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									Proportional Detector (Counter)

         Noreen Marwat
            NORI, Isb
• These detectors operate in region III of Fig.
  8.3, where there is a built-in amplification
  (~106) of the primary ionization through the
  production of secondary ionization.
• A sufficient amount of current is produced by
  a single ray for it to be counted, and the
  current is directly proportional to the energy
  of the radiation.
• Hence, a proportional detector, unlike the
  ionization chamber, can be used to count
  individual events and to determine the energy
  of the radiation
• Proportional counters require a sufficient
  amount of expertise in their construction and
  use. Their stability, with time and voltage
  fluctuations, is not as good as that of the
  ionization chambers.
• Proportional counters are rarely used in
  nuclear medicine.
Geiger-Mueller Detector (Counter)
• The operating voltage of GM detectors is in the region
  IV.
• The incoming particle in this case causes a discharge in
  the gas, and the amount of the current produced is
  more or less independent of the energy of the
  radiation and of the voltage.
• Once the discharge is established in the gas by a ray,
  how does one stop the process so that the detector is
  ready for the next ray? This is accomplished chemically.
  Small amounts of halogens or their organic compounds
  are introduced in the gas as impurities.
• These impurities, known as chemical quenchers,
  absorb the ultraviolet light produced during the
  discharge and the energy from the secondary ion pairs.
  Absorption of energy by the quencher molecules
  causes the discharge to stop but leads to their
  dissociation.
• However, within a short time, most quencher
  molecules are able to recombine to form the original
  molecule. Therefore, there is little depletion of the
  quencher. It takes from about 50 to 200 µsec to quench
  the discharge. During this time, the GM counter does
  not respond to another radiation; therefore, this is
  approximately the dead time of the detector and is
  nonparalyzable.
• The maximum usable count rate for a typical GM
  counter is 60,000 counts/min. GM detectors are the
  most sensitive of the gas-filled detectors.
 Scintillation Detectors (Counters)
• A variety of substances known as phosphors
  scintillate (produce light) under the influence
  of high-energy radiation.
• This property is used for the detection of
  radiation by instruments known as
  scintillation detectors
• for the detection of the light generated in a
  material, it should be able to be transmitted
  out of the material itself.
• In liquids, it poses no serious problem.
• For solids, only single crystals or glasses can
  be used because in powders (microcrystals),
  light is absorbed and scattered at the
  boundaries of the microcrystals, thereby
  causing significant and variable loss of light
  before its detection.
• Two other types of substances,
  thermoluminescent and photoluminescent,
  are also used as radiation detectors.
• These substances do not produce light
  immediately after interacting with radiation
  but can store some of the energy given by the
  radiation for a long time
                Scintillator
• The number of scintillating substances
  (scintillators) is large. Anthracene,
  naphthalene, various plastics, alkali halide
  crystals doped with or without impurities such
  as NaI(Tl) or CsF, lead tungstate, bismuth
  germanate, and aromatic compounds such as
  terphenyl can be used as scintillators.
   Mechanism of Light Production
• Because the mechanism of light production in a
  substance under the influence of radiation is
  complex and not well understood, there are no
  theoretical laws to predict the behavior of a given
  substance in this regard. Briefly, a γ-ray loses its
  energy in a scintillator through the photoelectric,
  Compton, or pair-production mechanisms. The
  electrons thus generated then lose their energy
  within short distances through ionization and
  excitation of the scintillator molecules
• The ion pairs thus produced then combine
  among themselves or with other atoms or
  molecules of the scintillator and produce
  certain excited states that, during their
  subsequent decay, emit light. The nature and
  quantity of these excited states determine the
  amount, color, and phosphorescent decay
  time of light.
       Properties of Scintillator
• A scintillator is primarily characterized by
• intrinsic efficiency,
• amount of light produced per unit of absorbed
  energy (light conversion efficiency),
• and the time in which emission of light takes
  place (phosphorescent decay time).
• The intrinsic efficiency of a scintillator for a γ-ray
  of a given energy depends on its linear
  attenuation coefficient, which in turn depends on
  the atomic number and the density of the
  material
• The amount of light produced per unit of
  absorbed energy, which in effect determines the
  energy resolution of these detectors, and the
  phosphorescent decay time, which is an
  important parameter for the dead time of these
  detectors, vary from substance to substance
• Light in a scintillator is produced in a very small
  volume, mainly determined by the range of
  photoelectrons or Compton recoil electrons produced
  in the scintillator.
• For energies of x- or γ-rays less than 1 MeV, this range
  does not exceed more than a millimeter in a NaI(Tl)
  crystal or other scintillators used in PET. From this small
  volume of production, light travels in all directions.
• The direction of most of the light toward the PM tube
  is achieved by coating the outside surface of the
  scintillator, except the side facing the PM tube, with a
  light reflector such as magnesium oxide. The coupling
  of the crystal assembly to the PM tube is also
  important because of possible light loss at the interface
  of the crystal and PM tube. This loss is generally
  minimized by the use of optical grease.
  Scintillator for Scintillation Camera,
                  NaI(Tl)
• Of all the known scintillators, sodium iodide
  crystals doped with small amounts of
  thallium, NaI(Tl), are most widely used in
  nuclear medicine, particularly in the
  scintillation camera. Its moderate density (d =
  3.67 g/cm3) and effective atomic number (Zeff
  = 45) make it very efficient for the detection of
  x- or γ-rays in the energy range of 30-500 keV
• The amount of light produced per unit of absorbed energy
  in a NaI(Tl) crystal is one of the highest, despite the fact
  that sodium iodide crystals without thallium doping do not
  produce much light. The presence of small amounts of
  thallium (one part in 106) enhances the light output by a
  factor of 10 or more. The phosphorescent decay time,
  which eventually determines the dead time of a
  scintillation detector, is ~0.23 µsec and is adequate for the
  amounts of radioactivities presently used in nuclear
  medicine. In addition, the technology required to grow
  these crystals in large sizes and various shapes is well
  advanced, making their use more economical compared
  with many other scintillators.
           Scintillators for PET
• Since in PET, the energy of the radiation to be
  detected is very high, 511 keV as opposed to 140
  keV for 99mTc,
• NaI(Tl) is not a good choice as a scintillator
  material as its detection efficiency drops off
  rapidly with the energy of γ ray.
• Instead, another scintillator, Bismuth Germanate,
  commonly known as BGO, has been the
  scintillator of choice.
•
• BGO has a big advantage over NaI(Tl) in the
  detection efficiency at 511 keV. However, its light
  output/keV of absorbed energy, which
  determines its energy resolution, is very low (1/7
  that of NaI) and therefore it has poor energy
  resolution compared to a NaI(Tl) detector.
• Poor energy resolution means poor scatter
  rejection Also, its phosphorent decay time is
  slightly longer than NaI making its dead time
  slightly longer than that of the NaI(Tl) detector
Associated Electronics
                    PM Tube

