Topic 1. Electronics for Radiation Detection Systems

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Topic 1. Electronics for Radiation Detection
Systems

• Introduction
• Preamplifier
• Amplifier
• Pulse Height Analyzers
• Display Mode
• Cathode Ray Tubes

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Introduction
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Preamplifier

• To amplify the relatively small signal from the
  detectors
• To match the impedance levels between the
  detector and subsequent components
• To shape the signal pulse for optimal subsequent
  processing

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A few points

• The output from the preamplifier is VVo
  exp(-t/RC) where Vo Q/C and RC is the time
  constant, typical 20-200 u sec for nuclear
  medicine detectors.
• The amplification for scintillation detectors is
  small (5-20) because the signals from the
  detectors have already been amplified by
  photo-multiply tubes (105-1010).
• Higher amplification is required for
  semiconductor detectors (103-104) due to small
  detector signals.
• Preamplifier is located as close as possible to
  the detector to maximise the signal to noise
  ratio (often in single unit).

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Amplifiers

• To amplify the still small signals from the
  preamplifier (1-1000).
• To reshape the slow decaying pulse from
  preamplifier into a narrow one (for high count
  rate and increasing the S/N rate etc,).
• Requirements for shaping preserve the input
  signal information such as pulse height and rise
  time.

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RC Shaping

• Differentiate circuit
• The output is a rapid rising pulse with decay
  constant td RC which is smaller than that in
  preamplifier.
• The amplitude of output is proportional to the
  rising portion of the input and insensitive to
  the tails.
• It discriminates against low frequency noise.
• Integration circuit
• output pulse rises with time constant.
  VVo(1-e-t/RC).
• It discriminates against high frequency noise.

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RC Shaping (continued)

• Differentiate plus Integration circuit
• The output amplitude is determined by the input
• Time constant is shortened (0.25-5 u sec for
  scintillation and semiconductor detectors, in
  contrast to 50-500 u sec in the preamplifier).
• Only one polarity (except for some small negative
  overshoot at the end)

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RC Shaping (continued)

• Double differentiation plus integration circuit
• Output is bipolar
• Shorter rising time and longer total duration
  than unipolar output
• Preferred for high counting rate

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Baseline Shift and Pulse Pileup

• Baseline Shift is caused by the negative
  component of the output (at the end of the pulse)
• Pulse Pileup is caused by high counting rates
  that they fall on top of each other.

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A few Points

• Baseline shift and pulse pileup are caused by
  high counting rates
• Both problems can be reduced by shortening time
  constant but also reduce the energy resolution
  and S/N ratio.
• Double differentiate bipolar amplifier and short
  time constant (0.025-0.5 u sec) are commonly used
  for NaI(Tl) detectors
• Unipolar and longer time constant (0.5-8 u sec)
  for semiconductor detectors (achieve high energy
  resolution).

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Pulse-Height Analyzers

• Basic Functions
• Single Channel Analyzers
• Time Methods
• Multi-channel Analyzers

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Basic Function

• The amplitude of output signal is proportional to
  the energy of the radiation event detected
• Selective counting of those pulses within certain
  amplitude resulted in counting of selective
  energy range
• A certain energy range or interval is called
  energy channel

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Single Channel Analyzers

• Counting only those within a single energy range
• Composed of three parts Lower Level
  Discriminator (LLD), Upper Level Discriminator
  (ULD) and Anticoincidence
• Percentage window a certain percentage of the
  windows central voltage.
• A single channel analyzer without ULD is a
  circuit called discriminator

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Timing Method

• Determine the timing of radiation event is
  important in Nuclear Medicine applications
• There are a number of timing methods available
  but two of those are often used in nuclear
  medicine leading-edge and zero-crossing.
• Leading-edge uses the rising portion of the input
  pulse to trigger the lower level discriminator
  which depends on the pulse amplitude (suffer
  certain amount of inaccuracy--5 to 50 nsec for
  NaI(Tl)).
• Zero-crossing requires bipolar pulses and is more
  accurate (4 nsec for NaI(Tl)).

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Multichannel Analyzers

• Simultaneous recording of multiple energy
  radiations.
• The principle of the popular Multichannel
  Analyzer (MCA) is different from the single
  channel analyzer
• The center of the Multichannel analyzer is the
  analog-to-digital converter (ADC)
• A memory is required for the sorting of energy
  channels (energy ranges, energy spectrum).

