6720 Lecture 4_measurement

# 6720 Lecture 4_measurement - Lecture IV 1 2 3 4 5 6...

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1 PHYS 6720 - Lecture IV 1 Lecture IV Radiation Measurements 1. Polychromatic x-ray radiation 2. Revisit of old definitions 3. Quality of X-ray 4. Ion chamber detectors 5. Other single detectors 6. Detector arrays PHYS 6720 - Lecture IV 2 IV-1 Polychromatic x-ray radiation Spectral distribution For a typical x-ray tube: d Φ /dE PHYS 6720 - Lecture IV 3 IV-1 Polychromatic x-ray radiation Spectral distribution function Φ (E) Let Φ (E) = number of photons with energy between 0 and E per unit of area, then d Φ /dE = number of photons with energy at E within dE per unit of energy and per unit of area E d Φ /dE Φ (E) E E max PHYS 6720 - Lecture IV 4 IV-1 Polychromatic x-ray radiation Fluence distribution function Ψ (E) If d Ψ /dE = radiation energy within dE of E per unit of energy and per unit of area = Ed Φ /dE then Ψ (E) = radiation energy between 0 and E per unit of area = and Ψ total = total radiation energy per unit of area = fluence = 0 E d Ed E dE Φ max 0 E d E dE Φ PHYS 6720 - Lecture IV 5 IV-1 Polychromatic x-ray radiation Discretization In practical cases, distribution functions are given by a table of discrete values versus E i with i=1, …, n The integration should be then replaced by Æ 0 E d E dE Φ 1 () n ii i EE = ΔΦ PHYS 6720 - Lecture IV 6 Absorbed dose D For a monochromatic x-ray beam with photon energy E Δ E ab (E)= μ a Δ xNE Æ D = For a polychromatic x-ray beam with energy spectrum of d Φ /dE D= = or max 0 E ab ab E d E md E μ ρ Δ Φ = Δ IV-2 Revisit of old definitions N Δ x Δ m= ρ A Δ x max 0 E ab d dE dE Ψ ab dD d dE dE Ψ = ab ab E E m Δ Δ

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2 PHYS 6720 - Lecture IV 7 IV-2 Revisit of old definitions Absorbed dose D air and exposure X in air • Almost all ionized particles in air due to incident radiation are singly charged and paired • The number of ion pairs is proportional to the total energy of radiation beam • The average absorbed energy per ion pair is given by W=33.85(eV) (see ICRU Report 31, 1979) • For an exposure of X units of (C/kg), the absorbed dose in dry air is given by D air (Gy)=33.85X PHYS 6720 - Lecture IV 8 IV-2 Revisit of old definitions Absorbed dose D air and exposure X in air Example: absorbed dose in air at an exposure of 1(C/kg). Given: X=1(C/kg) Find: D air =? D air =(W # of electrons)/mass of air =(W X)/e =33.85(eV) × 1.6 × 10 -19 (J/eV) × 1(C/kg)/1.6 × 10 -19 (C) =33.85(Gy) For an exposure of X units of (R), the absorbed dose in air is given by D air =33.85 × 100(rad/Gy)X(R) × 2.58 × 10 -4 (C/kg/R) D air (rad)=0.873X D air (Gy)=0.00873X PHYS 6720 - Lecture IV 9 IV-2 Revisit of old definitions Exposure X in air Energy dependence Total exposure () 1 1 0.873 0.873 air ab air dD dX E d E dE dE dE μ ρ Φ == max 0 1 0.873 E ab total air d X Ed E dE Φ = (cm 2 /kg) (1/keV.cm 2 ) 1 1 ( ) 0.873 n ab air i i i i E E = ≈Δ Φ (R) (keV) PHYS 6720 - Lecture IV 10 IV-2 Revisit of old definitions Exposure X in air after attenuation over d thickness dependence for monochromatic photons Thickness dependence for polychromatic photons (,) exp{ } t dX E d dX E d dE dE =− max 0 1 ( ) e x p { } 0.873 E ab total air t d X dE d d E dE Φ 1 1 ( ) e x p { ()} 0.873 n ab air i i t i i i E EE d = Φ x=d x=0
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## This note was uploaded on 04/25/2010 for the course PHYS 6720 taught by Professor Hu during the Spring '10 term at East Carolina University .

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6720 Lecture 4_measurement - Lecture IV 1 2 3 4 5 6...

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