> What is the specific radioactive activity of pure natural bismuth compared to pure natural potassium?

These are the kinds of questions that occur to me when I am cleaning my countertop with Windex.  I can't help it.  I figured the potassium was more radioactive and I am correct.

What is the specific radioactive activity of pure natural bismuth compared to pure natural potassium?
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For pure natural potassium, the specific activity is about 31 Bq/g (31 decays per second per gram). This comes almost entirely from the naturally occurring isotope Potassium-40, which makes up about 0.0117% of natural potassium.

For pure natural bismuth, which is essentially 100% Bismuth-209, the specific activity is only about 3.2 × 10⁻⁶ Bq/g because its half-life is roughly 2 × 10¹⁹ years.

Comparison

Natural potassium    ~31 Bq/g
Natural bismuth    ~3.2 × 10⁻⁶ Bq/g

So natural potassium is about 10 million times more radioactive per gram than natural bismuth. 📊

Another way to view it:

1 gram of potassium: ~31 decays/s
1 gram of bismuth: ~0.000003 decays/s ≈ 1 decay every 3.6 days

Even though bismuth is technically radioactive, its activity is so low that for most practical purposes it behaves as a stable element. In fact, ordinary human tissue (because of its potassium content) is far more radioactive than a piece of pure bismuth metal.
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