Soils radium content

In U.S. EPA Office of Radiation Program s New House Evaluation Program (NEWHEP), two builders in the Denver area, two in Colorado Springs, and one in Southfield, Michigan, installed various radon-resistant features in houses during construction. A sampling of subsequent measurements of indoor radon, adjacent soil gas radon, and soil radium content is summarized in Table 31.6.36... 

As shown in Table I, radium content of surface soils not associated with U mining or milling varies by slightly more than an... 

Soil is a mixture of solid materials, air and, usually, water and organic matter. The radium content of soil often reflects that of the rocks from which the solid materials are derived by physical and chemical activity. The observed ranges are from 0 to 20 Bq kg for ultrabasic rocks (dunite) to 1 to 1835 Bq kg for igneous metamorphic rocks (gneiss) (Wollenberg, 1984). While these ranges are broader than those for measured for soils, the mean values for rocks, excluding alkali rocks, is consistent with the means observed for soils. 

The radium-228 content of fly ash has varied from 1.8 to 3.1 pCi/g (0.07 to 0.12 Bq/g) (Eisenbud and Petrow 1964). If it is assumed that the total radium content of fly ash is 5 pCi/g (0.19 Bq/g), and that 1% of the ash generated at all coal-fired power plants in the United States escapes into the atmosphere, then an order-of-magnitude estimate of the amount of radium released each year would be 2.2 Ci (81,000,000 kBq) (Roy et al. 1981). Eisenbud and Petrow (1964) estimated that a single 1000-megawatt coal-fired power plant will discharge about 28 mCi (1,037,000 kBq) of total radium per year. Radium-226 has been detected in soils in industrial regions at levels up to 8.1 pCi/g (0.30 Bq/g) (Jaworowski and Gryzbowska 1977). 

Radon ( Rn) measurements made by the ARCAS provide a simple, reliable, real-time indicator of the relative maritime or continental nature of the air over coastal or oceanic areas (8). With a half-life of 3.8 days, Rn originates from the decay of Ra, a member of the decay chain. At least 98% of Rn originates from land masses (9). The radon flux at the surface depends on the radium content of the soils and rocks, the permeability of the source materials, atmospheric pressure, soil moisture, and vegetative cover (10). Relatively abrupt changes in the radon concentration over the ocean usually indicate changes in air masses and the passage of frontal systems. 

If someone constructs a building over soil or rock that has a high radium content (or uses stone with a high radium content to build the foundation ), the radon gas can percolate through the basement and accumulate in the house. Couple this with the need to build more energy-efficient, well-insulated dwellings. 

Measurements of radon in soil are expressed in terms of levels in soil-gas. However, these measurements do not directly relate to rates of radon released to the atmosphere. Factors which affect radon soil-gas levels include radium content, soil porosity, moisture content, and density. Technically, measurement of soil-gas is difficult and there are few studies which report such data. 

As stated in Section 5.2.1, soil is the primary source of radon. As such, radon is not released to soil but is the result of radioactive decay of radium-226 within the soil. The radon concentration in the soil is a function of the radium concentration, the soil moisture content, the soil particle size, and the rate of exchange of air with the atmosphere (Hopke 1987). Hopke (1987) states that normal soil-gas radon measurements are in the range of 270 to 675 pCi radon-222/L of air (10,000 to 25,000 Bq/m However, levels exceeding 10,000 pCi radon-222/L of air (370,000 Bq/m ) have been documented. 

Because radon is a gas, its occurrence in soil is most appropriately referred to as its occurrence in "soil-gas," which is in the gas or water-filled space between individual particles of soil. Factors that affect radon soil-gas levels include radium content and distribution, soil porosity, moisture, and density. However, soil as a source of radon is seldom characterized by radon levels in soil-gas, but is usually characterized directly by emanation measurements or indirectly by measurements of members of the uranium-238 series (National Research Council 1981). Radon content is not a direct function of the radium concentration of the soil, but radium concentration is an important indicator of the potential for radon production in soils and bedrock. However, Michel (1987) states that average radium content cannot be used to estimate radon soil-gas levels, primarily due to differences in soil porosity. 

