Ionizing Radiation Types

Alpha particles

An alpha particle is a tightly bound collection of two protons and two neutrons. It is identical to a helium-4 (⁴He) nucleus. Indeed, its ultimate fate after it loses most of its kinetic energy is to capture two electrons and become a helium atom.

Alpha-emitting radionuclides are generally relatively massive nuclei. Almost all alpha emitters have atomic numbers greater than or equal to that of lead (⁸²Pb). When a nucleus decays by emitting an alpha particle, both its atomic number (number of protons) and its number of neutrons are reduced by two and its atomic mass number is reduced by four. For example, the alpha decay of uranium-238 (²³⁸U) to thorium-234 (²³⁴Th) is represented by:

The left superscript is the atomic mass number (number of protons plus neutrons), the left subscript is the atomic number (number of protons), and the right subscript is the number of neutrons.

Common alpha emitters emit alpha particles with kinetic energies between about 4 and 5.5 MeV. Such alpha particles have a range in air of no more than about 5 cm (see figure 1). Alpha particles with an energy of at least 7.5 MeV are required to penetrate the epidermis (the protective layer of skin, 0.07 mm thick). Alpha emitters generally do not pose an external radiation hazard. They are hazardous only if taken within the body. Because they deposit their energy in a short distance, alpha particles are high linear energy transfer (LET) radiation and have a large radiation weighting factor; typically, w _R=20.

Figure 1. Range-energy radiation of slow alpha particles in air at 15 and 760 m

Beta particles

A beta particle is a highly energetic electron or positron. (A positron is the anti-particle of the electron. It has the same mass and most other properties of an electron except for its charge, which is exactly the same magnitude as that of an electron but is positive.) Beta-emitting radionuclides can be of high or low atomic weight.

Radionuclides that have an excess of protons in comparison with stable nuclides of about the same atomic mass number can decay when a proton in the nucleus converts to a neutron. When this occurs, the nucleus emits a positron and an extremely light, very non-interacting particle called a neutrino. (The neutrino and its anti-particle are of no interest in radiation protection.) When it has given up most of its kinetic energy, the positron ultimately collides with an electron and both are annihilated. The annihilation radiation produced is almost always two 0.511 keV (kiloelectron volt) photons travelling in directions 180 degrees apart. A typical positron decay is represented by:

where the positron is represented by β⁺ and the neutrino by n. Note that the resulting nuclide has the same atomic mass number as the parent nuclide and an atomic (proton) number larger by one and a neutron number lesser by one than those of the original nuclide.

Electron capture competes with positron decay. In electron capture decay, the nucleus absorbs an orbital electron and emits a neutrino. A typical electron capture decay is given by:

Electron capture is always possible when the resulting nucleus has a lower total energy than the initial nucleus. However, positron decay requires that the total energy of the initial atom is greater than that of the resulting atom by more than 1.02 MeV (twice the rest mass energy of the positron).

Similar to positron and electron capture decay, negatron (β^–) decay occurs for nuclei that have an excess of neutrons compared to stable nuclei of about the same atomic mass number. In this case, the nucleus emits a negatron (energetic electron) and an anti-neutrino. A typical negatron decay is represented by:

where the negatron is represented by β^– and the anti-neutrino by`n Here the resulting nucleus gains one neutron at the expense of one proton but again does not change its atomic mass number.

Alpha decay is a two-body reaction, so alpha particles are emitted with discrete kinetic energies. However, beta decay is a three-body reaction, so beta particles are emitted over a spectrum of energies. The maximum energy in the spectrum depends on the decaying radionuclide. The average beta energy in the spectrum is approximately one-third of the maximum energy (see figure 2).

Figure 2. Energy spectrum of negatrons emitted from ³²P

Typical maximum beta energies range from 18.6 keV for tritium (³H) to 1.71 MeV for phosphorus-32 (³²P).

The range of beta particles in air is approximately 3.65 m per MeV of kinetic energy. Beta particles of at least 70 keV energy are required to penetrate the epidermis. Beta particles are low-LET radiation.

Gamma radiation

Gamma radiation is electromagnetic radiation emitted by a nucleus when it undergoes a transition from a higher to a lower energy state. The number of protons and neutrons in the nucleus does not change in such a transition. The nucleus may have been left in the higher energy state following an earlier alpha or beta decay. That is, gamma rays are often emitted immediately following alpha or beta decays. Gamma rays can also result from neutron capture and inelastic scattering of subatomic particles by nuclei. The most energetic gamma rays have been observed in cosmic rays.

