Solar Neutrinos:

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Group members: Richard L. Hahn; Minfang Yeh; J. Keith Rowley (retired); Zheng Chang ; Alexander Garnov

 

Neutrino studies currently are a very hot topic in science. Why? Because there are strong indications from several experiments that the neutrino has new properties, such as non-zero rest mass (so-called "New Physics") that are not included in standard theories of elementary particles. Theory may have to be recast to explain the experimental results.

 

Neutrinos and the sun.

The neutrino was proposed by Wolfgang Pauli in 1930, but it took 26 more years before the neutrino was actually discovered (detected). Pauli proposed the existence of the neutrino as a solution to a frustrating problem in nuclear beta-decay that was studied in the laboratory, namely  that examination of the reaction products always indicated that a variable amount of energy was missing. Pauli concluded that the products must include a third almost invisible particle, one which didn't interact strongly enough for it to be detected. Enrico Fermi called this particle the "neutrino" (for the "little neutral one"). In 1956 Reines and Cowan discovered the neutrino interactions in their experiments at a nuclear reactor (Reines was jointly awarded the Nobel Prize in physics in 1995).

Today, neutrinos are known to be tiny, possibly massless, neutral elementary particles, which interact with matter via the weak nuclear force. The weakness of the weak force gives neutrinos the property that matter is almost transparent to them. It is fascinating that the neutrino, which was "invented" to solve a laboratory nuclear physics problem, must also be invoked to explain energy production in our sun and in all other stars. We now know that nuclear fusion and decay processes, which occur within the stellar core, produce copious amounts of neutrinos. As shown in the figure, the standard solar model predicts that these reactions produce several groups of neutrinos, with differing fluxes and energy spectra. The figure also shows the ranges of detection of existing solar neutrino experiments in different shades of blue, to illustrate that they sample different portions of the solar neutrino energy spectrum. Three of these experiments, plus a new one, are discussed below.

Since neutrinos rarely interact with matter, they pass through the sun and the earth (and us) virtually unhindered. Other sources of neutrinos include exploding stars (supernovae), relic neutrinos (from the birth of the universe) and nuclear power plants. For example, the sun produces over two hundred trillion trillion trillion neutrinos every second, and a supernova blast can unleash 1000 times more neutrinos than our sun will produce in its 10-billion year lifetime. About 65-billion neutrinos from the sun stream through every square centimeter on the Earth every second, yet we are oblivious to their passage in our every-day lives.

BNL and Measurements of Solar Neutrinos.

 

Radiochemical Neutrino Detectors

 

       The field of solar neutrino research had its birth in the BNL Chemistry Department, where Raymond Davis and colleagues developed a radiochemical method to separate and detect the few radioactive atoms formed by capture of solar neutrinos in a huge target. This first solar neutrino experiment, in the Homestake Mine in South Dakota, used the isotope, 37Cl, as the target in 680 tons of an organic liquid, perchloroethylene. Neutrino capture on the 37Cl, with an energy threshold of 0.814 MeV, produces radioactive 37Ar, a gas, which is removed from the target, purified, and counted. The results of this experiment revealed a "solar neutrino problem": The number of measured solar neutrinos was only about one-third of the value predicted from solar theory (note that popular accounts are available on the Web about the development of the field of solar neutrino research).

Another radiochemical neutrino detector was developed at BNL, using 71Ga as the target. Neutrino capture on the 71Ga produces radioactive 71Ge with an renergy threshold of 0.233 MeV. This 71Ge can be  emoved from the liquid target in the form of gaseous GeCl4, chemically purified, and converted to GeH4 gas for counting. Two gallium detectors based on this scheme were constructed and operated. The BNL Solar Neutrino Group participated in GALLEX at the underground Gran Sasso National Laboratory in Italy, where 30 tons of gallium in the form of a 100-ton aqueous solution of gallium trichloride served as the target; SAGE at the Baksan Neutrino Observatory in Russia instead used 57 tons of liquid gallium metal. The results from both gallium experiments confirmed the "deficit" of solar neutrinos, by observing only 60% of the expected neutrino flux. The GALLEX experiment ended in 1998. Subsequently it became the Gallium Neutrino Observatory, GNO, which uses the original GALLEX target. BNL is not a member of GNO.

From these experiments, and the Kamiokande and Super-Kamiokande neutrino detectors in Japan, the consensus has developed in the scientific community that the reason for the observed deficit of solar neutrinos is that the neutrinos oscillate. In other words, the electron-flavor neutrinos that are produced in beta-decay processes in nuclear reactions in the solar interior can be transformed into the other two known neutrino flavors, those of the muon-neutrino and the tau-neutrino. These neutrinos are not produced in the sun's nuclear reactions. In this scenario, the measured solar neutrino flux is artificially low since these other neutrino flavors are not readily observed by most neutrino detectors, and certainly not at all by the radiochemical neutrino detectors. Note that for this process to occur requires that at least one of the neutrino types must have non-zero rest mass. Since the current Standard Electroweak Model carries the assumption of massless neutrinos, proof of the existence of neutrino mass would be a major new discovery, leading to major changes in the theory what has been dubbed New Physics.  

