Topic 5' Counting Systems I

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Topic 5. Counting Systems (I)

• NaI(Tl) well counter
• Liquid and gas flow systems

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NaI(Tl) Well Counter

• Detector characteristics
• Detection Efficiency
• Sample volume effects
• Assay of absolute activity
• Shielding and background
• Energy calibration
• Multiple radionuclide source counting
• Deadtime
• Automatic multiple-sample systems
• Applications

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Detector Characteristics

• Standard well counter (4.5x5cm crystal and
  1.6x3.8 well)
• Background shielding (gt5cm lead)
• Energy resolution 10-15 FWHM for 137Cs (poor
  light transfer between NaI(Tl) crystal and PM
  tubes-- light reflection and scattering within
  well surface)

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Well Counter
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Detection Efficiency

• High geometric efficiency (near 100)
• Good for counting of small sample activity
• Intrinsic efficiency e depends on crystal
  thickness and ? ray energy (e100 for 1.3-4.5
  well thickness and ? ray energy less than 150
  keV).

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Detection Efficiency (2)

• Photo-peak fraction decreases with increasing ?
  ray energy (scattering increases as ? ray energy
  increased within 0-2 MeV)
• Photo-peak fraction increases with increasing
  well detector size (more multiple scattered
  photons added into the photopeak)

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Detection Efficiency (3)

• Intrinsic photopeak efficiency is the product of
  intrinsic efficiency and photo-fraction epefp
• Intrinsic photopeak efficiency may be used to
  estimate the photo-peak counting rate.

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Sample Volume Effects

• Geometric efficiency of the well counter depends
  on the sample positioning
• Geometric efficiency will decrease if the sample
  is displaced on top of the well.
• For a constant activity, increase the volume by
  diluting the solution will decrease the counting
  rate
• For a constant activity concentration, increase
  the volume will increase the counting rate and
  reach a maximum when the sample exceeds the top
  of the well.

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Assay of Absolute Activity

• Two methods are used for the determination of
  absolute activity from the counting rate
  calibration table and calibration (standard)
  source
• Long-lived radionuclides are used as calibration
  (mock) source 137Cs for 131I, 129I for 125I
  and 57Co for 99mTc.
• Samples absolute activity X is given by X
  ?A(mock)R(sample)/R(mock), where A is mock
  activity and ? the ratio of emission frequencies

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Shielding and Background

• External sources of background radiation are
  minimised by surrounding the detector with lead
• The thickness of the lead is typically 2.5-7.5cm.

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Energy Calibration

• Single channel analyzer attached to well counter
  is generally calibrated with 137Cs (1000
  divisions for 1Mev)
• Adjust the amplifiers gain until maximum
  counting rate is observed with the window setting
  from 642 to 682 (662 is the centre).
• 137Cs calibration is not accurate for energies
  below 100 keV because the non-linear energy
  response of NaI(Tl) detectors (re-calibration
  with low energy source)

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Multiple Radionuclide Source Counting

• There is a crosstalk interference if multiple
  radionuclide source is counted simultaneously.
• Correction is required in order to determine the
  true counts for each radionuclides photopeak.
• The count ratios of its photo-peak to other
  radionuclides photo-peak energies should be
  measured by using single radionuclide sample and
  for each radionuclide involved.
• For a two radionuclides system, the true counts
  for each radionuclide are Nt1(N1-R12N2)/(1-R12R21
  ) and Nt2(N2-R21N1)/(1-R12R21) where R12 and R21
  are the count ratios of pure sample at these two
  photo-peak energies (R12 from pure sample 2 etc.)

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Deadtime

• Deadtime could be easily a problem for well
  counter because its high detection efficiency
• For a small activity of 1 µCi with a deadtime 4
  µsec, about 5,000 cps could be lost if the
  detection efficiency is 100 (use
  RtRtexp(Rtt)).
• Well counter is therefore good for detection of
  small radioactivity.

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Automatic Multiple-Sample Systems

• Automatic multiple sample systems are necessary
  for counting large number of samples or repeated
  tests
• The main problem of the multiple sample well
  counters is the background shielding on top of
  the wells
• SCA, MCA and computers are all being used for the
  interface with the detectors.

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Multiple-Sample System
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Multi-Sample Through-Hole System
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Relative Efficiency
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Applications

• NaI(Tl) well counters are mainly used for in
  vitro assay in nuclear medicine (? or x rays)
  radioactivity in blood and urine sample,
  radiopharmaceutical quality control etc.
• NaI(Tl) can also be used for ß emission by
  detecting bremsstrahlung radiation.

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Liquid and Gas Flow Counting

• NaI(Tl) well counters can be used in conjunction
  with gas or liquid chromatographs (identification
  of different chemical compounds)
• Chromatographs are used to separate and identify
  different chemical compounds by selective
  retention or movement of chemical species in
  certain media
• Different radioactive species are identified by
  comparing radioactivity with the flowing chemical
  species (mass signals).

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