13.4.1 Dose Units

Previously in this book, the radioactive radiation has been characterized by the half-life, the type, the energy or energy distribution of the emitted particles or elec­tromagnetic radiation, and the activity. These properties are not sufficient to char­acterize the biological effects of radiation because the effects are due to the absorbed energy and the subsequent material changes induced. These effects are characterized by different terms, the radioactive doses.

The most common radiation effect is ionization. The different types of the radio­active radiation ionize the substances directly or via the electrons formed in the scattering processes. Thus, as a first approximation, the biological (or other radio­logical) effects are characterized by the number of the ions produced in the air under the effect of the radiation. The number of the ions related to the irradiation dose (or ion dose) and expressed as coulomb per kilogram (C/kg) in dry air. It should be pointed out that ionization produces electrons and positive ions in equal quantity, so the total number of the charged particles (ions + electrons) is double the ion dose. The previous unit of irradiation dose was 1 rantgen (R), which expresses the number of ions in 1 cm3 of air. The relation between the two is the following: 1 R = 2.58 X 10"4 C/kg.

The biological effects are due to the absorbed radiation energy. This is defined as an absorbed dose in 1 kg of material and expressed in joules per kilogram (J/kg). This unit has its own name, namely gray (Gy). This means that 1 J/kg = 1 Gy (gray). The previous unit of absorbed dose was rad: 1 Gy = 100 rad.

The irradiation and absorption doses can be related by taking into consideration that the formation of one ion and one electron demands 53.9 X 1019 J, assuming the average composition of the air. Since the charge of an electron is 1.6 X 1019 C, the formation of 1 C requires 33.7 J of energy. Therefore, a 1 C/kg irradiation dose is equivalent to a 33.7 Gy absorbed dose in air, assuming total absorption.

As discussed in Chapter 5, different radioactive radiations have different interac­tions with matter. This also applies to the biological effects. The biological effects of the different radiation types are taken into account by the radiation weighting factors, such that the absorbed dose is multiplied by the radiation weighting factors. In this way, the so-called equivalent dose is obtained. The unit of the equivalent dose is the sievert (Sv): Sv = radiation weighting factor X Gy. The value of the fac­tors is very different for each type of radiation. The radiation weighting factor for X-ray and gamma radiation has been chosen to be 1. However, the radiation weighting factor is 5—20 for neutrons, depending on the neutron energy, 5 for protons, and 20 for alpha particles and fission products. The previous unit of equiv­alent dose was rem: 1 Sv = 100 rem.

The effect of the radiation depends not only on the type and energy of the radia­tion but also the sensitivity of the organs and tissues to the radiation. This different sensitivity to stochastic radiation damage (see Section 13.4.4) is considered in the Publication 60 published by the International Commission of Radiological

Protection (CRP), in the Euratom basic standards for radiation protection dated May 1996 by the tissue weighting factor: 0.20 for gonads; 0.12 for colon, bone marrow (red), lung, and stomach; 0.05 for bladder, chest, liver, thyroid gland, and esophagus; 0.01 for skin, bone surface, and others. The sum of the tissue weighting factors is 1 for the whole body. The dose of a whole human body is the effective dose. To calculate the effective dose, the individual organ dose values are multi­plied by the respective tissue weighting factor and the products added. The unit of the effective dose is sieverts.

If the radiation is present for a long time, the dose rate is used, which is defined as the ratio of the dose and the time of irradiation. The background radiation, for example, is expressed in mSv/year (see Table 13.5).