Radiation: Interactions With Matter

Type of physical science: Nuclear physics

Field of study: Nuclear techniques

Charged and neutral particles and electromagnetic radiation penetrate matter and alter it in various ways. Knowledge of the basic physics of radiation interaction and energy transfer gives insight into the physical, chemical, and biological effects of radiation, and provides the basis for radiation detection, measurement, and control.

Overview

The primary effects of radiation interaction with matter are displacement of electrons (ionization) and atoms from lattice sites, excitation of both atoms and electrons without displacement, and the transmutation of nuclei. Energetic charged particles, such as electrons, positrons, protons, and α particles, always produce primary ionization and, depending on energetic conditions, usually produce primarily atomic displacements. Neutrons produce ionization only as a secondary process (the secondary electrons and ions produced in collision help ionization); the primary result is atomic displacement. Photons (electromagnetic radiation such as X rays and γ rays) produce only ionization or excitation as the primary effect; atomic displacement is a secondary result. Nuclear transmutation can, in principle, be produced by any kind of radiation, but it occurs to an appreciable extent because of neutrons in materials with high cross-section probability.

The interaction of electromagnetic radiation (for example, photons such as X rays or γ rays) with matter is more complex than the interaction of charged particles. When a photon interacts with matter, it might be absorbed and disappear or it might be scattered with or without loss of energy. As the energy of a photon is increased, the processes known as classical scattering, the photoelectric effect, the Compton effect, pair production, and photonuclear reactions can occur. The relative probability of each process is described by the "absorption/attenuation coefficient" or the cross section. As photons pass through a material, the radiation intensity and/or energy decreases with distance x. The decrease is referred to as "attenuation." Here, the intensity, the number of photons arriving at a given point per unit area across the beam per unit time, is measured in "good geometry" (by placing the detector far from the absorber with a narrow slit in front--that is, collimating it). Thus, attenuation is overall reduction in intensity including all absorption (radiation energy deposition in the material) and scattering (deflection of radiation caused by interaction with matter) processes. If the detector is not collimated and is placed close to the absorber, it will respond to a large percentage of the scattered photons as well as those that have escaped all interactions. This arrangement ("poor geometry") serves to measure energy transfer or the amount of energy that is actually deposited locally in the absorber, which is called absorption. Attenuation and absorption have practical applications; absorption coefficients are considered for shielding problems and attenuation coefficients are considered for narrow-beam conditions.

The number of photons dN, removed from the beam during the interaction with matter, is proportional to the thickness dx, and the number of incident photons No with a proportionality constant μ. Thus, dN = -μ N_odx. Here, μ is called the linear attenuation or absorption coefficient, depending on the detector geometry, and is measured in units of cm^-1. One can define other attenuation/absorption coefficients from the linear coefficient (μ). Mass coefficient μ_m = μ/ρ is measured in cm²gram^-1. Atomic coefficient μ_A = μA/ρN is measured in cm²atom^-1. Electronic coefficient mu_e = μ_A/ρNZ is measured in cm²electron^-1. Here, ρ is density, A is atomic weight, Z is atomic number, and N is Avogadro's number. The atomic and electronic absorption coefficients, which have the dimensions of an area, are often called "cross sections" and are measured in barns (1 barn = 10^-24 centimeters squared). The total linear attenuation coefficient can be written as the sum of several components: μ = μ_PE + μ_CT + μ_PP. (μ_PE, μ_CT, and μ_PP are linear attenuation coefficients for the photoelectric effect, the Compton effect, and pair production, respectively.)

In the photoelectric effect, an incoming photon interacts with an atomic electron and disappears, transferring its energy completely to the electron. Then an energetic photoelectron is ejected by the atom from one of its bound shells, mostly from the K shell of the atom. The kinetic energy of the photoelectron is given by E_e- = hv - E_b, where E_b is the binding energy of the electron, and hv is the energy of the incident photon. The photoelectric effect is predominant in the energy range from 100 to 500 kiloelectronvolts, and it decreases with photon energy and increases as the atomic number of the absorber is increased.

In the Compton effect, the incident photon interacts with a free or loosely bound electron. In this interaction, the photon of energy hv is scattered through an angle θ with an energy hv' (<hv), while the electron recoils with a kinetic energy E_e- at an angle Φ. E_e- = hv - hv'. The Compton effect is important in the energy range from about 100 kiloelectronvolts to 1 megaelectronvolt, and the Compton absorption coefficient is proportional to the number density of atoms and the atomic number of the material.

