In the two tables below some of the physical parameters are given. With regard to dose, the a-particle emitters are the most important. They all have a range of just a few micrometers in tissue – and if they are in the lungs, all their energy is deposited within a thin layer of cells in the bronchia. The radon family from the uranium-series is by far the most important.
4. Radon
Isotope Decay-
type Half-life Energy released (in MeV)
Rn-220 a 55.6 seconds
a 6.28
Po-216 a 0.15 seconds
a 6.78
Pb-212 β 10.6 hours β 0.34, 0.58
g 0.239 Bi-212 a (36%)
β (64%) 60.6 minutes
a 6.05
β 2.25 g 0.04
Po-212 a
a 8.78
Uranium – radium series
Thorium series
Isotope Decay-
type Half-life Energy released (in MeV)
Rn-222 a 3.82 days
a 5.49
Po-218 a 3.05 minutes
a 6.0
Pb-214 β 26.8 minutes β 0.59, 0.65 g 0.053, 0.35 Bi-214 β 19.8 minutes β 1.51, 1.0, 3.18
g 0.61, 2.42 Po-214 a 0.164 milliseconds
a 7.69
Trachea Bronchia
With regard to the radiation dose it is easy to understand that the a-particles from the radon daughters (i.e. the polonium isotopes) with energies from 5.49 MeV to 8.78 MeV are the most important. The a-particles deposit all their energy within a few tens of micrometers in tissue – and because of the high LET a large radiation weight factor is used (ICRP uses a factor of 20 for a-particles).
It is a long way to go from the measurements of radon in the air to an estimation of the effective doses from the radon and thoron families. In the following we shall try to give you some information about radon in the atmosphere, how it is measured, and how effective doses are estimated.
The radon sources
The two radon isotopes Rn-222 and Rn-220 are formed from the radium isotopes Ra-226 and Ra-224.
The distribution of radium follows the distribution of uranium (U-238) and thorium (Th-232).
It is important to note that radon is a noble gas, whereas all the other isotopes in the two families are metals. The gas may be trapped in the soil, but it has also a chance to be released to the air. When radon disintegrates, the metallic daughters will be attached to dust or other solids.
The radon concentrations in the soil within a few meters of the surface are important for the entry into the atmosphere. The main mechanism for this is molecular diffusion.
The concentrations of Rn-222 in soil vary over many orders of magnitude from place to place and show significant time variations at any given site. Because of the short half-life of thoron (Rn-220) in the soil, a large fraction of the atoms will disintegrate before they reach the surface.
Radon
Wall Radon-daughters
Aerosol particle
Deposition
Deposition
= Radon-daughters
= Rn-222 or Rn-220 This is an illustration from a masther Thesis at the University of Oslo (Camilla Lunder). It describes how radon is released to air – and how the decay products (the radon daughters) become attached to dust particles in the air.
The dust, as well as the daughters may depos- it on the walls – or deposit in the lungs upon inhalation. Inhaled radon (the noble gas) will probably not be stopped in the lungs, but follow the air out again.
The cilia (small hairs) along the air ways move in a sweeping motion, and thus stop the dust particles. It can be mentioned that cigarette smoke will damage the celia with the result that more dust particles with radon daughters may deposit – yielding a larger dose to the bronchia of smokers.
Another point: Due to the short half-life for Rn-220 (55.6 seconds) these atoms have a short time to move away from where they are formed.
(Courtesy of Camilla Lunder)
Radon measurements
The methods for measuring radon has developed considerable during the latter years. Today we have instruments that make it possible to follow the radon concentration as a function of time, the concen- tration of the different radon daughters, as well as the fraction not bound to particles. For indoor radon measurements we can use charcoal canister sampling and thermoluminescence dosimeters (TLD).
This method consists of a small box with carbon powder and two TLD crystals. One crystal is placed in the middle of the box whereas the other is outside the box. Radon daughters from the air will become trapped in the carbon powder and subsequently expose the crystal in the box. The crystal outside the box serves as a control for the cosmic radiation and gamma radiation from the walls of the room. The difference between the two TLD-crystals yields information on the average radon concentration during the exposure period. The first mapping of radon in Norwegian dwellings were performed using this method by A. Storruste and his students at the University of Oslo.
Another new and popular method is the solid-state nuclear track detectors. The method is based on the fact that a-particles make tracks in certain materials such as polymers, minerals or glass. The tracks are made visible upon etching and then examined microscopically. The size and shape of these tracks yield information about the mass, charge, energy and direction of motion of the particles.
Radon concentrations are determined by measuring the emitted alpha particles and the track detector has become the state-of-the-art for environmental radon monitoring.
Radon into the houses
Several possibilities exist for the release of radon into houses (see illustration on the next page).
The main sources are the rock or soil on which the house is built, as well as the water supply. The rock formations under a house always contain some radium and the radon gas can penetrate into the house through cracks in the floor and walls of the basement. Furthermore, the building materials is also a source for radon.
Another source for radon is the water supply. Water from wells, in particular in regions with radium- rich granite, may contain high radon concentrations. When water is the carrier, the radon gas is readily released.
The indoor radon concentrations usually show an annual variation with the largest values during the winter. This is quite usual in places with frost and snow on the ground. The frost makes it difficult for the gas to diffuse out of the surrounding ground and to be released directly into the air. Consequently, the radon gas is more likely to seep into the house. It is the radon concentration averaged over a long-time that is important with regard to health risk and, consequently, remediation. Decisions on what action to take are generally based on an average annual level.
This drawing illustrates how radon may enter a house. The radon gas (originating from radium) comes into the house from the ground, from the building materials and sometimes from the water. Well water contains in some areas (for example in Finland) quite large amounts of radon which is released when used in showers or in the kitchen. Drinking the water increases the amount of radiation to the body.
In this connection we are interested in inhaled radon daughters.
(Courtesy of Terje Strand, Norwegian Protection Agency)
Radon in the water
Radon from building materials
Radon from the ground