54 Chapter 1. Properties and Sources of Radiation
α-particles are extremely stable particles having a binding energy of about 28.8
MeV. The protons and neutrons are held together in a very stable configuration by
the strong nuclear force.
F.1 Sources ofα-Particles
Even with very low penetration capability,α-particles have found many applications
in different fields. A lot of research has therefore gone in the direction of designing
effectiveα-particle sources. Fortunately there are a host of naturally occurring ra-
dioisotopes that emitα-particles in abundance. A particle accelerator can then be
used to increase their energy up to the required energy. Apart from these radioactive
sources, other sources with higherα-particle fluxes have also been developed.
Accelerator Based Sources
Just like protons and neutrons,α-particles are also produced during collisions
of high energy particles with fixed target materials. At very high incident particle
energies the so called spallation reactions tear apart the target nuclei into its con-
stituents. Since the 2 protons plus 2 neutrons configuration of a nucleus is extremely
stable therefore such reactions produceα-particles as well together with protons and
neutrons.
One example of such a reaction is the collision of high energy neutrons (25-65
MeV) on a cobalt-59 target, which produces one or moreα-particles per collision.
Radioactive Sources ofα-Particles
There are numerous radionuclides that emitα-particles, some of which are listed
in Table.1.6.6.
1.6.G FissionFragments
Nuclear fission is a process in which an unstable nucleus splits up into two nuclides
and emits neutrons. Two types of fission reactions are possible: spontaneous fis-
sion and induced fission. We saw earlier that spontaneous fission occurs in some
radionuclides, at least one of which, i.e. californium-252, occurs naturally. These
radionuclides are extensively used as neutron sources.
In induced fission, when a fissionable nucleus, such as uranium, captures a neutron
it goes into an unstable state and eventually breaks up in two heavy parts. In this
process it also emits some neutrons. The two heavier particles are known as fission
fragments. The reaction can be written as
FM+n→FF 1 +FF 2 + (2-4)n, (1.6.8)whereFMrepresents fissionable material (such as^23592 U),nis the neutron, andFF 1
andFF 2 are the two fission fragments.
Most of these fission fragments are unstable and go through a series of nuclear
decays before transforming into stable elements. They generally have unequal masses
but in a sample of many fissionable atoms, some of the fragments are always of equal
mass (see Fig.1.6.10 for an example of uranium-235 fission fragment spectrum).