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<p>This work is an investigation into collimator designs for a deuterium-deuterium
(DD) neutron generator for an inexpensive and compact neutron imaging system that
can be implemented in a hospital. The envisioned application is for a spectroscopic
imaging technique called neutron stimulated emission computed tomography (NSECT).
</p><p>Previous NSECT studies have been performed using a Van-de-Graaff accelerator
at the Triangle Universities Nuclear Laboratory (TUNL) in Duke University. This facility
has provided invaluable research into the development of NSECT. To transition the
current imaging method into a clinically feasible system, there is a need for a high-intensity
fast neutron source that can produce collimated beams. The DD neutron generator from
Adelphi Technologies Inc. is being explored as a possible candidate to provide the
uncollimated neutrons. This DD generator is a compact source that produces 2.5 MeV
fast neutrons with intensities of 1012 n/s (4π). The neutron energy is sufficient
to excite most isotopes of interest in the body with the exception of carbon and oxygen.
However, a special collimator is needed to collimate the 4π neutron emission into
a narrow beam. This work describes the development and evaluation of a series of collimator
designs to collimate the DD generator for narrow beams suitable for NSECT imaging.</p><p>A
neutron collimator made of high-density polyethylene (HDPE) and lead was modeled and
simulated using the GEANT4 toolkit. The collimator was designed as a 52 x 52 x 52
cm3 HDPE block coupled with 1 cm lead shielding. Non-tapering (cylindrical) and tapering
(conical) opening designs were modeled into the collimator to permit passage of neutrons.
The shape, size, and geometry of the aperture were varied to assess the effects on
the collimated neutron beam. Parameters varied were: inlet diameter (1-5 cm), outlet
diameter (1-5 cm), aperture diameter (0.5-1.5 cm), and aperture placement (13-39 cm).
For each combination of collimator parameters, the spatial and energy distributions
of neutrons and gammas were tracked and analyzed to determine three performance parameters:
neutron beam-width, primary neutron flux, and the output quality. To evaluate these
parameters, the simulated neutron beams are then regenerated for a NSECT breast scan.
Scan involved a realistic breast lesion implanted into an anthropomorphic female phantom.</p><p>This
work indicates potential for collimating and shielding a DD neutron generator for
use in a clinical NSECT system. The proposed collimator designs produced a well-collimated
neutron beam that can be used for NSECT breast imaging. The aperture diameter showed
a strong correlation to the beam-width, where the collimated neutron beam-width was
about 10% larger than the physical aperture diameter. In addition, a collimator opening
consisting of a tapering inlet and cylindrical outlet allowed greater neutron throughput
when compared to a simple cylindrical opening. The tapering inlet design can allow
additional neutron throughput when the neck is placed farther from the source. On
the other hand, the tapering designs also decrease output quality (i.e. increase in
stray neutrons outside the primary collimated beam). All collimators are cataloged
in measures of beam-width, neutron flux, and output quality. For a particular NSECT
application, an optimal choice should be based on the collimator specifications listed
in this work.</p>
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