Nuclear physics

Nuclear physics

Radioactive decay Nuclear fission Nuclear fusion Classical decays Alpha decay · Beta decay · Gamma radiation · Cluster decay Advanced decays Double beta decay · Double electron capture · Internal conversion · Isomeric transition Emission processes Neutron emission · Positron emission · Proton emission Capturing Electron capture · Neutron capture R · S · P · Rp Fission Spontaneous fission · Spallation · Cosmic ray spallation · Photodisintegration Nucleosynthesis Stellar Nucleosynthesis Big Bang nucleosynthesis Supernova nucleosynthesis Scientists Henri Becquerel · Marie Curie · Pierre Curie

Nuclear physics is the branch of physics concerned with the nucleus of the atom. It has three main aspects: probing the fundamental particles (protons and neutrons) and their interactions, classifying and interpreting the properties of nuclei, and providing technological advances.

Forces

Nuclei are bound together by the strong force. The strong force acts over a very short range and causes an attraction between nucleons (protons and neutrons). The strong nuclear force is so named because it is significantly larger in magnitude than the three other fundamental forces (weak, electromagnetic and gravitational). The strong force is highly attractive at only very small distances which, combined with repulsion between protons due to the electromagnetic force, allows the nucleus to be stable. The strong force felt between nucleons arises due to the exchange of gluons. The study of the strong force is dealt with by quantum chromodynamics (QCD).

Nuclear models

Nucleons in the nucleus move about in a potential energy well which they themselves create arising from their interaction with, and movement with respect to, each other. Nucleons can interact with each other via 2-body, 3-body or multiple-body forces. The fact that many nucleons interact with each other in a complicated way makes the nuclear many-body problem difficult to solve.

There broadly exists two types of nuclear models which attempt to predict and understand characteristics of nuclei. These are microscopic and macroscopic nuclear models. Microscopic nuclear models approximate the potential which the nucleons create in the nucleus. Individual interactions are combined as linear sums of potentials. Almost all models use a central potential plus a spin orbit potential. The difference between models is then defined by the 3-body potential used, and/or the shape of the central potential. The form of this potential is then inserted into the Schrodinger equation. Solution of the Schrodinger equation then yields the nuclear wavefunction, spin, parity and excitation energy of individual levels. The form of the potential used to determine these nuclear properties indicates the type of microscopic model. The shell model and deformed shell model (Nilsson model) are two examples of microscopic nuclear models.

Macroscopic nuclear models attempt to describe such attributes as the nuclear size, shape and surface diffuseness. Rather than calculating individual levels, macroscopic models predict nuclear radii, degree of deformation and diffuseness parameter. A simple approximation for the nuclear radius is that is is proportional to the cube root of the nuclear mass.

$R\propto A^{1/3}$

This implies that all nuclei are spherical and their radius is directly proportional to the cube root of their volume (volume of a sphere = $4/3\pi R^{3}$). Nuclei can also exist in a deformed shape and thus a degree of deformation ,$\beta _{2}$, can be included to take this into account. The fact that the nucleus may not be entirely incompressible is also considered by the diffuseness parameter $\delta$. An example of a macroscopic model is the droplet model of Myers and Schmidt.

Some quite successful attempts have been made to combine the microscopic and macroscopic models together. These so called mic-mac models begin with a nuclear potential, solve the Schrodinger equation and proceed to predict macroscopic nuclear parameters.

Protons and Neutrons

Protons and neutrons are fermions, with different value of the isospin quantum number, so two protons and two neutrons can share the same space wave function. In the rare case of a hypernucleus, a third baryon called a hyperon, with a different value of the strangeness quantum number can also share the wave function.

History

The binding energies of the protons and neutrons are on the order of 1 percent of their relativistic rest masses, so non-relativistic quantum mechanics can be used with errors usually smaller than those from other approximations. Once the chemists of the 18th century had elucidated the chemical elements, the rules governing their combinations in matter, and their systematic classification (Mendeleev's periodic table of elements) and John Dalton had, in 1803, applied Democritus's idea of atom to them, it was natural that the next step would be a study of the fundamental properties of individual atoms of the various elements, an activity that we would today classify as atomic physics. These studies led to the discovery in 1896 by Becquerel of the radioactivity of certain species of atoms and to the further identification of radioactive substances by the Curies in 1898. Ernest Rutherford next took up the study of radiation and its properties; once he had achieved an understanding of the nature of the radioactivity, he turned around and used radiated particles to probe the atoms themselves. In the process he proposed in 1911 the existence of the atomic nucleus, the confirmation of which (through the painstaking experiments of Geiger and Marsden) provided a new branch of science, nuclear physics.

Following Rutherford's work, physicists around the world began trying to "split" the atom. The first to achieve this were two of Rutherford's students, John Cockcroft and Ernest Walton, who divided an atom using a particle accelerator in 1932. In 1938, the German physicists Otto Hahn conducted the first successful experiment in nuclear fission.

In the 1940s and 1950s, it was discovered that there was yet another level of structure even more fundamental than the nucleus, which is itself composed of protons and neutrons. Thus nuclear physics can be regarded as the descendant of chemistry and atomic physics and in turn the progenitor of particle physics.

Experiments with nuclei continue to contribute to the understanding of basic interactions. Investigation of nuclear properties and the laws governing the structure of nuclei is an active and productive area of research. Practical applications—nuclear power, smoke detectors, cardiac pacemakers, medical imaging devices, and so on—have become common.

External links

• Nuclear Reactor Technology Training - Boiling Water Reactor Plant. Retrieved October 26, 2007.

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