Interaction of Matter with Radiation PDF

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Radiation PhysicsInteraction of Radiation with MatterSELF-STUDY GUIDEM. Wajira Sanjaya PereraM. in Nuclear Science 2020/Department of Nuclear ScienceFaculty of ScienceUniversity of Colombo How heavy charged particles (HCP) interact  All energetic ions with mass of one atomic mass unit or greater ar...

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Radiation Physics Interaction of Radiation with Matter SELF-STUDY GUIDE

M. Wajira Sanjaya Perera M.Sc. in Nuclear Science 2020/2021 Department of Nuclear Science Faculty of Science University of Colombo

1. How heavy charged particles (HCP) interact  All energetic ions with mass of one atomic mass unit or greater are defined as heavy charged particles. Examples: protons, alpha particles, muons, fission fragments.  Basically, HCP interact with matter through Coulomb forces between their positive charge and the negative charge of the electrons from atomic orbitals.  In general, charged particles transfer energy mostly by: o Excitation. The charged particle can transfer energy to the atom, raising electrons to higher energy levels. o Ionization. Ionization can occur, when the charged particle have enough energy to remove an electron. 2. How electrons and positrons interact.  Electrons and positrons are also classified as charged particles but not heavy. They have the same absolute charge (e- and e+).  The interaction is mediated by electromagnetic force/ Coulomb force  They are interaction process with matter can be classified as follow. i. Elastic scattering: Incident electron is scattered without any change in energy. ii. Inelastic scattering: It leads to excitations and ionizations of atoms of the medium, and is called “Collisional Stopping Power”. iii. Bremsstrahlung: The charged particle is decelerates while interacting with the atomic nucleus electric field. It emits electromagnetic radiation. iv. Positron annihilation: A positron may annihilate while interacting with an electron, then both particles disappear and two photons are emitted with the same energies. 3. Differences between HCP interact and electrons/ positrons interact. i. Most of these differences are based on the different dynamics of the collision process. When a heavy particle collides with a much lighter particle (electrons in the atomic orbitals), the laws of energy and momentum conservation predict that only a small fraction of the massive particle’s energy can be transferred to the less massive particle. The actual amount of transferred energy depends on how closely the charged particles pass through the atom and it depends also on restrictions from quantisation of energy levels. ii. Fast electrons lose energy at a lower rate compared with HCP. For equivalent energy, the specific energy loss of electrons is much lower than HCP. 4. How gamma rays interact  Gamma rays, also known as gamma radiation.  This is an electromagnetic radiation (no rest mass, no charge) of very high energies.  Gamma rays ionize matter primarily via indirect ionization.  There are three key interaction mechanisms with matter. i. Photoelectric effect a. The photoelectric effect dominates at low-energies of gamma rays. b. The photoelectric effect leads to the emission of photoelectrons from matter when light (photons) shines upon them. ii. Compton scattering a. Compton scattering dominates at intermediate energies. b. It is the scattering of photons by atomic electrons Pair production iii. a. Pair production dominates at high energy more than 1.02MeV. b. In this interaction the gamma photon disappears and is replaced by an pair of electron and positron. 5. Summary Uncharged particle Interaction 1 X rays, gamma rays

Charged particle Interaction Alpha, Proton, Electrons, Positrons, Fission fragments

2 3 4 5 6 7 8

Interacts with matter gives rise to the release of charged particles Small no. of interactions with large energy losses Can’t interact with electrons or nucleus by long range coulomb force Less interaction cross-section Zero rest mass of photon Photon can’t decelerates Travels with light velocity

Transfer their energy to the medium through which they pass Large no. of interactions with small energy losses Can interact with electrons or nucleus by long range coulomb force More interaction cross-section Non zero rest mass of particle Charged can decelerates Velocity comparable with respect to light velocity

6. Learn about the Z (atomic number) and E (energy) dependence of the σ (cross section) (Cross section is a measure of the probability of interaction) Variation of the Photo electric cross section with the Z Variation of the Compton cross section with the Z Variation of the Pair production cross section with the Z Total cross section σ

Variation of the Photo electric cross section with the E Variation of the Compton cross σC ∝ Z section with the E Variation of the Pair production σ Pair ∝ Z 2 cross section with the E 4.5 2 σ =σ Pe ( ∝ Z ) +σ C ( ∝ Z)+ σ Pair (∝ Z ) σ Pe ∝ Z n , 4 ≤ n

1 E3.5 1 σC ∝ E

σ Pe ∝

σ Pe ∝ ln E

7. Dependence of cross section with energy for different materials

8. Linear attenuation coefficient, mass attenuation coefficient, mass energy absorption coefficient, relationship between microscopic cross section & macroscopic cross section, mass energy absorption coefficient Term

Linear attenuation coefficient

Notation

μ

Details a) the fractional decrease of the gamma ray beam intensity per unit thickness b) Also known as macroscopic cross section c) μ= τ ( photoelectric ) +σ ( Compton) +θ(Pair ) d) I = I o e− μt , I= Transmitted Photon intensity, Io= Incident Photon intensity, t= thickness of the absorber/ path length e) Units cm-1

Mass coefficient

attenuation

a) μm

Atomic attenuation coefficient μa

relationship between microscopic cross section & macroscopic cross section

9. mass energy absorption coefficient

−¿

μen

μ μm= ρ

, ρ=¿ density of the medium

b) Units= g-1cm2 c) For given gamma ray energy, μm does not change with the physical state of a given absorber. a) the fraction of gamma ray beam that is attenuated by a single atom μ , N is the no.of atoms per cm3 b) μa= N c) Also known as microscopic cross section d) Units = atom-1cm2 a) the number of nuclei N per unit volume multiplied by the cross section σ is converted in to the macroscopic cross section. b) ∑ ¿ Nσ = μ c) the cross section has units of area measures in units of barn, 1barn= 10-24cm2. a) μen =μ photoelectric + f 1 μ compton +f 2 μ pair , f1= fraction of energy absorbed in Compton event, f2= fraction of energy absorbed in pair production event b) In photoelectric  Full absorption of energy

9. Half value layer, μ relation Half Value Layer a) Absorber thickness required to reduce the intensity of the original beam by (HVL) one half. Io 0.693 ln 2 → HVL= 2 b) , H VL= , μ=¿ I = I o e− μt → =e−μHVL μ μ Io attenuation coeffiecent c) A beam with high HVL can penetrate more than the beam with low HVL 10. Interaction of neutrons, three modes. Relative importance with energy  Neutrons are uncharged like gamma and therefore no coulomb forces.  But unlike gamma rays, Neutrons interact with the Nucleus of atom.  Neutrons can travels through many centimeters of matter without any type of interaction.  As a result of interaction, the neutron may either totally disappear and be replaced by one or more secondary radiation, or else energy or direction of the neutron is changed.  There are 03 types of interactions: 1

Elastic scattering (n, n)

2

Inelastic scattering (n, n*)

3

a) Neutron collides with a target nucleus and bounces off in a manner similar to that in a billiard ball type collision. b) Total kinetic energy of the system is conserved. c) Most likely interaction between fast neutrons and low atomic number Z d) Most important process for slowing down neutrons e) Low Z materials are good moderators because they slow down neutrons. a) Incident neutron momentarily captured by targeted nucleus b) neutrons re-emit with less energy c) Nucleus left in excited state d) Nucleus relaxes by emitting charged particle or gamma e) Example: 16O(n, n*) 16O* with 6.1MeV gamma rays a) Important in slow neutrons (En...