Radioactive Decay and Gamma Radiation

Radioactive decay

• Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation
• There are three main types of radioactive decay: alpha decay (α-decay), beta decay (β-decay), and gamma decay (γ-decay).

Properties of radioactive energy

• Gamma radiation is the most relevant type of radioactive energy because it has the right properties to be measured by gamma-ray spectrometry.
• While many naturally occurring elements have radioactive isotopes, only K and the U and Th decay series have radioisotopes that emit gamma rays of sufficient energy and intensity to be measured by gamma-ray spectrometry.

Gamma-ray radiation

• Gamma radiation is caused by the decay of 238U, 235U, 232Th, and 40K.
• Each decay system emits gamma rays with different energies, which can be measured using a spectrometer.
• For example, the gamma-ray spectrum of 238U is different from the gamma-ray spectrum of 235U.

eU and eTh

• eU is equivalent uranium. It refers to the amount of uranium isotopes measured and adjusted to account for all uranium isotopes present.
• U concentration estimates are based on the measurement of 214Bi and 214Pb isotope abundances, which occur far down in the radioactive decay chain and may not be in equilibrium with U.
• Therefore, estimates of U concentration are usually reported as eU as these estimates are based on the assumption of equilibrium conditions.
• eTh is equivalent thorium. It refers to the concentration of Th estimated by gamma ray spectrometry under the assumption that the Th decay series are in equilibrium.

Mean radioelement concentration in various rocks

• Killeen (1977) provided data on the mean radioelement concentration in various rock types.

Radiometric Survey techniques

• Both ground and airborne techniques can be used for radiometric surveys, but airborne surveys are more common.
• Airborne surveys should be conducted at heights between 50m and 200m because 50% of gamma radiation is absorbed by the detector at 200 m and 75% at 50 m.
• Flight heights (h) are typically 80 to 100 m.
• Flight lines should be 2h apart.

Data Processing

• Corrections must be made for: height, cosmic rays, aircraft radiation, and atmospheric radon.
• Spectrometers are then calibrated so the gamma radiation can be directly converted into U, Th, and K concentrations.

Ternary Image Representation of Data

• Radiometric data can be represented using a K-Th-U ternary image.

Applications of Radiometric Surveys

• Geological mapping: Radiometrics can be used to map both bedrock and surficial sediments.
• Mineral exploration: Radiometrics can be used to explore for:
  • U
  • Granite-related elements like Sn, W, etc.
  • Rare elements like Be, Zr, Y
  • Carbonatites, which are sources of REEs, P, Nb
  • Precious and base metal mineralizations
• Environmental applications: Radiometrics can be used to assess:
  • Radon hazards
  • Nuclear accidents

Ratio of concentrations

• Data processing in mineral exploration typically involves looking at ratios of concentrations, which can be diagnostic of hydrothermal alteration.