Radiation

As already mentioned, an unstable nucleus will eventually become more stable by emitting particulate and/or electromagnetic radiation. The type of radiation emitted will depend on the type of instability. If a nucleus has too many neutrons for the number of protons (i.e. it is below the line of stability) it will tend to become more stable by essentially converting a neutron to a proton and emitting an electron. Electrons emitted from the nucleus are called beta particles (β-radiation). Typically, additional electromagnetic energy will also be emitted. Electromagnetic energy from the nucleus is called gamma radiation (γ-radiation).

If a nucleus has a large number of neutrons and protons, it is very heavy and will be located at the upper right end of the nuclide chart. If it has too many neutrons and protons it will be unstable, radioactive, and tend to become more stable by emitting a particle consisting of two neutrons and two protons. This particle is called an alpha particle (α-radiation).

It is also possible that some radioactive materials emit neutrons. If α-emitting radionuclides are mixed with material of light elements (e.g. beryllium) the nuclear reactions of the α-particles with light nuclei lead to the emission of neutrons (neutron radiation). Fission of ²³⁵U will also lead to the emission of neutrons.

There are other processes in which nuclei can become unstable, and other processes by which they reach stability, but for our practical purpose the emission of alpha, beta, gamma and neutron radiation are the most important processes.

2.3.1. Ionization

There are many other types of radiation energy to which humans are exposed. These include light, heat, radio and TV waves, ultra-violet, infrared, and microwave radiation. The 14

major distinction between these and the radiation from the nuclei of atoms is that only the latter can cause ionization.

Ionization of an atom occurs when an electron is removed from a neutral atom thereby leaving a positively charged ion (Fig. 5). This process of ionization carries advantages and disadvantages. It is advantageous in that it enables the radiation to be detected, and it also enables the radiation to be shielded. However, the disadvantage is that the ionization of atoms in the human body causes harmful biological effects.

charge -1 ejected electron charge +1

ionized atom

path of alpha, beta, gamma or X-radiation

FIG. 5. The ionization process 2.3.2. Alpha radiation

An alpha particle is actually the nucleus of a helium atom because it has two protons.

Due to the fact that it is a heavy particle and that it has a charge of +2, an alpha particle will give up its energy within a very short distance mostly by causing ionization. The implication of this is that alpha radiation is not very penetrating. This in turn means that it can be easily shielded. In fact most alpha particles cannot penetrate the dead layer of cells on the skin surface and therefore do not present any hazard while the alpha emitting radionuclide is external to the body. However, if the material becomes ingested or inhaled into the body then the alpha particles can ionize atoms in living cells. The rate of ionization in this case is very high and significant cell damage can occur. Another implication of the lack of penetrating power is that it makes alpha radiation difficult to detect. Special instruments with very thin windows or even without windows are required. In summary then (see Fig. 6), alpha radiation:

 Is not very penetrating, and can be shielded even by a sheet of paper;

 Is a significant internal hazard;

 Is detected only by special instruments.

2.3.3. Beta radiation

Beta particles, because they are electrons, are very much smaller and lighter than alpha particles. They are subsequently more penetrating but will travel in zigzag paths through materials. Their rate of ionization is much less than that of alpha particles. The penetration range of beta particles depends on their energy and the density of the material they are passing through. A beta particle of typical energy will not penetrate a thin sheet of metal, and will only travel about 10 mm in tissue. Hence, beta-emitting radionuclides are a hazard to skin and eyes as well as a hazard if they are incorporated into the body. Ease of detection of beta radiation 15

depends on the energy. However, all but the lowest energies can be detected fairly easily. In summary then (see Fig. 6), beta radiation:

 Is more penetrating than alpha radiation, but can be shielded by a sheet of metal, and is an external hazard to the skin and eyes;

 Is an internal hazard;

 Its detection is dependent on the energy of the radiation.

FIG. 6. The penetrating power of external radiation: alpha, beta and gamma 2.3.4. Gamma radiation

Gamma radiation is electromagnetic radiation similar to radar, radio and TV, microwave, light, ultra-violet, and infrared radiation. However, gamma radiation has higher energy, higher frequency and shorter wavelength than these similar forms of radiation. It also causes ionization whereas the others do not ionize at all. X-rays can be generally regarded as lower energy gamma rays that are machine produced instead of coming from a radioactive atom.

Gamma radiation is very penetrating depending on the energy of the radiation. High density material, or a large bulk of material, is required to shield gamma radiation.

Consequently, it is relatively easy for gamma radiation to completely penetrate the body. In summary then (see Fig. 6), gamma radiation:

 Is very penetrating, but can be shielded by dense materials such as lead and steel;

 Is an external and an internal hazard;

 Is easily detected at very low levels.

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2.3.5. Neutron radiation

In addition to the existence of neutrons in the nucleus, it is possible to have free neutrons as a form of radiation. Neutrons are unique among the types of radiation, in that they only have interactions with other nuclei (nuclear reactions). These interactions can be:

 Elastic scattering: For example, if the neutron hits a hydrogen atom (even in water), the moving neutron and the proton at rest behave like billiard balls. As the neutron and the proton have about the same mass, the neutron will be stopped completely in a central collision and the proton will carry away the full kinetic energy of the neutron. In non-central collisions the neutron and the proton share the kinetic energy. The neutron is slowed down.

 Inelastic scattering: A part of the kinetic energy of the neutron is absorbed by the target nucleus that in turn emits γ radiation.

 Neutron capture: The neutron is captured by a nucleus forming a new isotope of the nucleus with which it interacted. This is called neutron activation, because the resulting nuclei are radioactive and emit a characteristic γ radiation.

 Other types of nuclear reactions are possible, including fission.

 Neutrons are very penetrating and the ease with which they can be shielded and detected depends heavily on their energy. They can cause significant cell damage by indirect ionization and other processes as they pass through the body.

In summary then, neutron radiation:

 Is very penetrating, but can be shielded by hydrogenous material for fast neutrons, and by cadmium or boron for slow thermal neutrons;

 Is an external and internal hazard;

 Is detected only with special instruments.

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