Particle Acceleration

FIELDS OF STUDY: Astrophysics; Cosmology; Stellar Astronomy

ABSTRACT: Particle acceleration is any process that increases the energy and speed of particles, or small units of matter, such as protons and electrons. This can be achieved by means of a device or an object, natural or artificial, that is able to accelerate the particles to nearly the speed of light. Particle acceleration has important scientific, medical, industrial, and security applications.

Increasing Speed and Energy

In the early twentieth century, scientists believed that the radiation on Earth came from rocks on the surface. To prove this theory, scientists measured radiation from the top of the Eiffel Tower and other heights, trying to prove that radiation decreased with altitude. They learned that while radiation levels did decrease, the rate was far less than anticipated. Seeking answers, Austrian physicist Victor Hess (1883–1964) took to a hot-air balloon in August 1912 to measure the radiation in the atmosphere. At first, he found no clear differences. However, when the balloon reached a height of about 5,300 meters (17,388 feet), Hess discovered that the radiation levels were three times as high as at sea level. He concluded that radiation must be reaching Earth from outside its atmosphere. This particular radiation became known as cosmic rays.

Cosmic rays were originally thought to be a form of electromagnetic radiation, hence the name "rays." However, in the late 1920s, they were found to consist of highly energetic charged particles, awakening researchers to a wide range of previously unknown particles smaller than an atom. These included antiparticles, or particles with the same properties as ordinary particles but the opposite electrical and magnetic charges, such as antiprotons and positrons (antielectrons). Scientists began trying to determine what forces in the universe could create such high-speed, high-energy particles.

While this research was going on, other scientists began looking for ways to replicate these speed and energy levels in particles on Earth. The first manufactured particle accelerators were designed in the late 1920s and built in the early 1930s. By the 1950s, researchers were regularly using such devices to learn more about fundamental particles and matter. However, this achievement did not stop them from trying to track down the natural particle accelerators in space.

Natural Particle Accelerators

The key to finding the source of cosmic rays was another form of radiation, discovered more than a decade before Hess’s balloon ride revealed the presence of radiation from beyond Earth’s atmosphere. In 1900, Paul Villard (1860–1934), a French chemist and physicist, was conducting research into x-rays and cathode rays. He identified a new type of ray with high energy and a short wavelength. These rays stood out to Villard because they were able to "see" deeper than an x-ray and because they were not affected by either electric or magnetic fields. These newly found rays were called gamma rays.

In the decades that followed, researchers found that gamma rays could point the way to natural accelerators. Because gamma rays are unaffected by magnetic fields, it is easier to trace them back to their source. Study of gamma rays has led scientists to explore the powerful shock waves from supernovas and supermassive black holes as possible natural accelerators. In 2005 and 2006, scientists working with Suzaku, a joint Japanese Aerospace Exploration Agency (JAXA) and United States National Aeronautics and Space Administration (NASA) x-ray observatory, were studying a white dwarf star known as AE Aquarii, which might be a natural particle accelerator. The star also shows some qualities of a pulsar, releasing light in bursts. This meant that pulsars might also be natural sources of cosmic rays. Researchers continue to search the universe for objects with the power to imbue particles with great energy and speed.

Manufactured Particle Accelerators

Beginning in the 1930s, researchers have had access to increasingly advanced manufactured particle accelerators. These accelerators produce streams of charged particles—protons, electrons, and sometimes even atoms—that have high energy and travel at near the speed of light.

There are two main types of particle accelerators, linear and circular. In a linear accelerator, the particles travel in a straight line; in a circular accelerator, they travel in a loop or ring. The type of accelerator used depends on the experiment or procedure being conducted. Both linear and circular accelerators work in similar ways. The particles are placed in a metal beam pipe that is vacuum sealed to keep out dust and air, which could interfere with the experiment. Electromagnets are used to push and steer the particles as they move through the beam pipe. Electric fields are placed along the pipe and can have their current switched from positive to negative. This creates radio waves that help speed up clusters of particles as needed for the experiment. Most accelerators include targets, such as a piece of metal or other particles, where a beam of particles can be directed to cause a collision. They also have detectors that record the results. These collisions are orchestrated in order to study what happens when different types of particles encounter various targets. Because of this, some particle accelerators are also called colliders.

