The appropriateness of an underground facility for a particular experiment depends on a number of its characteristics. Typically the most important of these is the effective depth of the laboratory and therefore the degree to which backgrounds associated with cosmic rays are reduced. In addition to the vertical distance from the surface, often referred to as the facility’s “overburden” or “vertical overburden,” the structure, density, and makeup of the earth above the laboratory impact the penetration capability of cosmic rays. A facility’s depth is therefore “normalized” by measuring the actual cosmic ray intensity at that facility and then expressing its depth in terms of meters of water equivalent (m.w.e.), or the equivalent depth of water that would reduce the cosmic ray intensity to the measured amount. Figure 2.1 shows the measured drop-off in cosmic ray muon intensity as a function of m.w.e. Typically, the depth of a facility expressed in m.w.e. is roughly 2.65 times its vertical overburden expressed in meters.
An underground facility’s appropriateness for a given experiment also might be impacted by the absence or availability of active shielding that can be shared by several experiments. This typically is a “shield” outside the inner main detector that is often itself an active detector. By measuring activity at the shield and providing that information to the experiment, the influence of the surrounding environment on the inner main detector can be estimated and accounted for in data analysis.
Still other important characteristics include how the laboratory space is accessed, whether that access is shared, and the nature of the rock surrounding the laboratory. Horizontal shafts that allow the use of vehicles to bring equipment and supplies into and out of the laboratory space typically are preferred to vertical shafts, especially where those shafts, and the lifts in them, are fairly small. Many laboratories coexist with working mines or vehicular tunnels, so access to the laboratory space can at times be limited. The type of rock from which the laboratory was excavated can be of importance, especially for larger experiments. As the size of experiments grows, the density and stability of the surrounding rock and its ability to support the weight of the experimental apparatus could become an issue.
Finally, the comprehensiveness and location of the support facilities for an underground laboratory can be important general characteristics. Most of these facilities are located on the surface and typically include shared items such as electricity, communications, and cold water for the cooling of the experimental apparatus. Further, each underground laboratory needs a support team to care for safety, technical support, transportation between the surface and underground, and the like, which can vary significantly from facility to facility.
Underground research facilities are scattered throughout the world. Figure 2.2 shows the size and effective depth, in m.w.e., of the principal underground laboratories, including the program proposed for DUSEL. The remainder of the chapter discusses the characteristics of the principal laboratories located or planned in North America—DUSEL, Soudan, and SNOLAB—as well as Gran Sasso, the largest underground laboratory in the world; Kamikande, the largest Asian laboratory; and, briefly, CDUSEL/China JinPing Laboratory (CJPL), a laboratory being developed in China that is expected to be large, on the scale of DUSEL. Appendix D contains more detailed information about all of the principal laboratories shown in Figure 2.2. In developing this material, the committee drew from the results of two recent comprehensive surveys of underground laboratories.1
1 A. Bettini. 2011. Underground laboratories. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 626: S64-S68; E. Coccia. 2010. Underground laboratories: Cosmic silence, loud science. Journal of Physics Conference Series 203: 012023.
The proposed DUSEL program was developed by a group of researchers based at the University of California at Berkeley pursuant to an award granted by NSF. The program calls for a multilevel facility at the Homestake mine, an abandoned gold mine in Lead, South Dakota. In addition to surface facilities, the principal underground facilities would be located at the 800-, 4,850-, and the 7,400-ft levels, with the opportunity to place additional small experiments at various other levels, depending upon the demands of the research.
The surface campus, as planned, consisted of approximately 27,000 m2 of research and administrative space, with 1,100 m2 of that for assembly of experiments. Administrative and science support, including shops, offices, and assembly sites, were to be located at this level. It was also envisioned to have a separate maintenance and operations campus and a facility for education and outreach. The size of the facility at 800 ft depended, to a large degree, on whether there would be a liquid argon detector for the neutrino oscillation/proton decay experiments, as discussed in Chapter 3, in the section entitled “Neutrino Physics.”
At 4,850 ft, it was proposed to have 25,000 m2 of total working space, with 6,200 m2 devoted to science. This level would be the principal deep underground laboratory space and the site for the water Cherenkov detector(s), if chosen for the neutrino oscillation and proton decay experiments, as well as the dark matter and nuclear astrophysics experiments. The deepest level, at 7,400 feet, would consist of 7,000 m2 total space, with 1,300 m2 dedicated to research. It would be available for possible dark matter and neutrinoless double-beta decay experiments, as well as ecohydrology, geoscience, and subsurface engineering experiments.
