The Soudan Underground Laboratory


Welcome to the Soudan Underground Research Site, a unique physics laboratory operated by the School of Physics and Astronomy of the University of Minnesota-Twin Cities. The first physics experiments at Soudan began in 1980. The current laboratory was constructed between 1984 and 1986. The Soudan Laboratory site is leased by the University of Minnesota from the State of Minnesota, Department of Natural Resources (DNR). The lab is located in the Soudan Underground Mine State Park, where the DNR preserves the oldest iron mine in Minnesota.

The Need for an Underground Facility

The Soudan Laboratory is one of a handful of deep underground physics laboratories located at sites around the world. The major reason for the deep underground location is to shield very sensitive energy detectors from cosmic rays, naturally raining down on the earth from outer space. These cosmic ray particles deposit tiny flashes of energy in all matter which they traverse. A large number of such flashes can easily overwhelm sensitive experiments which are done in deep underground labs, masking the expected effects.

In an underground facility, the number of cosmic ray particles is considerably reduced by absorption in the rock above. At the Soudan lab, the number of cosmic ray particles is cut by a factor of 100,000 from the number at the surface. As a result, the current Soudan 2 detector needs to cope with only a few million interruptions per year due to cosmic rays rather than the trillions of cosmic rays per year that would pass through a similar detector on the earth's surface.

Underground physics labs similar to the one at Soudan but with different experiments are located in mines in South Dakota, Ontario and Japan. There are also deep underground physics labs in Russia, France and Italy, which are located underneath mountains and accessed by long, horizontal tunnels.

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Physics Underground

Most underground physics experiments study the nature of the fundamental forces between the tiniest bits of matter. There are four such forces or interactions. Scientists believe that all of them are likely related. The weakest of these interactions is gravity, the force which is familiar to us as both the reason why objects fall towards the earth's surface and why the earth and the other planets move in orbits around the sun. A second familiar force is electromagnetism, which is responsible for electromagnets and magnetic compasses and all forms of electricity. The strongest of the four forces is the strong or nuclear force which binds the nuclei of atoms together. The last force, known as the weak interaction, is important for the thermonuclear or burning reactions which power our Sun and for some very slow, radioactive decays.

Understanding these forces is important for several reasons. Two of these forces, electromagnetism and the strong force, are the basis for much of the 20th Century technology that enormously affects our daily lives. For example, electromagnetic waves form the basis for television and other modes of communications. Electromagnetism also plays a key role in determining the behavior of semiconductors, which implement all kinds of electronics. The strong or nuclear force determines the technology for release of nuclear energy, either by fission or by thermonuclear fusion. Past experience suggests that tomorrow's technologies will likely be built on the base of today's science. Advances will be based, among other things, on discoveries about the fundamental interactions.

A second reason is that these forces are key to answering the important questions which people have asked for millenia. How was the universe formed? What will eventually happen to it? Is there a beginning or an end to either space or time? The fundamental forces are also important to answering recent questions such as: Will the universe expand forever? Does matter live forever? Is there dark matter? The knowledge related to all of these questions is part of our culture. One role of science is to contribute to our humanity by advancing our worldwide culture.

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Physics Experiments at Soudan

Physics experiments at Soudan focus on three major topics: the stability of matter (nucleon decay), the nature and interaction patterns of the cosmic rays and particles called neutrinos, whose properties are a sensitive test of the mechanisms of the fundamental interactions. Scientists working at Soudan have collected data on all three of these questions for several years. These studies will likely continue for at least a few more years. In addition, a large collaboration of scientists has proposed a major new initiative to measure neutrino mass, which would send a beam of neutrinos 730 km through the earth from Fermilab, near Chicago, to the laboratory at Soudan. A large new detector would be added to the current instrumentation at the Soudan Laboratory, if this plan is approved.

Experimenters working on different physics questions share use of the apparatus and often work together, using the same data. Currently, the major instrumentation at Soudan is the Soudan 2 detector is a 1,000 ton device optimized to search for proton decay. It is located in a 690 m deep underground laboratory on the 27th level of the Soudan Mine. A 60 square meter energy detector, located on the earth's surface near the entrance to the Soudan Mine, and a nearby array of sensitive light detectors, which measure atmospheric light generated by cosmic rays, work with the underground detector. In addition, the proposed neutrino mass experiment would use a new detector called MINOS, which would be located in an new underground room adjacent to the existing laboratory.

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The Search for Nucleon Decay

Nucleons (protons and neutrons) make up the nucleus or center of all atoms. No clear occurrence of nucleon decay has yet been reported. It may also be so rare that it is extremely difficult to detect. In the Soudan Laboratory, researchers will be able to observe the spontaneous disintegration of protons and neutrons, the basic constituents of matter, even if these particles have a life as long as one hundred thousand million billion trillion years; (that's 10^32 years, or 10 followed by 32 zeros!)

