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Ionization Chambers

Ionization chambers are used to detect and measure ionizing radiation. This presentation describes the theory and applications of ionization chambers.

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Ionization Chambers

  1. 1. Measurement of Ionizing Radiation DILSHAD KP | DEPARTMENT OF PHYSICS | UNIVERSITY OF CALICUT | KERALA
  2. 2. Skin Erythema Dose (SED) is the received quantity of x- or 𝛾- radiation that causes diffuse redness over an area of skin after irradiation SED depends on: - The type of skin - The quality of radiation - The extent of skin exposed - Dose fractionation - Differences between early and delayed skin reactions In the early days measurement of absorbed ionizing radiation were done on the basis of chemical and biological effects such as: • Radiation effects on photographic emulsions • Changes in the color of chemical compounds • Reddening of the human skin Because the amount of radiation required to produce the erythema reaction varied from one person to another, it was a crude and inaccurate way to measure radiation exposure.
  3. 3. In , ICRU adopted the as the unit of measuring 𝑥- and 𝛾- radiation exposure The SI unit of 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 is 𝑐𝑜𝑢𝑙𝑜𝑚𝑏 𝑝𝑒𝑟 𝑘𝑖𝑙𝑜𝑔𝑟𝑎𝑚 (𝐶/𝑘𝑔), but the special unit is Old definition of roentgen: ICRU defines as the quotient of 𝑑𝑄 by 𝑑𝑚 where 𝑑𝑄 is the absolute value of the total charge of the ions of one sign produced in air when all the electrons (negatrons or positrons) liberated by photons in air of mass 𝑑𝑚 are completely stopped in air Current definition of roentgen: 1 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝑢𝑛𝑖𝑡 = 3.333 × 10−10 𝐶 1 𝑐𝑚3 𝑜𝑓 𝑎𝑖𝑟 𝑎𝑡 𝑆𝑇𝑃 = 1.293 × 10−6 𝑘𝑔
  4. 4. An 𝑥-ray beam passing through air sets in motion electrons by various interaction. These high-speed electrons produce ionization along their tracks. Because of the electric field produced by the voltage applied across the ion- collecting electrodes, the positive charges move toward the negative electrode and the negative charges move toward the positive electrode. This constitutes a current. The collected charge of either sign can be measured by an electrometer.
  5. 5. 102 104 106 108 1010 1012 200 400 600 800 1000 IonrecombinationRegion Ionization Region Proportionality Region Limitedproportionality Region Geiger Muller Region ContinuesDischarge
  6. 6. 100 101 102 103 104 105 150 250 350 Saturation IonrecombinationRegion ProportionalityRegion  The charge collected by the electrode in the ionization chamber will be less than the charge produced in the gas. This is because of within the gas.  An ionization chamber is said to be to the degree that such ionic recombination is absent.  Ionic recombination can be decrease by increasing the applied voltage. But, higher applied voltage will result in , in which the free electrons gain enough kinetic energy to produce secondary ionization in the gas (extra ionization). It is desired in proportional chambers, but not in ionization chambers.
  7. 7. Also known as “ ” Used at the National Bureau of Standards (NBS) to calibrate cavity ion chambers with low photon energy
  8. 8.  It is enclosed in a lead shielding box to exclude scattered 𝑥-rays from elsewhere.  At the front the box is a tungsten-alloy diaphragm that is aligned with the 𝑥-ray beam central axis.  The cavity chambers are centered at the axial point, 𝑃 to be compared with the free-air ionization chamber.
  9. 9.  Inside the box, there are three coplanar electrodes on one side of the beam and a parallel high-voltage electrode opposite. The guard electrodes are grounded and the collecting electrode is connected to a charge measuring device, electrometer. These electrodes are all parallel to the 𝑥-ray beam axis and equidistant from it.
  10. 10.  The guard wires provide a uniform electric field between the electrodes. These wires are electrically biased in uniform steps to establish parallel equipotential planes between the electrodes. The guard electrodes also assist in producing field uniformity.
  11. 11. According to the definition of roentgen, the by the photons in a specified volume (region of ion collection) and the
  12. 12. The electrons like colored will remain in the volume 𝑉′ and thus all their ionization will be collected and measured. 𝑉 is the actual volume of origin of secondary electrons whose ionization we wish to measure. These ionizations are collected from the volume 𝑉′ by the collector electrode.
