Research And Development Of Associated Alpha Particle Detector

In recent years,Associated Particle Imaging(API)is a nondestructive testing technology to detect the three-dimensional(isotopic)distribution of hidden explosives,drugs,special nuclear materials,and some other prohibited substances.The hardware of API system mainly consists of 1)a D-T neutron tube with associated alpha particle detector,2)a sample scanning rotating platform and 3)an array of neutron and/or γ detectors.The associated alpha particle detector is used to determine the time and direction of the tagged neutrons.A better time resolution of the said detector means better performance of the API system to reject scattered neutrons and background gammas,in another word,a better signal-to-noise ratio;meanwhile,a better spatial resolution of the detector is a key factor the API system to resolve the detected objects with a higher precision.Therefore,the research and development(R&D)of an associated alpha detector with high time and spatial resolution is one of the keys to the API system development.In recent decades,many different types of associated alpha particle detectors have been developed,including semiconductor to scintillator based ones.For the scintillator-based detectors,the plastic scintillator,ZnS:Ag,ZnO:Ga,and YAP:Ce have been studied.From the neutron tube manufacturing point of view and also the availability and price-performance ratio of these scintillation materials,the YAP:Ce scintillator is a better choice.Thus,the YAP:Ce scintillator is chosen as a main component to make the associated alpha detector reported in this paper.The detector consists of a 0.5mm-thick YAP:Ce scintillator,a quartz glass light guide,and a position sensitive photomultiplier tube(PSPMT)read out by a custom-made Anger-logic electronics.To optimize the thickness of the light guide for achieving a best spatial resolution,the developed detector with different thick light guide was characterized with an Am-241a source and spatial resolution testing targets.The experimental results show that the best achievable spatial resolution of the detector with a 5mm-thich quartz light guide is 0.7 mm.To obtain the timing resolution of the detector,a Na-22 isotopic source and a fast timing plastic scintillation detector are used.Limited by the low sampling rate(1GSa/s)of digitizer-based data acquisition system,a lower time resolution of 1.50ns is obtained.In fact,the time resolution is much better the one obtained experimentally.This result implies that a digitizer qith a higher sampling rate is needed to characterize the time resolution of the detector.