1.1 Radon Physics, Geology & Decay Chain

1.1 Radon Physics, Geology & Decay Chain

Radon is a naturally occurring, radioactive noble gas that is colorless, odorless, and tasteless. For Radon Measurement Professionals (RMPs), mastering the fundamental physics, geological origin, nuclear decay chain, and measurement units of radon is essential for accurate testing, data interpretation, and client communication.

The Origin of Radon: Uranium-238 Decay Chain

Radon-222 (²²²Rn) is not generated independently; it is an intermediate radioactive decay product of Uranium-238 (²³⁸U), a primordial radionuclide present in varying concentrations throughout the Earth's crust since the formation of the solar system. Uranium-238 undergoes a long series of radioactive transformations, decaying through Thorium-230 (²³⁰Th) to form Radium-226 (²²⁶Ra).

Radium-226 has a radioactive half-life (t_1/2) of approximately 1,600 years. When Radium-226 undergoes alpha decay, it releases an alpha particle and transforms into Radon-222:

${}^{226}_{88}\mathrm{Ra} \rightarrow {}^{222}_{86}\mathrm{Rn} + {}^{4}_{2}\alpha$

Because Radium-226 is a solid element bound within soil minerals and rocks, it remains trapped in place. However, Radon-222 is an inert gas. Unlike its solid predecessor isotopes, radon does not react chemically with surrounding soil minerals. Once formed, radon gas can migrate through interstitial pore spaces in soil, fractures in bedrock, and pathways in building foundations into indoor environments.

Radon Decay Products (RDPs) and Nuclear Chain

Radon-222 has a radioactive half-life of 3.823 days (approximately 3.82 days). As Radon-222 decays, it transforms into a series of short-lived radioactive isotopes known as Radon Decay Products (RDPs), historically called radon progeny or "radon daughters."

Unlike radon gas itself, RDPs are heavy metal atoms (polonium, lead, and bismuth) that carry an electrostatic charge upon creation. They readily attach to airborne dust particles, aerosols, walls, furniture, and airway passages within the human respiratory tract.

The short-lived decay chain of Radon-222 progresses as follows:

1. Polonium-218 (²¹⁸Po): Half-life of 3.10 minutes. Emits a high-energy alpha particle (6.00 MeV) to form Lead-214.
2. Lead-214 (²¹⁴Pb): Half-life of 26.8 minutes. Emits a beta particle and gamma radiation to form Bismuth-214.
3. Bismuth-214 (²¹⁴Bi): Half-life of 19.9 minutes. Emits a beta particle and gamma radiation to form Polonium-214.
4. Polonium-214 (²¹⁴Po): Half-life of 164 microseconds (0.000164 seconds). Emits a very high-energy alpha particle (7.69 MeV) to form Lead-210.
5. Lead-210 (²¹⁰Pb): Half-life of 22.2 years. Represents a long-lived breakpoint in the decay series, eventually decaying through Bismuth-210 and Polonium-210 to form stable Lead-206 (²⁰⁶Pb).

Types of Ionizing Radiation

To understand radon health physics, professionals must distinguish between the three primary forms of ionizing radiation emitted during nuclear decay:

1. Alpha Particles (α): Composed of two protons and two neutrons (a Helium-4 nucleus). Alpha particles are heavy, positively charged (+2), and move relatively slowly. Because of their large mass and high charge, they possess high Linear Energy Transfer (LET). They deposit all their kinetic energy within a very short distance (microns in human tissue, roughly 1 to 2 cell diameters). While blocked by a sheet of paper or the dead outer layer of skin (stratum corneum), alpha particles cause intense localized ionization when emitted inside the living bronchial epithelium.
2. Beta Particles (β): High-speed electrons emitted from the nucleus when a neutron converts into a proton. Beta particles have low mass, a -1 charge, and moderate penetration power (blocking requires several millimeters of aluminum or plastic).
3. Gamma Rays (γ): High-energy electromagnetic photons emitted alongside particle decay. Gamma rays have no mass or charge and possess high penetration power (requiring dense lead shielding).

Geology, Soil Mechanics & Gas Transport

Radon generation and indoor accumulation depend directly on geological factors:

• Parent Rock Lithology: Rocks with elevated radium-226 content include granite, dark organic shales (e.g., Chattanooga or Marcellus shale), phosphate deposits, metamorphic gneiss, and glacial tills containing granitic gravels.
• Emanation Power: The fraction of radon atoms generated by radium decay that escape from the mineral matrix into the pore space of soil.
• Soil Permeability: Coarse gravel and clean sand exhibit high permeability (10⁻⁸ to 10⁻¹⁰ m²), allowing rapid soil gas movement. Dense clay exhibits extremely low permeability (10⁻¹⁴ to 10⁻¹⁶ m²), restricting gas movement unless fractured.
• Advection vs. Diffusion: While radon can move by molecular diffusion from areas of high to low concentration, advection (bulk air flow driven by air pressure differentials between the soil and the building interior) is the primary mechanism bringing radon into homes. Thermal stack effects, mechanical ventilation, and wind driving negative pressure in basements draw soil gas into the building.

Units of Measurement & Physical Calculations

In the United States, indoor radon concentrations are measured in picocuries per liter of air (pCi/L). One Curie (Ci) equals 3.7 × 10¹⁰ disintegrations per second. A picocurie is one-trillionth of a Curie (10⁻¹² Ci).

• 1 pCi/L represents 2.22 radioactive disintegrations per minute per liter of air (dpm/L).

Internationally and in scientific SI units, radon concentration is expressed in Becquerels per cubic meter (Bq/m³). One Becquerel (Bq) equals one disintegration per second.

• Conversion Factor: 1 pCi/L = 37 Bq/m³. Conversely, 1 Bq/m³ ≈ 0.027 pCi/L.
• The EPA Action Level of 4.0 pCi/L corresponds to 4.0 × 37 = 148 Bq/m³.

Working Level (WL) and Equilibrium Ratio (ER)

While radon gas is measured in pCi/L, the radiation dose to lungs comes from short-lived RDPs. The cumulative concentration of decay products is measured in Working Levels (WL).

• Working Level (WL): Defined as any combination of short-lived radon decay products in one liter of air that will release 1.3 × 10⁵ MeV (mega-electron volts) of potential alpha energy.
• Equilibrium Ratio (ER): The ratio between the actual RDP potential alpha energy (expressed in WL) and the theoretical maximum energy if RDPs were in full secular equilibrium with the parent radon gas concentration (expressed in pCi/L):

$\text{ER} = \frac{\text{WL} \times 100}{\text{pCi/L}}$

Under typical indoor residential conditions with normal air ventilation and surface plate-out of attached particles, the Equilibrium Ratio is approximately 0.4 (40%). This means at an indoor radon level of 4.0 pCi/L, the decay product concentration is typically 0.4 × (4.0 / 100) = 0.016 WL.