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3.10.1 Conversion of collected data to DCGL units

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3.10.1.1 Surface activity: Conversion of counts to activity

3.10.1.1 Surface activity: Conversion of counts to activity

When measuring surface activity, it is important to account for the physical surface area assessed by the detector in order to make probe area corrections and report data in the proper units (i.e., Bq/m2, dpm/100 cm2). This is termed the physical probe area. A common misuse is to make probe area corrections using the effective probe area which accounts for the amount of the physical probe area covered by a protective screen. Figure 3.13 illustrates the difference between the physical probe area and the effective probe area. The physical probe area is used because the reduced detector response due to the screen is accounted for during instrument calibration.
The conversion of instrument display in counts to surface activity units is obtained using the following equation:

Bq/m2 = (Cs/Ts) / (εT x A) …………………………………………………………… (3-26)

where:
Cs = integrated counts recorded by the instrument;
Ts = time period over which the counts were recorded in seconds;
εT = total efficiency of the instrument in counts per disintegration, effectively the product of the instrument efficiency (εi) and the source efficiency (εs);
A = physical probe area in m2.

Figure 3.13 The physical probe area of a detector
Figure 3.13 The physical probe area of a detector

To convert instrument counts to conventional surface activity units, Equation 3-26 can be modified as shown in Equation 3-27:

(dpm / 100 cm2) = ( Cs/Ts ) / (εT x (A/100)) ………………………………………… (3-27)

Where:
Ts is recorded in minutes instead of seconds, and A is recorded in cm2 instead of m2.
Some instruments have background counts associated with the operation of the instrument. A correction for instrument background can be included in the data conversion calculation as shown in Equation 3-28. Note that the instrument background is not the same as the measurements in the background reference area.

Bq/m2 = ( Cs/Ts – Cb/Tb ) / (εT x A) ……………………………………………… (3-28)

where:
Cb = background counts recorded by the instrument;
Tb = time period over which the background counts were recorded in seconds.
Equation 3-28 can be modified to provide conventional surface activity units as shown in Equation 3-29.

( dpm / 100 cm2 ) = ( Cs/Ts – Cb/Tb ) / (εT x (A/100)) ………………………… (3-29)

where Ts and Tb are recorded in minutes instead of seconds and A is recorded in cm2 instead of m2.

The presence of multiple radio-nuclides at a site requires additional considerations for demonstrating compliance with a dose- or risk-based regulation. As demonstrated in Section 3.3.6.2 , a gross activity DCGL should be determined.

Example 3.20: Calculation of a Derived Concentration Guideline Level for a an area contaminated by multiple radio-nuclides

Consider a site contaminated with 60Co and 63Ni, with 60Co representing 60% of the total activity. The relative fractions are 0.6 for 60Co and 0.4 for 63Ni. If the DCGL for 60Co is 8,300 Bq/m2 (5,000 dpm/100 cm2) and the DCGL for 63Ni is 12,000 Bq/m2 (7,200 dpm/100 cm2), the gross activity DCGL is 9,500 Bq/m2 (5,700 dpm/100 cm2) calculated using Equation 3-29.
When using the gross activity DCGL, it is important to use an appropriately weighted total efficiency to convert from instrument counts to surface activity units using Equations 3-26 through 3-29. In the above example, the individual efficiencies for 60Co and 63Ni should be independently evaluated. The overall efficiency is then determined by weighting each individual efficiency by the relative fraction of each radionuclide.

3.10.1.2 Soil activity: Conversion of radionuclide concentration and exposure rates to DCGL’s

Analytical procedures, such as alpha and gamma spectrometry, are typically used to determine the radionuclide concentration in soil in units of Bq/kg. Net counts are converted to soil DCGL units by dividing by the time, detector or counter efficiency, mass or volume of the sample, and by the fractional recovery or yield of the chemistry procedure (if applicable).

Instruments, such as a PIC or micro-R meter, used to measure exposure rate typically read directly in mSv/h. A gamma scintillation detector (e.g., NaI(Tl)) provides data in counts per minute and conversion to mSv/h is accomplished by using site-specific calibration factors developed for the specific instrument.

In-situ gamma spectrometry data may require special analysis routines before the spectral data can be converted to soil concentration units or exposure rates.