Caesium-137 alienation around the Chernobyl nuclear power plant. Together with caesium-134, iodine-131, and strontium-90, caesium-137 was among the isotopes distributed by the reactor explosion that constitute the greatest risk to health. The mean contamination of caesium-137 in Germany following the Chernobyl disaster was 2000 to 4000 Bq/m. This corresponds to a contamination of 1 mg/km of caesium-137, totaling about 500 grams deposited over all of Germany. In Scandinavia, some reindeer and sheep exceeded the Norwegian legal limit (3000 Bq/kg) 26 years after Chernobyl. As of 2016 the Chernobyl caesium-137 has decayed by half, but could have been locally concentrated by much
Caesium inside old stars and by the R-process in supernova explosions. The only stable caesium isotope is Cs, with 78 neutrons. Although it has a large nuclear spin (+), nuclear magnetic resonance studies can use this isotope at a resonating frequency of 11.7 MHz. The radioactive Cs has a very long half-life of about 2.3 million years, longest of all radioactive isotopes of caesium. Cs and Cs have half-lives of 30 and two years, respectively. Cs decomposes to a short-lived Ba by beta decay, and then to nonradioactive barium, while Cs transforms into Ba directly. The isotopes with mass numbers of