Radioactive Cesium 137

Today we will explore the radioactivity and uses of Caesium 137!

Caesium is probably best known for its high reactivity and low melting point of only 28.5°C and it was first discovered in 1860 by two German scientists, Robert Bunsen and Gustav Kirchhoff. It has atomic number of 55 and has only one natural isotope, Caesium 133 which is stable. Caesium 137 on the other hand is a radioactive and it is produced by a nuclear fission of Uranium 235 and it is commonly found in nuclear waste and fallout. It has a medium-short half-life of 30.1 years and a single gram has an activity of 3.215 TBq (86.9 Ci).

TG-36 spark gap tube containing <1 uCi of Cs-137


Since Caesium 137 is one of the main radioactive elements found in nuclear fallout, it is very often used as a calibration source for radiation detectors. It also has many applications in the medical industry where it can be used in radio-therapy to fight cancer or to sterilise medical equipment. It can also be found in thickness gauges, flow meters and in gamma-ray well logging devices. Because Caesium 137 wasn’t produce before 1945, it can be used to date wine and detect counterfeits.

Health risks

Caesium 137 is one of the most dangerous isotopes found in nuclear fallout because of its strong gamma-rays but unlike Strontium 90, if ingested it is distributed around the body more or less evenly and it has a short biological half-life of 70 days.

Caesium 137 in mushrooms collected in Belarus

Radioactive decay & gamma spectroscopy

Caesium 137 is a beta and a gamma emitter. In 94% of the cases, it emits a beta particle (511 keV) turning into a metastable Barium 137m which then emits a gamma-ray (662 keV) before becoming stable. In the remaining 6% of cases, Caesium 137 decays directly into stable Barium 137 by a beta emission (1172 keV).

Caesium 137 has a very characteristic gamma spectrum with two large peaks at 31 keV and 662 keV which make it a very popular calibration source for gamma spectrometers.

TG-36 tube, RAYSID gamma spectrometer FWHM <8.5%

My samples

As of right now, I got two samples of Cs137. The first one is a TG-36 Spark Gap Tube produced by CP Clare. According to the date code, it was produced in the September of 1985 and it originally contained <1uCi of Cs137. Today the activity drop to <0.43uCi but it is still detectable and it measures just over 1 uSv/h on my RAYSID gamma spectrometer and about and about 450 CPM on my Ludlum Model 3 with 44-9 probe at 1cm.

TubeOriginal activity
CK1097-15< 280 pCi / 10.4 Bq
TG-20A< 1 uCi / 37 kBq
TG-30<5 uCi / 185 kBq
TG-36< 1 uCi / 37 kBq
TG-53< 5 uCi / 185 kBq
TG-77< 0.9uCi / 33.3 kBq
TG-133< 5 uCi / 185 kBq
TG-162< 5 uCi / 185 kBq
XG-1684< 1uCi / 37 kBq
Table with some of the tubes that used Cs-137

My other source is made from ashes of Belarusian mushrooms which contain the fallout from Chernobyl. They clock at almost 250 CPM on my Ludlum at 1cm distance but when measured in a lead castle with my RAYSID, the activity increased only by 6.5 CPS.


  • I have recently purchased TG-71 spark gaps which also contain Cs-237. On RaysID one has 5.4 uSv/h (850 CPS/ 51kCPM) and two others 270 and 450 CPS (on RaysID). So this one is pretty hot 🙂

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