Rare Nuclei Produced by Solar Neutrinos Deep in a Colorado Molybdenite Deposit May Show

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SCIENCE IDEAS Solar Variability Glacial Epochs, and Solar Neutrinos by George A. Cowan and Wick C. Haxton Rare nuclei produced by solar neutrinos deep in a Colorado molybdenite deposit may show that the earth’s most recent glacial epoch was triggered by a sudden reduction in the sun’s energy output.

e are all made of star stuff,” version of four protons into helium-4 (Fig. energy, has remained constant apart from a says Carl Sagan. This trib- 1). The standard stellar theory depicts the monotonic increase of 30 per cent over the ute to the stars as the cosmic main sequence as a relatively simple, steady- lifetime of the sun. This increase tracks the w factories of the heavy ele- state period in a star’s evolution. Thus, any rises in the temperature and helium-4 abun- ments tacitly accepts one of the great ex- failure of the standard theory to predict the dance of the solar core as its supply of trapolations of modern physics. We have present behavior of the sun could indicate a hydrogen is depleted. postulated complex thermonuclear reactions serious flaw in our stellar physics. To the extent that the earth’s geologic and occurring deep inside the stars as the source We believe that there is disturbing and biologic history provides a record of the of stellar energy and the natural progression controversial evidence that such flaws may solar luminosity, we can check the predic- of these reactions as the basis of stellar exist. Part of the evidence is provided by the tions of the standard model. There appear to evolution. Yet our first-hand knowledge of earth’s climatic history, and part by a con- be a number of inconsistencies. The low stellar structure is limited, consisting largely temporary experiment that directly moni- initial luminosity predicted by the standard of surface observations. How certain, then, is tors the thermonuclear reactions in the solar model suggests a primordial climate for the our understanding of the processes govern- core. The evidence suggests that variations in earth quite different from today’s, yet the ing synthesis of the elements deep within the rate of solar energy generation occur, paleoclimatic record shows no evidence for fiery stellar cores? perhaps induced by periodic mixing of the any significant climatic evolution. On the The sun, by merit of its proximity, core. We propose a new experiment that other hand, there is evidence for unexplained provides unmatched opportunities for testing may test the long-term stability of our sun. variability, even periodicity, in the sun’s our theories of stellar processes. By any behavior over shorter time scales (Fig. 2). criterion it appears a pedestrian star, some- Solar Variability? Sunspot activity has waxed and waned in a what smaller and fainter than the average of regular eleven-year cycle since 1715. In the its neighbors. Formed some 4.8 billion years The principal energy output of the sun is preceding seventy years, termed the ago, the sun has now progressed through the visible electromagnetic radiation leaking Maunder Minimum, sunspot activity was half of the main sequence of its evolution, a from its surface. According to the standard nearly absent, and, according to European phase in which almost all of its energy is model, the solar luminosity, or the rate at records, persistently cold weather took its derived from hydrogen “burning,” the con- which the sun radiates electromagnetic toll on crops. Corroborating evidence for a