  13. 13. The electrons like colored, which originate within the volume 𝑉′ , will carry some of their kinetic energy out of 𝑉′ and the remaining ionization will produce there. These ionizations will not collected by the collector electrode, but will go to the grounded guard electrode. This ionization lost must be replaced by the electrons like colored, which originate in the beam outside of volume 𝑉. Consequently the volume 𝑉′ as a whole in charged particle equilibrium. That is the ionization produced by all of the electrons originating in the beam within 𝑉 is equal to all of the ionization produced within 𝑉′ , and the correct amount of charge is thus measured.
  14. 14. Conditions to exist the in the volume 𝑉′ : 1. The distance from the boundaries of 𝑉 to each end of the lead box must be greater than the maximum electron range in air 2. The beam intensity (photon fluence per unit time) must remain constant across the length of the volume 𝑉 During CPE, we are neglecting the small effects of scattered photons, bremsstrahlung, and ionic recombination
  15. 15. If ∆𝑄 is the charge collected in Coulombs and 𝜌 is the density of air in 𝑘𝑔 𝑚3 , then the exposure (in 𝐶/𝑘𝑔) at the centre of the specified volume (point P) is: where, 𝑨 𝑷 is the cross-sectional area of the beam at point P (in 𝑚2 ) 𝑳 is the length of the collecting volume (in 𝑚) To change 𝑿 𝑷 from 𝐶/𝑘𝑔 to 𝑅 (1 𝑅 = 2.58 × 10−4 𝐶/𝑘𝑔):
  16. 16. Suppose 𝑓1 and 𝑓2 are the distances of the X-ray source to the diaphragm (D) and point P, respectively. Now, the exposure of the X-ray beam at D and P can be relate by the inverse square law as: Similarly, by the inverse square law, the area of cross-section at D and P can be relate as: So, the exposure (in 𝐶/𝑘𝑔) at D is: To change 𝑿 𝑫 from 𝐶/𝑘𝑔 to 𝑅 (1 𝑅 = 2.58 × 10−4 𝐶/𝑘𝑔): where, 𝑨 𝑫 is the cross-sectional area of the beam at D (in 𝑚2 )
  17. 17. If ∆𝑄 is the charge collected in the volume 𝑉′ , the exposure (in 𝐶/𝑘𝑔) at point D is: where, is the air attenuation coefficient (in 𝑚−1 ) is the distance from D to P (in 𝑚) To change 𝑿 𝑫 from 𝐶/𝑘𝑔 to 𝑅 (1 𝑅 = 2.58 × 10−4 𝐶/𝑘𝑔): NOTE : Typically 𝜇 ≅ 2 − 3% per meter of air and 𝑒 𝜇𝑥 ≅ 1.02
  18. 18. 1. As the photon energy increases, the range of the electrons liberated in air increases rapidly. This necessitates an increase in the separation of the plates to maintain electronic equilibrium. 2. Large electrode separation creates problems of non-uniform electric field and greater ion recombination. 3 𝑀𝑒𝑉 photons produce electron tracks 1.5 𝑚 long. This thickness of air will attenuate this photon beam by 5.4% and so large corrections are required to correct for the attenuation by the thickness of air between the diaphragm and the sensitive volume. Although the plate separation can be reduced by using air at high pressures, the problems still remain in regard to air attenuation, photon scatter, and reduction in the efficiency of ion collection.
