RADIATION SOURCES
Units and Definition
Activity (Radioactivity) Definition Activity: Rate of decay (transformation or disintegration) is described by its activity Activity = number of atoms that decay per unit time Units of Radioactivity Bq (Becquerel) SI unit 1 Bq = 1 disintegration / sec Ci (Curie) Traditional unit Activity ascribed to 1 g of Ra-226 3.7 x 10 10 Bq
Specific Activity Specific Activity: Activity per unit mass Unit Bq/g, Ci/g Calculation Activity = λn SA = Activity per gram of nuclide If N per gram of a nuclide is known, SA can be calculated N(per gram) = NA M 23 6.02 x 10 (atoms/mole) = M (g/mole) SA = Activity Mass NA = λ M M (atomic mass) A(atomic number) N SA = λ A A If half - life (instead of decay constant is given) SA = Ln2 N t A 1/2 A
Electron Volt (ev) The electron volt is a unit of energy Amount of kinetic energy (KE) gained by a single unbound electron when it accelerates through an electrostatic potential difference of 1 volt in vacuum 1 ev = 1.602 10 19 J Electric energy is defined as E = UQ where E = electric energy (J) U = electric potential difference (V) Q = charge (C). Therefore 1 ev corresponds to E = 1 ev = 1e x (1.602 x 10-19 C/e) x 1 V = 1.602 x 10-19 J (-) (+) - 1V
Energy of Electromagnetic (EM) Wave Radiation Energy of x- or gamma rays where E = energy h = Plank constant (6.626 x 10-34 J sec) n= frequency c = speed of light (3 x 10 8 m/s) l = wave length
Radiation Sources
Radiation Groups By Ionization Ability By Type By Ionization Process Ionizing radiation - Particles, EM waves Non-ionizing radiation - UV (Ultraviolet), Visible light, IR (Infrared) Particles - α, β, e, p, n, heavy charged particle Electromagnetic (EM) waves - γ-ray, x-ray Direct Ionizing radiation - α, β, e, p Indirect ionizing radiation - n, γ, x
Radiation Types Type Origin Charge Source Note Alpha He nucleus + 2e Heavy nuclides (z > 83) Alpha decay Monoenergetic Beta (-) Electron e - e Beta (-) decay Internal conversion electron Auger electron Beta continuous energy Electron Monoenergetic Beta (+) Positron e + + e Beta (+) decay Pair production Neutron n No Reactor (α,n) reaction: - 241 Am(α, n) 9 Be Cf-252 Positron Annihilation Penetrate well in medium γ - ray EM wave No Excited nucleus Monoenergetic X ray EM wave No Excited atom Bremsstrahlung Characteristic x-ray X-ray machine Bremsstrahlung continuous energy Characteristic x-ray monoenergetic
Alpha Radiation Source 1. Alpha decay of heavy nuclei
Alpha decay of heavy nuclei Unstable atomic nucleus emits an alpha particle (energetic He nucleus) and transforms into another atom. Generally, alpha decay occurs among heavy atoms Decay process Example 226 222 88 Ra 86Rn + 4 2 He
Kinetic energies of alpha and recoil Most (98%) kinetic energy (Q-value) goes to alpha particle Characteristics of alpha decay Alpha particles are monoenergetic! Generally alpha energy ranges 4 MeV ~ 6 MeV Strong correlation between alpha particle energy and half-life of the parent isotope: high energy short half-life (Table 1.3 in Knoll)
Electron Source 1. Beta decay 2. Internal Conversion 3. Auger Electron
Beta decay Unstable atomic nucleus eject electron from the nucleus A process that a neutron in a nucleus is transformed into a proton and an electron. Decay process Example 14 14 0 0 C N + β ν 6 7 1 + 0
Kinetic energies of beta and recoil Most of the kinetic energy go to the β-particle like α-decay? NO!!! Beta particles are observed to show a spectrum of kinetic energies Continuous energy of beta Particle Therefore, another particle must also be emitted so as to conserve energy Antineutrino
Pure Beta Emitter Most beta sources emit not only beta-particle but also γ-ray Most beta decays populate an excited state of the product nucleus, so that the subsequent de-excitation γ-rays are emitted together with beta particles in many common beta sources. Some radionuclides emit only beta-particles called pure beta emitter Some radionuclides decay (beta decay) directly to the ground state of the product. Therefore, they do not emit γ-rays Examples of pure better emitters H-3, C-14, P-32, S-35, Cl-36, Ca-45, Ni-63, Sr-90/Y-90, Tc-99m, Pm-147, Tl-204 Table 1.2 in Knoll
Internal Conversion Excited nucleus interacts with an orbital electron, causing the orbital electron to be emitted from the atom. Analogues to emission of Auger electrons in atomic process. But, internal conversion electron is nuclear radiation while Auger electron is atomic radiation Not to be confused with the more similar photoelectric effect. In internal conversion, the nucleus does not emit an intermediate real gamma ray
What happens to the K or L-Shell vacancy resulting from internal conversion? Characteristic x-ray emission Auger electron emission
