Year
2011
Abstract
Basically all commercial neutron-counting systems employ He-3 tubes for the high-efficiency detection of thermal neutrons. These systems are highly insensitive to gamma rays (~1/1e6 sensitivity), thus another system (typically plastic scintillator) is used to detect gamma rays in mixed neutron/gamma-ray fields (portal monitors for instance). However, due to a large number of He-3 systems deployed in the field and the decrease in He-3 production, there is currently a severe shortage of He-3 gas. Therefore, novel neutron detection systems are needed, especially because a large number of detection systems are needed to meet current security demands. One class of detectors that has been explored is capture-gated scintillation detectors which consist of a standard scintillation detector (plastic or liquid) that has been modified to include a neutron capturing isotope. The capture-gated neutron-spectroscopy principle is based on detecting scattering-capture events. Creation of a scattering pulse results in a relatively large neutron-capture probability due to the previous energy reduction of the incoming neutron in the scattering collision(s). The scattering neutron pulse from the scintillator is accepted only if a subsequent capture neutron pulse is detected that is reasonably close in time (typically of several microseconds). Also, the amplitude of the scattering pulse can be used to estimate the incident neutron energy, because the amplitude is related to the initial neutron energy. Consequently, the pulse height distribution measured with a capture-gated detector includes information on the energy spectrum of the incident neutrons. Various capture-gated organic scintillation detectors have been studied for neutron spectroscopy applications in the past. A novel capture-gated detector was developed at Brigham Young University. This detector consists of several layers of cadmium and plastic scintillator and this detector shows a promise for robust identification of SNM. In the paper, we compare measurement results for typical neutron sources (Cf-252, Pu-239/Be) to reveal the difference in the response of the detector to spontaneous and alpha-n sources (Cf-252 is used as a surrogate for Pu-240).