ionization chambers ii

Ionization Chambers II

Cavity Ionization Chambers

Cavity Ionization Chambers

• Cavity ionization chambers come in many varieties, but basically consist of a solid envelope surrounding a gas- (usually air-) filled cavity in which an electric field is established to collect the ions formed by radiation

Cavity Chambers (cont.)• Cavity chambers offer several advantages over

free-air ion chambers:1. They can be made very compact, even for high-energy

use, since the range of the secondary electrons in the solid wall material is only ~10-3 as great as in atmospheric air

2. They can measure multidirectional radiation fields, while free-air chambers require nearly monodirectional beams aligned to pass perpendicularly through the aperture

3. Through the application of cavity theory, the absorbed dose can be determined in any material of which the cavity wall is made

Cavity Chambers (cont.)4. Cavity chambers are capable of great variety in design, to

permit dose measurements of charged particles and neutrons as well as photons. Free-air chambers are designed exclusively for x rays, mainly below 300 keV, and do not lend themselves to modification for other kinds of radiation

5. Gas cavities can be designed to be thin and flat to measure the dose at the surface of a phantom and its variation as a function of depth, or can be made very small to function as a probe to sample to dose at various points in a medium under irradiation

6. Collected charge can be measured in real time by connecting the chamber to an electrometer, or the chamber can be operated without cables if it is a condenser-type cavity chamber

Thimble-Type Chambers

• Spherical or cylindrical chambers (as shown schematically in the following diagram) having gas volumes of 0.1 – 3 cm3 are the most common forms of cavity ion chambers

• Such chambers, especially the spherical designs, are reasonably isotropic in their sensitivity to radiation except for attenuation in the connecting “stem”

Fully guarded spherical thimble-type cavity ionization chamber. Cylindrical types may be regarded as elongated

spherical chambers.

Thimble-Type Chambers (cont.)

• Conventionally such “thimble” chambers, as they are sometimes called, are irradiated at right angles to the stem axis when monodirectional beams are measured

• This not only avoids stem attenuation but also minimizes the length of the stem and cable that are irradiated, thus reducing the possible influence of radiation-induced electrical leakage in the cable insulation

Fully Guarded Chambers

• The high voltage (HV), usually 200-500 V, is shown in the previous diagram applied to the chamber wall, with the collector connected to the electrometer input at or near ground potential

• The insulator arrangement shown exemplifies a fully guarded ion chamber, by which is meant that electric current leaking through (or across the surface of) the HV insulator is intercepted by a grounded guard electrode (“guard ring”) that extends completely through the insulator assembly in the stem

Fully Guarded Chambers (cont.)

• Thus this current cannot reach the collector and affect the measured charge

• The inner insulator separating the collector from the guard electrode has practically no potential difference across it; thus little leakage occurs

Fully Guarded Chambers (cont.)

• The insulator-and-guard assembly is shown in this design to be covered by an overhanging lip of the chamber wall

• This is done to avoid instabilities caused by charge collection on the insulator surfaces

• Without this covering lip the ions from a substantial fraction of the chamber volume would be delivered to the guard electrode instead of the collector, and that fraction would strongly depend on the pattern of ionic charge stuck on the surface of the insulators

• The covering lip limits the affected gas volume only to the thin underlying crevice, thus practically eliminating this source of instability

Gas Flow

• A gas connector is also shown in the figure, allowing the chamber to be filled (and continuously replenished by flowing) with a gas other than air, or with pure dry air in place of ambient atmosphere

• This feature is not present in most designs, but is important for neutron dosimetry, where tissue-equivalent and other special gases are employed

Chamber Wall Thickness

• For dose measurements in fields of photons or neutrons under CPE or TCPE conditions, thus allowing relatability to Kc, thimble chamber walls should be made thick enough to

a) keep out of the cavity any charged particles that originate outside of the wall, and simultaneously

b) provide at the cavity an equilibrium charged-particle fluence and spectrum that is fully characteristic of the photon or neutron interactions taking place in the wall material

Wall Thickness (cont.)

• For photon fields the required wall thickness can be taken (conservatively) as being equal to the range of the maximum-energy secondary electrons set in motion by the photons in the wall itself or in other nearby media

• In this connection it should be remembered that photoelectrons from nearby high-Z beam collimators may be more energetic than the maximum-energy Compton-recoil-electrons generated in low-Z walls, in which case requirement (a) above is more stringent than (b)


• The ionization charge Q produced in the mass m of gas is related to the absorbed dose in the cavity gas Dg by

where (̅W/e)g for the gas has values which will be discussed in subsequent lectures

• Dg can in turn be related to the absorbed dose Dw in the inner layer of the wall through the application of appropriate cavity theory

g

g

Q WD

m e


• Dw is equal to (Kc)w under CPE conditions, and is proportional to it under TCPE conditions

