L. Hahn (retired); Minfang Yeh;
James Cumming (retired);
Liangming Hu, Pankaj Sinha, Richard Rosero,
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.
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
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”.
|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 (PMT’s) 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 PMT’s .
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 goal of doing real-time spectroscopy of the lowest energy, and most intense, solar neutrinos, those from the pp continuum. LENS aims to employ an organometallic LS, with ~10% In, to detect these neutrinos, with the indium serving as the target for neutrino capture. The Q-value = 0.114 MeV in 115In is well below the 0.420-MeV upper cutoff of the pp solar neutrino spectrum. The initial stage of this experiment, called MiniLENS, involves development of a 0.25-T prototype of the In-LS neutrino detector.
The conceptual design of such a detector is completed. Recent progress in LENS R&D has shown that the In-loaded scintillator can indeed be produced in a stable form with suitable characters for neutrino detection. It is estimated that the LENS detector will require 10-20 tons of natural indium.
This proposed experiment has the potential to achieve significant new scientific results in the area of double beta-decay. By making use of the existing infrastructure and neutrino-detector components at SNOLab, by filling the existing SNO acrylic vessel with Nd-LS, it has the possibility of overtaking several other planned double beta-decay experiments that are still being planned. SNO+ has received funding from Canadian agencies and is in the process of preparing underground activies including AV rolled-down and liquid purification capacity and production facility. It’s expected to start the liquid production in 2012. Yeh is the convener for the design of Nd-LS production and is leading the R&D effort of 2nd phase double-beta experiments with other double-beta isotopes.
The project passed several major programmatic reviews culminating in the Lehman CD-3b Review that released OHEP project funds. The U.S. funding level for Daya Bay is $34 M. Civil construction at Daya Bay has been ongoing since October 2007. The Beneficial Occupancy (BO) of LS Hall was granted by Chinese government in Oct. 2010. The collaboration is currently in preparation for the liquid production. Yeh is the leading person for Gd-LS production and QA/QC.
This proposed experiment aims to explore the new physics beyond standard model by the neutrino interactions and transformations from a high intensity neutrino beam from FNAL at the distance of >1,000 kilometers straight through the earth to the largest particle detectors ever built. The detectors, two 100-kT water-equivalent, could be housed in the proposed Deep Underground Science and Engineering Laboratory in South Dakota. DUSEL would be the world’s deepest underground laboratory and shield the LBNE neutrino detectors from cosmic particles. The LBNE project office is at FNAL, while the detector-project office is at BNL.
Organometallic Liquid Scintillator
Metal-loaded liquid scintillators (M-LS) and unloaded liquid scintillators (LS) that are required for successful neutrino and antineutrino detection are being studied at BNL. The work in the development of LS, especially with metal loading, has generated great interest in the fields of particle and nuclear physics. Many interests of exploring the possibility of developing a variety of metal-loaded LS in proposed new experiments, not only for detection of sub-MeV neutrinos, super-nova neutrino and dark matter searches; but also for application of national security and reactor monitoring. There are only a very few groups in the world capable of conducting the type of R&D that applies (nuclear) chemistry to forefront physics experiments.
We have developed and refined recipes for preparing the M-LS, involving organic carboxylates (and/or RP=O organic phosphine oxides, see list below) to complex the metal, and scintillating solvents such as PC and LAB. These recipes are being translated into processes that can be applied at the multi-ton chemical scale. This research at BNL focuses on these chemical questions, including determination of the chemical species that constitute the M-LS. Key chemical and nuclear-chemical characteristics (NIM-A) are (a) long-term chemical stability; (b) high optical transparency; (c) high light production by the scintillator; and (d) ultra-low impurity content, mainly of natural radioactive contaminants, such as U, Th, and Ra, and of chemical contaminants (NIM-A) that can reduce the light output or light transmission. We also are doing R&D on the determination and reduction of the levels of background impurities, and on the chemical compatibility of the organic LS with transparent plastics, such as acrylics, that will be the material used to construct the detector vessels. Some assay methods will involve using low-level spikes of uranium, thorium, or radium.
