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Radioactive decay occurs when an unstable atomic nucleus loses energy by emitting particles or electromagnetic waves. These emitted particles or electromagnetic waves are called radiation. Some elements are far more radioactive than others. For example, the element radium is more radioactive than uranium. Also, isotopes tend to be less stable because they contain a larger number of neutrons than non-isotopes of the same element. Radioactive isotopes are called radioisotopes.
Types of Decay
The decay products were given the alphabetic names alpha, beta, and gamma, ordered by their ability to penetrate matter. Alpha decay is seen only in heavier elements greater than atomic number 52, tellurium. The other two types of decay are seen in all of the elements. These three modes can be broken down even further.
Table of modes of radioactive decay
Radionuclides can undergo a number of different reactions, summarized here. A nucleus with mass number A and atomic number Z is represented as (A, Z). The column "Daughter nucleus" indicates the difference between the new nucleus and the original nucleus. Thus, (A − 1, Z) means that the mass number is one less than before, but the atomic number is the same as before.
Alpha, Beta, Gamma Composition
Alpha particles carry a positive charge, beta particles carry a negative charge, and gamma rays are neutral. Alpha particles are more massive than beta particles. By passing alpha particles through a very thin glass window and trapping them in a discharge tube, researchers found that alpha particles are equivalent to helium nuclei. Other experiments showed the similarity between classical beta radiation and cathode rays; they are both streams of electrons. Likewise, gamma radiation and X-rays were found to be similar high-energy electromagnetic radiation.
The three types of radiation have different levels of penetrating power. Penetrating power refers to the energy with which the radiation particles are ejected from the atom. The higher the energy, the more the radioactive decay will penetrate a substance.
Penetration of the modes of decay
Alpha particles can be completely stopped by a sheet of paper. Beta particles can be stopped by aluminum shielding. Gamma rays can only be reduced by much more substantial mass, such as a very thick layer of lead.
An alpha particle ($\alpha$) is made up of two protons and two neutrons bound together. This type of radiation has a positive charge. An alpha particle is sometimes represented using the chemical symbol He2+, because it has the same structure as a helium atom missing its two electrons—hence the overall charge of +2. Their massive size means alpha particles have very low penetration power. Penetration power describes how easily the particles can pass through another material. Since alpha particles have a low penetration power, the outside layer of the human skin, for example, can block these particles.
Alpha decay occurs because the nucleus has too many protons. A nucleus with too many protons causes repulsion between these like charges. To reduce this repulsion, the nucleus emits an α particle. Examples of this can be seen in the decay of americium (Am) to neptunium (Np).
In radioactive nuclei with too many neutrons, a neutron may be converted into a proton, an electron, and another particle, called a neutrino. The high energy electrons that are released in this way are called beta particle. Beta particles ($\beta$) have a higher penetration power than alpha particles; they are able to pass through thicker materials such as paper.
During beta decay, the number of neutrons in the atom decreases by one, and the number of protons increases by one. Since the number of protons before and after the decay is diﬀerent, the atom has changed into a diﬀerent element.
When particles inside the nucleus collide during radioactive decay, energy is released. This energy can leave the nucleus in the form of waves of electromagnetic energy called gamma rays. Gamma radiation ($\gamma$) is part of the electromagnetic spectrum, just like visible light. However, unlike visible light, humans cannot see gamma rays because they are at a higher frequency and a higher energy. Gamma radiation has no mass or charge. This type of radiation is able to penetrate most common substances, including metals. The only substances that can absorb this radiation are thick lead and concrete.
Gamma decay occurs if the nucleus' energy level is too high. Since gamma rays are part of the electromagnetic spectrum, they can be thought of as waves or particles. In gamma decay, a ray or a particle called a photon is released, and the atomic number and atomic mass remain unchanged.