Detecting Almost Nothing

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Once the Standard Solar Model has specified the neutrino flux, the predictions are extended to specific experiments that detect neutrinos of different energies, tte neutrino reaction rate with atoms in these detectors is so slow that a special unit has been invented to specify the experiment-specific flux. Ms Solar Neutrino Unit, abbreviated SNU and pronounced "snew", is equal to one neutrino interaction per second for every trillion trillion trillion, or 1036, atoms. And even then, the predictions are only a few SNU per month for even the largest most-massive, detectors that were first constructed.

Of course, it isn't easy to catch the elusive neutrino, tte vast majority of neutrinos pass right through matter, but there is a finite chance that a neutrino will interact with some of it. When this slight chance is multiplied by the prodigious quantities of neutrinos flowing from the Sun, we conclude that a few of them will occasionally strike an atom's nucleus squarely enough to produce a nuclear reaction that signals the presence of an invisible neutrino, tte neutrino detector must nevertheless consist of large amounts of material, literally tons of it, to allow interaction with even a tiny fraction of the solar neutrinos and measure their actual numbers.

Unlike a conventional optical telescope, that is placed as high as possible to minimize distortion by the obscuring atmosphere, a neutrino telescope is buried beneath a mountain, or deep within the Earth's rocks inside mines. Ms is to shield the neutrino detector from deceptive signals caused by cosmic interference, ttere, beneath tons of rock that only the neutrino can penetrate, detectors can unambiguously measure neutrinos from the Sun. Otherwise, neutrino detectors near the Earth's surface would detect high-energy particles and radiation produced by other energetic particles, called cosmic rays, interacting with the Earth's atmosphere. But neither the primary cosmic rays nor their secondary atmospheric emissions can penetrate thick layers of rock.

ttus, solar neutrino astronomy involves massive, subterranean detectors that look right through the Earth and observe the Sun at night or day. tte first such neutrino telescope, constructed in 1967 by Raymond Davis, Jr. (1914- ), was a 615-ton tank located 1.5 kilometers underground in the Homestake Gold Mine near Lead, South Dakota (Fig. 3.3). tte huge cylindrical tank was filled with 378 thousand liters of cleaning fluid, technically called perchloroethylene or "perc" in the dry-cleaning trade; each molecule of the stain remover consists of two carbon atoms and four chlorine atoms.

Most solar neutrinos passed through the tank unimpeded. Occasionally, however, a neutrino scored a direct hit with the nucleus of a chlorine atom, turning one of its neutrons into a proton, emitting an electron to conserve charge and transforming the chlorine atom into an atom of radioactive argon. Only neutrinos more energetic than 0.814 million electron volts, abbreviated 0.814 MeV, triggered the nuclear conversion. None of the Sun's abundant proton-proton neutrinos have enough energy to cause this transformation, but the Homestake chlorine experiment was sensitive to the much rarer, higher-energy neutrinos produced by the less common boron-8 fusion reactions in the Sun.

tte new argon atom rebounds from the encounter with sufficient energy to break free of the perc molecule and enter the surrounding liquid. Because the argon is chemi-

FIG. 3.3 Underground neutrino detector The original solar neutrino detector located 1.5 kilometers underground, in the Homestake Gold Mine near Lead, South Dakota, to filter out strong signals from energetic cosmic particles. The huge cylindrical tank was filled with 378 thousand liters of cleaning fluid. When a high-energy solar neutrino interacted with the nucleus of a chlorine atom in the fluid, radioactive argon was produced, which was extracted to count the solar neutrinos. This experiment operated for more than 25 years, always finding fewer neutrinos than expected from the Standard Solar Model - see Fig. 3.4. (Courtesy of Brookhaven National Laboratory.)

FIG. 3.3 Underground neutrino detector The original solar neutrino detector located 1.5 kilometers underground, in the Homestake Gold Mine near Lead, South Dakota, to filter out strong signals from energetic cosmic particles. The huge cylindrical tank was filled with 378 thousand liters of cleaning fluid. When a high-energy solar neutrino interacted with the nucleus of a chlorine atom in the fluid, radioactive argon was produced, which was extracted to count the solar neutrinos. This experiment operated for more than 25 years, always finding fewer neutrinos than expected from the Standard Solar Model - see Fig. 3.4. (Courtesy of Brookhaven National Laboratory.)

cally inert, it can be culled from the liquid by bubbling helium gas through the tank; and the number of argon atoms recovered in this way measures the incident flux of solar neutrinos.

