Friday, 27 July 2012
Monday, 28 May 2012
Particle physics is the study of the fundamental
constituents of matter and the forces of nature. Modern particle physics “involves the biggest, most complicated
experiments in the history of science, with the fastest computers, the coldest
temperatures and the strongest magnets on earth.(Particle Physics UK) It has featured as one of
the largest scientific media stories of recent years. Rumours inclyded that the
particle collisions taking place at the Large Hadron Collider (LHC) at CERN
near Geneva would produce black holes into which the Earth could be swallowed.
However, how much do most people know about the type of science that is being
carried out at CERN, as well as at other places around the world?
The high-energy particle physics being carried out near
Geneva comes under the category of accelerator physics. The other category,
non-accelerator physics, is where natural processes have produced the particles
and we detect their effects. Each has their own advantages and disadvantages. “In
cosmic rays, nature provides particles at energies far beyond anything we can
contemplate achieving on earth [at accelerators]. However these rays come at
random and are much less intense than beams made at accelerators. It was the
desire to replicate the cosmic rays under controlled conditions that led to
modern high-energy physics at accelerators.” (Close, 2004) However,
unveiling the tiniest constituents of matter with accelerators is only half the
battle. Physicists also need extraordinary particle detectors to be able to
observe what happens in high-energy collisions.
Detectors are instruments that count particles, visualize
tracks, measure particle energies, record time-of-flight and identify different
particles. Detectors can be as tiny as computer chips or as big as apartment
houses, containing thousands of tons of steel and other material. Detection is
possible only through the interaction between particles or radiation and matter
as they pass through. For example,
“when a charged particle travels
through a gas, it leaves behind a trail of ionised atoms. A whole range of
particle detectors, from the cloud chamber to the wire spark chamber, depends
on sensing this trail of ionisation in some way.” (Close, 2004)
p.63 However “ways of detecting subatomic
particles are more familiar than many people realise. The crackle of a Geiger
counter, and the light emitted when electrically charged particles, such as
electrons, hit specially prepared materials forming the picture on our
television screen, are but two”(Close, 2004)
p.62.
In this project I will survey the various methods of particle
detection. Professor P. M. S. Blackett, F.R.S., Nobel Laureate said in (Wilson & Rochester, 1952) pVII, that “this
world of sub-atomic events is one which can be easily visualised and understood
without the aid of complicated mathematics or the mastery of deep theories.” A
good example of which is the cloud chamber. Without going into “the subtle
intricacies and uncertainties of modern fundamental theoretical physics” it is
still possible for an experimenter to investigate particle interactions, and by
so doing to enter “the world of energetic elementary particles”.
Types of Particle Detector
Scintillators
Scintillators are materials
which emit light when high energy particles hit them. There are two types of
scintillators: the inorganic and the organic type. In the inorganic kind atoms
are ionised, which can be read as an electrical signal. In the organic type,
usually plastic, the signal is in the form of photons of light emitted as the
material’s electrons ‘de-excite’ after having collided with the incident
particle. (FermiNationalAcceleratorLaboritory, 2004) All materials
scintillate to some extent but the light is usually absorbed again before it
has a chance to reach the edges of the material. (Allison, 2006)
The Fermi National Accelerator
Laboratory website describes how “detectors
based on recording scintillation light were used from the earliest days of
nuclear research.” Probably the most famous and most significant particle
experiment of all time, Ernst Rutherford’s alpha scattering from gold foil in
19??, used a circular scintillation screen coated with zinc sulphate to measure
the angle at which the alpha particles were deflected, as shown in figure 1.
Each flash was recorded manually and the evidence gathered gave a very strong indication
for there being a small, dense nucleus, as opposed to the ‘plum pudding’ model
that had existed before. However, with modern scintillators the signal can be
measured by using light-sensitive devices called photomultiplier tubes, as I
did in my project detecting muons at Queen Mary, University of London.
An example of a scintillator
that we have probably all come into contact with is the conventional television
display. The screen emits light when bit by the electron beam from the back. (Allison, 2006) Other
examples include the “phosphor
coating on the inside face of a cathode-ray tube” (p.162)
However scintillators give poor spatial information. A
modern variant uses bundles of scintillating optical fibres. However this
method still has considerable draw backs.
Cloud Chamber
Invented by C.T.R Wilson in 1910, the cloud chamber or
expansion method made the tracks of particles visible for the first time. Detectors
such as cloud chambers and bubble chambers work on the principle of having
their contents in an unstable, so that even a small disturbance in the charges,
such as may be applied by a charged particle, is enough to collapse that state.
Cloud chambers use an atmosphere of supercooled alcohol. This means that the
alcohol is at a temperature
or pressure where it would normally be liquid, but it has been cooled or had
the pressure removed so rapidly that this has not had time to happen. Then,
when a charged particle ionises some of the atoms this is enough to collapse
this state and trigger the condensation process, causing a trail of droplets in
the particles path. These tracks can be illuminated and photographed. Professor
Frank Close describes in Particle Physics 2004 how, “when illuminated, the tracks stand out like the
dust motes in a sunbeam.” Figure 2 shows Wilson’s cloud chamber set-up.
