Physicists use particle accelerators to study the nature of matter and energy. The massive machines accelerate charged particles (ions) through an electric field in a hollow, evacuated tube, eventually colliding each ion with a stationary target or another moving particle. Scientists analyze the results of the collisions, attempting to probe the interactions governing the subatomic world. (The collision point is usually located in a bubble chamber, a device that records the tracks of ionizing particles as rows of tiny bubbles in a liquid-filled chamber.) The paths of the accelerating particles may be straight, spiral, or circular. Both the cyclotron(spiral path) and the synchrotron (circular path) use an increasingly strong magnetic field to control the paths of particles. Although smashing particles may initially appear to be an odd technique of studying them, particle accelerators have enabled scientists to learn more about the subatomic world than any other device. Particle Accelerators are devices used to accelerate charged elementary particles or ions to high energies. Particle accelerators today are some of the largest and most expensive instruments used by physicists. They all have the same three basic parts:
Here are the different types of accelerator:
Another machine, first
conceived in the late 1920s, is the linear accelerator, or linac,
which uses alternating voltages of high magnitude to push particles along
in a straight line. Particles pass through a line of hollow metal tubes
enclosed in an evacuated cylinder. An alternating
voltage is timed so that a particle is pushed forward each time it goes
through a gap between two of the metal tubes. Theoretically, a linac of
any energy can be built. The largest linac in the world, at Stanford University,
is 3.2 km (2 mi) long. It is capable of accelerating electrons
to an energy of 50 GeV (50 billion, or giga, electron volts). Stanford's
linac is designed to collide two beams of particles accelerated on different
tracks of the accelerator.
The American physicist
Ernest O. Lawrence won the 1939 Nobel Prize in physics for a breakthrough
in accelerator design in the early 1930s. He developed the cyclotron,
the first circular accelerator. A cyclotron is somewhat like a linac
wrapped into a tight spiral. Instead of many tubes, the machine has only
two hollow vacuum chambers, called dees, that are shaped like capital
letter Ds back to back .A magnetic field, produced by a powerful electromagnet,
keeps the particles moving in a circle. Each time the charged particles
pass through the gap between the dees, they are accelerated. As the particles
gain energy, they spiral out toward the
edge of the accelerator until they gain enough energy to exit the accelerator.
Q1) Explain why the particles are accelerated as the move across the gap but move at constant radius in either dee.
Q2) Show that the maximum speed a proton could have in a dee of radius R and strength B is given by (ignoring relativistic effects.)
vm = BeR / mp
this for protons in a 1.20m diameter cyclotron of field strength
show that the frequency of the alternating p.d must be 7.61 MHz.
are accelerated, they undergo a large increase in mass at a relatively
low energy. At 1 MeV energy, an electron weighs two and one-half times
as much as an electron at rest. Synchrocyclotrons cannot be adapted to
make allowance for such large increases in mass.
Q4) Using the relativistic formula for mass, calculate the speed of an electron that has a mass two and a half times its rest mass.
is the most recent and
most powerful member of the accelerator family.
A synchrotron consists of a tube in the shape of a large ring through which
the particles travel; the tube is surrounded by magnets that keep the particles
moving through the center of the tube. The particles enter the tube after
already having been accelerated to several million electron volts.
Particles are accelerated at one or more points on the ring each time the
particles make a complete circle around the accelerator. To keep the particles
in a rigid orbit, the strengths of the magnets in the ring are increased
as the particles gain energy. In a few seconds, the particles reach energies
greater than 1 GeV and are ejected, either directly into experiments or
toward targets that produce a variety of elementary
particles when struck by the accelerated particles. The synchrotron principle
can be applied to either protons or electrons, although most of the large
machines are proton-synchrotrons.
Q5) Show that
the radius of curvature of the path of particles of momentum p and
charge q in a synchrotron is given by the formula R = p / q B where B is the field strength.
Q6) A synchrotron of radius R has four straight sections of length L each. If the period of the radio frequency oscillator corresponds to the time of one revolution,show that
(a) The speed of the particles must be
(b) by considering the relativistic momentum of particles of mass M , that the magneticfield strength of the synchrotron is given by
In synchrotrons a computer is used to maintain this relation between magnetic field and oscillator frequency.
By the early 1980s,
the two largest proton-synchrotrons were a 500-GeV device at CERN and a
similar one at the Fermi National Accelerator Laboratory (Fermilab) near
Batavia, Illinois. The capacity of the latter,
called Tevatron, was increased to a potential 1 TeV (trillion, or tera,
eV) in 1983 by installing superconducting magnets, making it the most powerful
accelerator in the world. In 1989, CERN began operating
the Large-Electron Positron Collider (LEP), a 27-km (16.7-mi) ring that
can accelerate electrons and positrons to an energy of 50 GeV.
(5)STORAGE RING COLLIDER
A storage ring collider accelerator is a synchrotron that produces more energetic collisions between particles than a conventional synchrotron, which slams accelerated particles into a stationary target. A storage ring collider accelerates two sets of particles that rotate in opposite directions in the ring, then collides the two sets of particles. CERN's Large Electron-Positron Collider is a storage ring collider. In 1987, Fermilab converted the Tevatron into a storage ring collider and installed a three-story-high detector that observed and measured the products of the head-on particle collisions.
Accelerators are used
to explore atomic nuclei, thereby allowing nuclear scientists to identify
new elements and to explain phenomena that affect the entire nucleus. Machines
exceeding 1 GeV are used to study the fundamental
particles that compose the nucleus.
Several hundred of these particles have been identified. High-energy physicists hope
to discover rules or principles that will permit an orderly arrangement of the proportion of subnuclear particles. Such an arrangement would be as useful to nuclear science as the periodic table of the chemical elements is to chemistry. Fermilab's accelerator and collider detector permit scientists to study violent particle collisions that mimic thestate of the universe when it was just microseconds old. Continued study of their findings should increase scientific understanding of the structure of the universe.
Problem based on the decay scheme below:
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