Answer to Question #536 Submitted to "Ask the Experts"

Q

What would you expect the radon emanation factor for a radium sulfate impregnated gold foil to be? Would there be any radon hazard for such a source (say 1 microCi)?

A

EMANATION FACTOR I do not have a direct answer to this part of your question. Is the radium sulfate actually impregnated into the foil, or rather is it deposited or plated on the surface? I don't know of any reference for radon emanation from radium impregnated in metals, but I did not do a search of the chemical literature. Offhand, I would expect the emanation from impregnated radium to be less than for surface deposits. In a very old reference ("Emanation Methods," Chapter 9 of Bonner and Wahl, Radioactivity Applied to Chemistry, John Wiley and Sons, 1951, pg 293) Wahl states that deposition of the parent of a radioactive inert gas on a preformed solid usually gives a source of higher emanating power than if the parent is incorporated into the solid by coprecipitation. He also cited a report where the emanating power of a preparation of radium deposited on a lead nitrate preparation was reduced from 18.7% to 1.4% after the preparation had been annealed to allow radium ions to diffuse into the crystals. To me, this is an indirect indication that the emanation factor for an impregnated foil should be less than from a surface-deposited source. When I addressed this question to one of my colleagues, he said that he believed that the emanation factor for radium incorporated in barium sulfate was rather low (perhaps 10%) and that he would guess that the emanation from pure radium sulfate would be similar but he was not sure. He commented that this may be a case where it is easier to make the measurement than to guess the result. There are several possible methods for measuring an emanation factor; two of these include a direct measure of emanated radon gas and a gamma-counting method. In the emanated radon gas measurement method, the radium content of the material or source must be known or measured. The material or source is sealed in an originally radon-free container and, after a period of time, the radon content of the air inside the container is measured by scintillation cell or other radon sampling method. The emanation factor is determined from the radium content of the source and the radon released to the container. The gamma-counting method measures the gamma radiation from radon daughters in order to determine radon content. In this method, the source is sealed up without any free air space. A count of the "package" or container immediately after sealing indicates the radon retained in the source when it is allowed to freely emanate. Following sealing, radon that would otherwise have been emanated is retained in the sealed package and after radon has built up to equilibrium with the contained radium (about three weeks) the package is counted again. The difference between the initial and the equilibrium counts indicates the radon that would be emanated if the source were not sealed; the ratio between the emanated radon and the equilibrium radon is the emanation factor. There are various versions of these two methods. RADON HAZARD FROM A 1 µCi SOURCE: Radon hazard would be assessed by predicting the airborne radon concentration that might occur in an enclosed working space containing the radium source. To do this we need to know, or make assumptions about, the activity of the source (to determine the radon production rate), the radon emanation factor, the volume of the space, and the ventilation rate of the space. The steady-state radon concentration in a space with constant input of radon would be: C (pCi L^-1) = I (pCi s^-1)/[V(L) x (s^-1)] Where I = radon input, pCi s^-1, V = space volume, L, and = effective reduction constant, s^-1 = radon decay constant, r (2.11×10^-6 s^-1) +ventilation turnover constant, v. For the purposes of the assessment, consider the following sets of conditions: Radon source: Consider emanation factors of 100% (E =1.0, worst case) and 10% (E = 0.1, alternative case). Since 1 µCi radium produces 2.1 pCi radon s^-1, radon input is 2.1 pCi s^-1 (worst case) and 0.21 pCi s^-1 (alternative case). Space volume: a nominal small volume of 1 m³ (1,000 L) and a small room 10'×10'×8' or 23,300 L. Ventilation rates: None, v = 0, = 2.11×10^-6 s^-1, Ventilation of 0.5 air changes per hour, v =1.39×10^-4 s^-1, = 1.41×10^-4 s^-1, High ventilation of 2 air changes per hour, v =5.56×10^-4 s^-1, = 5.58×10^-4 s^-1. Steady-state airborne radon concentrations, for emanation factors of 100% and 10%, are calculated to be: Space Volume of 1 m³ (1,000 L): No ventilation, = 2.11×10^-6 s^-1: 1000 pCi L^-1 and 100 pCi L^-1 0.5 ach, = 1.41×10^-4 s^-1: 15 pCi L^-1 and 1.5 pCi L^-1 2 ach, = 5.58×10^-4 s^-1: 3.8 pCi L^-1 and 0.38 pCi L^-1. Room 10'×10'×8' (23,300 L): No ventilation, = 2.11×10^-6 s^-1: 43 pCi L^-1 and 4.3 pCi L^-1 0.5 ach, = 1.41×10^-4 s^-1: 0.64 pCi L^-1 and 0.064 pCi L^-1 2 ach, = 5.58×10^-4 s^-1: 0.16 pCi L^-1 and 0.016 pCi L^-1. For any reasonable emanation factor, room size, and ventilation rate, the calculated airborne radon concentrations meet the EPA indoor radon guideline of 4 pCi L^-1 Charles E. Roessler, CHP, Ph.D.