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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/Such apparatus is of value owing to the ease with which internalcontamination can be measured; it is particularly useful in caseswhere a workeris suspected of having undergone appreciable internalcontamination. It is not necessary to take samples, and care mustonly be taken to ensure that there is no interfering skin contamination.The characteristic features of the spectrometry method areits high sensitivity, adequate reliability and precision, great flexibilityand convenience of use. However, the equipment is expensiveat present and can only be used by experts.(2) Indirect measurement by monitoring of excreta. Since radioelementsare eliminated by excretion according to more or less wellknownlaws, it is possible to estimate from the quantity of nuclidesin the excreta the quantity present in the organism at a given time.The mode of elimination depends on the nature of the radio-element:uranium and plutonium are excreted in urine, strontium in sweatand urine. Radiochemical analyses are generally carried out onurine and occasionally on faeces and breath.Radiochemical techniques generally consist of the followingstages: preparation of samples; chemical isolation of radionuclides;quantitative determination of the latter by measurement of radioactivityafter calibration with a control sample; exact identificationof the radionuclides.The sampling of excreta in reality requires more care than atfirst appears, if it is to give a true picture of the degree of elim i­nation of the substance under consideration. The ideal procedureis to collect specimens over a period of 24 hours. However, inpractice, quantities equivalent to those excreted in 24 hours areoften used. This method is relatively easy for the sampling of urinebut not so easy for that of faeces. Specimens of both urine andfaeces are collected in flasks or Polythene bags, and a check mustbe made that there has been no excessive absorption of the radionuclideon the walls of the receptacle used for collection. Breathis collected in large balloons having inlet and exhaust valves. Thiskind of sampling can be carried out only in specialized laboratories.It is always useful to separate the various contaminating radionuclideswith a view to measuring the activity of each. Separationis effected by the physico-chemical methods of co-precipitation, adsorption,ion exchange, etc. .Quantitative determination of the radionuclides is effected bymeasuring the alpha, beta or gamma activity, using counters of the

This publication is no longer validPlease see http://www.ns-iaea.org/standards/G eiger-M uller, proportional or scintillation type, carefully calibratedby means of control samples. Special precautions shouldbe taken where there is a possibility of natural radioactivity (e. g.potassium-40) interfering with the artificial activity to be measuredin the specimens.Finally, steps must be taken to identify with absolute certaintythe radionuclide or nuclides to be detected. This operation iscarried out by the methods of chemical analysis, radioactive decay(if the half-life is sufficiently short, i. e. a few hours or days), absorption(for beta emitters), spectrometry (for alpha and gammaem itters) and tracks in nuclear emulsions (for alpha em itters).Special radiochem ical techniques have been developed to facilitatequantitative determination of radionuclides present in theexcreta, particularly the urine.The method adopted for this type of examination must meetcertain requirements: in particular it must be specific, sensitive,accurate and rapid. These conditions are, however, seldom fulfilledby any one technique.Often variants exist for different radionuclides. Methods whichare very highly sensitive but complex in application are used foroccasional but extremely important examinations (e. g. followingan accident). Other methods, less accurate and less sensitive buteasy to apply, are suitable for routine examinations. A list ofmethods of particular importance for medical toxicological analyseshas recently been prepared [16].In the great majority of cases, examinations are carried out onurine samples. The results can be used either to prove the existenceof even a very slight degree of internal contamination, correspondingto normal working conditions, or to determine the degree of internalcontamination following an accident. It may be supposed that eliminationtakes place according to a simple exponential law and thatcontamination has occurred in a regular manner and finally reachesa certain equilibrium. Given such conditions, the body burden ofan individual for a particular radionuclide may be determined fromthe radioactivity of the excreta. Such conditions are, of course,ideal; the nearest approach to them is in cases where radioactivematerials are absorbed in quantities that vary little from day to dayand are eliminated very slowly. On the basis of the fraction excretedper unit time, the total body burden of the organism may beestimated with some accuracy. In most cases, however, the nuclidesare eliminated in a manner which does not facilitate estimation of89

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