University of Oslo University
Department of Physics Principles of dosimetry – The ionization chamber
FYS-KJM 4710
Audun Sanderud Ionometry 1)
• Ionometry: the measurement of the number of ionizations
University of Oslo University in substance Department of Physics • The number of ionizations are used as a measure of the radiation dose • Air filled ionizing chamber (thimble): Ionometry 2)
• High voltage between central and outer electrode University of Oslo University • Air ionized, electrons emitted from the atoms Department of Physics • The electrons moves to the positive electrode • Current induced • An electrometer then count the number of total charges Q (of one type of charged +/–) • Q is proportional with the dose to the air volume Exposure
• The exposure, X: the number of charges Q (ether positive
University of Oslo University or negative) which is produced in the gas with mass m as
Department of Physics a result of the radiation: dQ X = dm • The number of charges produced in the gas most be
proportional with the dose; X ∝ Dair
• The value connecting X and Dair is the mean energy expended in a gas per ion pair formed W The mean energy per ion pair,W
• Definition of W : University of Oslo University ─ Charged particles with kinetic energy T0 is stopped Department of Physics inside the gas:
N
─ Energy deposit per ion pair detected:
W=NT0 (the bremsstrahlung of electrons ought to be corrected for) ─ Mean energy per charge: W NT = 0 eQ Dose to air, Dair 1)
• Air has W e = 33.97 J/C University of Oslo University • Then the dose to air becomes: Department of Physics
NT0 QW⎛⎞ ⎛⎞ W Dair = = ⎜⎟ = X ⎜⎟ mme e ⎝⎠air ⎝⎠ air • Thereby: when measuring the number of charges
produced per unit of mass air, the Dair can be determent – independent of the type and energy of the ionizing radiation ( We is close to constant for all electrons and photon energies) Dose to air, Dair 2)
• When CPE is present inside the ionizing chamber, will
University of Oslo University the dose as a result of the photon exposure be given by:
Department of Physics CPE ⎛⎞µen ⎛⎞W D=K=air c,air Ψ⎜⎟= X⎜⎟ ρ e ⎝⎠air ⎝⎠air • The exposure can thereby be expressed by: −1 CPE ⎛⎞µen ⎛⎞W X= Ψ⎜⎟⎜⎟ ρ e ⎝⎠air ⎝⎠air • If the primary field is charged particles, Bragg-Gray theory is used: −1 B-G ⎛⎞dT⎛⎞ W X= Φ⎜⎟⎜⎟ ρdx e ⎝⎠air ⎝⎠air Exposure, example 1) • An electrometer and an air filled ionizing chamber 3
University of Oslo University (volume = 0.65cm ) measure the number of charges
Department of Physics Q=50nC in 2 minutes – the radiation source is 100 keV monoenergetic photons (CPE assumed) • Exposure: QQ 50×10C-9 X= = = = 0.064 C/kg m ρV 1.2×10-3 g/cm 3 ×0.65cm 3 • What is the energy fluence of the photon field? -1 ⎛⎞µen ⎛⎞W Ψ = X⎜⎟⎜⎟ () (µ/ρ)en from table ρ e ⎝⎠air ⎝⎠air 1 = 0.064 C/kg × × 33.97 J/C = 0.093 J/cm2 0.0234 cm2 /g Exposure, example 2) • What is the dose to air and what is dose rate? ⎛⎞W University of Oslo University Dair = X⎜⎟ = 0.064 C/kg × 33.97 J/C = 2.2 J/kg = 2.2 Gy e Department of Physics ⎝⎠air ΔD 2.2 Gy D =air = = 1.1 Gy/min = 18.3 mGy/s air Δt2 min • If the ionizing chamber is placed in water what is the dose to water? water Dµwater⎛⎞ en 0.0256 = ⎜⎟ = = 1.094 Dair ⎝⎠ρ air 0.0234
Dwater = 1.094 D air = 1.094× 2.2 Gy = 2.4 Gy Exposure, example 3) • If the same exposure is produced by 100 MeV protons,
University of Oslo University what is the energy fluence of that field?
