Into the nucleus
Soon after the turn of the century (in 1911), the New Zealand scientist Ernest Rutherford carried out one of the most significant experiments in physics. He directed a beam of alpha particles (helium nuclei) at a thin sheet of gold foil. Most of the particles passed straight through. Some were deflected slightly from a straight-line path, and a few were scattered through quite large angles, Rutherford deduced that the positively –charged alpha particles were scattered by near collisions with the positively-charged nuclei at the center of the gold atoms-he had proved the existence of the atomic nucleus. Such scattering techniques are now central to the study of atomic and subatomic structures.
It is now known that every atom contains a nucleus. Whose size is about 10,000 times less than that of the atom itself. Most of an atom is empty space. With the exception of hydrogen, every nucleus contains neutrons and protons (collectively called nucleons). It is the so-called strong nuclear force that holds the nucleus together; if the nucleons could not exert this force, the nucleus would fly apart because of the electrical repulsion between the closely packed protons. It turns out that this force is the strongest in nature, being 100 times more intense than the electrostatic force and 1045 times stronger than gravity.
The simple view that the atom is the fundamental component of matter has changed to acknowledge that the atom has structure. The question then arose as to whether of r not the basic subatomic particles also have structures, and are not genuinely fundamental.
The theory of quantum mechanics views all forces between particles as being the result of the exchange of some intermediary particle that carries the force. Because of the very short range of the strong nuclear force, the exchange particle carrying it must be relatively massive about 200 times that of and electron. This prediction leads to the identification of the exchange particles with mesons, which are so-called because their masses lie between those of and electron and a proton.
Mesons are continually being exchanged between the particles in the nucleus, as are the mass less photons, which are the carriers of the electrostatic force. Thus, the nucleus is in a state of unceasing, seething activity, with particles appearing and disappearing, but with the neutrons and the protons remaining relatively unchanged.
For technical reasons physicists have put forward the idea that protons, neutrons, and mesons are themselves made up of even more basic particles known as quarks. It such particles do exist; it is theoretically possible to generate the basic properties of many subatomic particles simply by combining a number of different types of quarks. But quarks have to have some odd characteristics to do this. For example, they must have charges that are multiples of one-third of the charge of and electron (which was believed to be a unit charge and indivisible).
Very high –energy scattering experiments have revealed the presence of a structure within protons similar to that expected from the quark theory. Despite many attempts, however, there has yet to be a convincing demonstration of the existence of a single quark outside its host particle. Some physicists believe that this is because there is a force between quarks that becomes stronger as they are separated, preventing them form appearing singly.
Experiments to probe the inner structures of both atomic and subatomic particles are frequently carried out using accelerators. These machines, which can be either circular or linear in form, use magnetic and electric fields to accelerate the particles in them to extremely high velocities, thus making them collide. The collisions often create new particles and the higher the energy of the collision, the more massive are the particles produced.
Consequently, the incentive in this branch of physics is always for higher and higher impact energies, and some accelerators achieve this by smashing together two beams of particles head –on. Others use a storage ring, in which electric and magnetic fields around a circular path give successive “kicks” to the particles until a high energy is attained. The particles are then release into the main collision path. Where they collide with the target particles.
To enable the result of a collision to be analyzed, it is necessary to record the tracks of the particles produced. Bubble chambers are frequently used for this purpose. Liquid hydrogen is heated in a chamber to just over its boiling point, but is prevented from boiling by being kept under pressure. Just before the particles enter the chamber, the pressure is released, causing bubbles to form about the particles and along their tracks. The tracks are photographed, and the shapes, lengths, and angles between the various lines can be used to identify the particles concerned.
Nuclear stability and radioactivity
For an atomic nucleus to remain stable, the forces at work within it- the electrostatic and strong nuclear forces-must remain in some sort of balance. If a nucleus contains too many neutrons, it is unstable because the strong nuclear force favors pairs of nucleons, and pairs of pairs. A nucleus with too many protons, on the other hand, is unstable because of the relatively strong resultant electrical repulsion between the positively-charged protons.
Unstable nuclei try to become more stable essentially by ejecting material. In scientific terms, this increases the amount of binging energy per nucleon. Einstein’s theory of relativity shows that the energy E equivalent to a mass m is given by e=mc2, where c is the velocity of light. The binging energy I therefore related to the difference between the sum of the masses of the individual particles that make up a nucleus and the actual total mass of the particles when gathered together in the nucleus.
The spontaneous process that takes place when and unstable atomic nucleus tries to become more stable is called radioactivity, it is accompanied by the emission from the nucleus of mass in the form of alpha particles or beta particles, sometimes accompanied by the emission of energy in the form of gamma rays.
An alpha particle consists of two protons and two neutrons. Beta decay, on the other hand, involves the emission of and electron or a positron. Gamma rays are a form of short-wavelength electromagnetic radiation, resembling high –energy x rays.
From these definitions, it is clear that when a “parent” nucleus decays by alpha-emission the resulting “daughter” nucleus has a mass lower by 4 (and an atomic number lower by 2);the daughter is a different , lighter element. Thus the metal radium has transmuted into radon.
In beta decay, the mass of the nucleus remains unchanged but the atomic number increases or decreases by one, depending on weather and electron or positron is emitted.
The greater the number of radioactive nuclei in a given sample, the grater is the number of nuclei undergoing transformation; the rate of activity also varies exponentially with time. Therefore, for any particular radioactive element, there is characteristic time called the half –life—the time after which the number o f radioactive nucleus in the sample has de-creased to half the number originally present. The common, nonfissile isotope of uranium, decays by the alpha process and has a half-life of 4,500 million years.
It is also possibly to cause artificial radioactivity by bombarding a nucleus with high-velocity neutrons. In the best-known example, uranium I struck by neutrons, and each uranium nucleus splits into two roughly equal parts, a process accompanied by the released of two or three neutrons and a great amount of energy.
Above a certain critical mass of such a fissile material, it is possible to ensure that there is always at least one neutron produced by each transformation capable of disrupting another nucleus, and so on in a chain reaction. If this is allowed to run away, out of control, the result is a nuclear explosion of vast power; this is the basis of the atomic bomb. But if the chain reaction id moderated and controlled, the enormous energy produced by the fission process can, for example, be used to heat water and produce steam to drive turbines and then generate electricity. This is the way in which a nuclear power station works.
Light elements can also be used to produce energy. By fusing together the nuclei of two such elements, it is possible to form a new nucleus whose mass is slightly less than the original ones. The “lost” mass appears as energy according to Einstein’s relation e=mc2. To be able to fuse, the two nuclei must have sufficient energy to overcome the electrostatic repulsion resulting from their charges. Such energy can be provided only by temperatures of hundreds of millions of degrees. In the stars, however, such temperatures are common place, and the fusing of hydrogen to produce helium keeps them shining for thousand of millions of years.
Scientists on earth have already demonstrated the uncontrolled power of nuclear fusion in the hydrogen bomb. By contrast, one area of active research in practical nuclear physics is concerned with finding a method of controlling fusion, so that it can be used productively to generate electricity.