Auger electron When an electron is removed from a shell of an atom, an electron from a outer shell may fall into the vacancy, resulting in Characteristic x-ray: Release of energy in form of an emitted photon Auger electron: The energy is transferred to another electron, which is ejected from the atom Note: only important for light elements (low Z) Light element normally Auger electron Heavy element normally x-ray
Positron Source 1. Positron (β + ) decay 2. Pair production
Positron (β + ) decay Unstable atomic nucleus eject positive electron (positron) from the nucleus A process that a proton in a nucleus is transformed into a neutron and an positron. Decay process Example 22 Na 22 0 + 11 10Ne + + 1β + 0 0 ν
Kinetic energies of beta (+) and recoil Positron particle also has continuous energy like beta particle Another particle called neutrino is emitted to conserve energy (c.f., antineutrino for beta decay) Fate of ejected positron Annihilation
Photon (Electromagnetic Radiation) Source 1. Gamma ray following beta decay 2. Annihilation radiation 3. Gamma rays following nuclear reactions 4. Bremsstrahlung (Continuous x-ray) 5. Characteristic X-rays 6. Synchrotron radiation
Gamma ray following beta decay Most beta decays populate an excited state of the product nucleus, so that the subsequent de-excitation γ-rays are emitted together with beta particles in many common beta sources. (c.f., pure better emitter) Examples
Annihilation radiation Ejected positron loses its kinetic energy by exciting or ionizing atoms through its path Finally it is annihilated by colliding with electron. The annihilation process creates 2 gamma rays with energy of 0.511 MeV to exactly opposite direction (180 ) This process is used for PET (Positron Emission Tomography)
PET (Positron Emission Tomography) One of recently famous medical imaging techniques is PET (Positron Emission Tomography) PET imaging uses annihilation process to make images PET is a nuclear medicine imaging technique which produces a threedimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer)
Gamma rays following nuclear reactions Nuclear reactions may result in product nucleus left in an excited state Therefore, subsequent de-excitation γ-rays are emitted to subsequently de-excite the production nucleus Examples 4 9 12 * 1 12 * 12 α + Be C + n (Subsequently C C + 2 4 6 0 6 6 γ 4 13 16 * 1 16 * 16 α + Be O + n (Subsequently O O + 2 6 8 0 8 8 γ (4.44 MeV)) (6.13MeV)) Gamma rays are also commonly emitted following the absorption of thermal neutrons by typical nuclei Neutron-capture gamma rays
Bremsstrahlung (Continuous x-ray) When a higher-energy electron is deflected in the electric field of an atomic nucleus, electromagnetic radiation is produced by the deceleration of the electron called Bremsstrahlung (breaking radiation) Electrons can lose its energy through bremsstrahlung from zero to total energy. Therefore, photon energy ranges from zero to electron s total energy. Continuous energy X-ray machines are made based on this interaction
Characteristic X-rays When there is a vacancy in electron orbit (by photoelectric effect, internal conversion, etc) one of the outer electrons falls down into the vacant slot. When this happens, a photons is emitted The emitted photon energy is equal to the difference of those electrons binding energies E (photon) = BE (inner shell) BE (outer shell) Emitted photon has mono-energetic energy (discrete energy)
Synchrotron radiation When a beam of energetic electrons is bent into a circular orbit a small fraction of the beam energy is radiated away during each cycle (electromagnetic theory) Used for accelerator to generate high intensity and tunable energy of the available source.
Neutron Sources 1. Spontaneous fission 2. Radioisotope (α, n) sources 3. Photoneutron source 4. Reactions from accelerated charged particles
Spontaneous fission Many of the transuranic heavy nuclides have an appreciable spontaneous fission decay probability Several fast neutrons are promptly emitted in each fission event The most common spontaneous fission source is Cf-252 (t 1/2 =2.65 year).
Radioisotope (α, n) sources Neutron is emitted by (α, n) reaction Example 4 9 12 1 α + Be C n 2 4 6 + 0 Energetic alpha particles are available from alpha decay of a number of radionuclides (e.g, Rn, Ra,.) Therefore, these neutron sources are made by alpha emitter + target (Table 1.6 in Knoll) Be is the most commonly used target
Photoneutron source Some radioisotope gamma-ray emitters can also be used to produce neutrons when combined with an appropriate target material Only 2 target nuclei (Be-9 and H-2) are practically significant 9 8 1 Be + hν Be + n (Q -1.666 MeV) 4 4 0 = 2 1 1 1 H + hν 1H + 0n (Q = -2.26 MeV)
Reactions from accelerated charged particles Reaction involving protons, deuterons, and so can generate neutrons Such charged particles are accelerated by accelerator Two of he most common reaction of this type used to produce neutrons: D D Reaction : D T Reaction : 2 2 3 1 1 H + 1H 2He + 0n (Q = 3.26 MeV) 2 3 4 1 1 H + 1H 2He + 0n (Q = 17.6 MeV)