Direct Penetrating Radiation Monitoring Systems: Technical Evaluation for Use at Area G, Los Alamos National Laboratory

Recent advances and commercialization of electret-ion-chamber (EIC) technology for photon measurements prompted us to consider EKs as a replacement for our TLD system. After laboratory tests indicated that both systems gave adequate results for controlled exposures, throughout 1998 we conducted field tests with paired TLDs and EICS, in LANL technical areas and in public areas. We had approximately 30 paired sampling sites at Area G. 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. At the sites outside Area G, the TLD readings for the first two quarters were statistically significantly higher than those of the EICS: group average exposures were 38 and 36, compared with 33 mR (both quarters) for the EICS; during quarter 3, the 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. At the Area G sites,we noticed that several of the TLDs gave much higher readings than their co-located EICS; we believe that the TLDs were over-responding by ∼50% to the low-energy (60-keV) gamma radiation associated with 241Am, whereas the EICS were responding accurately. We conclude that EICS are more accurate at a wide range of gamma energies and are preferable to TLDs in environments where a significant fraction of the photons are low energy