Adaptation of the Eberline AMS-4 Noble Gas Monitor for Use with Accelerator Produced Positron Emitting Gases
K. B. Welch
The Eberline AMS-4 is a real-time portable air monitor designed and factory calibrated for noble gases such as Kr-85 and Xe-133. Thomas Jefferson National Accelerator Facility operates a 4+ GeV electron accelerator designed for basic nuclear research, and a (currently) 50+ MeV electron accelerator which drives an IR Free electron laser. Gaseous positron emitting radioisotopes of oxygen, nitrogen, and carbon are produced during the operation of both facilities and the AMS-4 is used for periodic measurements at a series of locations to confirm calculations and/or to light signs when concentrations in excess of 0.1 DAC may be present in accessible areas. The calibration of the AMS-4 using a monte-carlo simulation of the detector response and a mock gas standard is discussed. 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. A real time computer based neutron monitoring station was developed for use in testing the integrity of concrete shielding used around the Accelerator and beam dumps at SLAC. This system is used to measure neutron skyshine and direct neutron flux passing through the shielding during high power beam operations.
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.