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

Studies have shown that alpha and neutron radiation cause greater biological damage for a given energy deposition per kg of tissue than gamma radiation does. In other words, equal doses of, say, alpha and gamma radiation produce unequal biological effects. This is because the body can more easily repair damage from radiation that is spread over a large area than that which is concentrated in a small area. Because more biological damage is caused for the same physical dose (i.e., the same energy deposited per unit mass of tissue), one gray of alpha or neutron radiation is more harmful than one gray of gamma radiation. Quality factors are used to compare the biological effects from different types of radiation. For example, fast neutron radiation is considered to be 20 times as damaging as X-rays or gamma radiation. You can also think of fast neutron radiation as being of "higher quality", since you need less absorbed dose to produce equivalent biological effects. This quality is expressed in terms of the Quality Factor (QF). The Quality Factor of a radiation type is defined as the ratio of the biological damage produced by the absorption of 1 Gy of that radiation to the biological damage produced by 1 Gy of X or gamma radiation.

Table 1 Quality factors for various types of radiation.
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.