Topic 4' Problems in Radiation Detection and Measurement

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Topic 4. Problems in Radiation Detection and
Measurement

• Detection Efficiency
• Problems in the detection and measurement of beta
  particles
• Deadtime

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Detection Efficiency

• The Definition
• Geometric Efficiency
• Intrinsic Efficiency
• Energy Selective Counting
• Absorption and Scatter

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The Definition

• Emission Rate (assume ? rays per disintegration)
  ?(?rays/sec)3.7104(dis/µCi.sec) A(µCi) ?
  (?rays/dis)
• Detection Efficiency (with R the recorded
  counting rate) DR/ ?

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Detection Efficiency

• Detection efficiency depends on a number of
  factors absorption and scatter (F), geometric
  efficiency (g), intrinsic efficiency (e) and
  energy-selective counting (f).
• It can be expressed as DFg e f

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Geometric Efficiency

• Inverse square law I?/4pr2 where I is the
  intensity of radiation per unit area and ? is the
  source emission rate.
• For point source, geometric efficiency is
  gpa/4pr2 where a is the detector surface area
  and r is the distance from the point source.

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Geometric Efficiency (small distance r)

• Assume the detector is close to the source (small
  r), the area of the detector is SOr2 where
  O2p(1-cos?).
• The geometric efficiency is then
  gpS/4pr2Or2/4pr2(1-cos?)/2
• When source in contact with the detector ?90o,
  gp1/2 and when source is immersed in the
  detector material, ?180o, gp1.

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Intrinsic Efficiency

• Intrinsic efficiency is defined as e(no. of
  radiation interacting with detector)/(no. of
  radiations striking the detector)
• For ?-ray detector, eIo-Ioexp-µl(E)x/Io
  1-exp-µl(E)x where Io is the incoming ?-ray
  intensity and Ioexp-µl(E)x is the ?-ray
  intensity that pass through the detector without
  interaction.

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Effect of ? ray energy and detector thickness on
Intrinsic Efficiency

• For NaI(Tl) detector, the intrinsic efficiency
  increases with the increase of thickness x and
  decreases with the increase of the photon energy
  (x5cm, for most nuclear medicine energy, e1)
• For semiconductor detector, the intrinsic
  efficiency is also energy dependent (comparison
  with NaI(Tl) is complicated due to the coupled
  atomic number and the crystal density)

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Intrinsic Efficiency (gas filled detectors)

• For gas filled detectors, the intrinsic
  efficiency is very small for photons (?, X, e
  lt0.01) but good for particle radiations (a,ß,
  e1).
• Detection of ? rays by gas filled detectors is
  mostly via electrons knocked out from the wall by
  the ? rays.

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Energy Selective Counting

• Not all output signals are counted in energy
  selective counting
• Photo-fraction fp is defined as the ratio of the
  counts in the photo-peak over the signal counts
  produced by the detector.
• Photo-fraction depends on ? energy, detector
  material and detector size.
• Full spectrum counting provides the maximum
  possible counting rate (used for single
  radionuclide with small scattering).

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Non-uniform Detection Efficiency
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Detector Profiles
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Minimum Source-Detector Distances
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Coincident Detection(Multiple Emission)

• R1?1xD1xA
• R2 ?2xD2xA
• R12 ?1xD1x ?2xD2xA
• RR1R2-R12

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Absorption and Scatter (1)

• Absorption and scatter occur when radionuclide is
  embedded within some media (tissue for instance).
• Counting rate is decreased by absorption but may
  be decreased or increased by scattering depending
  on the numbers of the scattering from and into
  the detector.
• Absorption and scatter are dependent on the ?
  energy, depth within the medium and if the energy
  selective counting is used.

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Absorption and Scatter (2)

• Absorption and scattering decreases with increase
  of the ? energy
• Absorption may predominate if the radionuclide is
  embedded in greater depth.
• Scattering is increased initially and reaching a
  maximum and then decreased with the increase of
  energy.

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Determination of Detection Efficiency

• Use known calibration source
• Calculation by definition equation DR/?
• Take into account the emission frequency if the
  calibration source is differing from the
  radionuclide concerned (see next table).

