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Particle accelerator

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The Large Hadron Collider(LHC) is the largest particle accelerator in the world

The particle accelerator is a device made to increase the kinetic energy of an atomic or nuclear particle. Probably the earliest real accelerator was built by John D. Cockcroft and Ernest Walton at the Cavendish Laboratory in Cambridge, England. It was used to accelerate protons down a long, straight vacuum tube [1]. The American physicist, E. O. Lawrence, made the first cyclotron function in January, 1931, it had particles orbit in a circle 4.5 inches wide.[2] Today, most scientists use circular particle accelerators. The CERN (European Council for Nuclear Research) has the largest circular accelerator, named the Large Hadron Collider. The LHC has a 27 kilometer (17 mile) circumference with the possibility of accelerating particles to 7 TeV (Tera-electronvolts). [3][4]

Types of particle accelerators

There are two main types of particle accelerators, linear and circular (cyclic accelerators). The linear particle accelerators are classified by the way particles are accelerated; by fixed potential electric fields (as in tandem electrostatic accelerators) by magnetic induction of electric fields (as in television sets) and by radio frequency inductance. There are four types of circular particle accelerators (Cyclotrons, Synchrocyclotrons, Betatrons, and Synchrotrons). Today, most scientists use circular accelerators, because circular particle accelerators support more powerful energies with smaller sizes than linear particle accelerators and they can be used to analyze particles each cycle. [5]

Straight Line

Linear

The linear particle accelerators (linac) were mainly used before the circular particle accelerator was invented. Linear particle accelerators can not pass a beam of particles many times through the acceleration process because of its shape, therefore, the linear particle accelerator must accelerate the particles completely in their length. At present, the longest linear particle accelerator is the Stanford Linear Accelerator which is 3 km long. If scientists use the Stanford Linear Accelerator, they can not get results which need over 3 km of acceleration, and all detectors must be spaced along the 3 km path. [6] Linear accelerators have an advantage for very high speed electrons because there is no energy lost as there would be when the beam is curved.

Tandem Electrostatic

This accelerator uses the same basically linear design as linear particle accelerators[7], but it is divided into two stages to accelerate the particle. When scientists put the atom in the machine, it is allowed to pick up an electron to become a charged negative ion to gain energy by attraction to the positive vessel center. When the particle arrives at the center of the accelerator vessel, the ion loses electrons, therefore the ion changes to positive which continues on its path being repelled by the high positive voltage. [8]

The accelerator that split the atom, made in the 1950s

Circular

Cyclotron

Today, most particle accelerators are based on the circular type, because the circular particle accelerators do not have a required ending point, so the beam can continue until scientists have enough results for their study. Circular particle accelerators were first called Cyclotrons. The Cyclotron was developed in 1929 by Ernest O. Lawrence who taught at the University of California, Berkeley, and he and Dr. Livingston built the first cyclotron in 1932. Only 30 cm across, it accelerated protons to roughly 1.2 MeV. [9]

Cyclotrons had a limit of beam energy of only 15 MeV(Million electronvolts). The cyclotron consisted of a central cavity between two large, flat electromagnets. The cavity is like a large, hollow coin that is cut into two parts called Dees or D-electrodes. A radioactive ion source is placed in the center, and the strong magnets bend the path of the ions into a circle. The two D shaped hollow electrodes provide the empty space where the particles orbit, and a radio frequency generator hooked to the two Dees provides a way to accelerate the particles by attracting them first to one side and then to the other. As the speed increases, the particles move in a larger circle until they leave as a beam of particles and strike a detector. [10]

Synchrocyclotron

These cyclotrons were developed next by Lawrence since his Cyclotron has a relatively low 15 Mev limit of energy. Scientists needed a machine which can provide more energetic particles. Synchrocyclotrons, came next, followed by Isochronous Cyclotrons [11]. The Synchrocyclotron has a U-shaped beam profile with a big radius. In this machine, the radius is important because as the orbit radius increases, the magnet power varies to maintain beam speed. [12]

Betatron

In 1940, another kind of particle accelerator was developed from the Synchrocyclotron called the Betatron which accelerated electrons (which are also called beta particles) to useful energies. The American physicist Donald Kerst at the University of Illinois, changed the orientation of the magnets used so that the electrons were accelerated by magnetic induction.[13] The electrons circulate in a donut shaped vacuum tube and though the cyclotron was unable to accelerate electrons, the betatron could reach energies of 300 mev. [14] High energy electrons can be used to strike a metal target to create high energy x-rays, so betatrons are now sold for medical x-ray machines, and industrial x-rays.[15]

Synchrotron

This accelerator is the most modern machine. The biggest Synchrotron is the Large Hadron Collider (LHC) which was developed by CERN (the European Council for Nuclear Research). It has a 27 kilometer circumference and circular shape with constant radius and can support beams of up to 7 TeV (Tera electronvolts) of energy. It is usually organized as a large vacuum tunnel with the highest magnet power available to bend the path of particles to keep them in the circular tunnel. [16][17]

Uses

The Picture of a Particle accelerator model which is called Teilchenbeschleuniger

The Particle Accelerator is useful for basic science. In nuclear physics, the scientists study nuclear and condensed matter. Condensed matter shows high temperature with high densities, such as in conditions scientists postulate appeared in the Big Bang.[18]

