Particle Accelerator

A particle accelerator is a machine that accelerates elementary particles, such as electrons or protons, to very high energies. Electrons and protons were respectively discovered by Thomson and Rutherford in 1897 and in 1919. In 1932, positrons were discovered studying cosmic rays, followed by muons and pions and more other elementary particles. In this way, particle physics or high-energy physics was born. From the mid-1950s, large accelerators were built and began to produce the “new” heaviest particles. Thanks to particle accelerators, a deep understanding of the fundamental particles and physical laws that govern matter, energy, space and time has been possible. Moreover, nuclear physicists and cosmologists may use beams of bare atomic nuclei, stripped of electrons, to investigate the structure, interactions and properties of the nuclei themselves. In addition, condensed matter at extremely high temperatures and densities, such as might have occurred in the first moments of the Big Bang. According to Stephen Hawking, we could define particle accelerators as the closest things we have to time machines. Today, accelerators are widespread and performing a variety of tasks. For example, they are used for medical diagnoses; for creating tumor-destroying beams of particles; for killing bacteria to prevent food-borne illnesses; to develop better materials for the manufacturing of computer chips. Finally, particle accelerators play an important role in national security, including cargo inspection, stockpile stewardship and materials characterization.

Why are particle accelerators so important in nuclear and particle physics? To examine matter at the scale of an atom, ~10-10 m, the energies required are in the range of a thousand electron volts. An electron volt is the energy that a particle acquires when it is accelerated across a potential difference of one volt. At the scale of the nucleus, energies in the MeV (106 eV) range are needed. To examine the fine structure of the basic constituents of matter requires energies generally exceeding 1 GeV (109 eV). Moreover, most of the objects of interest to elementary particle physics do not exist as free particles in nature; they have to be created artificially in the laboratory. The famous E=mc2 relationship governs the collisions in particle accelerators: an energy E is required to produce a particle of mass m. Many of the most interesting particles are so heavy that collision energies of many GeV are needed to create them. It has been necessary to reach the tens of TeV (1012 eV) to validate the Standard Model discovering the Higgs boson and to explore new theories beyond the Standard Model.

Facilities and Experiments

Particle accelerators use electric fields to speed up and increase the energy of a beam of particles, which are steered and focused by magnetic fields. The beam of particles travels inside a vacuum in a metal beam pipe. The vacuum is crucial to maintain an air and dust free environment and allow to the beam to travel undisturbed. There are two basic classes of accelerators: electrostatic and electrodynamic accelerators. Electrostatic accelerators use static electric fields to accelerate particles. Two examples of these instruments are the Cockcroft–Walton generator and the Van de Graaff generator. The cathode ray tube in an old television set is another example coming from everyday life that can help us to understand how electrostatic accelerators work. The achievable kinetic energy for particles is determined by the accelerating voltage and it is limited by electrical breakdown. In electrodynamic accelerators, electromagnetic fields are used instead to accelerate particles. Both circular and linear accelerators are part of this category. In circular accelerators, a radio frequency field is usually used. It is located in an area of the machine at which particles cross several times and each time they are accelerated. The particles are immersed in a magnetic field that curves their trajectories, allowing to exploit a single structure of acceleration for an unspecified number of times, thus obtaining very high energies. The LINear ACcelerators or LINAC are so called because they accelerates particles along a linear trajectory. They are generally consisting of a set of resonant cells in which there is a variable electric field. The longitudinal dimension of the cells must be adapted to the increasing speed of the particles to preserve their synchronism with the field, a fundamental condition for acceleration. The main advantage of a LINAC is its ability to produce beams of charged particles at high energies and intensity, with low dispersion in energy and small diameter, therefore resulting in high quality beams. In addition, for these accelerators, the injection and extraction of the beams are simple. All types of accelerators can reach different energies and are used for different purposes. The accelerators can also be used in a cascade system.

FIG. 1: The LHC is the last ring (dark blue line) in a complex chain of particle accelerators. The smaller machines are used in a chain to help boost the particles to their final energies and provide beams to a whole set of smaller experiments, which also aim to uncover the mysteries of the Universe.
FIG. 1: The LHC is the last ring (dark blue line) in a complex chain of particle accelerators. The smaller machines are used in a chain to help boost the particles to their final energies and provide beams to a whole set of smaller experiments, which also aim to uncover the mysteries of the Universe.

