Plastic scintillator for fission neutron detection

Abstract

ABSTRACTThe difference in speed of neutrons (E—2 MeV) and gamma rays has been investigated as a mechanism for distinguishing neutron and gamma interactions in plastic scintillators. Successive scattering of neutrons in a scintillatorresults in pulses that are broader than those from gamma rays because of the longer time interval (. 3ns) between scatterings.However, the experimentally observed pulse widths for these neutrons appear to be insufficiently different from those ofgamma rays to permit discrimination on a pulse-by-pulse basis. An alternate technique using scintillators separated by an airgap has also been investigated using time-of-flight measurements. For this configuration the efficiency for neutron detectionhas been measured along with the interference from gamma rays when sources emitting both neutrons and gamma rays arepresent.Keywords: Plastic scintillator, fission neutron, neutron 1. INTRODUCTION Neutrons emitted during the fission process typically have energies of 0.5 to 2 MeV. These fast neutrons are often detectedby first slowing them down to thermal energies where they can react more readily in certain detectors, such as gas-filled tubescontaining 3He or BF3. Slowing of the neutrons is generally achieved by using a hydrogenous moderating material, suchas polyethylene, where successive collisions between the neutron and hydrogen atoms efficiently transfer energy away fromthe neutron. However, the slowing-down process requires time, and the information on the original energy of the neutron, itsdirection of travel, and the time of emission is lost. For many applications the detection of fast neutrons that retain thisinformation is highly desirable.Plastic scintillators offer one method for detecting neutrons. However, gamma rays can also interact in the plasticscintillator, and in mixed neutron and gamma-ray radiation fields, distinguishing neutrons from gamma-rays becomes thepreeminent task. Our work on neutron detection with plastic scintillators uses the difference in speed with whichneutrons and gamma rays travel as a means for distinguishing the two types of radiation. The relatively slow speed of even