Radiation Exposure Converter (roentgen, C/kg)

Understanding the radiation exposure converter — concept and principle

This converter changes radiation exposure between roentgens (R), milliroentgens (mR), coulombs per kilogram (C/kg) and millicoulombs per kilogram (mC/kg). Use it to read older survey meters and dosimeters, X-ray equipment documentation or historical records, which often use R or mR/h, and to relate them to the SI unit C/kg.

How it is calculated

result = value × (C/kg per "from" unit) ÷ (C/kg per "to" unit)

• 1 R = 2.58 × 10⁻⁴ C/kg exactly (0.258 mC/kg).
• 1 mR = 0.001 R.
• 1 C/kg ≈ 3,875.97 R; 1 mC/kg ≈ 3.876 R.

For a rate such as mR/h, the same factors apply because the time unit does not change.

The principle

Exposure measures how much X-ray or gamma radiation ionizes air: the electric charge of the ions produced per kilogram of air. The roentgen was defined as the exposure that produces one electrostatic unit of charge in 0.001293 g of dry air (1 cm³ at 0 °C and 1 atm). Restated in SI units, that equals exactly 2.58 × 10⁻⁴ C/kg. Exposure applies only to photons in air; it is not a dose to a person or an object.

Worked example

Default inputs: 1 roentgen to milliroentgens.

1. 1 R = 2.58 × 10⁻⁴ C/kg.
2. 1 mR = 2.58 × 10⁻⁷ C/kg.
3. 2.58 × 10⁻⁴ ÷ 2.58 × 10⁻⁷ = 1,000 mR.
4. The same 1 R is 0.000258 C/kg or 0.258 mC/kg.

How to read the result

To relate exposure to energy absorbed, use the average energy needed to make one ion pair in dry air, about 33.97 J per coulomb. One roentgen therefore deposits about 2.58 × 10⁻⁴ × 33.97 ≈ 0.00876 Gy (8.76 mGy) in air. For soft tissue the absorbed dose from 1 R is roughly similar, which is why older texts often treated 1 R, 1 rad and 1 rem as roughly equal for X-rays and gamma rays. Modern radiation protection uses gray (Gy) for absorbed dose and sievert (Sv) for effective dose instead.

Common mistakes

• Treating roentgen as a dose unit. R is ionization in air; Gy and Sv are doses.
• Using roentgen for alpha, beta or neutron radiation; it is defined only for X-rays and gamma rays.
• Mixing up mR and µR, a factor of 1,000.
• Forgetting the time unit when comparing a rate (mR/h) with a total (mR).

Frequently asked questions

How many C/kg is 1 roentgen?

Exactly 0.000258 C/kg.

Is a roentgen the same as a rem?

No. The roentgen is exposure in air; the rem (now the sievert, 1 Sv = 100 rem) is dose to people. For X-rays and gamma rays they are numerically close but not the same quantity.

Why is the roentgen still used?

Many older survey meters and documents are scaled in R or mR/h, so conversions are still needed, even though it is not an SI unit.

Sources: ICRU Report 85, Fundamental Quantities and Units for Ionizing Radiation (2011) · NIST Special Publication 811 (2008), Appendix B