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quiescent sun during the Maunder Minimum billion years major glacial epochs lasting exists in the reduced brightness and extent of several million years have occurred reg- the sun’s corona, a diminished number of ularly, separated by warmer periods lasting auroral displays, and an increased abun- several hundred million years. The latest dance of carbon-14 in the atmosphere. (The glacial epoch, the Pleistocene, began just latter two phenomena result from a de- three million years ago, and the proximity of creased emission of charged particles by the its onset indicates that the present is an sun.) Two other prolonged perturbations of atypical time in the earth’s climatic history. the earth’s climate during the past thousand The similarity of glacial time scales to years, the warm Twelfth Century Grand those governing fundamental solar processes Maximum and the cold Sporer Minimum of has been discussed extensively. The duration 1450 to 1540, also are correlated with of the glacial epochs is comparable to the (86%) periods of increased or decreased solar ac- thermal diffusion time of the solar core. or tivity. Their spacing corresponds to a fundamental Does such evidence of solar variability hydrogen-burning scale, the time required for (14%) indicate that the standard model is incorrect? the ratio of helium-3 to hydrogen (see Fig. 1) The short time scales of these events are to reach equilibrium over an appreciable characteristic of complex physical processes fraction of the solar core. These observations occurring in the sun’s convective envelope. have stimulated development of a number of (99.85%) For instance, the eleven-year cycle of solar nonstandard models in which variations in magnetic field reversals, which govern the solar output are coupled to these thermal sunspot activity, is thought to be maintained and nuclear time scales, by dynamo action associated with convec- Perhaps the best known of these models is tion and rotation. Thus, the climatic the “Solar Spoon” of Dilke and Gough. Both (0.15%) anomalies of the past millenium may indicate the time required to reach equilibrium be- merely some lack of detail in standard stellar tween helium-3 and hydrogen and the theory rather than a basic flaw. However, equilibrium ratio are sharply decreasing Fig. I. The proton-proton chain, or hy- these phenomena do demonstrate that varia- functions of temperature, which itself is a drogen burning, is postulated by stan- tions in the solar output have terrestrial sharply decreasing function of the distance dard stellar theory as the principal consequences. A more provocative question from the sun’s center. Thus, as hydrogen then becomes whether there exist some burning proceeds, equilibration produces mechanism of energy generation in the climatic tests of solar behavior over the large composition gradients over expanding sun during the current stage of its evo- longer time scales that might characterize portions of the solar core. Dilke and Gough lution. The net result of this chain of possible changes in the solar core, where the found that such gradients would permit nuclear reactions is conversion of four basic process of energy generation occurs. large-amplitude excitation of certain protons into helium-4, and the energy Evidence of long-term climatic variability nonradial solar oscillations deep in the solar released is carried off by photons, is found in the repeated advance and retreat interior, namely, the low-order gravity positrons, and neutrinos. The predicted of continental glaciers and in the quasi- modes. (The gravity modes thus differ from branching ratios for competing reactions periodic occurrence of major glacial epochs. the familiar five-minute solar oscillations, at various steps of the chain are given in The strong correlation between stages of which are acoustic modes confined to the parentheses, and the chain terminations sun’s surface.) The gravity modes become continental glaciation and the periods in helium-4 are boldface. Another chain (10,000 to 100,000 years) of the earth’s unstable when equilibrium between helium-3 of nuclear reactions, which involves orbital parameters (its eccentricity, obliquity, and hydrogen is achieved over a sufficient and precession) suggest that these changes fraction of the solar core, Dilke and Gough carbon, nitrogen, and oxygen isotopes, are governed by the earth’s orbital geometry found that the time required for this to occur contributes only a small amount to the rather than solar phenomena. In contrast, is 200 million years, and therefore gravity- sun’s energy output but can be important terrestrial explanations of the glacial epochs mode instability should have been reached for main-sequence energy generation in appear much less convincing. In the last early in the sun’s main-sequence lifetime, Yet stars more massive than the sun.

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Fig. 2. Evidence for variability of the sun over its recent past. of charged particles from the sun creates a magnetic field that Sunspots, dark areas in the photosphere caused by a lowered shields the earth from cosmic rays, carbon-14 production surface temperature, are obvious indications of solar varia- varies inversely with the strength of the solar wind. Shown in bility. As shown in (a), sunspot activity was notably absent (b) is the deviation of the carbon-14 abundance from the during the Maunder Minimum, the seventy-year period be- average from John A. Eddy, Science 192, 1189 (1976)]. The tween 1645 and 1715. But since 1715 sunspots have been Maunder Minimum is characterized by a deviation greater abundant, the number varying in a regular eleven-year cycle than ten parts per million, as are two other periods, the Sporer [from M. W a ldemeier, The Sunspot Activity in the Years Minimum and the Twelfth Century Grand Maximum. These 1610-1960 (Schulthess Polygraphischer Verlag AG, Zurich, periods correlate, in both date and magnitude, with climatic 1961)]. Modern evidence that the Maunder Minimum was a extremes: significantly lower temperatures persisted during the period of abnormal solar behavior has come from measure- Maunder and Sporer Minima, and the “Medieval Climatic ments of carbon-14 abundance in tree rings. This long-lived Optimum” coincides with the Grand Maximum [see W. L. isotope is formed when evaporation neutrons, which are Gates and Y. Mintz, Understanding Climatic Change (Na- generated in the upper atmosphere by cosmic-ray interactions, tional Academy of Sciences, Washington, D. C., 1975), induce (n,p) reactions on nitrogen-14. Since the “solar wind” Appendix A ].

