"New light" boosts neutron detection

20 Mar 2008

A sensitive neutron detector that measures the UV light emitted when neutrons are absorbed by helium atoms could help to improve the safety of nuclear power plants and high-power accelerators.

A new optical method that detects individual neutrons with greater sensitivity than ever before and can record them over a range of intensities has been developed by researchers at the National Institute of Standards and Technology (NIST) in the US.

The technique relies on detecting light, which the group calls Lyman alpha light, from a previously unknown source. Lyman alpha light is emitted when an electron jumps between the two lowest-energy states of a hydrogen atom, which happens when neutrons are absorbed by helium-3 atoms.

"The use of optical radiation as a means of detection allows us to leverage many recent advances in photonic technology to produce a better neutron detector," Charles Clark and Alan Thompson, researchers at NIST, told optics.org. "This detector has the potential to outperform the current technology, which is refined, mature and has little potential for improvement compared with our new technique."

Conventional neutron detectors – such as fission chambers, lithium-loaded scintillators and proportional tubes – rely on the interaction of fission neutrons with isotopes of boron, helium and lithium. While the NIST team's detector makes use of the same reaction as a conventional helium-3 proportional tube detector, it measures the Lyman alpha photons produced by the reaction products instead of ion pairs.

"Traditional helium-3 proportional tubes detect the ionization tracks produced by the by-products of the absorption reaction using charge multiplication techniques," explained Clark and Thompson. "These detectors require precise manufacturing and have the disadvantage that they cannot observe high rates because of charge build-up in the tube and pulse timing."

Photons ease detection
Lyman alpha light can be measured by a variety of optical detectors, which can be optimized for different applications. "Proportional tubes are limited to a cylindrical geometry while our detector has no such constraints, which is an advantage for some applications," commented Clark and Thompson. "Other benefits include decreased cost due to fewer requirements for mechanical precision, gas purity and high voltages. It is also mechanically and electrically robust and offers greater sensitivity over a wide dynamic range."

Tens of photons are produced when each neutron is absorbed, which leads to high detection efficiency. By knowing the number of neutrons and their wavelength, the density of helium-3 gas and the experimental geometry, the group can also determine the number of Lyman alpha photons produced for every neutron absorbed in the helium-3 gas.

"Because photons are produced on sub-nanosecond time scales, we do not expect to be rate-limited below at least several million absorptions per second," explained Clark and Thompson. "Also, because the photons are produced from the outgoing hydrogen ions, there should be very low backgrounds from gamma rays or any other source. The only significant source of energetic hydrogen ions in our cell is the neutron reaction."

The group's current prototype uses a solar blind photomultiplier tube, but for future configurations it will look at a variety of detectors, including charge-coupled devices, avalanche photomultipliers and silicon photomultipliers.