A Question of Depth
HOW DEEP MUST PHYSICISTS GO?
Physics experimenters can quantify the background for any given experiment. The signal rates for events caused by processes at the frontier of physics may be only a handful per year. But at the earth's surface, twenty thousand cosmic-ray muons per minute pass through each square meter. Several techniques exist for mitigating this background, and depth is only one of them. For certain experiments, depth may be the only available strategy; for others it may be one of several options.
Source: Tommy Phelps
The graph in Figure 4 shows the rate per nucleus of interactions of cosmic-ray secondary neutrons with energies above 100 MeV as a function of depth. Those neutrons are the most difficult component of the cosmic rays to shield. The adjacent panels show the signal rates for "WIMP" dark-matter particle interactions, for neutrino-less double beta decay, and for solar neutrinos. For many experiments, scientists have developed strategies to reject backgrounds, and not every cosmic-ray neutron will mimic a signal, but the comparison shows that it is easier to carry out such searches at great depths. Where signals from the new physics will appear is unknown, but the ranges on the scales to the right cover what is expected for three major physics campaigns. As experiments become bigger and more sensitive (moving down on the graph), the need for depth becomes more acute.
If the expected signal rate falls below the cosmic neutron background rate, an experiment may still be practical. Among the strategies that can be used are a) shielding, b) energy selection, c) association of an event in time with another event, d) topology of events. Even when those techniques are available, however, depth is still an advantage. Experimenters can dispense with costly shields required for each experiment at shallower depth in favor of the overburden in a shared deep facility. Deep experiments lessen the concern that an observed signal might actually be background. Depths in the range 4000-6000 m.w.e. are sufficient to meet the needs of this kind of physics. There is a depth limit, set by the rate of neutrino interactions, beyond which experiments obtain no further gains. Even the whole earth provides no shielding against them. That depth limit is about 10000 m.w.e.
Source: Tommy Phelps
The search for proton decay is a case in which current experimental limits from 2000 m.w.e. are already a million times lower than the neutron rates shown. The high energy release and specific event topology help make this possible. Neutrinos are the main background for many proposed decay modes for the proton, even at that modest depth. The next generation of proton decay detectors will be very large, and at depths determined by a careful balance of engineering capability, cost and physics requirements.
Not all physics experiments are sensitive to cosmic rays–gravitational wave detectors are not, but they face their own backgrounds from phenomena such as seismic surface waves, traffic noise, wind, waves, tides and temperature changes. A quiet location a few hundred meters underground meets their needs.
WHAT DO THE BIOLOGISTS NEED?
The frontier in subsurface biological research is the depth where the temperature exceeds 80 to 125°C. This boundary may lie thousands of meters below the surface, and can be reached by drilling. To study subsurface microbial processes also requires borehole arrays with close spacing. Drilling from an already deep location has three advantages: reduced drilling costs, high spatial control and improved control over contamination from surface organisms. In Figure 5, cost estimates for drilling to various depths up to 10000 m from the surface and from a laboratory 2500 m deep are compared. The deep stance gives a great advantage in cost.
HOW ABOUT GEOSCIENTISTS AND ENGINEERS?
The interests of geoscientists in fact extend to the center of the earth, and seismic and electromagnetic methods have been used to "see" through rock. The accuracy of these methods can be tested and improved in an underground laboratory. At depths of a few thousand meters the rock pressure and temperature are high enough to modify the way rock responds to such probes. Thus direct access to rock at great depth is of interest to geoscientists and petroleum engineers. The rock pressure at depth also becomes the major challenge to engineers who must design large cavities not only for science but also for a wide range of technical, mining and societal applications. But shallower depths are also important in these disciplines.
