Radiation Dosimetry (an Introduction To Radiation Physics) 4r4k3l

Radiation Dosimetry

(An Introduction to Radiation Physics)

Naslinda Noor Rizan, MSc Medical Physicist Gleneagles Penang [email protected]

Structure of the Atom (Bohr

Model) Orbiting electron e (- ve) Only allowed certain discrete energy. define shells or energy levels

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) K, L, M, N…..etc Max e- shell formula : 2n2

Nomenclature: A Z

X [email protected]

Physics of the Very Small (is Very Strange) Leptons

(build electrons)

1. e- & e- neutrino 2. muon & muon neutrino 3. tau & tau neutrino

Standard Model of Matter Particles (building blocks of atoms)

Quarks (build protons, neutron...etc)

1. Up & down 2. Charm & strange 3. Top & bottom Boson (carrier of the fundamental forces in the universe)

1. 2. 3. 4.

Gluon (strong force) Photon (electromagnetic force) Z & W (weak force) Graviton (cause gravity) [email protected]

Nuclide & Energy General 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 A

Potential energy increases (from inner to outer valence)

Isobar - nuclide with same A& Z but different energy state.

Binding energy increases (from outer to inner shell) Amount of energy needed to remove electron from it’s orbit [email protected]

Units & Formula

Atomic mass unit (amu) = 1/12 mass of C-12 = 1.67X10 -27 kg

Amu

e- = 0.000548 amu p+ =1.00727 amu neutron = 1.00866 amu

E = mc2 Where m = 1.66 x 10-27 kg. C = 3x108 m/sec , therefore

E = hv = hc λ

Mass per atom = Aw NA

E = 1.49 x 10-10 J (1Mev = 1.6 x 10-13 J) E = 931.25 Mev

Planck’s Constant, h = 6.626 070 040 x 10-34 J s Speed of light, C = 3x108 m/sec Avogadro’s No NA= 6.0221 ×1023 mol-1 (atoms ,molecules etc) Joules = kg.m2/s2 [email protected]

Units & Formula Kinetic energy

Energy at rest e- energy at rest = 0.511MeV

Total energy

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Ionizing Has enough energy to ionize matter directly or indirectly

Radi ation

Non-ionizing Cannot ionize matter, energy lower than ionizing potential

Directly ionising (charged particles - e-, p+, α particles, heavy ions Deposit energy directly in matter Indirectly ionising (neutral particles (photons, neutrons Deposit energy through 2 step process 1. Release charge particle 2. Charge particle deposit energy in matter Source Generated source Radioisotope X-ray tube

Linear Accelerator (LINAC) [email protected]

Characteristic x-ray : etransition between atomic shell

Bremstrahlung: enucleus Coulomb interaction

Production of Ionizing photon

Gamma ray: nuclear transition

Annihilation quanta: positron -e- annihilation

X ray tube output spectrum [email protected]

Manufactured source 1.X-ray tube 2.Linear Accelerator (basic principle: e- accelerating onto a metal target to produce photons)

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X-ray Tube

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Linear accelerator

(the very

basic) RF Source Waveguide Electron gun

Bending magnets Flattening filter

Target

Primary collimator collimators http://www.irsn.fr/fr/professionnels_sante/documentation/documents/syllabus_chapitre_5.pdf [email protected]

Rayleigh Scattering (low energy photon - why the sky is blue) Photoelectric Compton Scattering Pair Production Photon

Radiat ion intera ction e-

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 Characteristic x-rays positron annihilates at the end of its range Bremstrahlung giving two 511 keV photons (important in SPECT and PET imaging)

http://ozradonc.wikidot.com/photon-interactions

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Dominance of process

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.

http://ocw.mit.edu/courses/nuclear-engineering/22-01-introduction-to-ionizing-radiation-fall-2006/lecture-notes/energy_dep_photo.pdf [email protected]

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 termed as keV or MeV

(*e- output in LINAC is produce but removing target and flattening filter) [email protected]

Segre Chart

Radioactivity

Alpha Nucleus too big

Neutron rich

Beta Too many protons or too many neutrons

Stable

Decay mode Gamma 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

http://algebralab.com/practice/practice.aspx?file=Reading_TheBandOfStability.xml

Proton rich

α - Alpha Decay During 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.

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Too many neutrons or too many protons. One of the protons or neutrons is transformed into the other.

β-Beta Decay Beta+ (positron) a proton decays into a neutron, a positron, and a neutrino

Electron Capture decay by capturing one of

Betaa neutron decays into a proton, an electron, and an antineutrino

the electrons that surround the nucleus.

�Gamma

A nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation

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Formula s Half life, T

1/2

=

Decay constant, Activity,

ln2/ƛ

time taken for half or nuclei present to decay

ƛ = ln2/T1/2

A = A0e-ƛt

Units Bq (Baquerel) = 1 disintigration per second 1mCi = 37MBq 1MBq = 27µCi [email protected]