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Problems in the detection and measurement of beta
particles

• Liquid Scintillation Detectors are generally used
  for short ranged ß particle detection.
• A survey meter may be used to detect surface
  contamination by ß particles provided it has an
  entrance window sufficiently thin to permit the ß
  particles to enter the sensitive volume of the
  detector

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Gas Filled Detectors in ß Assay

• Sample self absorption (before reaching the
  detector) and backscattering (from sample and
  sample holder) are the problems.
• Self absorption depends on sample thickness and ß
  energy (strong absorption for low energy and
  thicker samples).
• Backscattering increases the sample counting rate
  (20-30).

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NaI(Tl) For High Energy ß

• NaI(Tl) detectors can be used to detect higher
  energy ß emitting radionuclides (32P, Emax1.7
  Mev, not 14C, Emax0.156MeV) by counting the
  Bremsstrahlung radiation.
• Greater activities are required (1000 times more
  than ? emitters) because of the low production
  efficiency of Bremsstrahlung radiation.

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Deadtime

• Cause of Deadtime
• Mathematical Models
• Window Fraction Effects
• Deadtime Correction Methods

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Causes of Deadtime (1)

• Deadtime is the time required for the counting
  system to process an individual event (it is also
  called pulse resolving time t).
• Several components (detector, pulse amplifiers,
  pulse high analyzer, scaler and computer
  interface) could contribute to the deadtime and
  the longest component determines the system
  deadtime.

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Causes of Deadtime (2)

• The deadtime of NaI(Tl) or semiconductor
  detectors are mainly caused by pulse amplifiers
  (pulse pileup and baseline shift, typical 0.5-5
  µsec)
• GM tubes are mainly caused by detectors pulse
  overlap (typical 50-200 µsec).
• Deadtime results in counting losses (overlap loss
  or window shift loss).

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Mathematical Models

• Counting systems can be classified as paralyzable
  and non-paralyzable type and most of nuclear
  medicine systems are paralyzable type.
• A non-paralyzable system is one for which, if an
  event occurs during the deadtime t of a preceding
  event, then the second event is simply ignored.
• A paralyzable system is one for which each event
  introduces a deadtime t whether or not that event
  actually was counted.

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Nonparalyzable Systems(1)

• The relationship between observed count rate Ro
  and true count rate Rt for the nonparalyzable
  system is given by

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Nonparalyzable Systems(2)

• The observed counting rate Ro has a maximum
  value. For nonparalyzable system, it is given by

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Paralyzable Systems(1)

• The relationship between observed count rate and
  true count rate for the paralyzable system is
  given by

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Paralyzable Systems(2)

• The observed counting rate Ro also has a maximum
  value. For paralyzable system, it is given by

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Deadtime Losses

• Deadtime losses is defined as (Rt-Ro) and the
  percentage losses (PL) is given by

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Window Fraction Effects

• Deadtime losses depend on total counting
  spectrum.
• Deadtime per event in the selected window depends
  on the fraction of the window ta t/wf
• Window fraction effect must be considered in
  comparing deadtime of different systems using
  energy selective counting

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Deadtime Correction Methods

• True counting rate can be calculated if we know
  the deadtime and the observed counting rate
  (either use the equation for nonparalyzed system
  directly or use graphic or approximation for
  paralyzed system).
• Deadtime can be calculated by using the maximum
  observed counting rates
• Deadtime can also be measured by using two source
  method

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The Two Source Method

• Two equal sources and three measurements, R1,
  R12, R2. Equal time should be used in short lived
  radionuclide to avoid decay correction.
• The dead times are calculated by

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Fixed Rate Pulser Method

• A fixed rate pulser is connected to the
  preamplifier of the radiation detector.
• Different input pulse rates (Pt) are generated
  (pulse hight larger than the photo-peak) and the
  observed counting rates (Po) are recorded.
• Real detector system is then connected to the
  preamplifier and the observed counting rates (Ro)
  are measured. The true counting rate (Rt) is then
  calculated by

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