Nuclear Physics | Research & Encyclopedia Articles

Nuclear Physics

Nuclear physics is the study of the interaction of particles in a nucleus: protons and neutrons. Its goal is to understand the behavior of the nucleus. In early days of nuclear physics, this involved study of radioactivity and the nature of the atom itself; by the end of the twentieth century, it was primarily concerned with the strong nuclear force, which binds the nucleus together despite the protons' electric charge repulsion pushing the nucleons apart.

Despite the sophistication of their field of study, nuclear physicists basically do variations on one experiment: the collision. All nuclear physics experiments consist of either a beam of particles hitting a stationary target or of two beams hitting each other. It is often compared to smashing a Swiss watch and trying to deduce its mechanism from watching what parts fly out. This method has had surprising success and his historically used such devices as the electrostatic accelerator, the linear accelerator, the cyclotron, the betatron, the cloud chamber, and the bubble chamber to control and observe collision experiments.

Although the nuclear model of the atom had not been proposed yet, nuclear physics began with the study of radioactivity at the end of the nineteenth century. In studying elements such as uranium, Antoine Becquerel and the Curies were able to determine that the atoms were emitting other particles. With further study, Ernest Rutherford and T. Royds showed that one of the particles emitted, the alpha particle, was actually a helium nucleus. At this time, the standard model of an atom was Thomson's "plum pudding" model. This model characterized the atom as a sphere of uniform positive charge with small dots of negative charge within it, like raisins in a plum pudding.

Further study led Rutherford to propose in the first nuclear model of the atom in 1911. Collision experiments had shown quite clearly that the Thomson plum pudding model would not work, so Rutherford proposed that the positively charged material was gathered in together in a central nucleus, while the negatively charged electrons were outside it. In 1913, Niels Bohr proposed that the electrons orbited the nucleus, somewhat like planets around the sun. While Bohr's planetary model had some flaws, it produced good approximations of the average electron radius in a hydrogen atom and provided explanations for mechanisms. When Erwin Schrödinger developed his quantum theory, including the wave equation, in 1926, more questions about the nature of the atom were answered.

With the basic structure of the nucleus determined, it remained for physicists to probe the internal structure of nuclei. In 1932, James Chadwick discovered the neutron, which allowed for the known mass of the proton to be used in calculating atomic masses, without resorting to variations on the plum pudding model with some electrons in the nucleus and some outside. This discovery made it clear that some force independent of charge must be binding the nucleons together, and that this force must have greater strength at short range than the Coulomb repulsion that arises between particles of like charge, such as protons. Several models of nuclear force structure have been proposed, each with varying advantages and points of accuracy or clarity.

One of the earliest models of nuclear structure was proposed in 1936 by Niels Bohr. The compound nucleus theory states that the nucleus can be made up of previous particles or nuclei that have been fused in a collision, and that the later decay of the new nuclei will be independent of their formation. Further postulates were necessary from Bohr and Wheeler in 1939 to explain nuclear fission. The "liquid drop model" envisions the nucleus as similar to a drop of water that will reach a mass for which surface tension is no longer sufficient to hold it together. Similarly, the nucleus is thought to fission when the strong force is overcome by the Coulomb repulsion. Some of the terms in the formula for calculating the strong force under the liquid drop model are actually the same as for calculating the force on a real liquid drop.

The shell model, proposed in 1949 by Maria Goeppert-Mayer, J. Hans, D. Jensen, Haxel, and Suess, proposes that nucleons follow a shell structure somewhat akin to the shell system of electrons. Considerations such as spin were necessary to determine how stable a nucleus would be. This refinement explained why the stability of a nucleus does not depend solely on its size, but also on its place in a nuclear version of a periodic table. As with electrons, each atom would have inner nucleons and valence nucleons.

The collective model was developed to address properties of the nucleus as a whole that were not addressed in the shell model, which only dealt with individual electrons. Aage Bohr, Mottelson, and Rainwater developed this model to take into account all of the valence nucleons as a whole, rather than merely the "outermost" valence nucleon. The calculations of the magnetic moments of the nucleus were much more accurate from this model than from previous ones. Nonetheless, it must be remembered that all nuclear models are approximations.

In 1964, the quark model of hadron interactions was devised by Murray Gell-Mann and Zweig. Testing the quark model required bigger, higher energy facilities for collisions. More importantly, it was a movement into the next field: particle physics. Nucleons were no longer presumed to be the most fundamental units of the atom. While studies of the strong force were still possible and necessary in traditionally nuclear methods and sizes, structure discussions had to include considerations at the quark level.

Nuclear physics arose from a combination of atomic physics and quantum mechanics; it has in turn given rise to particle physics. While nuclear physics is no longer the field probing the smallest particles known to man, it still has a vital role to play in testing theories about the strong nuclear force. It has also given rise to technologies ranging from medical imaging methods to nuclear power.