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The term ionizing radiation refers to those subatomic particles and photons whose energy is sufficient to cause ionization in the matter with which they interact. The ionization process consists of removing an electron from an initially neutral atom or molecule. For many materials, the minimum energy required for this process is about 10 electron volts (eV), and this can be taken as the lower limit of the range of ionizing radiation energies. The more common types of ionizing radiation are characterized by particle or quantum energies measured in thousands or millions of electron volts (keV or MeV, respectively). At the upper end of the energy scale, the present discussion will be limited to those radiations with quantum energies less than about 20 MeV. This energy range covers the common types of ionizing radiation encountered in radioactive decay, fission and fusion systems and the medical and industrial applications of radioisotopes. It excludes the regime of high-energy particle physics in which quantum energies can reach billions or trillions of electron volts. In this field of research, measurements tend to employ much more massive and specialized detectors than those in common use for the lower-energy radiations.
Radiation interactions in matter
For the purposes of this discussion, it is convenient to divide the various types of ionizing radiation into two major categories: those that carry an electric charge and those that do not. In the first group are the radiations that are normally viewed as individual subatomic charged particles. Such radiation appears, for example, as the alpha particles that are spontaneously emitted in the decay of certain unstable heavy nuclei. These alpha particles consist of two protons and two neutrons and carry a positive electrical charge of two units. Another example is the beta-minus radiation also emitted in the decay of some radioactive nuclei. In this case, each nuclear decay produces a fast electron that carries a negative charge of one unit. In contrast, there are other types of ionizing radiation that carry no electrical charge. Common examples are gamma rays, which can be represented as high-frequency electromagnetic photons, and neutrons, which are classically pictured as subatomic particles carrying no electrical charge. In the discussions below, the term quantum will generally be used to represent a single particle or photon, regardless of its type.
Only charged radiations interact continuously with matter, and they are therefore the only types of radiation that are directly detectable in the devices described here. In contrast, uncharged quanta must first undergo a major interaction that transforms all or part of their energy into secondary charged radiations. Properties of the original uncharged radiations can then be inferred by studying the charged particles that are produced. These major interactions occur only rarely, so it is not unusual for an uncharged radiation to travel distances of many centimetres through solid materials before such an interaction occurs. Instruments that are designed for the efficient detection of these uncharged quanta therefore tend to have relatively large thicknesses to increase the probability of observing the results of such an interaction within the detector volume.
Interactions of heavy charged particles
The term heavy charged particle refers to those energetic particles whose mass is one atomic mass unit or greater. This category includes alpha particles, together with protons, deuterons, fission fragments, and other energetic heavy particles often produced in accelerators. These particles carry at least one electronic charge, and they interact with matter primarily through the Coulomb force that exists between the positive charge on the particle and the negative charge on electrons that are part of the absorber material. In this case, the force is an attractive one between the two opposite charges. As a charged particle passes near an electron in the absorber, it transfers a small fraction of its momentum to the electron. As a result, the charged particle slows down slightly, and the electron (which originally was nearly at rest) picks up some of its kinetic energy. At any given time, the charged particle is simultaneously interacting with many electrons in the absorber material, and the net result of all the Coulomb forces acts like a viscous drag on the particle. From the instant it enters the absorber, the particle slows down continuously until it is brought to a stop. Because the charged particle is thousands of times more massive than the electrons with which it is interacting, it is deflected relatively little from a straight-line path as it comes to rest. The time that elapses before the particle is stopped ranges from a few picoseconds (1 × 10−12 second) in solids or liquids to a few nanoseconds (1 × 10−9 second) in gases. These times are short enough that the stopping time can be considered to be instantaneous for many purposes, and this approximation is assumed in the following sections that describe the response of radiation detectors.
Several characteristics of the particle-deceleration process are important in understanding the behaviour of radiation detectors. First, the average distance traveled by the particle before it stops is called its mean range. For a given material, the mean range increases with increasing initial kinetic energy of the charged particle. Typical values for charged particles with initial energies of a few MeV are tens or hundreds of micrometres in solids or liquids and a few centimetres in gases at ordinary temperature and pressure. A second property is the specific energy loss at a given point along the particle track (path). This quantity measures the differential energy deposited per unit pathlength (dE/dx) in the material; it is also a function of the particle energy. In general, as the particle slows down and loses energy, the dE/dx value tends to increase. Thus, the density with which energy is being deposited in the absorber along the particle’s track tends to increase as it slows down. The average dE/dx value for charged particles is relatively large because of their short range, and they are often referred to as high dE/dx radiations.
Interactions of fast electrons
Energetic electrons (such as beta-minus particles), since they carry an electric charge, also interact with electrons in the absorber material through the Coulomb force. In this case, the force is a repulsive rather than an attractive one, but the net results are similar to those observed for heavy charged particles. The fast electron experiences the cumulative effect of many simultaneous Coulomb forces, and undergoes a continuous deceleration until it is stopped. As compared with a heavy charged particle, the distance traveled by the fast electron is many times greater for an equivalent initial energy. For example, a beta particle with an initial energy of 1 MeV travels one or two millimetres in typical solids and several metres in gases at standard conditions. Also, since a fast electron has a much smaller mass than a heavy charged particle, it is much more easily deflected along its path. A typical fast-electron track deviates considerably from a straight line, and deflections through large angles are not uncommon. Because a fast electron will travel perhaps 100 times as far in a given material as a heavy charged particle with the same initial energy, its energy is much less densely deposited along its track. For this reason, fast electrons are often referred to as low dE/dx radiations.
There is one other significant difference in the energy loss of fast electrons as compared with that of heavy charged particles. While undergoing large-angle deflections, fast electrons can radiate part of their energy in the form of electromagnetic radiation known as bremsstrahlung, or braking radiation. This form of radiation normally falls within the X-ray region of the spectrum. The fraction of the fast-electron energy lost in the form of bremsstrahlung is less than 1 percent for low-energy electrons in light materials but becomes a much larger fraction for high-energy electrons in materials with high atomic numbers.