v1.0.1 / chapter 9 of 10 / 01 aug 08 / greg goebel / public domain
* The earliest telescopes were optical telescopes, allowing astronomers to view the Universe in visible light. In the 20th century, astronomers extended the range of telescopes to cover the entire electromagnetic spectrum, from radio and infrared regions, into the ultraviolet, X ray, and gamma ray bands. However, electromagnetic radiation isn't the only sort of particle that falls from the sky: ionized atomic nuclei hit the Earth's atmosphere continuously, with such "cosmic rays" providing hints on energetic processes in the Universe. Since the middle of the 20th century, astronomers have been setting up instrument systems to help determine the origins of cosmic rays. This chapter provides a survey of cosmic-ray detector systems.
* After the discovery of radioactivity at the beginning of the 20th century, scientists then discovered that there seemed to be a pervasive background radiation that was present almost everywhere. The radiation was believed to be coming from the Earth itself. In 1910, a Jesuit priest named Theodor Wulf (1868:1946) went up the Eiffel Tower in Paris to measure radiation levels with a "electroscope". This was a simple device consisting of a sealed gas-filled globe with a metal rod inserted in the top, connected to two thin gold leaves inside the globe. A static electric charge could be used to form the two leaves to spread apart; any radiation passing through the globe would ionize the gas, causing the charge on the leaves to discharge so that they would gradually fall back together.
If the radiation was actually coming from the Earth, it would be weaker at the top of the tower -- but the radiation levels were surprisingly high. Wulf suggested that this mysterious radiation might be coming from the upper atmosphere or space. He suggested that balloon flights might be conducted to confirm this notion.
In 1911:1912 an Austrian physicist named Victor Hess (1883:1964) made a series of ten balloon flights with an electroscope to investigate. Hess did discover that radiation increased with altitude. There was widespread skepticism over his findings, but a German researcher named Werner Kollhoerster made five flights of his own and provided confirmation. Kollhoerster's last flight was on 28 June 1914; that was the day Serbian extremists assassinated the heir to the throne of the Austro-Hungarian Empire, setting off World War I, which put pure scientific research on the back burner until the end of the war in 1918.
After the conflict, in 1922, the American experimental physicist Robert Millikan (1868:1953) conducted studies of his own on the matter, launching automated balloons from Texas and performing studies from the top of tall Pike's Peak in Colorado. He reported no rise in the level of radiation; his findings were correct, but it turned out that the level of cosmic radiation in those regions was unusually low. Hess and Kollhoerster hotly contested Millikan's findings; although Millikan was not noted for being flexible in his judgements, he was very thorough, and so he conducted further studies in the mountains of California in 1925. He was forced to concede that the radiation did exist, naming it "cosmic rays".
That would prove to be his only really positive contribution to the debate. Millikan insisted that cosmic rays were high-energy gamma rays, but in 1929 Kollhoerster and his colleague Walter Boethe built a "coincidence counter", using two proportional counter tubes that would go off when a single particle passed through both. After recording the passage of cosmic-ray particles through the two tubes, they placed a slab of gold between them, assuming it would block the cosmic rays. It would have if they had been photons, but it didn't, meaning they were charged particles with mass.
Millikan insisted that their experiment was in error. Kollhoerster and Boethe suggested that if cosmic rays were charged particles, not photons, then they would be deflected by the Earth's magnetic field, with the cosmic-ray flux strongest at the poles and weakest at the equator. Studies by various researchers, including Millikan, were afflicted by equipment and other problems and gave ambiguous results, but in 1932 one of Millikan's ex-students, Arthur Holly Compton (1892:1962), announced the results of a careful series of observations to show that cosmic rays did vary with latitude as would be expected if they were charged particles. Millikan bitterly attacked Compton's results and then, confronted with new evidence that confirmed Compton's conclusions, abruptly reversed himself, claiming that he and Compton were (and had been) in complete agreement.
The entire subject of cosmic rays ended up being an embarrassment to Millikan. Although he could be hidebound, he was still one of the finest experimental physicists of his generation. He was simply off his game when it came to cosmic ray studies, and he would hardly mention them in his memoirs. There was a widespread belief for a time that he had discovered cosmic rays, but Hess's work was well documented, and Hess received the Nobel prize for physics in 1936 for the discovery.