• Thus the measurement can be related to the photon energy fluence or to the neutron fluence, for thick-walled ion chambers


• If the chamber is designed to measure the absorbed dose at a point of interest in a charged-particle field, the volume must be small, and the chamber wall must be thin, relative to the range of the incident particles

• This applies whether the charged particles constitute the primary beam or are generated in the surrounding media by photon or neutron interactions


• If the wall and cavity gas are approximately matched in atomic number, there will be a balance between rays escaping into the wall from the cavity gas and vice versa, assuming the wall is thick enough to provide such a -ray equilibrium

• For practical purposes a wall thickness of 15 mg/cm2 (the range of a 100-keV electron) should suffice, as most rays resulting from electron-electron collisions have energies less than that


• For heavy charged-particle beams the rays are still lower in energy

• The optimal wall thickness for charged-particle beam measurements is too thin for practical construction as a thimble chamber, and flat pillbox designs with thin plastic film windows suggest themselves


• Assuming -ray equilibrium, and assuming here that the small chamber accurately samples the charged-particle field without perturbing it, the dose in the cavity gas can be related to that in the irradiated medium at the point of measurement through application of B-G cavity theory, employing an average ratio of collision stopping powers evaluated for the spectrum of incident charged particles (excluding rays, since they are taken to be in equilibrium)

Chamber Wall Material• Air is a medium of special interest for photon

dosimetry because of its role as the reference medium for the definition of exposure and its convenience as an ion-chamber gas

• So-called “air-equivalent” chamber wall materials are often used

• Air equivalence of the wall requires not only the matching of its mean mass energy-absorption coefficient to that of air for the photon spectrum present, but also the corresponding matching of the mean mass collision stopping powers for the secondary-electron spectrum present

Chamber Wall Material (cont.)

• These requirements cannot in general both be satisfied simultaneously, except that they are reasonably compatible where Compton effect is the dominant mode of photon interaction

• If the photoelectric effect is important, its Z-dependence is so much stronger than that of the stopping power that the latter matching requirement is disregarded


• A less rigorous but more common statement of chamber-wall air equivalence with respect to photons is provided by the effective atomic number ̅Z, which must be further specified for the type of photon interaction being considered


• For photoelectric effect the formula for ̅Z has the form

where

is the fraction of the electrons present in the mixture that belong to atoms of atomic number Z1, and so on; f1 is the weight fraction of that element present; and m has a value of about 3.5

• On this basis ̅Zair is found to have a value of 7.8

1 1 2 2m mmZ a Z a Z

1 1 1 1/ / /i i iia f Z A f Z A


• For dosimetry in charged-particle beams, the mean mass collision stopping power, derived by use of elemental weight fractions as weighting factors, is the most relevant quantity to be matched between the gas, wall, and reference media

• The average charged-particle energy obtained from

is adequate to represent the charged-particle spectrum for this purpose

max

0

1

T

TT T dT


• Inasmuch as the wall must serve as an electrode, it must be electrically conducting, at least on the inside surface

• Various plastics that are often employed as ion-chamber wall materials are generally electrical insulators; hence they need application of a conducting layer

• Some special materials, such as A-150 tissue-equivalent plastic, are made volumetrically conducting as a result of incorporation of graphite during manufacture

Chamber Wall Material (cont.)• The ion-collecting rod in a thimble chamber should

be made of the same material as the wall if possible, as cavity theories do not deal with inhomogeneous wall media

• However, the surface area of the rod is usually so much less than that of the wall that it will not have much influence unless the interaction cross sections in the rod are much larger than in the wall– 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

Insulators

• Polystyrene, polyethylene, and Teflon are all excellent electrical insulators for ion-chamber use

• Most other common plastics, such as PMMA, Nylon, and Mylar, are also acceptable in most cases

• Teflon in particular is more readily damaged by radiation than the others, and should be avoided where total doses exceeding ~104 Gy are expected

• However, its smooth “waxy” surface is the most tolerant of humidity in the air without allowing leakage currents to pass across

Insulators (cont.)

• Except for radiation-induced volumetric electrical leakage, most observed leakage is a surface phenomenon that is minimal for clean, polished surfaces and worsens where dirt and/or humidity are present

• A fiber or hair bridging an insulator often is the cause of such leakage, and a rubber syringe should be kept on hand for blowing away such debris– The breath is too humid for this purpose

Insulators (cont.)

