Particle accelerators
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Accelerators solve two problems for physicists:
Since all particles behave like waves, physicists use accelerators to increase a particle's momentum, thus decreasing its wavelength enough that physicists can use it to poke inside atoms.
The energy of speedy particles is used to create the massive particles that physicists want to study How do accelerators work?
Basically, an accelerator takes a particle, speeds it up using electromagnetic fields, and bashes the particle into a target or other particles
Surrounding the collision point are detectors that record the many pieces of the event. How to obtain particles to accelerate?
Electrons: Heating a metal causes electrons to be ejected.
A television, like a cathode ray tube, uses this mechanism.
Protons: They can easily be obtained by ionizing hydrogen.
Antiparticles: To get antiparticles:
first have energetic particles hit a target.
Then pairs of particles and antiparticles will be created via virtual photons or gluons.
Magnetic fields can be used to separate them. Accelerating particles
Accelerators speed up charged particles by creating large electric fields which attract or repel the particles.
This field is then moved down the accelerator, "pushing" the particles along. Accelerating particles
In a linear accelerator the field is due to traveling electromagnetic (E-M) waves.
When an E-M wave hits a bunch of particles, those in the back get the biggest boost, while those in the front get less of a boost.
In this fashion, the particles "ride" the front of the E-M wave like a bunch of surfers. Accelerator design
There are several different ways to design these accelerators, each with its benefits and drawbacks.
Fixed target: Shoot a particle at a fixed target.
Colliding beams: Two beams of particles are made to cross each other. Accelerator design
Accelerators are shaped in one of two ways:
Linacs: Linear accelerators, in which the particle starts at one end and comes out the other.
Synchrotrons: Accelerators built in a circle, in which the particle goes around and around and around... Fixed target experiment
A charged particle such as an electron or a proton is accelerated by an electric field and collides with a target, which can be a solid, liquid, or gas.
A detector determines the charge, momentum, mass, etc. of the resulting particles. Fixed target experiment
An example of this process is Rutherford's gold foil experiment, in which the radioactive source provided high- energy alpha particles, which collided with the fixed target of the gold foil. The detector was the zinc sulfide screen. Colliding beam experiments
Two beams of high-energy particles are made to cross each other.
The advantage of this arrangement is that both beams have significant kinetic energy, so a collision between them is more likely to produce a higher mass particle than would a fixed-target collision (with the one beam) at the same energy.
Since we are dealing with particles with a lot of momentum, these particles have short wavelengths and make excellent probes. Colliders Einstein's famous equation E=mc2 tells us that energy and mass are equivalent. Thus the energy of a particle beam can convert into mass, creating a fascinating wealth of additional particles, many of them highly unstable and not normally found in nature. However if the incoming beam is simply slammed into a stationary target, much of the projectile energy is taken up by the target's recoil and not exploitable. Much more energy is available for the production of new particles if two beams traveling in opposite directions are collided together. How they work?
something to accelerate the particles, something to bend them, something to focus them, a vacuum for them to travel through plus something to house the whole lot The basic principles
All particle beams start from a particle source. The simplest source is a hot wire, like the filament inside a light bulb. This is the kind of source used by television sets. Negatively charged electrons boil off the wire, and accelerate in a vacuum towards and through a positively charged electrode. Electromagnetic fields then sweep the beam across the screen. The points where the beam strikes the screen glow, building up a picture. A similar filament is also used in a linear electron accelerator Linacs accelerate particles to much higher energies than a television, but the principle is the same. In a linac, particles accelerate from one electrode to the next, gaining energy with each one they pass. Television
Televisions use the same principles as LINAC, but on a much smaller scale. Televisions and particle accelerators have a lot in common: a particle source accelerating electrodes (televisions have one, accelerators have many more) electromagnetic fields to deflect the particles... a particle detector (in a television, this is the screen) Basic components
Accelerating component Bending component Focusing components The race track The accelerating component: The cavity