A consensus emerged that cosmic rays were generally charged atomic nuclei moving at a high velocity through space that strike the Earth's atmosphere, generating a "cascade" of a million to a billion secondary particles known as an "air shower", with the particles scattered over an ellipse hundreds of meters wide when it hit ground. The particles in the air showers proved to be a gold mine for particle physicists, since the cascades contained short-lived particles not easily found in the laboratory. In the postwar period, up to the early 1950s, cosmic rays were investigated with balloons that carried stacks of photographic emulsions to high altitude to record the traces of these particles.
* Cosmic rays hit the Earth at a rate of about one thousand a second per square meter, and their energies don't seem to have any upper bound, though their numbers do unsurprisingly fall off as the energy level increases. About 90% are hydrogen nuclei (protons), 9% are helium nuclei (alpha particles), and the remaining 1% are (mostly) various heavier nuclei. Since they are charged particles, their paths through space are scrambled by galactic magnetic fields, making it difficult to determine the location of their origin.
There are two classes of cosmic rays, those with energies below 10^16 electron-volts (eV) and those above that level up to 10^20 eV or more. Astronomers believe the two classes arise from separate processes. The low energy cosmic rays are common, while the more interesting high energy cosmic rays are rare, with the entire Earth intercepting one about once every second. The low energy cosmic rays are not seen as particularly mysterious: the great Italian-American physicist Enrico Fermi suggested that ordinary charged particles could be accelerated to such energies over long periods of time by magnetic fields in our Galaxy. They are also produced by the solar wind from the Sun. Low energy cosmic rays are also not seen as particularly interesting, and for the most part users of modern cosmic ray observatories regard them as "background noise" that has to be screened out.
In contrast, nobody has any clear idea of where the superpowerful cosmic rays come from. They are so powerful that they have been said to have energies comparable to a brick thrown through a plate glass window, pretty impressive performance for a submicroscopic particle. They are generally referred to as "ultra-high energy cosmic rays (UHECRs)". Galactic magnetic fields aren't strong enough to push them around, and since they hit the Earth from all directions instead of along the plane of the Milky Way in our sky, they appear to be produced by extragalactic sources, possibly by supernovas or other "cosmic catastrophes" -- though some physicists have suggested they may arise from exotic processes, such as the decay of "magnetic monopoles".
As mentioned above, about 1% of cosmic rays are "mostly" relatively heavy nuclei. However, that 1% includes a thin flux of very high-energy gamma rays, in the 10^12 eV range, that cause air showers very similar to those created by cosmic rays, with about one gamma-ray event for every 100,000 cosmic ray events. Millikan's assertion that cosmic rays were gamma rays wasn't completely wrong -- but it was very close to completely wrong. These gamma rays aren't diverted by galactic magnetic fields and so can be traced back to a source by mapping the geometry of the air shower.
* There are four ways to observe cosmic rays:
It is of course possible to build "hybrid" detector systems that use more than one of these methods. Particle detector and radio detector systems work round the clock; air Cerenkov and fluorescence detectors can only really work on clear, moonless nights.
* The first cosmic ray "telescope" was implemented by researchers of the British Atomic Energy Research Establishment in Harwell in 1953. Following a suggestion from the well-known experimental physicist Patrick Blackett (1897:1974) that as particles generated by cosmic rays fell down through the atmosphere, they might emit Cerenkov light, the British researchers built a primitive "cosmic ray telescope" consisting of a 30 centimeter parabolic mirror in a trash can, with a photomultiplier tube at the focus. It worked in collaboration with an array of proportional counters spread over the grounds of the site.
The Cerenkov light associated with an air shower is faint, one astronomer later comparing it to "a 5-watt blue light bulb moving at the speed of light 5 kilometers away." Of course, such an "air Cerenkov" detector was also useless in cloudy weather, which is by no means uncommon in the UK. However, the very first night the telescope was put into operation, it picked up large Cerenkov light flashes about once every two minutes, with the flashes validated by readings from the proportional counter array. Further measurements suggested that the footprint of the Cerenkov light cone from the cosmic ray shower was very large, about 200 meters across.
There was obviously room for building bigger air Cerenkov telescopes to get a better notion of where the cosmic ray particles seemed to be coming from in the sky. Larger and more sensitive Cerenkov detectors were set up all over the world. However, the first cosmic ray telescope that was really worthy of the name wasn't set up until 1968, when the Whipple Observatory in the US went online. It honestly looked like a telescope of sorts, being in the form of a parabolic dish 10 meters wide made up of 248 flat hexagonal mirrors, feeding a detector system consisting of an array of 100 photomultiplier tubes. It was later supplemented by a second, roughly similar telescope at the same site to allow it to pinpoint the direction of air showers with more precision.
