[2] Other decay modes have very small probabilities and emit less energetic gamma rays.
[1][3] Krypton-85 is mostly synthetic, though it is produced naturally in trace quantities by cosmic ray spallation.
Krypton-85 is produced in small quantities by the interaction of cosmic rays with stable krypton-84 in the atmosphere.
[4] As of 2009, the total amount in the atmosphere is estimated at 5500 PBq due to anthropogenic sources.
[6] The most important of these human sources is nuclear fuel reprocessing, as krypton-85 is one of the seven common medium-lived fission products.
Current nuclear reprocessing releases the gaseous 85Kr into the atmosphere when the spent fuel is dissolved.
The cumulative global amount of krypton-85 released from reprocessing activity has been estimated as 10,600 PBq as of 2000.
[4] The 1979 accident at the Three Mile Island nuclear power plant released about 1.6 PBq (43 kCi).