2.2. Types of Particle Interactions 77
An interesting point to note here is that when we talk about the behavior of
radiation passing through matter, we are actually referring to the statistical out-
comes of interaction of particles with other particles in the material. We saw earlier
that at microscopic level, the particle interactions are mainly governed by quantum
mechanics and therefore the behavior of radiation can be predicted by statistical
quantities such as cross section. Depending on the type of force involved, particles
can interact with other particles in a number of ways. For example, the predominant
mode of interaction for charged particles and photons at lower energies is electro-
magnetic, while neutral particles are mainly affected by short range nuclear forces.
In the following sections, we will have a general look at some of the important modes
of interaction relevant to the topic of radiation interaction and detection.
2.2.A ElasticScattering.........................
Elastic scattering refers to a process in which an incident particle scatters off a target
in such a way that the total kinetic energy of the system remains constant. It should
be noted that scattering elastically does not imply that there is no energy transfer
between incident and target particles. The incident particle can, and in most cases
does, loose some of its energy to the target particle. However this energy does not
go into any target excitation process, such as, its electronic and nuclear states of the
target are not affected.
The best way to mathematically model the elastic scattering process is by con-
sidering both the incident and target particles to have no internal structure. Such
particles are generally referred to aspoint-likeparticles.
There are also special types of elastic scattering processes in which the incident
particle transfers none or very minimal energy to the target. A common example is
the Rayleigh scattering of photons.
2.2.B InelasticScattering........................
In inelastic scattering process the incident particle excites the atom to a higher
electronic or nuclear state, which usually comes back to the ground state by emitting
one or more particles. In this type of reaction the kinetic energy is not conserved
because some of it goes into the excitation process. However the total energy of the
system remains constant.
2.2.C Annihilation
When a particle interacts with its antiparticle, the result is the annihilation of both
and generation of other particles. The most common and oft quoted example of
this process is the electron-positron annihilation. Positron is the antiparticle of
electron having all the characteristics of an electron except for the unit electrical
charge, which in its case is positive. When it comes very close to an electron, the
twoannihilateeach other producing other particles. Depending on their energies,
different particles can be generated during this process. At low to moderate energies,
only photons are produced while at high energies other particles such asZbosons
can be produced.