Radioactive Dust - Decay of Radon daughter products

As an experiment, I wiped some dust from the TV screen onto a tissue, and placed it in front of the radiation detector. The reading went from a background reading around 10 CPM to around 1300 CPM, or 130 times the reading!

This graph shows the radiation (in Counts Per Minute, CPM) over time, as the daughter products decay. In addition to plotting the raw data, I've also estimated the initial amounts of Radium B (Pb₂₁₄), Radium C (Bi₂₁₄) and Pb₂₁₂:

Radium B and Radium C are decay products of Radon. The Radon is produced by Uranium that naturally occurs in the soil, after a rather long decay process. Radon has a half life of just under 4 days. In addition to being produced by local sources, Radon (and it's daughter products) can be blown in by the winds from distant locations. So just because you don't have a radon problem in your basement doesn't mean that you won't find radioactive dust in your house!

The Radon decays into Polonium, which then decays into Radium B, an isotope of Lead. This decays with a half life of about 27 minutes in Radium C, an isotope Bismuth. This decays with a half life of about 20 minutes into another Polonium isotope, which quickly (164 microseconds) decays into an isotope of Lead. The Lead decays with a very long half life of 22 years, so we don't get much radiation from it, or any further products.

The lead and bismuth isotopes both undergo beta decay, and it is their radiation we detect in this experiment. Notice that we initially start with much more Radium B than Radium C. The Radium B decays, producing more Radium C, initially at a rate faster than the Radium C is decaying. So The amount of Radium C starts to increase. Eventually, there is not enough Radium B decaying into Radium C, and the amount of Radium C starts to drop.

Here's the process:

• Radon (Rn₂₂₂) does an alpha decay into Polonium (Po₂₁₈) with a half life of 3.824 days.
• Polonium (Po₂₁₈) does an alpha decay into Lead (Pb₂₁₄) with a half life of 3.05 minutes.
• Lead (Pb₂₁₄) does a beta decay into Bismuth (Bi₂₁₄) with a half life of 26.8 minutes.
• Bismuth (Bi₂₁₄) does a beta decay into Polonium (Po₂₁₄) with a half life of 19.8 minutes
• Polonium (Po₂₁₄) does an alpha decay into Lead (Pb₂₁₀) with a half life of 164 microseconds.
• Lead (Pb₂₁₀) does a beta decay into Bismuth (Bi₂₁₀) with a half life of 22.3 years.
• Bismuth (Bi₂₁₀) does a beta decay into Polonium (Po₂₁₀) with a half life of 5.01 days.
• Polonium (Po₂₁₀) does an alpha decay into Lead (Pb₂₀₆) with a half life of 138.38 days.
• Lead (Pb₂₀₆) is stable.

The black line at the bottom of the graph shows the amount of Lead-212 present. This is a daughter product of Thoron, an isotope of Radon which is produced by Thorium, rather than Uranium. Thoron has a very short half life, about 1 minute. Lead-212 has a much longer half life, almost 11 hours.