GaAs finds a new application in neutron detectors
The researchers turned to GaAs in an effort to miniaturize the detectors, making them more portable for applications such as detection of smuggled fissile material at ports and airports, and for use by international nuclear non-proliferation inspectors. Presently, two types of neutron detector are in use, those based on a tube filled with gas that is ionized by neutrons, and those based on silicon.
The GaAs-based detectors are made by coating semi-insulating GaAs with isotopically enriched boron or lithium. A neutron striking the coating releases a cascade of charged particles (an alpha particle and a lithium ion in the case of a thermal neutron striking 10B) which excite free carriers in the GaAs active region. The carriers drift to the detector contacts under an applied voltage and the induced charge is detected and amplified.
Using GaAs rather than Si brings several advantages. The detectors can be made smaller, require less than 50 V operating power and operate at room temperature. Replacing Si with GaAs also improves the lifetime of the detector when used in areas with high radiation levels.
“The working portion of the wafer is about the diameter of a collar button, but thinner,” said Raymond Klann, who leads the detector development group at ANL. “It is fairly straightforward to make full-sized detector systems about the size of a deck of cards or smaller. Something that small can be used covertly, if necessary, by weapons inspectors to monitor nuclear facilities.”
Klann’s team has experimented with various coatings and detector configurations to make the devices even more efficient. The group found that detection efficiency was improved by etching cylindrical holes into the GaAs before the contacts and the boron layer were deposited. The holes also help with the adhesion of the boron layer to the GaAs.
To date Klann’s work has been funded by the US Department of Energy through the Laboratory Directed Research and Development Program. ANL is now looking for partners to develop the prototype detectors into commercial products.