MadSci Network: Physics
Query:

Re: How do you split atoms?

Date: Tue Feb 3 14:35:59 1998
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Physics
ID: 886365393.Ph
Message:

> I was wondering how they split atoms in a nuclear bomb. Do you have any idea?

The picture at right is a photograph of the Seminole test explosion, a 13.7 kiloton device exploded at Enewetak atoll on June 6, 1956. Photographs are courtesy of the Department of Energy's Nevada Test Site Archive.

You may know that all atoms are made of protons (which have a positive electrical charge), neutrons (which have no electrical charge) and electrons (which have a negative electrical charge). The electrons are very small items which zip around the outskirts of the atom; but at the center is a very heavy object, the nucleus, which contains only protons and neutrons. Since the protons all have like charges, they repel each other and want to fly apart; but if neutrons are thrown into the mix, the atomic nucleus can be stabilized. The stabilizing effect is called the strong nuclear force, and can only operate over very short distances (between neighboring protons or neutrons).

Small atoms typically have about the same number of protons and neutrons, but as atoms get larger, their nucleii need more neutrons to hold the protons together. For example, while carbon has 6 protons and 6 neutrons, iron (with 26 protons) needs 30 neutrons. Lead, which is the largest stable nucleus (one that won't fall apart by itself), has 82 protons and 125 neutrons.

It's the number of protons which defines the chemical element, so that all atoms of carbon have 6 protons. But, within certain limits, an atom can have any number of neutrons: carbon, for example, has stable atoms with 5, 6, and 7 neutrons in them. Atoms which are the same element but have different numbers of neutrons are called isotopes. Lead has four stable isotopes, all of which have 82 protons: 204Pb, 206Pb, 207Pb and 208Pb.

Very large atoms have too many protons and neutrons to be stable, so they tend to break down. Even the most stable, very large atoms throw off chips of themselves, called alpha-particles. An alpha-particle is made of two protons and two neutrons, and is an attempt by an atom which has too many protons and neutrons to reduce itself to a more reasonable size. For example, the most common isotope of uranium, 238U (92 protons, 146 neutrons), is an alpha-emitter and throws off alpha-particles.

Less stable nucleii, such as 235U or 239Pu (plutonium, 94 protons, 145 neutrons), undergo fission, or splitting into two more-or-less equal-sized pieces. This releases a lot of energy, since less energy is needed to hold the protons and neutrons in two balls than was needed to hold them together in a single ball. In fact, the two new atoms find themselves with more neutrons than they need, so they throw them off during the fission process. And these extra neutrons are what makes large-scale nuclear energy (such as a bomb or reactor) possible.

You see, fissionable nucleii like 235U or 239Pu are so unstable that they will fall apart if struck by a neutron. So, if the first atom to fall apart releases two neutrons, and each of them hit another fissionable nucleus, which then release four neutrons, which go on to cause four fissions and release eight neutrons... pretty soon you have a nuclear chain reaction. There are various ways of controlling this, but only one is used in a nuclear bomb.

In a fission bomb, the fissionable material is kept apart until the bomb is detonated. Then, conventional high explosive forces the fissionable material together into a critical mass (a block of material large and dense enough that the neutrons cannot escape without causing other atoms to fission). The chain reaction happens so quickly that the critical mass cannot blow itself apart before it explodes... and the result is a beautiful but terrifying fireball.

I am become Death, the shatterer of worlds.
-- J. Robert Oppenheimer, quoting the Bhagavad Ghita after the first A-bomb test in New Mexico


For more information on nuclear physics and nuclear weapons, try the following sites:

For information about peaceful nuclear power, try Joseph Gonyeau's Virtual Nuclear Tourist.

  Dan Berger
  Bluffton College
  http://cs.bluffton.edu/~berger


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