understanding of it in many different ways. Although we are oblivious of their presence, there are times when as many as 10¹⁴ invisible neutrinos pass through us in a second, carrying the secrets of the cosmos from distant galaxies and the remote past. Neutrinos are not only witnesses but also essential byproducts and messengers of star birth, as well as key players in explosive star death.

low probability of occurrence. It interacts with other particles only by exchange of the heavy gauge bosons W⁺, W^- and Z⁰.

In terrestrial laboratories, neutrinos can be used to probe extremely minute bits of matter, and thereby study the interior of matter at different levels of structure from atomic nuclei to valence and sea quarks. The

neutrino comes in three "flavors"; the electron neutrino, the muon neutrino, and the tau neutrino. Each is associated with a corresponding charged counterpart: the electron, the muon and the tau. The world of the neutrino becomes more interesting if neutrinos do turn out to have even a small mass. In this case, neutrinos of different flavor may be able to transform among themselves. This flavor mixing is forbidden in the conventional Standard Model of particle physics, in which neutrinos are massless and flavor hierarchy is strictly maintained.

Today, at the beginning of the 21^st century, it has become technologically possible to track and monitor the activities of this tiny particle of matter and understand its role in diverse areas of physics. This is not a simple task because the neutrino reveals itself only very reluctantly to our investigations.

Experimental neutrino physics involves severe difficulties arising from small event rates, long running times, and the complexity of large detector systems. Therefore a laboratory neutrino experiment requires maximum neutrino intensity and low background. From nuclear physics and stellar astrophysics points of view, the ideal neutrino projectiles would be those having energies of the order of those produced and encountered in stars during various stages of their lives.

Fortunately, such a made-to-order neutrino source can be found at the Spallation Neutron Source (SNS) under construction at the Oak

Ridge National Laboratory. The SNS will produce the most intense (two million watts) pulsed-proton beam in the world. The facility will consist of a 493-m-long linear accelerator, an accumulator ring with a radius of approximately 35 meters, and a target station where neutrons are produced. A second target station is planned in the future. Along with the production of an intense flux of neutrons, the reason for the SNS, an intense flux of neutrinos and antineutrinos will be produced as a natural by-product of spallation. The SNS will be the world's most powerful low-energy neutrino factory. The pulsed nature of the product neutrinos and their energy and time structure make them additionally attractive for neutrino research.

This document highlights the scientific prospects for ORLaND, the Oak Ridge Laboratory for Neutrino Detectors, a proposed underground laboratory adjacent to the SNS target building. ORLaND will be a user facility supporting a comprehensive program of low-energy neutrino physics. Unique in its potential to obtain neutrino-nucleus interaction data and enhance our understanding of neutrino physics and stellar astrophysics, ORLaND would be available to users from the international scientific community for dedicated neutrino and neutrino-nucleus science. As a facility for multiple experiments, ORLaND would become a center for the training of future generations of nuclear and particle neutrino scientists and astrophysicists.

The ORLaND detectors must be shielded from the neutrons produced by the SNS and from the effects of background cosmic rays.

The equivalent of an estimated 10 meters of steel is needed to shield ORLaND from the particles produced by the SNS. A standard laboratory sited above ground would be costly in terms of shielding. Positioning the detectors underground, but in close proximity to the SNS target, lets one maintain low backgrounds at modest costs and still maintain the advantages of high neutrino intensity.

The SNS will produce about 10¹⁵ neutrinos per second from the decay of stopped pions produced in the primary spallation target and also from the decay of product muons. The neutrinos will be produced isotropically and their time structure and flavor components will allow separation of charged and neutral current interactions involving different neutrino families. The neutrino energy spectrum makes ORLaND unique for neutrino-nucleus experiments related to solar and supernova physics as well as fundamental nuclear, electroweak, and particle physics.

Show Sidebar: SNS Neutrinos

The intrinsic properties of the neutrino may have literally cosmic significance. Whether neutrinos have a finite mass, which would allow flavor mixing, is a matter of profound importance in our understanding of the limits of the Standard Model as a basic description of nature. Extrapolating to the cosmic context, a non-zero neutrino mass affects our understanding

of galaxy formation and makes the cosmic neutrino population a serious contender for at least a portion of the hot dark matter in our universe and consequently an influential player in the evolution of the universe.

