TOPIC 3: Efficiency and Calibration

Let’s explore Efficiency and Calibration.

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DPM is a way of stating activity.  CPM is an indication of the pulses received by the survey meter.  But how do we relate the CPM received by the survey meter to the DPM emitted by the source?

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We have Efficiency.

DPM is a function of the radionuclide. It is the number of atoms that are changing from one form to another – in other words, disintegrating. The different types and energies of radiation travel in all directions from the radioactive source.

CPM is a function of the survey meter. It is the number of these radiations that can be detected by the meter. You can’t detect them all but you can detect a representation of them all.

If we can assign a percentage to that representation, we can determine the meter Efficiency.

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A large quantity of disintegrations will end up going in all directions.

Using the concept of probability, the decay products going through the survey meter is a fairly consistent percentage of the total decay products of the radionuclide. Every survey meter has its own particular “efficiency” of detecting radioactive material.

So how can the efficiency be determined?

Start with a source of known activity. We put that source a known distance from the detector.  We then determine the Counts Per Minute and divide it by the source’s DPM.  We have the efficiency.  Now we can go to any source, place it at the same distance, and through efficiency we can fairly accurately determine the source’s activity.

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We can estimate the activity by taking the CPM and dividing it by the efficiency to get the DPM.

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To estimate the efficiency of the survey meter, first measure and record background radiation (bkg) in CPM in a nearby location where there are no artificial sources of radiation. Now go back to the source. Place the source a certain distance away from the detector. Write down the cpm displayed on the meter. Subtract the background cpm to get the net cpm. Divide the net cpm by the dpm of the source to determine the efficiency.

For example, let’s use a known radioactive source with 30 µCi of Cs-137. For simplicity, let’s say there is zero background. Hold the survey meter exactly one foot away from the source and it reads 4,500 cpm on the X1000 scale. Convert the 30 µCi to dpm to come up with an efficiency of cpm to dpm.

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Calibration of radiation instruments gives us confidence in their readings and is performed at least annually. Some survey meters, such as those in the medical field, are calibrated semi-annually or quarterly or even daily. The majority of facilities use a third-party vendor and send there survey meters off to be calibrated.

Meters are calibrated by placing them in a radiation field of known intensity and then adjusting the Calibration potentiometers to display that intensity.

You can use a National Institute or Science and Technology (NIST)-traceable “button” source to check your survey meter.  But you cannot calibrate your survey meter unless you are licensed to do so.

Check the meter if you suspect damage (maybe it was left in a hot truck, it was dropped, etc.).

If the meter fails a check measurement, send it for repair and calibration.  If the cable is damaged, send it for repair and calibration.  If you do anything except change the batteries, send it for repair and calibration.

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Meters like the G-M that display in CPM must go through a two-step calibration process.

The first step is to ensure that the number of counts that the meter receives from the detector is accurate. This is accomplished using a “pulser.” A pulser is a device that pulses, just like the detector send. Once the meter has been calibrated to the pulser, it is then placed in front of a radioactive source.

In this second step of calibration, the radioactive source is set to generate 1 mR/hr of exposure and the detector is placed in the beam path.

The Counts per Minute that are equivalent to 1 mR/hr is written on the calibration certificate. For typical G-M survey meters, there are approximately 3,600 cpm per 1 mR/hr of exposure.

Each make and model of survey meter has its own conversion.

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We start on the high end of the scale.  In this instance, lets assume 2/3 full scale is 4.5 mR/hr.  We have a source where we can adjust the distance from the detector and place lead shielding in between.  We use the concept of distance and shielding to get the radiation field we need.  We place our detector in the radiation field and then adjust the potentiometer until it displays exactly the strength of the radiation field.

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Each scale is calibrated and the results recorded on a calibration sheet.

A decal is placed on the detector indicating when the meter was calibrated and at which facility.

A calibration certificate signed by the calibration facility is an official record that must be kept by the RSO.

The decal is not the official indication the meter has been calibrated. The certificate, with the signature, is the only official record.

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Most survey meters are calibrated using a Cs-137 source generating 661 keV gamma.  All survey meters respond equally at this energy level.  We’ve marked it on this chart.

So a survey meter calibrated for Cs-137 could measure Bismuth-214 fairly accurately because the decay energies are very similar. But it could over respond to lower energy radionuclides.

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Your Radioactive Materials License will tell you how often your equipment will need calibration.

For most industries, it is at least ANNUALLY.

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You can ONLY change out the batteries. Any other maintenance or repair invalidates the Calibration.

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A Ludlum survey meter calibrated using a Cs-137 source would indicate less of the Co-60 (almost half).

But for Co-60, we get lucky. Only half the actual radiation is detected, but there are two gammas for each disintegration.

So by chance, Co-60 can be measured reasonably well by the Ludlum survey meter without any adjustments of the numbers.