Geiger Counters

1
Geiger Counters
2
Higher Voltage

• As the voltage increases in a gas detector the
  ions collected increases.
• The proportional region ends.
• Streamer mode
• Geiger mode
• Continuous discharge

3
Continuous Discharge

• Continuous discharge is due to the breakdown of
  gas into a plasma.
• Each gas has a threshold
• Example neon lamps
• Discharge is bad for detectors.
• Individual signals lost
• The fixed discharge threshold can be used to
  regulate voltage.

NE-38 typical breakdown voltage 135 VDC
4
Multiple Avalanches

• In proportional mode a single ion pair results in
  an avalanche.
• With higher fields electrons in the avalanche
  cause x-rays that start new avalanches.
• The process stops when sufficient positive ions
  quench the avalanches.
• Ions slowly drift to cathode

5
Geiger-Müller Region

• In the Geiger-Müller (GM) region of operation
  there is a maximum amount of electrons produced
  in the avalanche.
• Ion pair count is independent of initial
  ionization.
• Plateau over range of voltage
• The electrons are collected quickly
• Less than 1 ms
• Quenching gas is needed to suppress the later
  pulse from positive ions.

6
Geiger Tube

• Most Geiger tubes use a cylindrical geometry.
• Grounded outer cathode
• High voltage anode
• There is usually a thin window to allow particle
  to enter without loss.
• The output is either from case or capacitively
  coupled.

output
C
-
R
V
7
Geiger Amplifier

• Typical Problem
• In a Geiger tube with 1 kV between electrodes, a
  0.5 MeV b particle produces a pulse that fully
  charges a 5 pF capacitor.
• What is the energy amplification?
• How many electrons are in the avalanche?

• Answer
• The energy in the capacitor is (1/2)CV2 2.5 x
  10-6 J.
• 0.5 MeV 8 x 10-14 J
• Gain is 3 x 107
• The charge Q CV
• Q 5 x 10-9 C
• A 5 pF capacitor

8
Dead Time

• The avalanche in the GM tube and pulse readout
  take a fixed time.
• Ions need to become neutral
• Dead time between pulses
• At right the counter has a 90 ms dead time.
• Fails near 10 kcount/s

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Making High Voltage

• A single transformer could convert 120 V AC into
  a high voltage AC.
• Rectify to get DC
• Voltage doublers and switching circuits can pump
  charge into capacitors.

• This circuit produces DC output at 2 times the
  zero-to-peak input AC.
• Can be extended in series to higher multiples.

10
Portable HV
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11
Pulse Beeper

• Portable Geiger counters often make audible
  clicks or beeps when an avalanche occurs.
• Convert pulse to greater duration
• Buffer signal digitally
• Drive inverter oscillator and speaker

9 V
Geiger tube input
470 KW
10 MW
4700 pF
1 MW
220 pF
ground
1 MW
12
GM Kits
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Circuit Description

• The 4049 Hex Inverting Buffer is set up as a
  square wave generator. The power MOSFET IRF830
  switches the current on and off to the primary
  windings of the mini step-up transformer. The
  output of the mini step-up transformer is fed to
  a voltage doubler consisting of two high voltage
  diodes D2 and D3 and two high voltage capacitors
  C4 and C5.
• The high voltage output from this stage is
  regulated to 500 volts needed for our GM tube by
  three zener diodes stacked one on top of the
  other (D4, D5 and D6). Diodes D5 and D6 are 200V
  zener diodes and diode D4 is a 100-Volt zener.
  Together (200 200 100 500), they equal 500
  volts. Five hundred volts is the optimum
  operating voltage for our GM Tube.
• The 500-volt regulated output is fed to the anode
  of the GM tube through a current limiting 10
  mega-ohm resistor R4. The 10 mega-ohm resistor
  limits the current through the GM tube and helps
  quench the avalanched ionization when a
  radioactive particle is detected.
• The cathode of the tube is connected to a 470K
  (R5) resistor. The voltage pulse across R5
  generated by the detection of radiation, feeds to
  the base of a 2N3904 NPN transistor, through a
  1-uF capacitor (C6).
• The NPN transistor clamps the output pulse from
  the GM tube to Vcc and feeds it to an inverting
  gate on the 4049. The inverted pulse signal from
  the gate is a trigger to the 555 Timer. The timer
  is set up in monostable mode that stretches out
  the pulse received on its trigger. The output
  pulse from the timer flashes the LED and outputs
  an audible click to the speaker via pin 3.

14
Limited Proportional Mode

• There is a transition region between proportional
  and GM.
• Extra avalanches occur
• Localized compared to GM
• Devices in this region are limited streamer or
  self-quenched streamer chambers.

15
Iarocci Tubes

• A popular application of limited streamer is the
  Iarocci tube.
• Array of rectangular tubes
• One conducting cathode surface
• Equivalent to multiwire proportional chamber

• Iarocci tubes can be operated in either limited
  streamer or proportional mode.
• Limited streamer mode gives greater signal
  strength.
• Proportional mode gives greater spatial precision.

16
Equipotentials

• Equipotential lines in an Iarocci tube are very
  similar to proportional tubes near the anode.
• Conducting plane is resistive.
• Graphite coat

17
Limited Streamer Tubes

• Typical LST is a multiwire unit (BaBar).
• Silver-plated wire 100 mm in diameter
• 8 wire cells per unit
• Quenching gas mixture Ar(3)Isobutane(8)C02(89
  )
• Resistive layer of graphite, with resistivity
  between 0.2 and 1 MW/square
• Operates at 4.7 kV plateaus 200 V wide
• Wire signals of the order of 150/200 mV
• Pulse 50 ns, sometimes an afterpulse
• Average charge per pulse of 300 pC

18
Tracking Detector

• Iarocci tubes used in tracking are arranged in
  layers.
• Hits in cells are fit to a track.
• Timing converted to distance from wire
• Fit resolves left-right ambiguity