The pair production process occurs only when the photon energy is greater than twice the rest mass energy of an electron. At these energies, a photon may interact with a nucleus and disappear, producing a pair of particles, an electron and a positron, with kinetic energies Ee- and Ee+. In this process, the nucleus acquires an indeterminate momentum but negligible energy Enuc. The conservation of energy yields hv = 2m_oc² + E_e- + E_e+ + E_nuc. The presence of a nucleus is essential for the conservation of linear momentum. Since the positron is an antiparticle, it eventually annihilates with an electron and emits annihilation γ radiation.

The other process that is possible only at high photon energies is a nuclear process called "photodisintegration." Photons of energy from 20 to 100 megaelectronvolts interact with the nucleus and eject protons, neutrons, and even α particles. These reactions are (γ,p), (γ,n), and (γ, α). Sometimes two or more particles may be ejected, giving rise to reactions such as (γ,2n) or (γ,np). In general, this nuclear process is unimportant at moderate energies.

Charged particle interactions with matter are different from those of photons. Charged particles lose energy primarily through the ionization and excitation of atoms, and the Coulomb interactions are effective. The charged particles can be grouped into two categories: β particles and heavy charged particles. The main difference between these two categories is the mass.

Since the β particle's rest mass is very small, the relativistic effects are important. A heavy charged particle can transfer a small percentage of its energy in a single collision, and its deflection is negligible. A heavy charged particle therefore travels in an almost straight path. A β particle, however, can lose a large percentage of its energy and undergoes large deflections.

Hence, it does not travel in a straight path. A charged particle traveling through matter sometimes produces a secondary electron with enough energy to form short ionization traits of its own. These are called "δ rays." A charged particle, while passing through matter, can undergo elastic and inelastic collisions with electrons and atoms. In an "elastic collision," the particle interacts with an atom and transfers some of its energy, keeping the total kinetic energy conserved. In an "inelastic collision," there is a loss of energy caused by electronic excitations and ionizations.

At low energies, atomic electrons are the main targets, and ionization and excitation processes are predominant. At higher energies, especially for β particles, nuclei are the target particles and the "bremsstrahlung" (breaking radiation) process predominates. In this process, X rays of continuous energy spectrum are produced by the negative acceleration (stopping) of beta particles in a nuclear field. This process is very important for β particles at energies higher than 10 megaelectronvolts in water. For extremely energetic (relativistic) particles, the Cherenkov effect is also an important process. "Cherenkov radiation" is light that can be an electromagnetic "shock wave" that occurs when a particle travels in a substance with a velocity larger than the speed of light. The Cherenkov effect serves as the basis for a type of radiation detector known as a Cherenkov counter.

In considering the energy loss of charged particles, it is convenient to speak of "linear stopping power," S(E), which is defined as the amount of energy loss dE over the unit length of the path of the particle dx in a given absorber; for example, S(E) = -dE/dx. The stopping power for heavy charged particles in matter is described by the Bethe formula. For a β particle, since the radiative process (bremsstrahlung) is competent, the total stopping power is the sum of two components: collisional (caused by ionization and excitation) and radiation. The energy loss rate caused by bremsstrahlung for 10-megaelectronvolt electrons passing through lead is approximately equal to the loss rate caused by ionization. In such cases, the "linear energy transfer" should be considered. "Linear energy transfer" is the amount of energy transferred to the material per unit length along the particle path. The linear energy transfer would be equivalent to stopping power if all the particle energy were locally absorbed in the material. Thus, for heavy charged particles, linear energy transfer and stopping power are nearly equal. Yet, for a fast electron, in which part of the energy lost by the particle is radiated away from the particle path, the stopping power is larger than the linear energy transfer.

The particles will have a distribution of energy ("energy straggling") after a group of particles passes through a given thickness of the material; this is caused by different types of interactions. A charged particle will travel a fairly well-defined distance ("path length") before losing all of its kinetic energy. The mean path length of many monoenergetic particles is called the "range." The statistical variation of the path length for monoenergetic particles is called "range straggling." Most of the energy is deposited at the end of the range. The peak in the energy distribution is referred to the "Bragg peak."

Like photons, neutrons are uncharged; therefore, they are not subjected to Coulomb forces and can travel appreciable distances without interacting with the electric cloud of an atom.

