Principles of applied dosimetry - illustrated by ionometry. Lesson FYSKJM4710 Eirik Malinen

Transcription

1 Principles of applied dosimetry - illustrated by ionometry Lesson FYSKJM4710 Eirik Malinen Ionometry Ionometry: the art of measuring ionizations Number of ionizations proportional to dose Air filled ionization chamber ( thimble ): ~ 300 V 1

2 Ionometry High voltage over inner and outer electrode Air is ionized; electrons are liberated Electrons collected at the positive electrode Induced current The number of charges produced is counted by an electrometer, which also provides the voltage Number of charges proportional to dose to Exposure Exposure, X : number of charges Q (either positive or negative) prodused in a gass of mass m: dq X = dm Number of charges per mass proprtional to dose: X D The quantity relating X to D is the mean energy per ion p, W 2

3 Mean energy per ion p Determination of W : Charged particles with kinetic energyt 0 is completely stopped in the gas: N Total energy loss: W = NT 0 Mean energy loss per charge detected W NT = 0 e Q Dose to, D For, W/e is J/C The dose to : D NT Q W W X m m e e 0 = = = Thus, by measuring the number of charges produced per mass unit of, D may be determined indepedent of the radiation quality ( W/e is close to being constant for all electron- and photon energies) 3

4 Dose to, D 2 If CPE is present in the ion chamber, the dose following photon irradiation is: μ W = =Ψ = CPE en D Kc, X ρ e The exposure is thus: X 1 CPE μen W =Ψ ρ e Hvis primærefeltet er ladde partikler, anvendes Bragg-Gray teori: Dose to, D 3 If the primary field is charged particles, Bragg-Gray theory may be used: dt D =Φ ρdx 4

5 Exposure, examples Electrometer and filled ion chamber (volume = 0.65 cm 3 ) is used to measure Q=50 nc over 2 min the radiation a 100 kev monoenergetic photons (CPE is assumed) Exposure: 9 Q Q C X = = = = C/kg m ρ V g/cm 0.65 cm Energy fluence: 1 μen W Ψ= X ρ e 1 = C/kg J/C = J/cm cm /g Exposure, examples 2 Dose to and dose rate: W D = X = C/kg J/C e = 2.2 J/kg = 2.2 Gy ΔD 2.2 Gy 1.1 Gy/min Δt 2 min D = = = If the ion chamber is placed in water, the dose to water is w Dw μen = = = D ρ D = D = Gy = 2.4 Gy w 5

6 Exposure, examples 3 If the same exposure is resulting from 100 MeV protons, what is the corresponding energy fluence? For protons, Bragg-Gray theory is used: D dt W =Φ = X ρdx e The proton energy is virtually constant over the cavity: Exposure, examples 4 Dose til : W D = X = C/kg J/C = 2.2 Gy e (must be equal to the dose from photons, since the exposure was the same) Dose til water: w Dw dt 7.29 = = = 1.13 D ρdx 6.43 D = 1.13 D = Gy = 2.5 Gy w Same exposure from photons and protons does not give the same dose to e.g. water! 6

7 Practical ion chamber dosimetry Problems with ion chambers is e.g. inacuracies in determining volume In practice, the ion chamber is calibrated at a point where the dose is known performed at a primary standards laboratory (PSDL) electrometer γ, e - Ion chamber H 2 O A given dose gives reading M Practical ion chamber dosimetry 2 For a given dose to water, D w, an ion chamber reading M is obtained. Thus: Dw M Dw = MND,w The calibration factor is: Dw ND,w = M Thus, the dose may be determined without using W/e, μ en /ρ etc 7

8 Practical ion chamber dosimetry 3 However, the calibration factor is dependent on the radiation type and energy, due to differences in absorption properties between water and. Keep in mind (μ en /ρ)- the (dt/ρdx)-ratios, and that M is proportional to D! Usually, the chamber is calibrated in a well defined field, e.g. 60 Co γ-rays (average energy 1.25 MeV) Corrections of the calibration factor, k Q, is thus introduced for other energies (radiation qualities, e.g. 15 MV X-rays) Practical ion chamber dosimetry 4 However, the calibration factor is (weakly) dependent on the radiation type and energy, due to differences in absorption properties between water and. Keep in mind (μ en /ρ)- the (dt/ρdx)-ratios, and that M is proportional to D! Usually, the chamber is calibrated in a well defined field, e.g. 60 Co γ-rays (average energy 1.25 MeV) Corrections of the calibration factor, k Q, is thus introduced for other energies (radiation qualities, e.g. 15 MV X-rays) 8

9 Practical ion chamber dosimetry 5 Absorbed dose to water: k Q : beam quality correction factor photons ~ Mean photon energy, MeV 9