Once again Year 13 Rooks Heath Physics students were lucky enough to attend the Particle Physics Masterclass at the Rutherford Appleton Laboratory, Harwell Campus, Didcot
The first activity of the day was a lecture entitled “A very brief guide to accelerators”
A very brief guide to accelerators
By Dr. Tom Williams
We have always wanted to investigate the very big and the very small.
The very big four hundred years ago
In 1609 Galileo, in Florence, improved the telescope, to give a magnification of x30
Galileo Galilei (15 February 1564 – 8 January 1642), often known as Galileo, was an Italian physicist, mathematician, engineer, astronomer, and philosopher who played a major role in the scientific revolution during the Renaissance.
A replica of Galileo’s Telescope
Galileo was able to use his telescope to investigate the Moon in 1610
The very small four hundred years ago
Robert Hooke was able to improve the compound microscope to give a magnification of x30
The image above right is the microscope manufactured by Christopher Cock of London for Robert Hooke. Hooke is believed to have used this microscope for the observations that formed the basis of Micrographia. (M-030 00276) Courtesy – Billings Microscope Collection, National Museum of Health and Medicine, AFIP).
Robert Hooke FRS (28 July [O.S. 18 July] 1635 – 3 March 1703) was an English natural philosopher, architect and polymath.
Modern portrait of Robert Hooke (Rita Greer 2004) based on descriptions by Aubrey and Waller; no contemporary depictions of Hooke are known to survive.
In 1665 Hooke published Micrographia, a book describing observations made with microscopes and telescopes, as well as some original work in biology.
Hooke was the first to apply the word “cell” to biological objects when he saw the structure of cork (above left). Above centre is his drawing of a flea and above right is his drawing of a louse.
Investigating the very big and the very small for probing matter – Today
The very big
The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area 2.5 arcminutes across, about one 24-millionth of the whole sky, which is equivalent in angular size to a 65 mm tennis ball at a distance of 100 metres. The image was assembled from 342 separate exposures taken with the Space Telescope’s Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and December 28, 1995.
The Hubble Space Telescope as seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and final human spaceflight to it
Almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known. By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe, with the associated scientific paper having received over 900 citations by the end of 2014.
The Hubble Ultra-Deep Field (HUDF) is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 24, 2003, through to January 16, 2004. Looking back approximately 13 billion years (between 400 and 800 million years after the Big Bang) it will be used to search for galaxies that existed at that time.
The above image is the Hubble Ultra-Deep Field image (full range of ultraviolet to near-infrared light) and includes some of the most distant galaxies to have been imaged by an optical telescope, existing shortly after the Big Bang (June 2014).
The very small
The Large Hadron Collider is the world’s largest and most powerful particle accelerator (Image: CERN)
The LHC recreates the Big Bang
In order to probe matter properly we need higher energies to give smaller wavelengths. The wavelength limits the size of the object that is being investigated. Maximum diffraction occurs when the wavelength is about the same size as the object being investigated and resolution improves with smaller wavelengths.
You need to use the quantum properties of particles for increasingly smaller particles
Wavelength = Planck’s constant/momentum
Cosmic Rays are the highest naturally occurring energy particles
Cosmic ray flux versus particle energy
Cosmic rays are immensely high-energy radiation, mainly originating outside the Solar System.
The image below shows cosmic Rays – illustration from Close F., Marten M. Sutton, Ch. 2002. The Particle Odyssey – A Journey into the Heart of Matter, Oxford: Oxford University Press
Interactions in the atmosphere involve a low flux of particles and the process is difficult to identify and control. Proper accelerators are needed.
Charged Particle Beams
The force on charged particle is given by the Lorentz Force
In physics, particularly electromagnetism, the Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields. If a particle of charge q moves with velocity v in the presence of an electric field E and a magnetic field B, then it will experience a force
The electric field E produces an acceleration on the charged particle
and the magnetic field bends the path of the charged particle
The electric field always acts from positive to negative
One of the earliest accelerators was built by John Cockroft & Ernest Walton at the Cavendish Laboratory in 1932.
Sir John Douglas Cockcroft, OM, KCB, CBE, FRS (27 May 1897 – 18 September 1967) was a British physicist. He shared the Nobel Prize in Physics for splitting the atomic nucleus with Ernest Walton, and was instrumental in the development of nuclear power.
Ernest Thomas Sinton Walton (6 October 1903 – 25 June 1995) was an Irish physicist and Nobel laureate for his work with John Cockcroft with “atom-smashing” experiments done at Cambridge University in the early 1930s, and so became the first person in history to artificially split the atom, thus ushering the nuclear age.
In the above left image you can see Walton and the machine used to “split the atom”. Its maximum accelerating voltage was around 1MVbefore it broke down. The electric field ionises the medium that the charges are passing through which limits the voltage.
