Emission of a beta particle produces an energy change in the nucleus of an atom. The energy change that is observed, however, does not add up to satisfy the Law of Conservation of energy. The conservation law can be balanced if another particle, called the neutrino is part of this process. In the time it takes to read just this sentence, more than a billion of them will have passed unnoticed through the reader’s body. Neutrinos may account for an appreciable portion of the mass of the universe, even tending to slow it’s rate of expansion.
Originally proposed in 1931 by Wolfgang Pauli to account for mass that was mysteriously lost from certain nuclear reactions,and first observed in 1956 by Dr Frederick Reines and Dr. Clyde Cowan, neurtinos either were beleived to have no mass or to have a negligible amount of mass. Ever since their discovery scientists wondered if this elusive particle that has no electric charge and is able to pass through the entire earth without hitting anything might not have some negligible amount of mass. Because they exist in great numbers in the universe, even a small mass could provide the “dark matter” that cosmologists beleive makes up the substance of the cosmos. Having a mass, neutrinos might also be able to change into neutrinos of other types, by a process called oscillation. They have no electrical charge and are believed to travel at or near the speed of light. Neutrinos react very little with matter and only rarely do they react with protons or neutrons through a force !
called “the weak force”. Their discovery enabled scientists to work out a conservation of evergy, spin, and momentum for beta decay.
There are six types of neutrinos, depending upon the subatomic particles with which they are associated: electon antineutrinos with electrons, electron neutrinos with positrons, muon neutrinos and antimeutrinos with muon reactions, and tau neutrinos and antineutrinos with massive tau particles. Mass is what determines the rate at which the wave function of a particle vibrates. If the waves of two neutrinos of different masses mingle, they beat against each other much like sound waves of different pitch . If neutrinos had no mass, their waves would have the same frequency and would not be able to beat at all.
A particle detector at Los Alamos National Laboratory has captured eight events that could be the first direct sightings of neutrino oscillations. If verified, the observations will prove as well that neutrinos have mass. However such experiments are not easy to carry out. In the Liquid Scintillator Neutrino Detector experiment, a beam of protons from an accelerator is shot into a water target. Particles are absorbed and detected and some implications involving neutrino mass are made, and although this could help solve the dark matter problem it contributes nothing to the major puzzle the scientists are concerned with: the solar neutrino problem. Only about half the number of neutrinos that the theory predicts should come from the sun are actually detected. The deficit might be explained by presuming that the particles change to muon neutrinos and therefore escape detection. But if neutrinos change type over short distances as in the LSND experiment, the oscillations !
would average out over the 92 million miles that seperates the sun from the earth.
Some other neutrino experiment have been undertaken to determine the number of neutrinos that actually reach the earth from the Sun and stars. One such experiment known as DUMAND, for Deep Underwater Muon and Neutrino Detector, Which was led by scientists from the Univ. of Hawaii, sought to install detectors in deep water off the coast of Hawaii’s Big Island. But leaks and broken connections caused one expensive problem after another, and money for the project by the Department of Energy was recently ended. And just in November 1996 a collaboration of Greek and US physicists began their experiment called NESTOR (Neutrinos fom Supernova and Tervolt sources, Ocean Ridge). They began by lowering instument-laden platforms resembling umbrellas into the deepest part of the Mediterranean Sea off the southwest coast of Greece to test a system that will look for neutrinos coming up from the ocean floor. The earth is used as a filter to screen out extraneous particles that could !
be mistaken for neutrinos from space.
Scientists are also discussing a proposed international neutrino detector that will be 1,300,000,000 cubic yards in volume, that will be filled with water or ice, and studded with hundreds of thousands of photo multiplier detection tubes. This huge detection unit will give experimenters a much better chance of capturing passing neutrinos.
In October of 1995 Dr. Reines was awarded the Nobel Prize in Physics for his work on neutrinos. Specifically, Dr Reines found that when a neutrino collided with a proton in water, the interaction produced a gamma ray. Excess energy in a nucleus can also be relieved by emission of gamma rays. This electromagnetic radiation carries less energy than would be discharged by an alpha or beta particle emission. In natural radioactivity, gamma rays are emitted in situations that do not require energy losses sufficient for a particle emission burt offer adequate energy loss for nuclear stability.