Sample Activity Measurement

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

however, which must be converted to an aerodynamic size to be useful for dose estimation. They are

generally expensive tools used mostly for research.

Respirable -Fraction Samplers. A number of respirable -fraction samplers have been developed, but the

cyclone separator is the most widely used and best characterized type. The cyclone is specified by NIOSH

and MSHA for personal respirable -mass sampling in coal mines. NIOSH and MSHA currently certify

entire sampling systems (PAS pump, cyclone, filter head, and filters) for personal respirable -fraction

sampling. This "system" approach may be modified as the result of recent research; however, it does

provide an interim standard for performance. The performance of cyclones, pumps, and filters may be

characterized to allow intermixing of sampling-train components in future work; at present, however,

theoretical prediction of performance of mixed systems is not reliable.

Cyclones are aerodynamic particle sizers, as are impactors, but have some different operating

features. They are not affected by loading, so dusty environments are not a problem, although filter

loading may limit sampling time. Cyclones are rated for performance at a particular flow rate.

Performance at other flow rates cannot easily be predicted and should be determined by testing. In

contrast, impactors do follow a simple, well-defined relation between flow rate and size separation.

Alternatives to mechanical methods of particle -sizing exist and other respirable -fraction separators

may be available in the future. Combined total and respirable -fraction samplers would be desirable; such

designs retain both the respirable and non-respirable fractions so that total airborne activity can be

estimated.

4.1.6 Sample Activity Measurement

Most sample analyses at uranium facilities are performed by quantifying the radioactivity by

counting the samples colle cted. Some fluorometric analyses are performed with equivalent sensitivity.

Kinetic phosphorescence analysis is available with substantially greater sensitivity.

Alpha Counting. Alpha particles can be counted with ionization, proportional, scintillation, or other

solid state detectors. The major drawback is that relatively little particle penetration, in the filter or in the

dust loading, can result in a low reading caused by self-absorption of the alpha particles.

Alpha Spectrometry. Measurement of the energy spectrum of alpha-emitters on a filter paper is

possible and very beneficial in some applications in identifying or verifying the identity of the isotopes

present. Typically, semiconductor detectors are the choice, and membrane filters or other surface-

collecting filters are used with very low dust loading.

Beta Counting. Thin-window GM, ionization, proportional, and solid state detectors are used for

beta counting. Because of the wide range of beta-particle energies of even a "single energy" emitter,

careful energy calibration is necessary. Beta counting results are less dependent on self-absorption

effects.

Beta Spectrometry. Beta spectrometry has recently become feasible through developments in tissue-

equivalent plastic detectors. For routine isotopic identification, this method is not as useful, but it may

provide valuable shielding information, etc.

Gamma Spectrometry. NaI and GeLi detectors can provide essential isotopic identification of

gamma-emitters.

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