Answer to Question #10904 Submitted to "Ask the Experts"

I'd like to know our weak points in calculating the activity concentration of radionuclide ¹³⁷Cs.

I have International Atomic Energy Agency (IAEA) standard reference materials (SRM), IAEA-154 and IAEA-156. I'd like to check the activity of ¹³⁷Cs emitted from these SRMs by using our new high-purity germanium detector. Would that be possible?

Unfortunately, the point source cannot be used as a standard in order to measure the ¹³⁷Cs activities of the IAEA-154, a whey powder material, and 156, a clover material, sources. The geometries of the two IAEA sources are not point source geometries, and the measurement efficiencies will not be the same as for a point source, as you have found. Because the IAEA sources are volume sources, each containing more than 150 g of material, the contained ¹³⁷Cs activity, which is distributed more or less uniformly throughout each source, is farther removed from the detector, on average than is a point source when the respective samples are placed directly on the detector, and the counting efficiencies will be less for the volume sources than for the point source. In addition, the volume sources will result in self attenuation of some of the gamma radiation within the sources, also reducing counting efficiency. The effects of sample geometry can often be reduced by removing the sample to a greater distance from the detector; when the sample is far enough from the detector that the distance from sample to detector is about five times the largest dimension of the sample, the geometry will simulate a point source; however, the effects of volume attenuation of the gamma radiation in the sample will still prevail.

The generally accepted manner to establish efficiencies for counting gamma radiation of a particular energy in samples of specified physical characteristics (geometry and mass density) is to prepare standards that contain a known amount of the radionuclide of interest (or gamma ray energies of interest) dispersed in a medium that closely simulates the samples of interest in both geometry and mass density. This process in itself is sometimes difficult and time consuming, usually requiring purchasing calibrated quantities of the radionuclide(s) of concern in liquid solution and transferring the liquid solution to the simulative medium in such a manner as to ensure that the radionuclide gets distributed uniformly in the medium.

One possible thing you could attempt to confirm the activity of at least the higher activity IAEA-154 sample would be to transfer a small quantity of the material (perhaps 0.5 to 1 g) to a small container, or spread it over a small area on a piece of material that presented little gamma attenuation (the area should be much less than the facial area of detector), and count this small sample for an extended time at a location close to the surface of the detector. The location must be such that after the sample has been counted, the point source could then be placed at the same location and also counted. It is often preferable to position each source a short distance from the detector, perhaps 1 cm or so, so that differences in sizes and positions of the sample and the point source will not have a significant effect. The small sample will present little attenuation, and will simulate a point source as long as its dimensions are small and that it and the point source can each be positioned at the same location.

This procedure assumes that the point source activity is calibrated and that there is no significant attenuation of photons in the point source or in the prepared source. Naturally, doing this procedure disturbs the integrity of the original IAEA-154 sample, and this may be a problem, depending on quality assurance requirements at your facility.

George Chabot, CHP, PhD