http://www.ck12.org Chapter 25. Nuclear Physics
Nazis were working on such a bomb, and so was the United States. Fortunately for us, the United States succeeded
and the Nazis did not. In fact, Nazi Germany was defeated before the United States developed the atomic bomb.
About seven weeks after the surrender of Nazi Germany, the first atomic bomb was tested (detonated) in a remote
section of New Mexico on July 16, 1945. With the war in the Pacific still going on, President Harry S. Truman
made the decision to use the atomic bomb. In August 1945, atomic bombs were dropped on the Japanese cities of
Hiroshima and Nagasaki, leading to the quick capitulation of Japan and ending World War II.
For nearly two decades after World War II, atomic tests (denotations) were conducted above ground. These explo-
sions, despite all the precautions, produced enormous, long-lasting environmental effects. The radiation produced
from atomic explosions (called atomic fallout) does not simply go away. The half-life of many radioactive particles
produced in those tests is measured in decades, hundreds, and even thousands of years. As we stated earlier, nuclear
radiation can have devastating effects, such as triggering cancer, on human beings. Those who witnessed the effects
of atomic radiation in Japan knew well the consequences of waging atomic war. The ill effects of exposure to atomic
radiation are not confined to only those who experience it. They can be passed on to future generations in the form
of genetic mutations. Soil, air, and water can all be irrevocably poisoned by atomic fallout.
We now use atomic energy for peaceful purposes. Nuclear reactors produce heat that is used to generate steam to
run turbines (generators). The dangerous by-products of nuclear reactions, however, are still with us. It remains a
challenge to find safe and secure ways of dealing with nuclear materials.
Learn more about nuclear fission by clicking the links below.
http://phet.colorado.edu/en/simulation/nuclear-fission
http://demonstrations.wolfram.com/IsotopeDecay/
Nuclear fusion
The process in which the nuclei of lighter elements combine and form heavier elements, with an accompanying
release of energy, is callednuclear fusion.
Fusion does not occur naturally on Earth, since it requires tremendous amounts of energy to bring protons close
enough for the strong force to overcome the electrostatic repulsion between them. We have managed to create
uncontrolled fusion events on Earth in the form of thermonuclear bombs (an atomic bomb triggers a thermonuclear,
or fusion bomb). Controlled fusion events have been achieved experimentally for only a tiny fraction of a second,
and for very small samples of materials.
Fusion does occur naturally within the stars. Within the core of a star, sufficient energy exists to force the protons
close enough together within the range of the nuclear strong force. Through a series of nuclear reactions, helium
nuclei are eventually formed, releasing about 27 MeV per transmutation. Though this amount is considerably less
than the energy released during the fission of a single Uranium-235 nuclei (200 MeV), the sun has vast amounts
of hydrogen. Fusing hydrogen into helium releases about 4× 1026 Jevery second within the sun. This is about 1
million times more energy than the energy currently used by the entire human race in a year!
All nuclei beyond beryllium are made in the cores of stars. Stars as massive as the sun can create nuclei of oxygen
(Z= 8 )but no greater. Stars with masses greater than eight times that of the sun can create nuclei up through iron
and nickel. Some very massive red giant stars can produce nuclei up though lead. Most of the nuclei beyond iron,
however, are produced during supernovae explosions, where the temperature inside the core of such massive stars
reaches in excess of a billion kelvins (the core temperature of the sun is merely 30 million kelvins).
A supernova explosion occurs when the process of fusion inside a star’s core is overcome by the gravitational force
seeking to compress the star. As the star collapses, due to the inward force of gravity (known as “gravitational
collapse”), enough energy is available to fuse nuclei beyond iron. Stellar compression continues until a “blast wave”
is initiated and the star explodes, exuding those nuclei heavier than iron. Only a star with a mass several times
greater than the sun has can become a supernova. With the exception of hydrogen, all of the elements in the periodic
table,Figure25.7, are created through fusion.