SIGNAL PROCESSING, PERFORMANCE METRICS, AND LIFETIME OF A 6LiF:ZnS(Ag) NEUTRON DETECTOR WITH WLS FIBERS AND SIPM PHOTODETECTOR
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Date
2020-01-20
Type of Work
Department
Computer Science and Electrical Engineering
Program
Engineering, Electrical
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Distribution Rights granted to UMBC by the author.
This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
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Abstract
The NIST Center for Neutron Research studies the molecular and molecular-magnetic structure of materials using neutrons as a probing radiation. To further the speed of neutron scattering instrumentation, the NCNR has commissioned the Chromatic Analysis Neutron Diffractometer or Reflectometer (CANDOR) instrument. The CANDOR instrument has the potential to increase measurement speed by a factor of 1000. Detectors for neutron scattering instruments need to have high detection efficiency (90%), very high gamma rejection (10E-7), and operate at high count rates (10 kHz). Additionally for the CANDOR instrument, a neutron detector must be extremely thin (< 2mm) and use an alternative to the scarce helium-3 absorber isotope. The CANDOR neutron detector uses 6LiF:ZnS(Ag) neutron scintillator to generate light pulses which are transported to a silicon photomultiplier (SiPM) via wavelength shifting (WLS) fibers. An optimized detector design yields approximately 100 photons per neutron capture. Specially designed signal processing schemes were developed to meet the neutron scattering requirements in spite of the large noise background produced by the SiPM photodetector and low light neutron signals. The signal processing algorithms use pulse shape discrimination (PSD), noise rejection filters, and maintain high count rates by compensating for the slow fluorescence decay of ZnS(Ag) neutron scintillator. The Receiver Operator Characteristic (ROC) curve was used as a metric for gauging detection efficiency and gamma rejection, while arrival time statistics were used for gauging detector deadtime and doublecount fraction. Two detectors were characterized in detail; one detector very dim, one very bright. These two end cases encompass the performance of all detectors used in the CANDOR instrument. At a 10E-7 detection ratio for 662keV 137Cs gammas and a negligible doublecount fraction, the absolute efficiency ranged from 87% to 91%, and deadtime from 2'sto 4'sfor the dim and bright end cases respectively. The photopeak, neutron transmission, and detection efficiency were measured for more than 1000 CANDOR detectors as a fitness test before installation in the CANDOR instrument. Most detectors passed, but there are manufacturing variations that still need to be eliminated for high manufacturing yields. The CANDOR detectors can withstand approximately 10E+12 neutrons per cm2 irradiation before their performance degrades to an unacceptable level. This translates to more than 20 years of expected service life.