Field Comparison of Electret Ion Chambers and Thermoluminescent Dosimeters at Los Alamos National Laboratory (LANL)
D. H. Kraig, W. A. Treadway
Recent advances and commercialization of electret ion chamber (EIC) technology for photon measurements prompted us to consider EIC as a replacement for our thermoluminescent dosimeters (TLDs) system. After laboratory tests indicated that both systems gave adequate results for controlled exposures, we carried out field tests with paired TLDs and EICs in Los Alamos National Laboratory (LANL) technical areas and in public areas throughout 1998. At each sampling site, we deployed three TLDs and three EICs. The EICs were contained in air-tight jars, each of which was placed in a Tyvek envelope and hung about 1 m above the ground. The dosimeters were read and, if necessary, replaced every three months. For the first two quarters, the TLD results were statistically significantly higher than those of the EICs. Average exposures for TLDs at locations outside of LANL technical areas were 38 and 36 compared to EIC exposures of 33 mR for quarters 1 and 2, respectively. During quarter 3, the off-site EIC average (40 mR) was higher than the TLD average (34 mR). And, during quarter 4, the two systems were statistically the same; EIC=42, TLD=41 with a P-value of 0.61. We are still evaluating these differences and performing additional laboratory studies to determine causes. We also had monitors at our waste disposal area. The TLDs at some of these locations were much higher than their co-located EIC. The TLDs were over-responding by ~50% to the low-energy (60 keV) gamma associated with Am-241 whereas the EIC were responding accurately. We conclude that EICs are more accurate at a wide range of gamma energies but that there are some difficulties with using them in the field, primarily keeping them clean and sealed from radon.
This abstract was presented at the 33rd Annual Midyear Meeting, "Instrumentation, Measurements, and Electronic Dosimetry", Field Use Session, 1/30/2000 - 2/2/2000, held in Virginia Beach, VA.