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Monitoring Requirements and Selection of Employees - doe-std-1128-98_ch10122
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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In Vitro Analysis - doe-std-1128-98_ch10124


DOE-STD-1128-98
5.3.3.1 In Vivo Counting
Direct bioassay (in vivo counting) is the measurement of radiations emitted from
radioactive material taken into and deposited in the body. Direct bioassay is
appropriate for detection and measurement of photons emitted by plutonium and its
decay products. Lung, wound, liver, and skeleton counting are examples of in vivo
monitoring most commonly used for plutonium and its progeny. Whole body
counting, commonly used for monitoring high-energy fission and activation products
in the body, is ineffective for direct measurement of plutonium due to the very low
energy of photons emitted from plutonium and its decay products unless the
plutonium is intimately mixed in a high-energy photon-emitting matrix, such as spent
fuel.
Some low-energy x-rays emitted by plutonium decay products are energetic enough
to escape the body. When direct bioassay is used, the detection system should be
calibrated for the radionuclides to be measured in the appropriate organs. All
calibration procedures, calibration records, and quality control data should be
maintained. Energies most commonly used for plutonium monitoring are the 17-keV
L X-rays and the 60-keV gamma of 241Am. Mixtures of spent fuel material can lend
themselves to whole body counting if the ratio of a readily detectable high-energy
gamma-emitter (i.e., 137Cs) to plutonium is known.
A plutonium facility should have the capability to detect and assess depositions of
plutonium in the lungs of radiation workers. The major objective of lung counting is
to provide measurements of suspected intakes triggered by workplace monitoring
results. Lung measurements should be made to provide an early estimate of the
magnitude of the intake and resulting lung deposition.
Two methods have been used to detect plutonium in the lung: the L x-ray method and
the americium-tracer method. The L x-ray method is based on the measurement of L
X-rays following the decay of plutonium. This method provides a direct
measurement of plutonium. The detection capability of the method may be on the
order of tens of nanocuries for plutonium and requires an accurate measurement of
the chest wall thickness (because of the large attenuation of the low-energy X-rays by
the rib cage and overlying tissues). Other problems that complicate the measurement
of L X-rays are (1) the difference in attenuation in muscle and fat, (2) the possibility
of nonuniform distribution of the plutonium in the lung, and (3) interferences from
radionuclides in other organs or from other radionuclides in the lung.
5-13


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