Exploring Krypton 85 and its radioactivity

Welcome back fellow radiation nerds, today we dive deep into the radioactivity and uses of Krypton 85!

Krypton is a colourless, tasteless and odourless noble gas and it was first discovered in 1898 by Wiliam Ramsay and Morris Travers. It has atomic number of 36 and its most common isotope is Krypton 84, which is stable.

Kr-85 on the other hand, is radioactive and it was first discovered in 1940 by Martin Kamen and Sam Ruben, during their research involving the neutrons bombardment of stable Krypton isotopes, at the University of California, Berkeley.

Krypton-85 is predominantly produced through human activities such as nuclear fission and the reprocessing of spent nuclear fuel, however, traces of Krypton 85 are also produced naturally by the interaction of cosmic rays with a stable Krypton-84 found in Earths atmosphere.

Some of the main uses of the radioactive Krypton 85 are in arc discharge lamps and in cold-cathode voltage regulator electron tubes. Since Krypton is a gas, it can also be used to find leaks in closed systems and small defects in aircraft components.

Radioactivity & Gamma spectroscopy

Krypton-85 is a radioactive isotope with a half-life of approximately 10.76 years. It decays by beta emission, transforming into stable rubidium-85 and in the process it also releases a gamma ray with an energy of 514 keV. 

Krypton 85 gamma spectrum, RAYSID Gamma Spectrometer (FWHM <7%)

My Sample

My sample comes in the form of an old DDR smoke detector produced by Robotron RFT. The model number is 70130 and it was used in industrial complexes. The actual radioactive source is a little glass bulb originally containing 18.5MBq of Kr-85.

Robotron office (Source: DNN.de)

Since the detector is couple decades old by now, the activity of Krypton 85 decreased but it is still detectable when measured with my RAYSID gamma spectrometer. When placed on the top of the detector, RAYSID showed an activity of 136 CPS and had no problem in detecting the peak at 514 keV.

German smoke detectors and the activity of their sources (source: geigerzaehlerforum)

The housing of the smoke detector acts like a shield and stops most of the beta radiation, which results only in a slight increase in the activity when measuring it with my Ludlum Model 3 Meter with a 44-9 probe. This however changes when the housing is removed and the glass bulb is exposed. At 5cm my Ludlum reads 500 000 CPM and it maxes out if brought any closer. RAYSID reads a gamma dose of 0.86 uSv/h (675 CPS) at 1cm distance.

Rundfunk RFD 70361 smoke detector source containing <18.5 MBq of Kr85

Overall I am very happy to finally get a chance to explore the radioactivity of Krypton 85 and share my experiences. I absolutely love shape and form of the source and I find it very interesting that this smoke detector uses Kr85 instead of much more common Am241. If you ever come by one of those detectors, they are a great find but be careful as the glass bulb is rather fragile and can easily break

Thank you so much for reading this post, I hope you enjoyed it and learned something new! If yes, please make sure to subscribe to the email list so that you get notified when new posts are added. Also feel free to check out my Ko-Fi page where you can donate a nice cup of radioactive coffee and support my work financially.

and remember, stay active!

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