Universityof Birmingham

The Spark Chamber

Birmingham High Energy Physics Group
Last updated Summer 2010
Stephen Bull, James Fleming and Jessica Cuttriss

A Spark Chamber is a device used for detecting charged particles, it was widely developed and used in the 1970s. Now, even though more up-to-date equipment has been developed, it is still used for educational purposes, due to its relative simplicity and its usefulness in researching subatomic particles. This web page is dedicated to providing information about Spark Chambers to those who have a basic understanding of particle physics, but wish to learn more.  Also, those who have a flair for physics may find the information useful in developing their understanding of  cosmic rays and charged particles.  Below is a table of contents to help you navigate your way around the site.

Why do people study Physics?

How does a Spark Chamber work?
 How does it relate to Physics?
Types of particle
Detectors and the Particle Physicist Electromagnetic Interference
What does a Spark Chamber detect?  References and Acknowledgments
Cosmic Rays  Contact us

Chamber Efficiency
Weird Events

Why do people study Physics?

"What is the world made of?" "What holds it together?" are the two questions which particle physicists ask themselves everyday - before they go about setting up experiments in order to test their theories, and eventually answer the questions. The world is made up of fundamental building blocks; fundamental is used in the sense that these so-called building blocks are simple and structureless,  and cannot be made up of anything smaller (elementary particles).

Atoms consist of a nucleus which is surrounded by a cloud of electrons.   Elements are arranged within the periodic table (left to right and top to bottom) in order of increasing atomic number.  Atomic nuclei have a charge +Ze, where e is the magnitude of the charge on an electron and Z is the atomic number.  Z determines the number of atomic electrons and hence the ordered position of that species of atom in the periodic table. This order generally coincides with increasing atomic mass. The periodicity of the periodic table is explained beautifully by the conservation rules of quantum mechanics. As it is possible to build a periodic table this suggests that atoms must be made of simpler building blocks and therefore are not elementary particles.

Protons and Neutrons Quarks
  The nucleus is made up of protons and neutrons. The proton is a positively charged particle having a mass of 1.673x10-27kg whereas the neutron has no electric charge and has slightly greater mass of 1.675x10-27kg.  Both proton and neutron have a radius of 10-15m and are held together by the strong nuclear force. Nuclear physics is another branch of physics in which the physics of the atomic nucleus is studied. Nuclear physicists are concerned with understanding the properties of nuclei in terms of the behavior of their constituents.   Protons and neutrons are not elementary particles, as they consist of sets of point-like particles called quarks held together by the exchange of particles called gluons.  Current understanding is that quarks and gluons have no sub-structure.  Quarks do not exist on their own, so cannot be separated from the proton or neutron in which they are contained. Electrons are a different type of fundamental particle which do not interact with gluons.

What is a spark chamber? What does it have to do with physics?

A spark chamber is a device which is used for detecting charged particles. It is one of many detector devices used by elementary particle physicists in the search for a greater understanding of the subatomic particles which make up the universe.

In current research, the spark chamber has become largely redundant, as it has been replaced by faster and much more sophisticated detectors with better time and spatial resolution.  For example, the OPAL experiment at the Large Electron-Positron (LEP) accelerator at CERN (homepage) used drift chambers, while ATLAS (homepage) makes extensive use of silicon detectors. These detectors measure the trajectories of different charged particles.

However, the spark chamber is still of great scientific value, in that it remains relatively simple and cheap to build (compared to the LHC at CERN, which took 20 years and as well as $9 billion dollars) and it enables an observer to view the paths of charged particles.

A picture of the portable spark chamber made at  The University Of Birmingham.

A time line to show the development and usage of the spark chamber throughout history.
1949 Keuffel First observed that discharge between parallel plates occurred along the path of a cosmic ray.
1953 Bella and Frazinetti, took photos of spark discharges and published them.
1955 (1) Used several parallel plate counters ( called "modules")

(2) Enhanced the spark with a triggered condenser discharge (e.g. Argon or Alcohol)

(3) Took stereo photographs.
1957 Harwell, Cranshaw and Da Beer looked into applying high voltage to the plates immediately after the charged particle had passed through the spark chamber. They also developed the triggering of the chamber.
1959 Fukui and Migamoto
1963 Alikhanian came up with the idea of leaving enough space between the plates so a spark could be observed.
>1970 The spark chamber became the primary device used to demonstrate and research sub atomic particles, before being replaced by bubble chambers, drift chambers, ...

