Many people have heard of the subatomic particles, proton, neutron and electron. But few, perhaps, have heard of the neutrino. The discovery and detection of this elusive little fellow make an interesting story.
To introduce the neutrino, I must first give a brief historical review of atomic structure. By 1911 it was known that atoms have a dense central nucleus surrounded by a cloud of electrons.
The nucleus contains two kinds of particles, protons and neutrons, each of about the same mass (the neutron is slightly the heavier), and each much heavier (1,840 times) than the electron. Protons and neutrons are bound together by the strong nuclear force.
Each proton is positively charged. Neutrons are uncharged, and each electron is negatively charged. The atom is electrically neutral, i.e. the number of protons equals the number of surrounding electrons.
It is easier to detect protons and electrons than neutrons, because of their electrical charge. When charged particles pass quickly through matter, they damage atoms and are easily sensed by a detector. Neutrons pass through matter without causing damage, until they collide with atomic nuclei, thereby either accelerating the charged nucleus forward or causing it to split up. These secondary effects cause damage, allowing the neutron to be detected.
In 1928, Paul Dirac deduced theoretically that positively charged electrons (positrons) also exist, and that a positron/electron pair could be made "out of nothing" provided enough energy was provided.
This was experimentally demonstrated. When gamma rays (pure energy) pass through matter, positron/electron pairs are produced. Also, when positrons and electrons meet they annihilate each other in a flash of gamma radiation. Positrons and electrons are "antiparticles" of each other.
Dirac's insight blossomed into the view that every particle has a counterpart in a mirror-world of anti-matter. Anti-protons and anti-neutrons were postulated and verified by experiment.
The neutrino was unmasked during investigations of the nature of radioactivity. Radioactivity was discovered in 1896 by Henri Becquerel who noticed that uranium salts could cast images of themselves on photographic plates even when wrapped in opaque paper. The salts shoot out invisible penetrating rays. Three types of rays were subsequently discovered: alpha particles (packets of two protons and two neutrons), beta particles (electrons), and gamma rays (pure energy). Each originates in the nucleus.
How can a beta particle originate in the nucleus? The beta particle actually originates in the neutron. A neutron can be envisaged as a close combination of an electron and a proton. When a neutron emits a beta particle it changes into a proton.
But, more puzzling still, experiments indicated that beta emission violated the principles of conservation of energy and angular momentum. Energy was vanishing. In order to balance the books, the Austrian-born physicist, Wolfgang Pauli, proposed in 1930 that beta emission also produced an invisible particle which carried away just enough energy to balance the books.
No experimental device at the time could detect this postulated particle, which is incredibly penetrating, devoid of charge and mass, and travels at the speed of light. Enrico Fermi termed this new particle il neutrino (the little neutral one).
What would induce a neutron to change into a proton plus an electron? In order to answer this, a fourth force of nature was proposed - the weak nuclear force (the other forces of nature are gravity, the electromagnetic force, and the strong nuclear force). The weak force converts neutrons into protons and neutrinos by affecting their insides. Neutrinos feel only the weak force which makes them interact very weakly with matter. In 1934 it was calculated that a column of water 1000 light years thick would be required to capture most of the neutrinos generated in beta emission.
The energy of the sun is derived from nuclear transformations, some of the beta kind. Up to one sixth of the energy is radiated away into space by neutrinos. Ten thousand billion neutrinos from the sun pass unnoticed through your body every second, and most of them speed on through the earth as if it wasn't there.
The physical detection of the neutrino posed a huge challenge - how to "feel" particles, most of which can pass through the earth leaving no impression. However, even with neutrinos, some are stopped by matter. Frederick Reines and Clyde Cowan eventually detected neutrinos in 1956 using nuclear reactors as a source of neutrinos.
Solar neutrinos constantly bombard the earth, and their detection by neutrino telescopes is shedding light on the nuclear processes in the sun. The earth is also showered from space by cosmic rays.
These rays produce confounding results and, therefore, the neutrino detector must be shielded from them by burying it deep in the earth. One detector is located a mile beneath the surface in a gold mine in South Dakota. It consists of a 100,000 gallon tank of tetrachloroethylene. When a neutrino is stopped by the fluid a chlorine atom is converted to argon, which is the signal of success.
If the neutrino telescope shows that currently accepted nuclear models of the sun are wrong, most cosmological theory for the past 30 years could be misguided. Finally, astronomers can detect less than 10 per cent of the matter in the universe. The rest is "dark matter" whose nature is unknown. There is now evidence that the neutrino has a tiny mass. If so, there are so many neutrinos in the universe, they may constitute much of the dark matter.
William Reville is a senior lecturer in biochemistry at UCC.