Most "rays", as we have discussed them up to now, are the result of electrons de-exciting and emitting electromagnetic radiation. However, some "rays" occur from the spontaneous breakup of the atomic nucleus, or radioactivity. Only certain elements display radioactive properties. These elements include uranium, plutonium, polarium, and radium. The radioactive elements emit three different types of "rays"-alpha, beta, and gamma rays. Alpha rays are charged positively, beta rays negatively, and gamma rays are neutral. I have been putting the word "ray" in quotations because only one of these three "rays" are actually composed of photons or other massless particles-gamma rays, which are composed of high frequency photons. Alpha rays are composed of a stream of helium nuclei, called alpha particles, and beta rays are composed of a stream of electrons, called beta particles. (Remember, though, that everything has wave properties!) Each of these rays has different penetration abilities. Alpha rays penetrate the least, beta rays penetrate the second most, and gamma rays can pass through almost anything.



Remember from previous lessons that the atomic nucleus is composed of protons and neutrons. In general, the particles that compose the atomic nucleus are called nucleons. Thus a positively charged nucleon is a proton and a neutrally charged nucleon is a neutron. Nucleons have a mass of almost two thousand times that of an electron. However, just as there are different energy levels in electron orbits, there are different energy levels in the nucleus. Gamma rays are the result of a change of energy levels in the nucleus. Alpha particles, which are composed of two protons and two neutrons, result from the actual breakup of the atomic nucleus. An alpha particle breaks off from the nucleus, and then is violently repelled by the positively charged nucleus. Beta particles are ejected from the nucleus when a neutron is changed into a proton, releasing an electron. Beta particles are unique among radiation in that there are no electrons in the nucleus before the neutron transforms. The electron is created during the transformation.


Nucleons themselves are composed of even smaller particles, called quarks. There are six quarks, all named with very traditional names: up, down, top, bottom, charm, and strange. The up and down quarks compose the proton and neutron. The proton is composed of the combination of two ups and one down quark. The neutron is composed of one up and two down quarks. See if you can figure out the charge of the up and down quarks. As of the year two thousand, physicists have not found anything smaller than quarks, and believe them to be the most elementary particles. That is, quarks themselves are not made up of smaller particles.



The periodic table of the elements is arranged, from left to right, by the number of protons in the nucleus of the element. The atomic number of an atom is the number of protons in its nucleus. The mass number of an atom is the mass of the atom. Since nucleons are so much heavier than electrons, the mass number of an atom is the number of nucleons the atom has. So, the number of neutrons can be determined by subtracting the atomic number from the mass number. Isotopes are atoms that have the same atomic number but different mass numbers. That is, atoms with the same number of protons but different numbers of neutrons. Some isotopes are stable. Over two thousand, though, are radioactive, and these are the ones we are interested in.



The force that keeps protons from repelling each other in the nucleus is called the strong force. It is an attractive force between nucleons at short distances. Very short distances. In fact, nucleons farther apart than 10-15 meters feel almost no attractive strong force. The electric force is still large at that distance, so protons that far apart repel each other. The fact that the strong force drops off dramatically with distance means that atoms with large nuclei are less stable than atoms with small nuclei. Neutrons are also important factors in the stability of an atom. One proton and one neutron are bonded more firmly by the strong force than two protons are two neutrons. In large atoms, however, having an equal number of protons and neutrons creates a large enough electric force to blow the atom apart. So large atoms have more neutrons than protons. This makes a larger nucleus, which decreases the strong force. Any such atom with more than eighty two protons is unstable, and undergoes alpha and beta decay.


An atom emitting an alpha or beta particle becomes a new element. This is called transmutation. Consider a thorium atom, with a mass number of 230. It is unstable, and emits an alpha particle. Remember that an alpha particle has two protons and two neutrons, and therefore a mass number of four. The thorium atom, having lost the two protons and neutrons, has its atomic number decreased by two and its mass number decreased by four, and becomes radium, an element with a mass number of 226. This process of transmutation will eventually cause the thorium atom to become an isotope of lead, which is stable. This process takes a tremendously long amount of time, though.



The possible uses of radioactive isotopes in research are endless. For example, scientists can place a radioactive isotope in a person's food, and then track the isotope throughout the person's body, thus learning the structure of the human digestive system. Other uses include studying other plant and animal systems and processes, measuring the performance of different car oils, even finding leaks in pipes!