 Detectors and the Particle Physicist

Detectors are devices used by elementary particle physicists in the search for a greater understanding of the subatomic particles which make up the universe. As well as detecting charged particles, detectors can be used to detect radiation, making them a useful tool in other aspects of science such as in nuclear physics experiments, nuclear medicine and geological exploration. This section gives a brief insight into the detectors which replaced the spark chamber.

A wide range of detector devices exist, such as the cloud, bubble and proportional chambers. All of the detectors along the  same fundamental principle: the transfer of some or all of the energy of a particle to the detector mass, where it is converted into a a more readily observable form such as visible light.
The form in which the converted energy appears depends on the detector and its design. Gaseous detectors such as drift and multi wire proportional chambers (MWPC) output a current signal, whilst scintillation counters produces a light pulse. Modern detectors are electrical in nature and output pulses which are analysed by electronic means.
Before the development of the proportional and drift chambers during the late 1960's, the spark chamber was widely used as a triggerable track detector. The spark chamber itself was a development of the spark counter.

The spark counter consisted of a pair of parallel plates with a high potential difference between them, in a gaseous atmosphere, used in the same way as a Geiger-Muller tube. The main difference between the spark counter and the spark chamber was the use of photographic rather than electrical recording, thus converting the device from a counter to a track locating device.
Modern experiments use more sophisticated detectors with a combination of higher spatial and temporal resolution, increased efficiency and readout time, and a lower 'dead time' (the time needed by the machine before it can record a new reading).


How does a spark chamber work?

 Circuit Diagram of the spark chamber. (The components in the dashed white box are the 'detector' components of the spark chamber.)

  Our spark chamber consists of 16 modules, each module  being approximately (47cm x 30cm x 0.8cm). The design is quite robust - a spark chamber in particle physics research would normally consist of much less material.  There are two scintillator detectors, each connected to a small photomultiplier tube,  one above  the uppermost module (visible below), and one underneath the lowest module (less visible in photograph). Photo of spark chamber modules
Moduals of spark chamber
Each module consists of a perspex frame, 0.8cm thick The detector consists of the 16 modules, mounted one on top of the other.

Module of spark chamber

On the top and bottom of these perspex frames are glued 3mm thick aluminium plates:

For each module, one aluminium plate is connected to ground while the other is connected to the High Voltage (HV) circuit.

The active volume of each module (within the perspex frame and between the aluminium plates) is filled with a noble gas mixture (70% Neon, 30%Helium). The gas flows in series from one module to the next, through all 16 modules.

At the top and bottom of the spark chamber, covering the active area of the modules, are plastic scintillation counters. When a charged particle passes through the scintillation counters ionisation is produced which subsequently de-excites, emitting visible light. The photons are detected by a photomultiplier tube. Since both the scintillation counter and phototube have a fast response time, the resulting electrical signals indicate in a very short time the passage of charged particles.

Types of Particle

 Here are some brief details of different types of particle mentioned throughout this web-site.

Baryons  Are particles made up of 3 quarks (see below)Examples include the proton (p), neutron (n)

Kaon  (K) Kaons are mesons (see below)
Leptons  There are three types of charged lepton: the electron, muon and tau, each of which has an associated neutrino (electron neutrino, muon neutrino, tau neutrino) The properties of each are summarised below.  For each lepton there is a corresponding antiparticle (positron, anti-muon and anti-tau). Each of these also has a corresponding anti neutrino.

Mesons are particles made up of a quark and an anti-quark.  Examples of mesons are the Kaon , Pion , Psi particles.

