Wilhelm Conrad Rontgen's sensational discovery of X-rays in 1895 was soon followed by Henri Becquerel's discovery of radioactivity and by J. J. Thomson's proof of the inde- pendent existence of negative electrons of small mass. Marie and Pierre Curie discovered the radioactive elements polonium and radium, and Paul Villard observed a new kind of extremely penetrating ray, later called gamma rays [23].
Paul Villard's main interest was in chemistry studies, which led him into his studies of
"radium rays", X-rays and cathode rays. His radioactivity experiments resulted in the unexpected discovery of gamma rays in 1900. Villard observed gamma rays as being dierent from X-rays because they showed bigger penetrating depth. He also discovered that they were emitted from substances that are radioactive and were not aected by magnetic and electric elds. Even though Paul Villard had discovered gamma rays, Ernest Rutherford recognised that they were in the form of electromagnetic (EM) light only with shorter wavelength than X-rays. The name gamma ray was given by Ernest Rutherford [24].
Chapter 2. Literature review 20
Gamma radiation which is also called gamma rays is an electromagnetic radiation (of high frequency and therefore consists of high energy photon) that is emitted by an unstable nucleus of an atom during radioactive decay. A nucleus which is in an unstable state may decay to a more stable state or ground state by the emission of energy as gamma radiation [8].
There are many other radioactive decays such as beta (β) and alpha (α) decays. Beta decay is when a nucleus with either too many protons or too many neutrons undergo β decay through the weak nuclear force, a neutron-rich nucleus will undergo β- emission (n → p+ +e- +ν¯) and a proton-rich nucleus will either emit aβ+ particle (p→n + e+ +ν) or be transformed by capturing an atomic electron in a process called electron capture.
Alpha decay is the radioactive emission of an α particle which is the nucleus of 4He, consisting of two protons and two neutrons.
Beta (β) particles have a negative charge and a small mass. Unlike γ ray they have very low energies and may be shielded by the aluminium material while alpha (α) particles are positively charged and are also not very penetrating. Gamma (γ) rays are the most penetrating particles since they are not charged and have a very short wavelength and high energy.
Figure 2.7 shows the ability of dierent types of ionizing radiation to penetrate dierent materials [25]. This study focuses more on detection of gamma rays and using their energies to identify the radionuclides produced from the nuclear reactions.
Figure 2.7. Illustration of the relative ability ofγ,βandαradiation to penetrate dierent materials, such as paper, aluminium and lead [25].
2.2.1 Interaction of Gamma-rays with Matter
Like neutrons, gamma rays have no mass and no charge and so they need an indirect way to transfer energy. Unlike charged particles (electrons and protons), the penetrating power of gamma rays is very high [26, 27]. The interaction of gamma radiation with matter causes ionization in matter via three major processes. These three processes are the photoelectric eect, Compton scattering and pair production. The mass attenuation coecient (µm) dened in Equation (2.26) is a measure of the probability of a γ ray interacting in the material.
µm = µ
ρ (2.26)
Where (µ= Nσ) is the linear attenuation coeent, σ is the total interaction cross section per atom of material and ρ is the physical density. It depends on the atomic number, Z of the atom of the absorbing material, and it varies with the γ ray energy (Eγ) and has contributions from all the three γ ray interaction processes that occur in the absorbing material [26]. The gamma ray interaction processes with matter are discussed in the next subsection.
2.2.1.1 Photoelectric Eect
In photoelectric absorption process, a photon undergoes an interaction with an absorber atom in which a photon completely disappears. In its place an energetic photoelectron is released by the atom from one of its outer most shells [9]. Since the energy of the gamma ray (≈ 0.5 MeV) is greater than the electron binding energy (which is about 10 to 100 eV), the energy of the free electron can be considered equal to that of the incident gamma ray. Therefore the photoelectric eect results in a peak known as photopeak in the photomultiplier spectrum at an energy same as the energy of an incident gamma ray.
Figure 2.8 shows a photomultiplier spectrum [28].
Energy of the photoelectron is given by:
Ee =hv−Eb (2.27)
where (Eb) is the photoelectron binding energy in its original shell. Binding energy is the energy required to remove an electron from an atom or a molecule andhv is the energy of
Chapter 2. Literature review 22
Figure 2.8. Photomultiplier spectrum that shows some processes of a gamma ray inter- acting with matter [28].
the incident photon. When an electron from an inner K or L shell is ejected, electrons from outer shells fall from their higher energy states to ll the vacancy [8]. The photoelectric eect is a predominent mode of interaction for gamma ray of low energies, the process is also enhanced for absorber materials of higher atomic number (Z). The approximation of the probability of the photoelectric absorption per atom is given by:
τ ∼= Zn
Eγ3.5 (2.28)
where exponent n varies between 4 and 5 over the gamma ray energy region of interest [9].
2.2.1.2 Compton Scattering
Compton scattering is a process in which a photon collides with a bound electron and the photon energy is considerably greater than the electron binding energy. After the interac- tion, the photon continues in a new direction with reduced energy and the electron attains enough energy to leave the atom. The Compton process is the most important absorption process for photons with energies of about 100 keV to approximately 20 MeV [25]. The Compton scattering results in a peak called the Compton Edge in the photomultiplier spectrum shown in Figure 2.8.
In this process, as shown in Figure 2.9, the incident gamma ray is deected through an angle θ with respect to its original direction. A photon transfers some of its energy to the electron which is called a recoil electron, because all scattering angles are possible. The energy transferred to the electron vary from zero to a large fraction of gamma ray energy.
The expression that relates the energy of the scattered photon and the scattering angle (θ) for any given interaction is given by Equation (2.29).
hv0= hv
1 + mhv
oc2(1−cos(θ) (2.29)
where moc2 is the energy of an electron (0.511 MeV) and hv is the energy transfered. For a small angle very little energy of a photon is transferred to an electron. Figure 2.9 shows the Compton scattering process [9].
Figure 2.9. Compton Scattering [9].
2.2.1.3 Pair Production
Pair production is a process whereby an interaction of the incident γ-ray in the strong Coulomb eld surrounding the nucleus results in complete transmutation of γ photon energy into an electron-positron pair. Formation of the electron-positron pair requires an energy twice the rest mass of the electron m0c2, hence the pair production interaction becomes signicant when the incident γ-ray energies exceed 1022 keV. The excess energy given by the photon energy minus 1022 keV (2×511 keV), is then shared between electron
Chapter 2. Literature review 24
and positron as kinetic energy. These particles are very short lived and they lose their kinetic energy very quickly by further collision with the electrons of the detector, followed by the spontaneous annihilation of a positron and an electron to produce two 511 keV γ- rays. For momentum to be conserved, the two photons are emitted at 180o to one another.
If the detector absorbs both annihilation γ-rays, their interaction contributes to the full- energy peak in the gamma ray spectrum measured. However, if one gamma ray of the annihilation leaves the detector, the interaction contributes to the single-escape peak at 511 keV. Moreover, if both gamma rays escape, the interaction contributes to the double- escape peak which is at 1022 keV [29, 30]. Figure 2.10 shows the production of positron and electron pair.
Figure 2.10. Pair production process [25].