The largest and perhaps best-known collider is the Large Hadron Collider (LHC). After beginning operation in 2008, the LHC became one of several accelerators that are part of the European Organization for Nuclear Research (CERN). The LHC’s accelerator ring is twenty-seven kilometers (seventeen miles) long and is located in an underground tunnel near Geneva, Switzerland. Researchers announced in 2012 that data from the LHC showed strong evidence of the elusive Higgs boson, a type of elementary particle that was believed but not previously proved to exist. The next year, the LHC was shut down for two years to undergo repairs. A few of its 1,232 main superconducting dipole magnets were replaced, and more than ten thousand electrical connections to the magnets were reworked to increase safety and performance. All the work done prepared the LHC to create more collisions with greater power.

The LHC is only one of a dozen or so accelerators in the CERN facility. In 2010, the US Department of Energy estimated that more than thirty thousand particle accelerators were in use around the world. This estimate includes smaller accelerators that are used for medical or industrial purposes rather than for research.

Large accelerators are often located underground, both for practical reasons—they take up a lot of space—and for safety due to the power of the particle beams they create. These large facilities are mainly focused on research, seeking to learn why the universe is the way it is. Scientists theorize that the universe began when a singularity—an infinitesimal, infinitely dense collection of matter—expanded outward with explosive force. This "explosion" is known as the big bang. By studying how particles react when they collide with other objects or each other, scientists seek to understand how the big bang happened, what it means for the universe, and what other unknowns await discovery.

Everyday Applications of Particle Acceleration

Not all particle accelerators are the size of small towns or used to study the collisions of the tiniest parts of the universe. Smaller accelerators are used around the world in a variety of different ways. These include sealing consumer packaging, inspecting cargo, and helping diagnose and treat illnesses.

Particle acceleration plays two main roles in medicine: diagnostic and therapeutic. Many of the radioisotopes used in diagnostic testing are created by particle acceleration, and accelerators can also be used to generate x-rays or gamma rays for diagnostic imaging. Other accelerated particles are be used to treat cancer and other illnesses, to develop new drugs to treat serious illnesses, and to help in the study of DNA.

Thousands of products used every day by people worldwide are created or enhanced through particle acceleration. Particle accelerators are used to harden the materials used in manufacturing, improve medical procedures and food service by destroying pathogens, treat cancer by noninvasive procedures, and seal the packaging of consumer goods, among other tasks. The semiconductors necessary for many of the electronic items that have become part of daily life are made with the help of particle acceleration. Accelerated particles may also play a role in a cleaner environment; studies have shown them to be an effective way to clean up everything from nuclear waste to polluted water and air. Microwaves have been used to sterilize sewage sludge, decontaminate soil, and treat industrial wastewater.

Additionally, the national security of a number of countries, including the United States, has benefited from particle accelerators. Inspecting cargo, determining the characteristics of various materials, and inspecting nuclear fuels are all done with the help of accelerated particles. These particles can also play a role in national defense through the use of laser technology.

Scientists and researchers continue to study and use particle acceleration in hopes of understanding how the universe came to be the way it is and what could happen to it in the future. For example, the study of neutrino beams created from muons (unstable subatomic particles heavier than electrons) can help in this research. Some important research areas include the formation of elements in stars, the big bang, and matter-antimatter imbalance. In the meantime, particle accelerators also help improve everyday safety and quality of life.

PRINCIPAL TERMS

• cosmic rays: extremely high-energy charged particles, primarily atomic nuclei, that travel through space at near light speed.
• gamma rays: a form of electromagnetic radiation with high energy and a very short wavelength.
• particle accelerator: a machine designed to increase the speed and energy of subatomic particles to extremely high levels.

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