|MadSci Network: Physics|
Claire, That is a good question to introduce the interaction of radiation with matter. I will try to illustrate what is happening on the atomic level with some examples that might be easier to understand, but are on a scale you are familiar with. You mention the three particles-the proton, the neutron, and the electron-that are the most commonly encountered when considering nuclear reactors and radioactive materials. These particles form two groups: the proton and electron are charged particles, and the neutron is an uncharged, or neutral, particle. The property of charge is the most important in understanding the interaction of these particles with matter, or, as you put it, other atoms (like those that make up a table). The electron has a charge of -1 (the negative sign comes from the original definition of the charge on an electron being negative). The proton has a charge of +1 where the plus sign indicates the opposite charge from the electron. All atoms (except the hydrogen atom consisting of one proton and one electron) have a nucleus containing protons and neutrons at the center. Almost all the weight of the atom is in the nucleus because the proton and the neutron each weigh about 1837 times as much as an electron. An atom is mostly empty space--a heavy center with light electron "balls" whirling around it to form a spherical cloud. Atoms of any element in a pure state are neutral, therefore there are as many electrons in orbits around the atom as there are protons in the nucleus. When an electron is knocked out of an atom, we say the atom is ionized, and has a positive charge. To knock an electron out of an orbital takes energy, so if another electron comes speeding through a group of atoms, the negative charges of the electrons will repel the speeding electron, and the positive nuclear charge will attract it. Each time the speeding electron knocks another electron out of an atom, it loses some of its energy. If the electron is attracted to a positively charged nucleus, and its path is changed so it appears the electron is going around a curve, the electron loses energy again. In each encounter, the energy loss is small, therefore it takes a lot of collisions and curves to slow the speeding electron to a speed where it can be captured by a nucleus that has an open orbital. The speeding electron is constantly being deflected, and its path is not a straight line. The number of atomic electrons and charges on the nucleus depends on the material the speeding electron is traveling through. If it is made of light elements such as hydrogen, carbon, or oxygen (like wood of a table), there will be fewer electrons in a cube of the material than if it were made of a metal such as iron, gold, or lead. There will be many more collisions and curves in the path due to the high positive charges on the nuclei of the heavier atome. Therefore, the speeding electrons will not get as far in those materials. While a thin piece of metal easily stops speeding electrons, a 1-inch thick table top will also stop almost all of the electrons. So, the electrons will probably not get through the table. A fast moving proton has a charge of +1, and will attract every electron it comes near, causing a large amount of ionization along its path. Since the proton weighs so much more than the electron, it is not deflected by a collision--just like a bowling ball would not be deflected by a collision with a ping-pong ball. The only time a fast moving proton would be deflected is if it comes very close to a nucleus. So generally, the path of a proton is a straight line. The proton causes so much ionization, with each ionization costing some energy, that its path is short. Imagine a bowling ball passing through a mass of pingpong balls. The bowling ball goes straight until it is stopped by the large number of collisions, each taking a part of its energy, both from the collision and from the interaction of the negatively charged electrons with the positively charged proton. When the proton comes to rest, it captures one of the free electrons an becomes an ordinary hydrogen atom. So we see that no matter if the charge on the speeding particle is positive or negative, the interaction with the charges of the orbital electron or the nucleus will slow it down quickly; just how quickly depends on the number of electrons and positive charges, so heavier elements slow them down faster. Now we come to neutrons. Neutrons are not charged, so they are unaffected by the charges of the electrons or of the nuclei. They do not attract or ionize the electrons, so they do not lose much energy if they hit an electron. The only way they can lose energy is by a direct collision with a nucleus. What is the best material to stop a neutron? Since we are dealing with collisions, let's use the billiard ball example. If the cue ball hits a bowling ball, it will bounce off, changing direction, but it will not lose much energy. So the first generality about stopping neutrons is that heavy elements may not be effective. Now let's look at the cue ball hitting another billiard ball. Any amount of energy can be transferred to the ball that is hit. The cue ball may come to a dead stop in one direct, head-on collision where it transferred all its energy to the ball it hit. The cue ball may also glance off at an angle with only some of its energy lost. Since the neutron-as-a-cue-ball has a weight of one, the greatest energy transfers will occur with nuclei-as-balls having a very low atomic weight. The lightest is normal hydrogen, atomic weight 1, and it is the most effective material to stop neutrons. The light elements are all effective. So water, plastic, wood, parafin, and light elements such as lithium and boron are very good neutron absorbers. Still, it may take many inches, or even feet, of these materials to absorb all the neutrons from a nuclear reactor or from a radioactive source that emits neutrons. If you placed a neutron detector on top of the table, and had a neutron source under the table, there would probably be a measurable amount of neutrons coming through the table. If you would like to read some more about this, some references are William Ehmann and Diane Vance, "Radiochemistry and Nuclear Methods of Analysis", pages 173 to 175, Wiley Interscience, New York, 1991 On the Internet, there is a question and answer format website from the Health Physics Society (their information is reviewed for accuracy): http://hps.org/publicinformation/ate/q609.html Radiation Basics-Neutrons: This covers History, Properties, and the Billiard Ball Model. http://hps.org/publicinformation/ate/q1094.html The paragraph on shielding and interactions of neutrons is very good. http://hps.org/publicinformation/ate/q2877.html This explains what happens after the neutron is slowed down; how may be captured by a nucleus, making that nucleus radioactive. To see how neutrons interact differently from electrons or x-rays, go to: http://www.spie.org/web/oer/june/jun00/cover1.html This site has some very good pictures taken using neutrons, and also has a good neutron basics section.
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