Neutrino Astrophysics: A Research Briefing (1995)

neutrino line in Figure 1). Owing to the expected high neutrino flux, a relatively modest fiducial mass of 100 tons of liquid scintillator will according to the predictions of the standard solar model, produce a rate of 5 events per day from the 0.86-MeV ⁷Be neutrino line by neutrino-electron scattering. The ⁷Be events can be separated from the lower-energy p-p events (maximum energy 0.43-MeV) by the energy of the recoil electrons and by the characteristic shape [2] of their recoil spectrum, which has a sharp cutoff near 0.67 MeV.

The theoretical prediction for the flux of ⁷Be neutrinos is more reliable than for the higher-energy ⁸B neutrinos observed by Kamiokande, with an estimated uncertainty of about 6 percent [2]. If the MSW interpretation is correct, then the observed flux of ⁷Be neutrinos will be greatly reduced relative to the predictions of the standard solar model. The BOREXINO measurement will also provide an essential datum for the interpretation of the gallium experiments, since ⁷Be neutrinos are predicted (by the standard solar model) to contribute significantly.

The BOREXINO detector is to be constructed with standard liquid scintillator materials that are adapted to the requirements of high purity and large scale that are necessary for a solar neutrino experiment. The requirement of radiopurity (uranium and thorium at less than ~10 ⁻¹⁶ g/g) is at least an order of magnitude more stringent than is required for the water Cerenkov detectors (like Kamiokande and SNO), but the extremely low solubility of naturally occurring compounds of the radioimpurities in the BOREXINO organic liquids compared to their solubility in water is an offsetting advantage. The radiopurity and other aspects of the experiment related to backgrounds will be tested in a 4-ton version of the experiment, currently under construction at the Gran Sasso underground laboratory and expected to become operational in 1995. A full-scale BOREXINO experiment (approximately 100 tons) is planned to be built in the same laboratory. The BOREXINO collaboration includes experimenters from Italy, the United States, and Germany.

An iodine radiochemical detector (all ¹²⁷I) will be somewhat similar to the existing chlorine detector but, per target atom, is expected to have a counting rate 5 to 10 times larger [26]. A multi-module 100-ton iodine detector, with about one quarter the number of target atoms of the 615-ton chlorine detector, is now under construction in the Homestake mine near the present chlorine detector. This detector, which is expected to begin operating in mid-1995, will have a solar-neutrino-induced ¹²⁷Xe production rate of the order of 0.5 to 1 atom per day.

The product ¹²⁷Xe atoms will be removed from the iodine detector by an automatic, computer-controlled extraction system with a 2-hour extraction period. There will be two extraction cycles per day, one in the early morning and another in the early evening. The daytime-produced ¹²⁷Xe will be accumulated in one charcoal trap, while the ¹²⁷Xe produced at night will be collected in a second charcoal trap. Once a month the ¹²⁷Xe from each of these two traps will be transferred to proportional counters and the number of atoms in each trap determined. A significantly larger nighttime than daytime production rate of ¹²⁷Xe would be a clear indication that muon-type (or tau-type) neutrinos are being reconverted to electron-type neutrinos as they pass through the earth and thus that the MSW effect is operative. The counting of the daughter product, ¹²⁷Xe, which has a 36-day half-life, is very similar to that for ³⁷Ar.

Calibration experiments will be performed to directly measure the cross sections for neutrino conversion of ¹²⁷I to ¹²⁷Xe. These experiments are necessary to determine the relative sensitivity of the detector to 7Be and 8B neutrinos. Expansion of the present iodine detector to 1000 tons may be proposed after operating experience is obtained with the 100-ton detector and the neutrino cross sections are measured.

We begin by summarizing the work currently being done on four active (electronic and cryogenic) detectors (ICARUS, HERON,

HELLAZ, and bolometers) and then describe the work that is being done on new radiochemical detectors.

ICARUS [27] is an innovative detector that utilizes liquid argon and a technique adapted from high-energy physics experiments, the time-projection chamber technique, to perform detailed event reconstruction. The unique detection reaction for this experiment is

ν_e + ⁴⁰Ar → e⁻ + ⁴⁰K*, (5)

where the ⁴⁰K excited state with the largest absorption cross section has a 5.9-MeV threshold. Neutrinos of all types can be observed in the same detector via reaction (2). A 5000-ton detector will be installed in the Gran Sasso laboratory and is planned to be operational in 1998. The main priority of this detector is a search for proton decay, but it will also be able to detect solar neutrinos. Extensive laboratory tests of a 3-ton laboratory prototype have been successful suggesting that excellent spatial and energy resolution may be possible. Detailed pattern recognition with the time-projection technique can, in principle, resolve the directly produced electron and the spatially separated gamma-ray cascade from the decay of the ⁴⁰K excited state. This event reconstruction can provide a measure of the incident neutrino energy and a demonstration that the energy deposition was caused by reaction (5). A crucial question that remains to be answered for the 5000-ton detector is whether an energy threshold as low as is required for solar neutrino research can be achieved.

Cold helium may be the purest material available; it offers the possibility of detecting very low energy depositions in either the liquid or the gaseous state. In addition, helium is relatively inexpensive. Two projects are under study that propose to use cold helium to detect the low-energy p-p and ⁷Be neutrinos. The HERON detector [28] uses ballistic phonon propagation in liquid helium in the superfluid state. The HELLAZ experiment [29] will use a time-projection chamber in a high-pressure helium gas.

The goal of HERON is to provide a real-time detector with a threshold of ~ 10 keV, suitable for measuring the recoil electron spectrum and the rate of neutrino-electron scattering by both the p-p and ⁷Be neutrinos. A detector with a 10-ton fiducial volume would yield about 20 events per day with the full standard model flux of electron-type neutrinos. The detector is based on the ballistic propagation of rotons (sound excitations) produced by electrons scattered in superfluid helium and the subsequent detection of heat pulses on an array of bolometers. The high multiplicity of rotons relative to ions is expected to be an important advantage in achieving a low threshold. Extensive tests of the detection method have been carried out using radioactive sources in a 3-liter prototype instrumented with a variety of bolometers. The promising results of these tests indicate that the position and direction of electron recoil may be measurable with the aid of a measured asymmetry in the radiated rotons.

In the HELLAZ detector [29], low-energy solar neutrinos will be observed using a detector of gaseous helium under high pressure that is maintained at low temperature. A time-projection chamber will determine the incident neutrino energy by measuring the track length and the recoil direction of electrons in the gas that are scattered by neutrinos. Initial measurements on a small laboratory prototype, as well as Monte Carlo simulations, suggest that adequate energy resolution may be obtainable even for the low-energy p-p neutrinos.

Neutrinos have neutral-current interactions with nuclei as a whole, analogous to neutrino-electron scattering. Neutrinos of all active types can participate in a neutral-current reaction,

ν_x + A → ν_x + A. (6)

The neutrino transfers some of its energy to recoil motion of the nucleus, designated A in equation (6). Small detectors (in the range of grams to a few hundred grams) are currently in use to observe double beta-decay or dark matter [30]. Most of these detectors, if applicable to