The University is required to determine the counting efficiency and minimal detectable activity (MDA) of liquid scintillation and gamma counters in order to properly evaluate wipe test results. Monthly laboratory wipe tests must be recorded in units of disintegrations per minute (dpm) as per NRC regulations. To ensure compliance, the following procedures must be performed monthly for, at a minimum, the least efficient isotope that is used in the lab.
A. Determination of Efficiency (E):
- Use a standard of known activity. Remember that 1 uCi = 2.2 x 10 6 dpm.
- Set the gain and discriminator levels (windows) according to the manufacturer’s recommendation for the isotope to be counted.
- Count a blank (background) and the standard for one minute to obtain counts per minute (cpm) for both.
- Determine the net cpm of the standard by subtracting the background cpm from the standard cpm.
- Calculate the efficiency (E):
E = net standard cpm/activity of standard in dpm.
- Divide cpm of wipe samples by the efficiency to convert to dpm.
B. Determination of the Minimal Detectable Activity (MDA):
- Count the blank (background) for one minute.
- Calculate the MDA:
SQRT (MDA (cpm) = 4.65 background (cpm))
- To obtain results in dpm, divide the MDA by the efficiency.
- Record all calculations and results in the lab notebook with the monthly lab wipes. The MDA of the counting instrument should be less than 100 dpm. If not, please notify REHS.