Safety_Series_025_1968 - gnssn - International Atomic Energy ...

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This publication is no longer validPlease see http://www.ns-iaea.org/standards/4. 1. 1. 2. Measurement of radioactive contaminationRadioactive contamination is the second form of irradiation towhich workers handling radioactive materials may be subjected.Contamination may occur in various ways. It may be limited to thesurface of the body, in which case it is termed skin contamination,or radionuclides may penetrate into the body through the skin, digestivetract or lungs, when it constitutes internal contamination.It is therefore necessary to evaluate skin contamination on the onehand and internal contamination on the other. Evaluation of thelatter can be made either directly, as is possible in some cases,or indirectly, by measuring the amount of radioactive material liableto enter the body or the amount of such material eliminated fromthe body via the excreta.4. 1. 1. 2. 1. Measurement of external contamination. The evaluationof external contamination is relatively easy. The contaminated areashould first be demarcated by rough monitoring and a broad distinctionmade between the various possible contaminants. Knowledgeof the kind of work being performed usually provides an indicationof what has occurred. In any event it is easy, with the help of theavailable detectors, to make an immediate distinction between alphaemitters on the one hand and beta or gamma emitters on the other.The detectors used are generally proportional counters, Geiger-Miiller counters or scintillation counters. They are modified tosuit requirements and the detecting surface is usually flat for usein relatively open areas, or in the form of a probe for work in placesdifficult of access.The readings are expressed in counts per unit time. Accountmust first be taken of geometrical factors: the shape and surfacearea of the detector must be known in order to derive the number ofcounts per unit time and per unit area from the number per unittime. It is then necessary to take account of physical factors inorder to derive a value in curies per unit area from the number ofcounts. For this purpose the disintegration scheme of the radionuclides,together with the efficiency of the counter with regard tothe different types of radiation, must be known.Obviously, an exact determination of external contaminationis extremely laborious unless the nature of the contaminating nuclidesis previously known. External contamination is therefore frequentlyevaluated only in terms of alpha- or beta/gamma-emitting radio86
This publication is no longer validPlease see http://www.ns-iaea.org/standards/nuclides. For this purpose maximum permissible limits have beenestablished irrespective of the nature of the contaminating radionuclide.Decontamination is regarded as necessary after accidentalcontamination above certain levels, the values of which, as establishedby a number of national organizations, are quoted in Appendix II tothe Manual on Safe Handling of Radioisotopes [15].4. 1. 1. 2. 2. Measurement of internal contamination. (1) Directmeasurement. In cases of internal contamination by radionuclides,it would appear theoretically highly advantageous to make an overallmeasurement of the contamination and thus to determine the bodyburden of radionuclides. Direct measurement is possible only ifthe radionuclides emit radiation capable of detection outside the body:at present this can be achieved, using spectrometry methods, onlyfor gamma-emitters. Various instruments may be used to makesuch measurements in vivo. Owing to their extreme sensitivity,they must be shielded most carefully from background radiation toensure accuracy of the results. For any given radionuclide thethreshold of detection depends on the effectiveness of the shielding.The materials most widely used for shielding such apparatus arewater, iron and lead. The types of apparatus used include ionizationchambers, liquid scintillators and crystal scintillators.The most frequently used apparatus is, however, the crystalgamma spectrometer. In its most usual form it consists of a detectingcrystal connected with a photomultiplier. The gamma raysemitted in the organism are absorbed by the crystal and producescintillations which in turn produce photoelectrons. The latter arethen amplified by the photomultiplier, and converted into electricpulses. These pulses are then transmitted to a single- or multichannelpulse-height analyser. With this equipment the radioactivityof the whole or of a particular part of the organism can be measured.An ionization chamber cannot achieve energy discriminationbetween gamma radiations, but this is to some extent possible ifliquid scintillators are used and is an easy task with crystal scintillators.In all cases the readings should be carefully interpreted,taking into account physical factors connected with the nature of thecontaminating radionuclides and geometrical factors connected withthe equipment. Calibration with regard to sources or phantoms isstill a delicate operation. However, the accuracy of the equipmentis generally sufficient to allow a correct evaluation of the bodyburden for certain radionuclides, such as iron-59, cobalt-60,zinc-65, ruthenium-106, iodine-131, caesium-137 and radium-226.87
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26Table 6-2. Fracture-specific EC i
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