Muons  Muons are leptons. They have a charge of -1 (electron charge). The muon is 200 times bigger than the electron. Most of the particles detected by the spark chamber are muons, as they are very penetrating, and have a long lifetime.
  Italian, for "little neutral one," These are very low mass particles which have no charge. There are many, many billions of neutrino's passing through your little finger's nail every second, and their main source is thought to be from the Sun. They are so unreactive, that if they were to travel through a light year worth of lead, there would only be 50% change that it would hit anything along the way.
Much experimental work is currently being done to understand the mass and related properties of the neutrinos.  The neutrino has a very, very small mass and may constitute the source of some of the so called 'Dark Matter' present in the universe.
Pions  Also known as pi-meson. Pions are elementary particles classified as a meson. The pion can be positively , negatively or neutrally charged. These charged pions decay into muons and neutrinos, the neutral pions convert directly into photons.
Positrons  These are identical particles to electrons, having the same characteristics. The only difference being that they are positively charged. This is an example of antimatter, and  releases gamma radiation when it collides with matter.
Quarks  One of the fundamental constituents which make up matter. To give an example:

It is well known that an atom consists of neutrons, protons and electrons. If you could probe to see inside a proton or a neutron, you would find that each contained three smaller, constituent particles called quarks.

There are six quarks, each with its own flavour which can be simplified to a type of quark.  These are: up (u), down (d), strange (s), charm (c), bottom (b) and top (t).
Particles with half integer 'spin' (angular momentum) are called fermions and consist of three quarks.
Particles with integer spin 'spin' are called bosons and consist of quark-antiquark pairs.
For example, the proton is made of a two u quarks and one d quark, the neutron consists of two d quarks and one u quark. The positively charged pion consists of one u quark and one anti-d quark.
The proton, neutron, and pion are by no means the only particles which are made of quarks. All strongly interacting particles, known as baryons or mesons, are made of quarks.

What does a spark chamber detect?

  There are two types of cosmic rays which exist, primary and secondary. Primary cosmic rays are the names given to cosmic rays when they are initially formed (Click here to find out where cosmic rays come from). However, primary cosmic rays are very rarely detected at ground level because they generally undergo collisions with atoms very high up in the atmosphere. These collisions produce a cascade of secondary cosmic rays which shower down through the atmosphere to the earth's surface. It is of this type of cosmic ray which the spark chamber detects.

The diagram below shows the primary cosmic ray colliding with the nucleus at about 60Km above ground level, where the collision produces a cascade of secondary particles, known as a cosmic-ray shower.

  A Cosmic Ray Shower
 It can be seen that the secondary cosmic rays include pions (which decay to muons, neutrinos and gamma rays) as well as positrons and electrons produced by muon decay and gamma ray interactions with atmospheric atoms.

Why can primary cosmic rays decay and produce secondary cosmic rays?

Primary cosmic rays, as discussed above have high energy, E. By the usage of Einstein's well known equation, E = mc2,it can be seen that the energy of the primary cosmic ray can be converted into new mass, i.e. new particles can be produced from the energy of the primary cosmic ray. These new particles are the secondary cosmic rays. After a while, and if enough energy is available to individual secondary particles, these can decay to produce yet more secondary particles.
It can therefore be appreciated that one primary cosmic ray can cause the production of many secondary cosmic rays which is what the spark chamber detects. The nearer one gets to ground level, the secondary particles become less and less energetic. So the energy of most secondary cosmic rays detected at ground level is a lot less then the energy of the original primary cosmic ray.
 Although the spark chamber will detect all secondary cosmic rays, it is mainly muons which it detects. This is because muons are very penetrating as they only interact electromagnetically. They also have a long life-time (only 2.2ms, but they travel at the speed of light, so they travel very far) and there are thus
lots of them around at ground level which the spark chamber detects.

Cosmic Rays

A spark chamber can be used to detect cosmic rays. Cosmic rays are high energy charged particles which typically have energies ranging between 106 - 1020 eV (around 10-13 - 10-10 Joules).  Although in these terms this does not sound a very large amount of energy, the energy is very concentrated; the particles are extremely small (typically 10-29kg and of the order 10-18m in radius, or 0.000000000000000001 metres in radius) it can be appreciated that this is a large energy for each of these minute particles to have.

Cosmic rays originate in outer space, (mainly from supernova explosions but also from stars see "where do cosmic rays come from" to read more) travel at the speed of light and strike the Earth from all directions.

