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

ICARUS

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

νe + 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.

HERON

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.

HELLAZ

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.

Bolometric Detectors

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,

νx + A → νx + 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



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