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. Ion-beam accelerators, which accelerate heavier particles, find extensive use in the semiconductor industry in chip manufacturing and in hardening the surfaces of materials such as those used in artificial joints.
Tens of millions of patients receive accelerator-based diagnoses and therapy each year in hospitals and clinics around the world. There are two primary roles for particle accelerators in medical applications: the production of radioisotopes for medical diagnosis and therapy, and as sources of beams of electrons, protons and heavier charged particles for medical treatment. The wide range of half-lives of radioisotopes and their differing radiation types allow optimization for specific applications.
Isotopes emitting x-rays, gamma rays or positrons can serve as diagnostic probes, with instruments located outside the patient to image radiation distribution and thus the biological structures and fluid motion or constriction blood flow, for example.
Emitters of beta rays electrons and alpha particles helium nuclei deposit most of their energy close to the site of the emitting nucleus and serve as therapeutic agents to destroy cancerous tissue. Radiation therapy by external beams has developed into a highly effective method for treating cancer patients. The vast majority of these irradiations are now performed with microwave linear accelerators producing electron beams and x-rays. Accelerator technology, diagnostics and treatment technique developments over the past 50 years have dramatically improved clinical outcomes.
Today, 30 proton and three carbon-ion-beam treatment centers are in operation worldwide, with many new centers on the way.
The Energy Department's National Labs played a crucial role in the early development of these technologies. Los Alamos National Laboratory helped develop linear accelerators for electrons, now the workhorses of external-beam therapy. Oak Ridge and Brookhaven National Laboratories contributed much of the present expertise in isotopes for diagnosis and therapy.
The Large Hadron Collider is the largest and most powerful collider in the world. It boosts the particles in a loop 27 kilometres in circumference at an energy of 6. The type of particles, the energy of the collisions and the luminosity are among the important characteristics of an accelerator. An accelerator can circulate a lot of different particles, provided that they have an electric charge so that they can be accelerated with an electromagnetic field.
The CERN accelerator complex accelerates protons, but also nuclei of ionized atoms ions , such as the nuclei of lead, argon or xenon atoms. Some LHC runs are thus dedicated to lead-ion collisions. The energy of a particle is measured in electronvolts. One electronvolt is the energy gained by an electron that accelerates through a one-volt electrical field. As they race around the LHC, the protons acquire an energy of 6. It is the highest energy reached by an accelerator, but in everyday terms, this is a ridiculously tiny energy; roughly the energy of a safety pin dropped from a height of just two centimetres.
But an accelerator concentrates that energy at the infinitesimal scale to obtain very high concentrations of energy close to those that existed just after the Big Bang. The instantaneous luminosity is expressed in cm -2 s -1 and the integrated luminosity, corresponding to the number of collisions that can occur over a given period, is measured in inverse femtobarn.
One inverse femtobarn corresponds to million millions potential collisions. CERN operates a complex of eight accelerators and two decelerators. These accelerators supply experiments or are used as injectors, accelerating particles for larger accelerators. Some, such as the Proton Synchrotron PS or Super Proton Synchrotron SPS do both at once, preparing particles for experiments that they supply directly and injecting into larger accelerators.
The Large Hadron Collider is supplied with protons by a chain of four accelerators that boost the particles and divide them into bunches. Imagining, developing and building an accelerator takes several decades. For example, the former LEP electron-positron accelerator had not even begun operation when CERN scientists were already imagining replacing it with a more powerful accelerator.
That was in , twenty-four years before the LHC started. Work is also being done on alternative acceleration techniques for example with the AWAKE experiment. Many accelerators developed several decades ago are still in operation. The oldest of these is the Proton Synchrotron PS , commissioned in Others have been closed down, with some of their components being reused for new machines, at CERN or elsewhere.
The amount of energy a particle acquires—measured in electronvolts eV —as it moves through an electric field is determined by the difference in electric potential between where it enters and exits the field. Higher potential means higher particle energy. Magnetic fields focus and steer the particle beam. To get an idea of what this looks like, the above GIF illustrates a linear setup with an alternating electric field accelerating red particles.
To accelerate particles, both cyclic and linear accelerators typically use alternating electric fields generated by electromagnetic waves. These can range from radio- to microwaves. The field in adjacent accelerating cavities are out of phase with each other, so that the field ramps back up right as the particles transition from one cavity to the other.
All of this action happens inside vacuum chambers to avoid contact with the atmosphere. This is vital because charged particles are so small that they can be easily bumped off course or lose energy through collisions with the air.
These accelerated particles can be smashed into targets or into each other if there are two beams accelerating in opposite directions. They can be as tailor-made as the interactions researchers want to observe. There are two main accelerator families: linear and circular. Within those, there are many designs. The three most common types of accelerators are linear accelerators, cyclotrons, and synchrotrons. Linear accelerators or linacs are so named because of their shape.
In a linac, particles are accelerated through a sequence of electric fields in a straight line, gaining energy the further they travel. Like cars drag racing down a highway, they only go in one direction, accelerating all the while. The more fields they pass through, the more they accelerate, and the more fields, the longer the linac. Before the advent of flatscreen TVs, many people had accelerators sitting in their living rooms.
It can accelerate particles up to 50 gigaelectronvolts GeV. In the circular family of accelerators, there are two main types: cyclotrons and synchrotrons. In a cyclotron, particle beams are steered through relatively weak electric fields many times, gaining energy while traveling outward in a spiral towards a target.
Invented around , the first cyclotron was only 4. The largest ever built is 59 ft 18 m in diameter. Electrodes provide an alternating radio-frequency voltage that switches between the two dees.
Synchrotrons are a type of circular accelerator that can reach very high energies. They do this by keeping the electric and magnetic fields synchronized with the particle beam as it gains energy.
Hence, the name. Unlike the spiral motion of a cyclotron, particles move around a circle inside a synchrotron.
As the particles accelerate, the electromagnetic field in the ring increases to keep pace. The bunch could be a few centimeters long, but only a tenth of a millimeter wide. These bunches contain something like 10 12 particles, a density that still falls far short of the number of atoms that would be in an actual noodle of that size.
So, where do we go from here?
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