The evaluation of thermally stimulated exoelectron emission detectors and electret ionization chambers for the purpose of measuring tritium on surfaces

Currently, there are two accepted means of characterizing tritium surface contamination. Gas flow proportional counters meastire fixed and removable tritium and are difficult to operate in confined spaces. Removable tritium is measured with smears which are counted on a liquid scintillation system. The focus of this research is the evaluation of two types of small, solid state detectors for in situ measurements of total tritium contamination (fixed and removable). One type of detector is the Thermally Stimulated Exoelectron Emission (TSEE) detector. The other type is the Electret Ionization Chamber (EIC). Both types of detectors were studied for linearity and reproducibility using a solid, 100 cm² tritium source (NIST traceable). Three variations of TSEE detectors were evaluated: Thin Film BeO, Ceramic BeO (Thermalox 995), and Carbon-doped Anion Deficient Al₂O₃. Only Thin Film BeO responded in a linear and reproducible fashion which had efficiencies (ratio of exoelectrons to incident beta particles) between 0.22 and 0.65 for one hour exposures. EICs also demonstrated a linear and reproducible response (2.55 V/hr of exposure). It was determined that an EIC response of 1 V/hr corresponds to an activity of approximately 79,000 dpm/100 cm². Strong correlations existed in the laboratory environment between both detector types and the stated surface emission rate of the source. Both BeO Thin Film TSEE detectors and EICs were tested in tritium contaminated areas at the Oak Ridge National Laboratory (ORNL) and at the Savannah River Site (SRS). Problems encountered within these facilities severely affected contamination estimates made with TSEE and EIC detectors. At ORNL, tritium surface contamination estimates were as follows: TSEE -1,200 to 5,900,000 dpm/100 cm²; EIC - 37,000 to 197,000 dpm/100 cm² At SRS, tritium surface contamination estimates were as follows: TSEE - 1,500 to 27,000 dpm/100 cm²; EIC - 50,000 to 360,000 dpm/100 cm².