Discoveries
made using the cloud chamber include “the
first example of an antiparticle, the positron.” (Close, 2004) p.65 Figure 3
shows the track of a positron moving upwards through a 3mm lead plate. Its
trajectory is curved by a magnetic field. It could either have been an electron
moving downwards or a positively charged particle moving upwards. The direction
was determined from the observation that the particle had lost energy going
through the lead plate and was therefore curving more in the magnetic field. It mass was
determined to be equal to that of the electron, thereby ruling out it being an
observation of a proton.(Observing the World of Particles)
Cloud chambers were
also used in the discovery of strange particles in cosmic rays. (Close, 2004) However,
cloud chambers took a very long time to re-set and were no longer adequate for
the higher energy particles being produced at accelerator experiments. This led
to the invention of the bubble chamber.
Bubble Chamber
The
bubble chamber Worked on much the same principle as a cloud chamber, but with
the active volume a liquid, heated close to its boiling point. If you remove
some of the pressure it would usually being to boil. However, if you do this
rapidly enough then the liquid enters a superheated state. The process that
then occurs when the liquid is disturbed is similar to the process that occurs in
the cloud chamber, apart from instead of condensing the liquid begins to boil
when disturbed, forming little bubbles of gas. It is these that line the trail
of the particle. Bubble chamber apparatus would involve the active volume of
the detector connected to a piston, which could lower the pressure quickly, and
would trigger a photograph of the chamber. The piston would then return and the
chamber would be re-set. Figure 5 shows the traces obtained by a bubble
chamber.
Bubble chambers superseded cloud chambers by the 1950s
because high-energy particles were now being produced at accelerators, which
would pass straight through a cloud chamber without interacting.
Bubble chambers would have a greater chance
of detecting these because, compared to the thin gas in a cloud chamber, the
liquid in them would have a much greater density of atoms, which meant there
was therefore a much greater chance of interaction. During the 1960’s and
1970’s a great deal of new elementary particles were discovered using the
bubble chamber. Originally designed by Donald Glaser, the design for the bubble
chamber was improved by Luis Alvarez, who, instead of filling it with water,
used liquid hydrogen at a temperature of 26K, an image from which can be seen
in figure 7. However, since the rate at which a bubble chamber can acquire and
analyse data is rather slow, bubble chambers are no longer used for research
purposes.
(FermiNationalAcceleratorLaboritory, 2004)
Photography
Discovered
by accident by Henri Becquerel after the photographic plates he left in a
drawer with specimens of uranium sulphate recorded their tracks, photographic
emulsions have been used ever since. The fine grains of silver halide in a
photographic emulsion are metastable and a change of state can be caused by the
energy deposited along the tracks of charged particles as they cross, much in
the same way light or X-rays do. (Allison, 2006) Developed by
Cecil Powell to give better sensitivity and a larger active volume, nuclear
emulsions were extensively used in the 1950’s as well as later on at particle
accelerators. Used in Powell’s discovery of the pion in 1947 as well roentgen’s
discovery of the X-ray, the nuclear emulsion’s high spatial resolution of about
one micron means that it is still used today investigating very short lived
particles. (Observing the World of
Particles)
The
advantages of the nuclear emulsion were that it was a “robust cheap cordless radiation monitor
which does not require calibrating” and traditionally “film badges were carried by all radiation workers to record the
time-integrated radiation dose to which they had been exposed.” (Allison, 2006) However
it was only able to give a two-dimensional position. They were therefore
replaced by three-dimensional detectors, such as wire chambers.
Wire Chambers
The ionisation trail of
particles through a gas-filled chamber can be made to accelerate towards
positive or negative terminals. By placing thin wires periodically throughout
the chamber in three dimensions and applying high voltages across them a signal
can be recoded of any ionisation that takes place within the chamber. When an
ion or electron is attracted towards a wire it is accelerated by the force and
so will ionise other atoms as it travels towards it. This means that once it
reaches the wire the signal is strong enough to be collected by a computer. By
analysing which wire the signal came from it is possible to obtain a precise
spatial measurement. Each particle will lead to several signals as it ionises
along its path and so a three-dimension trajectory and time of flight may be
obtained.
Cerenkov Detectors
Cerenkov
detectors are based on the phenomenon of Cerenkov radiation. Discovered by
Pavel Cerenkov in 1934, Cerenkov radiation is when particles give off blue
light when travelling faster than the speed of light in a particular medium. (Pavel A. Cherenkov - Biography) “Although light travels faster than anything
else in vacuum, particles can travel faster than light in gases, liquids or
solids. If charged particles travel faster than light in such a medium, they
give up some of their energy by emitting blue light. In a transparent medium
like water, ice, oil or gas, the blue light can escape and transmit the
information that a fast charged particle has passed through. Using various
media to optimize the size of the light cone around the particle's direction,
physicists build Cerenkov detectors that contain light-sensitive devices called
photomultipliers.” (FermiNationalAcceleratorLaboritory, 2004) Figure 8
shows the giant detector of Super-Kamiokande in Japan. It consists of a water
tank of diameter 39.3m and 41.4m in height surrounded by photomultiplier tubes,
which you can see being fixed in Figure 9.