Current works focus on the preparations with relatively high concentrations of metals loading in the organic liquid scintillator for nuclear and particle physics. The group has been developing LS-based neutrino detectors for (a) 200-ton, 0.1% gadolinium in LS (Gd-LS) to detect antineutrinos and measure the theta-13 mixing angle at Daya Bay; (b) 1000-ton, 0.1% neodymium in LS (Nd-LS) to measure neutrino-less double beta decay in 150Nd in the SNO+ experiment in the new SNOLAB in Sudbury (SNO+ is the successor to SNO and will use most of the physical facilities of the SNO experiment); (c) 125-t, 8% indium in LS (In-LS) to measure the lowest energy solar neutrinos from the pp, pep, and 7Be solar branches in the Low Energy Neutrino Spectroscopy (LENS) experiment either at the Kimballton Underground Research facility or at Deep Underground Science and Engineering Laboratory (DUSEL), with the prototype Mini-LENS to be built first.
Other metal-loaded LS are also developed for (1) reactor monitoring (Lithium, Gadolinium, and Boron), (2) short half-life calibration source (Yttrium) for liquid scintillation detectors; and (3) other double-beta decay candidates (Zirconium and Tellurium) at SNO+.
Yeh is the Daya Bay Level-3 manager for Gd-LS production and leading all M-LS production, (In, Nd and Gd) for LENS and SNO+.
Water-based Liquid Scintillator
A pure many tens of kilotons of liquid scintillator has great sensitivity for sub-MeV neutrinos and dark matter searches and can push the current limit of proton decay lifetime (~1033 yrs) by an order of magnitude lower. However due to the cost, ES&H and chemical safety, this large pure LS detector is currently not favored by funding agencies. The success of water-based liquid scintillator will provide a new generation, cost-effective and environmental benign, detection medium that could make the large PDK+ detector affordable and largely reduce chemical usage and waste. This also has great potential in national security application and reactor monitoring.
The main motivations of developing the water-based liquid scintillator are (1) to optimize the ES&H and chemical safety on DOE missions by the reduction of large quantity (tens of kT) organic liquid scintillator and (2) to create a significant cost-saving technology for future large-scale physics experiments. A large 50-kT, pure LS-equivalent detector, to reach the predicted sensitivity of proton-decay lifetime (1029-35 yrs), in addition to sub-MeV neutrinos, is the main physics interest of this R&D. New applicable detection medium to enhance national security and to replace the current scintillation cocktails motivates the interest. BNL developed a mass-producible recipe for W-LS that has been (1) stable for 1.5 years since synthesis; and (2) capable of producing scintillation light with fast decay time to test the SUSY favored PDK+ mode.
Material Compatibility Program for Liquid Scintillation and Water Cherenkov Detectors:
The material leaching in and deterioration by liquid are the great concerns of neutrino experiments. BNL group has the expertise and facilities to carry out compatibility tests in different detection mediums. R&D procedures for (1) performing aging tests to speed up the test time in response to the data-taking lifetime of experiment; (2) measuring temperature-activation coefficient (Q10) to precisely predict the material behaviors and liquid impacts; and (3) building a material database per liquid to benefit the science community, are under development. BNL is the leading institute of material testing for SNO, Daya Bay, SNO+, LENS, and the designated group for Long Baseline Neutrino Experiment (LBNE) beginning FY10. This program will provide a better understanding of material leaching behaviors in water and in liquid scintillator to ensure the detector achieving its goals at planned lifetime. Standard QA/QC procedures for material selections and cleaning will be developed. A database with detailed vendor and model information is under construction, which will benefit all science community in current and future experiments. A simulated model will also be built to incorporate any purification schemes for a clear assessment with purification facility, which could lead to significant cost saving.
This work is supported by the DOE Office of High Energy and Nuclear Physics.
This page was revised on November 24, 2010