  19. 19. They consist of a solid envelop surrounding a in which an electric field is established to collect the ions formed by radiation 1. They can be made very compact, even for high energy use 2. They can measure multidirectional radiation fields 3. Through the application of cavity theory, the absorbed dose can be determined in any material of which the cavity wall is made 4. They are capable to design for the dose measurements of charged particles and neutrons as well as photons 5. Gas cavities can be designed to be thin and flat and as a result point dose measurement is possible as a function of depth 6. Collected charge can be measured in real time by connecting the chamber to an electrometer (condenser type chambers can be operate without the cable)
  20. 20. The chamber wall is shaped like a “ ” They are designed with air cavity volume in the range of 0.1 − 3 𝑐𝑐
  21. 21.  The high voltage (HV), usually ± 200 − 500 𝑉 is applied to the thimble wall, with the central electrode (collector) connected to the electrometer input at or near ground potential.  The guard ring electrode that encircle around the insulator is also in ground potential (not shown in figure).  The insulator arrangement exemplifies a fully guarded ion chamber. That is
  22. 22.  In order to be equivalent to a free-air ionization chamber, the wall of the thimble chamber (thimble wall) should be . The most commonly used wall materials are made either of graphite (carbon), Bakelite or a plastic coated on the inside by a conducting layer of graphite or a conducting mixture of Bakelite and graphite.  The effective atomic number of the thimble wall is generally a little less than that of air. It is closer to that of carbon (𝑍 = 6)  The graphite coated inner surface of the thimble wall is act as one electrode. The other electrode is a rod of aluminum (𝑍 = 13) held in the centre of the thimble but electrically insulated from it. The ion collecting rod in thimble-type chamber should be made of the same material as the wall, if possible. However, an aluminum rod is sometimes used in an air-equivalent-walled chamber to boost the photon response below ~ 100 𝑘𝑒𝑉 by the photoelectric effect, thus compensating for the increasing attenuation of the 𝑥-rays in the wall.  Polystyrene, polyethylene, Nylon, Mylar and Teflon are all excellent electrical insulators for ion chamber use (Teflon should be avoided where total doses exceeding ~104 𝐺𝑦, to avoid radiation damage).
  23. 23. In dealing with air, as a mixture of molecules, it is convenient to describe the air by an effective atomic number. Air contains nitrogen (Z = 7), oxygen (Z = 8) and argon (Z = 18). Effective atomic number (Z) is the atomic number of an element with which photons interact the same way as with the given composite material. Since photoelectric effect is highly 𝑍 dependent, effective atomic number is considered only for photoelectric interactions by the photons of energy range from 30 𝑘𝑒𝑉 𝑡𝑜 80 𝑘𝑒𝑉. where, 𝑎1, 𝑎2, … 𝑎 𝑛 are the fraction of electrons/gram of each element to the total number of electrons in the mixture 𝑍1, 𝑍2, … 𝑍 𝑛 are the atomic numbers of each element in the mixture
  24. 24. Composition by weight: 𝑁𝑖𝑡𝑟𝑜𝑔𝑒𝑛 = 75.5%, 𝑂𝑥𝑦𝑔𝑒𝑛 = 23.2%, 𝐴𝑟𝑔𝑜𝑛 = 1.3% 𝑁𝑖𝑡𝑟𝑜𝑔𝑒𝑛 ⇒ 6.023 × 1023 × 7 × 0.755 14.007 = 2.2725 × 1023 ⇒ 𝑂𝑥𝑦𝑔𝑒𝑛 ⇒ 6.023 × 1023 × 8 × 0.232 15.999 = 0.6987 × 1023 ⇒ 𝐴𝑟𝑔𝑜𝑛 ⇒ 6.023 × 1023 × 18 × 0.013 39.948 = 0.0353 × 1023 ⇒ 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑔 𝑜𝑓 𝑎𝑖𝑟 = 𝑁𝐴 𝑍 𝐴 × 𝐹𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑏𝑦 𝑤𝑒𝑖𝑔𝑕𝑡 𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑔 𝑜𝑓 𝑎𝑖𝑟 = 3.01 × 1023 𝑎1 = 2.2725 3.01 = 0.7558 𝑎2 = 0.6987 3.01 = 0.2321 𝑎3 = 0.0353 3.01 = 0.0117 Z = 0.7558 × 73.5 + 0.2321 × 83.5 + (0.0117 × 183.5) 3.5 =
  25. 25. If the chamber is designed to measure the absorbed dose at a point of interest in a charged particle field : • Chamber volume must be small • Chamber wall must be thin For practical purposes a wall thickness of ≅ 15 𝑚𝑔/𝑐𝑚2 (the range of a 100 𝑘𝑒𝑉 electron) should suffice, as most 𝛿-rays resulting from electron-electron collisions have energies less than that.