Calorimeters
Calorimeters,
as the name suggests, are concerned with measuring the energy of an incident
particle. As particles move through matter they lose a fraction or all of their
energy due to interactions with it. This energy is deposited in the Calorimeter
and can be measured to give the energy lost and determine the total energy of
the incoming particle. They are made out of a variety of materials including “solid lead-glass, cesium iodide, liquid
argon and silica aerogel.” (FermiNationalAcceleratorLaboritory, 2004) Different
types of Calorimeters are made to absorb different types of particles. “Electromagnetic calorimeters measure the
energy of leptons (such as electrons) and photons (light particles) as they
interact with the electrically charged particles inside matter. Hadronic
calorimeters monitor the energy of particles containing quarks as they interact
with atomic nuclei.” (FermiNationalAcceleratorLaboritory, 2004)
Because
calorimeters attempt to absorb all of the energy of a particle moving through
it they are often of vast volumes. In modern detectors they are built around
other types of detector. By knowing the amount of energy present at the
collision point, and measuring the loss in energy during each interaction, the
types of particles produced may be pieced together. Figure 9 shows the readout
from the ATLAS detector at CERN. The outer ring is a calorimeter detector and
it surrounds other, smaller types of detector.
Silicon detectors
Silicon detectors work in a similar way to the wire chamber. A
series of silicon semiconductor strips a fraction of a millimetre wide are
built up on pads and a voltage applied across them.
“Charged particles passing
through the silicon create electric signals that exactly indicate which strips
the particles have crossed.” (FermiNationalAcceleratorLaboritory,
2004)
These strips are then connected to tiny electronic chips, which amplify and
feed the signal back to a computer. Spatial resolution in very good and is
constantly improving as smaller and smaller pieces of silicon can be
manufactured and the rate of data acquisition is extremely fast.
Neutrino detectors
Because neutrinos have no charge and possibly no mass* they
interact very little with matter. Therefore, as Frank Close describes it “the technique [for detecting them] is to have huge samples, such as swimming
pool volumes of pure water.” p.47 This is in the hope that you may get a
few interaction ever so often. However, the high number of neutrinos that are
passing through everything, as described in my project on detecting muons at
Queen Mary, University of London, does make this probability significantly
higher.
*the mass of the neutrino is a topic of great contention.
Some recent research has shown that it may have a very small, but significant
mass.
Modern Particle detectors
Depending on the type of accelerator and the particles and
forces to be studied, physicists combine various detection devices arranged in
intricate configurations, as briefly discussed above under Calorimeters. In the
case of colliding beams, physicists build a detector surrounding the point at
which the two beams collide. Like the layers of an onion, such a detector
contains successive layers of detection devices with different functions. Close
to the centre, physicists place precision tracking instruments such as silicon
detectors and wire chambers. The instruments are usually surrounded by
calorimeters that measure the energy of particles passing through. The outer
shell of a detector, farthest from the collision area, is devoted to detecting
muons, heavy electron-like particles that can travel a couple of kilometres
through rock and steel before decaying. (FermiNationalAcceleratorLaboritory,
2004)
Physicists have to consider which detectors are most
suitable for a particular experiment. As Allison describes, it is often the
form and rapidity of the output data that is often the deciding factor:
“Photographic dada are
cheap, of high quality and convenient in a low technology environment.
Electronic methods are often preferred because they can be linked straight to a
computer. Circumstances and resources determine which detector is used.” (Allison, 2006) p161
A great example of this is at the Atlas detector at CERN,
shown in Figure 11.
Accelerators
Modern
particle physics experiments used accelerators to fire particles either at a
static target or at each other and they use detectors to analyse what happens.
There are several types of accelerators; cyclotrons, synchotrons and linear.
Cyclotrons were one of the earliest types of particle accelerators and used
magnets to bend the paths of particles into a circle and electric fields to
accelerate them. Figure 12 shows a diagram of a cyclotron. Synchotrons, like
those used at the Large Hadron Collider (LHC) at CERN, are able to accelerate
multiple particles at the same time and also in different directions at the
same time, and are reaching ever higher energies. However, due to synchrotron
radiation (where the energy of the particles leaks away) they have to be made
with a large radius in order to achieve high energies. For example the LHC is a
twenty seven kilometre ring and as such is the largest experiment ever
conducted. Linear accelerators, such as the one at Stanford, are the most
precise, but unlike how the circular accelerators can accelerate the same
particle many times, it can only do so once. An exciting prospect under
construction will be the two linear accelerators aligned end to end which will
fire particles at each other, rather than at a static target as has been done
before.
Monday, 26 September 2011
Saturday, 30 July 2011
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