CRPA(R) Prep, September 2019 / Préparation à la désignation (A)ACRP, septembre 2019
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 that we use on the CRPA(R) exam and perhaps convince more members to challenge the exam.
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:
What is the net count rate in counts per minute (cpm) given the following information?
- Removable activity 3 Bq/cm2
- Area wiped 100 cm2
- Collection factor for the wipe 10%
- Instrument efficiency 50%
a. 15 cpm
b. 900 cpm
c. 90 cpm
d. 150 cpm
This is a straight-up calculation question, and experienced radiation safety people should be able to figure out the answer fairly quickly. That being said, a number of situations could make this question challenging. Maybe it’s been a while since you’ve had to do a calculation like this, or maybe your facility doesn’t deal with a lot of loose contamination or indirect checking of surfaces, or maybe you just don’t know how to do the calculation. If any of these apply, this is a perfect opportunity to employ an age-old exam technique: unit analysis.
We want to get from Bq/cm2 to counts per minute (cpm). Let’s keep that in mind while we figure out what is going on.
We have wipe that we used to sample an area of 100 cm2. The area is contaminated with 3 Bq/cm2 (which we assume is uniformly distributed over the area). We are given a collection efficiency of 10%, which is the percentage of the surface contamination that is transferred to the wipe (and is a good rule of thumb in the absence of empirical data). Finally, we have the instrument efficiency, which gives us how many cpm we see on the instrument for every disintegration per minute (dpm) in the sample for the geometry of this particular situation.
If we think about what is going on, we can use unit analysis to make sure we are on the right track for this calculation. Basically, we can calculate the activity captured on the wipe and then calculate the reading expected on our measurement device for that activity.
To calculate the activity on the wipe (in Bq collected), we multiply the removable contamination (in Bq/cm2) by the area wiped (in cm2), then we multiply that result by the collection factor (in Bq collected per Bq removable contamination).
activity on wipe
= removable contamination × area wiped × collection factor
= 3 Bq/cm2 × 100 cm2 × 10% (Bq/Bq)
= 30 Bq
So, we have 30 Bq on the wipe.
Note that the area units cancel and you are left with Bq. We are looking for a result in cpm. The efficiency for the counting system is given by cpm/dpm. So we need to convert our sample activity to dpm. By definition, the Bq is disintegration per second (dps), so dpm is simply dps multiplied by 60 (60 seconds in 1 minute).
To calculate the resulting count rate, we multiply the activity on wipe (in Bq) by the counting efficiency (in cpm/dpm), then by 60 (dpm/Bq).
count rate result
= activity on wipe × counting efficiency × 60 dpm/Bq
= 30 Bq × 50% cpm/dpm × 60 dpm/Bq
= 900 cpm
The Bq and dpm cancel, so you are left with an answer in cpm. The answer is b., 900 cpm.
The question for next time:
What is not an exemption under the regulations?
a. A Canadian Nuclear Safety Commission inspector possessing a cesium-137 check source
b. A company shipping a replacement source for an industrial radiographer
c. An individual possessing less than an exemption quantity of iodine-125
d. Naturally occurring radioactive material not used in development of nuclear power