Ionization Chamber Pocket dosimeter 1 Ionization Chamber Pocket

Ionization Chamber ØPocket dosimeter 1

Ionization Chamber ØPocket dosimeter 2

Radiation Quantities and Units ØRadiation measurements require specification of the radiation field at various points n n n At the source – Activity, m. A, k. Vp In flight – Exposure, fluence (d. N/d. A), energy fluence (d. E/d. A) At the first interaction point – kerma w Kinetic Energy Released in Matter n In matter – Absorbed dose, equivalent dose, effective dose w Radiation dosimetry is concerned with a quantitative determination of the energy deposited a medium by ionizing radiation 3

Radiation Quantities and Units ØPictorially Energy Deposition Source Transport First Interaction 4

Radiation Units ØActivity n 1 Bq (bequerel) == 1 disintegration / s w A common unit is MBq = 106 Bq n 1 Ci (curie) == 3. 7 x 1010 disintegrations /s w An earlier unit of activity and used in EPP w A typical HDR brachytherapy source is 10 -20 Ci w A typical radioactive source is the lab is ~ 10μCi w 40 K in your body is 0. 1 μCi = 3700 Bq 5

Radiation Units Ø Exposure n n n Defined for x-ray and gamma rays < 3 Me. V Measures the amount of ionization (charge Q) in a volume of air at STP with mass m X == Q/m w 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) n Units are C/kg or R (roentgen) w 1 R (roentgen) == 2. 58 x 10 -4 C/kg w Somewhat historical unit (R) now but sometimes still found on radiation monitoring instruments w X-ray machine might be given as 5 m. R/m. As at 70 k. Vp at 100 cm 6

Radiation Units Ø Absorbed dose n n 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) w 1 Gy = 1 J / kg = 6. 24 x 1012 Me. V/kg w 1 Gy = 100 rad n 1 Gy is a relatively large dose w Radiotherapy doses ~ 50 Gy w Diagnostic radiology doses 1 -30 m. Gy w Typical background radiation ~ 6 m. Gy 7

Radiation Units ØEquivalent dose n n n 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 w. R 8

Radiation Units Ø Effective dose n n Not all tissues are equally sensitive to ionizing radiation 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 w The stochastic risks are carcinogenesis and hereditary effects w Not intended for acute effects w In practice, most exposures are whole body 9

Radiation Units Ø Tissue weighting factors n Sums to 1 10

Radiation Units ØUnits of equivalent dose and effective dose are sieverts (Sv) n 1 Sv = 100 rem (roentgen equivalent in man) w 3. 6 (6. 2) m. Sv / year = typical equivalent dose in 1980’s (2006) w 15 m. Sv/ year = Fermilab maximum allowed dose w 20 m. Sv/year = CERN maximum allowed dose w 50 m. Sv/year = US limit w 3 -4 Sv whole body = 50% chance of death (LD 50/30) 11

Background Radiation ØAverage equivalent dose (1980’s) 12

Background Radiation ØAverage equivalent dose (2006) 13

Background Radiation Ø 1980’s versus 2006 14

Radiation in Japan Ø 20 m. Sv / yr = 2. 3 m. Sv/hr Ø 3/28 update n Reactor 2 @ 1 Sv / hr !!! 15

Fission Yield ØSome of the more harmful fission products are 90 Sr (29 y), 106 Ru (1 y), 131 I (8 d), 132 Te (3 d), 133 Xe (5 d), and 137 Cs (30 y) 16

Natural Radioactivity 17

Natural Radioactivity Ø Terrestrial n n n Present during the formation of the solar system Uranium, actinium, thorium, neptunium series 40 K Ø Cosmogenic n Radionuclides produced in collisions between energetic cosmic rays and stable particles in the atmosphere (14 C, 3 H, 7 Be) Ø Human produced n Nuclear medicine, fission reactors, nuclear testing Ø Cosmic rays n ~270 μSv / year (a bit more in Tucson) 18

Natural Radioactivity Ø Radon 19

Radon Ø 222 Rn (radon) is produced in the 238 U decay series 222 Rn → 218 Po + α (t =3. 8 days) 1/2 218 Po → 214 Pb + α (t =3. 1 minutes) 1/2 Ø Radon is a gas that can easily travel from the soil to indoors n Air pressure differences n Cracks/openings in a building Ø 218 Po can be absorbed into the lungs (via dust, etc. ) n The decay alpha particles are heavily ionizing n The ionization in bronchial epithelial cells is believed to initiate carcinogenesis 20 n n

Radiation Units ØKerma n n n 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) w 1 Gy = 1 J / kg 21

Radiation Units ØThe energy transferred to electrons by photons (kerma) can be expended in two ways n n n Ionization losses Radiation losses (bremsstrahlung and electron-positron annihilation) Thus we can write 22

Photon Attenuation Coefficients Review 23

Compton Scattering 24

Kcol and D as a function of depth 25

Relations ØKerma and energy fluence n n For a monoenergetic photon beam of energy E The energy fluence Y units are J/m 2 26

Relations Ø Exposure and kerma n n 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 Ø Absorbed dose and kerma Ø In theory, one can thus use exposure X to determine the absorbed dose n n Assumes CPE Limited to photon energies below 3 Me. V 28

Kcol and D as a function of depth b=D/Kcol 29

Kcol and D as a function of depth ØIn the TCPE region, b = D/Kcol > 1 n n Photon beam is being attenuated Electrons are produced (generally) in the forward direction 30

Bragg-Gray Cavity Theory ØThe main question is, how does one determine or measure the absorbed dose delivered to the patient (to within a few percent) n n 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 Ø Assumes n n 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) 32

