Appendix. General information about the nature of radiation and the material-
radiation interaction
Radioactivity is the process of spontaneous nuclear decay by which an unstable atom
(uranium, thorium, radium etc.) changes the atomic and mass number. Such elements are
considered radioactive. The radioactive decay has a strictly defined constant speed measured by
the half-life period (the time required for one half of the atoms to disintegrate). The radioactive
decay cannot be stopped or speeded up.
Any radioactive substance produces three types of radiation:
−
alpha-radiation (α-radiation) that is a positively charged particle flux (helium
nucleus) moving at 20000 kilometers/second;
−
beta-radiation (β-radiation) that is a negatively charged particle flux (helium
nucleus) moving at 20000 kilometers/second;
−
gamma-radiation (γ—radiation) that is a shortwave electromagnetic emission.
Gamma-rays characteristics are close to the X-rays, but it has far more energy and
speed (it travels at the speed of light).
Ionizing radiation can penetrate matters of dissimilar thickness and ionize air and living
cells.
Radioactivity is the measure of the radioactive substance expressed by the number of
radioactive transformations per unit of time. In the SI system the unit of radioactivity is one
radioactive transformation, i.e. the number of atoms which, within a particular time frame,
transform. This unit is called The Becquerel (Bq)
.
The non-SI unit of radioactivity is curie (Ci).
1 Ci = 3,7*10
10
Bq
The extent and form of injuries caused to biological tissues by ionizing radiation exposure
depend on the dose of absorbed radiation energy. To characterize it the term ‘absorbed dose’ is
used, i.e. the energy absorbed by the unit of exposed material. In the SI system the unit of
absorbed dose is Gray (Gy). 1 Gray (Gy) measures the absorbed dose, giving the energy of 1
joule transferred by ionizing radiations to the material of 1
1 Gy=1j/kg
To characterize the dose according to the ionization effect caused in the air it’s used so-
called ‘exposure dose’ of X-rays and gamma-radiation. This dose is based on the X-rays and
gamma-radiation ionizing action and it’s expressed by total electric charge of the ions charged
equally and formed in the unit of air volume at the state of electronic equilibrium.
The non-SI unit of absorbed radiation dose is roentgen (R).
Absorbed dose and exposure radiation dose delivered over a time period are called
absorbed dose rate and exposure radiation dose rate. They are expressed by the units Gy/s and
R/s respectively.
Equivalent dose (H) is the main dosimetric quantity in the sphere of radiation safety
introduced to estimate the stochastic health effects of any chronic ionizing radiation exposing
when the H value is not more than five maximum permissible doses a year.
To obtain the equivalent dose the absorbed dose must be multiplied by a
special multiplicator called the relative biological effectiveness coefficient ("RBE") or the
radiation quality factor (Q) in the elementary volume of the tissue.
The unit of the equivalent dose is Sievert (Sv). 1 Sievert is an equivalent dose of any
radiation absorbed by 1 kg of tissue that produces the biological effect that equals the effect
