Major physics research may be heading downward, toward the "inner space" of a very deep mine in South Dakota. The new National Underground Science Laboratory (NUSL) may be created specifically to house and provide shielding for a broad range of cutting-edge low-background physics experiments. Let me explain what’s going on.
If you measured the radiation present in our environment with sufficient sensitivity, you would find that the Earth is a rather radioactive place. Radon (half-life 3.82 days), a radioactive inert gas that decays by emitting energetic 8 MeV alpha particles, is produced by natural radium in the Earth’s crust. It seeps out of the ground to be inhaled by humans and to collect inside our buildings, in some regions in large enough quantities to become a health hazard. The common element potassium, an important nutrient for all living things including our bodies, has about 117 atoms of radioactive potassium-40 per million natural potassium atoms. The potassium-40 isotope decays with a half-life of 1.28 billion years, producing energetic electrons and gamma rays. This makes our bodies slightly radioactive so that, for example, we irradiate each other (fortunately at a very low level) when we sleep together. There is also plenty of potassium-40 in the Earth’s crust and core. The main reason for the Earth’s interior heat, the power source driving volcanic activity and plate tectonics, is the energy liberated by the radioactive decay of potassium-40 and other radio-nuclides within our planet.
In addition to the Earth’s natural radioactivity, we are being continually irradiated by particles from space. Cosmic rays, mainly protons, electrons, and gamma rays, bombard the Earth’s upper atmosphere, where they produce muons (or m leptons). These cosmic muons are energetic charged particles that penetrate through the shielding atmosphere and deep into the Earth’s interior. Cosmic muons also make almost every material on the Earth’s surface slightly radioactive. In addition, cosmic ray reactions with atmospheric nitrogen and oxygen create carbon-14 (half life 5,730 years), tritium (half life 12.3 years), and berillium-7 (half-life 53 days), all of which collect and decay in organic and other materials.
Our bodies have evolved and adapted in this radioactive environment, and there is little evidence that we are affected, either positively or negatively, by the natural radioactivity in which we live. However, environmental radioactivity and radiation make it difficult to perform radiation-sensitive physics experiments, particularly those in which the effect being measured resembles or is masked by the natural and comsogenic radioactive background. Such experiments can only be done after reducing the natural radioactive background. The favored way of doing this is to place the experiment deep underground to shield it from cosmic rays and environmental radiation.
The Homestake gold mine located in Lead (pronounced like the metal), South Dakota has excavations that penetrate over 8,000 feet into the Earth. For many years Homestake has been very productive, both as an active gold mine and as the site of the first solar neutrino experiment, a large chlorine-based detector mounted by Ray Davis and his coworkers in the 1970s. Now the Homestake mine is reaching the end of its useful life as a commercial enterprise, but it may receive new life as a center for cutting-edge physics.
The National Underground Science Laboratory is a major new scientific initiative. It is now in the proposal stage. It would take over operation of the Homestake mine, prevent it from being flooded and abandoned, and make it the home of low-background physics experiments in neutrino detection, studies of rare decay processes, and searches for dark matter particles.
In six of my previous Alternate View columns in Analog issues March-86, May-86, December-87, September-92, February-93, and January-99, I described supernova detection, solar and cosmic ray neutrino detection, and dark matter searches, all of which were conducted in underground laboratories. In almost all of these experiments, the measurements were made more difficult by the background radiation from cosmic ray muons. In this column, I'll describe some of the physics projects intended for the NUSL.
Solar Neutrinos - Measurements show that only about 1/3 of the electron neutrinos produced by hydrogen fusion in the Sun reach the Earth. The quantity of neutrinos predicted by the best models of solar fusion reactions is incompatible with the data. Something is wrong. The first evidence of this problem emerged 30 years ago when the neutrino detector in the Homestake Mine went into operation. Recent results from the heavy-water-filled SNO neutrino detector located deep in a mine in Sudbury, Ontario, Canada, indicate that the missing neutrinos have been converted into non-interacting neutrino species by a process called "neutrino oscillation". Essentially, as they travel through the medium of the Sun and through the intervening space, the electron neutrinos from solar fusion processes become mu and tau neutrinos, which do not interact with most neutrino detectors.
New solar neutrino detectors built for the NUSL would focus on directly detecting the converted "missing" neutrinos and on measuring the solar neutrino energy spectrum. About 98% of solar neutrinos have energies below 1 million electron volts. Most of the existing detectors have no sensitivity in this energy region, but new detectors are being designed to make measurements there. New detectors would also use "neutral current interactions" to directly detect the mu and tau neutrinos produced in the oscillation process.
Dark Matter - We have come to realize that the ordinary matter of our universe, the stuff that forms quasars, galaxies, stars, planets, people, atoms, quarks, ..., is only an insignificant fluff, a minor impurity that permeates the real material of the universe. We have learned in the last few years that about 70% of the mass-energy in the universe is somehow stored in the vacuum itself as "dark energy", while about 5% is in the form of normal matter. The remaining 25% of the mass-energy is in the form of "cold dark matter", mysterious invisible particles that are known to inhabit the haloes of galaxies.
