Ionization Chambers II

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Ionization Chambers II

• Cavity Ionization Chambers

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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

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Cavity Chambers (cont.)

• Cavity chambers offer several advantages over
  free-air ion chambers
• 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
• They can measure multidirectional radiation
  fields, while free-air chambers require nearly
  monodirectional beams aligned to pass
  perpendicularly through the aperture
• Through the application of cavity theory, the
  absorbed dose can be determined in any material
  of which the cavity wall is made

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Cavity Chambers (cont.)

• 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
• 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
• 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

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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

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

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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

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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

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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

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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

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

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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)

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Wall Thickness (cont.)

• 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

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Wall Thickness (cont.)

• 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

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Wall Thickness (cont.)

• 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

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Wall Thickness (cont.)

• 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

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Wall Thickness (cont.)

• 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

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Wall Thickness (cont.)

• 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)

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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

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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

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Chamber Wall Material (cont.)

• 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

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Chamber Wall Material (cont.)

• 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

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Chamber Wall Material (cont.)

• 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

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Chamber Wall Material (cont.)

• 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

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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

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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

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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

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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

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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

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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

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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

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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.
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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

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Condenser-Type Chambers (cont.)

• 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

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Condenser-Type Chambers (cont.)

• 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

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Flat Cavity Chambers Extrapolation Chambers

• Flat cavity chambers have several special
  advantages
• 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
• 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

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Flat Cavity Chambers (cont.)

• 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
• 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

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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

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Ionization chamber for dosimetry of fast-electron
beams
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Flat Cavity Chambers (cont.)

• 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

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Flat Cavity Chambers (cont.)

• 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

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Flat Cavity Chambers (cont.)

• 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 chambers capacitance
  can provide a check on the mechanical
  determination of the collecting volume
• where the numerical constant has units of F/cm

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Flat Cavity Chambers (cont.)

• 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

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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
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Flat Cavity Chambers (cont.)

• 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

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Flat Cavity Chambers (cont.)

• 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

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Flat Cavity Chambers (cont.)

• 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

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

• 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

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(No Transcript)
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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

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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

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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

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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

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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

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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

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Monitor Chambers (cont.)

• 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

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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.
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Monitor Chambers (cont.)

• 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