The accelerator complex at CERN, shown in FIG. 1, is a succession of machines that accelerate particles to increasingly higher energies. Each machine boosts the energy of a beam of particles, before injecting it into the next machine in the sequence. In the Large Hadron Collider (LHC), which is the last element in this chain, particle beams are accelerated up to the record energy of 6.5 TeV per beam. Most of the other accelerators in the chain have their own experimental halls where beams are used for experiments at lower energies. The proton source is a simple bottle of hydrogen gas. An electric field is used to strip hydrogen atoms of their electrons to yield protons. The first accelerator in the chain is the LINAC 2, which accelerates the protons to the energy of 50 MeV. They are then injected into the Proton Synchrotron Booster (PSB) and accelerated to 1.4 GeV. The next step is in the Proton Synchrotron (PS), which pushes the beam to 25 GeV. Finally, before reaching the LHC, protons are sent to the Super Proton Synchrotron (SPS) where they are accelerated to 450 GeV.

With the LHC energies, we can investigate inside the nucleus to reach the smallest particles known up to now. But first, we want to understand how the nucleus is made. Each nucleus consists of nucleons that are protons and neutrons. The shell model is the basic theory to describe the nuclear structure. It is based on the assumption that each nucleon inside the nucleus moves independently from the others in a spherically symmetric potential. The nucleons are arranged in shells and there are magic numbers which correspond to fully occupied shells. If a nucleus was made only of positively charged protons, it could never be stable due to the electromagnetic repulsion forces. Neutrons need to be present so that protons can bind together using the strong nuclear force that opposes the electromagnetic repulsion. The known nuclei or nuclides are represented in the Segré Chart or table of nuclides shown in FIG. 2. It is a two-dimensional graph in which one axis represents the number of neutrons (N) and the other represents the number of protons (Z). The stable nuclides occupy a region of the Segré chart called the valley of stability. There are no stable nuclides having an equal number of protons and neutrons in their nuclei with an atomic number greater than 20. Nuclei of a greater atomic number require an excess of neutrons for stability. The stability of nuclides is based on their binding energy. Binding energy is the energy which holds a nucleus together. Nuclei with very low or very high mass numbers (A = number of nucleons) have lesser binding energy per nucleon and are less stable because the lesser the binding energy per nucleon, the easier it is to separate the nucleus into its constituent nucleons (view FIG. 3).

FIG. 2: The Segré Chart or tables of nuclides. On the X-axis, there is the number of neutrons (N) and on the Y-axis, the number of protons (Z).
FIG. 2: The Segré Chart or tables of nuclides. On the X-axis, there is the number of neutrons (N) and on the Y-axis, the number of protons (Z).

With the LHC energies, we can investigate inside the nucleus to reach the smallest particles known up to now. But first, we want to understand how the nucleus is made. Each nucleus consists of nucleons that are protons and neutrons. The shell model is the basic theory to describe the nuclear structure. It is based on the assumption that each nucleon inside the nucleus moves independently from the others in a spherically symmetric potential. The nucleons are arranged in shells and there are magic numbers which correspond to fully occupied shells. If a nucleus was made only of positively charged protons, it could never be stable due to the electromagnetic repulsion forces. Neutrons need to be present so that protons can bind together using the strong nuclear force that opposes the electromagnetic repulsion. The known nuclei or nuclides are represented in the Segré Chart or table of nuclides shown in FIG. 2. It is a two-dimensional graph in which one axis represents the number of neutrons (N) and the other represents the number of protons (Z). The stable nuclides occupy a region of the Segré chart called the valley of stability. There are no stable nuclides having an equal number of protons and neutrons in their nuclei with an atomic number greater than 20. Nuclei of a greater atomic number require an excess of neutrons for stability. The stability of nuclides is based on their binding energy. Binding energy is the energy which holds a nucleus together. Nuclei with very low or very high mass numbers (A = number of nucleons) have lesser binding energy per nucleon and are less stable because the lesser the binding energy per nucleon, the easier it is to separate the nucleus into its constituent nucleons (view FIG. 3).

FIG. 3: Binding energy per nucleons as a function of the mass number. The fact that there is a peak in the binding energy curve in the region of stability near iron means that either the breakup of heavier nuclei (fission) or the combining of lighter nuclei (fusion) will yield nuclei which are more tightly bound (less mass per nucleon). Image Courtesy of Indira Gandhi Centre for Atomic Research
FIG. 3: Binding energy per nucleons as a function of the mass number. The fact that there is a peak in the binding energy curve in the region of stability near iron means that either the breakup of heavier nuclei (fission) or the combining of lighter nuclei (fusion) will yield nuclei which are more tightly bound (less mass per nucleon). Image Courtesy of Indira Gandhi Centre for Atomic Research

Magic numbers are well established for nuclei along the stability line and for most of the known unstable nuclei.

