GMC-320S Geiger Counter Test Materials

The exact order can vary substantially because activity, geometry, shielding, and the 320S detector’s response all matter.

RankSourceExpected usefulness with GMC-320SNotes1****BananaVery weak~65 Bq K-40 per 500 g. Good sanity check, but don’t expect a dramatic increase. Canadian Nuclear Safety Commission2****Potatoes / carrotsVery weakSimilar K-40 activity; potatoes 63 Bq/500 g. Canadian Nuclear Safety Commission3****Brazil nutsWeak103 Bq K-40/500 g, plus naturally occurring radium-226. Better than bananas. Canadian Nuclear Safety Commission4****Granite / granite countertopWeak–moderateUranium/thorium/potassium occur naturally in rocks. A large piece close to the detector is much more useful than food. Canadian Nuclear Safety Commission5****Pottery / ceramic containing natural mineralsModerateSome glazes and mineral-rich ceramics can produce measurable increases.6****Uranium glassModerate–strongA small piece can produce a very obvious response on a 320S.7****Uranium-glazed vintage potteryStrongParticularly good for demonstrating the 320S’s beta/gamma response. GQ’s forum has reports of very large increases with uranium-glazed pottery. GQ Electronics8****Thorium-containing old lantern mantleStrongSome older mantles used thorium compounds. Don’t burn, cut, crush, or otherwise disturb one.9****Uranium ore specimenVery strongNatural uranium ore can produce thousands of CPM on this class of instrument. One published GMC-320 example reports ~2,905 CPM. MCU Mall10****Commercially sold educational/check sourceVery strongA properly packaged, legally sold check source can give you a repeatable test signal. Don’t improvise with loose radioactive material.