Because the free neutrons are non-stable particles, it requires external,
i.e. it does not depend on the field of neutron sources.
The first reaction is α-n reaction as used by Chadwick when
he first discovered the neutron. For example, in an n-source
such as 241Am/Be, 241Am undergoes α-decay; the α-particle
can be absorbed by a light element such as beryllium, which
then decays by neutron emission.
(α, n) reaction can only take place if the first the kinetic energy of α-particle
is above Coulomb barrier of the target nucleus and second
the excitation energy of a compound nucleus, resulting after the capture
of α-particle, is higher than the binding energy of a neutron in the compound nucleus.
An alternative is a γ-n source. For example: Radioactive
decay sources have the advantage of being small and highly
portable, but they have low intensity and are always “on”.
They can be used for testing (e.g., of the neutron detectors), in medicine
(e.g., for activation analysis, cancer treatment with 252Cf needles),
and for low-resolution/low-flux radiography.
The reaction of (γ, n) reaction may occur if the excitation energy is higher than
the binding energy of a neutron in the nucleus. Usually the binding
energy of a neutron in the nucleus is 6–8 MeV and it is too much
for the reaction but the nucleus of 9 Be and deuterium 2 H have
abnormally low energy of binding it is enough for γ–n reaction.
The next sources, is basis of the (p, n) reactions. Bombarding a target
by protons, it is possible to obtain a monochromatic neutron source.
Because of the need of penetration through the Coulomb barrier,
the appropriate targets are limited to light nuclei.
The next sources based on a fusion reaction. This sources is
higher intensities can be produced by small accelerator-based neutron sources.
Over the years, these have evolved sufficiently, so that compact
portable sources are now commercially available from a range of vendors.
They are normally based on the “D-T” reaction: deuterium and tritium
They are widely used for industrial and security applications based
on the fast neutron radiography and prompt gamma activation analysis.
Last but not least, the neutron reaction in which an incident
neutron is absorbed by a target nucleus, resulting
in a splitting of the target nucleus into two new atoms.
The fission process often produces free neutrons and photons
(in the form of gamma rays), and releases a large amount of energy.