  26. 26. By compressing the volume of air required for electronic equilibrium, we can reduce its dimensions. In fact, the air volume required for electronic equilibrium can be substituted by a small air cavity with surrounding so that the electronic equilibrium is maintained in the air cavity. The wall thickness of a thimble chamber must be equal to or greater than the maximum range of the electrons liberated in the thimble wall. Since the density of the solid air-equivalent wall is much greater than that of free air, the thicknesses required for electronic equilibrium in the thimble chamber are considerably reduced. The wall thickness of the thimble for X-ray energy range 100 – 250 𝑘𝑉𝑝 is about 1 𝑚𝑚 , and in the case of 𝐶𝑜60 𝛾 -rays ( 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑒𝑛𝑒𝑟𝑔𝑦 ≈ 1.25 𝑀𝑒𝑉 ), it is approximately 5 𝑚𝑚. If the wall is not thick enough, a close-fitting buildup cap is required to provide electronic equilibrium for the radiation in question. These buildup caps are required when making measurements in free air.
  27. 27. Conditions for the direct measurement of exposure by a thimble chamber are: 1. The chamber must be air-equivalent 2. Its cavity volume must be accurately known 3. Its wall thickness must be sufficient to provide electronic equilibrium Under the above conditions, the exposure is given by where 𝑄 is the ionization charge liberated in the cavity air of density 𝜌 and volume 𝑣; 𝐴 is the fraction of energy fluence transmitted through the air-equivalent wall of equilibrium thickness. The factor 𝐴 is slightly less than 1 and is used here to calculate the exposure for the energy fluence that would exist at the point of measurement in the absence of the chamber.
  28. 28. Hence, thimble chambers are against  A free-air ionization chamber for low energy X-rays (up to few hundred kilovolts)  A standard cavity chamber (with nearly air-equivalent walls and accurately known volume) for higher energies (up to 𝐶𝑜60 𝛾-rays) It is almost to construct a thimble chamber that is exactly It is to determine accurately the directly
  29. 29. 1 2 3 4 5 6 7 0.2 0.4 0.6 0.8 1.0 Adequate to achieve electronic equilibrium Required wall thickness for maximum ionization Correction factor 1/𝐴 Increased attenuation of the beam in the wall Too few electrons are generated in the wall True exposure in free air (without chamber) is given by where, 𝑀 is the electrometer reading which is connected to the chamber This exposure value is free from the wall attenuation or the perturbing influence of the chamber
  30. 30. Special form of thimble-type chamber Operates while being irradiated
  31. 31. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
  32. 32. In 1955, Farmer designed a chamber that provided a stable and reliable secondary standard This chamber connected to a specific electrometer is known as the Baldwin-Farmer substandard dosimeter. The original design of the Farmer chamber was later modified by Aird and Farmer to provide
  33. 33. : Graphite or plastic such as PMMA (acrylic), nylon, A.E. (air-equivalent) plastic and T. E. (tissue-equivalent) plastic : Inner surface of the thimble wall coated with a conducting material : A thin aluminum rod of 1 𝑚𝑚 diameter. It delivers the ionization current to the electrometer : A cylindrical conductor that wraps around the insulator surrounding the central electrode in the stem of the chamber. It has two functions: 1. To prevent leakage current from the central electrode 2. To improve the signal to noise ratio : Cylindrical air cavity with a nominal volume of 0.6 𝑐𝑐. The cavity radius is ≈ 0.3 𝑐𝑚. : Polytrichlorofluoroethylene (PTCFE)
  34. 34. Sensitivity is directly proportional to the chamber sensitive volume The sensitivity of a Cutie Pie survey meter (600 𝑐𝑐) used for measuring low-level 𝑥 rays is approximately 1,000 times the sensitivity of a Farmer-type thimble chamber (0.6 𝑐𝑐) used for calibration of a treatment beam. It is the ionization
  35. 35. A chamber is known to have stem leakage if it records (the ionization produced anywhere other than chamber sensitive volume (chamber stem and cable)) It depends on: • Chamber design • Beam quality (modality and energy) • Irradiation conditions (the extent of stem and cable exposed to radiation) Fully guarded Farmer-type chambers have almost immeasurable stem leakage. However, the stem leakage must be checked periodically. Measurements are made with the chamber oriented in each of the two positions shown. A number of points in the field are selected for such measurements and correction factors are obtained as a function of the stem length exposed in the field relative to the length of the stem exposed during calibration
  36. 36. It is the voltage difference between the electrodes of an ion chamber Saturation Current  As the bias voltage is increased from zero, the ionization current increases at first almost linearly and later more slowly. The curve finally approaches a value for the given exposure rate.  The initial increase of ionization current with voltage is caused by incomplete ion collection at low bias voltages (ion recombination). This recombination can be minimized by increasing the field strength (𝑉/𝑐𝑚).
