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Part 2. IAEA Training Material on Radiation Protection in Nuclear Medicine. Radiation Physics. Objective.
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Part 2 IAEA Training Material on Radiation Protection in Nuclear Medicine Radiation Physics
Objective To become familiar with the basic knowledge in radiation physics, dosimetric quantities and units to perform related calculations, different types of radiation detectors and their characteristics, their operating principles, and limitations. Part 2: Radiation Physics
Content • Atomic structure • Radioactive decay • Production of radionuclides • Interaction of ionizing radiation with matter • Radiation quantities and units • Radiation detectors Note: Radiation units & quantities are in the process of undergoing consensus through ICRU and IAEA. There may be changes necessitating incorporation in this CD. Part 2: Radiation Physics
Part 2. Radiation Physics IAEA Training Material on Radiation Protection in Nuclear Medicine Module 2.1. Atomic structure
THE ATOM • The nucleus structure • protons and neutrons = nucleons • Z protons with a positive electric charge (1.6 10-19 C) • neutrons with no charge (neutral) • number of nucleons = mass number A • The extranucleus structure • Z electrons (light particles with electric charge) • equal to proton charge but negative Particle Symbol Mass Energy Charge (kg) (MeV) ---------------------------------------------------------- Proton p 1.672*10-27 938.2 + Neutron n 1.675*10 -27 939.2 0 Electron e 0.911*10 -30 0.511 - Part 2: Radiation Physics
Identification of an Isotope Part 2: Radiation Physics
Ernest Rutherford (1871-1937) Part 2: Radiation Physics
Electron Binding Energy • Electrons can have only discrete energy levels • To remove an electron from its shell E electron binding energy • Discrete shells around the nucleus : K, L, M, … • K shell has maximum energy (i.e. stability) • Binding energy decreasing when Z increases • Maximum number of electrons in each shell : 2 in K,8 in L shell, … Part 2: Radiation Physics
Ionization-Excitation Energy Part 2: Radiation Physics
De-excitation Auger- electron characteristic radiation Part 2: Radiation Physics
The NucleusEnergy Levels ENERGY Excitation Deexcitation Particle emission 0 MeV ~8 MeV Gamma ray Occupied levels The nucleons can occupy different energy levels and the nucleus can be present in a ground state or in an excited state. An excited state can be reached by adding energy to the nucleus. At deexcitation the nucleus will emit the excess of energy by particle emission or by electromagnetic radiation. In this case the electromagnetic radiation is called a gamma ray. The energy of the gamma ray will be the difference in energies between the different energy levels in the nucleus. Part 2: Radiation Physics
Isomeric Transition Normally the excited nucleus will undergo de-excitation within picoseconds. In some cases, however, a mean residence time for the excited level can be measured. The de-excitation of such a level is then called isomeric transition (IT). This property of a nucleus is noted in the label of a nuclide by adding the letter m in the following way: technetium-99m, Tc-99m or 99mTc Part 2: Radiation Physics
Nuclear Excitation • Energy particles photons Part 2: Radiation Physics
Nuclear De-excitation alpha-particle beta-particle Gamma radiation Part 2: Radiation Physics
Internal Conversion characteristic radiation conversion electron Part 2: Radiation Physics
Gamma Ray Spectrum(characteristic of the nucleus) Part 2: Radiation Physics
Photons are part of the electromagnetic spectrum IR: infrared, UV: ultraviolet Part 2: Radiation Physics
Part 2. Radiation Physics IAEA Training Material on Radiation Protection in Nuclear Medicine Module 2.2. Radioactive decay
Stable Nuclides long ranged electrostatic forces p Line of stability p n short ranged nuclear forces Part 2: Radiation Physics
Stable and Unstable Nuclides Too many neutrons for stability Too many protons for stability Part 2: Radiation Physics
RadioactiveDecay Fission The nucleus is divided into two parts, fission fragments. and 3-4 neutrons. Examples: Cf-252 (spontaneous), U-235 (induced) a-decay The nucleus emits an a-particle (He-4). Examples: Ra-226, Rn-222 b-decay Too many neutrons results in b- -decay. n=>p++e-+n. Example:H-3, C-14, I-131. Too many protons results in b+ -decay p+=>n+ e++nExamples: O-16, F-18 or electron capture (EC). p+ + e-=>n+n Examples: I-125, Tl-201 Part 2: Radiation Physics