There are currently several small experiments under way at the 4,850 foot level as part of the Sanford Underground Laboratory. This laboratory was established in anticipation of the full-scale implementation of the DUSEL program, but its future is uncertain in light of changes to the DUSEL program. For more information, see http://dusel.org.
Soudan Underground Laboratory (SUL) is the only general research underground laboratory currently located in the United States. This facility was developed in 1980 and installed in an abandoned iron mine in Minnesota. The underground structures include the 1,400 m2 principal laboratory space, which hosts a dark matter experiment and a low-background counting facility; a small, high-purity copper fabrication facility; and the 560 m2 main injector neutrino oscillation search (MINOS) laboratory. The MINOS experiment is the far detector in a neutrino oscillations experiment; its neutrino beam originates at Fermilab. MINOS is expected to run a few years more with a 2-year decommissioning period at the end of that time.
The laboratory is fairly shallow, with a vertical overburden of 700 m of rock, and access is through a small, vertical shaft whose size places some restrictions on installation capability. The laboratory offers some education and outreach to the general public. It coexists with a historic state park that offers mine tours, some of which utilize a visitor’s gallery in the MINOS laboratory. For more information, see http://www.soudan.umn.edu/.
The Sudbury Neutrino Observatory (SNO) was excavated in the 1990s in an operating nickel mine. The original SNO experiment, located in a 200-m2 area, had a very successful set of discoveries and has completed its run. To this original space, new structures have been added to form a new laboratory, the SNOLAB, which consists of a main hall with 270 m2 floor area and ceiling heights from 15 to 19.5 m, a service hall of about 180 m2, and a number of narrow volumes called “ladder labs.” The laboratory space is one of the deepest currently available, with a rock coverage of 2,000 m under a flat surface. A “cryopit,” designed to cope with the safety issues surrounding large volumes of cryogenic fluids, has also been excavated. The total underground laboratory area is 7,215 m2, of which 3,055 m2 is available for experiments.
The access is through a vertical shaft that is shared with the working mine and is available daily. All of the laboratory space will be maintained at Class 1500 cleanliness standards. On the surface a 3,159-m2 building hosts a clean room, laboratories, staging and assembly areas, and administrative space.
The scientific programme includes (1) the Project in Canada to Search for Supersymmetric Objects (PICASSO), which is searching for dark matter (2 kg) using the superheated bubbles technique; (2) the experiment SNO+, which is to be hosted in the former SNO cavity and will use a liquid scintillator in which 150Nd has been dissolved to study low-energy solar neutrinos, geoneutrinos, and double-beta decay; and (3) dark matter searches that include the Dark matter Experiment with Argon and Pulse shape discrimination/Cryogenic Low Energy Astrophysics with Noble gases (DEAP/CLEAN), currently operating with a prototype, and superCDMS, operating with bolometers. For more information, see http://www.snolab.ca/ or http://www.sno.phy.queensu.ca/.
Laboratori Nazionali del Gran Sasso (LNGS) is a national laboratory of Italy’s Istituto Nazionale di Fisica Nucleare (INFN). It is the largest underground laboratory in the world and serves the largest and most international scientific community, about 750 scientists from 26 countries. LNGS arose out of a proposal in 1979 that a large underground laboratory be built close to and in conjunction with the Gran Sasso freeway tunnel then under construction in central Italy (an opportunity that substantially reduced its cost). The Parliament approved the construction in 1982, and construction was completed in 1987.
Access is horizontal, via the freeway. The underground laboratory principally consists of three main halls, each with an area of about 2,000 m2 (100 m × 20 m), 18 m high. There are also ancillary tunnels that provide space for services
and small-scale experiments and two 90-m long tunnels built for two Michelson interferometers for geology studies. The total area is 17,300 m2, and the total volume 180,000 m3. The laboratory is reasonably deep, with a vertical overburden of 1,400 m. Services hosted on the surface campus include a full range of support and administrative facilities, and the laboratory organizes a number of outreach and education activities.
LNGS is operated as an international laboratory with an international scientific committee, appointed by INFN, which advises the director on the suite of experiments for the facility. The scientific program includes these:
• The search for τ neutrino appearance on the μ neutrino beam emitted from the Large Hadron Collider of the European Center for Nuclear Research (CERN), 732 km away. This is the main focus of the OPERA experiment, which uses emulsion techniques and a large (kiloton), sensitive mass, consisting of 150,000 bricks made up of lead sheets interleaved with emulsion layers.
• ICARUS, a general-purpose particle detector in a 600-ton liquid argon time-projection chamber.
