DETECTORS, ASTROPHYSICAL

Particle physics is defined more by the questions addressed than by the techniques used. Although accelerator-based experiments have been, and will be, the primary experimental tool for particle physics research, nonaccelerator measurements are playing an increasingly important role. The intellectual connection between particle physics and other fields, particularly astrophysics and cosmology (the study of the history and evolution of the universe), has grown significantly over the past decade, as great progress has been made on many questions of common interest. In most cases, these detectors use the same measurement principles as in accelerator-based detectors, but they are adapted for a much wider variety of environments. The detector choices are motivated by the physics being investigated.

Connections and Physics Goals

Laboratory techniques for fundamental physics investigations have long been successfully applied to astrophysical measurements. In turn, making increasingly precise measurements of astrophysical phenomena, over immense distance scales and energy scales, allows one to address fundamental questions, test limits of physical law in the most extreme environments and over immense distances, and study the relationships between physical law and the evolution of the universe. Some of the goals of this research include

• Exploring black holes: The goal is to understand the acceleration mechanisms producing ultra-high-energy jets from supermassive (10⁶ to 10⁹ solar mass) black hole systems, which are nature's highest-energy accelerators, and study the characteristics of these remarkable objects. Black holes provide an important laboratory for testing theories of gravity, which is the least well-understood of the fundamental forces. The fact that many black hole systems shine brightly in high-energy particles makes them especially interesting to particle physicists and high-energy astrophysicists;
• Finding the origin(s) of the highest-energy cosmic rays (energetic particles propagating in outer space). Historically, particle physics grew out of cosmic ray physics (and nuclear physics), yet the origin of the cosmic rays remains an unsolved problem;
• Understanding gamma-ray bursts, which are brief and intense flashes of gamma-rays from faraway explosions that appear to be the most powerful since the Big Bang;
• Uncovering galactic dark matter, a hypothetical new form of matter that is required to explain a variety of observations of galaxies and clusters of galaxies;
• Studying the cosmic microwave background (CMB), an important and remarkable fossil from the early universe;
• Testing inflation, currently the most widely accepted paradigm for models of the earliest stages in the evolution of the universe;
• Searching for other Big Bang relics, which are clues to the puzzle of the early universe and which may provide information about physics at energy scales far higher than those achievable with artificial accelerators;
• Detecting gravity waves, a clear prediction of general relativity (the theory of gravity) and an entirely new window for astronomical observations;
• Confirming and studying the dark energy, the generic explanation for the recently discovered apparent increase in the expansion rate of the universe;
• Most importantly, discovering the unanticipated.

Many of the detectors that address these physics topics are surveyed below. For reasons of scope and space, a number of important categories are omitted, most notably optical, microwave, and X-ray detectors. These instruments have provided spectacular results in astrophysics and cosmology, and they are playing increasingly important roles in particle physics. In coming decades, particularly as dark energy and dark matter are better understood, these detectors may become part of major particle physics experiments. In what follows, energy is expressed in units of electron volts (eV), with 1 keV = 10³ eV, 1 MeV = 10⁶ eV, 1 GeV = 10⁹ eV, and 1 TeV = 10¹² eV.

Gamma Rays

Gamma rays are the highest-energy photons, which are the quanta of electromagnetism. Gamma-

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ray photons are very similar to the photons of visible light humans see, except they are at least a million times more energetic. Astrophysical sources of gamma rays identify sites of extreme particle acceleration, such as neutron star and black hole systems, or signal decays of very massive particles that might have been produced in the early universe.

Gamma rays are so energetic that, when interacting with matter, pairs of new particles with opposite electric charge can be created when a portion of the photon energy is converted into the new particles' mass. The pair is usually an electron and its antimatter partner, the positron. If there is sufficient energy, this process of converting energy into matter continues and a shower of particles is produced. The number of particles in the shower is proportional to the kinetic energy of the initial particle. When a celestial gamma ray encounters the upper atmosphere, it interacts and is stopped by this process. If the gamma ray has very high energy (more than approximately 50 to 100 GeV), enough information about the shower reaches the ground, and the direction and energy of the initiating gamma ray can be measured by ground-based detectors. For lower-energy gamma rays, these measurements must be made above the atmosphere, in space. For both ground-based and space-based detectors, one of the most significant challenges is background identification and rejection. For every gamma-ray photon from the weakest astrophysical sources, there can be upward of 10,000 background particles (mostly charged cosmic rays). This fact has a strong influence on the instrument designs and sometimes sets the fundamental limitations on the scientific capabilities of a particular detector.

