Research Of Capture-gated Fast Neutron Detection Method

Since the special nuclear materials (SNM) were usually significant spontaneousfission neutron sources, neutron detection technology was very important inidentification of special nuclear materials. Traditionally,3He proportional counter (withmoderator) were widely used for neutron detection for its high neutron sensitivity andgamma rejection capability. However, the worldwide3He supply crisis had triggeredresearches of alternative neutron detection technologies. Here we present research of analternative fast neutron detector which was based on plastic scintillator.The detector was composed of many (20×20or40×40) detector cells. Eachdetector cell was a1cm×1cm×20cm cuboid plastic scintillator covered with thinneutron absorption material. Since the size of the detector cell was small enough, theplastic scintillator and the neutron absorption materials were approximatelyhomogeneously mixed. The whole detector volume was actually “repeatedly used”,instead of “shared”, by plastic scintillator and neutron absorber. In this case, themoderation (in plastic sintillator) and absorption processes (innatCd ornatGd) of fastneutrons were performed in the “same” space so that the volume competing betweenabsorber and moderator was avoided. As a result, the intrinsic fast neutron detectionefficiency increased from12.6%of the3He detector to22.3%of this detector. Throughconincidence measurement of signals of recoiled proton and neutron capture, gammarejection was also achieved.In simulation of MCNP, the detector was optimized for achieving a high intrinsicdetection efficiency. The optimization parameters were the height (H) of the detector,the width (W) of the scintillator, the neutron absorption mateial, the thickness (T) of theneutron absorption film and the number (N) of the detector cell. According to thesimulation results,natCd andnatGd were the best neutron absortion materials. Thedetector achieved a high intrinsic detection efficiency (22.3%for252Cf neutrons) whenH, W, T and N were respectively20cm,1cm0.01cm and1600.In experiment, a detector composed of20×20natCd covered detector cells wasused. An intrinsic detection efficiecncy of12.00±0.06%was achieved for252Cfneutrons. For a252Cf neutron (1841n/s) souce positioned11cm away, under remarkablely varied gamma background (a3.21×105Bq137Cs source positioned15cm,30cm and50cm away repectively), the neutron count rate was relative stable. Thedetector could also alarm the existence of neutron and gamma sources at the same time.The false rate was respectively smaller than7%and1‰for the252Cf source (51cmaway,60s measurement) and the137Cs source (50cm away,1s measurment).