Multi-element Neutron Energy Spectrometer

In the area of nuclear radiological emergency response and preparedness applications, interest in neutron detection stems from several factors. Unlike gamma rays, which are abundant in nature and present serious difficulties in differentiating a signal from a changing background, whose values are location specific, neutrons are rare and nearly homogenous in spatial distribution. Additionally, many special nuclear materials (SNM) emit neutrons either directly by spontaneous fission or produce neutrons indirectly through (α, n) reactions in nearby light elements. Also of importance in detection scenarios is the fact that neutrons are not easily attenuated. Typically neutron detection is done by simply counting the low energy thermal neutrons by employing pressurized helium tubes operated at high voltages. Not much emphasis is put on determining the energy of the incident neutrons. However, critical information can be obtained by analyzing the neutron energy given off from radioactive materials. In the detection of an SNM, neutron energy information from an unknown source can be of paramount importance. We have modeled, designed, and prototyped multi-element neutron energy spectrometers that contain three to five pressurized helium tubes of dimensions 2-inch diam. x 10-inch in length. Each individual helium tube has a specific amount of high density plastic neutron moderators to slow down the incident energetic neutrons to an accurately estimated energy. A typical spectrometer is a set of moderator cylinders surrounding detectors that have high efficiency for detecting thermal neutrons. The larger the moderator, the higher the energy of incident neutrons for which the detector assembly has matched detection efficiency. If all the detectors are exposed to the same radiation field and the efficiency as a function of energy (response function) of each of the detectors is known, the neutron energy spectrum can be determined from the detector count rates. Monte Carlo simulation results of response function calculations for different arrays of helium tubes with varying amount of moderators will be shown. Experimental evidence of effectiveness of a set of moderated helium tubes to measure the hardness of the incident neutrons will be demonstrated.