With the ability to sense smuggled nuclear materials, highly efficient neutron detectors are critical for national security. Currently, there are two classes of detectors that either use helium gas or flashes of light. These detectors are very large — sometimes the size of a wall.
The new material introduces a third class: a semiconductor that can absorb neutrons and generate electrical signals that can be easily measured. The semiconductor-based detector is also highly efficient and stable. It can be used both in small, portable devices for field inspections and very large detectors that use arrays of crystals.
When heavy elements such as uranium and plutonium decay, their atoms eject neutrons from their nuclei. Most neutron detectors are so-called scintillators that work by sensing ejected neutrons and then emitting light to alert the user. The new material is a semiconductor and does not emit light; rather, it directly detects electrical signals induced by the neutrons. In addition to security applications, neutron detectors are used in radiation safety, astronomy, plasma physics, materials science, and crystallography.
Whereas classic types of thermal neutron detectors have been in use since the 1950s, a practical semiconductor material has remained elusive. Excellent at absorbing neutrons, lithium quickly emerged as the most promising material for neutron detecting devices. But integrating lithium into a semiconductor and making it stable (lithium crumbles when it meets water) was difficult.
The researchers discovered the right combination of materials to make a working device that also keeps lithium stable. The new material — lithium-indium-phosphorous-selenium — is layered in structure and enriched with the lithium-6 isotope. The lithium is inside the layers, so water cannot reach it — an important feature of this material.
The resulting semiconductor neutron detector can detect thermal neutrons from even a very weak source — and can do so within nanoseconds. It also can discriminate between neutrons and other types of nuclear signals such as gamma rays, preventing false alarms. The material also contains a very high amount of lithium, so a smaller fraction of the material can absorb the same amount of neutrons as a giant device. This leads to devices small enough for portable inspections in the field.