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11. Dosimetry Fundamentals

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11. Dosimetry Fundamentals Outline • Introduction Dosimetry Fundamentals • Dosimeter model • Interpretation of dosimeter measurements Chapter 11 – Photons and neutrons – Charged particles • General characteristics of dosimeters F.A. Attix, Introduction to Radiological Physics and Radiation Dosimetry • Summary Introduction Dosimeter • Radiation dosimetry deals with the determination • A dosimeter can be generally defined as (i.e., by measurement or calculation) of the any device that is capable of providing a absorbed dose or dose rate resulting from the interaction of ionizing radiation with matter reading R that is a measure of the absorbed • Other radiologically relevant quantities are dose Dg deposited in its sensitive volume V exposure, kerma, fluence, dose equivalent, energy by ionizing radiation imparted, etc. can be determined • If the dose is not homogeneous • Measuring one quantity (usually the absorbed dose) another one can be derived through throughout the sensitive calculations based on the defined relationships volume, then R is a measure of mean value Dg Dosimeter Simple dosimeter model • Ordinarily one is not interested in measuring • A dosimeter can generally be considered as the absorbed dose in a dosimeter’s sensitive consisting of a sensitive volume V filled with a volume itself, but rather as a means of medium g, surrounded by a wall (or envelope, determining the dose (or a related quantity) for container, etc.) of another medium w having a another medium in which direct measurements thickness t 0 are not feasible • A simple dosimeter can be treated in terms of cavity theory, the sensitive • Interpretation of a dosimeter reading is the volume being identified as the central problem in dosimetry, usually “cavity”, which may contain a exceeding in difficulty the actual measurement gaseous, liquid, or solid medium g 1 Interpretation of dosimeter Simple dosimeter model measurements: photons and neutrons • The dosimeter wall can serve a number of functions simultaneously: • Under CPE condition – being a source of secondary charged particles that contribute CPE DK Y/ to the dose in V, and provide CPE or TCPE in some cases c en – shielding V from charged particles that originate outside the • Consider a dosimeter with a wall of medium w, thick enough wall to exclude all charged particles generated elsewhere, and at – protecting V from “hostile” influences such as mechanical least as thick as the maximum range of secondary charged damage, dirt, humidity, light, electrostatic or RF fields, etc., particles generated in it by the photon or neutron field that may alter the reading • If the wall is uniformly irradiated, CPE exists in the wall – serving as a container for g that is a gas, liquid, or powder near the cavity, therefore knowing D can calculate energy – containing radiation filters to modify the energy dependency w fluence Y of the primary field of the dosimeter Interpretation of dosimeter Interpretation of dosimeter measurements: photons and neutrons measurements: photons and neutrons • The dosimeter reading provides us with a measure • For neutrons the CPE condition results in of the dose Dg in the dosimeter’s sensitive volume CPE DKF F n • If the latter is small enough to satisfy the B-G conditions, we can find Dw from Dg Fn is kerma-factor, F is neutron fluence • The dose Dx in any other medium x replacing the • Therefore the dose D in the medium of interest x dosimeter and given an identical irradiation under x CPE conditions can be obtained from CPE F n x CPE DDxw for neutrons en / x Fn Dx Dw for photons w en / w Interpretation of dosimeter Interpretation of dosimeter measurements: photons and neutrons measurements: photons and neutrons • For higher-energy radiation (h 1 MeV or Tn • The exposure X (C/kg) for photons can in turn be 10 MeV), CPE fails but TCPE takes its place in derived from the absorbed dose Dair (for x = air) dosimeters with walls of sufficient thickness through this relation: • For photons TCPE condition provides relationship CPE e Dair TCPE XDair D Kcc1/ x K Y en W air 33.97 • For neutrons TCPE • This relationship can be extended for higher D K1 x K F F n energies where TCPE exists, dividing Dair by • Relating Dw to Y or F then requires evaluation of = D/Kc for each case 2 Interpretation of dosimeter measurements: photons and neutrons Media matching • The value of is generally not much greater than • If media of interest (x), wall (w), and gas (g) are the unity for radiation energies up to a few tens of MeV, same, the measurement directly provides the dose of and it is not strongly dependent on atomic number interest. If these media are matched: – Influence of cavity theory is minimized • Thus