Monoenergetic neutrons are obtained using beams of hydrogen ions (protons or deuterons1). These ions are accelerated to a given energy and directed at a target consisting of a very thin deposit on a metallic surface. The ions interact with the nuclei forming the thin deposit on the target, and this interaction produces neutrons. The deposit may be of scandium, lithium or titanium, with tritium or deuterium trapped inside.
The ion beam that generates the neutrons is obtained in two stages. Negative hydrogen or deuterium ions (H- or D-) are accelerated by a voltage into the center of the accelerator, where their charge is reversed when they pass through a nitrogen flow that strips them of two electrons. They therefore become positive (protons or deuterons respectively) and then, having had their charge reversed, they are accelerated for the second time, by the same voltage.
The energy of the neutrons emitted from the target depends on the type of particles used (protons or deuterons) and their energy, but also on the deposit on the target. Two magnets, the first positioned after the ion source and the second at the output from the accelerator, are used respectively to select the type of particles to be accelerated (protons or deuterons) and their energy. Because the monoenergetic peak resolution2 depends mainly on the type and thickness of the chosen deposit, several deposit thicknesses are available.
The energy of the neutrons produced also varies with the emission angle in relation to the ion beam direction. This property can be used to change the energy of monoenergetic neutron fields without changing the energy of the ion beam. However, a 0° angle (along the beam axis) is preferred whenever possible because the neutron fields produced have better characteristics3. The energy range of neutrons emitted at 0° is specified in the table below. With emission angles of up to +/-150°, the neutron energy range covered by the facility is 2 keV to 7.3 MeV and 12 MeV to 20.5 MeV.
Table of nuclear reactions used to create monoenergetic fields with the associated energy range for these fields on the ion beam axis. ©IRSN
(1) A deuteron is a hydrogen isotope consisting of a proton and a neutron.
(2) The ratio between the full width at half maximum, in terms of energy, of the monoenergetic peak and the mean energy of this peak.
(3) At this angle, the energy and flux are at their maximum, the contribution of scattered neutrons is minimal and the field is at its most homogeneous on the detector surface.