Furthermore, the occurrence of neutrino mass and associated flavor oscillations could influence several aspects of stellar astrophysics, including our knowledge of stellar burning, evolution and explosions. Recently,

impressive evidence for flavor oscillations has been reported from experiments by the Super-Kamiokande collaboration. This result and others will not close the book on oscillations, but will restrict further the viable options for modifying the Standard Model.

The strong suppression of antielectron neutrinos in the blend of neutrino species produced at the SNS makes ORLaND an attractive facility for oscillation experiments that feature the appearance of this flavor of neutrino.

that provide information on such cosmic explosions. Muon and tau neutrinos in the stellar core couple to the core material only through neutral currents, whereas electron neutrinos and antineutrinos couple through both neutral and charged currents. Therefore, the former decouple at higher density and temperature, and with higher-energy spectra. If neutrino oscillations converting higher-flavor neutrinos to electron-type neutrinos were to occur, muon and tau neutrinos could be converted to electron neutrinos and influence the supernova mechanism by adding to the heating behind the shock.

Show Sidebar: The Neutrino

The mechanism by which stars, especially our Sun, generate their prodigious quantities of energy has long been of interest to scientists. Our present understanding of the Sun is codified in the Standard Solar Model. This model assumes that the Sun is made

of a homogeneous mixture of gases powered by nuclear reactions in its core, primarily the fusion of hydrogen into helium with the release of energy. A small part of the energy appears in the form of neutrinos. Energy not emitted as neutrinos is transported from the core of the Sun, where it is produced, to the surface, where it is radiated into space, by photons and by the convective motion of hot gas.

Recently, it has become possible to verify this solar model directly through helioseismology, which infers the internal structure of the Sun from vibrational waves observed on its surface. The figure belowT is based on SOHO helioseismology and illustrates the deviation of sound speed in the solar interior from that expected theoretically as a function of radius. Note that the deviations are very small and peak in two general regions: the outer part of the energy producing region, and just inside the convective zone.

Although this hypothesis of hydrogen burning has existed for over six decades, it is only recently, with the advent

of large-scale neutrino detectors, that direct tests of this picture have become possible. In 1965 scientists began operating a detector that would detect neutrinos from the interior of the Sun and thereby confirm that the Sun's emitted energy derived from the fusion of hydrogen into helium.

electron neutrinos made in the solar core transform into another neutrino species during their flight from the Sun to the Earth.

An important and timely experiment at ORLaND that would aid in unraveling the solar neutrino problem would be the measurement of neutrino-deuteron scattering using the known spectrum of neutrinos from the SNS. It has been recognized for many years that a direct measurement of the hydrogen fusion cross-section is extremely difficult. However, at ORLaND, the inverse process, the neutrino reaction on deterium could be measured with an accuracy of better than two percent and would provide an important absolute calibration for the interpretation of data from current and future solar neutrino detectors.

Core collapse supernovae are among the most energetic explosions in the known Universe, releasing 10⁵³ ergs of energy in the form of neutrinos of all flavors that are emitted at a staggering rate of 10⁵⁷ neutrinos per second.

The explosion almost entirely destroys stars that are ten to twenty times as massive as the Sun. It is believed that core-collapse supernovae are the site for the production of many of the elements heavier than iron, and they are a key link in our chain of origins from the Big Bang to the formation of life on Earth. Current supernova theory revolves around the idea that the supernova shock wave, formed when the iron core of a massive star collapses due to gravity, rebounds at high densities and stalls as a result of energy losses to nuclear dissociation and neutrino emission. It is thought that an intense flux of neutrinos

emerging from the center of the collapsed star re-energizes the explosion and that the interactions between this intense neutrino flux and matter in the outer layers of the star produce a "neutrino nucleosynthesis" in which some of nature's rarest isotopes are formed. Presently, there is no other known astrophysical site for the production of the rare isotopes, ¹³⁸La and ¹⁸⁰Ta. Their existence may be strong evidence for the neutrino nucleosynthesis process.

After the explosion begins, it is believed to be the site for the astrophysical r-process, which is responsible for more than half of the Solar System's abundance of heavy elements and all of its transuranic elements. Thus, underlying the dynamics of core collapse supernovae as well as the associated nucleosynthesis are the basic interactions of neutrinos with matter.

