The neutrino is a type of fundamental particle with no electric charge, a very small mass, and one-half unit of spin. Neutrinos are one of the most abundant particles in the universe. Because they have very little interaction with matter, however, they are incredibly difficult to detect. Nuclear forces treat electrons and neutrinos identically; neither participate in the strong nuclear force, but both participate equally in the weak nuclear force. Particles with this property are termed LEPTONS. In addition to the electron (and it's anti-particle, the positron), the charged leptons include the muon (with a mass 200 times greater than that of the electron), the tau (with mass 3,500 times greater than that of the electron) and their anti-particles. The neutrino has neither shape nor size, neither mass nor electric charge, and yet it has momentum and spin. Its existence was postulated to account for anomalies in the distribution of energy, momentum, and spin, in various atomic transformations.
Before explaining why neutrinos were first postulated it will be necessary to briefly discuss the nature of neutrons, nucleons and neutron decay.
A neutron is an uncharged (electrically neutral) subatomic particle with mass 1,839 times that of the electron. Neutrons are stable when bound in an atomic nucleus, whilst having a mean lifetime of approximately 1000 seconds as a free particle. The neutron and the proton form nearly the entire mass of atomic nuclei, so they are both called nucleons.
Nucleons are the subatomic particles found in the nucleus. They are the constituent (proton or neutron) of an atomic nucleus. These particles make the nucleus spin. Their rates of spin are based on certain energy levels called its energy spectrum. If an atom is not spinning, its in its ground state, while if its spinning its in the excited state. The nucleons are arranged in orbits, based on the amount of activity they possess.
Protons are positively charged. All protons are identical, regardless of the element. The mass of the typical proton is roughly 1amu. 1 amu is often defined as 1/12 the mass of a carbon-12 atom. Protons are often used to figure out the atomic number.
Neutrons are electrically neutral. All neutrons are identical, and have more mass than do protons, but they still consider the mass to be 1amu.
The proton has a positive electric charge that is equal to the negative charge of the electron. The neutron has no electric charge. Although the mass of the neutron and proton are generally considered to be generally equivalent, the mass of the neutron must exceed that of the proton in order for the stable elements to exist. But the neutron can only exceed the mass of the proton by an extremely small amount an amount which is exactly twice the mass of the electron. That critical point of balance is only one part in a thousand. If the ratio of the mass of the proton to neutron were to vary outside of that limit chaos would result.
The proton's mass is exactly what it should be in order to provide stability for the entire universe. If it were any less or more, atoms would fly apart or crush together, and everything they are in which is everything would be destroyed. If the mass of the proton were only slightly larger, the added weight would cause it to quickly become unstable and decay into a neutron, positron, and neutrino. Since hydrogen atoms have only one proton, its dissolution would destroy all hydrogen, and hydrogen is the dominant element in the universe. A master Designer planned that the proton's mass would be slightly smaller than that of the neutron. Without that delicate balance the universe would collapse.
The mass of the neutron is greater then the combined masses of the proton and that of the electron. This larger mass means that it is possible (in principle) to make a proton and an electron from the amount of matter in a neutron. This leads to the most striking properties of the neutron---it's instability.
Figuratively speaking, if one to take a neutron outside of the nucleus and put it on a table it would not stay there for long. Within hundreds of a second, it would decay and in its place would be a proton. An electron and the third type of particle, the neutrino. If we watched a large number of free neutrons we would find that they would decay at irregular intervals, first one, then another, until all had completed the transformation into the decay products at which point there would be nothing left.
The average decay time that it takes a fixed fraction of the original neutrons to decay into half the original number of particles is called the half-life.
The question is, why is it that a free neutron decays in minutes but a neutron within a nucleus (such as the six neutrons in a carbon atom) not decay at all? The answer is that when a free neutron decays, the proton, that is a result of the decay, can go anywhere it wants, since there are no other protons around. But in carbon atom, in order for one of the neutrons to decay, there has to be room in the nucleus for the proton that would result from the decay. In other words, all the places that the decay proton could fit are already filled with other protons in the carbon nucleus. The end result is that neutrons in most nuclei cannot decay, simply because there is no place for the decay products to go. Its much like in a parking lot. When the proton parking lot is filled in the nucleus then there is no place for the neutron to "park" its decay proton.
However there are some nuclei that are unstable where the neutron can and does decay. When this happens the nucleus emits an electron and acquires a unit of positive charge because the resultant proton produced from the decay of the neutron now has a "parking space" within the nucleus. These emitted electrons from such nuclei were one of the original types of radiation detected. They were called BETA RAYS. This process was called beta decay. The electron emitted had a great deal of energy and travels a long way from its parent atom.
After decay we are left with an atom that has one more proton in its nucleus then it did before the decay. This means that the new atom has a net positive charge (and will most likely pick up a stray electron in its surroundings and become electrically neutral in the process).
When the neutron decays then two charged particles are created: a proton and an electron. The net charge of these two decay products is 0. Thus we can say that the total charge of the final decay product is 0, which is exactly the charge of the neutron. What this means is that in the beta decay of a neutron there is a a conservation of electrical charge where the net charge of the decay products is equivalent to the primary particle before decay. Thus the electrical charge is conserved before and after the reaction. This is known as the Law of the Conservation of Electrical charge.
Conservation laws play a very important role in physics, so it was natural to ask whether the other well known conservation laws hold in beta decay. For example there are laws that tell us that the energy of a system has to be the same before and after every reaction and the other laws tell us the same thing about momentum.
However with beta decay it appeared that one of the laws of physics were being violated. Scientists noticed that when atoms of a particular isotope underwent beta decay, they always lost the same amount of energy, but the electrons were ejected with a range of energies [Note: Atoms that have the same number of protons but different numbers of neutrons are called isotopes. The element hydrogen, for example, has three commonly known isotopes: protium, deuterium and tritium]
It appeared as if energy was being destroyed in the reaction, violating a concept known as the conservation of energy. They also noticed that the ejected electron and the recoiling nucleus didn't always move apart on a straight line, but sometimes did so at an angle. This violated another concept known as the conservation of momentum. Believing that the two conservation laws were valid, Pauli stated than an undetected particle must be produced during beta decay, one that would carry away the missing energy and momentum. Pauli postulated that another particle comes out of he nucleus along with the beta particle and it was this particle that was carrying the missing energy. This mysterious particle has some strange properties. It has no electrical charge and had what appeared to have very little mass, which might even be zero. All it had was a certain amount of energy that speeds at the velocity of light. This particle looked like it was a fictional item created just to balance the energy books.
No sooner had the existence of this mysterious particle been proposed then the physicists were sure that this particle existed. When the neutron was discovered and found to break down into a proton, releasing an electron which (as in beta decay) also carried a deficiency of energy then the physicists were even more certain. Enrici Fermi in Italy gave the particle a name---neutrino, which in Italian means "little neutral one."