50 Chapter 1. Properties and Sources of Radiation
whereSTis the spallation target andSFrepresentmspallation fragments. The
numberkof neutrons produced in this reaction depends on the type of the target
and the energy of the incident particle.
The targets used in spallation sources are generally highZmaterials such as lead,
tungsten, silver, or bismuth. However, it is also possible to generate neutrons by
bombarding light elements with high energy charged particles, such as protons. Two
examples of such reactions are the production of neutrons by bombarding lithium
and beryllium targets by high energy protons.
p+^73 Li →^74 Be+n
p+^94 Be →^95 B+nThe reader should note that the reactions such as these are not strictly classified
as spallation reactions since they do not involve the break up of the target nuclei
into several constituents. These reactions are more closely related to the nuclear
reactions we will discuss shortly.
A big advantage of spallation sources is that they produce neutrons with a wide
spectrum of energies ranging from a feweV to severalGeV. Another advantage is
their ability to generate neutrons continuously or in short pulses. The pulses could
be as short as a nanosecond.
Composite Sources
Spallation is not the only means of producing neutrons, that is, it is not abso-
lutely necessary to impinge nuclei with high energy particles to produce neutrons.
This can also be accomplished by exciting nuclei such that they emit neutrons dur-
ing the process of de-excitation. Fortunately for such a process to occur it is not
always necessary that the target particle carries a very high energy. In fact, incident
particles coming out of radioactive sources are more than sufficient to cause neutron
emission from some materials. In this book the sources made with such materials
will be termed ascomposite sourcesas they are made of combining two different
materials together.
A composite neutron source consists of a source of incident particles and a tar-
get that decays by neutron emission. The incident particle source can either be a
radioactive material or a small particle generator. In general, the composite sources
are made of a radioactive material acting as the source of incident particles mixed
in a target material. A common example is the plutonium-beryllium source, which
produces neutrons in the following sequence.
238
94 Pu →
234
92 U+α
α+^94 Be →^136 C∗
13
6 C∗ → 12
6 C+nThe first step of this process involvesα-decay of plutonium-238, which emitsα-
particles of energy around 5.48MeV with a half life of about 87.4 years. This
moderately long half life makes it suitable for long time storage in laboratories. The
α-particle impinges on the beryllium-9 target and transforms it into carbon-13 in
an unstable state, which ultimately decays into carbon-12 by emitting a neutron.
α-induced reactions are widely used to generate neutrons. Another common
example of an (α−n) neutron source is^241 Am−Be. Americium-241 has a half