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 μ I = I0e is the linear attenuation coefficient μ μ

mρ= is the mass attenuation coefficient μ is the energy absorption coefficient μ en is the energy transfer coefficient μtr en = μtr ()1− g where g is the fraction of energy that is lost in radiative processes

23 σ σ σ σ Compton Scattering σ = tr + sc C ν Cσ C σ T hv − hv′ σtr = = ν C C h C h ν hv′ sc = C C h similarlyμ for the mass energy transfer attenuatioρ nμ coefficient ν tr T ρ T N σ C = C = ν Av C h h A 24 Kcol and D as a function of depth

25 Relations ¾Kerma and energy fluence

For a monoenergetic photon beam of energy E μ ⎛ tr ⎞ K = Ψ⎜ ⎟ ⎝ ρ ⎠E

2 The energy fluence Ψ units are J/m

26 Relations ¾Exposure and kerma ⎛ e ⎞ ⎜ ⎟ X = Kcol()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) ⎛ Sρ ⎞ ⎛ S ⎞ Dmed = Dcav ⎜ ⎟ /⎜ ⎟ ⎝ ⎠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)

⎛ Lρ ⎞ ⎛ L ⎞ Dmed = Dcav ⎜ ⎟ /⎜ ⎟ ⎝ ⎠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.

Nuclear Associates

VICTOREEN INC

41 Ionization Chamber ¾0.6 cm3 Farmer chamber

42 Ionization Chamber

Cavity

Electrode

Sleeve

43 Ionization Chambers ¾Materials used

Central Electrode Wall Sleeve Aluminum A150 PMMA Graphite C552 PMMA Graphite

A150 = Tissue equivalent plastic C552 = Air equivlaent plastic PMMA = Polymethyl-methacrylate (lucite)

44 Ionization Chamber ¾Farmer 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 Ionization Chamber ¾Correction factors

Saturation

Recombination

Stem effects

Polarity effects

Environmental conditions

46 Ionization Chamber ¾Need to ensure chamber is used in the saturation region

47 Ionization Chamber ¾Stem 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 Ionization Chamber ¾The collection efficiency can be measured by making measurements at two different voltages (one low and one nominal) ¾Polarity effects can be measured by making measurements at both polarities and taking the average ¾Environmental 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 ¾Plane parallel chamber

59 Ionization Chamber ¾Roos 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 Ionization Chamber ¾Smoke detector

61 Ionization Chamber ¾As with the proportional chamber, charge is induced by the drifting charge carriers

Can be both ions and electrons or only electrons ¾Reasoning 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 Ionization Chamber ¾Consider

63 Ionization Chamber

1 1 CV 2 = n eEv+t + n eEv−t + CV 2 2 0 0 0 2 ch

Following Knoll, VR = V0 −Vch is given by n e V = o ()v+ + v− t R dC As we saw with the proportional tube, the motion of the charges generates a the signal by inducing a charge on the electrodes n e After the electrons are collected V = o ()v+t + x R dC n e After the ions are collected V = o ()d − x + x R dC

noe So Vmax = C 64 Ionization Chamber ¾In order to minimize the deadtime, we usually don’t wait for the ions to drift to the electrodes

Then n ex V = o max Cd ¾But in this case, the amplitude depends on the position of interaction

65 Ionization Chamber ¾The 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 n e n e V = 0 v−t and V = 0 R dC max C