Development of a TDC-based digital positron annihilation lifetime spectrometer

Positron Annihilation Lifetime (PAL) spectroscopyis widely applied in material and biomedical science. The conventional analog PAL spectrometer built by multiple Nuclear Instrumentation Modules gradually shows deficiencies in performance. Most digital PAL spectrometers strongly rely on the advanced Analog-to-Digital Converter (ADC) or oscilloscope with an ultra-high-speed sampling rate. In this paper, we developed a prototype system of the digital PAL spectrometer based on ordinary Time-to-Digital Converter (TDC) and ADC. Benefiting from the high-performance digitization devices and digital signal processing algorithms accomplished by Field-Programmable Gate Array (FPGA), the system achieves remarkable energy and lifetime resolution in a simple structure and at a much lower cost. Electronic test results show that the time resolution of the TDC reaches 67 ps and the Effective Number Of Bits of ADC is better than 10.77 bit. The energy resolution of 511 keV photopeak is 4.1% and the Full Width at Half Maximum of the lifetime spectrum reaches 210.6 ps when using Hamamatsu H3378-50 Photomultiplier Tubes with LaBr3:5% Ce3+ scintillators as detectors.