Every few months Davis and his colleagues flushed the tank with helium, extracting about 15 argon atoms from a tank the size of an Olympic swimming pool, ttat was a remarkable achievement considering that the tank contains more than a million trillion trillion, or 1030, chlorine atoms. And the scientists persisted for nearly thirty years, like aging hunters tending a trap, capturing a total of just 2,000 neutrinos, ttis implied that nuclear fusion reactions were indeed providing the Sun's energy, making it shine, which was the sole motivation for carrying out the experiment. But there was a small, unexpected problem with the result.

For nearly two decades, from 1968 to 1987, the Homestake detector always yielded results in conflict with the most accurate theoretical calculations, tte final experiment value was 2.55 ± 0.25 SNU, where the ± value denotes an uncertainty of one standard deviation. (A standard deviation is a statistical measurement of the uncertainty of a measurement; a definite detection has to be above three standard deviations and preferably above five of them.) In contrast, the most recent theoretical result using the Standard Solar Model predicts that the Homestake detector should have observed a flux 8.5 ± 1.8 SNU. So the tank full of cleaning fluid captured almost one-third the expected number of neutrinos (Fig. 3.4). tte discrepancy between the observed and calculated values is known as the Solar Neutrino Problem.

FIG. 3.4 Solar neutrino fluxes Calculated and measured solar neutrino fluxes have consistently disagreed for several decades. The fluxes are measured in solar neutrino units, or SNU, defined as one neutrino interaction per trillion trillion trillion, or 1036, atoms per second. Measurements from the chlorine neutrino detector (small dots) give an average solar neutrino flux of 2.55 ± 0.25 SNU (lower broken line), well below theoretical calculations (large dots) that predict a flux of 8.5 ±1.8 SNU (upper broken line) for the Standard Solar Model. Other experiments have also observed a deficit of solar neutrinos, suggesting that either some process prevents neutrinos from being detected or the method by which the Sun shines differs from that predicted by current theoretical models.

FIG. 3.4 Solar neutrino fluxes Calculated and measured solar neutrino fluxes have consistently disagreed for several decades. The fluxes are measured in solar neutrino units, or SNU, defined as one neutrino interaction per trillion trillion trillion, or 1036, atoms per second. Measurements from the chlorine neutrino detector (small dots) give an average solar neutrino flux of 2.55 ± 0.25 SNU (lower broken line), well below theoretical calculations (large dots) that predict a flux of 8.5 ±1.8 SNU (upper broken line) for the Standard Solar Model. Other experiments have also observed a deficit of solar neutrinos, suggesting that either some process prevents neutrinos from being detected or the method by which the Sun shines differs from that predicted by current theoretical models.

In 1987, another giant, underground neutrino detector, called Kamiokande, began to monitor solar neutrinos, confirming the neutrino deficit observed by Davis, ttis second experiment, located in a mine at Kamioka, Japan, and known as Kamiokande, consisted of a 4,500-ton, or 4.5 million liters, tank of pure water. Nearly a thousand light detectors were placed in the tank's walls to measure signals emitted by electrons knocked free from water molecules by passing neutrinos.

tte number of scattered electrons detected by Kamiokande indicated that the flux of high-energy neutrinos is just under half the neutrino flux expected from the Standard Solar Model, or to be exact 0.48 ± 0.07 times the predicted value. An incident neutrino must have an energy of at least 7.5 MeV to produce a detectable recoil electron in water, so Kamiokande had a much higher threshold than even the chlorine experiment, at 0.814 MeV. tte dominant source in both experiments is the rare boron-8 neutrinos, so they independently confirm an apparent deficit of high-energy solar neutrinos, although the observed deficit was in different proportions.

When an energetic solar neutrino collides with an electron in the water, the neutrino knocks the electron out of its atomic orbit, pushes it forward in the direction of the incident neutrino, and accelerates the electron to nearly the velocity of light. In water, the electron moves faster than the light it radiates, and as a result the electron produces a cone-shaped pulse of light about its path, tte faint blue glow is technically known as Cherenkov radiation, named after the Soviet physicist Pavel A. Cherenkov (1904-1958) who discovered the effect in 1934.

tte axis of the light cone gives the electron's direction, which is the direction from which the neutrino arrived. And since the observed electrons were preferentially scattered along the direction of an imaginary line joining the Earth to the Sun, the Kamiokande water experiment confirmed that the neutrinos are indeed produced by nuclear reactions in the Sun's core. After 1000 days of observation, Yoji Totsuka (1942- ), speaking for the Kamiokande collaboration led by Masatoshi Koshiba (1926- ), could therefore report in 1991 that:

"tte directional information tells us that neutrinos are coming from the Sun, [providing] the first [direct] evidence that the fusion processes are taking place in the Sun.16

