IMAGINE a tank which can hold 50,000 tonnes of water, located about a km under the earth. This is the latest neutrino detector which starts its operations in December in Japan. Located some 300 km west of Tokyo in a lead mine, the construction of the US $ 100 million Super-Karmokande experiment is complete, and scientists hope to have enough data soon to answer some of the most intriguing questions in particle physics and cosmology (Science, Vol 270, November 3, 1995).

The neutrino is one of the most elusive particles known to us. Proposed by Pauli in 1930 to explain some puzzles of radioactive decay, it is among the most weakly interacting particles, thus making its detection an experimentalist's nightmare. It was not until 1956 that F Reines proved that the particle actually existed - a feat for which he was honoured with a Nobel Prize in Physics.

We now know that there are three kinds of neutrinos - associated with the electron, the muon and the tau particle. They are not charged and only interact through the weak nuclear force. But certain questions regarding the properties of the neutrinos remain unanswered. For instance, it is still not known whether neutrinos have a mass and if they do, what is it? Though reports of a measurement of the neutrino mass had appeared earlier this year, they have yet to be confirmed.

Neutrinos are produced copiously in particle accelerators and nuclear reactors. They are also produced in stars like the sun, as a by-product of the nuclear reactions which produce the energy of the star. Within 40 years since Reines found them, several neutrino detectors have been operational. Most of them are in abandoned mines - deep underground - and use large quantities of water, heavy water or some other chemical like perchloroethylene (dry cleaning fluid) for detection of neutrinos. It is important for them to be underground so that the assault of cosmic rays hitting the surface of the earth is minimised.

Approved in 1991, the Super-Kamiokande is a much bigger and better version of the still ongoing Karmokande experiment. The experiment uses the fact that, of the many neutrinos passing through the large quantity of water, some will interact with an electron. The electron then emits a characteristic radiation (known as Cerenkov radiation) which is detected by 11,200 photomultiplier tubes surrounding the water tank.

With the Super- Kamiokande, scientists hope to get more data in a year's time than that accumulated by all other neutrino detectors over the years! This huge volume of data will help solve some of the outstanding puzzles in high energy physics as well as cosmology. Although initially assumed to be massless, many physicists now believe that the neutrinos may actually have a very small mass. This would imply, according to quantum mechanics, that the three kinds of neutrinos could oscillate into each other. We could start with a beam of electron type neutrinos, but after some time the beam could be a mixture of all the three types.

The issue of the neutrino mass is of profound importance in cosmology, high energy physics and astrophysics. In high energy physics, a neutrino with a mass will rule out certain models of particle physics and make our understanding of nature at the smallest scales more complete and firm. A neutrino mass will also have a bearing on the important cosmological question of whether we live in a closed or an open universe - the answer to which depends on the energy density of the universe; massive neutrinos could contribute to the total energy density.

This is because in the primordial universe, neutrinos were produced copiously. In 1987, the much smaller Kamiokande detector was instrumental in detecting the spectacular supernova 1987A. A supernova is an explosive event towards the end of the life of certain types of stars in which enormous quantities of energy is released. The detection and study of neutrinos from the 1987A helped astrophysicists gain important insights into the mechanism which operates in this cataclysmic event.

Closer to home, our own sun produces neutrinos when Boron-8 decays. Studying these neutrinos from the sun, we will be able to understand the precise mechanism of energy generation in the sun, as well as fathom its structure. The prediction of the number of neutrinos reaching earth - as well as their energy profile - is at variance with the observations. But the amount of data is very small to establish anything conclusively. Super- Karmokande will be able to generate enough data to settle the solar neutrino puzzle, as this conflict of theory and experiment is known as.

At a fraction of the cost of proposed particle accelerators, the Super- Karmokande will provide invaluable clues to the mysteries of the very small (particle physics) as well as those of the very large (cosmology).