Figure 3 is a picture of the decay scheme for cobalt-60 (⁶⁰Co). It shows a cascade of two gamma rays emitted in nickel-60 (⁶⁰Ni) with energies of 1.17 MeV and 1.33 MeV following the beta decay of ⁶⁰Co.

Figure 3. Radioactive decay scheme for ⁶⁰Co

Figure 4 is a picture of the decay scheme for molybdenum-99 (⁹⁹Mo). Note that the resulting technetium-99 (⁹⁹Tc) nucleus has an excited state that lasts for an exceptionally long time (t_½ = 6 h). Such an excited nucleus is called an isomer. Most excited nuclear states have half-lives between a few picoseconds (ps) and 1 microsecond (μs).

Figure 4. Radioactive decay scheme for ⁹⁹Mo

Figure 5 is a picture of the decay scheme for arsenic-74 (⁷⁴As). It illustrates that some radionuclides decay in more than one way.

Figure 5. Radioactive decay scheme for ⁷⁴As, illustrating competing processes of negatron emission, positron emission and electron capture (m₀ is the rest mass of the electron)

While alpha and beta particles have definite ranges in matter, gamma rays are attenuated exponentially (ignoring build-up that results from scattering within a material) as they pass through matter. When build-up can be ignored the attenuation of gamma rays is given by:

where I(x) is the gamma ray intensity as a function of distance x into the material and μ is the mass attenuation coefficient. The mass attenuation coefficient depends on gamma-ray energy and on the material with which the gamma rays are interacting. Mass attenuation coefficient values are tabulated in many references. Figure 6 shows the absorption of gamma rays in matter in conditions of good geometry (build-up can be ignored).

Figure 6. Atenuation of 667 keV gamma rays in Al and Pb under conditions of good geometry (dashed line represents attenuation of a poly-energetic photon beam)

Build-up occurs when a broad gamma-ray beam interacts with matter. The measured intensity at points within the material is increased relative to the expected “good geometry” (narrow beam) value due to gamma rays scattered from the sides of the direct beam into the measuring device. The degree of build-up depends on the geometry of the beam, on the material and on the energy of the gamma rays.

Internal conversion competes with gamma emission when a nucleus transforms from a higher energy state to a lower one. In internal conversion, an inner orbital electron is ejected from the atom instead of the nucleus emitting a gamma ray. The ejected electron is directly ionizing. As outer orbital electrons drop to lower electronic energy levels to fill the vacancy left by the ejected electron, the atom emits x rays. Internal conversion probability relative to gamma emission probability increases with increasing atomic number.

X rays

X rays are electromagnetic radiation and, as such, are identical to gamma rays. The distinction between x rays and gamma rays is their origin. Whereas gamma rays originate in the atomic nucleus, x rays result from electron interactions. Although x rays often have lower energies than gamma rays, this is not a criterion for differentiating them. It is possible to produce x rays with energies much higher than gamma rays resulting from radioactive decay.

Internal conversion, discussed above, is one method of x ray production. In this case, the resulting x rays have discrete energies equal to the difference in the energy levels between which the orbital electrons transit.

Charged particles emit electromagnetic radiation whenever they are accelerated or decelerated. The amount of radiation emitted is inversely proportional to the fourth power of the particle’s mass. As a result, electrons emit much more x radiation than heavier particles such as protons, all other conditions being equal. X-ray systems produce x rays by accelerating electrons across a large electric potential difference of many kV or MV. The electrons are then quickly decelerated in a dense, heat-resistant material, such as tungsten (W).

The x rays emitted from such systems have energies spread over a spectrum ranging from about zero up to the maximum kinetic energy possessed by the electrons before deceleration. Often superimposed on this continuous spectrum are x rays of discrete energy. They are produced when the decelerating electrons ionize the target material. As other orbital electrons move to fill vacancies left after ionization, they emit x rays of discrete energies similar to the way x rays are emitted following internal conversion. They are called characteristic x rays because they are characteristic of the target (anode) material. See figure 7 for a typical x ray spectrum. Figure 8 depicts a typical x ray tube.

Figure 7. X-ray spectrum illustrating the contribution of characteristic x rays produced as electrons fill holes in the K shell of W (the wavelength of x rays is inversely proportional to their energy)

X rays interact with matter the same way gamma rays do, but a simple exponential attenuation equation does not adequately describe the attenuation of x rays with a continuous range of energies (see figure 6). However, as lower energy x rays are removed more rapidly from the beam than higher energy x rays as they pass through material, the description of attenuation approaches an exponential function.

Figure 8. A simplified x-ray tube with a stationary anode and a heated filament

Neutrons

Generally, neutrons are not emitted as a direct result of natural radioactive decay. They are produced during nuclear reactions. Nuclear reactors produce neutrons in the greatest abundance but particle accelerators and special neutron sources, called (α, n) sources, also can yield neutrons.

