RADIOACTIVITY

a. It was indicated earlier that the negative electrons are bound to the atom by the attraction of the positive protons. Within the nucleus, however, only positive charges exist and these charges repel each other. Therefore, other forces strong enough to overcome the repelling forces of the protons must exist between nucleons. Since these forces are of very short range, acting only between nucleons close to one another, they are called strong nuclear forces. It is possible in isotopes of heavier elements for the electrostatic forces between protons to overcome these strong nuclear forces. If this is the case, part of the nucleus actually may break off and escape. In other cases, rearrangements may take place, which lead to more stable configurations within the nucleus (see para e below). Nuclei in which this happens are said to be unstable or radioactive. All isotopes with atomic number Z greater than 83 are naturally radioactive and many more isotopes can be made artificially radioactive by adding neutrons, protons, or groups of these to destabilize the normally stable configuration.

b. Natural radioactivity occurs in three basic ways. Many unstable nuclei emit a particle composed of two protons and two neutrons called an alpha particle (symbol a). This configuration is also the nucleus of a helium (He) atom and is very stable. To see what results from an emission, consider the isotope of uranium with 146 neutrons and 92 protons (U-238). This is a naturally-occurring isotope, but is radioactive and an alpha emitter (Figure 1-6). Since emission removes the two protons, the uranium changes to an entirely new substance, thorium, in the form of the thorium isotope with 90 protons and 234 nucleons (protons + neutrons).



Figure 1-6. Alpha emission from U-238.

c. It often happens that unstable nuclei emit high-speed electrons called beta particles (symbol β). They are called particles, not electrons, to indicate that they are electrons that originate in the nucleus instead of electrons that originate in atomic orbits outside the nucleus. The source of electrons in this form of radioactivity is interesting since, as was previously pointed out, there are no electrons in a nucleus. This paradox is explained when it is found that a neutron can transform into a proton and an electron. When this happens, the electron (β particle) is ejected from the nucleus while the proton is left behind. Figure 1-7 pictures a hypothetical example of emission showing the decay of a neutron into a proton. The proton remains within the nucleus and an electron escapes.



Figure 1-7. Beta emission.

d. The thorium isotope that was formed in the example on emission is also radioactive, but it is an emitter causing Th-234 to change to the element protactinium (Figure 1-8). Note that the protactinium mass number (A = 234) does not change in decay, but that the atomic number Z does increase from 90 to 91 because a proton has been gained. This gain comes about, as explained earlier, when the neutron breaks down, ejecting an electron and retaining the proton.



Figure 1-8. Beta emission from Th-234.

e. Previously it was said that an unstable configuration of neutrons and protons in a nucleus is sometimes made more stable by a rearrangement of the components with no particles emitted. Such changes are accompanied by radioactivity in the form of energy. With different configurations of the nucleus, the components are bound with different energies and so, upon rearrangement, energy is often released in the form of electromagnetic waves called gamma rays (symbol ). These are like light waves except that their frequencies are much higher and they are not visible to the human eye. It is important to note that no change in atomic structure accompanies emission (A and Z numbers remain the same) and the only effect upon the nucleus involved is to leave it with less energy and usually with less tendency for further decay.

f. Table 1-1 presents a summary of the effect of radioactive emissions on the structure of an atom.



Table 1-1. Radioactive Summary.

g. Alpha, beta, and gamma radiations are the primary emissions resulting from natural radioactive decay. There are many other types of emissions that occur from artificially-produced radioactive material. These artificial nuclei may emit neutrons, positive electrons, and other emissions not important to this discussion. The neutrons are especially important in applications of nuclear energy for they can cause nuclear fission, a reaction fundamental to nuclear explosions and the generation of nuclear power.

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