In the above centre image you can see the ‘voltage-multiplying’ circuit that used capacitors (also known as condensers) to produce a high voltage direct current from a much lower voltage alternating current. Cockcroft and Walton erected a column of five rectifier diodes and capacitors to produce a DC voltage four times greater than the transformer AC voltage.
On the 14th April 1932 Walton set up the tube and bombarded lithium with high energy protons. He then crawled into the little observation cabin set up under the apparatus and immediately saw scintillations of the fluorescent screen. The reaction was giving off alpha-particles.
Linear particle accelerator
A linear particle accelerator (often shortened to linac) is a type of particle accelerator that greatly increases the kinetic energy of charged subatomic particles or ions by subjecting the charged particles to a series of oscillating electric potentials along a linear beamline; this method of particle acceleration was invented by Leó Szilárd. It was patented in 1928 by Rolf Widerøe, who also built the first operational device and was influenced by a publication of Gustav Ising.
The linac accelerates charged particles in a straight line by generating a high frequency alternating electric field along the common axis of a series of hollow ‘drift’ tubes.
During the first half of each alternating field cycle, the particles in the gaps between the drift tubes experience an accelerating field since they are repelled by the similarly charged tube they are leaving, and are attracted by the oppositely charged one they are approaching.
During the second half of the cycle, when the field is in the opposite direction and would decelerate them, the particles travel (or drift) through the centre of the tubes and are shielded from its effect. In this way the particles only ever experience accelerating electric fields.
As the particles travel down the linac, the drift tubes increase in length to directly compensate for the increase in speed as they must spend the same length of time drifting through each tube.
The main advantage of linear accelerators is that the particles are able to reach very high energies without the need for extremely high voltages.
The main disadvantage is that, because the particles travel in a straight line, each accelerating segment is used only once. This means that the only way of achieving particle beams with even higher energy is to undertake the expense of adding segments to the length of the linac.
2 Mile Linear Accelerator, SLAC, Stanford
The Stanford Linear Accelerator Center has a 2 mile linear accelerator. In a single pass, it accelerates electrons to 25 GeV. However it is inefficient.
SLAC National Accelerator Laboratory, originally named Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Department of Energy Office of Science and located in Menlo Park, California.
Cyclotron: First circular particle accelerator built by Ernest O. Lawrence & Stanley Livingston at Berkeley in 1930. Energy = 80 keV, Diameter = 13cm
Ernest Orlando Lawrence (August 8, 1901 – August 27, 1958) was a pioneering American nuclear scientist, winner of the Nobel Prize for Physics in 1939 for his invention of the cyclotron.
Milton Stanley Livingston (May 25, 1905 – August 25, 1986) was an American accelerator physicist, co-inventor of the cyclotron with Ernest Lawrence
A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1932 in which charged particles accelerate outwards from the centre along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying (radio frequency) electric field. Lawrence was awarded the 1939 Nobel Prize in physics for this invention.
Diagram showing how a cyclotron works. The magnet’s pole pieces are shown smaller than in reality, they must actually be as wide as the dees to create a uniform field.
A cyclotron accelerates a charged particle beam using a high frequency alternating voltage which is applied between two hollow “D”-shaped sheet metal electrodes called “dees” inside a vacuum chamber. The dees are placed face to face with a narrow gap between them, creating a cylindrical space within them for the particles to move. The particles are injected into the centre of this space. The dees are located between the poles of a large electromagnet which applies a static magnetic field B perpendicular to the electrode plane. The magnetic field causes the particles path to bend in a circle due to the Lorentz force perpendicular to their direction of motion.
A radio frequency (RF) alternating voltage of several thousand volts is applied between the dees. The frequency is set so that the particles make one circuit during a single cycle of the voltage. Each time after the particles pass to the other dee electrode the polarity of the RF voltage reverses. Therefore each time the particles cross the gap from one dee electrode to the other, the electric field is in the correct direction to accelerate them. The particles’ increasing speed due to these pushes causes them to move in a larger radius circle with each rotation, so the particles move in a spiral path outward from the centre to the rim of the dees. When they reach the rim the particles exit the dees through a small gap between them, and hit a target located at the exit point at the rim of the chamber, or leave the cyclotron through an evacuated beam tube to hit a remote target, Various materials may be used for the target, and the nuclear reactions due to the collisions will create secondary particles which may be guided outside of the cyclotron and into instruments for analysis.
The image above left shows Lawrence’s 60 inch cyclotron, with magnet poles 60 inches (5 feet, 1.5 meters) in diameter, at the University of California Lawrence Radiation Laboratory, Berkeley, in August, 1939, the most powerful accelerator in the world at the time. Glenn T. Seaborg and Edwin M. McMillan (right) used it to discover plutonium, neptunium and many other transuranic elements and isotopes, for which they received the 1941 Nobel Prize in physics. The cyclotron magnet is at left, the beamline which analysed the particles is at right.
