As an unstable nucleus attempts to reach stability, it emits various types of nuclear radiation. Early nuclear scientists, like Marie Curie (1867–1934) and Rutherford, called the first three forms of nuclear radiation alpha rays, beta rays, and gammarays. They assigned the letters of the Greek alphabet to these phenomena in the order of their discovery. Curie prepared a diagram in her 1903 doctoral dissertation that described how alpha rays bent one way in a magnetic field, beta rays bent the opposite way in the same magnetic field, while gamma rays appeared totally unaffected by the presence of the magnetic field (see Figure 4.10). At the time, the precise nature of these interesting rays emanating from naturally radioactive substances, such as radium and polonium, was not clearly understood.
Before beginning our discussion of alpha decay, beta decay, and gamma decay, a few comments about the nature of radiation and radioactivity will prevent many common misconceptions. Radiation is a general term used by scientists to describe the propagation of waves and particles through the vacuum of space. The term includes both electromagnetic radiation and nuclear particle radiation. As shown in Figure 4.11, electromagnetic radiation has a broad continuous spectrum that embraces radio waves, microwaves, infrared radiation, visible radiation (light), ultraviolet radia X-rays, and gamma rays. Photons of electromagnetic radiation travel at the speed of light. With the shortest wavelength (λ) and highest frequency (ν),the gamma ray photon is the most energetic. By way of comparison, radio wave photons have energies between 10−10 and 10−3 electron volt (eV), visible light photons between 1.5 and 3.0 eV, and gamma ray photons between approximately 1 and 10 million electron volts (MeV).
One of the most important defining characteristics of radiation is energy. So scientists need a convenient measure of energy to make comparisons easier. For ionizing radiation, a common unit is the electron volt (eV)—the kinetic energy that a free electron acquires when it accelerates across an electric potential difference of one volt. The passage of ionizing radiation through matter causes the ejection of outer (bound) electrons from their parent atoms. An ejected (or free) electron speeds off with its negative charge and leaves behind the parent atom with a positive electric charge. We call the two charged entities an ion pair. On average, it takes about 25 eV to produce an ion pair in water. Ionizing radiation can damage or harm many material substances, including living tissue, by rapidly creating a large number of ion pairs in a small volume of matter.
The term nuclear radiation refers to the particles and electromagnetic radiations emitted from atomic nuclei as a result of nuclear reaction processes, including radioactive decay and fission. All nuclear radiations of interest here are ionizing radiations. The most common forms of nuclear radiation encountered in nuclear technology are alpha particles, beta particles, gamma rays, and neutrons. However, nuclear radiation may also appear in other forms, such as energetic protons from accelerator-induced reactions or the spontaneous fission of heavy nuclei. When discussing the biological effects of nuclear radiation, we include X-rays in the list of ionizing radiations. Although energetic photons, X-rays are not a form of nuclear radiation because they do not originate from processes within the atomic nucleus.
Radioactivity is the spontaneous decay or disintegration of an unstable nucleus. The emission of nuclear radiation, such as alpha particles, beta particles, gamma rays, or neutrons, usually accompanies the decay process. How do we measure how radioactive a substance is? There are two units of radioactivity: the curie (Ci) and the becquerel (Bq). The curie is the traditional unit used by scientists to describe the intensity of radioactivity in a sample of material. One curie of radioactivity is equal to the disintegration or transformation of 37 billion (37 × 109) nuclei per second. The unit is named for Marie and Pierre Curie, who discovered radium (Ra) in 1898. There is a good historic reason for this unusual unit. The curie corresponds to the approximate radioactivity level of one gram of pure radium—a convenient, naturally occurring radioactive standard that arose in the early days of nuclear science. The becquerel is the SI unit of radioactivity. One becquerel corresponds to the disintegration (or spontaneous nuclear transformation) of one atom per second. This unit was named for Henri Becquerel (1852–1908), who discovered radioactivity in