A correlation between the amount of radon in the soil gas (or water) and the levels of and Ra only exists for two extremes the radon concentration in the soil gas is likely to be very low or very high if the radium content of the source materials (rock or sediment) is very low or very high, respectively. Outside these two extremes other factors dominate in controlling radon concentrations, with those factors responsible for the radon transport processes being especially important. These factors include soil porosity and permeability, density, moisture, barometric pressure, temperature, thickness of the soil over bedrock, and, in some cases, the vmder-lying bedrock. 

Use of radium in detailed soil geochemical exploration Because of its weak mobility in relation to uranium, it is particularly useful to determine the radium content in the soil during detailed prospecting. The contrary geochemical behaviour of radium and uranium (mobility in reducing conditions, immobility in oxidizing conditions) may be used to interpret specific anomalies (swamps, stream beds, etc.). 

Rona E, Urry WD (1952) Radium and uraninm content of ocean and river waters. Am J Sci 250 241-262 Rosholt J, Doe B, Tatsnmoto M (1966) Evolntion of the isotopic composition of nraninm and thorinm in soil profiles. Geol Soc Am Bull 77 987-1004... 

Figure 1. Schematic illustration of factors influencing the production and migration of radon in soils and into buildings. Geochemical processes affect the radium concentration in the soil. The emanating fraction is principally dependent upon soil moisture (1 0) and the size distribution of the soil grains (d). Diffusion of radon through the soil is affected primarily by soil porosity ( ) and moisture content, while convective flow of radon-bearing soil gas depends mainly upon the air permeability (k) of the soil and the pressure gradient (VP) established by the building.

Fast moving neutrons emitted from a radioactive source (usually Radium-Berrylium or Americium-Beryllium) upon collision with a particle having mass nearly equal to its own, like hydrogen atom in the soil, release their energy and gets thermalized or slowed down. The thermalized neutrons are detected by a detector and recorded on a scalar. Usually BFg gas is used as detector of slowed down neutrons. Increased thermalization indicates higher water content of the soil. The zone of influence is normally about 15-20 cm arormd the detector. 

Natural radioactivity is formed particularly by long-lived isotopes, i.e. by those with half-lives of 10 to 10 years. These isotopes are usually widely scattered in the soil. The activity level depends particularly on contents of uranium, thorium, radium and potassium the radiation energy of these elements represents as much as 98% of the total energy of radiation of all the natural radioactive elements in the soil. 

For efficient release of radon into the air spaces of soil, the radon atom must be formed within 20-70 nm of the mineral surface for most common minerals. This distance is the recoil range of a radon atom at the instant of its formation from the decay of radium. The processes by which radon atoms escape from a given material are referred to as radon emanation. The emanation power or the coefficient of emanation is defined as the ratio of the number of radon atoms that escape from the solid to the number of radon atoms formed by radioactive decay of radium in the solid. The emanation power varies from about 0.02 to 0.7, depending upon the mineral structure and the water content. 

Radium is similar to calcium in chemical behavior and is absorbed from soil by plants and is passed up the food chain to man. Chemical factors such as the amount of exchangeable calcium in the soil will determine the rate at which radium will be absorbed by plants. The radium in food transferred from soil is known to be as variable as Cs contents of food. 

Soils contain varying amounts of uranium-238, which decays in several steps to radium-226, then to radon-222, a gas (see Figure 21.4). Some homes situated in areas of high uranium content have been found to accumulate radon gas. Radon has a half-life of 3.8 days, decaying by alpha emission to radioactive lead, bismuth,and polonium.These decay products can remain in the lungs and may lead to lung cancer. 

Humans are exposed to natural radiation soil is a major source of external and internal exposure of radiation. The external exposure from the soil is associated with gamma radiation and internal exposure with radon inhalation. Exposures of radiation derived from soil are different in each region. The aim of this study is to evaluate radionuclides content, Rn exhalation rates, radium equivalent activity and hazard indices in soil samples around Institute de Pesquisas Energeticas e Nucleares (IPEN) facilities. 

The nuclear density apparatus is a handy device to rapidly obtain the unit weight and water content of the soil. Soil particles cause radiation to scatter to a detector tube and the amount of scatter is counted. The scatter count rate is inversely proportional to the unit weight of the soil (Figure B.27 left). If water is present in the soil, the hydrogen in water scatters the neutrons and the amount of scatter is proportional to the water content (Figure B.27 right). The radiation soiu ce is either radium or radioactive isotopes of caesium and americium. 

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