The image above right shows M. Stanley Livingston and Ernest O. Lawrence (right) in front of Lawrence’s 27 inch cyclotron at the Lawrence Radiation Laboratory. The curving metal frame is the magnet’s core, the large cylindrical boxes contain the coils of wire that generate the magnetic field. The vacuum chamber containing the “dee” electrodes is in the centre between the magnet’s poles.
The frequency f of the charged particle = qB/2pm where q is the charge on the particle, B is the magnetic field strength and m is the mass of the particle.
The radius r of the path = mv/qB where v is the speed of the particle.
TRIUMF, Canada’s national laboratory for nuclear and particle physics, houses the world’s largest cyclotron. The 18 m diameter, 4,000 tonne main magnet produces a field of 0.46 T while a 23 MHz 94 kV electric field is used to accelerate the 300 μA beam. The TRIUMF field goes from 0 to about 8.128m radius with the maximum beam radius of 7.874m. This is because it requires a lower magnetic field to reduce EM stripping of the loosely bound electrons. Its large size is partly a result of using negative hydrogen ions rather than protons. The advantage is that extraction is simpler; multi-energy, multi-beams can be extracted by inserting thin carbon stripping foils at appropriate radii. TRIUMF is run by a consortium of eighteen Canadian universities and is located at the University of British Columbia, Vancouver, Canada. Current = 18,500 A.
TRIUMF is Canada’s national laboratory for particle and nuclear physics. The acronym derives from the word TRI University Meson Facility, indicating the historical fact of its establishment by a group of three Canadian universities. TRIUMF is considered Canada’s leading nuclear science research institute, and is consistently regarded as one of the leading subatomic physics research centers on the international level. TRIUMF’s headquarter is located on the south campus of the University of British Columbia in Vancouver, British Columbia. TRIUMF houses the world’s largest cyclotron, a source of 500 MeV protons, which was named an IEEE Milestone in 2010. TRIUMF’s activities involve particle physics, nuclear physics, nuclear medicine, and materials science.
Principal Components of a Synchrotron
A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the guiding magnetic field (bending the particles into a closed path) is time-dependent, being synchronized to a particle beam of increasing kinetic energy. The synchrotron is one of the first accelerator concepts to enable the construction of large-scale facilities, since bending, beam focusing and acceleration can be separated into different components. The most powerful modern particle accelerators use versions of the synchrotron design. The largest synchrotron-type accelerator is the 27 kilometre circumference Large Hadron Collider (LHC) near Geneva, Switzerland, built in 2008 by the European Organization for Nuclear Research (CERN).
Edwin McMillan constructed the first electron synchrotron in 1945, although Vladimir Veksler had already (unknown to McMillan) published the principle in a Soviet journal in 1944. The first proton synchrotron was designed by Sir Marcus Oliphant and built in 1952.
Synchrotrons consist of large circular devices where charged particles travel in evacuated pipes under the influence of magnets which are positioned around the circumference of the circle. Acceleration is achieved by the application of radio frequency electric fields at RF cavities along the circumference of the ring. The magnetic fields must be increased synchronously with the acceleration in order to keep the particles on the constant radius path. Such accelerators can be used with protons or electrons, and even with heavier positive ions.
In a Collider, bunches of particles/antiparticles circulate in opposite directions.
The vacuum chamber is a metal pipe where air is permanently pumped out (by the vacuum pumps) to avoid that the accelerated particles collide with normal matter (like air molecules) and annihilate or get deflected off course.
Inside the pipe, particles are accelerated by electric fields. These are provided by Radio-Frequency (RF) cavities. Each time charged particles traverse an RF cavity, the electric field inside the cavity gives them a “kick”, i.e. some of the energy of the radio wave is transferred to them and they are accelerated. To make a more effective use of a limited number of RF cavities, accelerator designers can force the particle beam to go through them many times, by curving the beam trajectory into a closed loop. That is why most accelerators are roughly circular.
The curving of the beam’s path, to make sure the particles stay within their circular track, is usually achieved by the magnetic field of dipole magnets (which have a North and a South pole, like the well-known horseshoe magnet). They are also called “bending magnets”. This is because the magnetic force exerted on moving charged particles is always perpendicular to their velocity – perfect for curving the trajectory! The higher the energy of a particle, the stronger the field that is needed to bend the particle’s path. This means that, as the maximum magnetic field is limited (to some 2 Tesla for conventional magnets, some 10 Tesla for superconducting ones), the more powerful a machine is, and the larger it needs to be.