• One should avoid touching an insulator, especially with the fingers, as skin oil causes persistent leakage and is difficult to remove

• Pure ethyl or methyl alcohol is sometimes helpful in cleaning insulators by wiping the surface with a cotton swab, then drying with a syringe

• After such attempts one should not expect instant improvement; several hours may be needed for the insulator to return to normal

Insulators (cont.)• Mechanical stress of an insulator (e.g., bending a

cable) can cause apparent leakage currents due to polarization effects, and rubbing the surface of an insulator can produce surface charges by the triboelectric effect that may take a long time to dissipate, during which leakage currents will be observed

• The forward projection of electrons in high-energy photon interactions can transport charge through an insulator and thus cause a high potential difference to develop between electrodes of a capacitor

Insulators (cont.)• Charged-particle beams incident on a thick insulator

will build up charge wherever the particles stop at the ends of their paths

• When large blocks of insulating plastics such as acrylic or polystyrene are used as phantoms and irradiated to high doses by electron beams, the charge buildup due to stopped electrons may cause electric fields strong enough to influence the paths of primary or secondary electrons in the medium

• This condition can persist for hours or even days, distorting the dose distribution in subsequent photon or electron irradiations

Condenser-Type Chambers

• It is sometimes advantageous to design a thimble chamber to operate without external connections while being irradiated

• One option for accomplishing this is to connect the chamber electrodes in parallel with a capacitor, built into the stem of the chamber as shown in the following diagram

Schematic diagram of a Victoreen-type condenser ion chamber. Ions are produced in both of the air compartments, but there is no electric field to collect ions from the stem compartment at left, which behaves like a Faraday cage.

Condenser-Type Chambers (cont.)

• The capacitor (and chamber) are then charged up by temporarily connecting them across a potential P1 (typically 300V), which establishes an electric field in the chamber

• When the chamber is irradiated, the positive ions are drawn to the wall and the negative ions to the collector (for the polarity shown in the diagram)

• Thus the charge stored in the capacitor-chamber combination is diminished, and the potential is decreased to a new value P2


• If the combined capacitance is C, the charge collected from the chamber during irradiation is

Q is most accurately determined as the difference between the charge Q1 measured by connecting the unirradiated device across the input of a high-gain charge-integrating electrometer and the charge Q2 similarly measured after irradiation

• The radiation sensitivity of such a chamber is directly proportional to the chamber volume, and inversely proportional to C

1 2 1 2Q Q Q C P P


• If the final voltage P2 is allowed to fall too low, recombination of charge in the chamber can cause the collected charge Q to be significantly less than the charge produced by ionization of the gas in the chamber

• This can be detected by observing a lack of proportionality between Q and the irradiation time at a constant dose rate

Flat Cavity Chambers; Extrapolation Chambers

• Flat cavity chambers have several special advantages:

1. They can be constructed with thin foils or plastic membranes for one or both of the flat walls, causing only minimal attenuation or scattering of incident electrons or soft x-rays

2. The gas layer can be made as thin as 0.5 mm, allowing sampling of the dose with good depth resolution, especially advantageous in regions where the dose changes rapidly with distance

Flat Cavity Chambers (cont.)

3. In some designs the thickness of the gas layer is made variable, for example by an adjustable screw, thus allowing extrapolation of the ionization per unit gas-layer thickness to zero thickness

– This in effect removes the influence of perturbation due to the presence of a finite cavity in a phantom, for example, and further increases resolution of dose vs. depth

4. The dose at the surface of a phantom can be measured by extrapolation, and the buildup vs. depth can be observed by adding thin sheets of phantom medium over the entrance foil

Flat Cavity Chambers (cont.)• On the other hand, flat-geometry chambers are

generally more complicated in design than thimble chambers, and more difficult to construct

• Boag devised the chamber shown in the following diagram for electron-beam dosimetry

• As shown, it contains three graphite-coated mica foils, but thinly aluminized Mylar would do as well

• Capability for extrapolation of the air-layer thickness is not provided, but could be by using spacer rings or machining a screw around the rim

Ionization chamber for dosimetry of fast-electron beams


• Such a chamber can be used to study surface dose enhancement due to electron backscattering from a phantom, for example, since it contains no thick electrodes

• The collecting electrode in this chamber is insulated from the surrounding guard electrode by a clean scratch through the colloidal graphite coating on one side of the middle foil


• Notice that in this and most other flat chamber designs, the guard electrode serves primarily to provide a uniform electric field, thus allowing the radius of the collecting volume to be defined by the collecting-electrode radius plus the half-width of the insulating scratch or groove around it

• In some flat-chamber designs the guard ring also stops leakage currents from the HV electrode, as in fully guarded thimble chambers


• Guarded flat chambers can be viewed as plane capacitors having a capacitance proportional to the area of the collecting volume, and inversely proportional to the plate separation

• A simple measurement of the chamber’s capacitance can provide a check on the mechanical determination of the collecting volume:

where the numerical constant has units of F/cm

148.85 10Q a

CP s


• Commercially available flat chambers used to measure surface dose and dose buildup have been commonly designed with a thin foil entrance wall, but a thick conducting back wall comprising the collecting electrode and the surrounding guard electrode, as schematically shown in the following diagram