• Solar neutrino physics and geoneutrinos with the 1,300-ton liquid scintillator Borexino detector.
• The detection of low-energy neutrinos from the gravitational collapse of galactic objects with the 1,000-ton liquid scintillator Large Volume Detector (LVD) experiment.
• Dark matter searches with LIBRA (250-kg sensitive mass of NaI crystals), CRESST2 (an ultracryogenic CaWO4 detector), XENON (liquid xenon) and WARP (liquid argon)
• Neutrinoless double-beta decay experiments with GERDA (enriched 76Ge), CUORE (TeO2 bolometers), and COBRA (CdZnTe semiconductor detectors).
• Nuclear reaction of astrophysical interest with a 400-kV accelerator with LUNA2.
A special facility is dedicated to low radioactivity measurements, and the laboratory also supports several experiments in geology, biology, and the environment. For more information, see http://www.lngs.infn.it/.
The Kamioka Observatory is operated by the Institute for Cosmic Ray Research, University of Tokyo. It was established in 1983 by M. Koshiba as the Kamioka Underground Observatory. The original purpose of this observatory was
to conduct the Kamioka neutron decay experiment (KamiokaNDE); later, a neutrino observatory, Super-Kamiokande (SuperK), was built, which is currently the largest underground experiment. The Tokai-to-Kamioka long-baseline neutrino oscillation experiment (T2K) recently began operations. It is a third-generation neutrino oscillation experiment on an intense off-axis beam of muon neutrinos (vμ) produced at the J-PARC accelerator facility 295 km from the SuperK detector. The main goal of T2K is to measure the oscillation of vμ to ve and to measure the value of θ13.
The average vertical overburden for the research space is 1,000 m, and access is horizontal by vehicle, with no interference from mining activity. The average number of scientific users is more than 200.
The underground structures comprise the following:
• Hall SK (50-m diameter) hosting Super-Kamiokande;
• Clean room (10 × 5 m2) with XMASS prototype;
• Hall 40 (L-shape, 40 m × 4 m arms) hosting the purification tower for XMASS and the NEWAGE experiment on dark matter;
• Hall 100 (L-shape, 100 m × 4 m arms) with CLIO, a prototype of gravitational antenna (to be terminated in 2013) and a laser displacement detector;
• The new Hall A (15 × 21 m2) hosting XMASS 800 kg; and
• The new Hall B (6 × 11 m2) hosting CANDLE on double-beta decay, to be occupied until 2012.
Small areas are available in the abandoned mine. The underground large cryogenic gravitational antenna (LCGT), which has baseline lengths of 3 km × 3 km, was recently approved. Further enlargements are under development in order to accommodate more experiments. Buildings for offices and computer facilities are available on the surface.
In the same mountain, the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) experiment is operated by the Neutrino Centre, Tohoku University. KamLAND is designed to detect electron antineutrinos and to provide important results for neutrino oscillation by measuring antineutrinos from the commercial power reactors surrounding the site. For more information, see http://www-sk.icrr.u-tokyo.ac.jp/index_e.html
Recently, the China Deep Underground Science and Engineering Laboratory (CDUSEL) (also known as CJPL), a project for the world’s deepest, and possibly
largest, underground laboratory, was launched in China.2 The facility plans to take advantage of infrastructure being developed by the Ertan Hydropower Development Company (EHDC) in the course of installing a series of 21 hydroelectric power stations on the Yalong River in central China. A system of tunnels 17.5 km long will cut a big U-turn in the river under the 4,193-m-tall JinPing mountain. This system will have a flat area available for development as an underground laboratory that provides at its greatest depths a 2,500 m vertical rock overburden and more than 1,500 m vertical overburden in 70 percent of the directions. The access will be horizontal, from both sides.
Two small experimental halls 5 × 5 × 30 m3 are under construction; their relative size is shown in Figure 2.2. The final size of the laboratory has not been publicly disclosed, although it has been reported that the laboratory will be designed as an international facility, open to the world community. Ventilation, laboratory-grade power supply, and germanium detectors with their shielding will be installed. The muon flux (expected to be very low, on the order of 20 per m2 per year), the neutron flux, and radon concentration in the air will be measured shortly. A working group including scientists and engineers from Chinese institutions and universities as well as EHDC has been established to develop plans for this facility.
2 Qian, Yue. 2010. Status and prospects of China JinPing deep underground laboratory (CJPL) and China dark matter experiment (CDEX). Presentation at the TeV Particle Astrophysics 2010 Conference, Paris, France.