CERN (Conseil Européen pour la Recherche Nucléaire) is located in a northwest suburb of Geneva, Switzerland and is the world’s leading center for collaboration on nuclear research. One of its many activities involves the study of particle physics.
Particle physics is conducted in machines known as particle accelerators (aka particle colliders). These use electromagnetic fields to accelerate particles to high speeds and focus them into a fine beam. The world’s largest and most powerful particle accelerator, the Large Hadron Collider (LHC) began operation at CERN in 2008.
The LHC (Figure 1) is a 27 kilometer (16.8 mile) ring of superconducting magnets held at temperatures colder than outer space. Within this machine, subatomic particles smash together at near light speed in an ultrahigh vacuum. It has allowed scientists unique insights into the fundamental building blocks of matter. One of the key advances that make the LHC possible is an impressive array of vacuum technologies, consisting of several separate vacuum systems. These include cryogenic vacuum pumps operating at 1.9 Kelvin (-271°C or -456°F), turbomolecular pumps, and getter pumps using non-evaporable getter (NEG) coatings, a technology that was born and developed at CERN.
Below we will discuss the development of particle colliders, the fundamental technologies behind their operation, and how all this lead to the development of the Large Hadron Collider.
In grade school, we learned that matter was made up of atoms, which consisted of electrons orbiting a nucleus containing protons and neutrons (Fig. 2). For many years it was thought these were the fundamental particles of matter, and they could not be broken down further into anything smaller. It is primarily through the use of colliders like the LHC that smaller, more fundamental particles have been revealed; over 200 so far. One of the most amazing characteristics of the atom is how much empty space it consists of.
Although atoms are extremely tiny, electrons, protons and neutrons are much smaller yet. The diameter of an atom is about 10-11 meters but its nucleus is 1/10,000 of this. To understand the relative sizes of the atoms and their nuclei, as well as their masses, it is useful to make a comparison to tangible objects. If an atom were the size of a football stadium, for example, the nucleus would be smaller than a marble. The electrons would be orbiting at the perimeter of the stadium, and everything in between the marble and the perimeter would be empty space. The atomic nucleus is so dense, that the mass of that marble would be over 104 billion (104,000,000,000) kilograms or 114 million tons.
The electrons move very fast around the nucleus, each forming a “shell”, which accounts for the volume of the atom. Although quantum theory is necessary to explain the motion of electrons around the nucleus, it can be said in laymen’s terms that the distribution of electrons is in a spherical shape.
The concept of matter being composed of fundamental particles began in ancient Greece. In the 5th century BC, Greek philosopher Democritus first proposed that the universe consists of empty space as well as indivisible particles he called ‘atomos’ (“un-cuttables” in Greek). Interestingly, he was also the first to suggest that the idea that the Earth is located in a galaxy consisting of many stars. In 1802 English Chemist John Dalton furthered atomic theory, using it to explain compounds and chemical reactions. Dalton is also famous for being the originator of Dalton’s law of partial gases. In 1897 Joseph Thomson, a British scientist, discovered the electron through his research using cathode ray tubes. He recognized that atoms consist of much lighter, smaller particles he called ‘corpuscles’, which we now know as electrons. In 1911 New Zealand physicist Ernest Rutherford performed his famous gold foil experiment. Rutherford directed high-velocity particles (helium nuclei) at a thin sheet of gold and found most of the particles went right through the gold foil and only a few bounced back. This indicated the foil, although a solid, consisted of much more empty space than solid material. Rutherford postulated the planetary model of the atom in which electrons orbited around the nucleus like planets around the sun. He was later the first to identify the atomic nucleus, the proton, as well as alpha and beta particles. A giant among physicists, Rutherford also discovered that atoms of heavy elements have a tendency to decay, leading to the carbon dating technique still used in historical research.
The Invention of the Particle Accelerator
In 1932 after four years of development, British physicist John Cockcroft and Irish Physicist Ernest Walton designed the first particle accelerator. The device was called an electrostatic accelerator since the machine produced low energy protons in a small glass chamber above an eight-foot tall evacuated tube, and they were then accelerated down the tube in a fine beam using a voltage multiplier. One of the critical technologies making this machine possible was the development of the vacuum pump (Fig. 3).
