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Radiation Dosimetry Radiation Dosimetry AGEN-698 Advances in Food Engineering Introduction Radiation dosimetry is the branch of science that attempts to quantitatively relate specific measurement in a radiation field to chemical and/or biological changes that the radiation would produce in a target Radiation When radiation interact with matter it produces: Excited and ionized atoms and molecules A large number of secondary electrons Exposure It is defined for gamma and X- rays in terms of the amount of ionization they produce in air Units: roentgen [R] – 1 R = 2.58x10-4 C/kg ∆Q is the sum of all charges of one sign produced in air when ∆Q all the electrons liberated by Exposure = photons in a mass ∆m of air ∆m are completely stopped in air Applies only to electromagnetic radiation and the mass and roentgen refer only to air Absorbed dose The energy absorbed per unit mass from any kind 1J 107 erg of ionizing radiation in 1Gy ≡ = = any kind of matter kg 103 g Traditional unit: rad = erg = 104 = 100rad 100 erg/g g New SI unit: gray (Gy) Absorbed dose is often referred to simply as the dose Dose equivalent The absorbed dose to achieve a given level of biological damage (e.g., 50% cell killing) is often different for different kinds of radiation H = QD Equivalent dose, H, is defined as the product of the absorbed dose D and the dimensionless quantity Q, which depends on LET Dependence of Q on LET LET [MeV/cm] Q 3.5 or less 1 3.5-7.0 1-2 7.0-23 2-5 23-53 5-10 53-175 10-20 γ-rays, X-rays, electrons, 1 positrons of any LET Measurement of Exposure Exposure can be measured operationally with ‘free-air’ or ‘standard’ ionization chamber Free-air ionization chamber X-rays emerge from the target T of an X-ray tube Enter the free-air Q L chamber thru a circular area A B aperture of are A defining D T E e1 a circular cone TBC of P e2 P' F G rays C Parallel plates Q and Q’ Q' area A' in the chamber collect the d ions produced in the d' volume of air between them and the center P’ Free-air ionization chamber The plates collect all of the ions between them Not only those originated Q L from X-ray interactions area A B in the volume DEFG D T E e1 P e2 P' Some ions produced by F G X-ray escape this C volume area A' d Q' Some ions produced outside this volume are d' also collected Free-air ionization chamber If the distance P to DG is large enough (10 cm for 300 keV X-rays) Q electronic equilibrium L area A B will be realized D T E e1 There will be almost P e2 P' F G exact compensation C between ionization lost area A' from the volume DEFG d Q' by electrons (e2 that d' escape and e1 that enter) Free-air ionization chamber The distance P to DG should not be so large As to attenuate the beam Q L significantly between P area A B D and P’ T E e1 P e2 P' F G So, when the charge q is C collected, the exposure P’ Q' area A' is: d q q E = = d' P' M ρA' L Free-air ionization chamber In practice, one prefers to know the exposure Ep at P rather than Ep’: Q L 2 area A B d' D E = E T E P P' e1 d P e2 P' F G 2 C d' A = A' Q' area A' d d 2 d' d' q q EP' = = d ρA' L ρAL Air-wall ionization chamber Free-air ionization chamber is not practical instrument for measuring routine exposure Chambers can be built with walls of a solid material having photon response properties similar to those of air Air-wall ionization chamber Built as a capacitor A central anode, insulated, is given an initial charge from a charge-reader plastic device (attached before wearing) When exposed to photons, the secondary electrons liberated in the walls and enclosed air tend to neutralize the charge on the anode and anode lower the potential difference between it and the wall The change in potential difference is directly proportional to the total ionization produced (so exposure) Air-wall ionization chamber After exposure to photons, measurement of the change plastic in potential difference from its initial value (fully charged chamber) can be used to find exposure anode Direct-reading pocket ion chamber are available Measurement of absorbed dose Bragg-Gray principle Means of relating wall ionization gas measurements