There are two basic types of ground-based gamma-ray detectors: airshower Cerenkov telescopes (ACTs) and extended airshower (EAS) array detectors. Cerenkov light is emitted when a charged particle travels through a medium at speeds faster than light propagates in that same medium. The emission pattern is somewhat like the wake made by a speedboat in water. ACTs detect the Cerenkov light emitted by the electrons and positrons in the gamma-ray-induced shower. Large reflectors collect the light and focus it onto an array of sensors (usually phototubes) that record an image of the shower. The size, shape, and orientation of the image identify the shower and provide a measure of the gamma-ray energy and direction. The Whipple observatory, located on Mount Hopkins in Arizona, is one of the pioneering ACTs. The detector consists of a 10-meter reflector dish, comprising 248 mirror segments, and a camera array of approximately 500 phototubes. Other ACT observatories include CANGAROO (Australia), HEGRA (Canary Islands), and CAT (France). Two other experiments, STACEE in New Mexico and CELESTE in France, use large arrays of solar energy collectors for extended sensitivity to lower gamma-ray energy: during the daytime, these facilities are solar energy power plants, and during the nighttime, they detect gamma rays from space. Future detectors in planning or under construction use arrays of ACTs that can work together or independently. These include HESS and VERITAS. Note that gas Cerenkov detectors are also used in accelerator-based detectors for particle identification but in very limited volumes; in contrast, ground-based gamma-ray ACTs use the atmosphere as the detection medium, and they therefore have enormous collecting areas capable of detecting faint gamma-ray sources.

ACTs can only operate on cloudless, moonless nights, so EAS detectors are also being used for ground-based gamma-ray astrophysics. When the gamma-ray energy is high enough (typically greater than 500 GeV), the resulting air shower particles can reach the ground, and EAS detectors directly sense the passage of these particles. The number and geographic distribution of the particles gives a rough measure of the gamma-ray energy, and the relative arrival times of the particles across the array give a measure of the gamma-ray direction. Relative to ACTs, EAS detectors have much higher operating efficiency because they can make measurements day and night, and they also have a much larger field of view; however, they also typically have much worse measurement precision. The two techniques are therefore complementary. An example of a gamma-ray EAS is the Milagro detector, located in the Jemez mountains of New Mexico. The detector is an enclosed pool of water, about the size of a football field, with an array of over 700 phototubes. When airshower particles pass through the water, they emit Cerenkov light that is detected by the phototubes.

Space-based high-energy gamma-ray detectors use the pair conversion process even more directly. The gamma ray converts inside the active volume of the instrument, and the electron and positron trajectories are detected using the same kind of charged particle tracking detectors found in accelerator-based experiments. The combined tracks give the gamma ray direction. The energy is measured in a calorimeter placed behind the tracking detectors. Space-based calorimeters are quite similar to accelerator detector calorimeters, except they are far smaller and less massive due to launch volume and weight limitations. Other significant differences are that space-based detectors must withstand the accelerations and acoustic shocks from the launch vehicle; they must operate on far less power; they must work reliably without any possibility of repair (though access inside immense accelerator-based experiments is becoming equally difficult); and, of course, they must operate in the environment of space. The most successful space-based high-energy gamma-ray instrument was EGRET, aboard the Compton Gamma Ray Observatory (GRO) that was launched in 1991. EGRET had a gasbased spark chamber tracking detector and a single calorimeter. GLAST, which is planned for launch in 2006, uses modern particle physics techniques (precision silicon strip tracking detectors and a segmented calorimeter) and will provide much greater sensitivity. The Italian mission, AGILE, which is a miniature version of GLAST using similar technology and with sensitivity comparable to EGRET, is scheduled to launch in 2003. Since GRO was deorbited in 2001, AGILE will fill an important gap in time until GLAST launches.

Ground-based and space-based gamma-ray detectors have complementary capabilities, and results from each are even more scientifically significant when combined together. The next generation of instruments will, for the first time, have significant overlap in energy coverage, providing new opportunities for important crosschecks and comparisons of results.

