Answer to Question #1864 Submitted to "Ask the Experts"

Q

What are the most appropriate methods for the calibration of a ¹³⁷Cs source?

A

How you'd calibrate depends on the activity level of the source. Nevertheless, to do it properly, you'll need a similar ¹³⁷Cs source that is traceable to a national standards organization (for example, NIST in the United States). If it is a small sealed check source in the fraction of a microcurie (µCi) range, a sodium iodide scintillation counter peaked into the ^137mBa 662 keV gamma would be appropriate. If the source has activity in the tens of µCi or in the mCi range, a cross calibration using fixed geometry and an ion chamber would be my approach. If you have a calibrated ion chamber that has a flat energy response, and you know the air dose rate at a specific distance, you could use the inverse square relationship and ¹³⁷Cs exposure rate constant to back-calculate activity. You might assume little air and source encapsulation attenuation, depending on the source construction. Again, with a calibrated ion chamber, an alternate first approximation would be to measure the old "exposure rate" in roentgens per hour at a foot. The relationship X = 6CEn would be good for +/- 20%; where X is the measured R/hr at 1 foot, 6 is a conversion factor, C is the number of curies (Ci), E is the energy in MeV, and "n" is the fractional gamma yield per decay. For ¹³⁷Cs, E= 0.662 MeV and "n" would be 0.85 photons per beta disintegration. Rearranging, you'd have C = X/6En.

David J. Allard, CHP