Ruth Craig, age 12, of Vineland, Ontario, Canada, for her question:

What exactly are neutrinos?

The mighty atom is invisibly small and no instrument can observe its fantastic details. However, we cannot see thunder, though we can detect it indirectly from the sound it makes. Scientists probe the atom indirectly, by studying the behavior of its energetic particles. Chemical changes reveal its shells of negative electrons. The nucleus is harder to probe. But one big mystery was solved when radioactivity introduced neutrinos.

Chemists see the atom as a positive nucleus, with shells of negative electrons ready to perform chemical changes. Nuclear physicists see the atom as a fuzzy unit of particles, pulsing in wave like energies. They traced the laws that govern numerous atomic nuclei as they break apart in natural radioactivity. Here the key word is conservation.

In ecology, conservation means saving the earth's resources by sensible recycling processes. In radioactive decay, numerous atomic nuclei break apart. But the process conserves the original matter, or mass and energy    and also obeys strict conservation laws on other levels. Or so it seemed.

In this dynamic process, numerous nuclei shoot off various particles and energies in alpha, beta and gamma rays. In alpha and gamma rays, it was possible to measure and prove the laws of conservation. The original nuclei equaled the daughter nuclei, plus the energy emitted and the mass of the lost particles. In alpha and gamma rays, the radioactive substance balanced the books of conservation. But. some of the energy was unaccounted for in beta rays. Beta particles are high speed electrons and some had less energy than they should. Or so it seemed.

If this were true, it would upset the laws of conservation on several levels. Scientists reasoned that this was not likely. Most likely they had failed to detect something. So, in 1930, they invented the sort of particle that could balance the books. They would have detected a positive or negative charge. So the stranger must be an infinitesimal neutral particle. They called it the neutrino, the little neutral one. It was predicted that the undiscovered neutrinos shared energy with the beta electrons and whisked it away undetected.

Meantime, mighty atom smashers were invented and several other nuclear particles were discovered. In 1956, .a mighty synchrotron was used to shoot protons at a target and send showers of nuclear fragments through an iron wall, 42 feat thick. Only neutrinos could get through this barrier. They traced their activities on sensitive recording equipment    and proved that neutrinos really exist and perform as predicted.

In radioactive decay, they do indeed carry off the quota of energy to balance the books of conservation. The electron mass is very small, but physicists suspect that the neutrino has no mass at all. But every second, it whips through 186,000 miles of air or water, solid metal or empty space. The massless neutrino travels non stop through the universe at the speed of light. Only a collision with an antineutrino can disintegrate it. But this belongs in the fascinating story of antimatter.