Radioactive Decay It is impossible to know at what time a certain radioactive nucleus will decay. It is, however possible to determine the probability l of decay in a certain time. In a sample of N nuclei the number of decays per unit time is then: Part 2: Radiation Physics
Activity The number of decaying nuclei per unit of time 1 Bq (becquerel)=1 per second Part 2: Radiation Physics
1 Bq is a small quantity • 3000 Bq in the body from natural sources • 20 000 000-1000 000 000 Bq in nuclear medicine examinations Part 2: Radiation Physics
Multiple & Prefixes (Activity) Multiple Prefix Abbreviation 1 - Bq 1 000 000 Mega (M) MBq 1 000 000 000 Giga (G) GBq 1 000 000 000 000 Tera (T) TBq Part 2: Radiation Physics
Henri Becquerel 1852-1908 Part 2: Radiation Physics
Maria Curie 1867-1934 Part 2: Radiation Physics
Parent-Daughter Decay A B C λ2 λ1 Part 2: Radiation Physics
Parent-Daughter Decay Secular equilibrium TB<<TA≈ ∞ Transient equilibrium TA≈ 10 TB No equilibrium TA≈ 1/10 TB Part 2: Radiation Physics
99Mo-99mTc 87.6% 99mTc 99Mo 140 keV T½ = 6.02 h 12.4% ß- 442 keV 739 keV T½ = 2.75 d 99Tc ß- 292 keV T½ = 2*105 y 99Ru stable Part 2: Radiation Physics
Irene Curie (1897-1956)&Frederic Joliot (1900-1958) Part 2: Radiation Physics
Part 2. Radiation Physics IAEA Training Material on Radiation Protection in Nuclear Medicine Module 2.4. Interaction of Ionizing Radiation with Matter
Ionizing Radiation • Charged particles • alpha-particles • beta-particles • protons • Uncharged particles • photons (gamma- and X rays) • neutrons • Each single particle can cause ionization, • directly or indirectly Part 2: Radiation Physics
Charged Particles Interaction with Matter heavy light Macroscopic Microscopic Part 2: Radiation Physics
TransmissionCharged Particles Alpha particles Beta particles Part 2: Radiation Physics
Mean Range of b-particles Radionuclide Max energy Range (cm) in (keV) air water aluminium ------------------------------------------------------------------------------------- H-3 18.6 4.6 0.0005 0.00022 C-14 156 22.4 0.029 0.011 P-32 1700 610 0.79 0.29 Part 2: Radiation Physics
Bremsstrahlung Photon Electron Part 2: Radiation Physics
Bremsstrahlung Production • The higher the atomic number of the X-ray target, the higher the yield • The higher the incident electron energy, the higher the probability of X-ray production • At any electron energy, the probability of generating X-rays decreases with increasing X-ray energy Part 2: Radiation Physics
X-ray Production • High energy electrons hit a (metallic) target where part of their energy is converted into radiation electrons Low to medium energy (10-400keV) High > 1MeV energy target X-rays Part 2: Radiation Physics
X-Ray Tube for low and medium X-ray production Part 2: Radiation Physics
Megavoltage X-ray Linac electrons target X-rays Part 2: Radiation Physics
Issues with X-ray Production • Angular distribution: high energy X-rays are mainly forward directed, while low energy X-rays are primarily emitted perpendicular to the incident electron beam • Efficiency of production: In general, the higher the energy, the more efficient is X-ray production - this means that at low energies most of the energy of the electron (>98%) is converted into heat - target cooling is essential Part 2: Radiation Physics
The Resulting X-Ray Spectrum Characteristic X-rays Bremsstrahlung Spectrum after filtration Maximum electron energy Part 2: Radiation Physics
Photons Interaction with Matter absorption scattering transmission energy deposition Part 2: Radiation Physics
Photoelectric Effect photon electron characteristic radiation Part 2: Radiation Physics
Compton Process scattered photon photon electron Part 2: Radiation Physics
Pair Production positron photon electron Part 2: Radiation Physics
Annihilation (511 keV) (511 keV) + + e- + (1-3 mm) Radionuclide Part 2: Radiation Physics
Photon Interaction Atomic number (Z) Photon energy (MeV) Part 2: Radiation Physics
Transmission-Photons d: absorber thickness m: attenuation coefficient HVL: half value layer TVL: tenth value layer Part 2: Radiation Physics