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Concepts of Radiation Chemistry. Concepts of Radiation Chemistry. Interaction of high energy and/or particles having high energies with matter. Sources: -charged particles ( p+, e-, , d , T , etc. ) - neutrons - photons. 4 .1 The Interactions of Particles with Matter.
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Concepts of Radiation Chemistry Interaction of high energy and/or particles having high energies with matter. Sources: -charged particles ( p+, e-, , d , T , etc. ) - neutrons - photons
4.1 The Interactions of Particles with Matter • Nuclear radiation (either particle or EM) can only be detected via their interactions with matter. • For example: If interactions is small, such as neutrino, then can’t be detected. • To understand how one detects or measures radiation, on Must understand the various interacting phenomena between radiation and matter. • Degraders used to reduce energetic beams of particles • Calculated absorbance in radiation therapy. • Interactions with radiation of all kinds are essentially the same, BUT initial steps of interaction are quite different (i.e. excitation & ionization of atoms & molecules) for - Charged particles • Neutrons • EM (photons • Due to lightness of electron, sometimes charged particle interactions are separated into positive versus negative species.
4.1 (a) Charged Particles • Really two types: electrons versus others. • When charged particle (i.e./ p+ , ) traverses through matter, it loses energy mainly by excitation & ionization of electrons in the medium. • Often, E(particle) >> IP of medium. • E-loss • by particle in each encounter is very small • Let: m mass of electron • M mass of particle • Due to conservation of momentum • Therefore, many, many encounters are required to stop the particle. • Since there are a “large” number of electrons in medium, one can treat problem as that of a continuous loss of energy. • And Because of large of number of encounters, get little fluctuations in the Eavg lost per collision.
4.1 (a) Charged Particles • (ii) Range • distance traveled by particle before it stops losing all Ek . • well defined for heavy ions. Note for electron: straggling more pronounced due to greater loss of energy and scattering by medium.
4.1 (a) Charged Particles • (iii) Stopping Power • Using a simple model of a heavy charged particle hitting an electron • Can show that momentum given to electron is equal to the impulse of Force times time. • After much mathematics … equation (4.9) [Ng-p.119]
4.1 (a) Charged Particles • (iv) Range Energy Relationship R decreases for heavier absorbers.
4.1 (a) Charged Particles • (v) Bremsstrahlung Effects • If particle has very high energy (i.e. high velocity), then energy loss can be due to either • Transfer via ionization to medium (-dE/dx), or • Bremsstrahlung Radiation (“Braking Radiation”), especially true for electrons.
4.1 (a) Charged Particles • (vi) Energy Depth Dose Effects • The fact that –dE/dx for heavy charged particles is very great at very low energies has important applications in Nuclear Radiation Therapy. Most of the energy is deposited near the end of the range. So a beam of heavy charged particles can be used to destroy cancer cells at a given depth in the body without destroying other healthy cells if the energy is carefully chosen so that most the E-loss occurs at proper depth.
4.1 (b) Neutrons • Since neutrons are uncharged, they do not interact with electrons in matter. • Neutrons lose energy via either nuclear scattering or capture process. • As neutrons go through medium, neutrons will be removed from the beam due to any scattering or absorption: Where: = total capture –cross section for all scattering and absorption rocesses ( cm2 ) n = number of target nuclei per cm3 in sample x = path length of sample So number of neutrons decrease exponentially with distance x; and there is no range.
4.1 (c) Photons ( , X-rays ) • Just like neutrons, photons do not lose E continuously through the medium. • Intensity decreases exponentially. • Three important processes that remove photons from a beam are: • (i) Photoelectric effect at low E’s. • (ii) Compton scattering at intermediate E’s • (iii) Pair production at high E’s. • Beam can be attenuated using known capture cross sections for both three processes.
4.2 Radiation Absorption Terminology (Biological) • Biological effects of radiation mainly due to ionization produced. • Three values used to measure these effects. • (i) Roentgen (R) • Amount of radiation that produces 0.33 nC in 1 cm3 of dry air (0.001293 g) at 00C and 1 atm (STP) • (ii) Rad (rad = radiation absorbed dose) • Amount of radiation that deposits 0.01 Joule of energy per kilogram of material • SI unit is Grey (Gy); 1 rad = 0.01 Gy • 1 R = 8.7x10-3 J/kg deposited; 1 R 1 rad • Note that one rad of alpha can do roughly as much biological damage as 10 rad of gamma. • The deposition of energy is usually referred to as Linear Energy Transfer (LET), which is energy deposited per unit length.
4.2 Radiation Absorption Terminology (Biological) • (iii) rem (roentgen equivalent in man) • Measured relative to 1 rad of beta or gamma radiation • 1 rem = 1 rad x RBE • Where: RBE(QF) = relative biological effectiveness factor (quality factor)