The Whipple telescope was followed by a series of other air Cerenkov detector telescopes:
CANGAROO I led to the "CANGAROO II" telescope, which went online in 1999. CANGAROO II featured a mirror 7 meters in diameter, made up of 114 80-centimeter mirrors, focused on an array of 522 PMTs. A year later, it was expanded to 10 meters by adding 54 more mirrors. Three more identical telescopes were added into 2004 to form the "CANGAROO III" array of four telescopes.
* The latest and most impressive air Cerenkov telescope is the "High Energy Stereoscopic System (HESS)", a quadruple array of cosmic-ray shower telescope in Namibia, South-West Africa. Each of the four telescopes has a mirror grid with a diameter of 13 meters and an area of 107 square meters and features a 960 pixel light detector. HESS, which of course was named in honor of Victor Hess, was built by a consortium of researchers from South Africa and Europe.
Work is now underway on a successor to the pioneering Whipple telescope. The "Very Energetic Radiation Imaging Telescope Array System (VERITAS)". Like HESS, VERITAS will consist of four air Cerenkov telescopes, each 12 meters in diameter and based on the original Whipple telescope design.
* Detectors specifically designed to pick up cosmic rays -- as opposed to electroscopes, which just registered the presence of some kind of ionizing radiation -- go back to Kollhoerster and Bothe's coincidence counter of 1929, but the first cosmic ray detector system that was appropriate for wide-scale deployment wasn't developed for two more decades.
In the late 1940s, physicist John A. Simpson of the University of Chicago developed a "neutron monitor" that could determine the energy ranges of neutrons produced by an air shower. It used paraffin (candle wax) or polyethylene to slow down the neutrons, with the slow neutrons then being detected by a specialized variant of the proportional counter tube. Although a normal proportional counter tube can only pick up charged particles, the neutron monitor tubes were filled with the helium-3 isotope. A slow neutron could be absorbed by a helium-3 nucleus to become helium-4, emitting energy that caused ionization of a path through the gas, causing the counter to "fire" and report the event. There are variations on the same theme that use different filler gases. The tubes are generally arranged as arrays to give data on the direction of the paths of the neutrons.
During the 1950s, Simpson pushed a program to deploy neutron monitors to a network of mountain and other high-altitude sites around the world. This network has been expanded since that time, and provides a global map of cosmic-ray flux intensity.
Cosmic ray air showers also produce charged muons that can be observed with conventional proportional counters or, more frequently, scintillation detectors. Since muons have some surface penetration ability, detectors can be placed underground to shield them from confounding events by other particles.
* An array of particle detectors is required to obtain more information on cosmic rays. One of the first particle detector arrays was set up by the University of Leeds in the UK, with a grid of detectors set up in the mid-1960s over about twelve square kilometers of farmland in the vicinity of Haverah Park in North Yorkshire. There were 29 detectors, consisting of tanks of pure water with PMTs observing Cerenkov radiation from particles passing through the water.
The Haverah Park array was finally shut down in 1987, having been one of the first installations to observe UHECRs, picking up a total of four; people had problems believing in them at first. A smaller array, the "Gamma Ray Experiment (GREX)", made up of 36 scintillation detectors, operated at the same site from 1986 into 1993.
* The "High Energy Gamma Ray Astronomy (HEGRA)" array was built by the Max Planck Institute in Germany and went online on La Palma in the Canary Islands in 1987. The original array consisted of 250 scintillation counters spread over a grid 180 meters on a side. 17 muon detector "towers" were included in the array as well. An air Cerenkov detector capability was added after the creation of the particle detector array. Originally, an array of 49 PMTs arranged as a 7 x 7 grid was set up, with each PMT staring up into the sky through a fisheye lens; the array was later expanded to 97.
They were followed by the "Cerenkov Light Ultraviolet Experiment (CLUE)", an array of eight full Cerenkov telescopes set up among the earlier detectors. Each telescope featured a 1.8 meter parabolic mirror and, unusually, features a multiwire proportional counter detector system, not a PMT array. The HEGRA arrays were finally shut down in 2002 to make way for the MAGIC air Cerenkov telescope, discussed above.
