How Much Radioactive Material is in a Smoke Alarm?

Introduction

I was watching this Youtube video below on Americium and they mentioned that Americium is used in small quantities in smoke detectors. I thought it would be a nice mathematics exercise to compute the mass of Americium-241 in a smoke detector. I also thought it would be another good excuse to try out Mathcad Prime 2.0.

Warning on the video – Youtube often puts advertisements at the beginning. I have no control over what they put there.

Background

There is a very complete discussion of smoke detector operation at this web site, but I will give a brief description of how they work here. For a more complete discussion of Americium and its history, try this document.

The radioactive source produces alpha particles that ionize the air in a small chamber, which makes the air conductive. This conductivity can be sensed by electronics. When the air is clear of smoke, the ionized air conducts a given amount of electrical current. During a fire, smoke particles enter the chamber and reduce the conductivity of the air. The electronics sense this change in conductivity and activate the alarm.

This post will examine the amount of the radioactive material used in the home smoke detectors.

Analysis

Figure 1 shows a photograph of the smoke sensor from a home smoke detector I found on the Wikipedia.

Figure 1: Wikipedia Photograph of the Radiation Source within a Smoke Detector.

We see that the smoke sensor is actually labeled with the phrase “1.0 µCi 37k Bq”. This means that the radioactive source generates 1 µCurie of radiation, which equals 37,000 Bequerels (Bq). A Bq means a single decay event, which for Americium-241 means the generation of an alpha particle.

Figure 2 shows my calculations for determining the mass of Americium-241 needed to make 37,000 Bq.

Figure 2: Americium 241 Mass Calculations.

I calculate that there must be 0.29 µgrams of Americium-241 in the smoke detector. This agrees with the value given from other sources, like here.

Conclusion

I am amazed that people can actually measure out 0.29 micrograms of anything for a consumer product. As far as Mathcad Prime 2.0 goes, I am becoming more comfortable with it. Mathcad 15 has been a good friend for the last couple of years, but I am excited by where I see Mathcad Prime going. I am looking forward to seeing what is in Mathcad Prime 3.0, which is scheduled for release next year.

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