Neutrino Astrophysics: A Research Briefing (1995)

solar neutrinos, would operate by detecting in a target crystal the phonons that are generated by nuclear or electron recoil or by reaction products produced by reaction (6).

Two recent suggestions for bolometric experiments indicate the state of development of this technique. These proposals appear futuristic but are based on sound physics principles and existing data. The first suggestion [31] is to use crystals of NaBr to detect the pep and ⁷Be neutrino lines. A 100-ton (100,000-element) solar neutrino detector would be required. The second suggestion [32] is to use crystals of LiF at low temperature to detect gamma-ray lines produced by neutral-current and charged-current reactions in a 4-ton detector.

Fluorine (¹⁹F) is a potentially good detector of the higher-energy solar neutrinos (threshold 3.2 MeV). The material is relatively cheap and can be incorporated into a scintillator. The neutrino absorption cross section can be calculated accurately [2]. A prototype fluorine detector has been proposed in Russia [33].

The proven chlorine solar neutrino detector would benefit from higher count rates, and a 3000-tonne perchloroethylene experiment has been under development in the Baksan underground laboratory in Russia [34].

The most attractive features of a lithium detector are its accurately known absorption cross section and its predominant sensitivity to CNO and ⁷Be neutrinos [35]. A Li experiment in which ⁷Be is extracted from metallic lithium is under development in Russia [34]. The principal experimental difficulty is to find an efficient way of counting the ⁷Be nuclei produced by neutrino capture on ⁷Li. Research is under way to develop a cryogenic detector for the ⁷Be nuclei.

In summary, there are a number of potentially important solar neutrino detectors that can be developed for new experiments.

Higher-energy neutrinos that are most accessible experimentally are in the TeV (10¹² eV = 10⁶ MeV) energy range. Astrophysical neutrinos with these energies can be produced by high-energy collisions among nuclei or between protons and photons that produce secondary particles (pions and muons), which can decay into neutrinos. In contrast to the situation with regard to neutrinos from the sun, we have no evidence, either from tested astrophysical theories or from actual experiments, that such high-energy neutrinos are produced in astronomical sources in sufficient quantities to be detectable by instruments under construction. If they are, the environment in which they are created may be very unusual, making the search for their existence an exciting, exploratory activity.

These high-energy neutrinos can be detected via the reaction of a muon-type neutrino with a nuclear particle, leading to the neutrino being transformed into a charged muon. A multi-TeV charged muon will travel many kilometers in earth or in water. The neutrino target volume is thus the area of the muon detector times the range of the muon. Target volumes of billions of tons may be achieved in this way. The fast-moving muons can be detected by observing the blue flash of Cerenkov light (see discussion of the Kamiokande experiment) they produce when traversing a transparent medium, specifically water or ice. Detectors with effective areas of a few hundred square meters have been built in mines, four new instruments are being built with areas in the 20,000-m² range, and plans are being discussed for one or more instruments in the 1-km² class [36].

A first generation of natural neutrino detectors was built in the 1960s, and these succeeded in detecting the first neutrinos of local cosmic-ray origin. Although their areas and angular resolution were limited, a few of these experiments succeeded in observing neutrinos at approximately the rate expected from known cosmic-ray interactions with the atmosphere, and the first limits were placed on high-energy extraterrestrial neutrinos.

A second generation of neutrino telescopes was built in the late 1970s, originally designed to search for proton decay. Among these were the