* The Japanese constructed a ground particle detector array at Akeno in Japan. The "Akeno Giant Air Shower Array (AGASA)" consisted of 111 scintillator particle detectors placed on the surface and 27 buried detectors (to look for muons). The minimum spacing of the array was about 1 kilometer, with the nodes in the array in an irregular grid, and the entire array covered about 100 square kilometers. The nodes in the array were linked by fiber optics. The AGASA array went online in 1991; on 3 December 1993, it detected a UHECR with an energy of about 2E20 eV, the second most powerful cosmic ray observed in the 20th century.
A ground particle detector array, the "South Pole Air Shower Experiment (SPASE)" was set up very close to the South Pole in Antarctica. It consisted of 24 scintillation detectors, spaced on a 30 meter grid. It ran from 1988 to 1998, when it was replaced by "SPASE 2", which features 130 scintillation detectors with a "low profile" casing to avoid snow drifting. There are also a set of small air Cerenkov detectors dispersed among the SPASE-2 scintillation counters.
* The Cerenkov telescopes were one approach to hunting cosmic rays; another approach was to build arrays of particle or atmospheric fluorescence detectors set up over a wide area.
Systems to observe air fluorescence were originally developed by the US Los Alamos National Laboratory in the early 1960s, as a means of estimating the actual yield of nuclear tests. The first system to observe the fluorescence of cosmic rays was developed by a Cornell University group under Kenneth Griesen and put into service in 1967. It was an array of 500 PMTs divided into ten modules, with a Fresnel lens focusing the light on the PMTs in each module. The experiment ran for several years, but the detectors weren't sensitive enough and the site, in New York state, was less than optimum. In terms of obtaining useful observations, the experiment was a failure.
In 1976, researchers from the University of Utah had a bit more success using a demonstrator instrument set up at Volcano Ranch, near Albuquerque, New Mexico. There were three detectors, each consisting of a 1.8 meter mirror focusing light on an array of 14 PMTs. Since precise focusing wasn't needed, the mirrors had a spherical figure. The mirrors made the detectors twenty times more sensitive than the Cornell detectors, and the "seeing" in the New Mexico deserts is excellent.
The Volcano Ranch demonstrators provided proof that the air fluorescence could be practically used to observe air showers, and the University of Utah group went on to implement a full-scale fluorescence detector array, the "Fly's Eye", at the US Army Dugway Proving Grounds in Utah. Dugway was not far from the University of Utah; was at relatively high altitude; and had useful isolation from confounding sources of light, while still having access to electricity and communications.
The Fly's Eye consisted of 67 detectors oriented to cover the full sky above. The detectors very much like the Volcano Ranch detectors, though the mirrors were a bit smaller and there were 12 or 14 PMTs, depending on the position of a detector in the array. The initial array went online in 1981; in 1986, a second array, known as "Fly's Eye 2", went online 3.4 kilometers from the original site, renamed "Fly's Eye 1". The two arrays provided a stereo view of an air shower that permitted better characterization of its geometry and position.
On 15 October 1991, the Fly's Eye dual array picked up an air shower caused by a UHECR with an energy estimated at about 3.2E20 eV, the most powerful cosmic ray event discovered in the 20th century. There had been skepticism that the UHECR events reported by the Haverah Park array actually existed, but the 1991 event did much to convince people UHECRs were for real.
The Fly's Eye dual array was shut down in 1993 to make way for a new "Fly's Eye High Resolution (HiRes)" array. HiRes has the same basic architecture and function as the earlier Fly's Eye array, with two arrays of detectors at the Dugway Proving Grounds, though in the case of HiRes the separation is 12.6 kilometers. However, as the name implies, HiRes has much higher resolution. The "HiRes 1" sub-array has 22 detectors, while the "HiRes 2" sub-array has 22 detectors. Each detector has a wider mirror -- actually four one-meter mirrors fitted together -- and has 256 PMTs, organized in a 16 x 16 grid. HiRes 1 went online in 1996; it was joined by HiRes 2 in 1999.
* A much more ambitious detector, named the Auger Project -- after Pierre Auger (1899:1993), the French scientist who first investigated air showers -- is now under construction by a worldwide collaboration of researchers. The first of two, roughly identical, arrays in the Auger Project is now under construction on the pampas of Argentina around the town of Malargue. It is a hybrid array, with both particle and fluorescence detectors.
The particle detector part of the array is essentially an improved follow-on to the Haverah Park array, consisting of a total of 1,600 tanks of water, each with a capacity of 11,370 liters, sealed to keep out external sources of light and fitted with three PMTs to detect the passage of air shower particles through their emission of Cerenkov light. The tanks, which are insulated to keep them from freezing up, are spaced on a 1.5 kilometer grid, covering a total of 3,600 square kilometers. The spacing will allow detection of a single air shower by five to ten detector tanks. Nodes make use of off-the-shelf technology such as cell phones, GPS receivers, and solar cells.