The most critical ingredients in determining the properties of a nucleus are the number of nucleons and the ratio N/Z of neutrons to protons. The extremes in these quantities define the limits of nuclear existence. To create new elements we can add neutrons or protons to the existing nuclei. When neutrons are successively added to a nucleus on the stability lines, the binding energy of the last neutron decreases steadily until it vanishes and the nucleus decays by neutron emission. The position in the Segré chart where this happens is called the neutron drip line, which is farther from the valley of stability than the proton drip line, placed at the opposite side of the stability line. The location of the neutron drip line is known only for nuclei with mass up to around 30 amu (atomic mass unit which is approximately the mass of one nucleon). Therefore, the study of nuclei with large neutron excess (neutron-rich nuclei) is focused to find the position of the neutron drip line, but most of all on the investigation of how interactions between the nucleons, the nuclear density and the size in nuclei vary for exotic N/Z ratios. These changes are expected to lead to different nuclear symmetries, magic numbers and new excitation modes.

SPES is an INFN project to develop a second-generation Radioactive Ion Beam facility. This facility is presently in the construction phase at the Italian National Laboratories in Legnaro (LNL). SPES is the acronym for “Selective Production of Exotic Species”. The aim of the SPES project is to provide high intensity and high-quality beams of neutron-rich nuclei to perform forefront research in nuclear structures, reaction dynamics and interdisciplinary fields such as medical, biological and material sciences. It is based on the ISOL (Isotope Separation On-Line) method: a target, the ion source and an electromagnetic mass analyzer to separate different nuclides are coupled in a series. This apparatus is said to be on-line when reaction products of interest are slowed down and stopped in the system. We start from a proton beam, which is delivered by a Cyclotron accelerator with an energy of more then 40 MeV and a beam current of 200 A. Neutron-rich radioactive ions will be produced by Uranium fission at an expected fission rate in the target of the order of 1013 fissions per second. The exotic nuclides will be re-accelerated by the ALPI superconducting LINAC at energies of 10 MeV per nucleon and higher, for masses in the region of A=130 amu, with an expected rate on the secondary target of 108 pps (particle per second). NEDA (NEutron Detector Array) is a new generation array of neutron scintillator detectors that will be used in experiments with stable and radioactive beams at European accelerator facilities such as SPES. The goal of the experiments for which NEDA was designed are measurements of level densities and fission dynamics of neutron-rich nuclei. The most important characteristics of NEDA are: a good efficiency of neutron detection at energies between 1 to 20 MeV; a very good neutron-gamma discrimination which allows to perform experiments in a high gamma-ray background environment and at a high count rate; a resolution which allows the detection of events with neutron multiplicity larger than one (emission of a single neutron); a modular design to arrange the detectors around the target and couple them with other devices maximizing the geometrical detection efficiency and energy resolution. A regular hexagon was chosen as the starting point for the design of the NEDA geometry since it is the most suitable polygon for both clustering detectors and coupling to a circularly shaped photomultiplier tube (PMT), minimizing the uncovered area by the PMT. NEDA detectors will have the shape of a uniform hexagonal prism depicted in FIG. 4 left, with a depth of 20 cm and the length sufficient for the detection of neutrons with energies up to 10 MeV and a side of 8.1 cm, to make it suitable for a 5” diameter PMT; the largest commercially available PMT with flat window. The volume of the unit is about 3 liters and will be filled with EJ301 liquid scintillator. The thickness of the walls of the chamber is 3 mm, large enough to provide mechanical stability to the detector. The PMT used for NEDA is the Hamamatsu R11833-100-03.

To choose the design of the NEDA array it is important to take into account the geometrical efficiency, the target-to-detector distance for the best possible TOF discrimination of events and the granularity. In FIG. 4 – right, two configurations for a mixed array of NEDA and Neutron Wall (NW) cells are shown. NW is one of the first ancillary array of neutron detectors. The first NEDA detector units, available before completing the construction, have been used in combination with the old detectors. The two array configurations were proposed for the AGATA campaign at GANIL in France. AGATA (Advanced Gamma Tracking Array) is a new generation 4π spectrometer built from Germanium detectors. Configuration (a) allows to cover a larger solid angle with a smaller number of NEDA detectors, while (b) has a slightly larger granularity in the forward angles. The detector composed by AGATA (@ 145 mm from the target), NEDA (54 cells @ 510 mm) and NW (42 cells @ 650 mm) has been used for the identification of rare neutron-deficient evaporation residues produced by fusion-evaporation reactions.

FIG. 4: On the left, the NEDA detector unit is shown. On the right, two configurations for a mixed array of NEDA and NW cells are represented. They were proposed to be used together with the AGATA gamma detector at GANIL.
FIG. 4: On the left, the NEDA detector unit is shown. On the right, two configurations for a mixed array of NEDA and NW cells are represented. They were proposed to be used together with the AGATA gamma detector at GANIL.
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