In Vivo Counting - STD-1128-98master0135

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

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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