Answer to Question #8813 Submitted to "Ask the Experts"

I am in the process of ordering a ¹³¹I mock rod source to use with our phantom. In the past we have used a 3.7 × 10³ Bq ¹³³Ba source, but I have been told we need a combo ¹³³Ba and ¹³⁷Cs. If so, where do I purchase one?

These mixed ¹³³Ba-¹³⁷Cs sources seem to be less available than they had been in the past. They have the advantage that the ¹³³Ba provides photons in the 360 keV energy region to simulate the dominant photons from ¹³¹I, and the ¹³⁷Cs produces 662 keV photons, which are close to the less dominant 640 keV photons from ¹³¹I. If you use such a source you must be aware that the relative photon composition will change over time because the ¹³³Ba half-life is about 10 years compared to the 30-year half-life of ¹³⁷Cs. I cannot provide a name of a supplier in which I am confident, but I would try a company such as Isotope Products (now under Eckert & Ziegler). Contact the company and discuss your needs.

I should point out that you can effectively use a ¹³³Ba source under some conditions. In particular, if you use this single source you should do a one-time comparison measurement with a calibrated ¹³¹I sample so that you can properly set up your spectrometer and assess the relative counting efficiencies in the determined window region. Generally the window region that you define for counting of the dominant photons (about 360 keV) for ¹³¹I will be about the same as the window region for counting ¹³³Ba (dominant gamma energy also about 360 keV). The ¹³¹I counting efficiency will often be somewhat greater than the ¹³³Ba counting efficiency because of the higher yield of photons in the region of interest, but this can vary depending on how you set the counting window.

Good luck.

George Chabot, PhD, CHP