This table is used for computing the detection efficiency. Efficiencies are necessary for converting between counts per minute (cpm) on the instrument and disintegration per minute over 100 square centimeters (dpm/100cm2), which is the common unit for a building surface DCGLw.
In the Fraction column, the proportion of activity must be assigned to the nuclides and must sum to one. In this case, Am-241 accounts for 60% of the activity and U-235 accounts for 40% of the activity.
Next, the instrument efficiency for each nuclide must be entered in the Inst Eff column. This is the proportion of nuclide disintegrations (between 0.0 to 1.0) that is detected by the instrument.
Surface efficiency must also be entered in the Surf Eff column. This is the proportion of disintegrations that allow energy to escape the surface of the material to be detected by sensors. Surface efficiency is between 0.0 and 0.5.
These efficiency values are obtained through calibration processes that are outside the scope of this document.
Total efficiency of each nuclide is computed as:
\(\text{Eff}_{\text{nuclide-total}} = \text{Fraction} \times \text{Eff}_{\text{instrument}} \times \text{Eff}_{\text{surface}}\)
All the total nuclide efficiencies are summed and reported in the Tot Eff column. In the example above, \(\text{Eff}_{\text{activity-total}}\) is 0.2263.
Gross DCGLw (in dpm/100cm2 units) is computed as:
\begin{equation} DCGL_{Gross} = \frac{1}{\frac{f_1}{DCGL_1}+\frac{f_2}{DCGL_2}+..\frac{f_n}{DCGL_n}} \end{equation}
Gross DCGLw in disintegration per minute (dpm) can be computed as:
\begin{equation} DCGL_{dpm} = DCGL_{Gross} \times \frac{Area_{probe}}{100} \end{equation}
where \(Area_{probe}\) is the surface area of the instrument probe in square centimeters.
If all the efficiencies have been entered, Gross DCGLw (in cpm) can be computed as:
\begin{equation} DCGL_{cpm} = DCGL_{Gross} \times Efficiency_{total} \times \frac{Area_{probe}}{100} \end{equation}