Reactor Theory (Neutron Characteristics)
There is an abundant supply of high energy gammas in a reactor that has been operated because
many of the fission products are gamma emitters. All water-cooled reactors have some
deuterium present in the coolant in the reactor core because a small fraction of natural hydrogen
is the isotope deuterium. The atom percentage of deuterium in the water ranges from close to
the naturally occurring value (0.015%) for light water reactors to above 90% deuterium for heavy
water reactors. Therefore, the required conditions for production of photoneutrons exist.
The supply of gamma rays decreases with time after shutdown as the gamma emitters decay;
therefore, the photoneutron production rate also decreases. In a few particular reactors,
additional D O (heavy water) may be added to the reactor to increase the production of
photoneutrons following a long shutdown period.
Installed Neutron Sources
Because intrinsic neutron sources can be relatively weak or dependent upon the recent power
history of the reactor, many reactors have artificial sources of neutrons installed. These neutron
sources ensure that shutdown neutron levels are high enough to be detected by the nuclear
instruments at all times. This provides a true picture of reactor conditions and any change in
these conditions. An installed neutron source is an assembly placed in or near the reactor for
the sole purpose of producing source neutrons.
One strong source of neutrons is the artificial nuclide californium-252, which emits neutrons at
the rate of about 2 x 10 neutrons per second per gram as the result of spontaneous fission.
Important drawbacks for some applications may be its high cost and its short half-life (2.65
Many installed neutron sources use the ( ,n) reaction with beryllium. These sources are
composed of a mixture of metallic beryllium (100% beryllium-9) with a small quantity of an
alpha particle emitter, such as a compound of radium, polonium, or plutonium. The reaction
that occurs is shown below.
The beryllium is intimately (homogeneously) mixed with the alpha emitter and is usually
enclosed in a stainless steel capsule.
Another type of installed neutron source that is widely used is a photoneutron source that
employs the ( ,n) reaction with beryllium. Beryllium is used for photoneutron sources because
its stable isotope beryllium-9 has a weakly attached last neutron with a binding energy of only
1.66 MeV. Thus, a gamma ray with greater energy than 1.66 MeV can cause neutrons to be
ejected by the ( ,n) reaction as shown below.