Answer to Question #8177 Submitted to "Ask the Experts"
Category: Instrumentation and Measurements
The following question was answered by an expert in the appropriate field:
There are a variety of container types that are suitable for solid samples, the specific types depending on the sample sizes and compositions. If you are analyzing certain small-volume crystalline samples such as BaSO4 that may contain coprecipitated radium, as when a Ba(Ra)SO4 precipitation is used to separate radium in a chemical operation, a small-volume plastic container may be sufficient, especially if the sample is contained on a filter paper, as might be the case if a Ba(Ra)SO4 separation has been done. In such a case there is often little radon emanation out of the crystalline matrix, and a simple snug-fitting cover may be adequate for routine analysis.
For larger-volume samples, such as soil or some sediments, containing radium, the radon emanation factor may be considerably larger, and more care must be taken to avoid significant loss of radon. In such a case a moderately thick-walled plastic container with a tight-fitting cover, often taped around the seam or sealed with a nonpermanent adhesive to minimize leakage, may be acceptable. Since the sample may have to sit for about one month to ensure secular equilibrium of the radon and subsequent short-lived progeny with the radium, it is important that the container provide an effective barrier against leakage.
If no special treatment is used to reduce radon emanation from the sample, you should also attempt to minimize the amount of free-air volume above the sample in the closed container. This is to reduce the amount of radon that may effuse from the sample into the air space, thus changing the counting geometry and the associated counting efficiency that applies to the gamma-emitting radon progeny. Some authors have found that by mixing activated charcoal with the solid sample, the emanation of radon may be notably reduced.
A popular container for relatively large-volume solid or liquid samples is the Marinelli beaker with a lid. These are designed with an inverted well that fits over the germanium or NaI detector being used for analysis; the sample volume then surrounds the detector on the top and curved sides, thus providing a more favorable counting geometry than would result from placing the same volume sample in a container on top of the detector. Taping of the seam around the lid may be necessary to minimize leakage.
Plastic containers may be fabricated of a number of materials, such as polypropylene, high-density polyethylene, polystyrene, and some others. Other materials such as stainless steel and glass have also been used, especially if long-term sample storage is anticipated, as might be the case especially for standards that are prepared and are intended for intermittent use, possibly over a year or more. These materials minimize concern with long-term diffusion of radon into or through the walls of the container. For such standards, prepared for long-term storage and reuse, the cover should be sealed in a positive fashion. This may involve such measures as gluing the cover in place.
Regarding attenuation of the emitted gamma rays within the sample, you are correct in recognizing this as a potential concern. The concern grows as the mass of the sample being analyzed increases and/or as the energies of the photons of interest become smaller.
For example the mass attenuation coefficient, μ/ρ, for 295 keV photons from 214Pb in sand (silicon dioxide) is about 0.146 cm2g-1; if the sand has a density, ρ, of 1.6 g cm-3, the linear attenuation coefficient is then 0.234 cm-1. If N photons of this energy had to travel 5 cm through a sand sample to reach the detector, the number arriving at the detector would expectedly be reduced to Ne-μ(5) = 0.31N.
For higher-energy photons such as the 1.12 MeV photons from 214Bi, the value of μ would be about 0.097 cm-1 and the analogous number reaching the detector would be Ne-(0.097)(5) = 0.62N, about twice as high as the number of 295 keV photons.
Because photons are emitted from throughout the volume of the sample, the extent of attenuation is variable depending on where in the sample the photons originate. It is possible to do theoretical calculations to integrate over the entire volume, taking account of the variable path lengths traversed. To predict the detector response, in theory, one must also consider the angle of incidence on the detector and the path length available through the detector. Depending on the size and shape of the sample volume, Monte Carlo-type probabilistic simulations may be the most effective means of carrying out some of these calculations.
Many analysts have preferred to perform experimental evaluations of the impact of self-absorption on the overall detection efficiency for the photon energy of interest. This involves making up appropriate standards that closely simulate, in mass, volume, and composition, the samples to be measured. The samples may be prepared with known amounts of the specific radionuclides of interest (e.g., 226Ra) and evaluated at the photon energies of interest so that appropriate counting efficiencies can be directly determined. In some other instances researchers have prepared samples using standards (not necessarily of the same radionuclide[s] that will be measured in samples) that include a wide range of photon energies so that gamma detection efficiencies may be determined for a range of energies and samples that emit any photon energy within the range that has been evaluated may be analyzed.
You can find a lot of information on the Internet related to the topics of interest. Here is a link to an abstract of a paper by Noguchi Masayasu, et al. titled "Correction Methods of .GAMMA.-Ray Self-Absorption in Bulk Sample" that appeared in Radioisotopes in 2000 (this is a Japanese publication that has been translated into English). A second is a 2003 paper from Elsevier Publishing, "Determination of Self-Absorption Corrections for Gamma Analysis of Environmental Samples: Comparing Gamma-Absorption Curves and Spiked Matrix-Matched Samples" by Fegan, et al. An article by Stoulos et al. that appeared in Applied Radiation and Isotopes in 2004, "Measurement of Radon Emanation Factor from Granular Samples: Effects of Additives in Cement," discusses some emanation considerations.
If you have access to Science Direct you can see the latter two articles in their entirety. Otherwise, you might get copies by trying to reach the authors directly or through the journals (for a fee). Other articles by some of these same authors and others and some of the references referred to in the articles might be helpful. Many other publications are available, depending on your resources. If you are a member of the Health Physics Society you can gain access to articles in the Health Physics journal through Members Only access. You can use the search routine to find possible papers of interest. There are also some questions and answers available in the HPS Ask the Experts website that might be of interest—e.g., Questions 7823 and 5505.
Good luck in your pursuits.
George Chabot, PhD, CHP