Low Energy Solar Neutrino Detection

The following sections are included:

• Preface

• Contents

The following sections are included:

• Phenomenological summary

• Theoretical considerations

• What experiment should be done?

• References

In this talk, I review, from the phenomenological point of view, solutions to the solar neutrino problem, which are not provided by the conventional neutrino oscillation induced by mass and flavor mixing, and show that they can provide a good fit to the observed data. I also consider some simple implications for low energy solar neutrino experiments.

GNO (Gallium Neutrino Observatory) is monitoring the low energy solar neutrino flux at Laboratori Nazionali del Gran Sasso (Italy) with a 30 tons gallium target. After an overview of the main features of the detector, I report the recent results, and I discuss the ongoing R&D and the plans for the future.

MOON(Molybdenum Observatory Of Neutrinos) for low energy neutrinos and present status of double beta decays are briefly reported. MOON is a high sensitive detector for spectroscopic studies of Majorana ν masses with sensitivity of m_ν ~ 0.03 eV by double beta decays(ββ) of ¹⁰⁰Mo and real-time studies of low energy solar ν's and supernova ν's by inverse beta decays of ¹⁰⁰Mo. Neutrino-less double beta decays(0νββ)are sensitive to ν-masses and weak interactions beyond the Standard Model(SM). The present limits on the 0νββ half-lives for ⁷⁶Ge, and ¹⁰⁰Mo, ¹¹⁶Cd, ^128,130Te ¹³⁶Xe give upper limits of 0.3 ~ 1.3 eV and 1.1 ~ 4 eV, respectively, depending on the nuclear matrix elements M^0ν. They lead to stringent limits on the right-handed currents, the Majoron-ν couplings, and other terms beyond SM. New generation detectors for 0νββ with sensitivities of m_ν = 0.01 ~ 0.1 eV are being developed.

Uncertainties involved in the nuclear matrix elements relevant to nuclear double beta decay and low-energy neutrino capture are discussed from the view point of nuclear structure and nuclear transition mechanism.

This contribution describes the science and the technology of the BOREXINO experiment for sub-MeV solar neutrinos at the underground Gran Sasso laboratory. The paper shortly summarizes the material presented in reference 1. The talk was given by M.G. Giammarchi. This work is dedicated to the memory of our colleague Sandro Vitale.

The KamLAND experiment can explore world lowest mass difference region of neutrino oscillation parameter with an artificial neutrino source. It observes anti-neutrinos from various power plants spread in Japan with about 175 km baseline. This baseline is two order of magnitude longer than those of previously performed reactor experiments ^1,2. This challenging measurement requires very low background environment as low as 10^-13 g/g Uranium and Thorium impurities. In order to achieve this very low impurity level, various techniques (detector design, material selection, construction scheme etc) have been invested. These efforts have been rewarded rather quickly and current achievement doesn't hinder even the ⁷Be solar neutrino observation requiring 10^-16 g/g U and Th impurities. Various background sources relevant to the reactor and solar neutrino observation are considered and description of efforts to eliminate those backgrounds are the main issue of this manuscript. Physics goal and current successful construction status are also highlighted.

HERON (for HElium Roton Observation of Neutrinos) is an R&D project whose aim is to have a high-rate, real-time detector of p-p and ⁷Be solar neutrinos. The neutrino target material is superfluid helium—a medium which can be made free of internal radioactive background. The elastic neutrino interactions are detected by measuring, on wafer calorimeters mounted above the liquid, the UV photons and phonon/rotons generated by the recoil electron. This paper discusses the status of the project with particular emphasis on progress toward control of backgrounds from sources external to the liquid helium target volume.

Possibility to use ultra pure liquid Xenon as a low energy solar neutrino detector by means of ν+e scattering is evaluated. A possible detector with 10 tons of fiducial volume will give ~14 events for pp-neutrinos and ~6 events for ⁷Be neutrinos with the energy threshold at 50 keV. The detector can be built with known and established technologies. High density of the liquid- Xe would provide self-shields against the incoming backgrounds originating from the container and outer environments.

A search for weakly interacting massive particles (WIMPs) is one of the purposes of XMASS project. The 1 kg liquid xenon detector, which has strong background discrimination power against electron recoil, has been constructed for WIMPs direct detection and prepared the experimental setup in the Kamioka underground laboratory. The structure and the performance of the detector were briefly described.

