Download What does a spark chamber detect?

Spark chamber
1 & 3: The cosmic-ray particle crossing the scintillator causes light to be emitted. A part of
this light travels to the photomultiplier, where it is converted into an electronic signal. The
discriminator gives a binary (yes/no) response, producing a square-wave signal if the output
from the photomultiplier is above a noise threshold, or no signal otherwise
2: As the cosmic-ray particle crosses the neon-helium gas mixture inside the chamber, it
causes ionisation along its path, locally decreasing the electrical resistance.
4: A length of cable is used to delay the signal from discriminator 1 by the time taken for the
cosmic-ray particle to travel between scintillator 1 and scintillator 2.
5: When the coincidence unit records signals arriving simultaneously from the two
discriminators, it triggers the switching on of the high-voltage supply for a short time
interval. This creates large electric fields between neighbouring aluminium sheets in the
spark chamber.
6: The large electric fields create current flows along the paths of lowest electrical resistance,
meaning where the cosmic-ray particle caused ionisation. These current flows are seen as
sparks.
The spark chamber was developed between the late 1940s and the early 1960s, with
contributions from many people. It is a variation on a particle detector first demonstrated by
Hans Geiger and Walther Müller at the University of Kiel, in 1928. Spark chambers were the
first widely used track-visualisation devices that allowed triggering. This meant that they
could be used with an independent logic circuit that triggered activation of the detector when
specific conditions were satisfied. In most studies, only a tiny fraction of particle interactions
are of interest, and having detectors that can be triggered so as to select these is essential.
Large-volume spark chambers were used in the discovery of the muon-type neutrino, in 1962.
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been replaced by faster and much more sophisticated detectors with better time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 ElectronPositron (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 Our spark chamber consists
components in the dashed white box are the 'detector' of 16 modules, each
module
being
components of the spark chamber.)
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).
Each
module
consists
of a
perspex
frame,
0.8cm
thick The
detector
consists
of the 16
modules,
mounted
one on
top of the
other.
On the
top and
bottom of
these
perspex
frames
are glued
3mm
thick
aluminiu
m 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 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.
Neutrino 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
Electric
Charge
Mass
Average Penetration
Depth in Air
Summary
100m
Very energetic form
of light
3.7GeV +2e
0.1m
Equivalent structure
to He nucleus
0
1km
Very energetic form
of light
Primary cosmic Mixed Mixed
rays
approx. 10km
Mostly Hydrogen
and Helium nuclei
Secondary
cosmic rays
approx. 10km
Mostly muons
X-rays
0
Alpha rays
Gamma rays
0
0
Mixed Mixed
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:
http://www.sr.bham.ac.uk )
(C) Radiocarbon dating resulted from cosmic ray research (see
http://www.cq.rm.cnr.it/c-14.html )
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?
The
simple
explanatio
n 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
willdischa
rge down
the
ionised
track of
the
cosmic
ray, and
hence the
characteri
stic
"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.