Neutron detectors

The operation of nearly all neutron detectors is based on a two-step process:

1. Neutron Conversion (the key reaction): A neutron interacts with an isotope within a converter material, resulting in a nuclear reaction that emits one or more charged, ionizing particles, which are measured. The most suitable converter is chosen based on the neutron energy.

• Thermal Neutrons (low energy, ~0.025 eV): The most common reaction is ³He(n,p)³H or ¹⁰B(n,α)⁷Li. These reactions have high cross-sections (probability) for thermal neutrons and release significant kinetic energy in the form of a proton (p) and triton (³H), or an alpha particle (α) and lithium nucleus.
• Fast Neutrons (high energy, >0.1 MeV): Detection relies on elastic scattering with light nuclei, like hydrogen (in a plastic scintillator or polyethylene moderator). The neutron transfers a portion of its kinetic energy to the proton, creating a "recoil proton" that ionizes the moderator medium. Alternatively, threshold reactions like (n,p) or (n,α) on materials like zirconium or silicon can be used.

2. Detection of the Secondary Particle: The charged particles produced in the first step (protons, alpha particles, tritons, etc.) are then detected by a primary radiation detector. The signal from this detector is processed and counted.

• Gas-filled detectors (like proportional counters) detect the ionization caused by the charged particle traversing the gas.
• Scintillation detectors detect the light pulses produced when the charged particle excites molecules in a scintillating material.

To make a detector sensitive to fast neutrons, it is often surrounded by a moderator—a material like polyethylene or water containing light elements. The moderator slows down (thermalizes) fast neutrons through successive elastic collisions, allowing them to be detected via the high-probability thermal neutron capture reactions.