Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
4 METHODS OF INDIVIDUAL AND WORKPLACE MONITORING
4.1 Introduction
(246) This Chapter briefly describes the main measurement techniques, their
advantages and their limitations for individual monitoring. In most cases, assessment
of intakes of radionuclides may be achieved by body activity measurements, excreta
monitoring, air sampling with personal air samplers, workplace measurements or a
combination of these techniques. The choice of measurement technique will be
determined by a number of factors including the radiation emitted by the radionuclide,
the availability of equipment, the biokinetic behaviour of the contaminant and the
likely radiation dose.
4.2 Body Activity Measurements (In Vivo Measurements)
(247) In vivo measurement of body or organ content provides a quick and
convenient estimate of activity in the body. It is performed with one or more photon
detectors placed at specific positions in relation to the subject being measured. It is
feasible only for those radionuclides emitting radiation that can be detected outside
the body. In principle, the technique can be used for radionuclides that emit: X or
radiation: positrons, since they can be detected by measurement of annihilation
radiation; or energetic b particles that can be detected by measurement of
Bremsstrahlung radiation (e.g. 90 Y, produced by the decay of its 90 Sr parent).
(248) The detectors used for in vivo measurements are usually partially shielded and
the individual to be measured can be placed in a shielded, low background room to
reduce the interference from ambient sources of radiation.
(249) Direct (in vivo) bioassay is likely to be the monitoring method of choice if the
radionuclide is a high yield, high energy gamma-ray emitter or decays by positron
emission (with emission of annihilation radiation), unless the material is excreted
rapidly from the body. The gamma-radiation emitted by such radionuclides is strongly
penetrating, and so is readily detected using scintillation or semiconductor detectors
positioned close to the body. If the material is absorbed rapidly from the respiratory
tract, and is then either distributed uniformly in body tissues (e.g. 137 Cs in most
common chemical forms), or is distributed preferentially among a number of organs,
(e.g. 59 Fe) then whole body monitoring should be chosen. If the radionuclide deposits
preferentially in a single organ such as the thyroid (e.g. 125 I, 131 I), then partial body
monitoring of the relevant organ should be chosen. In the case of materials that are
absorbed less rapidly from the respiratory tract (e.g. insoluble forms of 60 Co oxide),
lung monitoring is preferable to whole body monitoring soon after the intake, as it
gives a more accurate measure of lung deposition and retention than a whole body
measurement.
(250) Direct bioassay is also useful for some radionuclides that emit photons (X- or
-rays) at lower energies and/or with lower yields (e.g. 241 Am, 210 Pb, 144 Ce). However,
in the case of radionuclides that mainly emit X-rays below 25 keV with low yields
(notably, the alpha-emitting isotopes of plutonium and curium) direct bioassay may
not achieve the sensitivity required for radiological protection purposes.
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