Interactions of Charged Particles with Matter Nick Harding Clinical Scientist Radiotherapy Department Castle Hill Hospital Hull & East Yorkshire Hospitals NHS Trust email: nicholas.harding@hey.nhs.uk

TYPES OF RADIATION α 4 +2 β 1/1800 ±1 Radiation Mass Electric Charge Speed 15,000 km/s α 4 +2 270,000 km/s β 1/1800 ±1 300,000 km/s γ / X α – radiation: Helium nuclei β – radiation: Electrons and positrons γ/X – radiation: Electromagnetic radiation

CHARGED PARTICLES Charged particles (e.g. α2+, β±, p+) interact Electromagnetically (i.e. via Coulombic forces) Elastic and inelastic collisions involve collisions with e- and nuclei in absorbing material interactions with e- most common Radiative collisions – Bremsstrahlung charged particles accelerated by electric field of nucleus

INDIVIDUAL INTERACTIONS Charged particles suffer many interactions along their path energy loss considered a continuous process At each interaction charged particles are deflected/scattered Charged particles may pass near a nucleus suffers large deflection most pronounced for light particles

PATH VERSUS RANGE Particle path length Particle range the actual distance the particle travels is dependent on the mass of the particle Particle range the actual depth of penetration of the particle in matter is dependent on the mass and kinetic energy of the particle the traversing material (i.e. atomic number Z)

PATH LENGTH (α) a-particles Path = Range large mass particles dense linear ionisation track Path = Range

PATH LENGTH (β) β-particles Path > Range small mass particles multiple scattering events follow tortuous path Path > Range

α RANGE (in water) Range of α-particles of energy of 4 MeV dependent on traversing material Material Range (mm) Air 25 Tissue 0.014

α RANGE (in water) Range of α-particles of energy of 8 MeV dependent on traversing material Material Range (mm) Air 70 Tissue 0.042

β- RANGE Electron range is dependent on kinetic energy of electrons traversing material (e.g. water) Electron Energy (keV) Range (mm) 20 0.01 40 0.03 100 0.14 400 1.30

β+ RANGE F-18 positron energy Emax = 0.6 MeV dependent on material Maximum Range (mm) Lead 0.05 Glass 0.9 Water 2.4 Air 2000!

SPECIFIC IONISATION Specific ionisation number of primary/secondary ion pairs produced per mm expressed in ion pairs (IP/mm) increased with the electrical charge of the particle decreased with incident velocity of the particle

SPECIFIC IONISATION

RADIOTHERAPY

LINEAR ENERGY TRANSFER The linear energy transfer (LET) is defined as: the amount of energy deposited per unit length (eV/mm) The LET of a particular type of radiation determines the biologic consequence of radiation exposure high LET radiations (α-particles, protons, electrons) low LET radiations (γ-rays and X-rays)

INTERACTION MECHANISMS Electromagnetic interactions extend over some distance not necessary for particle to make direct collision can transfer energy simply passing close by atomic internal energy quantised only certain energy values can be transferred may or may not excite and/or ionise atoms

INTERACTION MECHANISMS (1) Elastic collisions with orbital electrons with atomic nucleus Inelastic collisions

ELASTIC COLLISIONS No energy transfer Low-angle diffusion Coulomb interaction with electron cloud High-angle diffusion Coulomb interaction with atomic nucleus Atom is not ionised/excited

INELASTIC COLLISIONS Energy transfer Incident electron loses energy Ionisation of the atom Excitation of the atom

INELASTIC COLLISIONS Ionisation Excitation ejection of a bound electron characteristic radiation or Auger electrons Excitation bound electron “jumps” to higher energy state

AURORA

AURORA

INTERACTION MECHANISMS (2) Radiative collisions with atomic nucleus Involves the emission of radiation when a charged particle is accelerated

RADIATIVE COLLISIONS Inelastic collision Charged particle is deflected with electric field of nucleus Charged particle is deflected by the positive charge of nucleus with a loss of kinetic energy Bremsstrahlung (braking radiation) X-rays Energy of X-ray is equal to the energy lost by the electron

RADIATIVE COLLISIONS

X-RAY TUBE

PRODUCTION OF X-RAYS

X-RAY SPECTRUM

Radionuclide Therapy P-32 Sm-153 Ra-223 I-131 Polycythaemia rubra vera Thrombocythaemia Sm-153 Bone metastases Ra-223 I-131 Thyrotoxicosis Thyroid cancer

Xofigo® THERAPY Used in medical therapy Ra-223 dichloride (Xofigo®) Used to treat prostate cancer once it has spreaded to the bones Radium acts like calcium

Radionuclide Therapy P-32 Sm-153 Ra-223 I-131 Polycythaemia rubra vera Thrombocythaemia Sm-153 Bone metastases Ra-223 I-131 Thyrotoxicosis Thyroid cancer

How does it work?

How does it work? Some of the 131I is accumulated in the thyroid gland The remainder is excreted in urine faeces perspiration Saliva 131I is based on the radiation-induced cell damage caused by the high-energy radiation emitted } possible risk of contamination

How does it work? Irradiated thyroid cells lose the ability to multiply themselves Total mass of the gland is steadily reduced The thyroid gland is totally ablated (i.e. 131I ablation) Patient has no thyroid after the therapy The patient is then prescribed with T3 or T4

POSITRON ANNIHILATION

PET ISOTOPES positron annihilation positron emission (~0.6-1.7 MeV) photon (511 keV) neutrino positron emission (~0.6-1.7 MeV) up to “a few mm”

Properties of PET Isotopes Wide range of half-lives, generally shorter than conventional NM Mainly cyclotron produced but some generators

Variation of Resolution with Positron Energy Wide range of positron energies higher energy → worse spatial resolution

Physical Limits on Resolution in PET

PET WHOLE BODY SCANS 2004 2005

INTERACTION MECHANISMS (3) Cherenkov radiation a particle moves faster than the speed of light in a material Shock wave of EM radiation

INTERACTION MECHANISMS Inelastic interactions produce: heating visible light fluorescence Bremsstrahlung characteristic x-ray radiation secondary electrons Auger electron production

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