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today we find no evidence for large- amplitude oscillations of the characteristic frequency (about one hour). The explanation provided by the Solar Spoon is that such violent oscillations would induce sudden mixing of the solar interior and thus destroy the equilibrium conditions for gravity-mode instability. Further. by en- riching the core with hydrogen and helium-3, the mixing increases the rate of thermonuclear energy generation until it ex- ceeds the energy dissipation rate. The core then expands and cools. The cooling causes a decrease in nuclear reaction rates, which in turn leads to a suppression of the solar luminosity by 5 per cent for a period of about three million years. A period of elevated luminosity of somewhat longer duration then follows. When this transient mixing phase passes, the sun again burns in thermal equilibrium for 200 million years, the time required to re-establish the nuclear equilibrium necessary for gravity-mode instability. The duration and spacing of’ the transient The Solar Neutrino Flux nearly all pass unaffected through the sun’s mixing stages nicely match those of the —A Test of Solar Models outer layers and, eight minutes later, through glacial epochs. It is also widely believed that the earth. They retain, in their energy and in reduction of the sun’s luminosity by 5 per their flux, a detailed record of the nuclear cent would induce major climatic changes In light of these uncertainties, it is essential reactions that created them (Table I). Thus, a and that periodic mixing, by softening the to find more direct tests of the assumptions successful program of solar neutrino spec- long-term luminosity increase, would yield a made in the standard solar model. Un- troscopy could test the principal assumption primordial value more acceptable than that fortunately, conventional optical studies of of the standard model. Furthermore, since of the standard model. Furthermore, Sagan the sun provide little information about the neutrinos are the only particles that escape and Young contend that extinct Martian nuclear reactions occurring deep in the solar immediately from the sun, they are the lone rivers indicate an ice-age climate for Mars core. Photons, after completing their ten- contemporary monitor of activities in the coincident with the earth’s Pleistocene million-year journey outward to the sun’s solar core. epoch, which further suggests the existence surface, clearly retain no detailed memory of Precisely because neutrinos react so of extraterrestrial controls. Yet we should these parent reactions. Yet Nature has weakly with matter, terrestrial experiments bear in mind the circumstantial nature of provided a more direct means of probing the to detect them are enormously difficult. At these arguments: other explanations for the solar interior—the neutrino. present only a single solar neutrino measure- glacial epochs, such as the changing distribu- The nuclear reaction chains postulated by ment has been made, that of Raymond tions of the continents and oceans. may be the standard model as the mechanism of Davis, Jr., and collaborators. This radio- equally plausible. In addition, the suggested solar energy generation (see Fig. 1) include chemical experiment is based on the inverse mode of solar variability leaves unexplained a number of weak interactions (electron other glacial phenomena, such as the steady captures and beta decays) that produce 37Ar, which is induced by those solar neu- cooling of the oceans in the ten million years neutrinos. Neutrinos react with matter so trinos with energies greater than 0.81 million preceding the Pleistocene epoch. weakly that, once born in the solar interior, electron volts. The target consists of a

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100,000-gallon tank of tetrachloroethylene cally sensitive to the central temperature of relation indicated by the standard model

