In Vivo Counting - doe-std-1128-98

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.

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