Bragg-Gray Cavity Theory Ø Spencer-Attix modification n n Accounts for delta rays that may escape the cavity volume In this case, one uses the restricted stopping power (energy loss) 33

Calibration of MV Beams Ø Protocols exist to calibrate the absorbed dose from high energy photon and electron beams n n 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) Ø TG-21 n n Outdated but conceptually nice Based on cavity-gas calibration factor Ngas Ø TG-51 n n New standard Based on absorbed dose to water calibration factor ND, w for 60 Co 34

Ionization Chamber Ø Ionization chambers are a fundamental type of dosimeter in radiation physics Ø Measurement of the current or charge or reduction in charge gives the exposure or absorbed dose n n Free-air ionization chamber Thimble chamber Plane parallel chamber Pocket dosimeter 35

Ionization Chamber ØCurrent mode n Current gives average rate of ion formation of many particles ØPulse mode n Voltage gives measure of individual charged particle ion formation 36

Ionization Chamber ØFree-air chamber 37

Ionization Chamber Ø Used as a primary standard in standards laboratories Ø Used to measure X Ø Guard wires and guard electrodes produce uniform electric field Ø E ~ 100 -200 V/cm between plates Ø Assumes CPE Ø Limited to E<3 Me. V (if pressurized) because of electron range 38

Ionization Chamber ØFree-air chambers are not so practical however n Instead one uses an ion chamber with a solid, air equivalent wall 39

Ion Chambers EXRADIN A 12 Farmer EXRADIN A 17 Farmer EXRADIN A 12 thimble EXRADIN A 3 Spherical Chamber EXRADIN 11 Parallel Plate Chamber EXRADIN mini thimble 40

Ionization Chamber Capintec Inc. ØVendors Nuclear Associates VICTOREEN INC 41

Ionization Chamber Ø 0. 6 cm 3 Farmer chamber 42

Ionization Chamber Cavity Electrode Sleeve 43

Ionization Chambers Ø Materials used Central Electrode Aluminum Graphite n n n Wall A 150 C 552 PMMA Graphite Sleeve PMMA A 150 = Tissue equivalent plastic C 552 = Air equivlaent plastic PMMA = Polymethyl-methacrylate (lucite) 44

Ionization Chamber ØFarmer chamber n n Farmer type has a graphite wall and aluminum electrode For CPE , amount of carbon coating and size of aluminum electrode is adjusted so that the energy response of the chamber is nearly that of photons in free air over a wide range of energies Since an exact air equivalent chamber and knowledge of V is difficult, in practice they must be calibrated against free air chambers for low energy x-rays Nominal energy range is 60 ke. V – 50 Me. V 45

Ionization Chamber ØCorrection factors n n n Saturation Recombination Stem effects Polarity effects Environmental conditions 46

Ionization Chamber ØNeed to ensure chamber is used in the saturation region 47

Ionization Chamber Ø Stem irradiation cause ionization measured by the chamber so a correction factor will be needed n Found by irradiating the chamber with different stem lengths in the radiation field 48

Ionization Chamber ØThe collection efficiency can be measured by making measurements at two different voltages (one low and one nominal) ØPolarity effects can be measured by making measurements at both polarities and taking the average ØEnvironmental conditions are corrected to STP by 49

Beam Calibration with Water Phantom 50

Electrometer This device displays the measured values of dose and dose rate in Gy, Sv, R, Gy/min, Sv/h, R/min. 51

Ion Chamber and Electrometer Setup PTW Ion Chamber Electrometer 52

Ion Chamber and Electrometer Setup 53

Calibration Summary 54

Verification of the dose for treatment plan 55

Calibration of Novalis System 56

Novalis System at Department of Radiation Oncology, UA 57

Calibration of Novalis System 58

Ionization Chamber ØPlane parallel chamber 59

Ionization Chamber Ø Roos or advanced Markus type n n Used for precise dose measurements of electron beams w Nominal useful electron energy from 2 to 45 Me. V For surface dose from gammas, current arises from backwards Compton scattering 60

Ionization Chamber ØSmoke detector 61

Ionization Chamber ØAs with the proportional chamber, charge is induced by the drifting charge carriers n Can be both ions and electrons or only electrons ØReasoning goes as follows n n If response time > collection time, energy is conserved Energy to move the charges comes from the stored energy in the capacitor 62

Ionization Chamber Ø Consider 63

Ionization Chamber 64

Ionization Chamber Ø In order to minimize the deadtime, we usually don’t wait for the ions to drift to the electrodes n Then Ø But in this case, the amplitude depends on the position of interaction 65

Ionization Chamber Ø The solution to this feature is the Frisch grid n n The motion of the ions to the cathode and of the electrons to the grid is ignored because of the location of the load resistor Once the electrons pass the grid, using arguments as before 66

Radiation Units ØParticle fluence and flux n n Fluence F = N/A Flux (fluence rate) f = N/At Usually used to describe photon beams but may also be used in describing charged particle beams One can think of the particles being incident on a sphere of cross-sectional area A w Hence fluence is independent of incident angle n Units are m-2 (fluence) and m-2 s-1 (flux) 67

Radiation Units ØEnergy fluence and flux n n n Energy fluence Y = E/A Energy flux y = E/At Units are J/m 2 (energy fluence) and W/m 2 (energy flux) ØAlthough photon and energy fluence and flux are used in calculations, they are not easily measured 68

Radiation Units Ø Most realistic beams are polyenergetic and a spectrum must be used for fluence and energy fluence 69