NukeWorker Forum
Career Path => Radiation Safety => Topic started by: RFaunt on Nov 08, 2014, 09:03
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I'm currently a student in a Radiation Protection program in SC, and curiosity has led me to post to the forum. Can anyone explain to me or put me on the path to the answer for why Cs-137 is so widely used as a calibration source in radiation detection instruments? I know that Cs-137 decays to Ba-137m approx 95% of the time which gives off a 662 keV photon. Is there something distinct about that energy level which makes it a reliable calibration source? I've also entertained the idea that the 30.08 year half life could play a factor as well. Any direction or input would be greatly appreciated.
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You will want to familiarize yourself with the ISO 4037 series.
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Can anyone explain to me or put me on the path to the answer for why Cs-137 is so widely used as a calibration source in radiation detection instruments?
Might be due to being a commonly found fission fragment isotope one could encounter in a radiological facility, as opposed to F-18? ;)
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You are absolutely correct in that it has a relatively long half life and is an actual fission product common to a nuclear plant. It is also a conservative choice because it's photon energy is below that of the average photon energy found at a nuclear power plant. All good!
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Thank you to each of you for your responses. I appreciate you all taking the time to shed some light on this for me. My apologies in the delayed response to GLW and HydroDave63. I was out celebrating the Marine Corps birthday for the weekend with some brothers of mine. I'll definitely work on getting myself spun up on the ISO 4037 series; I may return with more questions.
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Follow up question: Would the energy dependent response of GM detectors play a factor in the preference of Cs-137 as calibration sources on site? The 662 keV photon falls in the small window of accurate dose rate response range for Geiger Muellers, as they overcompensate at low energies and undercompensate at higher energies.
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Yes, That is correct. At calibration labs, the source(usually or exclusively Cs-137 at varying activity levels), or the beam is usually measured with a very quality directly NIST traceable Ion Chamber and a good calibrated quality accurate multimeter. The Ion chamber response is not photon energy dependant at 662 kev range interval. This calibrated beam is then used to measure and calibrate GM and other instrument's response in mR/hour, R/hour CGray/hour, etc. GM type instruments are not usually used to calibrate a radiation beam or a photon field. Good Luck at your studies.