Dosimetry by PulseMode Detectors II

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Dosimetry by Pulse-Mode Detectors II

• Scintillation Dosimetry
• Semiconductor Detectors for Dosimetry

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Scintillation Dosimetry Introduction

• Many transparent substances, including certain
  solids, liquids, and gases, scintillate (i.e.,
  emit flashes of visible light) as a result of the
  action of ionizing radiation
• By using a sensitive light detector such as a
  photomultiplier (PM) tube, the light emitted can
  be converted into an electrical signal

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Introduction (cont.)

• A light photon incident on the photocathode of a
  PM tube may release an electron, which is then
  numerically amplified as much as ? 107 in passing
  through the dynode chain in the tube
• Either the output electrical pulses from numerous
  such events can be counted (with or without
  pulse-height analysis), or the output current can
  be measured

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General schematic design for a scintillation
detector for dosimetry applications. A light
pipe optically couples the scintillator to the
photomultiplier tube. The scintillator is
otherwise enclosed in an optically opaque and
internally reflective envelope that may also
filter out short-range radiation and may
additionally serve as a CPE buildup layer for
indirectly ionizing radiation.
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Introduction (cont.)

• Scintillators have been widely applied as
  detectors of ionizing radiation, especially in
  nuclear physics
• Very fast decay times, down to 10-9 s, make
  organic liquid and plastic scintillators
  excellent choices for coincidence measurements
  with good time-resolution, and they can occupy
  whatever volume shape and size one wishes

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Introduction (cont.)

• Scintillators, especially NaI(Tl), also have been
  used extensively for x- and ?-ray energy
  spectrometry but have been largely replaced by
  Si(Li) and Ge(Li) semiconductor detectors for
  best energy resolution
• However, scintillators continue to be widely used
  for those ?-ray spectrometry applications in
  which resolution is less critical, because of
  lower cost and the greater convenience of
  operating the detector at room temperature
  instead of inside a cryostat

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Light Output Efficiency

• Only a very small part of the energy imparted to
  a scintillator appears as light the rest is
  dissipated as heat
• In typical situations 1 keV of energy is spent
  in the scintillator for the release of one
  electron from the PM tubes photocathode
• However, the large gain available in the PM tube
  and external amplifiers still provides an
  adequate output signal

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Efficiency (cont.)

• The light generated in a scintillator by a given
  imparted energy depends on the LET of the charged
  particles delivering the energy
• In typical organic scintillators, increasing the
  particle LET decreases the light output for a
  given energy imparted, as can be seen in the
  following diagram

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Light output vs. particle energy for electrons
and protons stopped in the plastic scintillator
NE-102. The light output is proportional to
electron energy, but not to proton energy.
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Efficiency (cont.)

• The light response from electrons that spend
  their full track length in the scintillator is
  found to be proportional to their starting energy
  above about 125 keV
• For protons, the light output is only about 15
  as great as for electrons at 1 MeV, rising to
  about 40 as great at 10 MeV
• The technique of pulse-shape discrimination
  allows the separation of dose components on the
  basis of particle LET

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Efficiency (cont.)

• For dosimetry of ?-rays or electrons, either the
  PM-tube output should be measured as an electric
  current or the pulse-heights must be analyzed and
  calibrated in terms of dose, as discussed for
  proportional counters
• Simple counting of pulses without regard to their
  size is not a measure of the dose in a
  scintillator

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Scintillator Types

• For most dosimetry applications where soft tissue
  is the dose-relevant material, organic plastic
  scintillators such as NE-102, organic liquids
  such as NE-213, and the organic crystals stilbene
  and anthracene are the most useful because they
  are made mostly of the low-atomic-number elements
  C and H
• Thus they do not overrespond to photons through
  the photoelectric effect, and the hydrogen
  content makes the (n, p) elastic-scattering
  interaction the main process for fast-neutron
  dose deposition, as it is in tissue

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Characteristics of some scintillators
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Light Collection and Measurement Scintillator
Enclosure

• A light reflector, optimally a thin layer of MgO
  powder, is useful to maximize light-collection
  efficiency from a scintillator
• If the scintillator has polished surfaces, all
  the light incident from the inside is reflected
  if the angle of incidence is greater than the
  critical angle
• The MgO reflector will recapture most of the
  light that escapes at smaller angles

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Scintillator Enclosure (cont.)

• For small or thin scintillators (plastic ones may
  be as thin as 20 ?m) one should keep in mind
  cavity-theory considerations
• Simplest dosimetric interpretation for indirectly
  ionizing radiation calls for surrounding the
  scintillator by a nonscintillating layer of the
  same composition, thick enough to provide CPE

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Scintillator Enclosure (cont.)

• In the case of plastic scintillators a shell of
  Lucite will usually suffice, surrounding the thin
  reflector
• Outside of this an opaque covering such as
  aluminum foil is required to exclude ambient
  light
• NaI and CsI scintillators require hermetic seals,
  as they are hygroscopic

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Light Collection and Measurement Light Pipe and
PM Tube

• The exit surface of a scintillator is optically
  coupled to the PM-tube photocathode through a
  light pipe, usually consisting of a solid
  cylinder of polished Lucite
• The interfaces are filled with an optical
  coupling agent such as high-viscosity silicone
  oil or transparent epoxy cement
• Ideally all materials along the optical path
  should have nearly the same refractive index as
  the glass face of the PM tube, ? 1.5

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Light Pipe and PM Tube (cont.)