Astronomers discovered dark matter while using the Doppler-shift technique to measure the velocities of bright stars orbiting distant galaxies and galactic clusters. Roughly speaking, any orbiting object must have a velocity that depends on the square root of the amount of mass within the spherical volume enclosed by its orbit. As the distance R of the bright star from the center of the galaxy or cluster increases, it is expected that the star's orbital velocity should first increase in proportion to R, as long as it is at a radius within which the stars of the galaxy are distributed fairly uniformly. When the bright star is at a large radius outside the galaxy, so that the sphere of radius R encloses essentially all of the mass of the galaxy or cluster, the velocity of the star should fall off as the inverse square root of R. Thus, the average mass and mass distribution of a galaxy can be estimated by measuring the velocities of a number of bright stars at various distances from the galactic center.
To the surprise of astronomers, the velocities measured in this way were much larger than expected from a galactic star count, and it was found that the orbital velocity continued to increase with R far beyond the visible radius of the galaxy. This is convincing evidence that the visible mass of a galaxy, even after correcting for non-luminous "brown-dwarf" stars and interstellar gas, is much too small and too localized to account for the observed orbits. The conclusion from this work is that most of the mass of galaxies comes from some mysterious dark matter, which is distributed much more broadly than the visible stars in the galaxies studied. This mystery of the origin of this extra mass has come to be known as "The Dark Matter Problem". It is one of the outstanding unsolved problems of contemporary astrophysics.
The Dark Matter Problem has prompted a close examination of all theoretical possibilities for the missing mass of the universe. Theoretical particle physicists over the years have predicted scores of could-be particles that have never been observed in the laboratory, and many of these have become "dark-matter candidates", particles which, if they existed, might account for the hidden mass of the universe. Among these dark matter candidates are axions, massive neutrinos, and more generally, a whole class of weakly interacting massive particles otherwise known as "WIMPs".
In a typical neutrino or WIMP search, a large dense volume of sensitive detector material must be placed underground in a low- radiation environment. The detector material is chosen so that interaction with the particles of interest, e.g., dark matter WIMPs, produce detectable change in detector material or directly generate a detectable signal. Up to now, searches for axions and WIMPs have been unsuccessful. The NUSL facility would provide an ideal environment for constructing the next generation of dark matter detectors.
Proton Decay - Quantum mechanics tells us that the black holes themselves are slowly fizzing away to nothingness by boiling off Hawking radiation. But perhaps most devastating of all, we now have reason to consider the possibility that a fundamental building block of the universe, the proton which is the core of every hydrogen atom, has only a "limited warranty" which may run out in about 1034 years or so. With perhaps that half-life, protons (along with all nuclei containing protons) will "decay", releasing much energy as they are transformed into lighter particles. The process ends with a positron and some neutrinos and gamma rays replacing the proton.
Every year the Sun may be losing about 1018 protons (about a 10 microgram's worth) in this way. This is not much of a loss, but it is irreversible, and it adds up. In 1037 years or so, all of the protons in the universe might be gone. The universe would then be empty of all complex matter. There would be no galaxies, no stars, no planets, no organisms, no molecules, no atoms, no nuclei. No matter at all would be left except for miscellaneous electrons and positrons seeking a final annihilation and leaving behind only gamma rays.
Why should something as obviously stable as a proton be unstable at its roots? The Buddha gave the reason in about 485 BC: "All composite things decay." And protons may indeed decay because they are composite. We have learned in the past two decades that protons are not "fundamental" as has been previously supposed, but rather are composite particles made of three "quark" constituents. It is the interactions and transformations of these quarks that may permit the proton to occasionally decay. The strong, weak and electromagnetic forces are aspects of the same underlying physics, with "bridges" between the forces, and between their manifestation in quarks and leptons. For this reason, a quark can, in principle, decay into leptons, with an accompanying release of energy. If this happens to a quark in a proton, the proton decays. This decay process was first predicted in the 1980s, and several large experiments were mounted to try to observe the phenomenon. These first-generation experiments have not observe proton decay. However, the predicted threshold for detection lies not far below their sensitivity level. If the NUSL project is funded, it is likely that a new proton decay search using the excellent shielding of that facility would be constructed to push down the level where the decay process might be observed.
Supernova Neutrinos - A byproduct of the unsuccessful searches for proton decay in the 1980s was that the large Kamiokande water-filled detector operated by the Japanese was able to observe the neutrons from SN1987A. Several other large proton-decay detectors that might also have observed the neutrinos were off-line at the time. Since that event, the designers of all large water-filled detector experiments have considered their potential as detectors of supernova neutrinos. If the NUSL facility is funded, it is likely that it will provide a permanent site for a supernova neutrino detector, either as a stand-along detector or as a part of a detector that is simultaneously searching for other phenomena.
There are a number of other ideas for experiments at NUSL, but I'll stop here. The National Science Foundation is presently considering this initiative. Watch this column for further developments.
The NUSL Proposal:
See the "National Underground Science Laboratory at Homestake" web site, http://mocha.phys.washington.edu/NUSL
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