Answer to Question #179 Submitted to "Ask the Experts"

Q

I am trying to confirm the existence of a chemical named "Radium Zinc Sulfite" past or present. Can you tell me in what form one would be able to obtain it and its specific alpha activity? Also, what is the specific alpha activity of pure Radium? Thank You!

A

We have been unable to locate any information on "radium zinc sulfite." We suspect that zinc sulfite is being confused with zinc sulfide. A mixture of a radium salt and zinc sulfide was a common radioluminous preparation for such purposes as self-luminous dials and markers. Alternatively, it is possible that zinc sulfite may have been part of a proprietary luminous product, but we have not come across any information to support this. When people talk about "radium," they usually are referring to the isotope ²²⁶Ra (radium-226) of radium. The specific activity of ²²⁶Ra is approximately 1 Ci/g (curie/gram)--the curie was originally defined as the activity associated with one gram of radium. In SI units this is 3.7x10¹³ Bq/kg (becquerel per kilogram) or 3.7x10¹³ transformations per second-kilogram. Since the alpha particle yield for ²²⁶Ra is essentially one alpha particle emission per nuclear transformation, the alpha particle specific activity is 1 Ci/g or 3.7x10¹³ Bq/kg. However, anything other than a freshly separated radium sample will also have alpha activity from the series of radioactive decay products of ²²⁶Ra. The amount of decay product activity will depend upon the time after separation (what fraction of equilibrium has been attained) and the fractional emanation loss of gaseous ²²²Rn (radon-222) which is the first member of the decay product series. As an upper bound, a tightly sealed sample would retain all the ²²²Rn. In this sealed sample, the alpha activity will build up at a rate initially governed by the 3.8 day half-life of ²²²Rn and within a few weeks reach a value four times that from the pure ²²⁶Ra alone. With further time, there will be some additional buildup of alpha activity due to the alpha-emitting final member of the decay series, ²¹⁰Po (polonium-210), but this buildup would be tracked in terms of decades since it is governed by the 22-year half-life of one of the intermediates in the chain, ²¹⁰Pb (lead-210). In summary, the specific alpha activity would be:

• For a sample of ²²⁶Ra freshly separated from its decay products--1 Ci/g.
• For a sample sealed to prevent the escape of ²²²Rn--Building up with time to a value of 4 Ci/g after a few weeks.
• For the sealed sample--Further slow buildup over decades to an eventual equilibrium value of 5 Ci/g (after centuries).
• For a sample not totally sealed to prevent loss of ²²²Rn–A value intermediate between 1 Ci/g and 5 Ci/g (or 4 Ci/g in the short term), depending upon the time after preparation and the radon emanation factor for the particular preparation.

Radon emanation factors will depend upon the radium compound and the form (such as plated, thin precipitate, solid, liquid, etc.) and may vary from near zero to nearly 100 percent.

Charles E. Roessler, CHP, Ph.D.