A measurement of important neutrino-nucleus cross sections would provide an experimental foundation for the many theoretically-predicted cross sections used in supernova models. Given the strong correspondence between the SNS neutrino spectra and those found in supernovae, ORLaND will provide an outstanding

to mass numbers of about 100 are needed to accurately simulate the early phase of the explosion when the shock wave loses an amount of energy equivalent to about ten percent of the mass of the Sun. This loss has a significant impact on the dynamics of the supernova explosions. One goal of the proposed ORLaND facility will be to measure the cross section for electron neutrino charged-current capture on iron nuclei. The same technique used to measure the cross section for neutrino capture on iron can be used to measure the neutrino capture cross section on many other nuclei, such as: ⁷Li, ⁹Be, ¹¹B, ²⁷Al, ⁴⁰Ca, ⁵¹V, ⁵²Cr, ⁵⁵Mn, ⁵⁹Co, ⁹³Nb, ¹¹⁵In, ¹⁸¹Ta, and ²⁰⁹Bi.

In the elementary quark model, protons and neutrons are described as simple bound states of up (u) quarks and down (d) quarks: uud in the proton and udd in the neutron. The quarks are thought to be trapped within the proton and neutron by the strong interaction, a phenomenon known as confinement.

The interactions between quarks are due to the exchange of other strongly interacting particles called gluons. The basic "rules of interaction" are precisely described by quantum chromodynamics, which says that quarks and gluons exchange gluons and that a gluon can transform spontaneously into a quark-antiquark pair. Thus, protons and neutrons have a more complicated structure than the naive uud and udd picture would suggest.

A gluon can transform into any quark-antiquark pair; the most important of these new contributions are expected to contain strange quarks and charm quarks, since these are the next lightest quarks after the up quark and

the down quark. This pair production effect will lead to new components in the nucleon wave function corresponding to both hidden strangeness and hidden charm. It is surprisingly difficult to identify these novel components of the proton and neutron. This is partly because the fundamental quantum numbers of electric charge, baryon number, and spin are not changed for these new components. For this reason, the amount of hidden strangeness and hidden charm in the nucleon remained a matter for theoretical speculation until the advent of the recent hidden-strangeness experiments.

Hidden-strangeness experiments take advantage of the weak parity-violating coupling of the strange quark to determine the strangeness component in the proton and neutron. Since pure electromagnetic coupling conserves parity but the much more feeble weak interaction does not, one can extract this strange-quark weak coupling amplitude by careful searches for angular asymmetry in electron-nucleon and electron-nucleus scattering. Preliminary experiments have been carried out in the last few years but the results appear contradictory at present.

In this confused situation, one would ideally prefer an independent experiment using a different technique. One experiment that has

been proposed at ORLaND is to measure quasielastic neutrino-nucleus cross sections. This has advantages over parity-violating electron scattering because neutrinos take part in the weak but not the electromagnetic interaction, so the complications of large electromagnetic effects and associated radiative corrections are not present. In addition, such scattering proceeds through Z⁰ weak boson exchange, coupled dominantly to the axial vector current; this axial matrix element is one where the hidden-strangeness component should be most readily observed. A measurement of the ratio of proton to neutron quasielastic cross sections is very attractive experimentally because it is predicted to be quite sensitive to hidden strangeness in the nucleon, and because several uncertainties cancel in this ratio.

The heavy civil construction of the detector hall must be completed on a schedule that is compatible with the construction of the SNS target halls. We have attempted to identify those parts of the construction that are time critical with respect to the SNS construction schedule. Out of the $34M cost of the detector hall, about $8M will be required to complete the time-critical heavy civil construction. This part of the project (basically excavation and forming of the shell of the target hall) could be completed in about 11 months. Completion of the entire project would require about three years from the start of construction, depending on the funding profile. We have not yet made a careful analysis of facility operating costs, but an estimate of $2M to $3M per year is probably reasonable.

We should emphasize that the experimental equipment costs given here are estimates for representative experiments. ORLaND will operate as a user facility, and a Program Advisory Committee will select the actual experiments.

as a part of the ORLaND Development Project. We are indebted to the Design Laboratory of the University of Tennessee under the direction of Professor Cary Staples for providing the cover design. Please address any comments or questions to beene@mail.phy.ornl.gov.