Units of Radiation Dosimetry

For a complete characterization of the biological effect of ionizing radiation , it is necessary to know the quantities defining the degree of effect.

Units of radiation dosimetry

Emission

Emissions are among the basic quantities that characterize a radiation source. The emission value indicates the number of radiation particles emitted by the source per unit of time. The size of the emission is s-1.

Activity

If the radiation source is radioactive, the content of the emission is related to the quantity of activity A of the source. The activity of the source is defined as the number of radioactive transformations in a given amount of radionuclide per unit of time. The unit of activity is 1 Bq (becquerel). 1 Bq represents one radioactive transformation per 1 s. The measure of activity is s-1.

Volumetric activity

The volume activity av expresses the ratio between the activity of the source A and the volume V of a certain substance. The unit is Bq.m 3. The volume activity dimension is s -1.m -3.

Exposure

Exposure X (irradiation) is a characteristic of the X-ray and γ radiation field. This quantity is defined using the ionizing effect of radiation in a given environment (air by default). The exposure is given by the ratio of the electric charge ΔQ of the ions, which were created by the braking of electrons or positrons, and a certain mass Δm of the volume element of air.

X=ΔQ/Δm

Graphic showing ICRU protection dose quantities in SI units.

The main unit is 1 C.kg-1. The size of the exposure is Askg-1.

Exposure speed

Exposure speed (exposure power) dX/dt is characterized as an increase in exposure in a given time interval.

dX/dt = ΔX/Δt

The unit is A.kg -1. The dimension of exposure speed is Am 2.s -2.

Absorbed dose

The absorbed dose D is defined as the ratio of the average energy ΔE of ionizing radiation absorbed by a volume element of air of a certain mass Δm.

D=ΔE/Δm

The unit is 1 Gy (grey) = 1J.kg-1. The dimension of the absorbed dose D is m 2.s-2.

Batch rate

The dose rate (dose rate) dD/dt is expressed by the dose increment ratio ΔD in the time interval Δt.

dD/dt = ΔD/Δt

The unit is Gy.s-1.

Dose equivalent

The dose equivalent H correlates with the magnitude of the biological effects of various types of ionizing radiation. It is used for radiation hygiene purposes and has the meaning of a modified dose H = DQN , where Q is a quality factor (so-called quality factor) that expresses the quality of radiation in terms of biological effects. N represents the product of other factors that describe the irradiation conditions. The unit of dose equivalent is 1 Sv (sievert). The dimension of the quantity is m 2 s-2.

References

  • JIŘÍ, Beneš – JAROSLAVA, Kymplová – FRANTIŠEK, Vítek. Základy fyziky pro lékařské a zdravotnické obory : pro studium i praxi. - edition. Grada Publishing, a.s., 2015. 236 pp. ISBN 9788024747125.
  • BENEŠ, Jiří. Základy lékařské biofyziky. - edition. Karolinum, 2007. 201 pp. ISBN 9788024613864.