Example 4.4. Improving Detection Capabilities of Air Sampling Using Averaging

DOE-STD-1121-98

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

n/n

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

than" values.

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

4.5.1 Measurements

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.