Cosmic Rays

Cosmic rays are high-energy particles that propagate through space. The vast majority of cosmic rays are protons, and their observed energies range over more than twelve orders of magnitude to more than 10²⁰ eV. There are many important mysteries to solve in this area, but the question of most direct interest to particle physicists is the origin of the highest-energy cosmic rays. At present, the acceleration mechanism is not understood, and it is possible that some new physics at very high energy scales is required. Compounding this mystery is the fact that, while the universe is mostly transparent to most protons traveling through interstellar space, protons with energy greater than a particular cutoff energy, called the Greisen-Zatsepin-Kuzmin (GZK) energy, collide and interact with the pervasive CMB photons, causing them to lose energy. (Below the GZK cutoff energy, there is not enough energy to cause the reaction with the CMB photons.) One would therefore expect a reduction in the number of detected particles with energy greater than the GZK energy. Surprisingly, there is evidence that the number actually increases. However, the statistical evidence is not conclusive, and this must be confirmed by more than one experiment with greater statistical significance. Since the flux of these highest-energy particles is very low (approximately one particle per square kilometer per century!), gathering useful numbers of events within a reasonable amount of time requires instruments with enormous collecting areas.

As with ground-based gamma-ray detectors, the enormous collecting area is achieved by using the atmosphere as the principal medium. Showers of particles in the atmosphere, initiated by the cosmic ray particle, are detected either directly with EAS arrays (see the Gamma-ray Section above) or indirectly via collection of the light emitted by the shower particles. For these measurements, the light comes from atmospheric nitrogen that is excited by the developing airshower. Unlike Cerenkov light, this nitrogen fluorescence light is emitted isotropically, which greatly increases the effective collecting area of the sensors.

The world's largest cosmic ray EAS array is the AGASA array in Akeno, Japan. It has 111 particle detectors spaced approximately 1 kilometer apart, covering a total area of about 100 square kilometers. The AGASA array has detected the largest number of cosmic ray events with estimated energy greater than 10²⁰ eV. The HiRes experiment, in Utah, is a nitrogen fluorescence detector consisting of a set of collecting mirrors and phototube cameras. By imaging the light from two different perspectives, the properties of the airshower (and therefore the primary particle) can be better determined. HiRes has an effective collecting area that is larger than that of AGASA, but it has a lower observing efficiency. The Pierre Auger Observatory, under construction in Argentina, will combine the two techniques by using both types of detectors. Some of the airshowers will be observed by both types of instruments, allowing important cross-checks and a combined analysis. Auger will have a much larger collecting area, consisting of 1,600 EAS particle detectors covering 3,000 square kilometers, along with 24 fluorescence detector telescopes. A second Auger installation in the Northern Hemisphere is also being planned. Pushing the nitrogen fluorescence technique to its ultimate implementation, OWL, a space-based light collector that will view a large fraction of the entire atmosphere, is being considered. OWL would have an effective aperture approximately 1,000 times larger than that of AGASA.

Another important puzzle is the asymmetry between matter and antimatter in the universe. If models of the early universe are correct, matter and anti-matter were produced in equal amounts, so why does matter apparently dominate today? The AMS experiment, which will fly on the International Space Station, is being built to search for an anomalous flux of antimatter particles in space. AMS has silicon strip charged particle tracking detectors and a strong magnet to measure the momentum and electric charge of particles passing through it. Combined with other sensors in the experiment, the particle type can be identified. AMS, and related experiments such as PAMELA, will carry into space investigations that have been done on high-altitude balloons by experiments such as BESS and CAPRICE.

Neutrinos

There are several astrophysical sources of neutrinos, including solar neutrinos, which are produced in the core of the sun as a fusion reaction byproduct; atmospheric neutrinos, which are produced in airshowers initiated by cosmic rays; neutrinos from supernovae within our galaxy; and extragalactic neutrinos, which are expected from supermassive black hole accelerator systems and gamma-ray bursts. Neutrinos would also be produced by decays of most hypothetical massive relic states from the Big Bang and early universe.

Neutrinos interact only via the weak nuclear force (and, of course, gravity), so detecting them usually requires very massive instruments with which the neutrinos have a practical probability of interacting. There are two categories of weak nuclear interactions, called neutral current (NC) and charged current (CC), and the neutrino can participate in both of them. In a NC interaction, the neutrino scatters off a particle in the detector (nucleus or electron), imparting some of the neutrino's energy and momentum to that particle. After the collision, the neutrino escapes the detector. In a CC interaction, the neutrino is converted into a charged lepton (electron, muon, or tau, depending on the type of neutrino), which can be detected directly, and the nucleus is also converted to that of a neighboring element in the periodic table or is broken up if enough energy is transferred to it. By measuring the trajectory of the emerging charged lepton, the initial direction of the neutrino can be inferred. There are many different kinds of neutrino detectors, but they all rely on NC and/or CC interactions in some manner. The main distinction is the method of detecting the deposited energy and emerging lepton. In addition, the detector setup is optimized to study a particular source of neutrinos. All of these detectors operate deep underground to minimize backgrounds.

