Download Geiger Counters

Geiger Counters
Higher Voltage
• As the voltage increases
in a gas detector the ions
collected increases.
• The proportional region
ends.
– Streamer mode
– Geiger mode
– Continuous
discharge
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
NE-38: typical breakdown
voltage 135 VDC
• The fixed discharge threshold
can be used to regulate voltage.
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
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.
Geiger Tube
+
-
R
V
C
• Most Geiger tubes use a
cylindrical geometry.
output
– 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.
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
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.
Portable HV
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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
ground
1 MW
1 MW
220 pF
GM Kits
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Circuit Description
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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 megaohm 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.
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.
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.
Equipotentials
• Equipotential lines in an Iarocci
tube are very similar to
proportional tubes near the anode.
• Conducting plane is resistive.
– Graphite coat
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
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