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Radiation Dosimetry Introduction
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Radiation Dosimetry
(An Introduction to Radiation Physics)
Naslinda Noor Rizan, MScMedical Physicist
Gleneagles [email protected]
Structure of the Atom (Bohr Model)
Nucleus contains protons (+ve) and neutrons(no charge)
Nucleus contains protons (+ve) and neutrons (no charge)
P +N = A (mass no)
No of protons =Z (atomic no)
Orbiting electron e (- ve)
Nomenclature: A Z X
Only allowed certain discrete energy.
define shells or energy levels
K, L, M, N…..etc Max e- shell formula : 2n2
Physics of the Very Small (is
Very Strange) Standard Model of Matter Particles
(building blocks of atoms)
Leptons(build electrons) Quarks
(build protons, neutron...etc)
Boson(carrier of the fundamental forces in the universe)
1. e- & e- neutrino2. muon & muon neutrino3. tau & tau neutrino
1. Up & down2. Charm & strange3. Top & bottom
1. Gluon (strong force)2. Photon (electromagnetic force)3. Z & W (weak force)4. Graviton (cause gravity)
Nuclide & EnergyGeneral term for atoms with specific number protons and neutrons. (atom = nuclide + e-)
Isotope = nuclide with same Z (e.g 1H1, 1H2, 1H3 )Isotone = nuclide with same neutrons (N)Isomer - nuclide with same AIsobar - nuclide with same A& Z but different energy state.
Potential energy increases (from inner to outer valence)
Binding energy increases (from outer to inner shell)Amount of energy needed to remove electron from it’s orbit
Units & FormulaAtomic mass unit (amu) = 1/12 mass of C-12 = 1.67X10-27 kg
Amu E = mc2
Where m = 1.66 x 10-27 kg.
C = 3x108 m/sec , therefore
E = 1.49 x 10-10 J (1Mev = 1.6 x 10-13 J)
E = 931.25 Mev
e- = 0.000548 amup+ =1.00727 amuneutron = 1.00866 amu
E = hv = hc λ
Planck’s Constant, h = 6.626 070 040 x 10-34 J sSpeed of light, C = 3x108 m/secAvogadro’s No NA= 6.0221 ×1023 mol-1 (atoms ,molecules etc)Joules = kg.m2/s2
Mass per atom = Aw
NA
Units & FormulaKinetic energy
Total energy
Energy at rest
e- energy at rest = 0.511MeV
Radiation
IonizingHas enough energy to ionize matter directly or indirectly
Non-ionizingCannot ionize matter, energy lower than ionizing potential
Directly ionising (charged particles - e-, p+, α particles, heavy ionsDeposit energy directly in matter
Indirectly ionising (neutral particles (photons, neutronsDeposit energy through 2 step process
1. Release charge particle2. Charge particle deposit energy in matter
Source
RadioisotopeGenerated source
X-ray tube Linear Accelerator (LINAC)
Production of Ionizing photon
Characteristic x-ray : e- transition between atomic shell
Bremstrahlung: e- nucleus Coulomb interaction
Gamma ray: nuclear transition
Annihilation quanta: positron -e- annihilation
X ray tube output spectrum
Manufactured source1.X-ray tube
2.Linear Accelerator
(basic principle: e- accelerating onto a metal target to produce photons)
Linear accelerator (the very basic)
Electron gun
WaveguideBending magnets
Target
Primary collimator
collimators
Flattening filter
RF Source
http://www.irsn.fr/fr/professionnels_sante/documentation/documents/[email protected]
Radiation interaction
Photon
e-
Rayleigh Scattering (low energy photon - why the sky is blue)PhotoelectricCompton ScatteringPair Production
Characteristic x-raysBremstrahlung
http://ozradonc.wikidot.com/photon-interactions
conversion of photon into a moving electron (e- then goes on to ionize the medium) scattering of a photon by a
(free) electron that leads to a moving electron and a lower energy photon (most important in radiotherapy) conversion of a photon into a
matter/antimatter pair of electrons.The two moving electrons share the remainder of the initial photon energy. Eventually the positron annihilates at the end of its range giving two 511 keV photons (important in SPECT and PET imaging)
Dominance of process
http://ocw.mit.edu/courses/nuclear-engineering/22-01-introduction-to-ionizing-radiation-fall-2006/lecture-notes/energy_dep_photo.pdf
photon energy and atomic number, Z of the absorbing material
For both the photoelectric effect and pair production the photon is totally absorbed while for Compton scattering a photon of degraded energy remains.
Mv or MeV??To produce a 6MV photon in linac you need a 6MeV electron but the output would be in a spectrum with max energy of 6MeV (photon).
Therefore for convenience we refer to photon from x-ray tube using its electric potential (MV or kV)
Electron output and radioisotope has mono-energetic energy and is correctly termedas keV or MeV
(*e- output in LINAC is produce but removing target and flattening filter)[email protected]
Radioactivity
Unstable parent unstable atomic nuclei – don’t have enough binding energy to hold the nucleus together due to an excess of either protons or neutron
Alpha
Beta
Gamma
Segre Chart
Decay mode
http://algebralab.com/practice/practice.aspx?file=Reading_TheBandOfStability.xml
Neutron rich
Proton rich
Stable
Nucleus too big
Too many protons or too many neutrons
α - Alpha DecayDuring alpha decay, an atom's nucleus sheds two protons and two neutrons in a packet that scientists call an alpha particle.
alpha particle is a fast moving helium nucleus. Alpha particles carry a charge of +2 and strongly interact with matter. They travel only a few inches through air and can easily be stopped with a sheet of paper.
β-Beta DecayToo many neutrons or too many protons. One of the protons or neutrons is transformed into the other.
Beta+ (positron)Electron Capture a neutron decays into a
proton, an electron, and an antineutrinoa proton decays into a
neutron, a positron, and a neutrino
decay by capturing one of the electrons that surround the nucleus.
Beta-
𝛾- Gamma Decay
A nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation
Formulas Half life, T1/2 = ln2/ƛ
Decay constant, = ln2/Tƛ 1/2
Activity, A = A0e- tƛ
Units Bq (Baquerel) = 1 disintigration per second1mCi = 37MBq1MBq = 27µCi
time taken for half or nuclei present to decay
