2.2. Types of Particle Interactions 79
Eγ= 511 keV Eγ= 511 keV−
Figure 2.2.1: Depiction of electron-
positron annihilation process in
which two photons are produced. To
ensure conservation of energy and
momentum, each of these photons
must have an energy of 511keV and
they must travel opposite to each
other.is possible that only a single photon is produced in the process. Similarly production
of three or more photons is also possible. However the probability of production of
single, three or more photons at low energies is very small and can be safely ignored
(39).
The electron-positron annihilation process at low energies is used in a special type
of medical imaging called Positron Emission Tomography or PET. In this process
images are produced by detecting photons that are emitted as a result of this process.
We will discuss this technique in the chapter on imaging.
Since all particles have their inverse counterparts in nature, the annihilation
process is in no way limited to electrons and positrons. For example, protonsp
and anti-protons ̄palso get annihilated when they approach each other. In fact,pp ̄
collisions at the Tevatron collider at Fermilab in USA have given us immense insight
into the fundamental particles and their interactions.
2.2.D Bremsstrahlung..........................
Bremsstrahlung is a German word meaningbraking radiation. It refers to the process
in which decelerating charged particles emit electromagnetic radiation. All charged
particles can emit this kind of radiation provided they have enough energy. Gen-
erally speaking if a charged particle has energy much greater than its rest energy
(Erest≡E 0 =m 0 c^2 ,wherem 0 is its rest mass) emits Bremsstrahlung if it encounters
resistance while moving through a medium.
In pure Bremsstrahlung process there are nodirectelectronic or nuclear transi-
tions involved. However the radiation emitting particle may excite or ionize atoms
and excite nuclei of the medium as it decelerates. These excitations and ionizations
may lead to emission of other particles, such as x-rays andγ-rays with characteris-
tic energy peaks in the spectrum. These peaks are generally superimposed on the
continuous Bremsstrahlung spectrum and are therefore clearly distinguishable. The
most common example of this phenomenon is the emission of x-rays as we saw in
the previous chapter. The electrons, as they strike the anode, emit not only char-
acteristic x-rays but also Bremsstrahlung. In fact, as the electron has a very small
mass as compared to other charged particles, its Bremsstrahlung is one of the most
commonly encountered radiations in laboratories.