The cosmic rays strike the Earth's surface at the rate of about 1 cosmic ray every square centimetre every minute. Perhaps a more meaningful way to look at this is that during an average nights sleep a person will have on average of a million cosmic rays traveling through their body!

Cosmic rays are very penetrating; so much so that they have also been detected under the ground, such as in the London Underground system and down deep mines. The table below gives some comparison of penetration for different types of radiation, so that cosmic rays can be compared.

Radiation Mass Electric Charge Average Penetration Depth in Air Summary
X-rays 0 0 100m Very energetic form of light
Alpha rays 3.7GeV +2e 0.1m Equivalent structure to He nucleus
Gamma rays 0 0 1km Very energetic form of light
Primary cosmic rays Mixed Mixed approx. 10km Mostly Hydrogen and Helium nuclei
Secondary cosmic rays Mixed Mixed approx. 10km Mostly muons

How are cosmic rays useful in science?

(A) The large energies of cosmic rays allow them to be used to smash up atoms from which more can be learnt about the atom and the structure of matter.

(B) The source of cosmic rays are of interest to astronomers and cosmologists (see the University of Birmingham Astrophysics web page: )

(C) Radiocarbon dating resulted from cosmic ray research (see )

Where do cosmic rays come from?

All the light and the heat which we receive on the Earth comes from the sun, so it seems reasonable to suspect that cosmic rays may also come from the sun. This however, is not the case. There are two pieces of experimental evidence which back up this statement.

Firstly, it is well known that during the day it is light and at night it is dark, in other words a variation of light intensity seen on Earth occurs during a day (24 hour cycle). Similarly the hottest part of the day occurs between 11am and 3pm, the coolest part occurring in the early hours of the morning. In other words there is a heat variation during the day. Based on these facts, it is reasonable to assume that if cosmic rays came from the sun, they too would have some daily variation in intensity. For example, the intensity of cosmic rays received by the Earth would be large around lunchtime, and much less in the middle of the night. However no such variation is observed. Indeed the cosmic rays received by the Earth is (nearly) constant at all times during the day.
The second reason why cosmic rays cannot come from the sun, is that the sun is not able to give out particles with the high levels of energy which cosmic rays are known to have.

Evidence also exists to suggest that cosmic rays cannot exist outside of our galaxy. In order to understand why this is so it is first necessary to consider what exactly our galaxy looks like.

The milky way galaxy can be modeled as a 'fried egg'

The galaxy has a large number of stars at its centre and fewer further out. In fact, photos of our galaxy actually support this idea; at the center, a "mush" or "mix" of stars, where the large amount of light coming from them prevents us from seeing them separately. In all, our galaxy contains about 100,000 million stars. The Earth is found towards the edge of the galaxy. The galaxy is a spiral shape and is rotating at 290km/s which is equivalent to 640,000mph. This means that all the stars and planets within the galaxy are also rotating. Therefore the Earth is rotating and moving through space, relative to neighboring galaxies, with a speed of 640,000mph. The maximum speed a car can legally travel in Britain is 70mph, 9143 times slower than the speed with which the Earth is moving through space. When Usain Bolt broke the 100m world record, he ran at a speed of around 0.01km/s. Concorde travelled a little faster when it was still in service, and was capable of speeds around 0.63km/s.

The fact that the Earth is moving through space implies that the intensity of the cosmic rays coming from outside of space would be greater on the side of the Earth facing the direction from which the cosmic rays came.

To make this clearer, consider the figure on the left. If cosmic rays come from outside of the galaxy, say traveling (as shown) from right to left, then the number of cosmic rays received on face A must be greater than the number received on face B, (as the purple cosmic ray intensity shadow shows)

Cosmic rays from outside of our galaxy?
Furthermore, if cosmic rays come from outside of the galaxy "cosmic ray intensity" would change at different times during the day, due to the fact that the Earth is spinning on it's own axis. No such variation is observed. Indeed the cosmic ray intensity on all points on the Earth's surface are roughly the same, at all times during the day. Therefore it is thought cosmic rays have no spatial or time variation.