We describe an approach to the study of neutrino masses that combines quantum optics techniques with radiation detectors to obtain unprecedented sensitivity. With it the search for Majorana neutrino masses down to ~10 meV will become accessible. The experimental technique uses the possibility of individually detecting Ba⁺-ions in the final state of ¹³⁶Xe double-beta decay via resonant excitation with a set of lasers aimed at a specific location in a large Time Projection Chamber. The specificity of the atomic levels provides tagging and, together with more traditional event recognition parameters, greatly suppresses radioactive backgrounds.

A method for detecting low energy solar neutrinos is presented. In the CLEAN approach, condensed neon or helium is used as a scintillation medium for the real-time detection of neutrino-electron scattering events. The resulting extreme ultraviolet scintillation light is wavelength shifted for detection by photomultiplier tubes. Radioactive isotopes can be removed from the scintillator through cryogenic distillation and by freezing out impurities onto cold surfaces. Gamma ray backgrounds are minimized through the use of a shielding layer of high purity liquid or solid neon.

Double beta decay is indispensable to solve the question of the neutrino mass matrix together with ν oscillation experiments. The most sensitive experiment since eight years — the HEIDELBERG-MOSCOW experiment in Gran-Sasso — already now, with the experimental limit of 〈m_ν〉 < 0.26 eV excludes degenerate ν mass scenarios allowing neutrinos as hot dark matter in the universe for the small angle MSW solution of the solar neutrino problem. It probes cosmological models including hot dark matter already now on the level of future satellite experiments MAP and PLANCK. It further probes many topics of beyond Standard Model physics at the TeV scale. Future experiments should give access to the multi-TeV range and complement on many ways the search for new physics at future colliders like LHC and NLC. For neutrino physics GENIUS will allow to test almost all neutrino mass scenarios allowed by the present neutrino oscillation experiments. At the same time GENIUS will cover a wide range of the parameter space of predictions of SUSY for neutralinos as cold dark matter. Further it has the potential to be a real-time detector for low-energy (pp and ⁷Be) solar neutrinos. A GENIUS Test Facility has just been funded and will come into operation by end of 2001.

We built a low background detector based on a 1 m³ time projection chamber to measure the elastic cross section at low energy. The detector has been installed close to a nuclear reactor in Bugey and it is running since about 1.5 years. After having reduced the electron background by almost 4 orders of magnitude we are now taking data to be sensitive to a neutrino magnetic moment in the region below 10^-10 Bohr magnetons. The MUNU detector is the first one doing neutrino spectroscopy in the MeV region by measuring both the energy and the direction of the recoiling electron. Its potentialities as a low background prototype of a TPC for the spectroscopy of the low energy neutrinos from the Sun (pp and ⁷Be) are discussed.

²²²Rn and ²²⁰Rn coming from the atmosphere or from ²³⁸U- and ²³²Th-contamination within a detector is of special concern in experiments, which aim to measure particles in the energy region below 1 MeV. Radon is omnipresent in the environment at concentrations of several orders of magnitude above what would be tolerable in these detectors. In addition radium, the progenitor of radon, must also be reduced to very low levels of concentration in the detector construction materials in order to reduce the radon emanation. The different nature of the samples, which have to be controlled, (e.g. nitrogen gas, water, or even complete photomultiplier tubes) requires adequate assay techniques.

In this article the muon induced spallation background in forthcoming and future solar neutrino experiments like BOREXINO, KAMLAND, LENS and LIQUID XENON is beeing discussed. The results of cross section measurements at the SPS muon beam at CERN to determine the muon-induced in-situ production of radioactive isotopes in scintillator based targets are presented as well as the calculated and estimated background rates for the solar neutrino experiments mentioned above.

A sensitive determination method for trace elements in ultra pure detector components of low background experiments is presented in this article. Neutron Activation Analysis and radiochemical separation methods are applied for the detection of primordial radioactive trace elements. The focus on the detection of ²³⁸U led the sensitivity to the sub fg/g range for some isotopes which are of interest for neutrino- and other low background experiments. Results for an organic scintillator investigated are presented.

I discuss why we need solar neutrino experiments below 1 MeV. I also express my prejudices about the desired number and types of such experiments, emphasizing the importance of p-p solar neutrino experiments.

The following sections are included:

• List of Participants

• Scientific Programme