Proton decay detectors, which are very large-volume instruments that can detect small energy releases within those volumes, are often also excellent neutrino observatories. These detectors have made important observations of neutrinos from a supernova, atmospheric neutrinos, and solar neutrinos. An example is the Japanese Super-Kamiokande detector, a huge, 50-ton, underground imaging water Cerenkov detector. The inner portion of the detector is viewed by 11,146 phototubes. The cone of Cerenkov light projects a ring onto the array of phototubes, and the character, location, size, and shape of the ring tell the particle type, energy, and direction. In addition to neutrino interactions within the volume of the detector, the instrument can also detect CC interactions that occur nearby in the surrounding rock, effectively instrumenting the Earth. Airshower neutrinos from the other side of the Earth travel through the Earth and can produce upward-going muons that are seen in the detector. Analysis of these events has provided important evidence for neutrino oscillations. The Sudbury Neutrino Observatory (SNO) in Canada is a similar type of detector, but the active medium is heavy water (D₂O) instead of ordinary water. This allows distinct measurements of the CC and NC interactions and provides other important cross-checks for neutrino oscillation measurements. The Soudan II detector, located in an underground laboratory in Minnesota, has very similar physics goals but uses a different detection principle. The heart of the detector is a 960-ton iron calorimeter with gaseous charged particle sensors. Borexino, at the Gran Sasso laboratory in central Italy, has as its main emphasis the study of solar neutrinos. The detector consists of 300 tons of liquid scintillator, viewed by 2,200 phototubes. ICARUS is a large-volume charged particle tracking detector, called a time projection chamber (TPC), whose target and detection medium is not gas but rather liquid argon.

Isotope experiments, rather than viewing the interactions immediately when they happen, detect the presence of small numbers of converted element nuclei indicating neutrino CC interactions after the fact. The Homestake detector, located in a gold mine in South Dakota, was constructed to measure the flux of solar neutrinos. Six hundred tons of tetrachloroethylene served as a target for the neutrinos. A CC interaction converted the ³⁷Cl nuclei into ³⁷Ar, which could then be detected by its radioactive decay. The number of argon nuclei tracked the number of neutrino interactions during the running period. This instrument uncovered a surprising deficit of solar neutrinos relative to theoretical expectation, providing evidence that neutrinos have mass. Two other detectors of this type, Gallex at Gran Sasso, Italy, and the Russian-American SAGE experiment, use gallium as the target medium.

Other detectors search for higher-energy neutrinos by detecting the products of CC interactions in the matter near the instruments. The usual signature is the detection of upward-going muons (the flux of downward-going muons is dominated by muons from cosmic-ray induced airshowers). The MACRO detector, in Gran Sasso, is a large-area detector designed to search for magnetic monopoles, but it is also quite effective for searching for high-energy neutrino fluxes using this technique. Instead of using optically opaque rock as the target medium for the neutrino interactions, one can use large, naturally occurring volumes of water or ice. The AMANDA experiment is an array of phototubes placed deep in the Antarctic ice, 1.5 to 2.2 kilometers under the surface. Upward-going high-energy muons from CC neutrino interactions produce Cerenkov light in the ice, which is collected by the phototubes. The arrival times of the light at the spatially distributed phototubes gives a measurement of the muon trajectory and hence the original neutrino direction. A vastly expanded version of AMANDA, called ICECUBE, is currently being planned. The BAIKAL experiment in Siberia, the NESTOR experiment in Greece, and the ANTARES experiment in France use water instead of ice as the medium. The detection principles are the same as for the ice experiments, though the details of the light propagation are different.

