Physics and Engineering of Radiation Detection

76 Chapter 2. Interaction of Radiation with Matter

whereprepresents the momentum. The subscriptsiandfstand for the initial and
the final states of the system (such as before and after collision) and thevectorsum
is carried over all moving particles taking part in the collision.
Conservation of angular momentum ensures that there are no preferred directions,
that is, the space is isotropic and the laws of physics are the same in all directions.

E.3 ConservationofElectricalCharge...............

Most of the particles we know hold a property called electrical charge. For example,
an electron carries a unit charge of 1. 602 × 10 −^19 coulomb while a proton has the
same charge with positive polarity. However there is one fundamental difference
between the charge carried by these two particles. The electron does not seem to
have any internal structure and is therefore thought to carry the unit charge. On
the other hand the proton is composed of three quarks each carrying a fractional
charge. The total charge is still the so calledunit chargebut internally the charge
is divided between the constituents of the proton. A neutron is also composed of
three quarks but in such a way that the fractional charges of these quarks cancel
out and the net charge on the neutron appears to be zero. The law of conservation
of electrical charge states that the unit charge can neither be created nor destroyed.
This law eliminates some of the interaction possibilities. For example, a beta decay
can notbe written as
n→e+γ,

since it would imply the creation of electrical charge. Instead the correct reaction is

n→p+e+ ̄ν.

2.2 TypesofParticleInteractions

Radiation carries energy and whenever it interacts with a detection medium it may
depositsomeorallofittotheparticlesinthemedium. Theresultissomeformof
excitation. This excitation can form a basis of signal formation that can be detected
and measured by the processing electronics. Some of these excitations are:

Ionization

Scintillation

Excitation of lattice vibrations

Breakup of Cooper pairs in superconductors

Formation of superheated droplets in superfluids

Excitation of optical states

However not all of these can always be effectively used for particle detection. The
most commonly used excitation mechanisms that are used for this purpose are the
ionization and scintillation.