Facilities for Deep Science
Physics laboratories (blue) are listed with their depths in meters water equivalent. Laboratories for research into the long-term (~million-year) isolation of high-level nuclear waste, shown in red, are listed with actual depth. The NELSAM laboratory (black) is for earthquake research.
Facilities for Deep Science
The 21st century exerts increasing demands to go underground. Expanding and developing populations put growing pressure on surface space, driving mass transit systems, hydroelectric plants, energy storage, waste disposal facilities and a host of other systems underground. The depletion of shallow mineral and energy resources sends prospectors ever deeper in search of essential raw materials and new sources of energy. Yet despite the demand for subsurface space and resources, major engineering obstacles to underground use remain.
THE NEED FOR UNDERGROUND FACILITIES AT DEPTH
Although some investigations can be carried out at intermediate depth, it is clear that the frontier of underground science is at great depth. For physicists and astrophysicists, this is simply the quietest environment, with the lowest background noise from cosmic muons that limit the sensitivity of searches for neutrinoless double beta decay and dark matter, as well as for certain solar neutrino interactions (Figure 3). Depth is also the frontier for geomicrobiologists, who require pristine environments to study underground microbes in situ, with minimum disturbance of the underground environment. Starting from the lowest possible depth, biologists could drill down to measure to what temperature and pressure life continues to exist. Geoscientists also need to understand rock under the constant influence of biology, water, chemicals, heat and stress. Experiments in boreholes extending several kilometers below the lower levels of a deep underground laboratory would also be very valuable to them. Moreover, they require an opportunity to vary these parameters on a large volume of rock for long intervals of time. They must also go underground in order to verify, or ground-truth, the new computational and remote-sensing techniques that are being developed to make the earth "transparent." Such studies are complementary to those currently done in relatively shallow underground laboratories devoted to waste storage research (Figure 3). The challenges of large depth for engineering will push the envelope of underground design and construction methods.
Interest in deep science is growing, because of its success and compelling nature. The trend is particularly strong in physics and astrophysics, with the recognition that discovering the nature of the quantum universe requires a coordinated program for discovery based on underground experiments combined with observations in space and experiments at particle accelerators. Similar trends exist in geomicrobiology, which emerged as a field some 20 years ago, and in the geosciences and engineering, where dependence on the subsurface as a multifaceted resource is growing.
Figure 2 shows as an example the increase in the number of physicists involved in the search for weakly interacting massive particles that may form the dark matter in the universe, following a seminal paper by Goodman and Witten 20 years ago. The recent discovery at underground experiments that the neutrino has mass has intensified interest in the search for neutrinoless double beta decay.
The importance of the scientific questions argues for carrying out several experiments with similar scientific goals using different technologies as a protection against unexpected technical difficulties, such as unanticipated backgrounds in physics experiments. Besides providing an important cross check on results, comparisons between experiments often provide significant additional information. Because experiments are increasing in scale and complexity, their life cycles become longer and longer (10-15 years for large physics experiments). To maintain reasonable progress, it is necessary to start installing the next generation experiments while existing ones are still in active operation.
It is difficult to make an exact prescription of the need for underground space: a simple scientific wish list is unrealistic, and the need depends on scientific priority, on funds allocated to underground experiments in each of the fields, and on the scientific policy pursued by the agencies (e.g. with respect to the number of experiments pursuing parallel objectives). But the long-term trend is clear: in the next decades the demand for dedicated long-term research space underground will increase.
Global Context
Like most modern scientists, underground researchers are deeply involved in international collaboration. This global cooperation applies not only in physics–as at the Sudbury Neutrino Observatory in Canada or in Japan's KamLAND and Super-Kamiokande experiments–but increasingly in geosciences and biology, as at the Natural Earthquake Laboratory in South African Mines. The trend toward internationalization accelerates with the increasing size and complexity of experiments. The costs of some projects– large detectors for neutrino physics and proton decay experiments, for example–mandate regional or interregional coordination. Worldwide, strong scientific interest has prompted universities and institutes in several countries to build and operate underground facilities. Figure 3 compares the volume and the depth of the major dedicated underground scientific laboratories in the world (page 35).
