CRPA(R) Prep, March 2020 / Préparation à la désignation (A)ACRP, mars 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 are interested in obtaining the CRPA(R) designation, we recommend you apply soon to ensure you can write the exam in June 2020! The application deadline is April 9, 2020.
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:
Let’s take a look at the solution to the question from the last issue. We went back to some calculations.
A gamma counter produces a measurement of 1,805 counts over a five-minute period. What are the count rate and the associated standard deviation?
a. 361 ± 115 cpm
b. 361 ± 75 cpm
c. 361 ± 9 cpm
d. 361 ± 11 cpm
This is another good question for all you calculation lovers out there! We took it up a notch this time with the inclusion of uncertainties and error. Like last time, you shouldn’t have any problems solving the problem with general knowledge of counting instruments and associated calculations. Things can get tricky with uncertainties, though—but that’s why you prepare for these exams!
As I have mentioned many times in previous CRPA(R) Prep columns, when it comes to instrumentation and counting, my favourite “go-to” resource is Knoll’s Radiation Detection and Measurement. There’s an entire section of the book dedicated to the propagation of errors in calculations. This book is part of the recommended reading list in the CRPA Radiation Safety Professional Registration Process Document on the CRPA website.
Getting back to our question, count rate is given by:
We know the count rate in this example is 361 cpm. (We didn’t even need to calculate this—it’s the only option provided!) The corresponding standard deviation for total counts is given as:
So, the standard deviation for counts is given by the square root of 1,805.
Now, here is the key piece of information. Typically, it is assumed that time is measured with very small uncertainty—so small that we can, by convention, consider it constant.
So, even without heeding the obviously sound advice of the Registration Subcommittee and using the recommended reading list, you can reach into the far recesses of your mind—go back to your early lab courses—and recall error propagation.
Error propagation for division by a constant (in this case, time) involves finding the fractional error. In this example, that is given by:
So, our final answer before rounding looks like 361 ± 8.497 cpm. By some conventions, the uncertainty error is always rounded up. (You don’t become MORE certain going to one signification figure.) In any case, the closest answer is c, 361 ± 9 cpm.
Some people might be inclined to calculate the rate first, then take the square root of that result (361 ± 19 cpm). Others might want to quote only the uncertainty in the counts (361 ± 42 cpm). In this particular example, you will know right away that you are not on the right track, as those answers are not offered for this question. Other questions might not be as generous and may have distractor answers that include common miscalculations. Be careful out there! Check your work!
The question for next time:
You have a 555 MBq Ni-63 sealed source in permanent storage. How often must a leak test be conducted?
a. Once a month
b. Every 12 months
c. Every 24 months
d. Only when the source is disposed of
If you would like to see a particular type of question covered in this column, feel free to send me an email!
1. Knoll, G.F. (2000). Radiation Detection and Measurement (4th ed.). Hoboken, NJ: John Wiley & Sons.