 

 

Real-Time Neutrino Detectors

SNO, the Sudbury Neutrino Observatory

  
            SNO is a new solar neutrino detector that was  constructed in Canada to search for definitive evidence of this postulated new neutrino physics. BNL joined this collaboration in early 1996. The SNO neutrino detector began taking data in October 1999.

SNO was designed to detect neutrino interactions as they occur in real time with energies > 5 MeV. It is situated in a specially constructed underground clean area, at the 6800-foot level of the Creighton mine, which is operated by INCO, the International Nickel Co., near Sudbury, Ontario. The detector contains 1000 tons of ultra-pure heavy water, D2O, in a 12-meter wide transparent acrylic plastic vessel, surrounded by 7000 tons of ultra-pure light water, H2O, which acts as shielding. The D2O, with a value of about $300 million, is being lent by the Canadian Government. In the H2O, 9600 photomultiplier tubes (PMTs) surround and view the acrylic vessel, detecting the Cerenkov light produced in the D2O by neutrino interactions and thus measuring the neutrino energy-spectra and fluxes.

Although SNO is not a radiochemical neutrino detector, chemistry still plays a crucially important role in the SNO project. For the detector to function properly, the amounts of radioactive impurities, such as those in the U-238 and Th-232 decay chains, and Rn-222 from the mine air, must be reduced to extraordinarily low levels (e.g., 10-15 gram Th per gram D2O). Other chemical contaminants must also be removed completely, since both the D2O and H2O must be optically transparent to allow the Cerenkov light to reach the PMTs .

The deuteron in the D2O in SNO makes it unique among neutrino detectors, since it can observe all three neutrino flavors. The electron neutrino is the only flavor that can convert the D into 2 protons + a negative electron. This electron provides the signal for the so-called "charged current" (CC) neutrino reaction. However, all three neutrino flavors are equally effective in breaking apart the D into its constituents, a proton + a neutron. This neutron provides the signal for the "neutral current" (NC) interaction. If SNO were to measure the NC rate to be greater than the CC rate, this would be definitive proof, a "smoking gun", for the existence of neutrino oscillations. The cause of the solar neutrino problem would be the transformation of some of the solar electron neutrinos into the other flavors. The physics community is excited by this prospect for new physics. A spin-off of such a result is that massive neutrinos could account for the "missing mass" that is required for a closed universe.

Preliminary results from sno were presented in June 2000 (SNO at the Neutrino 2000 Conference). The time is fast approaching when SNO will present its first results in an article to be published in the open literature.

 

 

LENS, the Low-Energy Neutrino Spectrometer

 

Many theoretical scenarios of neutrino oscillations predict strong effects at low energies, in the region of  <1 MeV. To date, only the radiochemical detectors have been sensitive to solar neutrinos at such low energies. A major thrust in the solar neutrino field is the development of new real-time detectors that can operate at such low energies.

One such detector, Borexino, is being built in Italy with the goal of detecting the main neutrino line from Be-7 line by observing elastic scattering in a liquid scintillator. Note that elastic scattering has contributions from both the charged-current and neutral-current neutrino interactions.

Another concept is LENS, the Low-Energy Neutrino Spectrometer, which has the ambitious aim of doing  real-time spectroscopy of the lowest energy, and most intense, solar neutrinos, those from the pp continuum below 0.42 MeV. The basic idea of LENS is to detect the charged current by neutrino capture in a target with a very low Q-value, such as 176Yb. Since LENS will also be able to detect Be-7 neutrinos, comparison of the signals from LENS and Borexino has the potential to provide definite evidence of Be-7 neutrino oscillations.

The basic neutrino signature in LENS is a delayed coincidence

e + 176Yb --> 176 Lu* + e-
                                           
  176 Lu + g

where the occurrence from the same location in the detector of pulses from the electron and the g ray within a time interval of some tens of nanoseconds in principle provides a neutrino-capture tag. This coincidence tag, proposed a long time ago by R. Raghavan, provides an excellent method to overcome the huge natural backgrounds that occur in the 1 MeV energy region .

The design of such an experiment is still under active investigation in the recently formed LENS International Collaboration. The basic idea at present is to use an organic liquid scintillator loaded with Yb to ~ 10% to detect the neutrino signal. Recent progress in LENS R&D has shown that this scintillator can indeed be produced in a stable form suitable for neutrino detection. It is estimated that the LENS detector will require 10-20 tons of natural Yb.

 

This work is supported by the DOE Office of High Energy and Nuclear Physics.

 This page was revised on July 27, 2004