The purpose of this section is to describe particle accelerators and detectors. Modern machines are based on earlier ones, so it is helpful to present a brief history of accelerators and detectors. A particle accelerator is a machine designed to accelerate charged particles. This acceleration is usually achieved with strong electric fields, magnetic fields, or both. A simple example of a particle accelerator is the Van de Graaff accelerator see Electric Potential. This type of accelerator collects charges on a hollow metal sphere using a moving belt.
When the electrostatic potential difference of the sphere is sufficiently large, the field is used to accelerate particles through an evacuated tube. Energies produced by a Van de Graaff accelerator are not large enough to create new particles, but the machine was important for early exploration of the atomic nucleus. Charged particles produced at the beginning of the linac are accelerated by a continuous line of charged hollow tubes.
The voltage between a given pair of tubes is set to draw the charged particle in, and once the particle arrives, the voltage between the next pair of tubes is set to push the charged particle out. In other words, voltages are applied in such a way that the tubes deliver a series of carefully synchronized electric kicks Figure.
Modern linacs employ radio frequency RF cavities that set up oscillating electromagnetic fields, which propel the particle forward like a surfer on an ocean wave. Linacs can accelerate electrons to over MeV. Electrons with kinetic energies greater than 2 MeV are moving very close to the speed of light. In modern particle research, linear accelerators are often used in the first stage of acceleration. Accelerating Tubes A linear accelerator designed to produce a beam of MeV protons has accelerating tubes separated by gaps.
What average voltage must be applied between tubes to achieve the desired energy? Hint :. Since and the proton gains 1 eV in energy for each volt across the gap that it passes through. The ac voltage applied to the tubes is timed so that it adds to the energy in each gap. The effective voltage is the sum of the gap voltages and equals MV to give each proton an energy of MeV.
Solution There are gaps and the sum of the voltages across them is MV. Therefore, the average voltage applied is 0. Significance A voltage of this magnitude is not difficult to achieve in a vacuum. Synchrotrons are aided by the circular path of the accelerated particles, which can orbit many times, effectively multiplying the number of accelerations by the number of orbits. This makes it possible to reach energies greater than 1 TeV. Check Your Understanding How much energy does an electron receive in accelerating through a 1-V potential difference?
The next-generation accelerator after the linac is the cyclotron Figure. A cyclotron uses alternating electric fields and fixed magnets to accelerate particles in a circular spiral path.
A particle at the center of the cyclotron is first accelerated by an electric field in a gap between two D-shaped magnets Dees. As the particle crosses over the D-shaped magnet, the particle is bent into a circular path by a Lorentz force.
The Lorentz force was discussed in Magnetic Forces and Fields. Assuming no energy losses, the momentum of the particle is related to its radius of curvature by. This expression is valid to classical and relativistic velocities. The circular trajectory returns the particle to the electric field gap, the electric field is reversed, and the process continues. As the particle is accelerated, the radius of curvature gets larger and larger—spirally outward—until the electrons leave the device.
Watch this video to learn more about cyclotrons. A synchrotron is a circular accelerator that uses alternating voltage and increasing magnetic field strength to accelerate particles to higher energies. Charged particles are accelerated by RF cavities, and steered and focused by magnets. Steering high-energy particles requires strong magnetic fields, so superconducting magnets are often used to reduce heat losses. As the charged particles move in a circle, they radiate energy: According to classical theory, any charged particle that accelerates and circular motion is an accelerated motion also radiates.
In a synchrotron, such radiation is called synchrotron radiation. This radiation is useful for many other purposes, such as medical and materials research. The Energy of an Electron in a Cyclotron An electron is accelerated using a cyclotron. If the magnetic field is 1. The exit momentum of the particle is determined using the radius of orbit and strength of the magnetic field. The exit energy of the particle can be determined the particle momentum Relativity.
Another method looks at how much a particle ionises the matter that it passes through, as this is velocity-dependent and can be measured by tracking devices. If a charged particle travels faster than light through a given medium, it emits Cherenkov radiation at an angle that depends on its velocity. Alternatively, when a particle crosses the boundary between two electrical insulators with different resistances to electric currents, it emits transition radiation, the energy of which depends on the particle's velocity.
Collating all these clues from different parts of the detector, physicists build up a snapshot of what was in the detector at the moment of a collision. The next step is to scour the collisions for unusual particles, or for results that do not fit current theories.
How a detector works Just as hunters can identify animals from tracks in mud or snow, physicists identify subatomic particles from the traces they leave in detectors Accelerators at CERN boost particles to high energies before they are made to collide inside detectors.
Tracking devices Tracking devices reveal the paths of electrically charged particles as they pass through and interact with suitable substances. Particle accelerators caught the attention of the general public back in when particle physicists at CERN — the European Organization for Nuclear Research — discovered the Higgs boson; colloquially referred to as the God Particle. This breakthrough discovery resulted from decades of research and experimentation, alongside billions of dollars of investment into the now renowned large hadron collider LHC.
Electromagnets steer and focus the beam of particles while it travels through the vacuum tube. Electric fields spaced around the accelerator switch from positive to negative at a given frequency, creating radio waves that accelerate particles in bunches. Particles can be directed at a fixed target, such as a thin piece of metal foil, or two beams of particles can be collided.
Particle detectors record and reveal the particles and radiation that are produced by the collision between a beam of particles and the target. Particle accelerators are essential tools of discovery for particle and nuclear physics and for sciences that use x-rays and neutrons, a type of neutral subatomic particle.
Particle physics, also called high-energy physics, asks basic questions about the universe. With particle accelerators as their primary scientific tools, particle physicists have achieved a profound understanding of the fundamental particles and physical laws that govern matter, energy, space and time.
Over the last four decades, light sources -- accelerators producing photons, the subatomic particle responsible for electromagnetic radiation -- and the sciences that use them have made dramatic advances that cut across many fields of research. Today, there are now about 10, scientists in the United States using x-ray beams for research in physics and chemistry, biology and medicine, Earth sciences, and many more aspects of materials science and development.
Worldwide, hundreds of industrial processes use particle accelerators -- from the manufacturing of computer chips to the cross-linking of plastic for shrink wrap and beyond. Electron-beam applications center on the modification of material properties, such as the alteration of plastics, for surface treatment, and for pathogen destruction in medical sterilization and food irradiation.
0コメント