(C2C14) located 1500 meters underground in the sun. If the standard model could be between photon and neutrino luminosities the Homestake gold mine at Lead, South modified to produce the observed solar would not at present be valid because the sun Dakota. (This depth of overburden is luminosity with a lower core temperature, would not yet have returned to thermal necessary to shield against cosmic-ray the boron-8 neutrino flux could be reduced equilibrium. Specifically, the calculations of muons, which also can initiate reactions to a level consistent with the Davis experi- Dilke and Gough show that the depression in leading to argon-37.) The argon-37 atoms ment. Although the requisite modifications the boron-8 neutrino flux following mixing are removed from the tank by periodic appear violent, such as postulating could be of sufficient magnitude and dura- flushing with helium and are counted by mechanisms that reduce the core opacity by tion to account for the results of the Davis detecting the Auger electrons emitted as they maintaining a homogeneous distribution of experiment. decay (with a half-life of 35 days) by electron elements with higher atomic weight, they How can such speculations be tested capture. cannot be dismissed a priori. quantitatively? Several suggestions have The measured neutrino capture rate is Other components of the solar neutrino been made in recent years for performing 1.95 ± 0.3 x 10-36 captures per chlorine-37 flux are much less sensitive to possible geochemical solar neutrino measurements, atom per second, which is equivalent to the modifications of the standard model. The that is, measurements of the concentrations production of one argon-37 atom in the flux of low-energy neutrinos from the driving of certain long-lived isotopes produced by entire target about every three days. This neutrino-induced reactions in natural ore rate is in sharp disagreement with the 8.0 x 2H + e+ + v, is effectively fixed by the bodies or salt deposits. Geochemical experi- 10-36 captures per chlorine-37 atom per observed luminosity and by the assumption ments enjoy a considerable advantage over second predicted by the standard solar that hydrogen burning is the solar energy their laboratory counterparts in that much model. The efficiency for argon-37 recovery source. If the flux of these neutrinos proves larger concentrations of product isotopes from the target has been demonstrated, and also to be strongly suppressed, one would accumulate over geologic times. Our concern the cross section for neutrino capture by conclude that some failure in our under- with solar variability over time scales on the chlorine-37 is known. Thus some flaw must standing of neutrino propagation, rather order of a million to ten million years exist in our prediction of the neutrino flux than solar physics, is responsible for the suggests another reason for pursuing such reaching the earth. solar neutrino puzzle. The best hope for experiments: they may provide a quantitative This “solar neutrino puzzle” has remained measuring these low-energy neutrinos may record of past conditions of the solar core. unresolved for more than a decade. The be an experiment based on the reaction v + An intrinsic difficulty associated with discrepancy may be a symptom of some geochemical experiments is the inability to fundamental difficulty with our understand- to the experiment appears to be the cost of control backgrounds. Various nuclear reac- ing of stellar physics. Alternatively, the sun the requisite quantity (50 tons, or about tions induced by energetic neutrons, protons, may burn as we expect, but the behavior of twice the non-Communist world’s annual and alpha particles can effectively swamp neutrinos over astrophysical distances may production) of gallium, which is estimated to the solar neutrino signal by producing the involve new physics, The latter suggestion be in excess of $25 million. isotope of interest at a greater rate than do has become increasingly plausible with re- the neutrinos. Sources of such particles in- cent indications of massive neutrinos and A Los Alamos Solar Neutrino clude cosmic rays and radioactive nuclides, neutrino oscillations (see “The Neutrino in Experiment such as thorium and uranium, which are 1980” in Volume 2, Number 1 of Los found in trace quantities throughout the Alamos Science). No less strong are the arguments for earth’s crust. Cosmic-ray backgrounds, The puzzle can be solved by mounting pursuing new neutrino experiments that which are due principally to reactions in- additional solar neutrino experiments. Alone, probe untested aspects of solar physics. Two duced by protons evaporated from nuclei the Davis experiment is not definitive be- years ago we became fascinated by sugges- after interaction with high-energy muons, cause it detects only a very small fraction of tions that secular variations in the sun’s will be unimportant provided the ore body is the solar neutrino flux, primarily those high- energy production could be responsible for deeply buried. (It was to minimize the cos- energy neutrinos produced in the beta decay both the solar neutrino puzzle and the pe- mic-ray background that Davis chose to of boron-8 (see Fig. 1 and Table I). This riodic occurrence of glacial epochs. If core mount his experiment deep in the Homestake branch of the proton-proton chain is criti- mixing initiated the Pleistocene epoch, the gold mine.) In contrast, the importance of

