How do you Determine A Custom Efficiency for Specific Isotopes Using The Radiation Alert® Family of Detectors?

The efficiency of a detector, expressed in percentages, is the probability that an emitted gamma ray will interact with the detector and produce a count, representing the probability that a gamma ray of a specified energy passing through the detector will interact and be detected.

The objective will be to define efficiencies for specific isotopes. Additionally, to quantify the data generated when using one of the Radiation Alert Wipe Test/Planchet systems. Efficiencies can be generated for a myriad of isotopes through a simple procedure. In the following example the efficiency of the Radiation Alert Ranger WTP to an alpha/beta source deposited onto a planchet will be determined. A similar procedure can be used to determine efficiencies for wipe tests.

This procedure assumes that an alpha/beta source of known activity has been prepared and applied to a planchet. In addition, the average normal background (CPM) has already been determined with the Wipe Test Plate in position.

The first step is to take a ten minute count with the planchet (source) sitting on the Wipe Test Plate (shorter count times may be used). Using the Wipe Test Plate assures reproducible geometry and that results will be comparable. Next, convert the gross counts obtained for the ten minute interval into counts per minute (CPMnet) above normal background using the following equation:

Equation 1

After  is obtained a value for the detector efficiency, E, can be determined.The equation for E is as follows:


It should be noted that both the count Rate and the activity must share the same units of time (i.e. CPM and DPM or CPS and Bq). The efficiency will always be less than 1 and have units of counts per disintegrations (c/d). The efficiency may also be expressed as a percent.

Finally, to convert a reading taken from a planchet using the Inspector + WTP into activity use Equation 4 or 5:

Equation 3


Equation 4

To determine activity when the efficiency is supplied in a percent, use either of the above equations and multiply the result by 100.

Finally, to convert DPM or DPS(Bq) into Curies(Ci) use Equation 6 or 7 :

Equation 5


Equation 6

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