Neutrons can collide with an atomic nucleus, which can scatter them elastically or inelastically, so that they will eventually lose their energy until it is of the order of the thermal energy. A neutron in this energy range is called a "thermal neutron." Neutrons interact with a nucleus by means of very strong, short-range nuclear forces. Thermal neutrons are captured by the nucleus, and a γ ray is emitted. Neutron capture or absorption may also emit energetic particles by means of reactions such as (n,p), (n,2n), and (n, α ). Some neutron capture reactions form compound nuclei that do not decay immediately. This process, known as "neutron activation," causes the compound nuclei to become radioactive. The decay time may be several years. The absorbed neutrons can also initiate the nuclear fission processes. These reactions strongly depend upon the type of atomic nucleus, and therefore the isotope effect is very important. The process of slowing down fast neutrons in matter by means of a series of mostly elastic scattering events is called "neutron moderation." The material in which it takes place is called the "moderator." If the energy of the fast neutron is sufficiently high, inelastic scattering with nuclei can take place, in which case the recoil nucleus is elevated to one of its excited states during the collision.

Applications

The subject of radiation interactions with matter ceived much attention from nuclear and material engineers, metallurgists, physicists, chemists, biologists, physicians, and even nonscientists. The various phenomena associated with the radiation effects on materials of neutrons, X rays, γ rays, charged particles, and fission fragments are of great scientific and practical interest. It is important to understand the different processes by which radiation interacts with matter, since they have many potential applications in radiation detectors, nuclear medicine, modification of materials, materials testing and analysis, nuclear shielding, and controlling health hazards.

Radiation effects and ion implantation in materials have become very important in technology because of their usefulness in the modification of materials. Radiation interactions in solids can influence physical properties (mechanical, oxidation, corrosion, wear, electrical, magnetic, thermal, optical, dimensional stability, phase transformations, amorphization, precipitation from solid solutions) and chemical properties (excitation, decomposition, polymerization, reaction rates, chemical yield). Some chemical reactions, mostly in insulators, are promoted by γ radiation that cannot be induced by other means. Numerous chemical systems have been observed that undergo chemical changes on exposure to penetrating X and γ radiations, such as the oxidation of ferrous iron compounds to ferric iron compounds, the reduction of ceric ions to cerous ions, the decolorization of methylene blue, the liberation of free iodine from iodine compounds, and the inactivation of certain enzymes in diluted aqueous solutions. Chloroform irradiation with X rays liberates hydrochloric acid in amounts proportional to the radiation energy absorbed. Acid formation was shown to be increased in water-saturated chloroform solutions by γ radiation. "Chloroform-dye dosimeters" are developed to measure the X-ray and γ-ray doses. Ozone (O₃), a strong absorber of ultraviolet photons, reduces the high ultraviolet intensity emitted by the sun to a value that is compatible with life.

Radiation interactions are also used to identify the material constituents. The "particle-induced X-ray emission" technique is widely used in trace element analysis of materials. In this technique, a thin sample of a material is bombarded with protons, α particles, or even heavy ions. The ionization of a K or L shell electron creates an electron vacancy that is filled by the higher level electron, which causes the emission of an X ray (10 to 100 kiloelectronvolts) characteristic of the target atom. "Rutherford backscattering" of α particles is another technique for material study, which is used mainly for analysis of solid surfaces. Large-angle scattering of α particles from a target that consists of a variety of isotopes of elements will produce a spectrum of α energies, each of which corresponds to a unique mass of the struck atom, from which one can infer the composition of the target. This technique is also used to find the depth profiles of ions implanted into a target material.

In certain low-energy (about 10 kiloelectronvolts) regions, the photoelectric effect cross section of a low atomic number material may be greater than that of a high atomic number material. This fact is utilized in the construction of filters that are used to separate a band of nearly monoenergetic radiation from a continuous X-ray spectrum. The photoelectric effect is also utilized in photoelectric cells. Photodisintegration has been the subject of many investigations based on the radiations from betatrons, synchrotrons, and other high-energy devices. The phenomena of fluorescence and phosphorescence are caused by the interaction of ionizing radiation with some solids. When ionizing radiation penetrates the target material, the target atoms are excited. The excited atoms de-excite quickly, emitting low-energy photons. If a large amount of light escapes from the medium, the material is called the "fluoresce." If the light escapes more slowly because of slow atomic de-excitation, the material is called the "phosphoresce." Defects called "color centers" can be produced in inorganic crystals such as sodium chloride by irradiating the crystal with X rays.