Schematic diagram of a flat chamber with thick back wall of conducting material, illustrating the cause of polarity differences observed in the measured output current resulting from radiation


• When such a chamber is placed in a -ray beam, electrons are knocked out of the back electrode by the Compton effect, constituting a positive current entering the electrometer

• If positive voltage is applied to the front foil, positive ions will arrive at the collecting electrode, adding to the Compton current

• For negative applied voltage the negative ions are collected, and the net negative current sent to the electrometer is the difference between the ion current and the Compton current


• Thus the true ion current may be obtained as the average of the currents measured with the two HV polarities

• This effect is most pronounced for a small plate separation and a thin front wall

• As the front-wall thickness is increased, an equilibrium is gradually established for the electrons entering and leaving the collecting electrode, and the inequality between polarities disappears

• With charged-particle beams a comparable, but more complicated, effect is observed when the particles stop in the collecting electrode, or knock out rays


• These problems can be avoided by using chamber designs such as that of Boag or the one shown in the following diagram

• The latter, recommended by the NACP, has a thin foil collector supported by (but insulated from) a thicker wall

• Few charged particles can start or be stopped within such a thin collector

Flat chamber designed not to exhibit polarity-difference effects. The collecting electrode is very thin (< 0.1 mm) and is mounted on a thin insulating layer ( 0.2 mm).


• Another kind of problem that may arise from faulty design of any type of ion chamber, but is more likely to affect flat chambers, is extracameral ionization, i.e., ionization that is collected from air spaces outside of the designated collecting volume

• Such unwanted contributions of ionization can drastically affect the outcome of an experiment if unnoticed, especially in extrapolation chambers that are supposed to approach zero volume

Extracameral Ionization• In (a) a flat chamber is shown, including an

insulating plate painted on both sides with colloidal graphite, and a circular scratch made to separate the collector C from the guard ring G

• A bare wire is shown attached to the collector and leading out to a coaxial-cable connection at the side, and thence to the electrometer input

• Since the radiation beam also irradiates the guard-ring area, air ions as shown (assuming +HV) may be collected by electric lines of force terminating on the wire, thus contributing measured charge from a region outside of the collecting volume

Extracameral Ionization (cont.)• In (b) a similar design is shown, except that now the

wire passes from the collector through the insulating plate and out through a bare spot on the grounded graphite back surface, then to a coaxial connection at the side, leading to the electrometer

• All conductors have surface contact potentials, some as great as ~1 V

• The difference in their magnitudes creates a weak electric field in any gas space between dissimilar surfaces

• Some of the ions created behind the chamber by the radiation field may be collected on the wire

Extracameral Ionization (cont.)

• The resulting extracameral charge may be considerable

• In this case, the effect can be easily detected by HV polarity reversal, since the extracameral ion collection is unaffected

• Thus it adds to the current in one polarity and subtracts in the other, and the ±average gives the correct current without the extracameral component

Extracameral Ionization (cont.)

• In (c) the wire is shown sealed inside the insulating plate itself until it reaches the coaxial connector, and (d) shows the coaxial cable connecting directly to the back of the insulating plate

• In either of these cases little or no extracameral effect will be observed

Transmission Monitor Chambers

• When radiation generators are not constant with time, due to power-line fluctuations for example, some kind of monitoring ionization chamber may be employed to allow normalization of results by dividing all radiation measurements by the corresponding monitor readings

Monitor Chambers (cont.)

• A thimble chamber can be used for this purpose, by simply positioning it at a convenient fixed location in the beam

• However, a thin flat chamber through which the beam passes on its way to the point of measurement has the advantages that it can be permanently installed and that it can monitor specifically the segment of the beam that is of greatest interest, or can monitor the whole beam if preferred


• A transmission chamber suitable for x-ray beam applications, rugged, and simple to construct is shown in the following diagram

• This chamber should be well vented to the atmosphere to avoid plate distortion due to changes in barometric pressure

• Relatively thick Lucite plates are shown, as they simplify construction by being self-supporting, but stretched foils could be substituted for electrons or soft x-rays

Simple design for a transmission ionization chamber. The size is optional, but the HV electrode should be larger in diameter than the ion collector, which in turn should cover the beam area to be monitored.


• Electrical contacts are made by bronze leaf springs that press against the inner colloidal graphite coatings when the plates are fixed in place

• The graphite coatings on the outside surfaces are both grounded by contact with the aluminum rim

• Electrical insulation for both the HV and collecting electrodes is provided by a border of bare Lucite around the edge, separating the graphited areas from the supporting rim

ionization chambers ii

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