Also shown are DUSEL's approximate volumes, both for the proposed first suite of experiments (red) and for possible extensions (pink).
Three observations can be readily made:
a) In terms of volume available underground, the field is dominated by Gran Sasso in Italy at 3000 m.w.e. (~1400 meters deep). Smaller facilities exist at the Japanese Kamioka laboratories, at the U.K.'s Boulby mine, and at Canfranc, in Spain. In spite of its large size, Gran Sasso is fully subscribed, and major expansion is underway at Canfranc to meet the demands of new experiments. In the U.S., physicists have mostly relied on Soudan. All these facilities are primarily devoted to physics experiments.
b) With the exception of the Natural Earthquake Laboratory in South African Mines (NELSAM), most of the earth sciences and microbiology underground research laboratory facilities are limited to relatively shallow depth (Figure 1). Limited facilities for hydrological and microbial studies in fractured granite are available at Aspö, Sweden, to a depth of 1200 m.w.e, (480 meters in rock). Some underground research laboratories developed originally for research related to long-term isolation of radioactive waste may be used for more general research in the geosciences and in microbiology.
c) There is a relative dearth of deep facilities: SNOLAB, 5990 m.w.e. (~2 km of rock), whose scientific facilities are currently being expanded, is the only very deep facility, and it is devoted only to physics experiments. Small facilities are available in the Fréjus/ Modane tunnel connecting Italy and France (4150 m.w.e.), and at Baksan in Russia (4700 m.w.e). The state of South Dakota has recently announced interim funding for the reopening for science of the 4850-foot level (4050 m.w.e.) of the abandoned Homestake Mine. The National Science Foundation, in its DUSEL solicitation process, has provided funding for the preparation of conceptual designs for a laboratory at Homestake and at the Henderson Mine, an active molybdenum mine in Colorado.
The last decade has seen a severe space crunch for dedicated subsurface science facilities, particularly at large depth. While SNOLAB may serve North American needs in the short term from 2007 to about 2012, in the longer term a serious shortage of space threatens the generation of physics and astrophysics experiments slated for 2012-2015, and for the decade that will follow. Moreover, SNOLAB will not provide significant opportunities for research in other fields. The same lack of space applies to geosciences, biology and engineering, where there are essentially no dedicated deep facilities offering long-term research access. Europe, already home to the largest underground laboratory, is aggressively expanding at Canfranc and considering an expansion at Fréjus/Modane to meet the demand.
The need for optimal use of existing facilities and for a deep national facility:
Existing facilities internationally will not provide a long-term solution to the space problem of the rapidly expanding field of deep underground science. To support this emerging field, the U.S. must optimally use its existing facilities (WIPP and Soudan); take full advantage of international opportunities, in particular at SNOLAB, Kamioka and Gran Sasso; and consider building facilities deep beneath its own soil.
Looking at the list of underground facilities worldwide given in Figure 3, it is striking that, at the moment, the U.S. is close to being the only developed country (specifically, in the G8 group) without an underground science laboratory deeper than 2000 m.w.e. Clearly, this puts the nation at a distinct disadvantage in underground research. Although the results of science and engineering experiments performed in other countries' facilities are shared worldwide, and U.S. scientists have been very successful in international collaboration, important benefits accrue to any country supporting world-class facilities in its own territory. These benefits include leadership in experiment and technology development; rapid follow-up of novel ideas; optimal coordination with other national assets, such as accelerators, seismographic arrays and biological facilities; and impact on the national economy through science and technology transfer and education. There is no adequate substitute for a national facility.
A deep national science and engineering laboratory would therefore fulfill three goals: provide increased deep underground access to U.S. and foreign scientists in a space-limited international environment, put the U.S. in a stronger strategic position in deep underground science, and maximize the benefits of underground research for national well-being.
