  Associated Particle Imaging (API) is a nuclear technique that allows for the
nondestructive determination of 3D isotopic distributions. The technique is
based on the detection of the alpha particles associated with the neutron
emitted in the deuterium-tritium (DT) fusion reaction, which provides
information regarding the direction and time of the emitted 14 MeV neutron.
Inelastic neutron scattering leads to characteristic gamma-ray emission from
certain isotopes, for example C-12, that can be correlated with the neutron
interaction location. An API system consisting of a sealed-type neutron
generator, gamma detectors, and a position-sensitive alpha detector is under
development for the nondestructive quantification of carbon distribution in
soils. This paper describes the design of the alpha detector, detector response
simulations, and first experimental results. The alpha detector consists of a
Yttrium Aluminum Perovskite (YAP) scintillator mounted on the inside of a
neutron generator tube. The scintillation light propagates through a sapphire
window to a position-sensitive photomultiplier tube mounted on the outside. The
16x16 output signals are connected through a resistive network for a 4-corner
readout. The four readout channels are amplified, filtered, and then digitized
for the calculation of the alpha position. First test results demonstrate that
an excellent alpha position resolution, better than the 1 mm FWHM required by
the application, can be achieved with this detector design.