Gravity Waves

Gravity waves have not yet been detected, but there is a good chance this situation will change dramatically over the coming decade as new detectors are completed and begin operation. According to theory, as a gravity wave propagates, space is stretched and compressed by tiny amounts. The size of the strain depends on the strength of the disturbance that created the waves. Typical astrophysical sources that are believed to generate gravity waves (supernova core collapse, neutron star and black hole mergers) are expected to cause strains around 10^-20 or smaller. Thus, a typical gravity wave passing through the solar system would change the distance between the earth and the moon by an amount that is less than the radius of a single proton! Such tiny changes can be detected using laser interferometry. The largest interferometer system under construction is LIGO. There are two LIGO sites, one in Hanford, Washington, and one in Louisiana. Each site has a 4-kilometer, two-arm interferometer with state-of-the-art control and noise reduction systems. LIGO will be able to detect gravity waves in the frequency band between a few Hz and a few thousand Hz, with strain sensitivity as low as 10^-23. Other gravity wave interferometers are VIRGO and GEO in Europe, TAMA in Japan, and AIGO in Australia. By comparing signals from these geographically distributed detectors, the direction of the wave can be inferred. Seismic noise and other terrestrial disturbances, along with the interferometer arm length, limit the ultimate capabilities of these detectors. A space-based interferometer system would not have these limitations. LISA, a space-based gravity wave interferometer, is an international project currently in planning. The LISA interferometer will be formed by three spacecraft separated by 5 million kilometers and will be able to detect gravity waves in the frequency band between 10^-4 Hz and 1 Hz with strain sensitivity as low as 10^-23. Together, ground-based and space-based gravity wave interferometers will open a completely new window through which scientists can observe the universe.

Direct Searches for Dark Matter

If the models are correct, we are immersed in a local density of particle dark matter equivalent to about one proton mass in every 3 cubic centimeters. The possibility that there is a flux of a new form of matter passing through the Earth undetected is extremely compelling to particle physicists. There are a number of creative, but indirect, ways to detect particle dark matter (e.g., searches for high-energy neutrinos from the sun and sharp peaks in the galactic gamma ray spectrum), but there is also the possibility of detecting these interesting particles directly. Although they interact only very weakly with ordinary matter, there is a small chance one of the dark matter particles will collide with a nucleus of ordinary matter, imparting approximately 10 keV of energy to the nucleus and causing it to recoil. The challenge is to detect these small recoil energy deposits, and exclude the backgrounds, in enough target detector material so that the rate of the collisions is measurable in a practical amount of time. Many years of new detector research and development have paid off, and there are now many direct dark matter detection experiments, only a few of which can be mentioned here. All of them are conducted in low-background environments, often deep underground.

CRESST is a cryogenic detector located in the Gran Sasso facility. It consists of sapphire target material and sensitive superconducting thermometers to detect the energy deposited by the dark matter particle interaction. The DAMA experiment, also in Gran Sasso, uses scintillators as target detectors. It is worth noting that, as of 2002, the DAMA group has detected a potential signal for dark matter events by examining the annual variation in the event rate in their 100 kg sodium iodide scintillation detectors; however, this must still be confirmed or refuted by other experiments. In particular, the CDMS experiment, which uses sophisticated silicon and germanium cryogenic detectors that can identify classes of backgrounds on an event-by-event basis, has failed to see events at the rate one would expect if the DAMA results were to be confirmed, but it is still too early to be conclusive. CDMS II, the next phase of the CDMS experiment, will operate in the Soudan mine in Minnesota and will achieve dramatically improved sensitivity. UKDMC is a collection of different detectors operating in the Boulby mine in the United Kingdom. These include sodium iodide scintillator detectors, liquid xenon scintillation and drift detectors, and a new kind of detector, called DRIFT, which can provide directional information about the dark matter event. Finally, searches for a different type of dark matter particle, called the axion, are being carried out by the appropriately named AXION experiment in the United States.

Nothing like these detectors is used in 2002 accelerator-based experiments. It is worth noting, however, that the same kinds of particles that could compose the galactic dark matter might also soon be produced and discovered in accelerator-based experiments, using very different measurement techniques, so the searches are complementary.

The adaptation of accelerator-based particle detection techniques for use in astrophysical detectors, along with advances in detector technologies targeted to solve unique experimental problems, has opened up the universe as a laboratory for fundamental physics. These investigations draw together the communities of particle physicists, astrophysicists, and cosmologists.

See also:Case Study: Gravitational Wave Detection, LIGO; Case Study: Super-Kamiokande and the Discovery of Neutrino Oscillations; Detectors; Detectors and Subsystems; Detectors, Collider; Detectors, Fixed-Target; Detectors, Particle

Bibliography

AMANDA. <http://amanda.uci.edu/>.

CRESST. <http://www.lngs.infn.it/site/>.

GLAST. <http://www-glast.slac.stanford.edu/default.htm>.

HEGRA. <http://wpos6.physik.uni-wuppertal.de:8080/>.

OWL. <http://owl.gsfc.nasa.gov/detector.html>.

Super-Kamiokande. <http://www-sk.icrr.u-tokyo.ac.jp/doc/sk/>.

VIRGO. <http://www.virgo.infn.it/>.

Steven Ritz