MCNP Analysis of a Phoswich Detector
A series of triple crystal phosphor sandwich detectors have been developed and constructed for testing at the University of Missouri-Columbia [1-7]. These detectors can simultaneously measure alpha, beta, and gamma radiation and utilize digital pulse shape discrimination to identify and separate radiation events coming from each of the separate phosphors. The research reported here uses Monte Carlo [8] software analysis to determine operating parameters for this detector system and optimizes its design for measuring trace amounts of alpha, beta and gamma-ray activity in effluent streams from nuclear waste cleanup processes. The previously designed, fabricated and tested phoswich detector [5] consisted of three scintillators placed on top of each other with a common diameter of 5.08 cm and viewed with a single photomultiplier tube. The scintillators (ZnS-0.00376 cm, CaF{sub 2}-0.254 cm and NaI-2.54 cm) interact preferentially with alpha, beta and gamma-ray radiation, respectively. This design allows preferential, but not exclusive, interaction of various radiations with specific layers. Taking into account and correcting for events that can occur in the ''wrong'' phosphor, this system was experimentally shown to have a 99% accuracy for properly identifying radiation coming from a mixed alpha/beta/gamma-ray source. In an attempt to better understand this system and provide design guidance for a detector system to be used in monitoring effluents from nuclear waste treatment facilities, this detector was modeled using MCNP [8]. This analysis [9] indicated that the thin ZnS layer adequately stops alpha particle energy, but greatly reduces beta detection efficiency to essentially zero at beta E{sub max} energies below 300 keV. The CaF{sub 2} layer, designed to keep any beta particle energy from entering the NaI detector results in an incorrect gamma-ray response that is approximately 23% of the NaI's response and is variable with energy. High energy beta events in the CaF{sub 2} can lead to Bremsstrahlung radiation being detected in the NaI. These errors must be corrected by system software.
- Research Organization:
- University of Missouri-Columbia (US)
- Sponsoring Organization:
- USDOE Office of Science and Technology (OST) (EM-50) (US)
- DOE Contract Number:
- FG07-01ID14118
- OSTI ID:
- 821939
- Report Number(s):
- Technical Paper No. 3; TRN: US0401327
- Resource Relation:
- Conference: Transactions of the American Nuclear Society, 86, Conference location not supplied, 06/10/2002--06/13/2002; Other Information: PBD: 12 Jun 2002
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
ALPHA PARTICLES
BETA DETECTION
BETA PARTICLES
BREMSSTRAHLUNG
GAMMA RADIATION
MONITORING
NAI DETECTORS
PHOSPHORS
PHOTOMULTIPLIERS
RADIATIONS
RADIOACTIVE WASTES
SHAPE
TESTING
TRACE AMOUNTS
RADIATION DETECTION
MCNP
PHOSWICH DETECTORS
NUCLEAR WASTE STREAM MONITORING