Also particle accelerator are used to study the relationship between matter, space, and time. In addition, the particle accelerator is used in medicine. X-ray machines use low energy versions of the accelerator to create the beams of electrons that create the x-rays. Carefully controlled x-ray machines can show clear pictures of our bodies, though emphasizing bones and providing only black and white views.[19]

History

The old Particle Accelerator which was developed by CERN

The early particle accelerator was invented by John D. Cockcroft and Ernest Walton, who worked in the Cavendish Laboratory, in Cambridge, England. When they started investigating, in 1930, they used a 200-kilovolt potential with a straight discharge tube to accelerate protons. But 200-kilovolts was too weak to overcome the electron barrier around the nucleus. Therefore, they made voltage multipliers to increase their power (the voltage multiplier can amplify a lower voltage to a higher voltage using capacitors and diodes as switches, their design is still used today). In 1932, Cockcroft and Walton received their first results from an eight foot long tube with a particle accelerator potential of 800,000 volts. They used lithium as their target, and showed that a nucleus of lithium was broken into two alpha particles. [20]

The American physicist Ernest O. Lawrence constructed a cyclotron which can accelerate heavier ions by driving them in a circle, avoiding the extra long tubes needed as the particles grow heavier. Lawrence first made a 4.5 inch diameter cyclotron in January of 1931, and then an 11 inch model by September. He continued to make bigger cyclotrons obtaining research money for nuclear and medical research.[21]

In 1940, Donald Kerst built the first betatron at the University of Illinois, in the United States. Larger betatrons were built at levels of 25 MeV, and finally, in 1949, 300 MeV.[22]

After World War II, a European made particle accelerator developed into the Proton Synchrotron in CERN (European Council for Nuclear Research). The Proton Synchrotron was the first major particle accelerator in Europe, it can reach 28 GeV (Giga-electronVolt). To reach a more powerful voltage, physicists needed to make a bigger accelerator. For example, CERN made the LEP Synchrotron, which has circumference of 26.6 kilometers. The LEP Synchrotron was made underground and is still used at CERN. [23]

The USA also tried to make a large particle accelerator, named the Superconducting Super Collider, in Texas. This project’s plan was to develop the greatest particle accelerator with the highest support energy. The planned 87.1 km circumference would have energy support up to 20 TeV (Tera-electronVolts). However, this plan was canceled in 1993 largely due to rising cost estimates.[24]

Today, the LHC (Large Hadron Collider) is the largest particle accelerator in the world. It was made by CERN, with a 27 kilometer (17mile) circumference and hopes to reach TeV (Tera-Volt) by colliding beams going in two different directions.[25] However, the LHC was stopped by a superconducting magnet problem in September of 2008,[26]. Since part of the collider was broken, it was shut down until the temperature increased to where people could enter to make repairs. Some magnets short circuited with sufficient arcing to cause soot damage in other components, so all components had to be checked. As of June, 2009, it was hoped that the LHC could restart in the fall of 2009 (nearly a year after the magnet failure). [27] In March of 2010, the LHC was planned to begin functioning at the 3.5 TeV level, which is only half of its planned energy. During the repair phase, 250 magnets were repaired or replaced, and new protective sensors were added which will hopefully prevent any new catastrophic magnet failures. Problems were found in copper bus bars which make it wise to limit power until repairs can be made in the planned shutdown starting in 2012. Full power twin beams of 7 TeV each can only be used after these repairs, probably starting in 2013. The half power level is still higher than any other accelerator, though the Fermilab Tevatron will continue to operate until the LHC is confirmed at the very high power levels.[28]

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References

  1. American Institute of Physics, History, Cockcroft and Walton
  2. American Institute of Physics, History, Lawrence and the Cyclotron
  3. The Large Hadron Collider (LHC) Machine Outreach Page
  4. Encyclopedia Britannica
  5. Wikipedia, Particle Accelerator
  6. Linear Particle Accelerator by Wikipedia
  7. ANTARES - The Nuclear Accelerator Australian National by Australian Nuclear Science and Technology Organisation
  8. Particle Accelerator by Wikipedia
  9. Cern Teachers Resources, accessed 15 June 2009
  10. American Institute of Physics, History, Early Particle Accelerators
  11. Oak Ridge National Laboratory Oak Ridge Isochronous Cyclotron Light-ion Driver Accelerator for Radioactive Ion Beam Production
  12. Wikipedia Synchrocyclotron
  13. University of Illinois at Urbana-Champaign Department of Physics
  14. Wikipedia Betatron
  15. Global X-ray Betatron
  16. Wikipedia Large Hadron Collider
  17. Wikipedia Synchrotron
  18. Egglescliffe Organization Particle Accelerators
  19. Wikipedia X-ray
  20. American Institute of Physics, History, Lawrence and the Cyclotron
  21. American Institute of Physics, History, First Cyclotron
  22. University of Illinois at Urbana-Champaign Department of Physics
  23. Wikipedia Particle Accelerator
  24. Wikipedia Superconducting Super Collider
  25. Wikipedia Large Hadron Collider
  26. CERN Press Release LHC Restart
  27. CERN Press Release LHC Restart
  28. Powerful Collider Set to Smash Protons, by Ron Cowen, Science News Web addition, accessed 15 Feb 2010.

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