Ionization Chamber. Pocket dosimeter

Transcription

1 Ionization Chamber Pocket dosimeter 1

2 Ionization Chamber Pocket dosimeter 2

3 Radiation Quantities and Units Radiation 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

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

5 Activity Radiation Units 1 Bq (bequerel) == 1 disintegration / s A common unit is MBq = 10 6 Bq 1 Ci (curie) == 3.7x10 10 disintegrations /s An earlier unit of activity and used in EPP A typical HDR brachytherapy source is Ci A typical radioactive source is the lab is ~ 10μCi 40 K in your body is 0.1 μci = 3700 Bq 5

6 Exposure Radiation Units 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

7 Radiation Units Absorbed 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 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

8 Equivalent dose Radiation Units 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) H T =D x w R 8

9 Effective dose Radiation Units Not all tissues are equally sensitive to ionizing radiation E = H T w T 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

10 Radiation Units Tissue weighting factors Sums to 1 Tissue or Organ Tissue weighing factor - w T 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

11 Radiation Units Units 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

12 Background Radiation Average equivalent dose (1980 s) 12

13 Background Radiation Average equivalent dose (2006) 13

14 Background Radiation 1980 s versus

15 20 msv / yr = 2.3 μsv/hr 3/28 update Reactor 1 Sv / hr!!! Radiation in Japan 15

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

17 Natural Radioactivity 17

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

19 Radon Natural Radioactivity 19

20 Radon 222 Rn (radon) is produced in the 238 U decay series 222 Rn 218 Po + α (t 1/2 =3.8 days) 218 Po 214 Pb + α (t 1/2 =3.1 minutes) Radon is a gas that can easily travel from the soil to indoors Air pressure differences Cracks/openings in a building 218 Po 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

21 Kerma Radiation Units 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=E tr /m Units are grays (Gy) 1 Gy = 1 J / kg 21

22 Radiation Units The 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

23 I = I 0 e Photon Attenuation Coefficients Review μ is the linear attenuation coefficient μ μ μ μ m en tr en μx μ = is the mass attenuation coefficient ρ is the energy absorption coefficient is the energy transfer coefficient = μ tr ( 1 g) where g is that is lost in radiative processes the fraction of energy 23

24 Compton Scattering σ C tr C tr T hv hv σ C = σ C = σ C hν hν sc hv σ C = σ C hν similarly for the mass energy transfer tr μc ρ = σ = T hν + σ μc ρ sc C attenuation coefficient = T N hν σ A Av C 24

25 K col and D as a function of depth 25

26 Relations Kerma and energy fluence For a monoenergetic photon beam of energy E K μtr = Ψ ρ The energy fluence Ψ units are J/m 2 E 26

27 Relations Exposure and kerma X W e = air K = col ( air ) e W 33.97eV ion pair = 33.97J / air C C J / ev / ionpair W air 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 19 27

28 Relations Absorbed dose and kerma ( 1 g) D = Kcol = K g is the radiative fraction g depends on the electron kinetic energy as the material under consideration The above relation assumes CPE well as In theory, one can thus use exposure X to determine the absorbed dose Assumes CPE Limited to photon energies below 3 MeV 28

29 K col and D as a function of depth β=d/k col 29

30 K col and D as a function of depth In the TCPE region, β = D/K col > 1 Photon beam is being attenuated Electrons are produced (generally) in the forward direction 30

31 Bragg-Gray Cavity Theory The 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

32 Assumes D S D Bragg-Gray Cavity Theory Cavity is small (< R electrons ) 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) med is cav = D S / ρ the average unrestricted mass collision stopping power = Q m cav S ρ W e = med IP kg ev IP cav ; ev IP = ev IP for air 32

33 Bragg-Gray Cavity Theory Spencer-Attix modification Accounts for delta rays that may escape the cavity volume In this case, one uses the restricted stopping power (energy loss) D L med is = D cav L ρ med L / ρ cav the average restricted mass collision stopping power 33

34 Calibration of MV Beams Protocols 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) TG-21 Outdated but conceptually nice Based on cavity-gas calibration factor N gas TG-51 New standard Based on absorbed dose to water calibration factor N D,w for 60 Co 34

35 Ionization Chamber Ionization chambers are a fundamental type of dosimeter in radiation physics Measurement 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

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

37 Ionization Chamber Free-air chamber 37

38 Ionization Chamber Used as a primary standard in standards laboratories Used to measure X X ( ) μx R = e A P Q Lρ Guard wires and guard electrodes produce uniform electric field E ~ V/cm between plates Assumes CPE Limited to E<3 MeV (if pressurized) because of electron range 4 38

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

40 Ion Chambers EXRADIN A12 Farmer EXRADIN A3 Spherical Chamber EXRADIN A17 Farmer EXRADIN 11 Parallel Plate Chamber EXRADIN A12 thimble EXRADIN mini thimble 40

41 Ionization Chamber Capintec Inc. Vendors Nuclear Associates VICTOREEN INC 41

42 Ionization Chamber 0.6 cm 3 Farmer chamber 42

43 Ionization Chamber Cavity Electrode Sleeve 43

44 Materials used Ionization Chambers Central Electrode Wall Sleeve Aluminum Graphite A150 C552 PMMA Graphite PMMA A150 = Tissue equivalent plastic C552 = Air equivlaent plastic PMMA = Polymethyl-methacrylate (lucite) 44

45 Farmer chamber Ionization Chamber 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 kev 50 MeV 45

46 Ionization Chamber Correction factors Saturation Recombination Stem effects Polarity effects Environmental conditions 46

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

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

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

50 Beam Calibration with Water Phantom 50

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

52 Ion Chamber and Electrometer Setup PTW Ion Chamber Electrometer 52

53 Ion Chamber and Electrometer Setup 53

54 Calibration Summary 54

55 Verification of the dose for treatment plan 55

56 Calibration of Novalis System 56

57 Novalis System at Department of Radiation Oncology, UA 57

58 Calibration of Novalis System 58

59 Ionization Chamber Plane parallel chamber 59

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

61 Smoke detector Ionization Chamber 61

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

63 Consider Ionization Chamber 63

64 Ionization Chamber CV0 = n0eev t + n0eev 2 Following Knoll, V = V SoV + ( v + v ) noe VR = t dc As we saw with the proportional motion of After the electrons are collectedv After the ions are collectedv max = noe C R 0 t + V ch R CV by inducing a charge on the electrodes 1 2 = 2 ch is given by tube, the the charges generates a the signal R noe dc = noe dc + ( v t + x) ( d x + x) 64

65 Ionization Chamber In order to minimize the deadtime, we usually don t wait for the ions to drift to the electrodes Then noex Vmax = Cd But in this case, the amplitude depends on the position of interaction 65

66 Ionization Chamber The solution to this feature is the Frisch grid 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 n0 e n0e V R = v t and Vmax = dc C 66