In thermonuclear reactors, neutrons have to be moderated in order to produce further fission processes, and moderators are selected for their ability to moderate without capturing neutrons. Heavy water (D₂O) and graphite (C) are used for this purpose. Low atomic mass number (A) materials such as hydrogen are much better at stopping fast neutrons than are high A elements such as lead. This is exactly the opposite of the photon case. Thus, wood is a better radiation shield against fast neutrons than is lead. Lead has a very low absorption cross section for thermal neutrons, too. Fast neutron detectors are made of low A materials, as are neutron shields, and are usually backed by a thermal neutron absorber.

The recoil following elastic scattering with a light target is used to measure the energies of fast neutrons. Metals used in nuclear reactor structures can be severely weakened by the high flux of energetic neutrons, which often leads to metal fatigue, so it is important to slow down the fast neutrons. Boron (B) graphite, and water are a few examples of moderator materials. Nuclear reactions such as (n, α ) in lithium 6 and boron 10, however, have large cross sections of thermal neutrons, about 1,000 and 4,000 barns, respectively. The energetic particles from this reaction can produce important radiation effects. Hence, transmutation is of great importance.

The slow and thermal neutron detectors are in fact based on (n,p) and (n, α) reactions and the resulting energetic p or α particle interactions.

Most radiation detectors are based on ionization in gases, liquids, or solids. The ionization effects are almost the same in gases and liquids, but the recombination is greater in liquids. In insulators such as diamonds, the ionization caused by the charged particle creates electron-hole pairs, and under the influence of applied potential, the electrons or holes move, giving rise to an electric current. The recombination of pairs poses problems, however, and in some cases it is advantageous to receive fast signals. Semiconductors such as silicon and germanium are tractable in this respect and thus are used as detectors. The effects in cloud chambers, bubble chambers, and photographic emulsions are caused by ionization. Charged particles cause light emission (scintillation) in some organic or plastic materials.

The interaction of radiation with matter has remarkable applications in nuclear medicine, for both diagnostic and therapeutic purposes. Patients take pelvic radium treatments for cancer of the cervix, spinal X rays for ankylosing spondylitis, and iodine 131 for hyperthyroidism. Radiation exposure may lead to leukemia and cancer in many cases. Diagnostic nuclear medicine has improved tremendously in recent years. X rays travel easily through soft body tissue but are strongly attenuated by bone, hence X-ray photography, which can reveal the detailed structure of the human skeletal systems, is invaluable in resetting broken bones. Its dental applications are well known. Medical imaging by γ-ray cameras is used to obtain images at specific depths of parts of the body. By choosing a pharmaceutical compound with a particular radioactive (γ-emitting) isotope that tends to accumulate in a specific organ, one can produce the complete picture of the organ from the spatial pattern of radioactive emissions.

Using this technique, one can even identify the location of a tumor in a brain. Positron-emitting isotopes are being studied to obtain positron-emission tomography imaging from the detection of annihilation γ radiation. Nuclear magnetic resonance (NMR) imaging has become more prevalent than X-ray, γ-ray, and positron-emission imaging techniques, since it does not require that a patient be exposed to ionizing radiation. Therapeutic nuclear medicine is widely used. Nuclear radiation is used in the destruction of unwanted or malfunctioning tissues in the body, such as cancerous tumors and overactive thyroid glands. This effect originates with the ionizing ability of nuclear radiation.

Radiation is also used in the sterilization of food and drugs. The principal purpose of treating foods with ionizing radiation is to secure their preservation. This can involve inactivation of several kinds of microorganisms that may contaminate foods and cause spoilage.

A second purpose has to do with the inactivation of foodborne pathogenic microorganisms. A third purpose, applicable to fresh fruits and vegetables, is to delay ripening or senescence, or to inhibit sprouting. A fourth purpose of food irradiation is to secure decontamination or disinfestation with regard to bacteria, yeasts, molds, and insects, and a fifth is to secure a chemical change in the food itself when such a change improves some characteristic of the food, or its processing. Radiation cannot be used to control degradation of a food that is caused by the chemical action of atmospheric dioxygen.

Context

Radioactivity was first discovered by Antoine-Henri Becquerel in 1896, within a year after Wilhelm Conrad Rontgen's discovery of X rays. Natural radioactivity was named, however, by Marie Curie in 1898. The rays emitted by radioactive nuclei were first classified in 1899 by Ernest Rutherford as α, β, and γ rays, according to their ability to penetrate matter and ionize air. In 1919, Rutherford bombarded nitrogen with α particles and observed scintillations on a zinc sulfide screen that were caused by protons, which have a much longer range in air than α particles. This was the first observation of artificial nuclear disintegration. In 1932, the neutron was discovered by Sir James Chadwick, the positron was discovered by Carl David Anderson, and the first nuclear reaction was observed by Sir John Douglas Cockcroft and Ernest Thomas Sinton Walton, who used artificially accelerated particles.

