Answer to Question #5735 Submitted to "Ask the Experts"

I have questions about using sand as shielding material for gamma radiation. Specifically, what is the density of the sand used for shielding purposes? Further, the HVT (half-value thickness) values and build-up factors for the same density are also not available. I have values of linear attenuation coefficient for the sand of density 2.2 g/cc for some energies. Could you please provide me with the data or some references? Also, would the sand present in different regions of the world vary much in its properties? If so, which one is the internationally accepted quality suitable for shielding purposes?

To my knowledge there is no "standard" sand used in shielding applications. The compositions and densities of sand vary somewhat from location to location depending on the source of the sand. Sand is basically rock that has been degraded through various physical/chemical processes; as such, it has a mineral composition with the most common constituent of sand in general use being quartz, SiO₂.

The density of sand, even if pure silica, may vary depending on grain size and the degree of packing of the sand and whether it is wet. By definition agreed upon by most scientists, sand grain size may vary between 0.05 mm (very fine) and 2 mm (coarse). While the density of pure crystalline quartz (single crystal) is about 2.65 g cm^-3, the densities of sand can range from about 1.4 to somewhat greater than 2 g cm^-3. The most commonly used values, in my experience, are 1.5 to 1.7 g cm^-3.

Following are a couple of websites that give some values of density for sand. The dry sand mentioned on the University of Washington site has a relatively uniform size range (0.5 to 0.85 mm) and a density of about 1.46 g cm^-3. The second site is from a company in the United Kingdom and shows a typical density of dry sand of 1.6 g cm^-3, but variations are shown for wet sand, packed sand, and sand containing other material (gravel). See paragraph 2.4 of document on the University of Washington website, and look for sand in the list of materials provided on the U.K. SiMetric company site. The commonly used computer code RESRAD (developed by the Environmental Assessment Division of Argonne National Laboratory under the sponsorship of the U.S. Department of Energy and the U.S. Nuclear Regulatory Commission) uses a default value of 1.5 g cm^-3 for the various soils (not necessarily pure sand) used in evaluating effects of radioactive ground contamination in the code.

You can calculate the mass attenuation coefficient for sand by assuming a composition of SiO₂ and using the expression μ/ρ = Σ(μ/ρ)_iw_i, where i refers to a given constituent (Si or O) in the compound, (μ/ρ)_i is its respective mass attenuation coefficient, and w_i refers to its respective weight fraction in the compound. If you don't want to bother with this you can likely use values of mass attenuation coefficients and buildup factors for normal concrete or for glass (e.g., borosilicate glass) as reasonable surrogates for sand. You can find values of mass attenuation coefficients for elements and compounds on these two respective National Institute of Standards and Technology websites (elements and compounds). There are various sources of build-up data available. You might have access to the Radiation Shielding text by Shultis and Faw that contains considerable useful information; it is available through the American Nuclear Society.

George Chabot, PhD, CHP