This beam of high-energy protons came out of the bottom of the tube and bombarded a target placed underneath it. Using this machine, they conducted the first controlled splitting of the atom. Cockcroft and Walton observed that upon bombarding a lithium sample with protons, helium was emitted (Fig. 4). They concluded that the lithium nucleus (consisting of 3 protons) had, with the introduction of another proton, had been split into two smaller helium nuclei, each consisting of 2 protons. This interpretation was later fully confirmed.
Advances in Particle Accelerator Design
By 1934, a different type of collider was invented by American physicist Ernest Lawrence at the University of California Berkeley. Lawrence, who had been lured from a faculty position at Yale University with promises of connections to the chemistry department, was a later contributor on the Manhattan Project. Lawrence’s cyclotron, as he called it, was a type of particle accelerator in which charged particles accelerate outwards from the center along a spiral path in a vacuum (Fig. 5). They are held on their trajectory by a magnetic field and accelerated by a rapidly varying this electric field. Lawrence was awarded the Nobel Prize in physics for this invention. The cyclotron was an improvement on the Cockcroft and Walton’s linear accelerator, but further improvements in this technology were to come.
A Basic Problem for Particle Accelerator Design to Overcome
As improvements in accelerator technology led to faster particle discharge velocities, scientists were faced with overcoming the effects of special relativity. According to special relativity, the mass of particles moving near the speed of light increases significantly in a non-linear manner, making further acceleration requires much more energy (Fig. 6) than would be expected in classical Newtonian physics.
These relativistic effects of a particle’s speed on its mass was originally measured by scientists in the early 1900’s when electrons were accelerated to various speeds approaching the speed of light, and their deflection measured as they passed through a magnetic field of known strength. It was discovered that at velocities approaching light speed, the electrons deflected less than would be predicted by the calculations of Newtonian physics, which govern the motion of physical bodies in our everyday world. When the deflections were measured it was realized the only explanation for their reduced deflection was that their mass was actually increasing at higher speeds. In fact, the experiments, repeated by many scientists, showed the increase in mass matched that predicted by Einstein’s equations for special relativity. This represented a huge challenge to the designers of particle accelerators, as extreme amounts of energy would be required to accelerate particles to speeds approaching the speed of light and allow the further breakthroughs of physics to be realized.
In order to overcome relativistic effects at high particle velocity, a new type of particle accelerator was required, one that could impart much greater energy to the particles before leaving the device. The result was the synchrotron, the early precursor to the Large Hadron Collider. Unlike the cyclotron, which accelerates particles in a spiral pattern before discharging them, the synchrotron carries the particles in a circular loop (Fig. 7). They are allowed to make many multiple passes, as on a racetrack, and continue to accelerate during each pass, until they reach the desired velocity and are discharged from the accelerator. The magnetic field strength that contains the particles is increased over time as the particles accelerate, to keep them on the “race track” even as they gain mass. In addition, the frequency of the magnetic field is increased to continue the acceleration as the particles gain mass while approaching light speed.
Measuring Energy Levels in Particle Accelerators
In order to comprehend the relative energy levels of particle accelerators, it is useful to understand the unit of measurement called the electron volt. The electron volt (eV) is the amount of energy gained (or lost) by the charge of a single electron moving across an electric potential difference of one volt. It is used in particle physics because a particle with charge q has an energy E = qV after passing through the potential V. Interestingly, the electron volt is also a unit of mass in particle physics, where units of mass and energy are often interchanged using Einstein’s famous formula E = mc2. Early particle accelerators generated less than 1 million electron volts (MeV). As the technology improved and better accelerators were developed, this increased to 1 gigaelectron volts (GeV), which is 1 billion electron volts. After its initial startup, the Large Hadron Collider was upgraded to achieve 2.36 TeV, or 2.36 trillion electron volts, which is over 5,000 times more powerful than Cockroft and Walton’s original accelerator.
The LHC uses this energy to accelerate two beams of protons to 99.999956% of the speed of light, before colliding them head-on. Each beam is made up of 2808 groups (referred to as bunches) of protons and each bunch contains over 1 x 1011 protons. Since protons are so tiny, this high number is required to increase the likelihood of a head-on collision, resulting in the desired liberation of smaller, more elementary particles. The study of these fundamental particles is the real goal of the LHC.