# Answer to Question #12522 Submitted to "Ask the Experts"

*Category: Radiation Basics — Radiation Quantities and Units*

The following question was answered by an expert in the appropriate field:

Could you please explain how kerma differs from absorbed dose? I'm confused because they both use the same units of measurement.

You are correct that the quantities absorbed dose and kinetic energy released per unit mass (kerma) both use the same units. In fact, kerma is fairly often used as a surrogate for absorbed dose, but the exposure conditions must meet certain requirements for the approximation to be acceptable. The difference between the two quantities lies in how they are defined.

Absorbed dose represents the average energy imparted to (or absorbed by) material per unit mass of the material by whatever type(s) of radiation are responsible for the exposure. Kerma is defined only for indirectly ionizing radiation as the radiation type(s) (either gamma rays, x rays, or neutrons for our purposes) and represents the energy transferred to a small mass of material per unit mass of the material by the kinetic energy of charged particles released in the mass by the initial interactions of the indirectly ionizing radiation(s). For gamma or x radiation, the quantity absorbed dose would be specified mathematically as dE/dm, and kerma as dE_{tr}/dm, both the numerators and denominators representing differential quantities.

The energy imparted to the mass is not necessarily the same as the energy absorbed in the mass and therein lies the possible difference in magnitude between the two quantities for a given irradiation condition. If secondary charged particle equilibrium exists in the differential mass element (under which conditions the energy emitted per unit mass of material is the same as the energy absorbed per unit volume), and if no significant energy is carried away from the mass element by secondary radiation processes, such as bremsstrahlung production, the absorbed dose and the kerma will be likely the same. Under conditions of secondary charged particle equilibrium the absorbed dose from monoenergetic gamma rays or x rays is given by

D = ΦE_{γ }µ_{en}/r,

where Φ is the photon fluence, E_{γ} is the photon energy, and µ_{en}/r, is the mass energy absorption coefficient for the photons of interest in the medium being irradiated. The kerma for similar radiations is given by

K = ΦE_{γ }µ_{tr}/r,

where µ_{tr}/r is the mass energy transfer coefficient for the photons of interest in the medium of interest. The value of µ_{tr}/r is always greater than or equal to µ_{en}/r, the extent of the difference naturally affecting the relative magnitudes of the absorbed dose and kerma.

I hope this explanation is helpful.

George Chabot, PhD