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Radioactive Samples & Measurements

Natural background


Common background radiation goes from 0.041μSv/h to 0.081μSv/h (3650 - 7200μSv/year). At my location, Measured to be in average 23 CPM or 0.135 μSv/h.

Potassium (K)

Potassium-40 (K-40) is perhaps the most common radioactive nuclide. Potassium is widespread in all living beings and is fundamental for cell signaling in e.g. neurons. Natural potassium is composed by three isotopes: K-39, K-40, and K-41. The 39 and 41 isotopes are stable and constitute 99.988% of total potassium. Potassium-40 is 0.0117% and is radioactive with an half-life of 1.252 billion years. In 98.28% of cases, K-40 decays to calcium-40 by β emission with a maximum energy of 1.31 MeV and an antineutrino. About 10.72% of the time it decays to argon-40 by electron capture, with the emission of a 1.460 MeV gamma ray. Both Ca-40 and Ar-40 are stable isotopes. Given the half-life of 1.252 billion years, the specific activity of potassium-40 is 254.5 KBq/g. As said, 1 g of natural potassium contains 0.117 mg of K-40, so 1 g of potassium has a specific activity of about 31 Bq.


A famous source of K-40 radioactivity is the banana dose, a curious unit of measure of the effective dose a human is subjected by eating a banana. Assuming a banana of 150 g, it contains about 360 mg of potassium per 100 grams. This totals to about 65 μg of K-40 that, with a specific activity of 264.5 KBq/g (see above), produces an activity of 17 Bq. Unfortunately, this is too small and hard to detect.

Potassium Chloride (KCl)

A more concentrated source of potassium, is potassium chloride (KCl) which can be bought easily online. I bought a bag of 454 g of KCl (one pound) which is said to be about 95% pure. Potassium chloride contains 52.44% of potassium, which means 26.4 mg of K-40. The activity is much higher than the banana dose, reaching 7 KBq. This is readily measurable. My SGS tube measures 18 CPM (background already subtracted), which converts to 0.11 μSv/h. Given the background is 23 CPM, my pound of potassium chloride causes an almost 2-fold increase in detected activity.

Thorium (Th)

Thorium is one of the few radioactive primordial element. It is formed during the core collapse of very massive stars and dispersed in space by the explosion of the resulting supernova. No stable isotopes of thorium exist – all are radioactive. Fortunately (?), thorium-232 has a half-life of 14.06 billion years, which is comparable to the age of the universe (13.8 billion years). This implies a good quantity of Th-232 can be found in natural rocks and minerals. Natural thorium is effectively pure Th-232: the second most abundant isotope is Th-230 (0.02%) and the others are present in traces. Th-230 has a half-file of 75.400 years and is a byproduct of uranium decay.

Th-232 is the head of the “thorium series” decay chain. The chain follows: Th-232, Ra-228, Ac-228, Th-228, Ra-224, Rn-220, Po-226, Pb-212, Bi-212. Here the chain splits: Bi-212 decays 64% of time to Po-212 and 36% of time to Tl-208. Both decay to Pb-208, which is stable. The chain contains 6 nuclides that alpha decay, and 4 which beta decay. The longest-lived isotope in the serie (after Th-232) is Ra-228, which has a half-file of 5.7 years. This implies that all Th-232 samples older than about 50 years (about 10 times the Ra-228 half-life) contains all 10 decay products of Th-232 in equilibrium, with Th-232 slowly decreasing and Pb-212 slowling accumulating. As an interesting note, a pure sample of Th-232 has a significantly lower radioactivity than a 50-years old sample, due to the missing radioactivity of all daugther isotopes. Assuming none of the Rn-220 is dispersed (it is a gas), an “equilibrium” sample would be about 10 times more radioactive than a pure Th-232 sample. Monitoring the radioactivity of a pure Th-232 sample with time we would observe a steady increase for the first 50 years or so, and then an essentially constant activity for millions of years.

Gas Mantle

As of May 2018, my SGS-5 geiger counter measures 287 CPM (background already subtracted), or 1.68 μSv/h.

Other sources to explore


Uranium glass

As of May 2018, my SGS-5 geiger counter measures 13 CPM (background already subtracted), or 0.08 μSv/h.


As of May 2018, my SGS-5 geiger counter measures 2011 CPM (background already subtracted), or 11.75 μSv/h.


smoke detector

As of May 2018, my SGS-5 geiger counter measures 258 CPM (background already subtracted), or 1.51 μSv/h.

Other possible sources…

Last update: 29 June 2018