Neutron Production
To produce a free neutron, a particle with sufficient energy is projected onto a nucleus in order to break it apart and release neutrons.
Neutron production mainly relies on three types of sources:
nuclear reactors, spallation sources and CANS sources.
Producing a free neutron
⚛ Nuclear reactor
A neutron is sent onto a fissile nucleus in order to trigger a fission reaction.
The produced neutrons can generate additional fissions or be extracted for scientific experiments.
Example: ILL in Grenoble, France
A neutron is sent onto a fissile nucleus in order to trigger a fission reaction.
The produced neutrons can generate additional fissions or be extracted for scientific experiments.
Example: ILL in Grenoble, France
🔬 Spallation source
A high-energy proton (> 100 MeV) strikes a very heavy nucleus.
The excess neutrons inside the target nucleus are then released.
Example: ESS in Sweden
A high-energy proton (> 100 MeV) strikes a very heavy nucleus.
The excess neutrons inside the target nucleus are then released.
Example: ESS in Sweden
🔹 CANS source
A proton with an energy of a few MeV strikes a lighter nucleus.
The nuclear reaction produces neutrons that are subsequently released.
Example: ICONE
A proton with an energy of a few MeV strikes a lighter nucleus.
The nuclear reaction produces neutrons that are subsequently released.
Example: ICONE
Moderating a free neutron
When a neutron is released from a nucleus, its energy is generally of the order of a few MeV.
To obtain neutrons suitable for neutron scattering experiments,
their energy must be reduced down to a few meV,
corresponding to an energy reduction factor close to 10⁹.
This energy reduction is achieved through successive collisions with light nuclei whose own thermal energy is sufficiently low.
Energy transfer during a collision
When a neutron collides with a nucleus, part of its kinetic energy is transferred to the nucleus.
If A represents the ratio between the target nucleus
mass and the neutron mass, the energy transfer depends on the mass
ratio and on the collision angle.
θ = 180°
The energy transfer is maximum.
The neutron energy after collision reaches its minimum value.
The energy transfer is maximum.
The neutron energy after collision reaches its minimum value.
Low A value
When the nucleus mass is close to the neutron mass, energy cooling is more efficient.
When the nucleus mass is close to the neutron mass, energy cooling is more efficient.
Choice of moderator
Light nuclei therefore allow a rapid reduction of the neutron energy through successive collisions.
Hydrogen
Deuterium
Carbon
Moderator materials
To moderate a neutron, a number of collisions n is required.
This number becomes smaller as the mass ratio A decreases.