CRPA(R) Prep, September 2018 / Préparation à la désignation (A)ACRP, septembre 2018
In the last issue of the Bulletin, we debuted the “CRPA(R) Prep” column. In each issue, we will propose a question or two similar to the questions on the CRPA(R) exam. The answers will be published in the next issue, along with new questions.
The intention is to give people a sense of how the questions are written for the CRPA(R) exam and their level of difficulty. Hopefully, we can encourage more members to take the exam and become Registered Radiation Safety Professionals (RRSPs).
Also, 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:
Here is the question from the last issue:
Calculate the effective half-life for a radionuclide with a physical half-life of 16 hours and a biological decay constant of 0.0068 per minute.
So let’s take a look at a proposed solution. There are lots of ways to tackle this problem. It’s pretty straightforward, but there are some small areas where people might get tripped up.
First, always make sure your time units are the same. It doesn’t matter which unit you choose, but they have to match—I converted everything to minutes.
Next, let’s identify some relevant questions to consider. The two most common are:
- λe is the effective decay constant
- λp is the physical (or radiological) decay constant
- λb is the biological decay constant
- t1/2e is the effective decay constant
- t1/2p is the physical (or radiological) decay constant
- t1/2b is the biological decay constant
You need to use either all decay constants or all half-lives. I chose to use decay constants to avoid the pesky denominators. You find the physical decay constant by using the physical half-life with the following well-known relationship:
I used 960 minutes (16 h) and got 0.00072 min−1 for the physical decay constant.
If you add this to the biological decay constant 0.0068 min−1, you get 0.0075 min−1 for the effective decay constant.
Finally, to get an effective half-life of 92 mins you use
Sometimes, the tricky exam coordinator will give you answers in different units, so you may have to do some unit conversion to find the correct multiple-choice answer.
SEE! That wasn’t so bad—you can totally nail this exam!
Here’s this issue’s question:
One tenth value layer (TVL) is equal to approximately how many half value layers (HVLs)?
Stay tuned for the next issue to check your answer and get another question!