The Geiger Tubes

What is a Geiger-Müller tube?

The Geiger-Müller tube is a gaseous ionization detector that uses the Townsend avalanche phenomenon to produce an easily detectable electronic pulse from as little as a single ionising event due to a radiation particle. It is used for the detection of gamma radiation, X-Rays, and alpha and beta particles. It can also be adapted to detect neutrons. The tube operates in the “Geiger” region of ion pair generation.

A Geiger Muller tube like other gas filled detectors has an anode and a cathode in an envelope that contains a gas at low pressure (for most tubes, in the range 50 to 100 torr). The main component is an inert gas such as helium, argon or neon, in some cases in a Penning mixture, and a quench gas of 5-10% of an organic vapor or a halogen gas to prevent multiple pulsing. There may also be a special ‘window’ of radiation transparent material either at one end of the tube or in its side. Some tubes also have an integral radiation shield as part of the outer walls where this is required for modifying the performance characteristic.

When ionizing radiation strikes the tube, some molecules of the fill gas are ionized, either directly by the incident radiation or indirectly by means of secondary electrons produced in the walls of the tube. This creates positively charged ions and electrons, known as ion pairs, in the gas. The strong electric field created by the tube’s electrodes accelerates the positive ions towards the cathode and the electrons towards the anode. Close to the anode in the “avalanche region” the electrons gain sufficient energy to ionize additional gas molecules and create a large number of electron avalanches which spread along the anode and effectively throughout the avalanche region. This is the “gas multiplication” effect which gives the tube its key characteristic of being able to produce a significant output pulse from a single ionising event.

The discharge is terminated by the collective effect of the positive ions created by the avalanches. These ions have lower mobility than the free electrons due to their higher mass and remain in the area of the anode wire. This creates a “space charge” which counteracts the electric field which is necessary for continued avalanche generation. For a particular tube geometry and operating voltage this termination always occurs when a certain number of avalanches have been created, therefore the pulses from the tube are always of the same magnitude regardless of the energy of the initiating particle. Consequently there is no radiation energy information in the pulses which means the Geiger-Muller tube cannot be used to generate spectral information about the incident radiation.

On its own, a Geiger-Müller tube only produces pulses. It becomes useful when it sits inside a radiation monitor, the instrument that counts those pulses continuously and converts them into a dose rate in µSv/h.

The Russian SBM-20 is a small/medium size Geiger Muller tube, designed to detect Gamma and Hard Beta radiation. The robust design made it a popular choice in commercial dosimeters all over the world. We just got a new batch of tubes that will join the project as uRADMonitor dosimeters.

What Radiation Can a Geiger-Müller Tube Detect?

A Geiger-Müller tube responds to ionising radiation, but which types it can actually see depends on its construction, above all on whether it has a thin entry window:

• Gamma and X-rays: detected by almost all tubes, though with low intrinsic efficiency (typically a few percent), as most photons pass straight through the gas.
• Beta particles: detected by metal-wall tubes like the SBM-20 for higher-energy (hard) beta, and far more efficiently by thin-window tubes.
• Alpha particles: only detected by tubes with a very thin mica end window, because alpha is stopped by even a few centimetres of air or a metal wall.
• Neutrons: only with a special conversion layer or fill gas (for example boron or helium-3); a standard tube does not respond to them.

Because the output pulse is always the same size, a Geiger tube counts events but cannot tell you the energy of each particle. To put a reading in context, see what a normal background radiation level looks like.

Types of Geiger-Müller Tubes

Geiger tubes come in a few distinct families, and choosing the wrong one for the job is the most common mistake in radiation detection:

• Side-window / metal-wall tubes (e.g. SBM-20): rugged, sealed in a steel cylinder, sensitive to gamma and hard beta. The workhorse of most dosimeters, including uRADMonitor units, but blind to alpha.
• End-window “pancake” tubes (e.g. LND712): a thin mica window lets alpha and low-energy beta reach the gas, giving a much larger sensitive area. The choice for surface contamination surveys.
• Energy-compensated tubes: fitted with a metal shield that flattens the energy response so the count rate tracks dose rate more accurately across a wide gamma energy range.

A second distinction is the quench gas. Organic-quenched tubes (ethanol or butane vapour) are cheap but wear out after roughly 10⁹ counts as the quench molecules are consumed. Halogen-quenched tubes self-replenish and last far longer, which is why they dominate modern instruments.

Key Performance Characteristics: Voltage, Dead Time and Lifespan

• Operating voltage and plateau: each tube has a voltage range (the “plateau”, around 400 V for an SBM-20) where the count rate is nearly independent of voltage. Running a tube on its plateau is essential for stable readings.
• Dead time: after each pulse the tube is briefly insensitive, typically 50 to 100 microseconds. This caps the maximum count rate and must be corrected for in high-radiation fields, otherwise the tube under-reports.
• Lifespan: halogen tubes can exceed 10¹⁰ counts; organic-quenched tubes age faster. A tube near end of life shows an unstable plateau and drifting counts.

If your readings look wrong or a counter has gone quiet, the tube is often the cause. Our guide on checking the Geiger tube shows how to test one and tell a healthy tube from a failing one.

Geiger Tube vs Geiger Counter

The two terms are often confused. The Geiger-Müller tube is the sensor, the sealed gas-filled detector described above. A Geiger counter is the complete instrument: the tube plus the high-voltage supply, pulse-counting electronics, display and power. The tube detects the radiation; the counter turns its pulses into a number you can read. For the full picture, see what is a Geiger counter. uRADMonitor builds Geiger counters around these tubes, from the KIT and MODEL dosimeters that feed the global live radiation map.

Geiger-Müller Tubes: Frequently Asked Questions

What gas is inside a Geiger-Müller tube?

An inert fill gas (helium, neon or argon) at low pressure, plus a small amount of quench gas, either an organic vapour or a halogen, to stop the discharge after each pulse.

Can a Geiger tube detect alpha radiation?

Only if it has a thin mica end window, like a pancake tube. Standard metal-wall tubes such as the SBM-20 detect gamma and hard beta but not alpha, which is stopped by the wall.

How long does a Geiger-Müller tube last?

Organic-quenched tubes last around 10⁹ counts; halogen-quenched tubes can exceed 10¹⁰ and are effectively limited by age and handling rather than count total.

What is the difference between a Geiger tube and a Geiger counter?

The tube is the detector; the counter is the whole instrument that powers the tube and counts its pulses. One is the sensor, the other is the device built around it.

Why can’t a Geiger tube measure radiation energy?

Because every pulse is the same magnitude regardless of the particle’s energy, so the tube can count events but carries no spectral information. Energy measurement needs a proportional counter or scintillation detector instead.

More information on Geiger tubes is available on:
Centronic Geiger Tube Theory
Wiki
An introduction to Geiger counters

New to radiation detection? Start with our guide:
What Is a Geiger Counter? Radiation Detectors Explained