Metal foam provides lightweight radiation shielding

Lightweight composite metal foams can absorb energy from impacts and block X-rays, gamma rays and neutron radiation, according to a study from North Carolina State University. The discovery means the materials could be useful in spacecraft, the nuclear industry and in medicine.

Aerospace and mechanical engineer Afsanah Rabiei originally developed the foams, which consist of hollow metal spheres of one metal dispersed in a matrix which can be of the same or a different metal, for military transport applications. They are mechanically strong, thermally insulative and lightweight, with their structure reducing the density compared with a bulk material, but Rabiei  wanted to determine whether they could provide structural support while also shielding from radiation. Her research involved comparing foams’ shielding properties against pure lead and the A356 grade of aluminium, metals that are currently used for shielding purposes. Each comparison used samples of the same weight, but differing volume.

The best results were obtained from a foam called high-Z steel-steel, which consists of stainless steel spheres dispersed in a matrix of high-speed T15 steel, an alloy containing trace amounts of vanadium and tungsten. The term High-Z refers to all the metals in the alloy having a large number of protons in their atomic nuclei; Rabiei’s team chose this alloy because  tungsten and vanadium both have good radiation shielding properties. The tungsten-containing foam was modified so that its density was the same as a foam made entirely from stainless steel.

The researchers found that the high-Z foam was as good as the bulk materials at blocking high-energy gamma rays, such as those emitted by radioactive caesium and cobalt, but even better at blocking low-energy gamma, such as the radiation from barium an americium. It outperformed bulk materials at blocking neutron radiation, and was bettered only by lead at blocking X-rays. “However, 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,” Rabiei said. “And our foams have the advantage of being non-toxic, which means that they are easier to manufacture and recycle.”

The size of the hollow steel spheres seemed to have little effect, as long as the ratio of wall thickness to diameter stayed constant; however, smaller spheres (around 2mm diameter) seem to be more efficient in X-ray applications. Rabiei believes that the foams could be particularly useful in making vessels to transport nuclear waste, in protecting equipment on-board exploratory spacecraft from the high radiation fluxes sometimes found in space without adding significantly to their weight, and in protecting patients from radiation doses in CT scanners. The team discusses its work in a paper in the journal Radiation Physics and Chemistry.