Geiger counters

1. Instructor
Gyan Rao Dhote

2. Geiger Counters

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

4. 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

5. 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

6. 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 µs
• Quenching gas is needed to suppress the later pulse from positive ions.

7. 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.
C
V
+
− R
output

8. Geiger Amplifier
Typical Problem
• In a Geiger tube with 1 kV
between electrodes, a 0.5 MeV
β 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

9. 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 µs dead time.
– Fails near 10 kcount/s
http://www.imagesco.com/articles/geiger

10. 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.

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
Geiger tube
input
+9 V
ground
10 MΩ
1 MΩ
4700 pF
1 MΩ
470 KΩ
220 pF

12. GM Kits
http://www.imagesco.com/articles/geiger

13. Limited Streamer Tubes
• Typical LST is a multiwire unit (BaBar).
– Silver-plated wire 100 µm 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

14. Geiger Counters