In contrast, the chlorine experiment cannot tell what direction the neutrinos are coming from. However, because the Sun is so close it should dominate the cosmic neutrino input to Earth, and no other known cosmic source could be providing the high-energy neutrinos the chlorine experiment detected for so long.

tte Japanese experiment had incidentally already detected, on 23 February 1987, a brief burst of just 11 neutrinos from a rare and distant supernova explosion known as SN 1987A. tte total energy emitted by all the undetected neutrinos during the explosion of the massive star is about equal to all the energy emitted by the Sun in its entire 4.6-billion-year history.

tte predicted flux of the high-energy boron-8 neutrinos from the Sun has a large uncertainty, about 37 percent, for it depends strongly on the uncertain temperature at the center of the Sun; the amount of these neutrinos varies roughly as the eighteenth power of the core temperature. Scientists therefore developed a method of detecting the more numerous low-energy, proton-proton neutrinos using gallium, a rare and expensive metal used in the red lights of hand calculators and other pieces of electronic equipment, tte predicted flux of the proton-proton neutrinos has only a 2 percent uncertainty, primarily because it is relatively insensitive to the core temperature.

When a low-energy neutrino has a rare, head-on collision with the nucleus of a gallium-71 nucleus, containing 31 protons and 40 neutrons, one of its neutrons is changed into a proton, emitting an electron to conserve charge and producing a nucleus of a germanium-71 atom, with 32 protons and 39 neutrons, tte energy threshold for this nuclear conversion is 0.23 MeV, so it should be sensitive to a sizeable fraction of the proton-proton neutrino flux, whose energy varies from 0 to 0.422 MeV, as well as the more energetic boron-8 neutrinos, tte germanium produced in this way can be chemically separated from the gallium, and counted by its radioactive decay; thereby providing a measure of the flux of incident neutrinos with energies above the 0.23 MeV threshold.

Unfortunately, it takes at least 30 tons of gallium to produce a detectable signal from solar neutrinos, tte total world production of gallium was about 10 tons per year, and a ton of gallium costs half a million dollars - a lot more than cleaning fluid! Moreover, a neutrino conversion of a gallium nucleus does not happen very often. Only about one such event is expected to happen each day in 30 tons of gallium. But this is serious business; so two international collaborations spent millions of dollars to place huge tanks of gallium underground.

In 1990, the Soviet-American Gallium Experiment, abbreviated SAGE, began operation at the Baksan Neutrino Observatory located in a long tunnel, some 2 kilometers below the summit of Mount Andyrchi in the northern Caucasus Mountains in Russia. SAGE used 60 tons of gallium metal kept molten in reactor vessels at about 303 kelvin. Asecond multinational experiment, dubbed GALLEX for GALLium Experiment, started operating in 1991 within the Gran Sasso Underground Laboratory, located 1.4 kilometers below a peak in the Apennine Mountains of Italy, tte GALLEX experiment used 30 tons of gallium in 100 tons of highly concentrated gallium-chloride solution.

tte GALLEX collaboration operated between 1991 and 1997, and a new series of measurements was then carried out at the same location by the Gallium Neutrino Observatory, abbreviated GNO, from 1998 to 2002. tte combined GALLEX and GNO result was 70.8 ± 4.5 SNU. Between 1990 and 2001, SAGE obtained a very similar measurement of 70.8 ± 5.3 SNU. tte two results are well below the predicted 131 ± 10 SNU using the Standard Solar Model for the gallium experiments.

tte low-energy neutrinos from the direct fusion of two protons, in the protonproton reaction, contribute just slightly more than half of the predicted gallium detection rate, or about 70 SNU of the predicted 131 SNU, and the high-energy boron-8 neutrinos contribute the rest of the predicted amount. When combined with the chlorine and the Kamiokande results, which show a deficit of high-energy neutrinos, the gallium experiments must have detected the low-energy proton-proton neutrinos, ttis was an important result, for it provided confirmation that hydrogen fusion makes the Sun shine.

tte results from the first four solar neutrino experiments are given in Table 3.1, where they are also compared to theoretical calculations using the Standard Solar Model, tte chlorine experiment detects about one fourth of the expected flux of neutrinos, and

TABLE 3.1 The first solar neutrino experiments

Measured Predicted Ratio:

Threshold Neutrino Flux Neutrino Flux Measured/ Target_Experiment Energy (MeV) (SNU)"_(SNU) Predicted

TABLE 3.1 The first solar neutrino experiments

Measured Predicted Ratio:

Threshold Neutrino Flux Neutrino Flux Measured/ Target_Experiment Energy (MeV) (SNU)"_(SNU) Predicted

Chlorine 37

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