There is a set of four "Fly's Eye" style arrays of fluorescence detector telescopes interspersed among the water tank grid. There is one telescope array or "eye" in the center of the particle detector grid, with the other three eyes around the rim of the grid. The eyes have a vertical field of view up to about 30 degrees above ground; the perimeter eyes have a horizontal field of view of 180 degrees, staring into the particle detector grid, while the central eye has a horizontal field of view of 360 degrees.
The perimeter eyes mount six fluorescence detector telescopes, while the central eye has twelve. Each individual telescope has a field of view of about 30 degrees along each axis, and has a Schmidt configuration, features a mirror with a diameter of 3.5 meters and a correcting lens. Each telescope uses a camera consisting of an array of 440 PMTs, organized as a 22 x 20 grid. This gives a total of 30 telescopes with 13,200 PMTs. The Argentina array will be joined by a twin in the US state of Colorado, with the two arrays giving coverage of both hemispheres of the Earth.
* The US group behind the Fly's Eye Hires and the Japanese group behind AGASA has followed up with a hybrid "Telescope Array" at Black Rock Mesa in southwest Utah, which went online in 2007. The Telescope Array features 576 scintillation detectors with plastic target media arranged over a grid about 730 square kilometers in area, plus three arrays of fluorescence detector telescopes with a total of 38 telescopes.
Each telescope has a mirror with spherical curvature and 3 meters wide, focused on an array of 256 PMTs in a 16 x 16 configuration; each has a field of view of 18 x 16 degrees, with the telescopes in each array lined up to provide wide sky coverage. Plans are being developed to extend the array with a hundred more detectors and two new telescope sites. The Telescope Array replaced the Fly'e Eye Hires and AGASA arrays, which were shut down. Working on the Dugway military reservation had become more difficult due to increased security concerns, with the military refusing access to foreign citizens.
* Cosmic-ray detector arrays do have (at least in part) a resemblance to telescopes, though it's hard to think of their particle detector elements as a telescope in any literal sense. However, over the past few decades, physicists and astrophysicists have been building large particle detector systems as "telescopes" to hunt for almost invisible particles falling from the sky. Although a detailed discussion of these systems is really more appropriate to a document on particle physics, it is worth at least briefly outlining them here.
The best-known of these nearly-invisible particles is the "neutrino", a ghostly particle that, on the average, can zip through light-years of lead before it is absorbed. Neutrinos are a product of radioactive decay, and when absorbed they recreate the decay process in reverse. Although the odds of this reverse reaction happening are very small, vast numbers of neutrinos are emitted from the core of our Sun at all times, and given a big enough target mass, there will be some probability of observing such a reverse reaction.
The first attempt to search the neutrino sky was performed in the 1960s by Raymond Davis (1914:2006) and his group from the US Brookhaven National Laboratory. Their system consisted of a tank containing 615 tonnes of perchloroethylene (C2Cl4), a dry cleaning fluid, stuck deep in the Homestead Mine in Lead, South Dakota, to reduce confounding radiation from other sources. Interaction of a neutrino with a chlorine atom would create a radioactive isotope of argon, which could be flushed out of the tank and picked up by proportional counters. The estimated rate of interactions was one per day, but the system was capable of picking up that rate.
The rate turned out to be about once every 2.5 days. This was very puzzling, since it meant either the system wasn't working right, the Sun wasn't working the way the astrophysicists thought it should, or that we didn't understand neutrinos as well as we thought we did.
Davis got a boost in 1983 when a "swimming pool" detector -- a big tank of pure water lined with PMTs to track Cerenkov radiation from neutrino interactions -- was put into operation a kilometer deep in the Mozumi Mine of the Kamioka Mining & Smelting Company in Japan. The tank was a cylinder 16 meters tall and 15.6 meters in diameter, containing 3,000 tonnes of pure water and lined with about a thousand PMTs. Its intended purpose was to hunt for the possible decay of protons into other particles, and so it was named the "Kamioka Nucleon Decay Experiment" or "Kamiokande". The detector could also pick up neutrinos caused by cosmic-ray collisions in the atmosphere, but the flux seemed definitely lower than predicted.
