Characterization of a New Neutron Calibration Range Using a "SNOOPY" and MCNP
S. O. Schwahn, P. V. Degtiarenko
The Radiation Control Group at the Thomas Jefferson National Accelerator Facility recently moved their calibration range from one building to another. This move presented the opportunity to reevaluate existing calibration practices. The new calibration range is fully automated. The sources are moved automatically and photodetectors are used to verify position. Detectors are placed in fixtures and are moved on a conveyor system, which ensures a placement reproducibility of approximately one millimeter. Neutron measurements were made with a "Snoopy" at six points on the range and, as expected, they differed significantly form the manufacturer data on the source, becoming relatively worse with distance. It was assumed that the bulk of the difference between measurement and calculation was due to scattering. The geometry of the range, source type, and detector response was then modeled in MCNP. The data from the measurements were compared to the output of the MCNP calculation: they matched almost identically. A function was derived using variance reduction techniques, which describe the dose equivalent rate as a function of distance and significantly extended the functional range of calibration measurements.
This abstract was presented at the 33rd Annual Midyear Meeting, "Instrumentation, Measurements, and Electronic Dosimetry", Abstracts Session, 1/30/2000 - 2/2/2000, held in Virginia Beach, VA.