A: 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 ultrahigh-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 LaBr 3 :5% Ce 3+ scintillators as detectors.
The gamma-ray spectrometer (GRS) is a powerful tool to acquire energy spectra. For the traditional analog GRS based on commercial modules and the digital GRS based on ultrafast digitization, it is difficult to meet the requirements of high resolution and high throughput simultaneously, and the energy spectrum cannot be obtained in real-time. Besides, energy spectra experiments require the use of radioactive sources. Prolonged exposure to the sources will harm the human body. In this paper, we introduce a digital GRS based on an ordinary analog-to-digital converter (ADC). To avoid human body exposure to radiation in spectra tests, we integrated a digital-to-analog converter based random pulse generator (RPG) on the board to simulate the real nuclear pulses. The algorithms were accomplished by a field-programmable gate array (FPGA). The electronic test results show that the effective number of bits of ADC reaches 11.78 bit, and the spectrometer works well when the average frequency of the input pulse is under 3 MHz. The energy resolution of the 511 keV photopeak reaches 4.05% when using LaBr 3 :5% Ce 3+ scintillator and 22 Na source. The energy spectrum is obtained through FPGA in real-time and can be delivered to the personal computer via Ethernet or displayed on the local screen. The RPG can generate simulated nuclear pulses with Gaussian distributed energy. The mean and sigma of the Gaussian curve and the event rate of output pulses can be configured.
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