4. Alpha radiation

In 1903, Ernest Rutherford (a New Zealander who worked in Cambridge, England most of his life) performed a simple and elegant experiment showing that the α-particle is the nucleus of the helium atom. Rutherford positioned one glass tube inside a second glass tube. The inner tube contained a radioactive source that emitted α-particles. The outer tube contained a vacuum and at each end there was an electrode. The α-particles passed through a thin window, picking up two electrons on the way, and entered the outer tube as a gas. When Rutherford turned on the high voltage between the electrodes, the tube emitted light at very specific wavelengths (specific colors). He compared wavelengths of this light with the wavelengths of light produced by a similar tube that he had filled with helium gas. The colors of the light were identical. Rutherford concluded that α-particle is simply the nucleus of a helium atom and that when the α-particles reach the outermost tube they have picked up two electrons to become helium atoms.

5. Beta and gamma radiation

Experiments have shown that the β-particle is a fast moving electron, whereas γ-radiation is an electromagnetic wave. Other examples of electromagnetic radiation are ultraviolet (UV), visible light, infrared and radio waves. Electromagnetic radiation is characterized by its wavelength or frequency. The wavelength is the distance from one wave peak to the next and the frequency is the number of waves passing a given point per second. Through quantum mechanics it is known that particles can be described as waves and vice versa. Thus, γ-rays and other electromagnetic radiation are sometimes described as particles and are called photons.

6. What is an isotope?

In several places throughout this book isotopes are mentioned. Some isotopes are unstable, and therefore radioactive, while others are stable, and thus nonradioactive. What is an isotope? An element can exist in several versions that are chemically equivalent but have different atomic weights. The atomic weight of an element can be changed by altering the number of neutrons in the nucleus.

This is illustrated above for the most common of the elements, hydrogen.

The nucleus of an atom consists of protons and neutrons (called nucleons). The number of protons determines the element and the number of nucleons determines the atomic weight. Isotopes are atoms with the same number of protons, but with different numbers of neutrons.

Isotopes are written using the symbol for the element, such as H for hydrogen, O for oxygen, and U for uranium. Furthermore, the nucleon number is used to separate the isotopes. For example the three hydrogen isotopes are written as H-1, H-2 and H-3 (you will often see the isotopes written as ¹H, ²H and ³H).

Since the hydrogen isotopes are so well known, they have attained their own names. H-2 is called deuterium and H-3 is called tritium. When tritium disintegrates it emits β-particle with an average energy of only 5.68 keV and maximum energy of 18.6 keV (1 ke V equals 1000 electron volts of energy).

In nature, 99.985% of hydrogen is the H-1 isotope. In ordinary water, only one out of 7000 atoms is deuterium. Due to nuclear processes in the atmosphere there are small amounts of tritium. Tritium is widely used in research.

Potassium is another example of an element that has radioactive isotopes. Potassium consists of 93.10% K-39, 6.88% K-41 and 0.0118% of the radioactive isotope K-40. The latter isotope is present because it has a very long half-life of 1.27 billion years. The Earth's crust contains a lot of potassium. In spite of the small fraction of K-40, the radiation from this isotope is quite important. All living organisms contain some radioactive potassium. For example a human being contains, on average, about 60 Bq/kg body weight of K-40.