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the backgrounds induced by particles from ground to be less than 10 per cent of the oxides at the controlled roasting temperature radioactive nuclides depends strongly on ore neutrino signal predicted by the standard (about 700 degrees Celsius) and passes into composition, nuclear reaction thresholds, solar model, the minimum depth of over- the gas stream, which consists largely of and Coulomb barriers, and conditions in burden for the typical host rock is 1340 sulfur dioxide and air, with efficiencies that Nature are usually such that these back- meters. can exceed 90 per cent. We believe that most grounds prove fatal. Unfortunately, the We know of one commercially developed of the rhenium and, presumably, the accom- magnitudes of these backgrounds are often molybdenite deposit that satisfies this depth panying technetium are then removed from apparent only after tedious calculations of criterion, the Henderson ore body under Red this gas stream by a scrubbing operation reaction cross sections. Mountain in Clear Creek County, Colorado prior to conversion of the sulfur dioxide to Thus, although our search for target (Fig. 3). The ore contains 0.49% sulfuric acid. isotopes yielded several candidates, the up- molybdenite on average, is currently being We had anticipated having to undertake per limits imposed by the background mined at a depth in excess of 1132 meters, the costly chemistry of extracting technetium calculations on the thorium and uranium and extends to a depth of more than 1500 from the effluent of the gas-scrubbing opera- contents of candidate ore bodies eliminated meters. Furthermore, for a geochemical ex- tion. Much to our delight, we learned from most of these. There remained two periment the effective depth is somewhat early discussions with AMAX officials that possibilities, both involving the production of greater since the top of the ore body at the this step also was performed at the con- technetium isotopes from molybdenum: time of its formation (about 25 million years version plant. The company had recently ago) lay 1500 to 1800 meters below the determined that the effluent contained surface. The present minimum depth of over- selenium in concentrations that could be burden is at the valley floor through which damaging to the environment, and last fall the deposit is entered. This floor resulted had installed a treatment facility to and from glacial scouring only 10,000 years ago. precipitate excess metal from the effluent. To determine the long-term boron-8 neu- Because of their similar chemistry, both trino flux we must extract approximately 10 rhenium and technetium should precipitate Fortunately, these reactions probe precisely million atoms (about 10–]5 gram) each of with the selenium. the time scale and neutrino-flux component technetium-97 and technetium-98 from 2000 With the problems of large-scale of most interest: the boron-8 neutrino metric tons of ore (equivalent to 10 per cent chemistry circumvented, the remaining hur- luminosity, which is the most sensitive of the daily yield of the Henderson mine). dle is the development of techniques for monitor of variations in the solar core The great miracle of this experiment is that a isolation and analysis of approximately 10 temperature, during and before the series of coincidences of Nature and com- million atoms each of technetium-97 and -98 Pleistocene epoch. (The half-lives of merce render such large-scale isolation of residing in kilograms of the selenium-rhe- technetium-97 and -98 are, respectively, 2.6 technetium both feasible and affordable. nium-technetium sludge. Although the task is and 4.2 million years; the reaction on The commercial world has made its con- formidable, progress made recently in the molybdenum-98 is induced only by the high- tribution in the fortuitous design of the chemistry and mass spectrometry of energy boron-8 neutrinos; and the reaction molybdenum recovery process, which technetium would appear to justify our op- on molybdenum-97 may sample in addition already includes most of the chemistry timism. Standard distillation, solvent extrac- the flux of beryllium-7 neutrinos, which are needed for technetium isolation (Fig. 4). tion, and ion chromatography procedures second only to boron-8 neutrinos in After being mined, the raw ore is finely will be followed in preparing a technetium sensitivity to the core temperature.) ground and concentrated by repeated flota- sample suitable for isotopic analysis, a final Is a geochemical measurement of tion. The resulting concentrate contains 85 task that Nature has greatly simplified. technetium feasible? The first requirement is to 90 per cent molybdenite. AMAX Inc., the Technetium is unique among the elements to locate a suitable ore body, that is, mine operator, then ships the concentrate by in having long-lived but no stable isotopes. molybdenum in sufficient quantity and at rail to a conversion plant in Fort Madison, Thus, the mass-spectrometric resolution for sufficient depth. Although molybdenum is Iowa, where the molybdenite is converted to counting 10 million atoms of technetium-97 not a common element, there do exist com- the trioxide by roasting with excess oxygen. and -98 is determined not by the concentra- mercial deposits of molybdenite (MoS2), an The concentrate also contains rhenium, an tion of a stable isotope but by that of a third accessory mineral in certain altered granitic element chemically similar to technetium. It unstable technetium isotope, technetium-99. rock. If we require the cosmic-ray back- is known that the rhenium forms volatile This isotope, whose half-life is 0.21 million

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years, occurs in the ore principally as a fission product of uranium-238. Resolution NW SE of technetium-99 and -98 appears feasible at the expected ratio (less than 7000) of the two isotopes. (Had Nature produced a stable technetium isotope with an abundance of only 1 part per billion in the ore, the ratio of the stable isotope to the mass-98 isotope would be 1015, and the experiment would be impossible with current technology.) In fact, the presence of technetium-99 at a level about 10,000 times that of technetium-98 may prove a great advantage: because its concentration in the ore can be inferred from the known uranium content, it can serve as a monitor of the overall chemical efficiency for recovering technetium. There are a number of points that must be addressed before we can state definitively that the proposed solar neutrino measurement is practical. We must demonstrate that the loss of technetium over geologic times in the reducing environment of the Henderson ore body is low by establishing that technetium-99 is close to secular equilibrium with its uranium-238 parent; that an appreciable fraction of the rhenium content of Henderson molybdenite can be recovered from the gas-scrubbing effluent; and that our theoretical estimates of the neutrino-capture cross sections, upon which our background estimates are predicated, are reasonable by measuring the Gamow-Teller strength dis- tributions in technetium-97 and -98 with forward-scattering (p,n) reactions. Yet we believe there are substantial reasons for optimism. Certainly the chemical and eco- Fig. 3. Geologic section of Red Mountain, Clear Creek County, Colorado. More than nomic aspects of the large-scale technetium 1300 meters below the mountain’s summit lies the Henderson ore body, one of the isolation are unusually advantageous. largest known deposits of molybdenum ore in the world. The existence of the Urad one Furthermore, there exists at this Laboratory body, a molybdenum deposit much closer to the surface, led geologists to suspect that the unique array of talents required to undertake a multidisciplinary endeavor of this Red Mountain might contain more molybdenum, and in 1965 the last of a series of test magnitude. (our present and future col- drillings intersected the fringe of the Henderson ore body. Geologic section from D. E. laborators include Alexander Gancarz, Ranta, W. H. White, A. D. Ward, R. E. Graichen, M. W. Ganster, and D. R. Stewart James S. Gilmore, Charles M. Miller, in Professional Contributions of Colorado School of Mines: Studies in Colorado Field Nicholas S. Nogar, A. Edward Norris, Geology, Rudy C. Epis and Robert J. Weimer, Eds. (Colorado School of Mines, Thomas L. Norris, Donald J. Rokop, Eliza- Golden, Colorado, 1976). beth N. Treher, and Kurt Wolfsberg, all of