• The main purpose of the light pipe in dosimetry
  is to remove the PM tube from the radiation field
  that the scintillator is measuring
• PM tubes are capable of responding to ionizing
  events occurring within their structure
• The interactions occur in different media than
  the scintillator, at different locations, and
  with variable gain factors
• Large doses can so damage a PM tube that its
  light sensitivity is permanently decreased

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Comparison with an Ionization Chamber

• Scintillators are often used as a more sensitive
  substitute for an ionization chamber in a ?-ray
  survey meter for health-physics applications
• It is instructive to consider what factors are
  involved in estimating the difference in current
  output from a scintillator and an ion chamber of
  the same volume

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Comparison with an Ionization Chamber (cont.)

• The analogue in a scintillator of W in a gas is
  the average energy spent by an electron per light
  photon produced
• For plastic scintillators this is around 60 eV
  (about twice that for gases)
• For good optical coupling 1/3 of the photons
  reach the photocathode (typical efficiency about
  15 and tube gain about 106)
• Thus for equal masses of chamber gas and plastic
  scintillator, the output current of the latter is
  3 ? 104 greater

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Pulse-Shape Discrimination

• In most scintillators, the promptly emitted light
  comprises nearly all of the observed
  scintillation
• In some materials a sizable longer-time-constant
  component exists that is LET-dependent
• Particles with denser tracks thus have a more
  pronounced component of longer decay time
  constant, as shown in the following diagram

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Time dependence of scintillation pulses in
stilbene, normalized to equal heights at time
zero, when excited by radiations of different LET
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Pulse-Shape Discrimination (cont.)

• Suitable electronic discrimination can be
  provided to count pulses of different lengths
  separately, correlated with the LET of the
  particles that produced them
• Thus it becomes possible to apply different dose
  calibrations to pulse heights for radiations
  having different LETs
• This feature is especially useful for dosimetry
  in combined neutron-?-ray fields

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Pulse-Shape Discrimination (cont.)

• Combinations of two different scintillators
  coupled to the same PM tube are useful for some
  dosimetry situations
• The scintillators chosen have different decay
  times so pulse-shape discrimination can be
  applied to separate the signals
• One thin scintillator can be used to stop a
  relatively non-penetrating component of radiation
  while a thicker scintillator behind the first
  interacts more strongly with more penetrating
  ?-rays

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Beta-Ray Dosimetry

• A plastic scintillator covered by a thin opaque
  window and coupled to the PM tube face can be
  used to measure the planar energy-flux density
  due to incident ?-rays, assuming that the
  scintillator is thick enough to stop them, and
  that the light output is proportional to ?-ray
  energy
• The distribution of dose vs. depth can be
  obtained from the reductions in light output
  observed when a series of tissue-equivalent
  plastic absorbing layers are placed over the
  front of the scintillator

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Semiconductor Detectors Introduction

• Semiconductor detectors have characteristics that
  make them very attractive as dosimeters, for
  measuring either dose or dose rate, as a
  substitute for an ion chamber
• They can also serve as a solid-state analogue of
  a proportional counter, since the ionization
  produced by a charged particle in traversing the
  sensitive volume of the detector is proportional
  to the energy spent, irrespective of LET, for
  particles lighter than ?s

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Introduction (cont.)

• Some internal amplification is even possible in
  the avalanche detector mode of operation, but
  external amplification is usually preferred
• The broad lack of LET-dependence is an advantage
  over scintillation detectors, allowing simpler
  interpretation of pulse heights in terms of
  energy imparted
• Semiconductor detectors may be employed as
  neutron dosimeters by measuring the resulting
  radiation damage done by the neutrons

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Basic Operation of Reverse-Biased Semiconductor
Junction Detectors

• The following diagram illustrates the operation
  of a typical reverse-biased semiconductor, the
  silicon p-n junction
• The bulk of the crystal consists of a p region
  having an excess of holes, while a thin layer
  at the surface is an n region having an excess
  of electrons
• Electrical conduction in each region occurs
  through motion of these majority charge carriers

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Reverse-Biased Detectors (cont.)

• Then a positive potential (10 103 V) is
  applied to the n-terminal relative to the
  opposite evaporated-metal surface contact,
  electrons and holes are pulled out of an
  intermediate region called the depletion layer,
  and current cannot then flow across the junction
  except for some leakage current
• If a charged particle passes through the
  depletion layer while the junction is in this
  reverse-biased condition, it forms electron-hole
  pairs by the usual collision processes

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Reverse-Biased Detectors (cont.)

• The mean energy spent per electron-hole pair in
  Si at 300 K is 3.62 eV for ?s and 3.68 for
  electrons, and in Ge at 77 K it is 2.97 eV for
  both
• These figures are only about one-tenth of the
  analogous W-values for gas ion chambers hence
  10 times as much ionization is formed in
  semiconductor detectors as in ion chambers for
  the same energy expenditure
• This also helps account for the good energy
  resolution of Si and Ge detectors

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Reverse-Biased Detectors (cont.)

