Dr. Zoomie – during my last inspection the regulators asked me what counting efficiency I was using when I counted smear wipes. I couldn’t give him and answer because I have no idea. How can I find that out? And what is counting efficiency anyhow – and why is it so important?
Let me start at the end of your question and work my way backwards.
When you’re counting, say, a smear wipe the number of counts your instrument shows only tells you how many counts it picked up, but you have no idea how much radioactivity that represents. The thing is, you’re required to report contamination levels in terms of the amount of radioactivity you measured, and radioactivity is a measure of the number of decays in a given amount of time.
So…say you’ve got a smear wipe beneath your pancake GM and it’s reading 400 cpm – do you need to be concerned?
One microcurie (µCi), for example, is the amount of material that undergoes 37,000 decays every second or 2.22 million decays every minute. But you can’t know how many decays are taking place unless you have a way to convert 400 cpm into decays per minute (dpm). This is where counting efficiency comes into play.
If the nuclide you’re measuring is a high-energy beta emitter such as Sr-90 then your counting efficiency is likely to be around 40% – this means that, if your meter is showing 40 counts, the sample you’re counting has experienced 100 radioactive decays. So if you’re converting cpm to dpm then you need to divide the cpm by the counting efficiency. In this case, it means that your 400 cpm reflects 1000 dpm of activity on your wipe – about 0.45 nCi.
So the next question is how to find out your counting efficiency. You can measure a radioactive source and calculate your counting efficiency, but my preferred method is to ask the company that’s calibrating my instruments to determine the counting efficiency for me. The big thing is that you need to tell the calibration lab what nuclides you’re counting so that they can give you the correct value. My pancake GM, for example, has a 40% counting efficiency for high-energy beta radiation like Sr-90, a 5% counting efficiency for lower-energy beta such as C-14, and less than 1% counting efficiency for even lower-energy betas such as tritium (H-3). I’d recommend you do the same – it’ll save you time and effort and the number you use will be more defensible than if you calculate it yourself.
So that ought to answer your questions, but the professor in me is urging me to explain what a counting efficiency of 40% actually means – what that number represents. And, as it turns out, there are two components to an instrument’s counting efficiency.
The first component is what’s called the “intrinsic” counting efficiency. This tells us, of the beta particles, alpha particles, or gamma ray photons that strike the radiation detector, how many are detected? In the case of high-energy beta radiation striking a pancake GM detector, 80% of the betas that strike the detector will register as a count. So the intrinsic counting efficiency of a pancake GM for high-energy betas is 80%
The other component is called the “geometric” counting efficiency. This tells us, of all the radiation emitted by a source, how much of it even strikes your detector. In the case of most radioactive sources, they emit radiation in all directions. That means that, even if the detector is sitting right on top of some contamination, 5only 0% of the radiation it gives off will even enter the detector.
Putting both of these together gives us the instrument’s absolute counting efficiency. If only 50% of the beta particles emitted by a Sr-90 source enter a detector and, of those that enter, 80% are turned into counts then the detector has an absolute counting efficiency of 80% x 50%, or 40%. Which is just what I expect to see when I’m counting Sr-90 with a GM detector!
And I think that covers your questions, and more.