Why is exposure different from absorbed dose?

Exposure (R or C/kg) measures ionization in air specifically—it describes the radiation field. Absorbed dose (Gy) measures energy actually deposited in any material. You can have the same exposure but different absorbed doses in different materials due to varying absorption properties.

Why do survey meters still use roentgens?

Historical convention and convenience. Ion chamber survey meters directly measure charge collection proportional to roentgens. The R/hr scale has familiar benchmarks. Modern meters often display both R/hr and equivalent mSv/hr, with automatic conversion factors.

What replaced exposure for modern dosimetry?

Air kerma (Kinetic Energy Released per unit MAss) in grays is now preferred. It measures energy transferred to air without the limitations of the ionization-based roentgen. For radiation protection, absorbed dose (Gy) and equivalent dose (Sv) are standard.

Exposition aux rayonnements Converter

About Radiation Exposure Conversion

Radiation exposure measures the ionization produced in air by X-rays or gamma rays. It quantifies the radiation field strength rather than the biological effect on tissue. Exposure is specific to photon radiation (X-rays and gamma rays) and to air as the medium—it's been largely replaced by air kerma for modern dosimetry, but legacy equipment and regulations still reference exposure. The concept remains important for understanding radiation survey instruments and historical dose records.

The SI unit is coulombs per kilogram (C/kg), measuring the electric charge of ions produced per mass of air. The older roentgen (R) remains widely used, especially for survey meters, radiation safety equipment, and historical records. Ion chamber instruments naturally measure exposure because they detect ionization. The approximate conversion 1 R ≈ 8.7 mGy in soft tissue enables rough dose estimation from survey readings.

Our converter handles radiation exposure units used in health physics, radiological surveys, and historical documentation.

Common Radiation Exposure Conversions

From	To	Multiply By
R	C/kg	2.58 × 10⁻⁴
C/kg	R	3,876
R	mR	1,000
mR	R	0.001
R	μR	10⁶
mC/kg	C/kg	0.001
R	mGy (in tissue)	~8.7
R/hr	mGy/hr (in tissue)	~8.7
C/kg	mC/kg	1,000

Radiation Exposure Unit Reference

Coulomb per kilogram (C/kg) – The SI unit of radiation exposure, representing the electric charge of ions (of one sign) produced per mass of air. 1 C/kg = 3876 R exactly. This is a very large unit for practical measurements—a lethal radiation exposure would still be only about 0.1 C/kg. Submultiples (mC/kg, μC/kg) are more practical for real measurements.

Roentgen (R) – The traditional exposure unit, named after Wilhelm Röntgen who discovered X-rays. 1 R = 2.58 × 10⁻⁴ C/kg exactly. Historically defined as producing 1 esu of charge per cm³ of dry air at STP (about 2.08 × 10⁹ ion pairs). For X-rays in soft tissue: 1 R ≈ 0.87 rad ≈ 8.7 mGy. Still standard for survey meters and diagnostic radiology.

Milliroentgen (mR) – 1/1000 R = 2.58 × 10⁻⁷ C/kg. The common scale for environmental monitoring, industrial radiography surveys, and diagnostic X-ray measurements. Dental X-ray: ~100-300 mR; chest X-ray: ~15-25 mR at skin surface.

Microroentgen (μR) – 1/1,000,000 R = 2.58 × 10⁻¹⁰ C/kg. Used for background radiation measurements. Natural background radiation: ~10-20 μR/hr depending on location. Sensitive survey instruments can detect single μR/hr changes.

Roentgen per hour (R/hr, mR/hr) – Exposure rate units common on survey instruments. Nuclear accident exclusion zones might be defined at 10-100 mR/hr; areas above 100 R/hr indicate severe contamination requiring immediate evacuation.