scintillation pulse is not only determined by scintillator and photo-electric device but also the parasitic capacitance and inductance of the read out circuits. The unideal scintillation pulse model adopted in the MVT degrades the spectral accuracy and the energy resolution. To address this problem, an energy calibration method is proposed in this paper. By measuring the energy of scintillation pulses with different pulse amplitudes with high sample rate ADC and the MVT respectively, a look-up table is generated by dividing the energy obtained in the ADC to that in the MVT. The look-up table is used as calibration coefficient to correct the energy in the all-digital data acquisition system (DAQ) based on the MVT. Our preliminary results show that with the proposed energy calibration method, the peak-to-valley ratio in the Cs-137 energy spectrum is increased by about 1.5 times and the energy resolution is improved from 6.1% to 3.8% at 662 keV with LaBr3/PMT detector, which is equivalent to that of a 5 GSPS ADC.