From Event: SPIE Optical Engineering + Applications, 2016
A pulsed neutron source is used to interrogate a target, producing secondary gammas and neutrons. In order to make
good use of the relatively small number of gamma rays that emerge from the system after the neutron flash, our detector
system must be both efficient in converting gamma rays to a detectable electronic signal and reasonably large in volume.
Isotropic gamma rays are emitted from the target. These signals are converted to light within a large chamber of a liquid
scintillator. To provide adequate time-of-flight separation between the gamma and neutron signals, the liquid scintillator
is placed meters away from the target under interrogation. An acrylic PMMA (polymethyl methacrylate) light guide
directs the emission light from the chamber into a 5-inch-diameter photomultiplier tube. However, this PMMA light
guide produces a time delay for much of the light.
Illumination design programs count rays traced from the source to a receiver. By including the index of refraction of the
different materials that the rays pass through, the optical power at the receiver is calculated. An illumination design
program can be used to optimize the optical material geometries to maximize the ray count and/or the receiver power. A
macro was written to collect the optical path lengths of the rays and import them into a spreadsheet, where histograms of
the time histories of the rays are plotted. This method allows optimization on the time response of different optical
detector systems. One liquid scintillator chamber has been filled with a grid of reflective plates to improve its time
response. Cylindrical detector geometries are more efficient.
Robert M. Malone, Robert A. Buckles, Anemarie DeYoung, Irene Garza, Daniel K. Frayer, Morris I. Kaufman, George L. Morgan, Andrew W. Obst, Robert S. Rundberg, Jim Tinsley, Tom B. Waltman, and Vincent W. Yuan, "Improving the time response of a gamma/neutron liquid detector," Proc. SPIE 9969, Radiation Detectors: Systems and Applications XVII, 99690D (Presented at SPIE Optical Engineering + Applications: September 01, 2016; Published: 12 September 2016); https://doi.org/10.1117/12.2238957.
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