Nuclear reactors produce neutrons when uranium (U) nuclei in nuclear fuel split, or fission. Indeed, the production of neutrons is essential in maintaining nuclear fission in a reactor.

Particle accelerators produce neutrons by accelerating charged particles, such as protons or electrons, to high energies to bombard stable nuclei in a target. Neutrons are only one of the particles that can result from such nuclear reactions. For example, the following reaction produces neutrons in a cyclotron that is accelerating deuterium ions to bombard a beryllium target:

Alpha emitters mixed with beryllium are portable sources of neutrons. These (α, n) sources produce neutrons via the reaction:

The source of the alpha particles can be such isotopes as polonium-210 (²¹⁰Po),
plutonium-239 (²³⁹Pu) and americium-241 (²⁴¹Am).

Neutrons are generally classified according to their energy as illustrated in table 1. This classification is somewhat arbitrary and may vary in different contexts.

Table 1. Classification of neutrons according to kinetic energy

A number of possible modes of neutron interaction with matter exist, but the two main modes for the purposes of radiation safety are elastic scattering and neutron capture.

Elastic scattering is the means by which higher-energy neutrons are reduced to thermal energies. Higher-energy neutrons interact primarily by elastic scattering and generally do not cause fission or produce radioactive material by neutron capture. It is thermal neutrons that are primarily responsible for the latter types of interaction.

Elastic scattering occurs when a neutron interacts with a nucleus and bounces off with reduced energy. The interacting nucleus takes up the kinetic energy the neutron loses. After being excited in this manner, the nucleus soon gives up this energy as gamma radiation.

When the neutron eventually reaches thermal energies (so-called because the neutron is in thermal equilibrium with its environment), it is easily captured by most nuclei. Neutrons, having no charge, are not repelled by the positively charged nucleus as are protons. When a thermal neutron approaches a nucleus and comes within the range of the strong nuclear force, on the order of a few fm (fm = 10^–15 metres), the nucleus captures the neutron. The result can then be a radioactive nucleus that emits a photon or other particle or, in the case of fissionable nuclei such as ²³⁵U and ²³⁹Pu, the capturing nucleus can fission into two smaller nuclei and more neutrons.

The laws of kinematics indicate that neutrons will reach thermal energies more rapidly if the elastic scattering medium includes a large number of light nuclei. A neutron rebounding off a light nucleus loses a much larger percentage of its kinetic energy than when it bounces off of a heavy nucleus. For this reason, water and hydrogenous materials are the best shielding material to slow down neutrons.

A monoenergetic beam of neutrons will attenuate exponentially in material, obeying an equation similar to that given above for photons. The probability of a neutron interacting with a given nucleus is described in terms of the quantity cross section. Cross section has units of area. The special unit for cross section is the barn (b), defined by:

It is extremely difficult to produce neutrons without accompanying gamma and x rays. It may be generally assumed that if neutrons are present, so are high energy photons.

Ionizing Radiation Sources

Primordial radionuclides

Primordial radionuclides occur in nature because their half-lives are comparable with the age of the earth. Table 2 lists the most important primordial radionuclides.

Table 2. Primordial radionuclides

Uranium and thorium isotopes head a long chain of progeny radioisotopes that, as a result, are also naturally occurring. Figure 9, A-C, illustrates the decay chains for ²³²Th, ²³⁸U and ²³⁵U, respectively. Because alpha decay is common above atomic mass number 205 and an alpha particle’s atomic mass number is 4, there are four distinct decay chains for heavy nuclei. One of these chains (see figure 9, D), that for ²³⁷Np, does not occur in nature. This is because it does not contain a primordial radionuclide (that is, no radionuclide in this chain has a half-life comparable with the age of the earth).

Figure 9. Decay series (Z = atomic number; N = atomic mass number)    

 Note that radon (Rn) isotopes occur in each chain (²¹⁹Rn, ²²⁰Rn and ²²²Rn). Since Rn is a gas, once Rn is produced, it has a chance of escape to the atmosphere from the matrix in which it was formed. However, the half-life of ²¹⁹Rn is much too short to allow significant amounts of it to reach a breathing zone. The relatively short half-life of ²²⁰Rn usually makes it a lesser health hazard concern than ²²²Rn.

Not including Rn, primordial radionuclides external to the body deliver on the average about 0.3 mSv annual effective dose to the human population. The actual annual effective dose varies widely and is determined primarily by the concentration of uranium and thorium in the local soil. In some parts of the world where monazite sands are common, the annual effective dose to a member of the population is as high as about 20 mSv. In                                                                                                                           other places such as on coral atolls and near seashores, the                                                                                                                           value may be as low as 0.03 mSv (see figure 9).