The Kamiokande detector was upgraded to the "Kamiokande II" system in 1985. Kamiokande II was capable of detecting the direction of detected neutrinos. The first revelation was that the flux of neutrinos from the far side of the Earth was much lower than the flux from the near side. The second revelation was in 1987, when Kamiokande II picked up a burst of neutrinos from a supernova explosion.
Incidentally, despite the fact that the Kamiokande detector had actually been constructed to detect the decay of the proton, it failed to do so, the likely reason being that the rate of proton decay was at least ten times lower than predicted. A much enlarged detector system, "Super-Kamiokande" or "Super-K", was installed, featuring a much bigger tank, about 40 meters tall and 40 meters wide, containing 50,000 tonnes of pure water and lined with about 11,000 PMTs. Unfortunately, in November 2001, the implosion of a PMT detector set off a chain reaction that caused over half of the detector's PMTs to implode as well. The detector was not back up and running until the spring of 2006.
* In any case, neutrino experiments became fashionable, and a number of new systems were implemented, the most advanced of which was the "Sudbury Neutrino Observatory (SNO)", built two kilometers under the ground in a nickel mine in Sudbury, Ontario, Canada. SNO was a detector built around 1,000 tonnes of heavy water in a spherical tank lined with PMTs; it was a much more capable system than Davis's tank of cleaning fluid, and was able to unravel the solar neutrino deficit.
The problem was a failure to understand the nature of the neutrino. There are actually several different kinds or "flavors" of neutrinos, and Davis's detector could only pick up one flavor, though that was the flavor expected to be emitted by the Sun. It turned out neutrinos seemed to change flavor as they passed from Sun to Earth, which is why the detection rate was so low. The "oscillation" of neutrinos was what caused the deficit of neutrinos from the far side of the Earth detected by Kamiokande II. The discovery helped win Raymond Davis the 2002 Nobel prize in physics.
Neutrino hunts continue to provide more information on cosmic processes. Large arrays of PMTs are being set up undersea or in Antarctic ice, with the water providing the interaction medium. An example of modern water array is the "Neutrino Extended Submarine Telescope with Oceanographic Research (NESTOR)", also rendered as "Neutrinos from Supernovas & TeV sources Ocean Range", being set up in the Mediterranean off the coast of Greece. In completion, NESTOR will consist of detectors mounted on seven towers, each 330 meters tall. The towers will have 12 hexagonal floors each, with PMTs at the corners of the floors. As far as Antarctic ice detectors go, a series of "Antarctic Muon And Neutrino Detector Array (AMANDA)" systems has been set up at the South Pole, to lead to "IceCube", with 5,000 PMT detector modules in a cubic kilometer of ice.
* Along with the neutrinos, searches have been performed to try to find the mysterious "dark matter" that makes up much of the Universe, but which otherwise seems undetectable.
Although neutrinos were once believed to be massless particles, the fact that they oscillate means they must have a slight mass. A massless particle must travel at the speed of light; since time slows down as the velocity of an object approaches the speed of light, if the neutrino were massless, it wouldn't experience time, meaning that it would be unchanging.
The fact that the neutrino seems to have mass provides one useful candidate for dark matter. Although only an upper limit on neutrino mass is known at present, if the neutrino mass is close to that upper limit, neutrinos could explain all the dark matter in the Universe. There are two other proposed forms of dark matter, known as "weakly interacting massive particles (WIMPs)" and "axions". Since dark matter should sleet through ordinary matter in much the same way that neutrinos do, most dark matter searches are performed in deep underground sites where the bulk of confounding events can be screened out.
WIMP hunts have been performed using scintillation detectors, such at the "UK Dark Matter Collaboration (UKDMC)" and the Italian-Chinese "Dark Matter (DAMA)" search; or with ionization detectors, as in the "Heidelberg Dark Matter Search (HDMS)", a German-Russian collaboration; and the "Cryogenic Dark Matter Search (CDMS)", which hopes to detect WIMPs by measuring the tiny rise in temperature from an interaction with a cryogenically cooled material.
Axions are different beasts. Although they can be picked up in principle by ionization detectors, some systems, like the "CERN Axion Solar Telescope (CAST)" actually looks a little bit like a telescope. According to theory, an axion should be converted to X-rays in a strong magnetic field, and so CAST consists of an X-ray detector telescope peering through a strong magnetic field.
So far, results of all dark matter experiments have been negative or inconclusive, though given the difficulty of picking rare dark matter events out of the noise, nobody ever expected progress to be easy.
