The effect of radiation depends on the amount you have received. The amounts of radiation received are referred to as doses, and the measurement of such doses is known as dosimetry.

## Absorbed Dose

A similar approach is used in radiation protection measurements, where the unit of ABSORBED DOSE is specified in terms of the amount of energy deposited by radiation in 1 kg of material. This unit is the gray, abbreviated Gy. An absorbed radiation dose of 1 Gy corresponds to the deposition of 1 joule of energy in 1 kg of material. Absorbed dose is given the symbol D. The gray is a measure of energy absorbed by 1 kg of any material, be it air, water, tissue or whatever. A person who has absorbed a whole body dose of 1 Gy has absorbed one joule of energy in each kg of body tissue.
 Gray (Gy): 1 Gy = 1 J/kg
The gray is a physical unit. It describes the physical effect of the incident radiation (i.e., the amount of energy deposited per kg), but it tells us nothing about the biological consequences of such energy deposition in tissue.

### Equivalent Dose

Table 1 Quality factors for various types of radiation.
Quality Factor
200 – 250 keV X-rays
1
γ-rays, β particles and electrons
1
Thermal neutrons (< 0.8 MeV)
3
Fast neutrons (>0.8 MeV), protons
10
α-particles
20
Heavy ions
20
The absorbed radiation dose, when multiplied by the QF of the radiation delivering the dose, will give us a measure of the biological effect of the dose. This is known as the equivalent dose. Equivalent dose is given the symbol H. The unit of H is the sievert (Sv). An equivalent dose of one sievert represents that quantity of radiation dose that is equivalent, in terms of specified biological damage, to one gray of X- or γ-rays. In practice, we use the millisievert (mSv) and microsievert (μSv). Equivalent dose, quality factor and absorbed dose are related by the expression:
 Sievert (Sv) H (Sv) = D (Gy) × QF
The sievert is the unit that we use all the time, because it is the only one that is meaningful in terms of biological harm. In calculating the equivalent dose from several types of radiation (we call this "mixed radiation"), all measurements are converted to Sv, mSv or μSv and added. Thus, the sievert allows us to add doses of different radiation types to obtain total effective dose.
 Example What is an individual's dose equivalent from 10 mGy of gamma rays, 5 mGy of β- particles and 10 mGy of fast neutrons? Dose Equivalent (mSv) = Absorbed Dose (mGy) x QF Gamma dose equivalent = 10 x 1 10 mSv Beta dose equivalent = 5 x 1 5 mSv Neutron dose equiv. = 10 x 10 100 mSv Total 115 mSv

A photon, as described by the Quantum Theory, is a "particle" or "quantum" that contains a discrete quantity of electromagnetic energy which travels at the speed of light, or 3×108 meters per second.