Radon is usually considered separately from other naturally occurring terrestrial radionuclides. It seeps into the air from the soil. Once in the air, Rn further decays to radioactive isotopes of Po, bismuth (Bi) and Pb. These progeny radionuclides attach themselves to dust particles that may be breathed in and trapped in the lungs. Being alpha emitters, they deliver almost all of their radiation energy to the lungs. It is estimated that the average annual lung equivalent dose from such exposure is about 20 mSv. This lung equivalent dose is comparable to a whole body effective dose of about 2 mSv. Clearly, Rn and its progeny radionuclides are the most significant contributors to background radiation effective dose (see figure 9).

Cosmic rays

Cosmic radiation includes energetic particles of extraterrestrial origin that strike the atmosphere of the earth (primarily particles and mostly protons). It also includes secondary particles; mostly photons, neutrons and muons, generated by interactions of primary particles with gases in the atmosphere.

By virtue of these interactions, the atmosphere serves as a shield against cosmic radiation, and the thinner this shield, the greater the effective dose rate. Thus, the cosmic-ray effective dose rate increases with altitude. For example, the dose rate at an altitude of 1,800 metres is about double that at sea level.

Because primary cosmic radiation consists mostly of charged particles, it is influenced by the earth’s magnetic field. People living in higher latitudes receive greater effective doses of cosmic radiation than those closer to the earth’s equator. Variation due to this effect is of the order
of 10%.

Finally, the cosmic-ray effective dose rate varies according to modulation of the sun’s cosmic-ray output. On the average, cosmic rays contribute about 0.3 mSv to background radiation whole-body effective dose.

Cosmogenic radionuclides

Cosmic rays produce cosmogenic radionuclides in the atmosphere. The most prominent of these are tritium (³H), beryllium-7 (⁷Be), carbon-14 (¹⁴C) and sodium-22 (²²Na). They are produced by cosmic rays interacting with atmospheric gases. Cosmogenic radionuclides deliver about 0.01 mSv annual effective dose. Most of this comes from ¹⁴C.

Nuclear fallout

From the 1940s through the 1960s, extensive testing of nuclear weapons above ground occurred. This testing produced large quantities of radioactive materials and distributed them to the environment throughout the world as fallout. Although much of this debris has since decayed to stable isotopes, small amounts that remain will be a source of exposure for many years to come. In addition, nations that continue to occasionally test nuclear weapons in the atmosphere add to the worldwide inventory.

The primary fallout contributors to effective dose currently are strontium-90 (⁹⁰Sr) and caesium-137 (¹³⁷Cs), both of which have half-lives around 30 years. The average annual effective dose from fallout is about 0.05 mSv.

Radioactive material in the body

The deposition of naturally occurring radionuclides in the human body results primarily from the inhalation and ingestion of these materials in air, food and water. Such nuclides include radioisotopes of Pb, Po, Bi, Ra, K (potassium), C, H, U and Th. Of these, ⁴⁰K is the largest contributor. Naturally occurring radionuclides deposited in the body contribute about 0.3 mSv to the annual effective dose.

Machine-produced radiation

The use of x rays in the healing arts is the largest source of exposure to machine-produced radiation. Millions of medical x ray systems are in use around the world. The average exposure to these medical x ray systems is greatly dependent on a population’s access to care. In developed countries, the average annual effective dose from medically prescribed radiation from x rays and radioactive material for diagnosis and therapy is on the order of 1 mSv.

X rays are a by-product of most high-energy physics particle accelerators, especially those that accelerate electrons and positrons. However, appropriate shielding and safety precautions plus the limited population at risk make this source of radiation exposure less significant than the above sources.

Machine-produced radionuclides

Particle accelerators can produce a large variety of radionuclides in varying quantities by way of nuclear reactions. Accelerated particles include protons, deuterons (²H nuclei), alpha particles, charged mesons, heavy ions and so on. Target materials can be made of almost any isotope.

Particle accelerators are virtually the only source for positron-emitting radioisotopes. (Nuclear reactors tend to produce neutron-rich radioisotopes that decay by negatron emission.) They are also being increasingly used to produce short-lived isotopes for medical use, especially for positron-emission tomography (PET).

Technologically enhanced material and consumer products

X rays and radioactive materials appear, wanted and unwanted, in a great number of modern-day operations. Table 3 lists these radiation sources.

Table 3. Sources and estimates of associated population effective doses from technologically enhanced material and consumer products

Source: NCRP 1987.

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