If you find yourself in possession of a few bags of high-potassium commercial fertilizer, or potassium chloride water softener tablets, or potassium chloride ice/snow melter, or potassium-based salt substitute, or even a bowl of Brazil nuts, try getting a radiation measurement from them with your Geiger counter. It’s not difficult to get a reading that’s twice the normal background radiation.
The key is the potassium: household items that are high in potassium are very slightly more radioactive than other objects. This is because elemental potassium has a naturally occurring isotope called potassium-40 (about 1 in every 8,000 atoms), which is very slightly radioactive. (We keep emphasizing very slightly so that you don’t panic at the sight of a bunch of bananas, which are high in potassium.)
This very slight radioactivity is more than enough to be detected by your Geiger counter.
Rather than using the preassembled Geiger counter, buy a kit, such as Electronic Goldmine’s Sensitive Geiger Counter Kit (sku C6979), and build it yourself. Note: This kit requires intermediate-level soldering skills; we recommend it for makers who have accomplished at least a few successful soldering projects.
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