Neutron Detection
Helium-3 is a most important isotope in instrumentation for neutron detection. It has a high absorption cross section for thermal neutron beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction
- n + 3He → 3H + 1H + 0.764 MeV
into charged particles tritium (T, 3H) and protium (p, 1H) which then are detected by creating a charge cloud in the stopping gas of a proportional counter or a Geiger-Müller tube.
Furthermore, the absorption process is strongly spin-dependent, which allows a spin-polarized helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in neutron polarization analysis, a technique which probes for magnetic properties of matter.
The United States Department of Homeland Security had hoped to deploy detectors to spot smuggled plutonium in shipping containers by their neutron emissions, but the worldwide shortage of helium-3 following the drawdown in nuclear weapons production since the Cold War has to some extent prevented this.
Read more about this topic: Helium-3