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Analog-to-Digital Converter

• Two types of ADC are used in nuclear medicine for
  MCA and the interface between scintillation
  cameras and computers Wilkinson or Ramp
  converter and successive approximation
• Both require time for the conversion which could
  be a bottle neck for the time resolution but is
  not a major problem for nuclear medicine
  application
• Both of the converters use binary number
  representation which means that the more bits the
  more accurate but requires more time and memory.

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Ramp ADC

• RC circuitry and clock oscillator
• Discharging time proportional to the amplitude of
  the input pulse (radiation energy)
• Clock oscillator produces pulse train that are
  counted in a counting circuit
• The number of the clock pulses counted are
  proportional to the discharging time which in
  turn proportional the radiation energy).

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Successive Approximation

• The input pulse is compared with one-half of the
  full scale
• The comparison voltage is then either increased
  or decreased by one half of its initial level
  depending on whether the pulse amplitude did or
  did not exceed the initial level.
• The process is repeated for several steps.

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Time to Amplitude Converter
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Scalers and Timers

• A device that only counts pulses is called a
  scaler
• An auxiliary device that controls the scaler
  counting time is called timer.

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Analog Ratemeters

• A analog ratemeter is used to determine the
  average number of events occurring per unit time.
  The average is determined continuously rather
  than over discrete counting time
• Linear vs logarithmic ratemeters V0knQRp vs
  V0klog(nQRp) - wider range of counting rate
• Ratemeter responds to the rate change has a time
  constant which can be adjusted (change the
  capacitor)

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Coincidence Unit
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Cathode Ray Tube (CRT)

• Electron Gun
• Deflection Plates
• Phosphor-coated Display Screens
• Focus and Brightness Controls
• Colour Cathode Ray Tubes

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Electron Gun

• Cathode Hot filament-Tungsten, thoriated
  tungsten, nickel coated with oxides of barium and
  strontium, etc
• Control grid a cape with a hole in its centre
  and a negative potential applied to control the
  passage of electrons.
• Accelerating anode similar to control grid but
  reverse in shape. Positive potential applied to
  accelerate the electrons.
• Focusing anode a second anode that further
  shapes the electron beam. A negative potential is
  applied to compress and focus the beam of
  electrons.

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Deflection Plates

• Deflection plates are used for the positioning
  the electron beams on the screen electronstatic
  or electromagnetic types.
• Electrostatic type applies voltages to the two
  sets of plates. Used for small screen with fast
  speed.
• Electromagenetic type uses two sets of wire coil.
  Used for large screen with a slower speed.

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Phosphor-coated Display Screen

• Electrons strike the screen (glass coated with
  phosphorescent materials) and release
  phosphorescent light.
• Persistence time the lifetime of the light
  emission from the phosphor.
• Persistence scope long persistence time up to a
  few minutes. Composed of storage mesh and flood
  gun etc. Used as visual monitor for patient
  positioning with the gamma camera.

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Focus and Brightness Control

• Second anode in the CRT controls the focus. It is
  a potentiometer that varies the potential applied
  to the anode.
• The control grid controls the brightness or
  electron intensity. Increasing the voltage
  (negative) decreases the intensity

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Colour Cathode Ray Tube

• Three electron guns produce different electron
  beams onto arrays of individual phosphors which
  in turn, produce three colours, red, green and
  blue.
• A total of 64 colours can be produced by mixing
  the three colours for human eyes.

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Oscilloscopes

• Oscilloscope is composed of a CRT, a signal
  amplifier and a time-sweep generator. It is used
  for displaying signal amplitude or frequency as a
  function of time.
• The signal amplifier is used to amplify the small
  signals to be displayed which is connected to the
  vertical deflection plates.
• The sweep signal is applied to horizontal
  deflection plates which sweeps the electron
  across the screen at a constant speed and is
  repeated.
• Often used in cardiac studies for nuclear medicine

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Television or Computer Monitors

• A CRT tube with the two deflection plates
  controlled by constant frequency time-sweep
  generators.
• Electron gun controls the intensity at each
  point.
• Active or retrace sweeps the electron gun is on
  or off.
• Most TV monitors use interlacing. The two sets of
  scan lines are called fields and the two
  interlaced fields is called frame. Each frame
  takes 1/30 or 1/25 sec depending on the frequency
  of the electricity.
• The resolution depends on the number of lines
  (65) for the vertical direction and the changing
  rate of the brightness during the horizontal
  sweep.

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