Department of Physics • Bragg-Gray theory is used: ⎛⎞dT⎛⎞ W Dair = Φ⎜⎟ = X⎜⎟ (W/e assumed to be 33.97 J/C) ρdx e ⎝⎠air ⎝⎠air • The proton energy is ~unchanged over the air cavity: −1 Ψ ⎛⎞WdT⎛⎞ ⇒⇒Ψ=ΦT0 = X⎜⎟⎜⎟ Teρdx 0 ⎝⎠air ⎝⎠air −1 ⎛⎞WdT⎛⎞ ⇒ Ψ= XT0 ⎜⎟⎜⎟ e ρdx ⎝⎠air ⎝⎠air 0.064 C/kg×100 MeV×33.97 J/C = = 0.034 J/cm2 6.43 MeV cm2 / g Exposure, example 4) • Dose to air: ⎛⎞W University of Oslo University Dair = X⎜⎟ = 0.064 C/kg × 33.97 J/C = 2.2 Gy
Department of Physics e ⎝⎠air (has to be the same as of photon, because it gave the same exposure) • Dose to water: water Dwater ⎛⎞dT 7.29 = ⎜⎟ = = 1.13 Dair ⎝⎠ρdxair 6.34
⇒×Dwater = 1.13 D air = 1.13 2.2 Gy = 2.5 Gy • The equal exposure of air by photons or protons give the equal doses to air but not to water! Ionizing chamber, use 1)
• The problem with the ionizing chamber is among other
University of Oslo University difficulties to precise decide the size of the air volume –
Department of Physics increase the uncertainty in dose • In practice is the chamber calibrated in a point of the radiation field where the dose is known – done at a primary standard laboratory (PSDL)
Electrometer
- Ionizing γ, e ,… chamber A certain dose gives a measured value M H2O Ionizing chamber, use 2)
• A certain dose to water Dwater gives a measured value M. University of Oslo University Then: Department of Physics Dwater ∝ M ⇔ Dwater = M·ND,water • The calibration factor of the chamber is then:
D N= water D,water M • The dose is then establish from the (measured)
calibration factor – do not have to use W/e, µen/ρ or dT/ρdx Ionizing chamber, use 3)
• But: the calibration factor is (weakly) dependent of
University of Oslo University radiation type and energy. Department of Physics • Usually the calibration takes place in a well known radiation field as that of 60Co γ-rays (mean energy 1.25 MeV)
• The corrections in the calibration factor, kQ, is then introduced for other radiation qualities (radiation qualities, Q) • The dose is then given by:
Dwater,Q = MQ·ND,water·kQ Ionizing chamber, use 4)
• kQ is named the energy correction factor; shown below in University of Oslo University the case of high energy photons: Department of Physics
~mean photon energy, MeV 1 3.5 6 Other methods 1)
• The method and theory explained is basically the same
University of Oslo University also in other measuring methods – the measurable unit M
Department of Physics is transferred into dose by a calibration factor • Example: EPR dosimetry. To calibrate are dosimeters radiated in a known radiation field (60Co-γ) to a known dose. The EPR-intensity of the dosimeters (M) is then proportional with the dose. The calibration factor of the
dosimeters will then be determent as described above. kQ most then be found if other radiation qualities if these are to be used. Other methods 2)
• Calorimetric: measure the temperature in detector – very
University of Oslo University good method in absolute dosimetry: Department of Physics
ε(1-δ) D1-( δ) ΔTemp = = hm h δ: thermal defect hΔtemp h: heat capacity ⇒ D = ()1-δ Other methods 3)
• Semiconductor dosimetry: current induced by the
University of Oslo University radiation over the depletion layer. Current proportional
Department of Physics with the dose rate.