  37. 37. If for a given exposure, the , then the chamber is said to show a polarity effect  Major causes of polarity effect: High-energy electrons (Compton electrons) ejected by high-energy photons constitute a current (Compton current) independent of air cavity ionization It is collected outside the sensitive volume (stem, cable and inadequately screened collector circuit points)  Magnitude of polarity effect depends on: • Chamber design • Beam energy and modality • Adequacy of bias voltage • Irradiation conditions  The errors caused by the polarity effect can be minimized but not eliminated by reversing the chamber polarity and taking the mean value of the collector current The difference between collector currents measured with positive and negative polarity should be < 0.5% for any radiation beam quality
  38. 38. It is the ratio of the number of ions collected to the number of ions produced  The chamber bias voltage should be within the saturation region to minimize recombination losses under the conditions such as: • Chamber design • At very high ionization intensity In the case of pulsed beams, significant loss of charge by recombination may occur even at bias voltages in the saturation region. Correction has to be applied for these losses. (To determine ion recombination correction 𝑃𝑖𝑜𝑛) Select two bias voltages 𝑉1 𝑎𝑛𝑑 𝑉2 such as 𝑉2 = 𝑉1 2 and take electrometer reading with them. The ratio of the two readings (𝑄1 𝑄2) is related to 𝑃𝑖𝑜𝑛. 1.02 1.04 1.06 1.08 1.10 1.12 1.14 1.10 1.20
  39. 39. The calibration laboratories provide chamber calibration factors for reference environmental conditions of temperature and pressure . If the ion chamber is open to the atmosphere, its response is affected by temperature and pressure of the air in the chamber cavity
  40. 40. 1. Minimal variation in sensitivity or exposure calibration factor over a wide range of photon energies 2. Suitable volume to allow measurements for the expected range of exposures (Sensitivity is directly proportional to the chamber sensitive volume) 3. Minimal variation in sensitivity with the direction of incident radiation 4. Minimal stem leakage 5. Calibrated for exposure against a standard instrument for all radiation qualities of interest 6. Minimal ion recombination losses (sufficient voltage, typically 300 𝑉)
  41. 41. Absorbed Dose Determination in External Beam Radiotherapy:
  42. 42.  Medium energy X-rays above 80 𝑘𝑉 and an HVL of 2 𝑚𝑚 𝐴𝑙  𝐶𝑜60 𝛾-radiation  High energy photon beams  Electron beams with energy above 10 𝑀𝑒𝑉 approximately  Therapeutic proton and heavy ion beams  Volume : 0.1 𝑐𝑐 𝑡𝑜 1 𝑐𝑐 (this size range provides sufficient sensitivity and point dose measurements)  Internal diameter : ≤ 7 𝑚𝑚  Internal length : ≤ 25 𝑚𝑚 • Chamber must be aligned in a way such that ; it should equilibrate rapidly with the ambient temperature and air pressure
  43. 43.  All electron energies (below 10 𝑀𝑒𝑉 their use is mandatory)  Low energy 𝑥-rays  Photon beams, only when a calibration in terms of absorbed dose to water is available  Proton and heavy ion beams (specially for beams having narrow SOBP)  At the center of inner surface of entrance window for all beam qualities and depths • For the requirements concerning scattering perturbation effects and effective point of measurement, the  The ratio of cavity diameter to the cavity height should be 5 or more (cavity height should be ≤ 2 𝑚𝑚)  Diameter of the collecting electrode should be ≤ 20 𝑚𝑚; to reduce the influence of radial non- uniformities of the beam profile  Width of guard electrode not smaller than 1.5 times the cavity height  Thickness of the front window should be 0.1 𝑔 𝑐𝑚2 or 1 𝑚𝑚 of PMMA; to make measurements at shallow depths possible

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