LOS ALAMOS SCIENCE/Summer 1982 53 SCIENCE IDEAS Recovery of Technetium from Molybdenum Ore

Molybdenum ore is ground to sandlike constituency in mills 8.5 meters in diame- ter and 4.3 meters long. Within the mill are steel balls somewhat larger than a softball that help pulverize the ore as the mill rotates at 10 revolutions per minute. (Photo courtesy of AMAX Inc.)

Molybdenite in the ore is separated from waste material by flotation. AS air is blown through a mixture of finely ground ore, water, and chemicals, particles of molybdenite cling to the air bubbles, which rise to form a froth on the surface. (Photo courtesy of AMAX Inc.)

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Fig. 4. Raw molybdenum ore is converted to valuable interest to Los Alamos scientists, for it contains extremely minute quantities of technetium formed over millions of years here. It is the sludge from the effluent treatment that is of by interaction of solar neutrinos with molybdenum.

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Los Alamos, and Anthony Turkevich of the isolation of technetium could have important Conclusions University of Chicago.) implications for unified theories of the strong The primary motivation for our efforts is and electroweak forces. These theories pre- A causal relation between solar variability the belief that a quantitative test can be made dict the existence of superheavy elementary and terrestrial climatic changes during the of nonstandard solar models that suggest a particles with masses 10 to 100,000 times past millenium can be demonstrated from the connection between the solar neutrino greater than that of the proton. If some records of sunspot activity and carbon-14 puzzle, the proximity of the Pleistocene abundance of integrally charged superheavy production. The solar neutrino puzzle, the glacial epoch, and the fundamental thermal particles X± was created in the early days of proximity of the Pleistocene epoch, and the and nuclear times of the solar core. Specifi- the universe, superheavy nuclei may now similarity between the spacing and duration cally, our proposed experiment can test the exist. In particular, the X– could bind elec- of the major glacial epochs and the funda- suggestion that solar mixing about four tromagnetically to ruthenium to form a mental time scales of the solar core then million years ago initiated the Pleistocene stable nucleus with the charge of technetium raise the possibility that solar variability may epoch and a persisting depression of the and chemical properties similar to those of also be the cause of the glacial epochs. high-energy solar neutrino flux. Clear mem- both technetium and rhenium. A principal Perhaps the lone monitor of the behavior of ory of the steady-state solar phase that commercial source of rhenium is the solar core over the relevant time scales is preceded mixing should be retained in molybdenum ore, since rhenium disulfide the solar neutrino flux. Because of a series of technetium-98 with its half-life of 4.2 million (ReS2) is often the primary impurity in fortuitous circumstances of commerce and years. Recovery of this isotope in a quantity molybdenite. Presumably, if RuX- exists, it Nature, Los Alamos scientists have an op- lower than that predicted by the standard should also concentrate in molybdenite. We portunity to read the geochemical record of solar model but significantly higher than that could demonstrate its existence by isolating that flux in the abundances of technetium-97 detected by the Davis experiment would in the mass spectrometer a minimally de- and -98 in a deeply buried molybdenite support suggestions of solar variability and flected “Tc” component with the isotopic deposit. Together with the Davis experiment, solar influence on terrestrial climate. distribution of ruthenium. Our experiment this reading should provide an unequivocal Another justification for this experiment should be capable of detecting an average test of possible variations in the central was pointed out recently by Cahn and RuX- concentration in the earth’s crust of 1 temperature of the sun during the Pleistocene Glashow. They suggested that chemical part in 1026. epoch. ■