Workplace Monitoring STCs

DOE-HDBK-1184-2004

3 WORKPLACE MONITORING FOR STCS

3.1 Observed Versus Actual Activity

Because of the difficulties associated with detection and quantification of STC levels on

surfaces and in sample media, two terms are used in this handbook to describe analysis

results. The observed activity is that activity that is detected by the prescribed analysis

method. Because of the self-shielding attribute of particulate STCs, the observed activity

(the activity detected by the sample analysis equipment, corrected for efficiency and

detector-to-sample geometric considerations) may not reflect the total quantity of

radioactive material present. The term actual activity is used to describe the total activity

present in STCs. In practice, the actual activity may be estimated from the observed

activity by use of appropriate conversions to adjust for particle self-shielding and other

factors that may interfere with accurate sample analysis. However, in many cases,

characterization of metal tritide species make it difficult to quantify and correct observed

activity to actual activity. As discussed in this document, the use of observed activity is

particularly relevant for internal dose assignment.

Current regulations require that certain radiation protection measures (e.g., contamination

area posting and access control, radioactive material labeling and control) be implemented

based on assessments of the quantities or concentrations of radioactive material present.

This handbook adopts the concept of observed activity (discussed above) as an

appropriate surrogate for the actual activity for STCs. 10 CFR 835 Ap pendix D, Footnote 2

provides surface contamination values in disintegrations per minute as meaning "the rate of

emission by radioactive material as determined by correcting the counts per minute

observed by an appropriate detector for background, efficiency, and geometric factors

associated with the instrumentation." The use of observed activity is consistent with this

provision. This handbook will demonstrate that certain other measures, such as individual

dose estimates, may be based on assessments of observed activity.

3.1.1

Conversion from Observed to Actual Activity

The conversion of analysis results from observed to actual activity is relatively

straightforward, once the various factors that introduce analytical uncertainties are

understood and quantified. First, corrections must be applied to convert the

analysis results presented by the instrument, usually counts per unit time, to the

desired activity units and to eliminate the more common analytical uncertainties

(e.g., uncertainties arising from detector efficiencies and counting geometry

considerations). Further discussion on the use of observed activity is discussed in

Section 5.2, especially considerations given to self-absorption of emissions that do

not escape the STC particle.

In more conventional sample counting applications, application of these corrections

will yield the actual activity present in the sample, usually in units of disintegrations

per minute per 100 square centimeters (dpm/100cm2) for surface contamination

monitoring or microcuries per cubic centimeter (Ci/cm3) for airborne radioactivity

monitoring. (These are the units used for surface contamination and airborne

radioactivity control criteria, respectively, in 10 CFR Part 835 ). However, due to

the physical characteristics of STCs, application of these corrections yields only the

observed activity. Further corrections may be made to account for that fraction of