In the radioactive decay of many radioisotopes, gamma rays accompany the beta particles; these radioisotopes are called beta–gamma emitters. Radionuclides that emit only beta particles without any accompanying gamma radiation are called pure beta emitters.
Beta particles emitted by any particular radioisotope have a spread of energies, or a spectral distribution, that extends from near zero to a maximum that is characteristic of that radioisotope, as illustrated in Figure 1. The maximum energies vary from one radioisotope to another, and span a range of energies extending from several kiloelectron volts to several megaelectron volts. The average energy beta from any particular radioisotope is, in most cases, approximately one‐third of the maximum energy.
Beta energy spectrum for 32P.
Source: Based on data from Radiological Toolbox v. 1.0.0.
The depth of penetration, or the range of the beta radiation in matter, increases as the energy of the radiation increases. In air, very low‐energy betas have a range of several centimeters, while high‐energy betas travel about 3 m in air per MeV of energy. Figure 2 shows the relationship between range and energy for beta particles.
The range of the beta radiation in Figure 2 is expressed in units of density thickness. Density thickness is related to linear thickness by
(3)
For example, a sheet of aluminum 1‐mm (0.1‐cm) thick, density = 2.7 g cm−3, has a density thickness of
(4)
Range–energy relationship for beta particles.
From Ref. 2.
The concept of density thickness is useful because different materials are almost equivalent in their ability to stop beta radiation if their density thicknesses are equal. In this context, a sheet of graphite, whose density is 2.2 g cm−3, is equivalent to 0.1 cm Al if its linear thickness t is
(5)
4.2.1 Interaction with Matter
The physical mechanisms by which charged particles, such as alpha and beta particles, transfer their kinetic energy to matter are relatively well understood. The most likely occurrence is a collision between the charged particle and one of the extranuclear orbital electrons in the energy‐absorbing matter with which the radiation interacts. When the orbital electron is struck with sufficient force, it is knocked out of the atom, and the atom is said to be ionized. Ionization is the process of separating an electron from an atom, thereby upsetting the electrical neutrality of the atom and producing a pair of electrically charged particles – the ejected electron, which is the negative ion, and the residual atom, which is a positive ion. Work must be done by the ionizing particle, the alpha or beta particle, to separate the electron from its atom. An alpha particle loses an average of about 35.5 eV of energy per ion pair produced in air or in soft tissue, while a beta particle loses an average of about 34 eV per ionizing collision. These “collisions” are in reality interactions between the electric fields associated with the ionizing particles and the electrons in the absorbing media. Because an alpha particle has a large mass, moves slowly, and is doubly charged, it has a high rate of ionizing collisions, and consequently a high rate of energy loss. This high rate of energy loss explains the very low penetration of alpha radiation into matter. The linear rate of ion production is called the specific ionization. In air, the specific ionization of alpha particles is of the order of 104–105 ion pairs per centimeter 5.
Beta particles have only a single charge and travel at speeds near that of light. As a consequence, the specific ionization of beta particles is relatively low. While an alpha particle produces of the order of 50 000 ion pairs per centimeter in air, a beta particle in air produces only about 100 ion pairs per centimeter. This difference in the specific ionization between the two types of radiation is important in health physics for several reasons. First, it accounts for the higher penetrating power of beta particles than that of alpha radiation. For example, a 1.71‐MeV beta penetrates tissue to a depth of about 8 mm, while a 5.3‐MeV alpha, which has three times the energy, penetrates only about 0.005 cm of tissue. Second, it is utilized in the design of radiation‐measuring instruments sensitive to this difference in order to be able to distinguish between the two types of radiation.
4.3 Bremsstrahlung
Bremsstrahlung (from the German meaning “braking radiation”) is the production of X‐rays when a charged particle undergoes a sudden change in velocity. When a high‐speed electron collides with an atomic nucleus electric field, there is an abrupt change in the particle's velocity, and a fraction of the particle's kinetic energy is converted into X‐rays. This fraction is extremely small for low‐energy betas and for low atomic numbered absorbers, but it increases with increasing energy and with increasing atomic number. For this reason, beta shields are made of materials of low atomic number. In practice, beta‐shielding material of atomic number higher than 13 (Al) is seldom used.
Bremsstrahlung production is of importance in two cases, the first being when it is deliberately used to generate useful X‐rays. In this application, electrons are emitted from the cathode in a specially designed high vacuum diode, Figure 3, and are accelerated across a high voltage (∼100 kV in medical diagnostic X‐ray units). When electrons strike the high atomic numbered tungsten anode, approximately 1/1000th of their kinetic energy is converted into electromagnetic X‐ray energy. The intensity of the X‐rays increases as the beam of accelerated electrons increase and as the accelerating voltage increases; the penetrating power of the X‐rays depends only on the high voltage, and increases with increasing high voltage. To protect against unwanted bremsstrahlung, X‐ray tubes are enclosed in lead shields that have shuttered apertures through which the useful beam escapes.
Second, bremsstrahlung X‐rays are an unwanted side effect of shielding betas or in an instrument or other device in which electrons are accelerated across high voltages, such as an electron microscope, a klystron microwave generator, or an electron beam metallurgical furnace. Since these devices are not intended to be used as an X‐ray source, these unwanted X‐rays can pose a serious hazard if the user or the industrial hygienist is unaware of their existence.
FIGURE 3 Stationary target X‐ray tube. X‐rays are formed via bremsstrahlung in the tungsten target. http://www.osha.gov/SLTC/radiationionizing/introtoionizing/ion3.gif.