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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" diam. x 10" 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.
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