Effect of atmospheric temperature on underground radon: A laboratory experiment.

The effect of atmospheric temperature on underground radon flow was investigated in a customized climate-controlled laboratory (CCL) system, which enabled us to isolate the impact of ambient atmospheric temperature variations on underground radon transport. The soil thermal gradients that developed, following atmospheric warming, acted as the driving force for the diffusive radon flow, resulting in a decrease in the radon concentration along the experimental column setup at a rate of ∼70 Bq∙m-3 per oC∙m-1 (∼0.4% of the radon concentration). When the ambient temperature decreased, compared to the soil temperature, an air-soil temperature difference developed along the column, which acted as a driving force for radon to flow along the column and promptly increased the radon concentration at a rate of ∼140 Bq∙m-3 per oC (∼0.8% of the radon concentration). The overall radon concentration changes under the experimental conditions were up to 30%. The changes in the molecular diffusion coefficient in the experimental temperature range were ∼7%, with thermal diffusion as a possible additional mechanism contributing to radon transport due to temperature. The cyclic changes in ambient temperature in the forced conditions experiments were found to be directly correlated with underground radon oscillations. The same frequency for ambient temperature and radon concentration, along the experimental column in low frequency warming-cooling cycles (i.e., 4-8 days), was found. This good correlation was lost at higher frequencies (two days or more), due to the asymmetrical response of radon to atmospheric warming and cooling. The results of this study explain some of the field observations in underground radon monitoring.

Monte Carlo modeling of scintillation detectors for continuous underground radon monitoring.

Nuclear simulation methods were applied to two systems that investigate radon transport within geological porous media: a) a laboratory system built to test, under controlled climate conditions, the effect of temperature on radon transport, and b) a field monitoring system comprising gamma and alpha detectors in an abandoned water well. The use of Monte Carlo simulations of NaI and BGO scintillation detectors in continuous underground radon measurements by gamma counting, to estimate the photon flux in the detector volume, is presented. The advantages of shielding side-view NaI detectors were demonstrated for a laboratory system containing ground phosphate rock, including avoiding high counting rates and reducing the effective source volume in radon transport studies. The gross gamma counting procedure was shown to result in a lower uncertainty than spectrometric measurement, by at least a factor of two, despite it being a simpler and more suitable procedure for field measurements. The calculation of simulated source volumes for a BGO detector in a borehole and the measurements in the field support the assumption that the gamma signal comes primarily from radon flowing in the bedrock's air-filled pores. As a practical outcome of this study, positioning the detector a few cm off-center from the borehole's axis increased the gamma counting efficiency; however, measurements in groundwater taken too close to the iron casing had a lower detection efficiency. The conversion factor from the scintillator signal to the radon activity concentration, for the laboratory system, was calculated. Monte Carlo simulations demonstrated the advantages of the gross counting procedures using gamma scintillation detectors in field underground high-frequency radon monitoring.

Detailed effects of particle size and surface area on 222Rn emanation of a phosphate rock.

The dependency of radon emanation on soil texture was investigated using the closed chamber method. Ground phosphate rock with a large specific surface area was analyzed, and the presence of inner pores, as well as a high degree of roughness and heterogeneity in the phosphate particles, was found. The average radon emanation of the dry phosphate was 0.145 ± 0.016. The emanation coefficient was highest (0.169 ± 0.019) for the smallest particles (<25 µm), decreasing to a constant value (0.091 ± 0.014) for the larger particles (>210 µm). The reduction rate followed an inverse power law. As expected, a linear dependence between the emanation coefficient and the specific surface area was found, being lower than predicted for the large specific surface area. This was most likely due to an increase in the embedding effect of radon atoms in adjacent grains separated by micropores. Results indicate that knowledge of grain radium distribution is crucial to making accurate emanation predictions.

Effectiveness of radioactive tungsten source in the prevention of restenosis in stented porcine coronary arteries.

PURPOSE: Intracoronary radiation has shown the potential to inhibit neointimal proliferation in porcine models of restenosis. The objective of this study was to determine whether intracoronary radiation using a new coiled wire of tungsten-188 ((188)W), a pure beta emitter (half-life 69.4 days) is safe. In addition, a dose of 0 Gy, 18 Gy, or 25 Gy prescribed to 2 mm from the center of the source and delivered intraluminally is sufficient to prevent restenosis and free from adverse effects. METHODS AND MATERIALS: Ten domestic swine underwent 13-mm stent implantation (SI) into two arteries, left anterior descending plus either the left circumflex or right coronary artery. After SI, a closed-end lumen radiation catheter was inserted to the treated artery and a 40-mm coiled (188)W source was manually delivered to cover the stented segment and its margins. A total of 20 arteries were randomized to treatment with a radiation dose of 0, 18 Gy, or 25 Gy delivered to 2 mm depth from the center of the source. Four weeks after the procedure, the swine underwent angiography and intravascular ultrasound using automated pullback at 0.5 mm/s. before being killed and the arteries perfusion fixed. Histopathologic and histomorphometric analyses were performed at 28 days after injury and radiation. RESULTS: Irradiation with (188)W at a dose of 25 Gy after SI significantly inhibited neointima formation (intimal area: 1.05 +/- 0.64 vs. 2.75 +/- 0.99 mm(2), p < 0.01) and at an 18 Gy dose of radiation (intimal area: 1.73 +/- 0.49 vs. 2.75 +/- 0.99 mm(2)), as compared to controls. One artery receiving 18 Gy and two arteries receiving 25 Gy were totally occluded at follow-up due to thrombus formation but no edge stenosis was observed in any of the irradiated arteries. CONCLUSIONS: Intracoronary radiation therapy using a new coiled wire of (188)W source delivered after SI appeared to be safe and well tolerated. The radiation doses demonstrated efficacy in reducing neointima formation in the porcine coronary stent injury model.