Answer to Question #9232 Submitted to "Ask the Experts"

I am conducting a test of contaminated items to determine their surface radioactivity. The items are contaminated with various concentrations of ¹³⁷Cs and ⁹⁰Sr/⁹⁰Y, but predominantly ⁹⁰Sr/⁹⁰Y. I am using an ion chamber (RO-2) to take direct readings and convert to Bq cm^-2. I have measured several ⁹⁰Sr/⁹⁰Y sources and need to derive a correction factor for dose rate to surface activity. Can you assist with a formula? Thank you in advance. 

It would be convenient if there were a simple formulation that would allow you to correlate the detector reading with a surface contamination level. Unfortunately, the response of the ionization chamber depends significantly on the geometry characteristics of the source.

The RO-2 ionization chamber, previously manufactured by Eberline Corp., is a relatively large-volume cylindrical chamber, having a volume of 208 cm³ and a diameter of 7.62 cm. It is equipped with two thin Mylar^TM windows, the first outer window being on the instrument case and the second being 0.4 cm away from and parallel to the first and on the flat end of the ionization chamber. If a surface beta-emitting source is placed close to or in contact with the outer window, the dose rate throughout the chamber will be very nonuniform because of the marked effect of the inverse square law. Thus, the reading on the chamber may represent just a small fraction of the dose rate at contact with the contaminated item and is not a good indicator of the actual dose rate near the surface of the contaminated material.

An example of this can be demonstrated by comparing the beta fluence rate from a point isotropic source, assuming no significant attenuation in the air or detector components, averaged throughout the detector volume, to the fluence rate at 0.4 cm (the distance from the outer window to the inner window) from the same source. This is fairly easy to do by using integral calculus. For brevity's sake I am not including the calculation here, but if you want the details you may contact me. The results of the calculation show that the fluence rate averaged throughout the RO-2 active volume would be 0.00802 S, where S is the beta particle emission rate from the source. For the same source at 0.4 cm, the fluence rate would be simply S/4p(0.4 cm)² = 0.497 S. Thus, the fluence rate, and associated dose rate, at 0.4 cm would be about 60 times greater than the respective quantities averaged through the detector volume. Since the detector reading is indicative of the dose rate averaged throughout the volume, it is clear that a large-volume detector, such as the RO-2, is not well suited to making measurements of surface sources of small spots of beta activity.

If the surface source is distributed over a significant area, the situation may be somewhat better but still difficult—e.g., for a source distributed evenly over a disc-shaped surface area of radius equal to that of the detector (3.81 cm), the fluence rate averaged through the detector volume would be 3 to 4 times less than the fluence rate at a point 0.4 cm above the center of the disc-shaped contamination. As the contamination area increases to dimensions greater than the detector window area, the difference between measured and actual dose rates expectedly would increase because the sides of the instrument case and detector wall would greatly attenuate the beta radiation such that the additional source area would contribute minimally to the instrument reading.

Additionally, the RO-2 chamber uses a central planar collecting electrode that is parallel to the entrance window and located at half the chamber depth. This electrode does not appreciably impact most gamma readings, but it does affect beta readings because of the attenuation that it presents. The ⁹⁰Sr beta particles would be more greatly affected by this than would the higher-energy ⁹⁰Y beta particles.

You say that you have measured several ⁹⁰Sr/⁹⁰Y sources. Important questions are (1) what were the geometry characteristics of these sources? (2) how did the ion chamber responses vary among the sources? and (3) how did the source geometries and material characteristics compare to the expected contamination geometries and the contaminated material characteristics. I expect you have the answers to questions 1 and 2, but you may not have a sufficient answer to the third question. Often when dealing with surface-contaminated items, one does not know the source characteristics—e.g., is the activity represented by a small spot of activity or is the activity distributed over the surface in some fashion? The other aspect of the third question has to do, especially, with the effective atomic number of the material on which the contamination resides—e.g., are you dealing with a steel tool, or are you concerned with a contaminated wooden bench? When dealing with beta radiation, the degree of backscatter is often greatly affected by the composition of the material on which the contamination is deposited. The effect is often most significant with high-energy beta emitters such as ⁹⁰Y. If contamination is on a steel surface, and the standard is backed by a low atomic-number material such as plastic, the detector response to the standard may be almost 100% lower than the response to the contamination present in the same amount and geometry as the standard. It is therefore desirable to use test standards that are backed by a material of similar atomic number to that of the material you are testing.

If you are unable to specify what contamination geometries you would likely be dealing with, and you have used test standards of varying geometries (point or small-area sources and larger-area distributed sources) with appropriate backing materials, you may elect to use the most conservative results to interpret surface contamination levels—i.e., select the highest conversion factor (highest value of Bq cm^-2 per unit exposure rate). This will yield conservative results, although this can be a problem when the results are above acceptable release limits, and better definition of a possibly more appropriate conversion factor could be beneficial.

Sorry I cannot provide a simpler and more definitive answer to your question.

George Chabot, PhD, CHP