Example 4.4. Improving Detection Capabilities of Air Sampling Using Averaging
The minimum detectable average concentration for repeated BZ or personal air samples over
a year or other period of time can be reduced by averaging the original raw data, as described in the
Appendix to NUREG-1400 (Hickey et al. 1993a, Strom 1993). The simplest case is when
independent activity measurements are made of a sequence of samples for which large numbers of
counts (i.e., more than 50) are collected and for which the following remain identical between
samples: background count times and rates, sample count times, and counting yields. In such a case,
the MDA for the sum of n samples is larger than that for a single sample: MDA(n) = /6AMDA(1).
Conversely, the minimum detectable average concentration (MDC) for n samples is smaller than the
minimum detectable concentration (MDC) for a single sample: MDC(n) = MDC(1)//n, when sample
volumes or masses are all equal, equal sample collection times are used, and collection efficiencies
are equal. Although the MDA for such pooled samples increases by /6, the volume or mass in which
this activity is found increases by a factor of n, resulting in a net decrease in MDC by a factor of
/6 = 1//6. In general, samples may have varying count times, background count rates, counting
efficiencies, collection efficiencies, and collection times. Exact time-weighted formulas for MDC
and decision level (DL) are given for the general case in the Appendix to NUREG-1400, and exact
formulas are provided for both large and small numbers of background counts (Hickey et al. 1993a).
This methodology is useful in situations where daily, weekly, or monthly concentration
measurements must be compared to an annual limit. It is also useful in determining the detection
capabilities of a measurement program. This work shows the importance of reporting measurements
and their standard deviations as observed, rather than "censoring" them by reporting them as "less
An alternative to averaging is to physically combine air filters containing long-lived material.
For example, if a worker had 200 separate personal air sample filters during a year, they could be
combined and the composite analyzed as a single sample. If the material were a penetrating photon-
emitter, the ensemble of filters could be counted directly by gamma spectroscopy. If the material
were an alpha-emitter, radiochemistry would be necessary.
4.5 MEASUREMENTS OF WORKPLACE RADON AND THORON CONCENTRATIONS,
POTENTIAL ALPHA ENERGY CONCENTRATIONS, AND MEASUREMENTS OF
(OR ASSUMPTIONS ABOUT) EQUILIBRIUM FACTORS
There are two objectives of radon/radon progeny monitoring and hence two sets of standards for
these measurements. The two monitoring objectives are 1) to characterize in real time the concentrations
that workers might be exposed to while in an area and 2) to establish the exposure of record that each
worker actually receives. In the Air Monitoring Guide (DOE 1999d), these two types of monitoring are
respectively referred to as air monitoring and air sampling. It will generally be found that meeting both
objectives is best achieved using two different types of instruments.
Instruments used for both purposes should measure either airborne radon or radon progeny
concentration. If materials containing thorium-232 or its progeny are known to be present in the area, the
instruments should also be capable of measuring airborne thoron progeny concentrations.