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Ionization Chamber ¾Pocket Dosimeter

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Ionization Chamber ¾Pocket Dosimeter Ionization Chamber ¾Pocket dosimeter 1 Ionization Chamber ¾Pocket dosimeter 2 Radiation Quantities and Units ¾Radiation measurements require specification of the radiation field at various points At the source – Activity, mA, kVp In flight – Exposure, fluence (dN/dA), energy fluence (dE/dA) At the first interaction point – kerma Kinetic Energy Released in Matter In matter – Absorbed dose, equivalent dose, effective dose Radiation dosimetry is concerned with a quantitative determination of the energy deposited a medium by ionizing radiation 3 Radiation Quantities and Units ¾Pictorially Energy Source Deposition Transport First Interaction 4 Radiation Units ¾Activity 1 Bq (bequerel) == 1 disintegration / s A common unit is MBq = 106 Bq 10 1 Ci (curie) == 3.7x10 disintegrations /s An earlier unit of activity and used in EPP A typical HDR brachytherapy source is 10-20 Ci A typical radioactive source is the lab is ~ 10μCi 40K in your body is 0.1 μCi = 3700 Bq 5 Radiation Units ¾Exposure Defined for x-ray and gamma rays < 3 MeV Measures the amount of ionization (charge Q) in a volume of air at STP with mass m X == Q/m Assumes that the small test volume is embedded in a sufficiently large volume of irradiation that the number of secondary electrons entering the volume equals the number that leave (CPE) Units are C/kg or R (roentgen) 1 R (roentgen) == 2.58 x 10-4 C/kg Somewhat historical unit (R) now but sometimes still found on radiation monitoring instruments X-ray machine might be given as 5mR/mAs at 70 kVp at 100 cm 6 Radiation Units ¾Absorbed dose Energy deposited by ionizing radiation in a volume element of material divided by the mass of the volume D=E/m Related to biological effects in matter Units are grays (Gy) or rads (R) 1 Gy = 1 J / kg = 6.24 x 1012 MeV/kg 1 Gy = 100 rad 1 Gy is a relatively large dose Radiotherapy doses ~ 50 Gy Diagnostic radiology doses 1-30 mGy Typical background radiation ~ 6 mGy 7 Radiation Units ¾Equivalent dose Not all types of radiation cause the same biological damage per unit dose Dense ionization (high LET) along a track causes more biological damage than less dense (low LET) HT=D x wR 8 Radiation Units ¾Effective dose Not all tissues are equally sensitive to ionizing radiation E = ∑ HT ⋅ wT T Used to compare the stochastic risk from an exposure to a specific organ(s) in terms of the equivalent risk from an exposure of the whole body The stochastic risks are carcinogenesis and hereditary effects Not intended for acute effects In practice, most exposures are whole body 9 Radiation Units ¾Tissue weighting factors Sums to 1 Tissue or Organ Tissue weighing factor - wT Gonads 0.20 Bone marrow – red 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Liver 0.05 Oesophagus 0.05 Thyroid 0.05 Skin 0.01 Bone surface 0.01 Remainder 0.05 10 Radiation Units ¾Units of equivalent dose and effective dose are sieverts (Sv) 1 Sv = 100 rem (roentgen equivalent in man) 3.6 (6.2) mSv / year = typical equivalent dose in 1980’s (2006) 15 mSv/ year = Fermilab maximum allowed dose 20 mSv/year = CERN maximum allowed dose 50 mSv/year = US limit 3-4 Sv whole body = 50% chance of death (LD 50/30) 11 Background Radiation ¾Average equivalent dose (1980’s) 12 Background Radiation ¾Average equivalent dose (2006) 13 Background Radiation ¾1980’s versus 2006 14 Radiation in Japan ¾20 mSv / yr = 2.3 μSv/hr ¾3/28 update Reactor 2 @ 1 Sv / hr !!! 15 Fission Yield ¾Some of the more harmful fission products are 90Sr (29y), 106Ru (1y), 131I (8d), 132Te (3d), 133Xe (5d), and 137Cs (30y) 16 Natural Radioactivity 17 Natural Radioactivity ¾Terrestrial Present during the formation of the solar system Uranium, actinium, thorium, neptunium series 40 K ¾Cosmogenic Radionuclides produced in collisions between energetic cosmic rays and stable particles in the atmosphere (14C, 3H, 7Be) ¾Human produced Nuclear medicine, fission reactors, nuclear testing ¾Cosmic rays ~270 μSv / year (a bit more in Tucson) 18 Natural Radioactivity ¾Radon 19 Radon ¾222Rn (radon) is produced in the 238U decay series 222 218 Rn → Po + α (t1/2=3.8 days) 218 214 Po → Pb + α (t1/2=3.1 minutes) ¾Radon is a gas that can easily travel from the soil to indoors Air pressure differences Cracks/openings in a building ¾218Po can be absorbed into the lungs (via dust, etc.) The decay alpha particles are heavily ionizing The ionization in bronchial epithelial cells is believed to initiate carcinogenesis 20 Radiation Units ¾Kerma Kinetic energy released per unit mass Defined for indirectly ionizing energy (photons and neutrons) Mean energy