I used to work extensively with the NIST radioisotope SRMs. I particularly remember working with SRM 4965a (https://shop.nist.gov/ccrz__ProductDetails?sku=4965

I used to work extensively with the NIST radioisotope SRMs. I particularly remember working with SRM 4965a (https://shop.nist.gov/ccrz__ProductDetails?sku=4965a&cclcl=en_US https://shop.nist.gov/ccrz__ProductDetails?sku=4965a&cclcl=e...). My wife was very worried when I told her that I'd be working with radioactive hydrochloric acid. Technically, that is what it is, but the acid is very dilute and the radioactivity is very low. It sure sounds dangerous though! I had to repackage it into a new container so that I could contain and draw off the radon gas, but still have the capsule remain wet so that the emanation rate would stay constant. My low-tech solution was to put a wet sponge in the chamber, so that the air would remain saturated with water vapor but that the condensation would not dilute the acid. It worked great for about 10 years. I passed that source on to another company, it might be working still. Radium sources tend to be rather long-lasting.