KEYWORDS: Lead, Sensors, Silicon carbide, Gamma radiation, Cadmium, Signal detection, Diodes, Signal to noise ratio, Semiconductors, Detector development
Pulsed neutron interrogation methods for detection of Special Nuclear Materials are being developed. Fast prompt
neutrons from thermal neutron-induced fissions are detected in the time intervals following 100-μs neutron bursts from a
pulsed D-T neutron generator operating at 1000 pulses per second. Silicon Carbide semiconductor neutron detectors are
used to detect fission neutrons in the 30-840 μs time intervals following each 14-MeV D-T neutron pulse. Optimization
of the neutron detectors has led to dramatic reduction of detector background and improvement of the signal-to-noise
ratio for Special Nuclear Material detection. Detection of Special Nuclear Materials in the presence of lead, cadmium
and plywood shielding has been demonstrated. Generally, the introduction of shielding leads to short thermal neutron
die-away times of 100-200 μs or less. The pulsed neutron interrogation method developed allows detection of the
neutron signal even when the die-away time is less than 100 μs.
Current requirements of some Homeland Security active interrogation projects for the detection of Special Nuclear Material
(SNM) necessitate the development of faster inspection and acquisition capabilities. In order to do so, fast detectors which
can operate during and shortly after intense interrogation radiation flashes are being developed. Novel silicon carbide (SiC)
semiconductor Schottky diodes have been utilized as robust neutron and photon detectors in both pulsed photon and pulsed
neutron fields and are being integrated into active inspection environments to allow exploitation of both prompt and delayed
emissions. These detectors have demonstrated the capability of detecting both photon and neutron events during intense
photon flashes typical of an active inspection environment. Beyond the inherent insensitivity of SiC to gamma radiation,
fast digitization and processing has demonstrated that pulse shape discrimination (PSD) in combination with amplitude
discrimination can further suppress unwanted gamma signals and extract fast neutron signatures. Usable neutron signals
have been extracted from mixed radiation fields where the background has exceeded the signals of interest by >1000:1.
A new neutron interrogation technique for detection of concealed Special Nuclear Material (SNM) is
described. This technique is a combination of timing techniques from pulsed prompt gamma neutron activation analysis
with silicon carbide (SiC) semiconductor fast neutron detector technology. SiC detectors are a new class of radiation
detectors that are ultra-fast and capable of processing high count rates. SiC detectors can operate during and within
nanoseconds of the end of an intense neutron pulse, providing the ability to detect the prompt neutron emissions from
fission events produced by the neutrons in concealed SNM on a much faster pulsing time scale than has been achieved
by other techniques.
Neutron-induced fission neutrons in 235U have been observed in the time intervals between pulses of 14-MeV
neutrons from a deuterium-tritium electronic neutron generator. Initial measurements have emphasized the detection of
SNM using thermal-neutron induced fission. Neutron pulsing and time-sequenced neutron counts were carried out on a
hundreds of microseconds time scale, enabling the observation of prompt fission neutrons induced by the die-away of
thermal neutrons following the 14-MeV pulse. A discussion of pulsed prompt-neutron measurements and of SiC
detectors as well as initial measurement results will be presented.
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