for media w and x not differing very greatly in – No need to know the radiation energy spectrum Z, the equations for D are still approximately valid x • Matching parameters: • If the dosimeter has too large a sensitive volume for – Atomic compositions the application of B-G theory, Burlin theory can be used to calculate the equilibrium dose D from – The density state (gaseous vs. condensed), which w influences the collision-stopping-power ratios for electrons – requires sensitive-volume size d parameter by the polarization effect Media matching in photon Media matching dosimeters (w=g) • w=g: if the wall and sensitive-volume media of the • The Burlin cavity relation dosimeter are identical with respect to composition g Dg g en d mw S (1 d ) and density, then Dw = Dg for all homogeneous Dw irradiations w • If w and g are matched, the doses Dw= Dg and • w=g=x: the dosimeter would be truly representative g g en of that medium with respect to radiation interactions, mwS 1 and Dx = Dw = Dg w • Unfortunately, dosimeters are only available in a • This relationship is hard to satisfy, especially for w finite variety, therefore have to rely on cavity theory and g of different atomic compositions D g Media matching in photon Matching the dosimeter to the dosimeters (w=g) medium of interest when w≠g • A more flexible and practical relationship • If the wall medium wg, matching to medium x g g en depends on the dosimeter volume size mwSn w • If the sensitive volume is small (d = 1 in Burlin’s cavity • The Burling relation then becomes independent of d theory), then the wall should be matched to medium x, to minimize the need for spectral information since Dg / Dw n • It is relevant, for example, if photons interact only • Dose in x can be obtained from by the Compton effect in g and w: en/ ~the mass w collision scattering power of the secondary electrons DDD ggw S g en ~ to the number of electrons per gram, NAZ/A; mw DDDx w x x n (Z/A) /(Z/A) g w B-G 3 Matching the dosimeter to the Correcting for attenuation of medium of interest when w≠g radiation • The problem arises from the difference in attenuation by • If the sensitive volume is large (d = 0 in Burlin’s media x (for example, water), w (thickness t) and g (radius r) cavity theory), the wall influence on the dose in the • The photon energy fluence reaching the center of a dosimeter medium g is entirely lost Y Y 1 en t en r dos 0 w g • Medium x should be matched to g and dose in x can be w g obtained from • On the other hand energy fluence reaching the water sphere g w g replacing the dosimeter DD ggDw en en en en Ywat Y0 1 wat (t r) wat DDDx w x w x x • The dosimeter reading should be multiplied by Ywat/Ydos to • For a general case of intermediate size cavity (0<d<1) correct for the difference of attenuation in determining the full Burlin equation can be used to obtain Dw from Dg dose to water at the dosimeter midpoint Importance of dosimeter wall Interpretation of dosimeter thickness measurements: charged particles • Dosimeter wall thickness requirement depends on the goal • The absorbed dose at a point in a medium x is the of a measurement: to obtain a quantity that depends on product of the charged-particle fluence (not the a) the characteristics of the local photon or neutron field, energy fluence) and the mass collision stopping the dosimeter should have a wall at least as thick as the power, assuming that CPE exists for -rays maximum range of the charged particles present, to provide CPE or TCPE Tmax dT Dx FT dT b) the characteristics of the local secondary charged-particle 0 dx field then the dosimeter wall and sensitive volume should c,x both be thin enough not to interfere with the passage of • The integration is over all energies in the primary incident charged particles (i.e., non--ray) charged-particle spectrum F(T) Interpretation of dosimeter Interpretation of dosimeter measurements: charged particles measurements: charged particles • Charged particle dosimeter needs a sensitive volume Approximate CSDA ranges of electrons and protons in water small (or thin) enough to satisfy the B-G conditions (nonperturbation of the charged-particle field), and all dose to be deposited only by crossers • The dosimeter wall should be thick enough to serve any essential functions (e.g., containment) • Thin flat pillbox- or coin-shaped dosimeters, • As a rule of thumb: neither the wall thickness nor that of the sensitive volume should exceed ~1% of oriented perpendicularly to the particle-beam the range of the incident charged particles direction, are used to satisfy these requirements • Ranges in other low-Z media are comparable 4 Interpretation of dosimeter Interpretation of dosimeter measurements: charged particles measurements: charged particles • One of the functions
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