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10. Cavity Theory

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10. Cavity Theory Outline • Problem statement Cavity Theory • Bragg-Gray cavity theory • Spencer cavity theory Chapter 10 • Burlin cavity theory • Dose near interfaces between F.A. Attix, Introduction to Radiological dissimilar media Physics and Radiation Dosimetry • Summary Cavity Theory: Problem Statement Bragg-Gray Theory • Homogeneous medium, Q wall (w) • Probe - cavity - thin layer of gas (g) • Charged particles crossing + - w-g interface gas • Objective: find a relation Dg between the dose in a probe wall to that in the medium (A) A fluence of charged particles crossing an interface between media w and g Dw (B) A fluence of charged particles passing through a thin layer of medium g • Basis for dosimetry sandwiched between regions contain medium w Fluence is assumed to be continuous across all interfaces; it is related to the dose Bragg-Gray Theory Bragg-Gray Theory • Charged particles of fluence • Two conditions: , kinetic energy T – Thickness of g layer is much smaller than the range of charged particles (medium g is close to w in atomic • Dg, Dw - absorbed doses on number) each side of the boundary – The absorbed dose in the cavity is deposited entirely by dT the charged particles crossing it • dx - mass collision stopping c • Additional assumptions: power, evaluated at energy T – Existence of CPE • Assuming continuous – Absence of bremsstrahlung generation across the interface – No backscattering 1 Bragg-Gray Theory Bragg-Gray Theory For differential energy distribution T, average mass For charge Q (of either sign) produced in gas of mass m by collision stopping power radiation, the dose in gas Tmax dT Q W Tmax dT ΦT dT Dg ΦT dT 0 dx 0 dx c,g m e g c,w S S m g Tmax m w Tmax Then the B-G relation expressed in terms of cavity ionization: T dT T dT 0 0 T T 1 max dT Dg Q W w 1 max dT Dw T dT Dw m Sg T dT 0 0 dx m e dxc,w c,g g This equation allows calculating the absorbed dose in the medium Dw m Sw w immediately surrounding a B-G cavity, based on the charge m Sg produced in the cavity gas, provided that the parameters are known Dg m Sg Bragg-Gray Theory Bragg-Gray Theory w • First Bragg-Grey Corollary: two different • As long as m S g is evaluated for the charged-particle gases filling the cavity spectrum T that crosses the cavity, the B-G relation requires neither CPE nor a homogeneous field of radiation Q V W / e g 2 2 1 2 • The charged-particle fluence must be the same in the m Sg T Q V W / e 1 cavity and in the medium w 1 1 2 • If CPE does exist in the neighborhood of a point of interest • Second Bragg-Grey Corollary: two chambers in the medium w, then the insertion of a B-G cavity at the (walls) of different volume and material point may be assumed not to perturb the “equilibrium w1 Q V en / S spectrum” of charged particles existing there 2 2 w2 m g Q V / S w2 1 1 en w1 m g Spencer derivation of B-G Spencer derivation of B-G • The absorbed dose at any point in the undisturbed • Consider a small cavity filled with medium g, medium w where CPE exists: surrounded by a homogeneous medium w that CPE contains a homogeneous source emitting N Dw Kw NT0 (MeV/g) identical charged particles per gram, each with • For an equilibrium charged-particle fluence spectrum e kinetic energy T0 (MeV) T the absorbed dose T • The cavity is assumed to be far enough from the 0 e dT Dw T dT outer limits of w that CPE exists 0 dxw • Both B-G conditions are assumed to be satisfied • Then the spectrum by the cavity, and bremsstrahlung generation is e N assumed to be absent T dT dxw 2 Spencer derivation of B-G Spencer derivation of B-G Start with mono- • Assuming the same spectrum on both sides, ratio energetic source of the dose in the cavity to that of the medium w of T0=2MeV T0 Dg 1 dT dxg g dT S D T dT dx m w w 0 0 w • Can be generalized to accommodate bremsstrahlung generation by electrons T0 e Dg 1 dT dxc,g • Example of an equilibrium fluence spectrum, T = N/(dT/dx), dT S g of primary electrons under CPE conditions in water, assuming D T 1Y T dT dx m w the continuous-slowing-down approximation w 0 w 0 0 c,w B-G Cavity Theory vs. Experiment Averaging of Stopping Powers • Extend a single starting energy T0 to a distribution T (spectrum): stopping power has to be integrated over the distribution T Averaging over T T max 0 each T0, and over dT dxc,g N dT T for each T0 T0 Dg 0 0 dT dxw g m Sw D Tmax w N T 1Y Y dT T0 0 w 0 0 0 • Ratio of collision stopping powers is a slowly varying Comparison of measured ionization densities (solid curves) in flat air-filled ion function, therefore average energy T may be used chambers having various wall materials and adjustable gap widths, with Bragg-Gray theory (tick marks at left) and Spencer theory (dashed curves), for 198Au rays B-G Cavity Theory vs. Experiment Spencer Cavity Theory • Experiments had shown deviations from B-G theory • Goals: to account for d-rays and cavity size effect • Dependence on cavity size and Z • Starts with two B-G conditions (narrow g region and • For walls of high atomic number d-ray dose produced by crossing particles) and two production becomes an issue additional assumptions (existence of CPE and absence of bremsstrahlung generation) – d-rays cross the cavity with the rest of electrons • Introduces mean energy D, needed to cross the cavity – they change spectrum, enhancing low energy part • Based on their energy T, electrons in a spectrum are • Spencer theory divided into „fast‟ ( T D ) and „slow‟ ( T D ) groups 3 Spencer Cavity Theory Spencer Cavity Theory e,d e Approximate Values of R(T0,T) = T/ T, the Ratio of the • For monoenergetic electron beam with T0 emitting N particles Differential Electron Fluences with and without d-rays per gram through a homogeneous medium w the absorbed dose is expressed in terms of restricted stopping power CPE T D NT 0 e,d S T, D dT w0 D T m w • The equilibrium spectrum including d-rays NR(,) T T e,d 0 T dT/ dx w • R(T,T0) – ratio of differential electron fluence, including d-rays to that of primary electrons alone Spectrum enhanced many-fold at low electron energies Spencer Cavity Theory Spencer Cavity Theory • Taking into account adjustment for electron spectrum, dose to the wall CPE T0 RTT(,) D NT N0 S T, D dT w0 D dT/ dx m w w • D regulates the cavity size; for D=0 • Dose to the cavity Spencer Cavity Theory Spencer Cavity Theory cavity size • Ratio of doses in cavity and wall T 0 RTT(,)0 DmgS(,) T dT D dT/ dx Dg w T0 RTT(,) Dw 0 DmwS(,) T dT D dT/ dx w • Works well for small cavities (electron range is much larger than the cavity size) • If actual spectrum of crossing charged particles is Values of Dg/Dw Calculated for Air Cavities by Spencer from Spencer Cavity known, can replace the ratio term Theory, vs. Bragg-Gray Theory Better agreement between Spencer and B-G for large cavity sizes 4 Spencer Cavity Theory Burlin Cavity Theory • The Spencer cavity theory gives somewhat better • -ray cavity theory, for intermediate cavity size agreement with experimental observations for small cavities than does simple B-G theory, by taking account of d-ray production and relating the dose integral to the cavity size • However, it still relies on the B-G conditions, and therefore fails to the extent that they are violated • In particular, in the case of cavities that are large small intermediate large (i.e., comparable to the range of the secondary charged particles generated by indirectly ionizing radiation), neither B-G condition is satisfied e1 – crossers e3 – stoppers e2 – starters e4 – insiders Burlin Cavity Theory Burlin Cavity Theory To arrive at a usefully simple theory Burlin made the following assumptions, either explicitly or implicitly: electrons generated in the cavity 1. The media w and g are homogeneous. - 2. A homogeneous -ray field exists everywhere throughout w and g. (This e means that no -ray attenuation correction) 3. Charged-particle equilibrium exists at all points that are farther than the max electron range from the cavity boundary 4. The equilibrium spectra of secondary electrons generated in w and g are electrons generated the same in the wall 5. The fluence of electrons entering from the wall is attenuated exponentially as it passes through the medium g, without changing its spectral distribution. 6. The fluence of electrons that originate in the cavity builds up to its equilibrium value exponentially as a function of distance into the cavity, according to the same attenuation coefficient that applies to the incoming electrons Burlin Cavity Theory • Cavity relation accounts for 2 sources of electrons Burlin Cavity Theory depositing dose g • Parameter d~1 for small, d~0 for large Dg g en d mw S (1 d ) cavities Dw w • The corresponding relation for 1 – d, • Parameter is related to the cavity size, expressed as representing the average value of /e L g g el e dl L throughout the cavity: w 0 w 1 e d e L e L L w dl el 0 w 1 e dl L g 0 g Le1 • l is the distance (cm) of any point in the cavity from the 1d e L e L wall, along a mean chord of length L g dl 0 g 5 Burlin Cavity Theory Burlin Cavity Theory e e • For the nonhomogeneous case where g w, ( w g) • Moreover, if the -value of the cavity medium for the wall • Theory works well for wide range of cavity sizes electrons is not the same as for the cavity-generated and materials electrons, due to a difference in spectral distributions, then • Parameter estimated for air-filled cavity in general g 16 dd (1 ) e 1.4 g Tmax 0.036 and hence etmax 0.01 dd1 • The
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