transferred to ionizing particles in the medium without concern as to what happens after the transfer K=Etr/m Units are grays (Gy) 1 Gy = 1 J / kg 21 Radiation Units ¾The energy transferred to electrons by photons (kerma) can be expended in two ways Ionization losses Radiation losses (bremsstrahlung and electron-positron annihilation) Thus we can write K = Kcol + Krad Kcol = K()1− g g is the fraction of energy transferred to electrons that is lost through radiative processes 22 Photon Attenuation Coefficients Review −μx I = I0e μ is the linear attenuation coefficient μ μ = is the mass attenuation coefficient m ρ μen is the energy absorption coefficient μtr is the energy transfer coefficient μen = μtr ()1− g where g is the fraction of energy that is lost in radiative processes 23 Compton Scattering tr sc σ C = σ C +σ C T hv − hv′ σ tr = σ = σ C C hν C hν hv′ σ sc = σ C C hν similarly for the mass energy transfer attenuation coefficient μ tr T μ T N σ C = C = Av C ρ hν ρ hν A 24 Kcol and D as a function of depth 25 Relations ¾Kerma and energy fluence For a monoenergetic photon beam of energy E ⎛ μtr ⎞ K = Ψ⎜ ⎟ ⎝ ρ ⎠E 2 The energy fluence Ψ units are J/m 26 Relations ¾Exposure and kerma ⎛ e ⎞ X = K ⎜ ⎟ col()air ⎜ ⎟ ⎝Wair ⎠ W 33.97eV 1.602×10−19 J / eV air = ⋅ e ion pair 1.602×10−19 C / ionpair = 33.97J / C Wair includes the electron’s binding energy, average kinetic energy of ejected electrons, energy lost in excitation of atoms, … On average, 2.2 atoms are excited for each atom ionized 27 Relations ¾Absorbed dose and kerma D = Kcol = K(1− g) g is the radiative fraction g depends on the electron kinetic energy as well as the material under consideration The above relation assumes CPE ¾In theory, one can thus use exposure X to determine the absorbed dose Assumes CPE Limited to photon energies below 3 MeV 28 Kcol and D as a function of depth β=D/Kcol 29 Kcol and D as a function of depth ¾In the TCPE region, β = D/Kcol > 1 Photon beam is being attenuated Electrons are produced (generally) in the forward direction 30 Bragg-Gray Cavity Theory ¾The main question is, how does one determine or measure the absorbed dose delivered to the patient (to within a few percent) The answer is to use ionization in an air ion chamber placed in a medium The ionization can then be related to energy absorbed in the surrounding medium 31 Bragg-Gray Cavity Theory ¾Assumes Cavity is small (< Relectrons) so that the fluence of charged particles is not perturbed (CPE) Absorbed dose in the cavity comes solely by charged particles crossing it (i.e. no electrons are produced in the cavity or stop in the cavity) ⎛ S ⎞ ⎛ S ⎞ Dmed = Dcav ⎜ ⎟ /⎜ ⎟ ⎝ ρ ⎠med ⎝ ρ ⎠cav S is the average unrestricted mass collision stopping power Q W ⎛ IP ⎞⎛ eV ⎞ ⎛ eV ⎞ eV Dcav = ⋅ = ⎜ ⎟⎜ ⎟ ; ⎜ ⎟ = 33.97 for air m e ⎝ kg ⎠⎝ IP ⎠ ⎝ IP ⎠ IP 32 Bragg-Gray Cavity Theory ¾Spencer-Attix modification Accounts for delta rays that may escape the cavity volume In this case, one uses the restricted stopping power (energy loss) ⎛ L ⎞ ⎛ L ⎞ Dmed = Dcav ⎜ ⎟ /⎜ ⎟ ⎝ ρ ⎠med ⎝ ρ ⎠cav L is the average restricted mass collision stopping power 33 Calibration of MV Beams ¾Protocols exist to calibrate the absorbed dose from high energy photon and electron beams End result is a measurement of dose to water per MU (monitor unit = 0.01 Gy) For a reference depth, field size, and source to surface distance (SSD) ¾TG-21 Outdated but conceptually nice Based on cavity-gas calibration factor Ngas ¾TG-51 New standard Based on absorbed dose to water calibration 60 factor ND,w for Co 34 Ionization Chamber ¾Ionization chambers are a fundamental type of dosimeter in radiation physics ¾Measurement of the current or charge or reduction in charge gives the exposure or absorbed dose Free-air ionization chamber Thimble chamber Plane parallel chamber Pocket dosimeter 35 Ionization Chamber ¾Current mode Current gives average rate of ion formation of many particles ¾Pulse mode Voltage gives measure of individual charged particle ion formation 36 Ionization Chamber ¾Free-air chamber 37 Ionization Chamber ¾Used as a primary standard in standards laboratories ¾Used to measure X Q −μx′ X ()R = −4 e AP Lρ ⋅2.58×10 ¾Guard wires and guard electrodes produce uniform electric field ¾E ~ 100-200V/cm between plates ¾Assumes CPE ¾Limited to E<3 MeV (if pressurized) because of electron range 38 Ionization Chamber ¾Free-air chambers are not so practical however Instead one uses an ion chamber with a solid, air equivalent wall 39 Ion Chambers EXRADIN A12 Farmer EXRADIN A3 Spherical Chamber EXRADIN 11 Parallel Plate Chamber EXRADIN A17 Farmer EXRADIN mini thimble EXRADIN A12 thimble 40 Ionization Chamber ¾Vendors Capintec Inc.
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