The time needed to conduct an experimental study of nucleon decay has been reduced to a few years in the Soudan experiment by arranging to monitor many particles simultaneously. Thus, by observing 10^32 nucleons, each with a 10^32 life expectancy, there is a good chance that at least one decay will be detected each year.

The Soudan 2 detector consists of nearly 1,000 tons of material, comprising 6 x 10^32 particles. If the nucleon lifetime is 10^32 years, scientists might expect to observe about 6 decays per year. The equivalent of more than two years of data from the full 1,000 ton detector has now been recorded. Scientists working on the experiment are analyzing that data, looking for possible decays.

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Cosmic Ray Studies

Cosmic rays are high energy radiation from space that naturally come down from the sky. Very energetic muons, a type of subnuclear particle, generated by cosmic ray primaries are able to penetrate rock to the depth of the Soudan laboratory. When measuring these muons, the Soudan 2 detector acts as a telescope with a selective filter. The filter is the rock overhead which screens almost everything other than these unique particles. The Soudan researchers have examined these particle tracks looking for point sources or "hot spots" in the sky. To date, Soudan data have suggested two very interesting objects as possible sources of particles which create high energy muons.

One of these objects is the binary star system known as Cygnus X-3. This star pair is a known source of x-rays and of very high energy radio flares. On January 20-23, 1991, during an intense radio flare, the Soudan detector recorded an unexpected number of muons from the direction of Cygnus X-3. A detector in the Canary Islands also observed an unexpected number of muons from the direction of Cygnus X-3 at the same time. Curiously, other detectors around the world observed nothing unusual.

The Soudan scientists have now analyzed all of the data collected on muons since the beginning of 1989. These data suggest a possible cosmic ray muon source near the direction of the powerful quasar 3C273. That possibility has been discussed at several international conferences, which, over time, will permit scientists with other detectors to verify this result.

Another cosmic ray study at Soudan is an attempt to measure the chemical composition of very high energy cosmic rays. There are two energy detectors located on the earth's surface near the entrance to the Soudan mine. One is a flat array of detectors called proportional tubes located in a house trailer parked about 100 m east of the mine shaft. This array, 15 m long by 4 m wide, measures the amount of energy left in the earth's atmosphere by a cosmic ray, while the Soudan 2 detector measures characteristics of the muons associated with the same cosmic ray. The correlation between surface and underground data yields information about the properties of the original cosmic ray, as it entered the earth's atmosphere.

A second kind of energy detector, located near the proportional tube array, is called an atmospheric Cerenkov detector. It also measures cosmic ray energy deposition in the atmosphere, but by a different technique. The Cerenkov detector is sensitive to very faint light produced in the atmosphere as the cosmic ray propagates downward. This light is so faint that the atmospheric Cerenkov detector is usable only on clear nights when the moon is not visible. It also operates in conjunction with the Soudan 2 detector in the deep underground laboratory.

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Neutrino Experiments at Soudan

Although the original purpose of the Soudan detector was the search for nucleon decay, researchers have also used data from the Soudan 2 detector to study the properties of neutrinos. The neutrino is an elusive particle whose existence was first hypothesized during the 1930's in order to explain some radioactive decays. Since neutrinos almost never interact with matter, they were not actually discovered for more than 20 years. Today, scientists believe that the entire universe may be bathed in a sea of neutrinos left over from the "Big Bang." If neutrinos have mass, they may comprise some or all of the intriguing "dark matter," which many scientists believe to have an important influence on the evolution of the universe.

Neutrinos come in three varieties, called the electron, muon and tau-type neutrinos. If neutrinos have mass, they can change from one type to another. Such changes or oscillations may account for the surprising results on neutrinos from the sun first observed at the Homestake Gold Mine in South Dakota. The Soudan 2 detector is currently measuring the number of electron and muon-type neutrinos produced by the cosmic rays. Comparison of observed and expected numbers may therefore provide information about neutrino mass. Preliminary data taken at Soudan confirm results from two other detectors indicating a shortage of muon-type neutrinos, supporting the idea that neutrinos do have mass.

This possibility is so important that further experiments on neutrino mass are planned at several laboratories worldwide. The proposed MINOS (Main Injector Neutrino Oscillation Search) experiment would generate neutrinos at Fermi National Accelerator Laboratory, about 70 km west of Chicago. These neutrinos would then be directed north northwest about 730 km through the earth to the laboratory at Soudan. A "near" detector located at Fermilab would be used to measure the relative numbers of the three types of neutrinos near the production point. Both a new 10,000 ton MINOS detector and the existing Soudan 2 detector would be used to measure the same ratios at the remote location. A change in the proportions of electron, muon and tau- type neutrinos between the near and far laboratories would indicate that neutrinos have mass. The magnitude of the neutrino mass and the strength of the overlap among neutrino types could then be measured using the same instrumentation. Since neutrinos almost never interact with matter, neutrino beams have no influence on people, animals and plants.

