Stephen D. Schery

Modeling radon transport in dry, cracked soil

A two-dimensional finite element code was used to investigate the effect of changes in surface air pressure on radon flux from soil with parallel, partially penetrating cracks. A sensitivity analysis investigates the effects of various crack dimensions, soil characteristics, and surface air pressure on radon flux from the soil surface to the atmosphere. Simulation results indicate that radon flux is most sensitive to soil properties; the diffusion coefficient is most important, followed by permeability and porosity. Radon flux is also sensitive to changes in barometric pressure, which cause variations in radon flux above and below the average diffusive flux. Sinusoidal variations in barometric pressure cause a net increase in the average radon flux from the soil, because increases in flux during periods of decreasing pressure are greater than the decreases in flux during periods of decreasing pressure of equal magnitude. Cracks were found to significantly increase radon flux from soils of low permeability. 33 refs. 19 figs., 1 tab.

A cyclotron time-of-flight facility☆

Abstract A description of the neutron time-of-flight facility at the University of Colorado 30 MeV isochronous cyclotron is given. The cyclotron is provided with a gated ion source to select one out of a predetermined number of rf cycles to accelerate ions. The system incorporates a beam swinger to rotate the direction of the incoming beam relative to a fixed flight path. Three detectors separated by 8° are used for a flight path of 9.5 m while an array of seven detectors are used for a single angle for a flight path of 30 m. The angular range available is 10°–160°. Beams of protons, deuterons and 3He are available with overall time resolution of 0.8–1.2 ns depending on the detailed circumstances. Examples of spectra from (p, n) and (3He, n) reactions are given.

Exhalation of radon and thoron: the question of the effect of thermal gradients in soil

Abstract Experimental measurements show no evidence for diurnal variation in exhalation of radon and thoron from soil due to convection induced by thermal gradients in the top few decimeters of soil as suggested by several authors. Estimates based on convective calculations indicate that even in the unlikely event of either vertical temperature gradients large enough to cause vertical instability, or horizontal gradients sufficient to cause significant convection, any effect would be too small to be detected. These same calculations suggest that it is difficult to conceive of cases involving typical thermal gradients in unfractured porous media such as soil where thermally induced convection would play an important role in transport of radon or thoron.

Thoron in the Environment

Thoron (220Rn), an isotope of the radon family, is produced in the earth’s crust at a rate comparable to that of common radon (222Rn). Thoron’s average activity concentration in soil gas and ground-level outside air is comparable to that of radon. Recent data from Europe and the United States indicate that in terms of the energy of the alpha particle decays of thoron’s progeny, its concentration in indoor air is significant, typically about half that due to radon progeny. This paper reviews current knowledge about thoron and its progeny in the outdoor and indoor environments and discusses issues involved in assessing whether or not it is a significant indoor pollutant.

Soil as a source of indoor 220Rn.

Two suggestions for sources of indoor 220Rn (thoron) have appeared in the literature: 1) building materials and outside air, and 2) soil beneath the house. Due to the difficulty of 220Rn measurement and limited data, both suggestions lack sufficient supporting evidence. We have investigated sources of indoor 220Rn in seven occupied houses in northern New Mexico, U.S. A two-filter system was used to measure indoor 220Rn levels continuously, and 220Rn progeny were measured with single filters and specialized alpha-track detectors. The amount of 220Rn entry from soil was curtailed by cutting off soil gas flow to the indoor air with subfloor depressurization mitigation systems. Four of the houses showed significant reductions in 220Rn with mitigation systems on. The average effect for these houses was to reduce indoor 220Rn levels by 70%. The other three houses had no clear reductions but in one of these houses, the mitigation system was not effective for stopping soil gas flow. Our results provide some of the most clear evidence to date supporting soil as an important source of indoor 220Rn.

