Basic Quantities and Units in Radiation Safety
To characterize and measure ionizing radiation, we need quantities that describe the source, the radiation field at the point of interest, and the energy deposited in the material with which ionizing radiation interacts. The values assigned to the various
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Basic Quantities and Units in Radiation Safety
Human senses are not capable of detecting ionizing radiation, thus its detection has to be done indirectly by the effects it causes. Ionizing radiation is the energy that comes from a source, travels through a medium and may be absorbed by it. Therefore, to characterize and measure ionizing radiation, we need quantities that describe the source, the radiation field at the point of interest, and the energy deposited in the material with which ionizing radiation interacts. Selection of the most appropriate quantity depends on the specific case. The values assigned to the various quantities may be obtained by calculations and/or measurements. The International Commission on Radiological Protection (ICRP) and the International Commission on Radiation Units and Measurements (ICRU) are international organizations who develop internationally accepted recommendations on radiation measurements, quantities, and units. The ICRU defines the units, and the ICRP recommends how they are used for radiation protection. In the United States, the main counterpart is the National Council on Radiation Protection and Measurements (NCRP), an organization chartered by the U.S. Congress.
4.1
Source Quantities
The starting point to evaluate the amount of radiation from a specific exposure situation is the source. A radioactive source—a radioactive material used as a source of radiation—is characterized by its activity and half-life. Activity is “the quantity A for an amount of radionuclide in a given energy state at a given time,” [“Reproduced with permission by the IAEA.”] also defined as the rate at which nuclear transformations occur [1]: AðtÞ ¼
dN ; dt
unit: s1
© Springer International Publishing Switzerland 2017 H. Domenech, Radiation Safety, DOI 10.1007/978-3-319-42671-6_4
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4 Basic Quantities and Units in Radiation Safety
In other words, activity is the number of disintegrations of a radionuclide per unit time. Where dN is the expectation value of the number of spontaneous nuclear transformations from the given energy state in the time interval dt. The International System of Units (hereafter referred to as SI) unit of activity is the reciprocal second (s−1), having the special name Becquerel (Bq): 1 Bq = 1 s−1 Becquerel replaces formerly activity expressed in curies (Ci), where 1 Ci = 3.7 1010 Bq. Activity values may be given in Ci (with the equivalent in Bq in parentheses) if they are being quoted from a reference that uses Ci as the unit. For each radionuclide, the physical quantity half-life (T½) is the time required for the nuclei population to decrease by a radioactive decay process by half. The activity also decreases by half by the same radioactive decay process, so the time taken for activity to decrease by half can be used as an alternative definition of half-life. Where it is necessary to distinguish this from other half-lives (e.g., biological half-life, effective half-life), radioactive half-life is used. The SI unit is the time unit (seconds, minutes, hours, or
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