Particle accelerators Agen-689 Advances in Food Engineering Accelerators 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