In addition to just curving the beam, it is also necessary to focus it. Just like a shot from a shotgun, a particle beam spreads out as it travels. Focussing the beam allows its width and height to be constrained so that it stays inside the vacuum chamber. This is achieved by quadrupole magnets (which have four poles), which act on the beam of charged particles exactly the same way a lens would act on a beam of light. They are also called “focussing magnets”.
Super Proton Synchrotron CERN, Geneva (6km circumference) with a very high vacuum
It is housed in a circular tunnel, 6.9 kilometres in circumference, straddling the border of France and Switzerland near Geneva, Switzerland.
A microwave cavity or radio frequency (RF) cavity is a special type of resonator, consisting of a closed (or largely closed) metal structure that confines electromagnetic fields in the microwave region of the spectrum. The structure is either hollow or filled with dielectric material.
A microwave cavity acts similarly to a resonant circuit with extremely low loss at its frequency of operation, resulting in quality factors up to the order of 106, compared to 102 for circuits made with separate inductors and capacitors at the same frequency. They are used in oscillators and transmitters to create microwave signals, and as filters to separate a signal at a given frequency from other signals, in equipment such as radar equipment, microwave relay stations, satellite communications, and microwave ovens.
In addition to their use in electrical networks, RF cavities can manipulate charged particles passing through them by application of acceleration voltage and are thus used in particle accelerators.
Radiofrequency cavities along the LHC accelerate particles and keep them in controlled bunches (Image: CERN)
Large Electron Positron Collider (LEP) 1989-2000
27 km circumference
3,000 bending magnets
800 focussing magnets
During 11 years of research, LEP’s experiments provided a detailed study of the electroweak interaction. Measurements performed at LEP also proved that there are three – and only three – generations of particles of matter. LEP was closed down on 2 November 2000 to make way for the construction of the Large Hadron Collider in the same tunnel.
Electromagnetic waves accelerate particles in the same way that waves propel surfers. Timing is vital!
In accelerator physics strong focusing or alternating-gradient focusing is the principle that the net effect on a particle beam of charged particles passing through alternating field gradients is to make the beam converge. By contrast “Weak focusing” is the principle that nearby circles, described by charged particles moving in a uniform magnetic field, only intersect once per revolution.
Quadrupole magnets – strong focussing of the beam. Beam is alternately focussed in the horizontal and vertical planes.
Sextupole for the correction of chromatic spread
Tevatron, FermiLab (Chicago)
The LHC collides two proton beams. The energy is up to 7000 GeV. The path is 27 km in circumference.
The Rutherford Appleton Laboratory has its own synchrotron – Diamond Light Source
It started operation in February 2007. It uses synchrotron radiation for studies at molecular/atomic level.
Diamond Light Source is the UK’s synchrotron. It works like a giant microscope, harnessing the power of electrons to produce bright light that scientists can use to study anything from fossils to jet engines to viruses and vaccines.
The machine speeds up electrons to near light speeds so that they give off a light 10 billion times brighter than the sun. These bright beams are then directed off into laboratories known as ‘beamlines’. Here, scientists use the light to study a vast range of subject matter, from new medicines and treatments for disease to innovative engineering and cutting-edge technology.
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced, for example, in synchrotrons using bending magnets, undulators and/or wigglers. It is similar to cyclotron radiation except that synchrotron radiation is generated by the acceleration of ultrarelativistic charged particles through magnetic fields. Synchrotron radiation may be achieved artificially in synchrotrons or storage rings, or naturally by fast electrons moving through magnetic fields. The radiation produced in this way has a characteristic polarization and the frequencies generated can range over the entire electromagnetic spectrum.
The International Linear Collider (ILC) is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, with the possibility for a later upgrade to 1000 GeV (1 TeV). The host country for the accelerator has not yet been chosen and proposed locations are Japan, Europe (CERN) and the USA (Fermilab). Japan is considered the most likely candidate, as the Japanese government is willing to contribute half of the costs, according to the coordinator of study for detectors at the ILC . Construction could begin in 2015 or 2016 and will not be completed before 2026.
An overview graphic of the planned ILC based on the accelerator design of the Technical Design Report
• 31 km long – e+e- collider.
• Collisions between bunches of 5 nanometres in height.
• 14,000 collisions/second.
• Energy 0.5 – 1 TeV
• 16,000 superconducting cavities made of pure niobium.
• Maximise accelerating gradient (31.5 MV/m).
Compact Linear Collider (CLIC)
• Energy 0.5 – 5 TeV.
• Room temperature.
• Accelerating gradient 100 MV/m.
• High frequency travelling wave structure (12 GHz).
The Compact Linear Collider (CLIC) study is an international collaboration working on a concept for a machine to collide electrons and positrons (antielectrons) head-on at energies up to several Teraelectronvolts (TeV). This energy range is similar to the LHC’s, but using electrons and their antiparticles rather than protons, physicists will gain a different perspective on the underlying physics.