• The amount of light produced in NaI(Tl)
  crystals or any other scintillator is quite small
  relative to that which the human eye can
  easily detect
• A PM tube is a light-sensitive device that converts light into
  measurable electronic pulses. It consists of a photocathode
  facing the window through which light enters, a series of
  metallic electrodes known as dynodes arranged in a special
  geometric pattern, and an anode—all enclosed in vacuo in
  a glass tube. When the light photon hits the photocathode,
  it produces an electron of low energy (0.1-1 eV) through
  photoelectric interaction. This photoelectron is then
  accelerated toward a dynode by the application of a voltage
  (between 50 and 100 V) to that dynode. As a result of this
  acceleration, the electron acquires sufficient kinetic energy
  (50-100 eV) to produce a number of secondary electrons
  when it collides with the dynode
                    Preamplifier
• The electric pulses arriving at the anode of a PM tube are
  small (microampere) and therefore require amplification to
  several volts before they can be further analyzed or
  processed. The output of a PM tube, however, cannot be
  directly fed into an amplifier because of the wide difference
  in the output impedance (an important electronic
  parameter) of a PM tube and the input impedance of an
  amplifier that results in signal distortion and attenuation. A
  preamplifier is a device that solves the problem of
  impedance mismatch. It is located next to the PM tube
  because long connecting cables at this stage also attenuate
  the signal. Output of the preamplifier, however, can be sent
  many feet away from the detector assembly without
  attenuation.
              Linear Amplifier
• A linear amplifier amplifies the incoming
  pulses in a direct proportion. The ratio
  between the amplitudes of the outgoing and
  incoming pulses is known as amplifier gain.
  The amplification can be changed by adjusting
  the gain control G specifically provided for this
  purpose.
     Pulse-Height Analyzer or Single-
            Channel Analyzer
• A pulse-height analyzer PHA or single-channel analyzer
  SCA is an electronic device that accepts the pulses
  whose voltage (height) lies between a preselected
  range and rejects the pulses whose voltage lies outside
  this range. An SCA is provided with two controls known
  as lower level and window that determine the range of
  the pulses to be selected. For example, if pulses with
  voltages varying from 1 to 10 are being produced by a
  linear amplifier in response to an x- or γ-ray, to select
  pulses whose voltage lies between 5 and 6, it is
  necessary to set the lower level of SCA at 5 V and the
  window at 1 V. The SCA will produce an output pulse
  only when the input pulses lie in this range
         Multichannel Analyzer
• A PHA or SCA selects only one range of pulse-
  height distribution. In a multichannel analyzer,
  there are many PHAs (1000 or more) that can
  simultaneously separate the pulses into
  multiple ranges; therefore, the whole
  spectrum of pulse-height distribution can be
  measured at one time.
             Scaler and Timer
• These devices comprise an electronic counter
  used for counting the pulses coming from an
  amplifier or SCA either for a specific time
  interval or until a predetermined number of
  counts has been collected. In either case, both
  the number of counts and the time during
  which they are collected are obtained.
                Rate Meter
• This is a device that, instead of giving the
  number of counts and the time period in
  which the counts were collected, yields
  directly the count rate (counts/min). With the
  ubiquitous use of computers, rate meters are
  now rarely used in nuclear medicine
  instruments.

								
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