in a incident photons gas to the absorbed scattered photons dose in some material from which a dosimeter can be fabricated Measurement of absorbed dose Photons lose energy in the gas by producing secondary electrons there The ration of the energy wall gas e1 deposited and the mass of e2 incident the gas is the absorbed dose photons scattered in gas photons This energy is proportional to the amount of ionization in the gas when equilibrium exists between the wall and the gas Measurement of absorbed dose The electron produced by a photon in gas, e1, enters the wall before losing all of its wall gas e1 energy e2 The electron produced in incident photons wall and stopped in the gas, scattered photons e2, compensates for e1. When wall and gas have the same atomic composition, the energy spectra of these electrons will be the same Measurement of absorbed dose For electronic equilibrium, the wall wall gas e1 thickness must be at e2 incident least as great as the photons maximum range of scattered photons secondary charged Gas in cavity enclosed by wall particles Bragg-Gary principle If a gas is enclosed by a wall of the same atomic composition and if the wall meets the thickness conditions, then the energy absorbed per unit mass in the gas is equal to the number of ion pairs produced times the W value divided by the mass m of the gas So, the absorbed dose in the gas is equal to the absorbed dose in the wall Bragg-Gary principle N W D = D = g w g m Where Ng = number of ions in gas W = average energy needed to produce an ion pair Bragg-Gary principle For wall and gas are of different atomic composition The cavity size and gas pressure must be small (so the secondary particles lose on a small fraction of their energy in gas) Dg Sw N gWSw Dw = = Sg mSg Sw/Sg = ratio of the mass stopping powers of the wall and gas Measurement of X and Gamma- ray dose This chamber satisfies the Bragg- C CO2 Gray conditions CO2 The dose Dc in the carbon wall can be obtained from the insulator measured ionization in the CO2 Graphite-walled CO2 chamber Dose measurement for charged- particle beams Dose or dose rate is determined by measuring the current from a thin- recorder walled ionization chamber placed at ion different depths in a chamber beam water target exposed to water the beam Dose rate is proportional to the current Dose calculations Charged-particle beams Point source of gamma rays Charged-particle Beams Uniform, parallel beam of charged particles normally incident on thick tissue slab . Consider a thin, disc- φ shaped volume element ∆x x with thickness ∆x in the x-direction and area A normal to the beam Fluence rate [cm-2 s-1] Charged-particle Beams Rate of energy deposition ϕ&A(−dE / dx)∆x dE D& = = ϕ&− ρA∆x dx Fluence rate [cm-2 s-1] Point-Source of γ-rays The dose rate in air from a point γ-ray source of strength C curies that emits an average total photon energy of E[MeV] per disintegration can be calculated The rate of energy released in the form of photons escaping from the source is: CE[CiMeV ]× 3.7 ×1010 sec −1 Ci −1 ×1.6 ×10 −6 erg / MeV = 5.92 ×104 CE[erg / sec] Point-Source of γ-rays Neglecting attenuation in air, we can write the rate of energy flow per unit area (=intensity) through the surface of a sphere of radius r (cm) surrounding the source: 4.71×103CE erg I = r 2 cm 2 sec Point-Source of γ-rays 1 cm2 The rate of energy absorption in a volume element of air, having a unit area and thickness dr at r is given by IµAdr where µA is the average energy -1 I absorption coefficient in cm for photons dr If the density of air is ρ [g/cm3], the r mass of air in the volume element in grams is ρdr: source 3 Iµ Adr 4.71 ×10 CEµ A erg D& = = 2 ρdr r ρ g sec KERMA Kinetic energy released per unit mass It includes energy that may subsequently appears as bremsstralung and Auger-electron energies The absorbed dose builds up behind a surface irradiated by a beam of particles to a depth comparable with the range of secondary charged particles generated The kerma, on the other hand, decreases steadily because of the attenuation of the primary radiation with increasing depth.
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