3. What is Radioactivity?
1. Ionizing radiation. 2. Radioactive elements. 3. The nature of radioactivity. 4. Alpha radiation. 5. Beta and Gamma radiation. 6. What is an isotope? 7. The radiation series. 8. The energy of radiation. 8.1 Description of radioactive sources. 8.2. Alpha (α) radiation. 8.3. Beta (β) radiation. 8.4. Gamma (γ) radiation. 9. The penetration of radiation. 10. Ionization. Excitation.
1. Ionizing radiation
Living organisms as well as non-living objects of the environment are affected by set of various factors of the physical nature including radiation (visible light, ultraviolet, infra-red, magnetic fields, radio-waves of various ranges). Ionizing radiation of natural radioactive elements and isotopes as well as space radiation also acts.
The common name for both radiation from x-ray machines and radioactive sources is ionizing radiation. The name indicates that the radiation has sufficient energy to ionize atoms and molecules. Ionization takes place when an electron is removed from its position in the atom or molecule. Since a molecule usually has no net charge to begin with, the loss of a negative electron leaves behind a positive ion. The electron can then end up on another molecule which then becomes a negative ion. The creation of positive and negative ions in matter is the signature of radiation, allowing us to detect and categorize it. Ionizing radiation is distinct from low energy radiations which include ultraviolet, visible, infrared, microwaves and radio waves that produce effects, for the most part, of a different nature.
2. Radioactive Elements
The atomic structure of most elements contains a nucleus that is stable. Under normal conditions, these elements remain unchanged indefinitely. They are not radioactive. Radioactive elements, in contrast, contain a nucleus that is unstable. The unstable nucleus is actually in an excited state that can not be sustained indefinitely; it must relax, or decay, to a more stable configuration. Decay occurs spontaneously and transforms the nucleus from a high energy configuration to one that is lower in energy. This can only happen if the nucleus gives off energy. The energy emitted by the relaxing nucleus is radiation. All radioactive elements have unstable nuclei; that is what makes them radioactive.
Thus, the radioactivity is considered as a spontaneous or artificial transformation of nucleus of atom of unstable isotopes of a chemical element from the basic state in other isotope of this or other element that is accompanied by emitting of energy of elementary particles or nucleus.
3. The Nature of Radiation
The energy emitted by an unstable nucleus comes packaged in very specific forms. In the years that followed the discovery of radioactivity, determining the kind of radiation emitted from radioactive compounds was of great interest. It was found that these radiations consisted of three types called: alpha (α), beta (β) and gamma (γ) radiations after the first three letters in the Greek alphabet (see Figure 3.1).
The nuclear emission transforms the element into either a new element or a different isotope of the same element. A given radioactive nucleus does this just once. The process is called decay or disintegration.
The law of radioactive decay asserts that the identical portion of available nucleus disintegrates per unit time. The measure of the number of disintegrations per unit time is called the decay rate. The decay rate is proportional to the number of radioactive atoms present.
FIG 3-1.
A radioactive atom possesses an unstable nucleus. This means that radioactive atoms will emit radiation sooner or later and convert into a more stable state. The types of radiation that may be emitted are called alpha (α), beta (β) and gamma (γ) radiation.
The evidence for the three types of radiation comes from an experiment in which the radiation from radioactive compounds was passed through a magnetic field. γ-rays passed through the field without disturbance, whereas the two other types were deflected from a straight line. Because it was known at that time that charged particles are deflected when they pass through a magnetic field, the conclusion was evident; γ-rays have no charge while α- and β-radiations consist of charged particles. The α-particles, deflected in one direction are positive whereas the β-particles, deflected in the opposite direction, are negative.