We conclude from these observations that the majority of cosmic rays must originate within our galaxy. The next question which inevitable follows is: where from within our galaxy do cosmic rays come from?
Some cosmic rays are formed from stars, but most come from Supernova explosions. As keen astronomers will know, a star "lives" for around 1 billion years. At the end of their lives, some stars explode. These are called Supernova explosions. About 100 supernova explosions occur during one year (within our "horizon" of space), and they are observed mostly by professional astronomers, although during February, 2001, the 5th ever supernova explosion seen by an amateur was recorded. Since this website was first made,  5 more significant t hey release atomic nuclei. These atomic nuclei are the source cosmic rays and are mostly hydrogen and helium nuclei. These source cosmic rays are known as Primary cosmic rays.
supernovae have been recorded. In our galaxy, a supernova only happens every 50 years, (on average.) When supernova explosions occur

Are cosmic rays harmful?
Primary cosmic rays could, potentially cause damage to cells in the body, and can cause cancer. Some scientists also have a theory, that cosmic rays may have altered Man's evolution, by altering DNA. However, it is highly unlikely that the primary rays will reach ground level (although it is not unknown); they will have already collided in the upper atmosphere. Secondary rays, which are common around ground level, are not harmful. If they were, or if the upper atmosphere didn't stop the primary rays, life on Earth may not have been possible.

Why does the Spark Chamber spark?

Spark formation sketch The simple explanation to this question is given in Stage 3 of 'How the Spark Chamber works'. When a cosmic ray has traveled through the detector, a large potential difference exists across each module. This is a very unstable situation, of which the modules cannot stay like it for any length of time. The plates must discharge.

 This discharge will occur along the easiest path possible. The easiest path is through the ionised track left behind in the Ne-He by the passage of a cosmic ray. Therefore the plates willdischarge down the ionised track of the cosmic ray, and hence the characteristic "spark" is observed, and the 'crack' from the discharge is heard.

Stages of spark formation within one module of the Spark Chamber
(adapted from Rice-Evans, 1974)

 A large potential difference across the closely spaced parallel plates of each module has associated with it an electric field in the vicinity of the plates. An electron  present in the active region of each module (created from the ionisation of the gas mixture as a charged particle traversed the chamber) is accelerated towards the anode plate.

 As the electron traverses the active region, its energy increases, which becomes great enough to cause ionisation when it collides with a gas molecule in its path. An additional electron is liberated which, after acceleration will also be able to ionise. This process continues and results in the formation of an "avalanche", which rapidly builds up. Electrons move towards the 'head' of the avalanche, whilst ions move in the opposite direction.

 When the number of electrons in the head approaches 106, the avalanche begins to slow down due to the attraction of the positive ions. When 108 electrons in the head is reached, an electric field within the avalanche is created, which is in the opposite sense to the electric field between the plates (shown in pink in figure 1c). Recombination of electrons and ions results within the avalanche, and photons are emitted isotropically from the "avalanche". The emitted photons cause ionisation of surrounding molecules in the vicinity of the original avalanche. The field in front and behind the original avalanche is enhanced, whilst the field around the sides are suppressed. Thus ahead and behind of the original avalanche, new avalanches rapidly form (figure d) until the old and new avalanches merge, forming a streamer (figure e). The extremities of the streamer grow (in approximately 10ns) until they arrive at the plates. Thus, the two plates of each module are connected by a low resistance conducting plasma of electrons and positive ions, which extends in a parallel direction to the electric field lines. A spark subsequently passes
between the two plates.

Contact Us

This web-site has been constructed by Stephen A. Bull as part of a final year undergraduate project and has been subsequently updated by James Fleming and Jessica Cuttriss as part of work experience placements in 2010.  The site will be maintained by the particle physics group.

Whilst every effort has been made to ensure that all the information given on this site is accurate and correct, no responsibility can be taken by the author or indeed the University for any inaccuracies found on these pages. We would appreciate any feedback and suggestions for improvements. Also anyone interested in learning more about Particle Physics at Birmingham, the School of Physics and Astronomy, or anyone interested in studying for a degree at the University of Birmingham may contact us. We will be delighted to hear from you.

Or write to us at:
Spark Chamber Correspondence
C/O Dr. J.A. Wilson
School of Physics and Astronomy
The University of Birmingham
B15 2TT