The (p, n) reaction to the isobaric analogue state of high-Z elements at 25.8 MeV

Abstract Angular distributions have been measured for the (p, n) reaction to the isobaric analogue state of the targets 197Au, 206Pb, 207Pb, 208Pb, 209Bi, and 232Th at a proton energy of 25.8 MeV. Results of distorted wave Born approximation calculations indicated that angular distributions were predicted well by both real and complex form factors, although there was less sensitivity to an imaginary surface term than found in previous studies for lower-Z elements. An isospin correction to parameters for proton elastic scattering as determined from (p, n) quasi-elastic scattering was used to predict neutron elastic scattering in an application of the isospin model due to Lane. Although the isospin correction appeared necessary, it was not sufficient to provide adequate agreement with neutron elastic scattering data. A microscopic interpretation of the form factor provided a model in which angular distributions could be predicted well by a real form factor involving only two free parameters. This model was also used to determine the ratio of the rms radii of the neutron matter distribution to the proton matter distribution in 208Pb and gave the result 〈r n 2 〉 1 2 /〈r p 2 〉 1 2 = 1.07±0.02 .

A two‐particle‐size model and measurements of radon progeny near the Earth's surface

Measurements of radon progeny in the attached-to-aerosol and unattached-to-aerosol states were made in central New Mexico. Simultaneous measurements of attached and unattached progeny at 0.2 and 2 m were carried out over a range of meteorological and terrain conditions. The ratio of the average progeny concentrations at 2.2 to 0.2 m was 1.06 for total progeny and 1.35 for unattached progeny, indicating a net downward flux, with the unattached progeny dominating removal to the Earths surface. Progeny/parent activity ratios greater than 1 were clearly detected (for example, at 0.2 m, the average 214Pb/218Po ratio was 1.43 ± 0.10), providing partial support for some previous observations. A two-particle-size model for radon progeny is able to account for the observed gradients, progeny/parent activity ratios greater than 1, and some trends in the experimental measurements as a function of meteorological conditions.

Relaxed Eddy Accumulator for Flux Measurement of Nanometer-Size Particles

ABSTRACT An investigation of the relaxed eddy accumulation (REA) technique to measure the flux of ultrafine (∼1 nm in diameter) aerosol particles using unattached radon progeny as a tracer and the construction of a prototype system based on the REA principle is reported. The system consisted of a sonic anemometer with a response frequency of 21 Hz, three screen/filter holders, a custom-built electronic circuit to control three electromagnetic inlet valves for sampling the up-, down- and neutral vertical winds, a high-capacity air blower and a portable PC. A 635-wire mesh screen/fiberglass filter combination was used in each intake to provide a separate measure of the unattached-to-aerosol and attached-to-aerosol radon progeny. The 9 cm-diameter 635 mesh screen, combined with an air flow rate of 230 L min−1, resulted in 50% penetration for 2.7 nm-diameter particles. Corrections for a system response delay of 125 ms and the screen collection and alpha counting efficiencies were incorporated into the flux ca...

Thoron and radon fluxes from the island of Hawaii

The accumulator method has been used to determine the flux of thoron and radon from 42 varied sites in Hawaii. This is the first part, aimed at measuring source terms, of a two-part study of the use of the decay product of thoron, Pb-212, as an atmospheric tracer. It was of particular interest to scientists at the baseline atmospheric sampling laboratory on Mauna Loa to know the origin of Pb-212 in their air samples. The observed average flux from lowland soils was 173 mBq m−2s−1 (4.7 pCi m−2s−1) on the wet eastern side of the island and 854 mBq m−2s−1 (23 pCi m−2s−1) on the west. Barren lava yielded 20 mBq m−2s−1, which is thought sufficient to contribute a significant amount of Pb-212 to the air stream after a path of a few miles over lava. A comparison of the radon flux measurements with a previous survey indicated that there are seasonal variations of radon and thoron fluxes.

Understanding Radioactive Aerosols and Their Measurement

Preface. 1. Radioactivity and Aerosols: An Introduction. 2. Radioactivity and the Interaction of Nuclear Radiation with Matter. 3. Measurement of Nuclear Radiation and Radioactivity. 4. Physical Behavior of Aerosol Particles. 5. Aerosol Measurement. 6. Radioactive Aerosols: Atmospheric Sources and Indoor Models. 7. Radioactive Aerosols: Tracer Applications and Outdoor Transport. Appendix A - Units of Measurement for Radioactivity and Aerosols. Appendix B - Explanation of Symbols. Cited References. Index.