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We present significant results in recent advances in the measurement of neutron energy. Neutron
energy measurements are a small but significant part of radiological emergency response applications.
Mission critical information can be obtained by analyzing the neutron energy given off from
radioactive materials. In the case of searching for special nuclear materials, neutron energy information
from an unknown source can be of importance. At the Remote Sensing Laboratory (RSL) of National
Security Technologies, LLC, a series of materials, viz., liquid organic scintillator (LOS), Lithium
Gadolinium Borate (LGB) or Li6Gd(BO3)3 in a plastic matrix, a recently developed crystal of Cesium
Lithium Yttrium Chloride, Cs2LiYCl6: Ce (called CLYC)[1], and normal plastic scintillator (BC-408)
with 3He tubes have been used to study their effectiveness as a portable neutron energy spectrometer.
Comparisons illustrating the strengths of the various materials will be provided. Of these materials,
LGB offers the ability to tailor its response to the neutron spectrum by varying the isotopic
composition of the key constituents (Lithium, Gadolinium [Yttrium], and Boron). All three of the
constituent elements possess large neutron capture cross section isotopes for highly exothermic
reactions. These compounds of composition Li6Gd(Y)(BO3)3 can be activated by Cerium ions Ce3+.
CLYC, on the other hand, has a remarkable gamma response in addition to superb neutron
discrimination, comparable to that of Europium-doped Lithium Iodide (6LiI: Eu). Comparing these two
materials, CLYC has higher light output (4500 phe/MeV) than that from 6LiI: Eu and shows better
energy resolution for both gamma and neutron pulse heights. Using CLYC, gamma energy pulses can
be discriminated from the neutron signals by simple pulse height separation. For the cases of both LGB
and LOS, careful pulse shape discrimination is needed to separate the gamma energy signals from
neutron pulses. Both analog and digital methods have been applied to obtain a clear gamma and
neutron energy spectrum in a mixed radiation field. A waveform digitizer manufactured by Agilent
Technology Inc. has been successfully used to digitize the signal and separate the gamma and neutron
signals to obtain a high gamma rejection ratio. These results along with some interesting data from a
plastic (BC-408) and 3He dual gamma-neuron detector will be presented.
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