August 27, 2015
Exposure to science fiction films is enough to give people a little knowledge of physics without the nuisance of getting a physics education. In this way, nearly everyone knows that lead will act as a shield from radiation, with thick slabs giving the best protection. Our "atomic age" was aided by the ubiquity of lead, a metal common in antiquity.
Lead was produced in antiquity by merely heating the mineral, galena (lead sulfide (PbS), in air to produce lead oxide (PbO). The lead oxide is then reduced in the presence of carbon (C) to form free lead; viz.,
2PbS + 3O2 -> 2PbO + 2SO2
Lead is a reasonably abundant element in the Earth's crust. As shown in the table, it's not as abundant as iron and a few of the other metals, but quite a bit more abundant than tin, silver and gold. Galena is also a source of silver.
It's not that lead has a magical property among the elements that enables its shielding effect. All other materials will shield from radiation, but most to a lesser extent. Lead is most often used for shielding, since it's an inexpensive material with significant shielding power. While other laboratories strive to remove all lead for environmental and health reasons, as I summarized in a previous article (Leaded Brass, November 29, 2010), the X-ray diffraction laboratory I used had lead everywhere, in sheets, and as an additive (up to about 30%) to Plexiglas for transparent shielding enclosures.
Ample shielding from X-rays in a typical diffraction laboratory for crystallography requires just small amounts of lead. That's because the energy of X-rays is in the range of just a few ten thousand electronvolts (~104 eV). Radiation, however, extends to very high energies. Gamma rays have energies an order of magnitude above that of X-rays and far beyond. Cosmic rays of energy 1020 eV have been observed.
The transmittance of radiation through a material, the measure of how well it acts as a shield, follows the law,
in which I/I0 is the ratio of intensity to initial intensity, μ/ρ is the mass attenuation coefficient of the specific material at the specific X-ray energy, and ℓ is the distance through the material. There are tables of μ/ρ for many materials at various X-ray energies. The mass attenuation coefficient of lead over a wide range of radiation energy is shown in the following graph. As can be seen from the graph, the shielding effectiveness is less at higher energies, so a greater thickness of lead is required.
Humans are not the only objects that need radiation protection. Electronic components in spacecraft need radiation shielding, also, along with protection from impacts from small pieces of space debris. A research team at North Carolina State University led by Afsaneh Rabiei, a professor of mechanical and aerospace engineering, has been investigating composite metal foams effective at both radiation shielding and absorption of the energy of high impact collisions.[2-3] I wrote about metal foams in a recent article (Low-Density Syntactic Foam Alloy, June 18, 2015).
The NCSU team prepared a variety of metal foams and compared their radiation shielding effectiveness at blocking X-rays, gamma rays and neutron radiation. For an accurate comparison, they normalized their results to the areal density. The matrix materials were 316 L stainless steel, high-speed T15 steel and some aluminum alloys mixed with 2-, 4- and 5.2- mm hollow steel spheres. The T15 steel alloy contains high concentrations of tungsten and vanadium and was designated "high-Z foam." Tungsten, because of its high atomic number of 74, has good shielding ability (lead has the atomic number 82).
Radiation test were performed using isotopes of cesium and cobalt as high energy gamma ray emitters, and isotopes of barium and americium as lower-energy gamma ray emitters. The high-Z foam was comparable to bulk materials in its ability to block high-energy gamma rays, but it was much better than even bulk steel as a shield for low-energy gamma rays. It also was better at blocking neutron and X-ray radiation, but it was not as good as lead as an X-ray shield. It is, however, lighter in weight and more environmentally friendly than lead.
The effectiveness of the foam's radiation shielding was not affected by the sphere geometry, as long as the ratio of wall thickness to the diameter of the spheres was constant. Small spheres, however, seemed to lead to foams of slightly better shielding. Quasi-static compression testing showed that the foams have good energy absorption capability. Says Rabiei,
PbO + C -> Pb + CO
"... We are working to modify the composition of the metal foam to be even more effective than lead at blocking X-rays - and our early results are promising... and our foams have the advantage of being non-toxic, which means that they are easier to manufacture and recycle. In addition, the extraordinary mechanical and thermal properties of composite metal foams, and their energy absorption capabilities, make the material a good candidate for various nuclear structural applications."
This work was supported by the US Department of Energy's Office of Nuclear Energy.
- X-Ray Mass Attenuation Coefficients on NIST web site.
- Shuo Chen, Mohamed Bourham, and Afsaneh Rabiei, "Attenuation efficiency of X-ray and comparison to gamma ray and neutrons in composite metal foams," Radiation Physics and Chemistry, Early Online Publication, July 8, 2015.
- Matt Shipman, "Study Finds Metal Foams Capable of Shielding X-rays, Gamma Rays, Neutron Radiation," North Carolina State University Press Release, July 17, 2015.
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