CRPA(R) Prep, January 2020 / Préparation à la désignation (A)ACRP, janvier 2020
In this section of the Bulletin, we introduce a question or two similar to the questions on the CRPA(R) exam. In the next issue, we will provide the solution. The intention is to give people an idea of the types of questions we use on the CRPA(R) exam and perhaps convince more members to challenge the exam.
The CRPA(R) designation is the highest level of competency recognized by CRPA at the Canadian level. To find out more, visit the CRPA website.
If you already have your CRPA(R) designation, we invite you to submit questions to earn points for your registration maintenance!
Question from the last issue:
We went back to some calculations…
A Cs-137 source has a current activity of 20.76 kBq. A total number of 579,605 counts are measured over a five-minute counting period. If the background count rate is 88.7 cpm, what is the detector efficiency for this counter?
Our last question was purely knowledge based, so if you didn’t know the answer or didn’t know where to find it in the provided set of regulations, you might have had to throw a dart!
This question is for all you calculation lovers out there. With general knowledge of counting instruments and associated calculations, you can figure this one out in your sleep. If you have only basic knowledge of radiation protection fundamentals, you should still be able to work out the answer. Even using unit analysis, you might be able to get the job done.
The CRPA Radiation Safety Professional Registration Process document (found here) is a good resource. Section 4 includes a recommended reading list with many resources that would address this particular topic. My favourite resource for this kind of question would be Knoll’s Radiation Detection and Measurement.
However, even if you have (misguidedly) ignored the registration subcommittee’s terrific advice to go through the recommended reading list, and you managed to register for the exam while having no idea what detector efficiency is, you can still take a stab at the question by inferring what is meant. Basically we want to know, how good is this fictional detector at detecting?
If you did (sensibly) heed the subcommittee’s advice to go through the recommended reading, you would know that the detector efficiency (in this case, specifically, the absolute efficiency, which you would have understood from the context of the question and information provided) is given in Knoll as:
Absolute Efficiency = Number of pulses recorded/Number of radiation quanta emitted by the source
The number of pulses recorded is a gross count of 579,605 in 5 minutes, or 115,921 counts per minute (cpm). When we subtract the background count rate of 88.7 cpm, we get a net count of 115,832.3 cpm. (Interestingly, the background count rate is so small compared to the measured rate that failure to include it should yield the same answer.)
The number of quanta is given as an activity (assuming one quanta per disintegration). Remember that the definition of a becquerel is one disintegration per second (dps). So we are actually given 20,760 dps.
Efficiency is usually described in units of counts per second (cps)/disintegrations per second (dps). So we need to divide our net measured count rate by 60 (seconds per minute) to go from cpm to cps to get our units to line up, which gives us 1,931 cps. Plug ’n’ play time!
Absolute Efficiency = 1,931 counts per second (cps)/20,760 disintegrations per second (dps)
Working that out gives us a detector efficiency of 0.093 (9.3%) so the answer is b.
The question for next time:
Let’s stay in the measurement and counting realm, but make it a little more difficult.
A gamma counter produces a measurement of 1,805 counts over a five-minute period. What is the count rate and associated standard deviation?
a. 361 + 115 cpm
b. 361 + 75 cpm
c. 361 + 9 cpm
d. 361 + 11 cpm
 Knoll, G.F. (2000). Radiation Detection and Measurement (4th ed.). Hoboken, NJ: John Wiley & Sons.