If the MINOS experiment is approved, the new detector would be built in a new laboratory located about 50 m east of the current Soudan laboratory. The MINOS experiment would likely begin data collection about the year 2000, thus extending the usefulness of the Soudan laboratory for at least another decade.

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The Soudan 2 Detector

The Soudan 2 detector is a gas ionization, time projection calorimeter, consisting of 224 independent modules, each 1 m by 1 m horizontally by 2.5 m high. The modules, each of which weighs about 5 tons are constructed inside gas-tight, steel boxes. The boxes are filled with a mixture of 85% argon, 15% carbon dioxide gas. Most of the mass in each module is located in 240 corrugated steel plates, which are layered horizontally giving the inside of each module the appearance of a large honeycomb.

If a nucleon were to decay, its mass energy would be partially converted into kinetic energy of lighter particles emerging from the point where the nucleon disappeared. The existence of nucleon decay can be inferred from a study of the observed tracks of these particles. There are three criteria for identifying a nucleon decay. (1) The mass energy of a nucleon (proton or neutron) is known and the total energy of the emerging particles (mass energy plus kinetic energy) must be equal to the initial mass energy of the nucleon. This criterion is conservation of energy. (2) The paths of the particles created by the decay should radiate in opposite directions, roughly isotropically. This criterion is conservation of momentum. (3) Finally, the nucleon decay must be spontaneous, that is, it should not be related to energy coming into the detector from its exterior.

Each of the Soudan 2 modules is a complex energy detector, which records the detailed position and time history of high velocity, charged particles which traverse the module. The process for recording this information is as follows: A charged particle passing through a detector module ionizes or ejects electrons from gas molecules along its flight path. The Soudan 2 modules are constructed so that most of the gas volume in each module is under the influence of an electric field. This field causes the ejected electrons to drift horizontally along channels in the module for distances of up to 50 cm. The electrons are then accelerated toward a collecting wire or anode. During this acceleration process, the electrons produce more ionization. This gas multiplication process eventually results in the collection by the anode of 10-50,000 times the number of electrons originally produced by the ionization process. These electrons produce a current pulse on the anode and simultaneously generate an induced current pulse on cathode strips located just behind the anode wires. These current pulses are amplified, stored and eventually transmitted to a computer for analysis. Using this information, the computer is able to reconstruct spatial and time information about the energy deposited initially, as well as the total amount of energy involved. This information can then be compared with expectations for nucleon decay events using three experimental criteria, in order to determine whether a decay occurred.

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The Experiment Team

People working on experiments at the Soudan Laboratory are staff members from universities and laboratories in the United States and abroad. The lab also has a permanent staff of technicians, who live in the local communities around Soudan. Personnel working on the Soudan 2 detector work for the University of Minnesota-Twin Cities, Argonne National Laboratory in Illinois, Tufts University in Massachusetts and from Oxford University and the Daresbury Rutherford Appleton Laboratory, both located in the England. The MINOS team includes individuals from these same institutions, as well as from Boston College, California Institute of Technology, Columbia University, Fermi National Accelerator Laboratory, University of Houston, Indiana University, Institute of Theoretical and Experimental Physics (Moscow), Lebedev Institute (Moscow), Lawrence Livermore Laboratory, Oak Ridge National Laboratory, Stanford University, University of Sussex, Texas A&M University and Western Washington University.

Fabrication of Soudan 2 detector components was done in Minnesota, Illinois, Massachusetts and England. Construction of the Soudan 2 laboratory began in 1984 and was completed in 1986. Detector installation began at that time and was completed in 1992.

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Host and the Sponsors

research at Soudan is currently sponsored by the governments of the United States of America and the United Kingdom of Great Britain and Northern Ireland as part of their support for fundamental research and graduate education in science. The State of Minnesota has provided the facilities of the Soudan Underground Mine State Park. The State and the University of Minnesota have contributed $1.2 million for laboratory construction. The total cost of the Soudan Laboratory and currently installed equipment exceeds $15 million. The anticipated cost of the new MINOS detector is about $40 million.

The Soudan Underground Mine State Park, supervised by Paul Wannarka, is unique among the state parks operated by the Minnesota Department of Natural Resources. The park is situated in a recreational setting of great beauty. With the historic mine as its centerpiece, the Soudan Mine Park stands as a monument to human striving. The first century of the mine's history was devoted to providing iron to help fuel the industrial revolution. Now, in its second century, it hosts a large- scale effort to better understand the subtle forces in the universe.

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