**3. Quantification of stochastic and deterministic effects**

The interaction of radiation with matter leads to the deposition of some or full of its energy in the absorption medium, the temperature of which may increase. Since the deposition energy is very low, this is not the main cause of consequent effects in the living tissues where the type of particles, the density of the energy lost per unit of the tracking sensitivity of different tissues exposed, and other factors play a more significant role. This is why the response of the body cannot be expressed by pure physical quantities, and other factors related to the tissue reactions to formed radicals are of primary importance.

The risk created by radiation to the human body cannot be expressed by means of only physical quantities and some specific quantities—we may call them *biophysical* rather than *physical quantities.* The biophysical quantities are based on the physical quantities weighted by specific factors taking into account the biological harm of various types of radiation as well as the sensitivity of particular organs and tissues to the exposure.

## **3.1 Physical quantities and units**

One of the first attempts to quantify radiation exposure to a person was based on ionizing abilities of radiation (at that time, only X-ray photos were assumed) where a unit *roentgen* was introduced as a measure of the ability of photons to ionize the air. Later on, the roentgen (R) became a unit of a quantity *exposure*, introduced by the equation

*Basic Radiation Protection for the Safe Use of Radiation and Nuclear Technologies DOI: http://dx.doi.org/10.5772/intechopen.108379*

$$X = \frac{dQ}{dm} \tag{1}$$

where *dQ* is the total charge of the ions of one sign generated by the electrons (negatrons and positrons) produced by photons in the mass of air *dm*.

The SI unit of this quantity is C kg�<sup>1</sup> , the relation with the old unit—roentgen (R)—is 1 R = 2.58 � <sup>10</sup>�<sup>4</sup> C.kg�<sup>1</sup> (exactly). Because of the definitions, the quantity of exposure could be applied in practice only to photons of energy up to about 300 keV [4].

Later on, when radiation protection had to address the results of interactions of other types of radiations, including beta, alpha, neutrons, and others, a universal quantity of (absorbed) dose was introduced. This is a universal physical quantity reflecting the deposition of radiation in any substance. The *dose* was introduced as follows:

$$D = \frac{dE\_i}{dm} \tag{2}$$

where *dEi* is the mean energy imparted to the matter of mass dm. The unit of the dose and the dose rate are Gy (gray) and Gy.h�<sup>1</sup> (gray per hour). Commonly, units mGy, μGy, and mGy h�<sup>1</sup> or μGy.h�<sup>1</sup> are frequently used. Before, for the old unit, the rad unit was in use, where 1 Gy = 100 rad [5].

The dose is considered to be a universal quantity in dosimetry, and it is a basis for most quantities used in radiation protection. It can be used for any type of radiation and for any medium or absorber.

The last physical quantity to be mentioned here is the *kerma (K)*, which is the acronym for Kinetic Energy Released per unit Mass. This quantity can only be used for photons and neutrons in any media. It is still widely used especially in computational dosimetry. The kerma is defined by the equation

$$K = \frac{dE\_{tr}}{dm} \tag{3}$$

where *dEtr* is the sum of the initial kinetic energies of all the charged particles liberated by uncharged particles in a mass *dm* of material. The medium should always be specified.

The special name for the unit of kerma is gray (Gy); the unit for the kerma and dose is thus the same. In addition, here, one can specify this quantity related to the unit of time as the *kerma rate*, defined as the kerma per second. The main unit for this quantity is analogical to the dose rate, i.e., Gy.s�<sup>1</sup> .

The illustration of the dose and kerma is shown in **Figure 5**, documenting their relationship. It is obvious that the kerma reflects the energy of secondary particles released by indirectly ionizing radiation at the point of interest, while the dose represents the energy absorbed by these particles. This absorption takes place at a certain distance from the origin of their production.

**Figure 5** shows the attenuation of photons in their penetration through the absorber where at the surface, the kerma has a maximum value and then shows a continuous decrease, while the dose is first increasing its value and after reaching the maximum; it decreases with the same rate as the kerma (equilibrium). This behavior is due to the fact that at a certain depth, the particles from the layer above contribute to the dose where the kerma is lower because of the attenuation.

**Figure 5.** *The relationship between the kerma and the dose depends on the depth.*
