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Nuclear Physics
I
INTRODUCTION Nuclear
Physics, the study of atomic nuclei,
and of their interactions with other nuclei and with individual elementary
particles 
II
DECAY OF NUCLIDES
Atomic nuclei consist of positively charged
protons and neutral, or uncharged, neutrons. The number of protons in a nucleus
is the atomic number, which defines the chemical element. Nuclei with 11
protons, for example, are nuclei of sodium (Na) atoms. An element can have
various isotopes, the nuclei of which have differing numbers of neutrons. For
example, stable sodium nuclei contain 12 neutrons, whereas those with 13 are
radioactive. These isotopes are notated as ®Na and ²Na,
where the left-hand subscript indicates the atomic number and the superscript
represents the total number of nucleons, or neutrons and protons. Any species of
nucleus designated by certain atomic and neutron numbers is called a nuclide.
Radioactive nuclides are unstable: they undergo spontaneous transformation into nuclides of other elements, releasing energy in the process. These transformations include alpha (á) decay (the emission of a helium nucleus, ¸He2+), and beta (â) decay or positron (â+) decay. In â decay a neutron is transformed into a proton with the simultaneous emission of a high-energy electron. In â+ decay a nuclear proton turns into a neutron with the emission of a high-energy positron. For example, 24Na undergoes â decay to form the next higher element, magnesium:
Gamma (ã) radiation, like light, is electromagnetic radiation, but by virtue of their much higher frequency, ã rays have far more energy. When á or â decay occurs, the resulting nucleus is often left in an excited (higher energy) state. Gamma rays are emitted as the nucleus drops to a lower energy state.
Any characterization of radioactive nuclide decay
must include a determination of the half-life of the nuclide, that is, the time
it takes for half of a sample to decay. The half-life of 24Na,
for example, is 15 hours. The types and energies of radiation emitted by the
nuclide are also important in characterizing the decay.
III
EARLY EXPERIMENTS
Radioactivity emitted by uranium salts was
discovered by the French physicist Henri Becquerel in 1896. In 1898 the French
scientists Marie Curie and Pierre Curie discovered the naturally occurring
radioactive elements polonium (84Po)
and radium (88Ra).
During the 1930s, Irène and Frédéric Joliot-Curie made the first artificial
radioactive nuclides by bombarding boron (5B)
and aluminium (13Al)
with á particles to form radioactive isotopes of nitrogen (7
N) and phosphorus (15P).
Naturally occurring isotopes of these elements are stable.
The German nuclear scientists Otto Hahn and Fritz
Strassmann discovered nuclear fission in 1938. When uranium is irradiated with
neutrons, some uranium nuclei split into two nuclei, each with about half the
atomic number of uranium. Fission releases enormous energy and is used in
nuclear fission weapons and reactors (see Nuclear Energy).
IV NUCLEAR REACTIONS Nuclear physics also involves the study of nuclear reactions: the use of nuclear projectiles to convert one species of nucleus into another. If, for example, sodium is bombarded with neutrons, some of the stable ®Na nuclei capture neutrons to form radioactive ²Na nuclei:
®Na + ¦n → ²Na + ã rays
Neutron reactions are studied by placing samples inside nuclear reactors, which produce enormous numbers of neutrons.
Nuclei can also react with each other, but being
positively charged, they repel each other with great force. The projectile
nucleus must have a high energy to overcome the repulsion and to react with
target nuclei. High-energy nuclei are produced in cyclotrons, Van de Graaff
generators, or other particle accelerators.
A typical nuclear reaction is the one that was used to produce artificially the next heavier element above uranium (°U), the heaviest element that occurs in nature (see Periodic Law). Neptunium (±Np) was made by bombarding uranium (mostly °U) with deuterons (nuclei of the heavy hydrogen isotope, ªH1) to knock out two neutrons, forming ±Np:
°U + ªH → ±Np + 2¦n
V
RADIOCHEMICAL ANALYSIS
Alpha particles, most of which are emitted by
elements with atomic numbers above 83, have discrete energies characteristic of
the emitting nuclide. Thus, á emitters can be identified by measuring the
energies of the á particles. The samples being measured must be very thin, as
á particles lose energy rapidly on passing through matter. Gamma rays also have
discrete energies characteristic of the decaying nuclide, so ã-ray energies can
also be used to identify nuclides. Because ã rays can pass through considerable
amounts of matter without losing energy, samples need not be thin. Beta-particle
(and positron) energy spectra are not useful for identifying nuclides; they are
spread over all energies up to a maximum for each â emitter. See
Particle Detectors.
Nuclear physics techniques are frequently used to analyse materials for trace elements—elements that occur in minute amounts. The technique used is called activation analysis. A sample is irradiated with nuclear projectiles, usually neutrons, to convert stable nuclides into radioactive ones, the activity of which is then measured with nuclear radiation detectors. For example, any sodium in a sample can be detected by irradiating the sample with neutrons, thereby converting some of the stable ®Na nuclei into radioactive 24Na and measuring the amount of 24Na by counting the â particles and ã rays emitted.
Activation analysis can (without chemical separation) measure quantities as small as a nanogram (a billionth of a gram or 0.03 billionth of an ounce) of about 35 elements in materials such as soil, rocks, meteorites, and lunar samples. Activation analysis can be used on biological samples, such as human blood and tissue; however, fewer elements can be observed in biological materials without chemical separations.
Desired radioactive isotopes can be produced for medical diagnoses and treatments, and for use as radioactive isotopic tracers. These are valuable in studies of the chemical behaviour of elements, in the measurement of wear in car engines, and in other studies involving extremely small amounts of material.
See also Physics; Quantum Theory.