Charged particles receive the energy needed to reach a speed close to that of light from sophisticated accelerating cavities like the one illustrated here. These cavities store up electrical energy, transferring a small amount to the particles each time they pass. They act like a short section of linear accelerator. The bending component: The dipole magnet
Magnets called dipoles are used to keep the particles moving in a circle. Each time more energy is pumped into the particles, the magnetic field has to be increased to prevent them from skidding off the ring. The focusing component: The quadrupole and sextupole
Other magnets, called quadrupoles and sextupoles, are used to keep the particles tightly packed within the beam. They work in much the same way as lenses do with light. The race track: The vacuum chamber
In particle accelerators, to ensure that particles are not lost by colliding with molecules of air, they travel inside a pipe, from which all the air has been removed. Vacuum pumps all around the ring ensure that there is even less matter inside the beam pipe than there is in outer space. The Large Electron Positron accelerator
The LEP is a collider. Its 3368 magnets bend two particle beams and keep them on orbit. Where negatively charged electrons bend one way, positively charged positrons bend the other. This allows LEP to circulate 90 GeV beams of electrons and positrons in opposite directions using the same magnets. The Super Proton Synchrotron (SPS), uses the same technique to circulate protons in one direction and anti-protons in the opposite direction. Charged particles accelerators
To induce nuclear reactions with positively charged particles (protons, alpha) Particles must have sufficient KE to overcome the barrier created by the repulsion between the positive charges of the particles and the nucleus Charged particles accelerators
To achieve higher KE the particles have to be ionized These ions can be accelerated through a potential difference thus acquiring some additional KE To obtain the desired KE: Production of the charged particles Acceleration thru the required potential difference Ion source – the principle
H2 Gas A gas is bombarded by energetic B1 B2 B3 Hot e Beam electrons Filament anode cathode The atoms of the gas are ionized S1 S2 Positive ions are produced vacuum
H+ Ions Ion source – the principle
H2 flows into region above H2 Gas
filament B1 B2 B3 Electrons are accelerated to Hot e Beam an anode (dV over B1-B2 = Filament anode 100 V) cathode Electrons passage thru the
gas cause ionization S1 Positive ions are extracted S2 by attraction to a negative electrode (dV over S1-S2 = 1-10 kV) into the accelerator vacuum region
Vacuum at beam extraction H+ Ions is 10-4 Pa, ionization area 10- 2 Pa Single-stage accelerators
Developed by Cockcroft-Walton - 1932 The total potential produced from a high- voltage generator is imposed across the accelerator Between the source and the target Single-stage accelerator
Principles The total potential produced from high voltage generator is imposed between the ion source and the target The KE of the particle is:
Ekin = nqV
# stages =1 Potential across acceleration gap
Charge of accelerated ions, C Single-stage accelerator
Recently, small versions of the Cockcroft-Watson accelerator Transformer-rectifier accelerators Used for acceleration of electrons or acceleration of deuterons for production of neutrons: 3 2 4 1 H +1H →2 He + n
Tritium targets are bombarded by accelerated deuterons Tunneling of the Coulomb barrier results in good yield for this reaction (even for 0.1 MeV) Single-stage accelerators
D2 molecules leak thru a heated palladium foil D2 Gas Accelerator tube into the vacuum of the Concentric electrodes ion source
There high frequency target electric field Ion source Particle path decomposed the D2 +1 molecules to form D 100 kV ions and electrons +<3 kV magnet
Radio +100 kV vacuum Cooling Ions are extracted with frequency water low negative potential Electron extractor to enter the High voltage generator acceleration tube with 2.5 keV KE Single-stage accelerators
The 100 kV is obtained from a transform and D2 Gas Accelerator tube rectifier unit coupled to a Concentric electrodes set of cylindrical electrodes connected by a target resistor chain Ion source Particle path The beam particles exit
the last electrode and drift 100 kV +<3 kV magnet thru a short tube and
strike the target (titanium Radio +100 kV vacuum Cooling water with absorbed tritium) frequency Electron extractor The target is cooled by High voltage generator water to minimize tritium evaporation Single-stage accelerators
With 100 keV and 0.5 mA This accelerator can D2 Gas Accelerator 10 tube produce 10 n/s with 14 Concentric electrodes MeV Can reduce KE to thermal values (0.025 eV) by target Ion source Particle path placing water or paraffin around the target 100 kV Flux of thermal neutron = +<3 kV magnet 108 n/cm2 Radio +100 kV vacuum Cooling Production rate of frequency water neutrons increases as the Electron extractor beam energy and beam High voltage generator current increases Van de Graaf accelerators (VdG)
Developed by van de Graaf in 1931 Can provide beams of higher energy than the single-stage C-W accelerators The tandem-VdG can produce 20 MeV protons and 30 MeV α-particles VdG can also accelerate electrons and positive ions of higher Z Van de Graaf accelerators (VdG)
