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Neutrinos and WIMPs

by John G. Cramer

Alternate View Column AV-13
Keywords: solar neutrinos, WIMPs, underground, gallium, weak interaction
Published in the May-1986 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 10/20/85 and is copyrighted © 1985, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.


    This AV column is about more "troglodyte physics", in this case for the detection of neutrinos. Deep underground in the Homestake Gold Mine, 4850 feet below Lead, South Dakota, is a giant 100,000 gallon tank of per-chloro-ethylene, a cleaning solvent made for use by commercial dry cleaners. But this cleaning fluid has been given another mission than removing stains from dresses and sports coats. Since 1968 it has been used in a massive effort to measure the temperature at the heart of the sun; it is a detector of solar neutrinos. Two decades ago Dr. Raymond Davis of Brookhaven National Laboratory set up this apparatus to detect the neutrinos emitted by one of the nuclear fusion reactions which are the sun's power source.

    The neutrino is the massless and electrically neutral weak-interaction partner of the electron, always traveling at the speed of light and rarely interacting with anything. The sun makes lots of neutrinos. About 61,000,000,000 neutrinos per second from the sun pass through each square centimeter of cross section on the surface of the Earth. If your body presents an area to the sun of 10,000 square centimeters, this means that 610 trillion neutrinos are passing right through your body in the second it takes to read this line. But you don't notice this; neutrinos can pass through light years of lead without impediment. They pass through your body and through the Earth as if neither was there. As you might imagine, this makes the detection of neutrinos very difficult ... but not impossible.

    Because neutrinos have such a low probability of interacting with matter, detecting them requires a scale of physics experiments which can best be described as Heroic. Davis detects neutrinos by collecting, every month or so, the few atoms of radioactive argon-37 gas which are produced when neutrinos from the sun interact with the 1031 or so chlorine atoms in his cleaning fluid. And over the years he has found that he can account for only about 1/3 of the number neutrinos that should be present according to our best present understanding of how nuclear fusion reactions work in the interior of the sun.

    When Davis' results first began to emerge about a decade ago there was furious experimental and theoretical activity to check all of the aspects of solar astrophysics to see if the factor of 1/3 discrepancy could be explained. The astrophysical modeling of the sun was carefully re-examined in every detail. New experimental tests of the nuclear physics underpinnings of the solar models were made, but produced no explanation of the dearth of neutrinos. There have also been a number of explanations given which might be characterized as far-out. Examples of these are the suggestion that neutrinos oscillate from one state to another as they travel from the sun's interior to the earth, or that the sun has an inert iron core, or that the fusion reactions in the sun may have actually have stopped, leaving the sun to run on its left-over heat. But so far, no consistent and satisfactory explanation has been given. Davis' low neutrino count today remains an unsolved mystery.

    In this AV column I want to describe some new techniques for detecting neutrinos from the sun. Then we'll end with a new idea which may explain both the solar neutrino problem and the Dark Matter problem (see "The Dark Side of the Force of Gravity", my AV column of 2/85).

    The next operating detector of solar neutrinos is going to be very big and very expensive. It's likely to be made of gallium, the 31st element in the periodic table. Gallium is a metal with a very low melting temperature. If you held a cube of it in your hand, it would melt into a mercury-like puddle. It's used to make the red LED indicator lights used in many electronic devices. And in a few years the semiconductor galluim arsenide may provide the basis for the next revolution in high-speed transistor electronics.

    Gallium will probably be used for neutrino detection because the isotope gallium-71 (31 protons and 40 neutrons) is converted to germanium-71 (32 protons and 39 neutrons) by solar neutrinos. This conversion process is sensitive to neutrinos of much lower energies than those detected by the Davis experiment. A large quantity of gallium would be placed in chemical solution in eighteen glass-lined 750 gallon tanks located deep underground, perhaps in the Homestake Mine. Radiochemical processing would recover and identify the germanium-71 made by solar neutrinos over the course of months and years. The measurement would give an independent check on whether our understanding of how the sun works is really incomplete.

But there is a problem. Gallium is a very expensive material, and there's not too much of it around. The gallium neutrino detector which has been designed needs 30 tons of the stuff, a good slice of the world supply worth more than $5,000,000. The operating cost of the experiment would be a comparable sum. Perhaps if the gallium were sold at the end of the experiment some of these costs (or more) could be recovered, but that's not the way the funding of scientific experiments is figured. And $10 million is large enough chunk of cash to dent the budgets of scientific funding agencies like the National Science Foundation and the Department of Energy. After gasping at the cost of this experiment for a number of years, the scientific community now seems to have arrived at the conclusion that the gallium neutrino experiment really is necessary and should be done soon. It is expected to start up in the next year or so.

