Neutrons and neutrinos, despite their similar names, are quite different entities. Both are electrically neutral particles, so they are unaffected by electromagnetic fields. That makes the measurement of these nuclear particles problematic.
Ernest Rutherford’s preliminary (1911) model of the atom consisted of varying numbers of electrons orbiting about a nucleus. Like planets orbiting the sun, the negative electrons were believed to remain in orbits because of their attraction to positively charged protons in the nucleus. But why did the protons, with like charges, not fly apart?
Rutherford’s erroneous answer in 1920 was that the protons in the nucleus must be held together by neutral particles consisting of protons and electrons that had somehow combined. For years theorists believed that the nucleus consisted of protons and what they called nuclear electrons.
Rutherford’s associate, James Chadwick, eventually demonstrated that the particle responsible for stabilizing the nucleus was an uncharged entity with a mass equal to the proton. Later it was found that the neutron’s mass is slightly greater than that of the proton, but Chadwick had the right idea.
For a long time the neutron was assumed to be an elementary particle. We know now that it is actually a hadron, comprised of quarks. More specifically it is a baryon, because there are three quarks. The neutron consists of two down quarks, each with an electrical charge of -1/3e (where e is the elementary charge equal to the negative of a single electron’s charge) and one up quark with an electrical charge of +2/3e. The bottom line is that the neutron is indeed neutral. As in the proton, which is also made up of quarks, the quarks are held together by the nuclear force, mediated by another kind of elementary particle called a gluon.
Individual neutrons can exist outside the nucleus, and they are known as free neutrons. They are a form of ionizing radiation by virtue of the fact that they possess enough energy to knock electrons in an atom out of orbit. When an atom loses an electron, it is no longer neutral, but becomes a positively charged ion. For this reason, ionizing radiation is a biological hazard, and depending upon the dose, it may be lethal. A high dose will cause instant death and a lower dose may cause radiation sickness or create genetic mutations leading to cancer years later.
On a more positive note, low levels of ionizing radiation may have played a role in the creation of random mutations that are a necessary part of natural selection, setting the stage for evolutionary progress.
Neutrinos come from stars, each the scene of an immense amount of nuclear fusion. This process takes place in the star’s core. Four protons join, becoming a single helium nucleus. Two of the protons become neutrons, and one neutrino is released. Each neutrino moves unimpeded through the star’s outer layers and is released into the surrounding space.
There are a vast number of these particles flying away from the sun. At the surface of the earth, each square centimeter receives 65 billion (6.5 X 1010) neutrinos per second. Most of them travel through the earth and exit through the far side.
Despite their vast number, these neutrinos are extremely difficult to detect. Due to their properties, they have little tendency to interact with other types of matter. Neutrinos resemble electrons except that they have no electric charge. For this reason, they are neither attracted nor repelled by electromagnetic forces. Consequently, they can pass unaffected through large material bodies such as the earth. It is like light passing through glass. Neutrinos are true elementary particles in that they cannot be further subdivided and have no discernible internal structure.
Wolfgang Pauli, five years after formulating his Exclusion Principle, proposed the neutrino to account for energy, momentum and spin in connection with beta decay. At this time, the particle was considered a type of neutron. In 1933, Henrico Fermi came up with the term neutrino, which denotes a “little neutron.”
In all there are three types of neutrinos, each of which exists in conjunction with a different particle that, unlike the neutrino, carries an electrical charge. They are ve, relating to the electron, vµ, relating to the muon, and vτ, relating to the tau particle.
Without charge, neutrinos are ghostly travelers that pass through large solid bodies such as the earth with little interaction. Consequently they are difficult to detect and elaborate equipment is required to gain information about them.
Neutrinos populate inter-galactic space. They were created shortly after the Big Bang and they accompany background radiation, which is responsible for the 1.9°K temperature that is measurable everywhere.
Compared to their close relative, the electron, we know relatively little about the neutrino. As higher-energy particle colliders go online, scientists anticipate that methods will become available for gathering more information about these elusive entities.
Both neutrons and neutrinos are an intimate part of neutron stars.
A neutron star results from a violent, cataclysmic event that occurs when a massive star contracts due to its immense gravity to a point where all that concentrated energy and mass explodes, producing what we perceive as a supernova. A supernova emits more radiation in its weeks- or months-long life than does our sun in its entire existence. It is brighter than an entire galaxy, emitting energy at the speed of light. This huge shock wave incorporates and carries along all dust and gas in its path as it rapidly expands.
Abruptly, because of the loss of the vast amount of energy, this star cools down to a “mere” 1,012°K. Its matter is highly compressed. The pressure is so great that electrons and protons fuse to create neutrons, in the process emitting neutrinos. The neutrons, under such immense gravity, are squeezed closely together, constrained only by the Pauli Exclusion Principle, which states that no two neutrons, or any other fermions, can occupy the same energy state and location at the same time. The bottom line is that the neutron star, with a radius of seven miles, is composed of material so dense that a matchbox-sized sample could weigh five trillion tons.
As the neutron star contracts, a number of strange changes take place. Just as figure skaters who draw in their arms spin much more rapidly, the shrinking star rotates on its axis at a far greater rate. That is because angular momentum, of which radius of the mass in question is a component, is always conserved. It is hard to conceive, but a neutron star may spin at a rate of over 40,000 rpm, double the speed of a woodworker’s router.
The intense magnetic field created by the great rate of rotation often makes the energy that is emitted highly directional, like the beam from a lighthouse. If earthly radio-telescope observers lie in the plane of this rotation, these objects will appear to blink at widely varying rates. Hence they are called pulsars, spelled with an “a.” (Pulser with an “e” is a digital troubleshooting instrument.) Actually pulsars do not pulse. The pulsing is an optical effect caused by the beam of energy periodically striking our receivers.
Because of the pulsar’s intense, highly directed energy, it is likely that increasingly sensitive instruments will be able at last to detect gravitational waves.