ABSTRACT
This work presents two experimental methods for the evaluation of 220Rn homogeneity in calibration chambers. The first method is based on LSC of the 220Rn decay products captured in silica aerogel. The second method is based on application of solid state nuclear track detectors facing the air of the calibration chambers. The performances of the two methods are evaluated by dedicated experiments. The repeatability of the LSC-based method, estimated as relative standard deviation of the LSC measurements of ten silica aerogel samplers exposed under the same conditions is found to be 1.6%. Both methods are applied to study thoron homogeneity in the commercially available 50 L AlphaGuard emanation and calibration container, which was empty and its fan was turned on. It was found that the 220Rn distribution in this case is homogeneous within 10%. Both methods are also applied to test the thoron homogeneity in the BACCARA chamber at IRSN during a thoron calibration exercise. The results show that, at the centre of the chamber where the inputs of the sampling systems of the radon/thoron detectors were put close to each other, the thoron inhomogeneity is less than 10%. However, regions of higher thoron concentrations are clearly identified near the walls and the upper part of the chamber, with 220Rn concentrations being up to 60% higher compared to the concentration at the reference point. These results highlight the importance of the control and assessment of thoron homogeneity in thoron calibrations and in the cases when radon monitors are checked for thoron influence.
ABSTRACT
This work presents a comparison of two different logics for imposing extending-type dead-time in TDCR measurements: the common dead-time (CDT) and the individual dead-time (IDT) counting logics. The CDT is implemented in the widely used MAC3 TDCR counting module and the IDT was recently implemented in the nanoTDCR counting device. The performance of the two counting algorithms is evaluated by three experimental setups and a dedicated Monte Carlo (MC) code for the simulation of realistic TDCR events. An excellent agreement is observed between the two counting logics for measurements of the pure ß-emitting radionuclides 3H, 14C, 63Ni and 90Sr/90Y for which the relative deviations in measured activities are in all cases less than 0.27%. For the measured 222Rn sources, we observe relative deviations up to 0.25% between the logical sum of double coincidences counting rates obtained with the two counting logics. The differences are in the double coincidence counting rate estimates which propagate to the estimates of the activity. Excellent agreement was observed between the CDT and IDT in MC simulated measurements of 3H where relative deviations are less than 0.24% for activities up to 70â¯kBq. The IDT counting algorithm seems to have an advantage in this particular case as it results in the same counting rates and calculated activities but with a significant reduction in the double coincidences dead-time.
ABSTRACT
This work presents a method for measuring the depth distribution of 222Rn activity in soil gas. The method is based on the capacity of polycarbonates to absorb 222Rn and on the possibility of performing sensitive measurements of 222Rn absorbed by the polycarbonates via liquid scintillation counting (LSC). The method is the following: cylindrical holes are drilled along a metal rod and Makrofol® N polycarbonate foils enclosed in polyethylene envelopes are placed in each hole. The rod is driven into the soil and kept for a certain time. As long as the rod is in the soil, the polycarbonate foils are exposed to the 222Rn concentration at their depth. At the end of the exposure the rod is pulled out and the foils are transferred to liquid scintillation (LS) vials filled with liquid scintillator. The 222Rn absorbed in the foils is then measured with a LS analyzer. The rod with the polycarbonate foils acts as a passive probe which senses the 222Rn concentration at different depths beneath the ground surface. The achievable minimum detectable 222Rn activity concentration with the equipment and conditions used in this study is around 12.5 kBq/m3. It can easily be lowered below 1 kBq/m3 if larger foils and low-background LS analyzers are used. Since the method does not require air sampling the depth distribution of 222Rn in the soil is unperturbed by the sampling. The spatial distribution and the maximum measurement depth are set by the distance between the holes and the depth to which the rod can be fixed into the ground. Results from in situ applications of the method in terrains with high 222Rn in soil-gas are reported, which demonstrate the feasibility and the usefulness of the proposed approach.