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Safety_Series_025_1968 - gnssn - International Atomic Energy ...

Safety_Series_025_1968 - gnssn - International Atomic Energy ...

This publication is no

This publication is no longer validPlease see http://www.ns-iaea.org/standards/It should be borne in mind that the above-mentioned elementaryreactions occupy a very short period of time, 10~17 - 10‘ 15 seconds.The excited or ionized atoms or molecules are very unstable andextremely reactive. As we shall see later, they undergo chemicalreactions with the formation of free, radicals or stable and unstablemolecules.It may be stated in general that, as a result of ionizing radiation,matter is permeated by electrons of various velocities andsimultaneously a considerable number of atoms are excited, i.e .become more reactive with respect to other atoms or m olecules.1.1.4. Radioactivity and radioactive decayAt the present time there are over forty radioactive elementswith high atomic weight which occur in nature. Besides that, a fewof the lighter elements, as for example potassium, rubidium andsome others, possess radioactive properties.The nuclei of radioactive nuclides as distinct from those ofinactive nuclides undergo spontaneous disintegration with the em issionof alpha, beta or gamma rays. Unstable nuclei are turned intostable nuclei after one or more such disintegrations. For a givenelement various nuclides, characterized by the different mass oftheir nucleus (due to the different number Of neutrons) or isotopes,may be recognized. Intermediate nuclides in a radioactive seriesof disintegrations are called radioactive daughters. The naturallyoccurring series are the uranium, thorium and actinium series.The disintegration of an unstable isotope is a random eventappearing with a certain probability per unit time. A half-life of aradioactive nuclide is the time required for a given amount of elementto decay to half its initial value. The half-life is a constant for anuclide and may vary according to the characteristic of the nuclide,from a small fraction of a second to several thousand million years.The activity of a radioactive sample is determined by the number ofdisintegrations occurring per unit time (see section 1.1.5).1.1.5. UnitsThe question of radiological units is under continuous considerationby the International Commission on Radiological Units andMeasurements (ICRU). This commission has stated that the additionof further units in the field of radiation dosimetry is undesirable and

This publication is no longer validPlease see http://www.ns-iaea.org/standards/has recommended that the use of each special unit be restricted toone quantity as follows:radrCntgencurieremfor absorbed dosefor exposurefor activityfor dose equivalentThe unit for absorbed dose - the rad - is defined as the amountof energy imparted to matter by ionizing particles per unit mass ofirradiated material at the place of interest. One rad corresponds to100 ergs per gram.The former term 'exposure dose' has been replaced by the term'exposure', expressed in rCntgens (R). The unit rCntgen (R) is thatquantity of ionizing radiation which will produce one electrostaticunit of charge in one cm 3 of air (or 0. 001293 g of air).Another important aspect of irradiation is the dose rate, whichis the dose delivered per unit time. For the biological end-effectit is important to consider not only the total dose but also the doserate.The curie (Ci) is the unit of activity of radioactive nuclides andis defined as the activity of that amount of a substance which undergoes3. 7 X 1010 disintegrations per second.It is important always to make a distinction between activitymeasured in curies and dose measured in rtfntgens or rads and evaluatedin rems. Activity is equivalent to number of disintegrations perunit of time, dose is a measure of energy absorbed at some point intissue.From the point of view of radiation protection it is important totake into consideration "LET - dependent factor by which absorbeddoses are to be multiplied to obtain.......... A quantity that expresseson a common scale for all ionizing radiations the irradiation incurredby exposed persons. The name recommended for this factor is thequality factor (QF). Provisions for other factors are also made.Thus a distribution factor (DF) may be used to express the modificationof biological effect due to non-uniform distribution of internallydeposited isotopes. The product of absorbed dose and modifyingfactors is termed the dose equivalent (DE).......... The unit ofdose equivalent is the 'rem '. The dose equivalent is numericallyequal to the dose in rads multiplied by the appropriate modifyingfactors" (ICRU [3]).9

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