# Time, Distance, and Shielding

Part of the Nuclear Science Merit Badge Digital Resource Guide: [Introduction & Overview](https://merit-badge.university/merit-badges/nuclear-science/guide/index.md)

*5. Radiation Protection*

> **5a.** Using a radiation survey meter and a radioactive source, show how the counts per minute change as the source gets closer to or farther from the radiation detector. Place three different materials between the source and the detector, then explain any differences in the measurements per minute. Explain how time, distance, and shielding can reduce an individual's radiation dose.

This requirement puts ALARA into action. Instead of talking about safety in theory, you measure how the setup changes the reading. That makes the three main protection tools feel real.

## What counts per minute tells you

A survey meter detects radiation events and reports a reading such as **counts per minute (CPM)**. A higher CPM usually means the detector is receiving more radiation events. If the source moves closer, the reading often rises. If it moves farther away, the reading often falls.

The reason distance matters is simple: radiation spreads out. When the detector is farther away, less of that radiation reaches it.

## What shielding shows

Different materials reduce radiation differently because alpha, beta, and gamma radiation do not all interact with matter the same way. A light barrier may stop alpha radiation. Denser or thicker material may be needed for beta or gamma radiation.

When you compare three shielding materials, do not just list the readings. Explain **why** the differences happened. Was the material denser? Thicker? Better suited to that type of radiation?

[How Materials Shield Radiation (PDF)](https://filestore.scouting.org/filestore/Merit_Badge_ReqandRes/Requirement%20Resources/Nuclear%20Science/shutterstock_1169023357.pdf): A visual guide that helps you compare how different materials reduce radiation in different situations.

Video: [Time, Distance and Shielding (video)](https://youtu.be/bNSKIbRNy3c?si=3WqSseNb_EbOeZAx)

Video: [Measuring Radiation (video)](https://youtu.be/GdZD_hJBdOE?si=p4mOulsJpBxDWH5j)

Video: [Using a Survey Meter: CPM vs Distance (video)](https://youtu.be/0-Yaeh3dOZk?si=Z1MokzEOznMdvlZD)

**Three ideas to say out loud** — These are the real lesson of the activity

- [ ] **Time**: less time near the source usually means less dose.
- [ ] **Distance**: more distance usually means a lower reading.
- [ ] **Shielding**: the right material can reduce what reaches the detector or person.

> **Safety First!**
>
> Only a qualified adult should provide the source, meter, and setup for this activity. Never point unknown materials at a detector just to "see what happens," and never handle sources unless you have direct instruction to do so.

If you want to explore a radiation issue that shows up in ordinary buildings, the radon option is next.

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- Previous: [Choose a Safety Investigation](https://merit-badge.university/merit-badges/nuclear-science/guide/req5/index.md)
- Next: [Radon at Home](https://merit-badge.university/merit-badges/nuclear-science/guide/req5b/index.md)