Radiation - Exposure

Measuring the Field: A Guide to Radiation Exposure

In the context of radiation protection, exposure is a measure of the ability of ionizing radiation to produce charged ions in a specific volume of air. It is a quantity that describes the intensity of a radiation field in the air, not the dose absorbed by a person or object within that field. Specifically, it quantifies the total electric charge of the ions of one sign (e.g., all the positive ions) produced in a unit mass of air when the air is fully irradiated. This measurement is only defined for X-rays and gamma rays, as charged particles like alpha and beta particles have their own distinct interaction properties.

The standard international (SI) unit for radiation exposure is the Coulomb per kilogram (C/kg). It is a direct measure of the charge produced in a mass of air. However, a much more historically common unit is the Roentgen (R), named after Wilhelm Röntgen, the discoverer of X-rays. The Roentgen is defined based on the amount of charge created in a specific volume of air under standard temperature and pressure conditions. While the Roentgen is no longer an official SI unit, it remains widely used in the United States on older radiation survey instruments and in some regulatory contexts. This converter facilitates the translation between these two units, which is essential for health physicists and radiation safety officers who need to interpret readings from various instruments and ensure compliance with safety standards.

Relevant Formulas and Concepts

• Definition of Exposure (Physics): The fundamental definition is X = dQ / dm, where 'X' is the exposure, 'dQ' is the total charge of the ions of one sign produced, and 'dm' is the mass of the air in which the charge is produced.
• Exposure Rate: This measures how quickly an exposure is being delivered, often expressed in units like Roentgens per hour (R/hr). This is typically what a radiation survey meter measures to assess the immediate hazard in an area.
• Relationship to Absorbed Dose (Physics): Exposure in air can be related to the absorbed dose in a different material (like tissue) at the same location. The absorbed dose (D) can be calculated from the exposure (X) by D = f * X, where 'f' is a conversion factor that depends on the energy of the radiation and the composition of the absorbing material. For practical purposes in health physics, 1 Roentgen of gamma ray exposure is often approximated to result in an absorbed dose of about 0.96 rads (or 9.6 mGy) in soft tissue.

A Deep Dive into Radiation Exposure Units

• Coulomb per kilogram (C/kg): This is the SI unit of radiation exposure. It is a direct and fundamental measure of the charge created per unit mass of air. While it is the official standard, it is less commonly used in everyday practice than the Roentgen.
• Roentgen (R): The Roentgen is a legacy unit of exposure for X-rays and gamma rays. One Roentgen is defined as the amount of radiation that produces 1 electrostatic unit (esu) of charge in 1 cubic centimeter of dry air at standard temperature and pressure. This corresponds to 2.58 x 10⁻⁴ C/kg.
• Milliroentgen (mR): A smaller, more practical unit equal to one-thousandth of a Roentgen. Radiation survey meters often measure in milliroentgens per hour (mR/hr).