Based on a compendium written by Prof. Per Hoff (University of Oslo, 2005)
Uploaded and edited by Prof. Jon Petter Omtvedt (University of Oslo, 2015)
- Stable and Unstable Nuclei
- The Compostion and Size of the Nucleus
- Law of Planck - Energy Units
Stable and Unstable Nuclei
In “standard” chemistry the atomic nucleus is considered as a stable, unchangeable system. We have also learned that many elements have several stable isotopes, i.e. nuclei with the same number of protons, but different masses due to different number of neutrons. There is about 270 different stable atomic nuclei or nuclides. The term nuclide describes a system with a given number of protons and a given number of neutrons, and the term will be frequently used in this chapter. The term radionuclide refers to an unstable nucleus, in other words a radioactive nuclide.
The total number of known nuclides is about 2000. Most of these nuclides are not found in nature, and have been produced and characterised in various nuclear physics or nuclear chemistry experiments. The majority of the nuclides are radioactive, and most of the nuclides have very short half-lives, less than one minute, and have little practical use. A large number of the radionuclides are, however, well- suited for practical purposes. Here we will give an introduction to the properties of radioactive nuclides.
Radioactive radiation was discovered in 1896, when Becquerel found that photographic plates were darkened in the vicinity of uranium salts. He also observed the ionising effect of the radiation in air. He had discovered strong phosphoresce from uranium salts already in 1880, but he did not have the right interpretation of the phenomenon then. In 1898, Schmidt and Marie Curie found similar properties in thorium (Th). In the same year, Marie and Pierre Curie isolated polonium (Po), a previously unknown element. Some months later, radium (Ra) was isolated, and in the coming years, the Curies isolated and characterised several new radioactive elements.
Marie and Pierre Curie additionally measured the heat emission from radium salts (under terrible conditions regarding radiation exposure), and found it to be 100 calories pr. hour and gram. The first speculations about the exploitation of these forces in war were published in St. Louis 4th of October 1903.
In 1899, Rutherford embarked on a more detailed study of the radiation itself and found two components, which he labelled α- and β-radiation. Deflection of the radiation in a magnetic field showed that the first consisted of heavy particles with positive charge, while the other consisted of light particles with negative charge. Additionally, a third type of radiation, γ-radiation, was discovered, more penetrating than the other two and not deflected in a magnetic field. These types of radiation will be discussed in detail later. A volatile radioactive compound evaporating out of uranium and thorium minerals was also discovered. This radioactivity disappeared quite fast, and was named “radium emanation”, later abbreviated to radon.
Several of the concepts envisioned in this period, were quite primitive. An example is the suggestion that the α-particle was a cloud of independent electrons, made by Bragg in 1904.
Already in 1903, Rutherford and Soddy found that the radiation was due to the transformation of one element into another element. This was a remarkably farsighted
theory, at a time when the concepts of nucleus and electrons of an atom were still unknown.
Within a short period of time, so many different radioactive atoms were found that the periodic table was overcrowded. In response, Soddy suggested in 1913 that several of the different types of radioactive atoms in reality were the same element. Failed attempts to chemically separate some of the different types of atoms supported this theory, since the chemical properties were identical. The term isotope thus came into use (isotope = at the same place) in chemistry. Two years earlier, Rutherford found, in his most important investigation, that the atom consisted of a heavy, positively charged nucleus surrounded by light electrons. It was soon realised that the charge of the nucleus and the atomic number were identical. Isotopes with different masses made Rutherford suggest the existence of a heavy, uncharged particle, the neutron, which was discovered by Chadwick in 1932. Irène Curie and Frédéric Joliot produced artificial radioactivity the following year. In this period, the first particle accelerators were built at several institutions. Additionally, radiation therapy of cancer using radioactive compounds became common. In Norway, The Norwegian Radium Hospital was founded in 1932.
Fission was discovered in 1938-39, and the consequences of this will be discussed in section 5.3.7.
The Compostion and Size of the Nucleus
In the atomic model suggested by Rutherford, which will suffice for our purpose, the atom consists of a small, dense nucleus, surrounded by electrons. The nucleus is composed of a number of protons (Z) and a number of neutrons (N). The sum of the numbers of protons and neutrons is the mass number (A = Z + N). The proton and electron have equal amounts of charge with opposite signs. A neutral atom therefore consists of equal numbers of protons and electrons.
The proton and the neutron have almost the same mass and are called nucleons. The proton is 1836 times heavier than the electron. Almost all the mass of an atom is thus in its nucleus.
The nucleus has a very small volume, ranging from a radius of 1.4·10-15 m forH to a radius of 8.7·10-15 m for uranium. The radius of the atom is significantly larger, ranging from 0.4·10-10 m for 1H to 2.6·10-10 m for Cs.
There is also a substantial difference between the binding energies for the nucleons and the electrons in an atom. To release the outermost electron of an atom, energy of typically 3-10 electron volts is required. To release a nucleon from an atomic nucleus, an energy of 3-10 million electron volts is typically required (this unit will be discussed in section 5.1.5).
Since radioactivity is a consequence of changes in the nucleus, the amount of energy released in a radioactive disintegration is correspondingly larger, compared to the amounts of energy released in processes in the electron shell.
A nuclide is characterised by the number of protons and neutrons. To describe a particular nuclide, we use the following notation:
where A = mass number
Z = proton number
N = neutron number
X = the chemical symbol of the particular element
Since the chemical symbol identifies the proton number and N = A – Z, Z and N are usually omitted when describing a nuclide. Thus we simply write AX since this notation gives an unambiguous description of the nuclide.
These notations will be used interchangeably in this compendium, depending on circumstance.
A radionuclide commonly used in nuclear medicine, carbon-14, is therefore:
Law of Planck - Energy Units
According to Planck, all radiation has particle characteristics, and may be described as a flow of energetic, massless particles, so-called photons, as:
E = hν
where h is Planck’s constant and ν is the frequency of the radiation. As we will look into shortly, γ-radiation is electromagnetic radiation released from the nucleus with definite frequencies, which also means that they have definite energies. When we are doing measurements on this radiation, we are always detecting singular photons. When specifying γ-radiation, the energy is stated, while the frequency of the γ-radiation is irrelevant.
In nuclear chemistry, energies are almost exclusively given in the unit electron volt (eV), which is defined as the energy a particle with one unit of charge gets when accelerated in a field of 1 volt. By inserting the unitary charge, we get:
1 eV = 1.6·10-19 J (corresponding to 96.4 kJ/mole)
Frequently, the units of kilo electron volt (keV), mega electron volt (MeV) and giga electron colt (GeV) are used, where:
1 keV = 103 eV
1 MeV = 106 eV
1 GeV = 109 eV