• Electrons have mobilities of 1350 cm/s per V/cm
  in Si and 3900 in Ge, at 300 K
• Hole mobilities are 480 cm/s per V/cm in Si and
  1900 in Ge, at 300 K
• Thus typically they can reach the boundary of the
  depletion layer in 10-7 10-8 s, producing a
  comparable voltage-pulse rise time
• A charge-sensitive linear preamplifier and linear
  voltage amplifier comparable to those used for
  proportional counters, but with suitably shorter
  time constants, are used to amplify the charge
  pulses for charge measurement or pulse-height
  analysis and counting

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Silicon Diodes without Bias

• Although the sensitivity is greater and the
  response time is less for Si diode detectors with
  reverse bias applied, for DC operation there is
  an advantage in operating without any external
  bias
• As the bias voltage is reduced to zero, the DC
  leakage current decreases more rapidly than the
  radiation-induced current
• The residual zero-bias radiation-induced current
  results from alteration of charge-carrier
  concentrations, and in turn gives rise to a
  potential difference between the electrodes

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Design of unbiased silicon p-n junction
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Operation of unbiased silicon p-n junction
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Silicon Diodes without Bias (cont.)

• The ranges of dose rate that are measured in
  radiotherapy applications (0.03 3 Gy/min)
  produce adequate output currents from an unbiased
  silicon diode detector with a typical sensitivity
  of 2 ? 10-11 A per R/min

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Lithium-Drifted Si and Ge Detectors

• These are prepared by diffusing Li ions into
  high-purity (but slightly p-type) Si or Ge
  crystals
• The Li ions lodge at interstitial positions next
  to the electron-acceptor sites, then capture
  electrons to become electron donor sites, which
  thereby neutralize the acceptor sites
• The crystal is then said to be compensated, by
  having the same number of electrons in the
  conduction band as it has holes in the valence
  band

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Lithium-Drifted Detectors (cont.)

• In this condition it acts like an intrinsic
  material, that is, one that is free of all donor
  and acceptor sites, being almost completely pure
• Drifted regions up to almost 2 cm in thickness
  can be achieved in this way, and the entire
  intrinsic volume acts as the dosimeters
  sensitive volume
• Changing the applied potential varies the
  electric field strength across this volume, but
  doesnt change its depth

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Lithium-Drifted Detectors (cont.)

• Si(Li) and Ge(Li) detectors can be made as thin
  as 10 ?m to serve as dE/dx measuring devices
  for charged particles passing through, by which
  is meant that they respond proportionally to the
  collision stopping power of the material
  (ignoring ?-ray production)
• Likewise they can serve as thin dosimeters, or to
  measure LET distributions of charged-particle
  fields

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Lithium-Drifted Detectors (cont.)

• Ge(Li) detectors are preferred over Si(Li) for x-
  or ?-ray spectrometry above 50 keV, or for
  energy-fluence measurements, because the higher Z
  (32) of Ge gives it a greater photoelectric cross
  section than Si (Z 14), so that Ge stops the
  beam more efficiently
• Si(Li) detectors are preferred for lower-energy x
  rays and for ?-ray dosimetry because their
  backscattering is much less

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Lithium-Drifted Detectors (cont.)

• One disadvantage of Ge(Li) and Si(Li) detectors
  is that, to maintain their energy resolution for
  spectrometry, they must be maintained and
  operated at liquid-N2 temperature
• Allowing Ge(Li) detectors ever to warm up to room
  temperature deteriorates them by allowing the Li
  ions to migrate, thus disturbing donor-acceptor
  compensation
• Si(Li) detectors usually may be allowed to reach
  room temperature without damage, because of lower
  Li-ion mobility

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Use of Si(Li) as an Ion-Chamber Substitute

• The density of Si is about 2.3 g/cm3, or about
  1800 times that of air
• Thus, considering also the W difference, a
  Si(Li) detector will produce about 18,000 times
  as much charge as an ion chamber of the same
  volume, in the same x-ray field, at energies (gt
  100 keV) where the photoelectric effect is
  unimportant

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Use of Si(Li) Junctions with Reverse Bias as
Counting Dose-Rate Meters

• Si(Li) detectors 1 mm thick have been used as
  probes for measuring the depth dose due to heavy
  charged particles, including pions
• The pulse height was found to be proportional to
  the energy spent by the particle in the sensitive
  volume of the detector
• Dose vs. LET results have been consistent with
  those of a Rossi proportional counter

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Fast-Neutron Dosimetry

• Silicon detectors are damaged by very high doses
  (gt 104 Gy) of electrons or x rays, but are much
  more sensitive to damage by fast neutrons
• Doses of 0.1 to 10 Gy (tissue) cause permanent
  defects in the Si crystal lattice, which act as
  traps for charge carriers
• As a result the resistance of the detector is
  effectively increased
• The voltage drop across the detector when a
  constant test current is passed through it in a
  forward direction increases gradually vs. dose