Making Sense of Radiation Units

If you are new to the nuclear industry, you might feel like you’ve stepped into a bowl of alphabet soup. You’ll hear terms like Rem, Sieverts, Becquerels, Counts, and Curies thrown around constantly.

Don’t panic. You don’t need to be a mathematician to stay safe. You just need to know which units measure risk (dose) and which measure mess (contamination).

1. Measuring Dose (The Risk)

When we talk about “Dose,” we are talking about the potential biological effect on your body.

The Two Main Systems: Just like miles vs. kilometres, the nuclear world is split between two systems.

North America: Often uses Rem and mRem.

International (Europe/Asia): Uses Sieverts (Sv) and millisieverts (mSv).

The Golden Rule of Conversion:
As mentioned in the video below, if you travel between sites, you might see different units. The conversion is simple:

1 Rem = 10 mSv

Pro Tip: Don’t try to constantly convert in your head while working. If your site uses mSv, think in mSv. If they use Rem, think in Rem. Stick to the local language to avoid confusion.

2. Measuring Contamination (The Mess)

When you use a meter to check for contamination (radioactive “dirt”), you will see different units depending on the instrument.

Raw Counts (CPM & CPS):

CPM (Counts Per Minute): Common in North America.

CPS (Counts Per Second): Common in Europe.

Important: These numbers tell you what that specific instrument is seeing. You cannot compare the CPM of a large floor monitor to the CPM of a small hand-held frisker. They have different “eyes” and see things differently.

Activity (Becquerels & DPM):

To standardize things, Health Physics calculates the actual amount of radioactive material, regardless of the tool used.

Becquerel (Bq): One atom decaying per second.

DPM: Disintegrations Per Minute.

Surface Contamination: You will often see limits written as Bq/cm². This balances the math so you can compare apples to apples.

3. Video: Units Explained

Watch this breakdown to see how these units relate to the ICRP limits and how to visualize the conversion between Rem and Sieverts.

In this video, we cover:

Prefixes: Why “milli” and “micro” matter.

The Ruler Analogy: Visualizing the 50 mSv (5 Rem) annual limit.

The “Back End” Work: How Health Physicists set the instrument limits so you don’t have to do the math in the field.