A rapidly moving belt accumulates positive charge as it passes an array of sharp spray points Steel tank
Which transfer electrons target Insulating supports from the belt to the spray Accelerating tube Ion source points pulley pulley A + + + + + + + + + + + + + + + + + + + + + + + + + + + belt The positive charge on the - E1 E2 belt is continuously transferred by the movement Removable lid of the belt away from the ground Van de Graaf accelerators (VdG)
At the high-voltage terminal, (a hollow metal sphere) another set of spray points neutralize the charges on the Steel tank belt by electrons emitted target Insulating supports from the spray points Accelerating tube Ion source This results in positive pulley pulley A + + + + + + + + + + + + + + + + + + + + + + + + + + + belt charge to the sphere - E1 E2 The continuous process of transferring positive charge Removable lid to the sphere can built a high potential on the sphere Van de Graaf accelerators (VdG)
The limit of the voltage that can be accumulated in the hollow electrode is
determined by the discharge Steel tank potential to the surrounding housing target Insulating supports If it is insulated by some Accelerating tube Ion source pulley pulley A pressurized gas (N2, CO2 of + + + + + + + + + + + + + + + + + + + + + + + + + + + belt SF6) about 16 MV can be - E1 E2 achieved Removable lid This can be used to accelerate protons to energy of about 15 MeV in a single stage Van de Graaf accelerators (VdG)
The energy of the beam produced by the VdG generator is extremely precise The current (10-100 µA) is less than that of other accelerators The beam current i (A) is: Net charge of the beam particle
i = qIo = ezIo
Incident particle current (particles/s) Particle charge (C) Multi-stage accelerators
The potential obtained from a high voltage generator can be used repeatedly in a multi-stage accelerator process The linear accelerator operates in this principle Wideroe Multi-stage accelerators
The accelerator tube consists of a series of cylindrical electrodes – drift tubes Ion source The electrodes are coupled to a Vacuum chamber Ln Drift tube radio frequency generator The high voltage generator target n-1 n n+1 gives a maximum voltage V The voltage is applied to the electrodes by the RF so that the electrodes alternate in the sign of the voltage at a constant ABV ~ frequency RF oscillator Wideroe Multi-stage accelerators
If the particles arrive at the gap between electrodes in proper phase with the radio Ion source frequency, the particles are Vacuum chamber Ln Drift tube accelerated across the gap target They receive an increase ion n-1 n n+1 energy of qV (for n electrodes = nqV) Inside the drift tubes no acceleration takes place ABV ~ RF oscillator LINAC
Is a particle accelerator which accelerates charged particles - electrons, protons or heavy ions - in a straight line. Charged particles enter on the left and are accelerated towards the first drift tube by an electric field. Once inside the drift tube, they are shielded from the field and drift through at a constant velocity. When they arrive at the next gap, the field accelerates them again until they reach the next drift tube. This continues, with the particles picking up more and more energy in each gap, until they shoot out of the accelerator on the right. The drift tubes are necessary because an alternating field is used and without them, the field would alternately accelerate and decelerate the particles. The drift tubes shield the particles for the length of time that the field would be decelerating. Cyclotons
The cyclotron is a particle accelerator conceived by Ernest O. Lawrence in 1929, and developed, with this colleagues and students at the University of California in the 1930s. Cyclotons A Cyclotron
Consists of two large dipole magnets designed to produce a semi-circular region of uniform magnetic field, pointing uniformly downward.
These are called Ds because of their D-shape.
The two D's are placed back- to-back with their straight sides parallel but slightly separated. A Cyclotron
An oscillating voltage is applied to produce an electric field across this gap.
Particles injected into the magnetic field region of a D trace out a semicircular path until they reach the gap.
The electric field in the gap then accelerates the particles as they pass across it. A Cyclotron
The particles now have higher energy so they follow a semi- circular path in the next D with larger radius and so reach the gap again.
The electric field frequency must be just right so that the direction of the field has reversed by their time of arrival at the gap.
The field in the gap accelerates them and they enter the first D again. A Cyclotron
Thus the particles gain energy as they spiral around.
The trick is that as they speed up, they trace a larger arc and so they always take the same time to reach the gap.
This way a constant frequency electric field oscillation continues to always accelerate them across the gap.
The limitation on the energy that can be reached in such a device depends on the size of the magnets that form the D's and the strength of their magnetic fields.