    But 30 tons of gallium! Surely there must be a better way to detect neutrinos. And perhaps there is. The semiconductor material silicon in its single crystal form at low temperatures has a very unusual property. A small amount of energy introduced into the crystal results in a large change in temperature. Recently, this characteristic of silicon has been used in the detection of X-rays with a technique called "bolometry" or energy measurement. A silicon crystal is cooled down to within a few thousandths of a degree of absolute zero. An X-ray stopped in this crystal deposits its energy and the accompanying temperature rise is registered by sensitive thermometers attached to the silicon crystal surface. The observed signal shows a tiny temperature rise for a few thousandths of a second before the cooling system restores the temperature to its former value. The technique should work equally well with neutrinos from the sun, which typically carry ten or more times as much energy as the X-rays which have already been detected with this technique.

    Calculations show that, given an interaction, detection of neutrinos by this bolometric technique should be quite easy. The severe problem is, of course, to use enough silicon to give a reasonable probability of having a neutrino interaction in the first place. This requires a large amount of silicon (up to 10 tons), somewhat less of a cheaper and more abundant material than the 30 tons of gallium mentioned above. The silicon bolometric technique looks very promising. And it offers direction sensitivity, energy resolution, and other advantages which we do not have the space to discuss here. But the technical design of the experiment will require several more years of hard work by a team of physicists before actual measurements can begin. So keep your fingers crossed ...

    Finally, I want to mention WIMPs. That perhaps misleading acronym stands for Weakly Interacting Massive Particles. As I have mentioned in past AV columns, various presently fashionable theories of fundamental particles, particularly grand unification (GUTs) and super-symmetry (SuSy) theories, have predicted a veritable bestiary of peculiar particles, none of which anyone has yet observed. There are squarks and winos, gluinos and higgsinos, selectrons, sneutrinos and photinos. These could-be particles have only one property that interests us at present: mass. The suggestion of some of these particle theories is that the lightest of these exotic particles could possibly still be around as remnants of the Big Bang. They should have a mass perhaps on the order of a proton mass but should not have strong or electromagnetic interactions with more normal matter. Thus this type of object is a weakly interacting (in the generic sense) and massive particle: in a word, it's a WIMP.

    My 2/85 AV column on Dark Matter discussed the possibility that axions, hypothetical particles of very small mass, are responsible for most to the mass of the universe. WIMPs are a more massive alternative to the axions which could provide an explanation for exactly the same puzzles: the high velocities of stars in galactic haloes, the synthesis of helium and lithium in the Big Bang, the formation of galaxies, and the requirement of the new inflationary scenario of the Big Bang that the universe must have exactly enough mass to be "critical" and 300 times more than we can easily account for.

    But it has been recently realized that, unlike axions, WIMPs can also provide the solution to another puzzle, the solar neutrino problem discussed above. Theorists at the University of California at Santa Cruz and at Harvard have shown that WIMPs can reduce the number of solar neutrinos detected by the Davis experiment. The scenario goes like this. Heavy WIMPs tend to be collected by the sun's very strong gravitational field and to move freely through the sun's volume, passing repeatedly through the central region and occasionally interacting with the normal matter of the sun by scattering. The solar model prediction of the number of neutrinos detected by the Davis experiment tells us that only the central 5% of the sun's volume, the region with the very highest temperature, produces 70% of the especially energetic neutrinos detected by Davis but contributes only 10% to the total energy output of the sun.

    But WIMPs could act as a rather good thermal conductor, carrying heat from the sun's center to other locations by their infrequent scatterings. The effect of this would be to reduce the temperature at the sun's center while raising it slightly elsewhere. This can reduce the production of the most energetic neutrinos from the sun without markedly reducing the sun's total energy production.

    There are some problems with this picture which must be worked out, but it does seem to provide a plausible way of dealing with the dark matter problem and the dark matter problem at a stroke. Moreover, it has the decided advantage of being testable. The modeling of the sun with WIMPs included would predict a reduced number of the energetic neutrinos detected by Davis's chlorine experiment, but no diminishment of the number which should be detected by the proposed gallium detector and the silicon bolometer described above. Therefore, the ratio of high energy neutrinos to medium and low energy neutrinos is a very valuable measure of the validity of the WIMP explanation, and this parameter should become available when the results of the new neutrino experiments become available. We may confirm these ideas and we may find the need for new ones. But in any case, we will be probing the very heart of the sun to learn how this most supremely important of all energy sources works. We will learn the secrets of the sun itself.


The Solar Neutrino Problem:
Solar Neutrinos and Neutrino Astronomy, Eds. N. L. Cherry, W. P. Fowler, and K. Lande, AIP Conference Proceedings No. 126, American Institute of Physics, (New York, 1985).

Silicon Bolometry:
B. Cabrera, L. M. Krauss, and F. Wilczek, Physical Review Letters 55, 25 (1985).

M. M. Waldrop, Science 229, 955 (1985);
W. D. Press and D. N. Spergel, Astrophysics Journal 296, 679 (1985);
J. Faulkner and R. L. Gilliland, Astrophysics Journal, to appear December (1985).

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