Energy struggling for electrons was first observed by Niels Bohr in 1915.

The International Commission on Radiological Units (ICRU) and Measurements (added later) was established in 1925 to deal with the problems of radiation measurement and standardization. In 1928, the ICRU established the "roentgen" unit, which is used to measure X-ray or γ-ray exposure (dosage). It is defined in terms of the ionization charge that is created by secondary electrons. A roentgen is the amount of X or γ radiation that liberates one statcoulomb of ions in one cubic centimeter of dry air at standard atmospheric pressure. In 1953, the ICRU established a new unit, the "rad" (radiation absorbed dose), which is the amount of radiation that deposits 100 ergs per gram of energy in any material. In SI units, the absorbed dose is measured in gray. The amount of biological damage has been found to depend not only upon the energy absorbed but also upon the spacing of the ion pairs; if the ion pairs are closely spaced, as in the case of ionization by α particles, the biological effect is enhanced. The "rem" (roentgen equivalent in man) unit is the dose that has the same biological effect as 1 rad of X or γ radiation. The corresponding SI unit for rem is the "sievert," which is equal to 100 rem.

Depending on the dose, the kind of radiation, and the observed end point, the biological effects of radiation differ widely. Some occur relatively quickly ("acute radiation syndrome"), while others may take years to become evident ("delayed somatic effects"). Radicals produced by charged particles in a biological system react in about 10-3 seconds. Some biochemical processes are altered almost immediately, in less than a second. Cell division can be affected in a matter of hours. In higher organisms, damage to the gastrointestinal tract and central nervous system appears within a matter of days, and hemopoietic death occurs in about a month. Other kinds of damage, such as lung fibrosis, may take several weeks to develop. Cataracts and cancer occur many years after exposure to radiation. Genetic effects are first seen in the next or subsequent generations of an exposed individual. A dose of less than 50 rem causes no short-term ill effects. A dose of between 50 and 300 rem brings on radiation sickness. A whole-body dose in the 400 to 500 rem range is classified as a "lethal dose" for about 50 percent of the people so exposed; death occurs within a few months. Whole-body doses greater than 600 rem result in death for almost all individuals. Because of the hazards of radiation, the ICRU recommended in 1925 (the year of its formation) that the annual permissible dose of radiation be one-tenth of the acute dose that produces only visible reddening of the skin ("erythema dose").

The federal government has established a dose limit that stipulates that an individual in the general population should not receive more than 170 millirems of man-made radiation each year, exclusive of medical sources, and that a person exposed to radiation in the workplace (a radiation therapist, for example) should not receive more than 5 rem per year from work-related sources.

Principal terms

ABSORPTION COEFFICIENT: a measure of a medium's ability to absorb radiation, but not to scatter or diffuse it

ATTENUATION: the loss of power suffered by radiation as it passes through matter

COMPTON EFFECT: the reduction of a photon's energy that is caused by its interaction with a free electron; part of the photon's energy is transferred to the electron

CROSS SECTION: the effective area that has to be attributed to a particular atom or nucleus to account for its interaction with an incident beam of radiation

PAIR PRODUCTION: the creation of an electron and a positron by the interaction between a photon and an atomic nucleus

PHOTOELECTRIC EFFECT: an electron emission caused by complete energy transfer from a photon to an atomic electron

RANGE: the average path length of a monoenergetic particle in matter as it almost comes to a stop

SCATTERING: the deflection of any radiation as a result of its interaction with matter

STOPPING POWER: a measure of the ability of a substance to reduce the kinetic energy of a charged particle passing through it

STRAGGLING: the statistical variation of the path length for monoenergetic particles

Bibliography

Knoll, Glenn F. RADIATION DETECTION AND MEASUREMENT. New York: John Wiley & Sons, 1979. Chapter 2, which covers radiation interactions, is particularly informative regarding relations among various parameters and contains a discussion of radiation exposure and dosage.

Krane, Kenneth S. INTRODUCTORY NUCLEAR PHYSICS. New York: John Wiley & Sons, 1987. Chapter 20 features applications of nuclear radiation, including applications in nuclear medicine.

Tait, W. H. RADIATION DETECTION. London: Butterworths, 1980. Chapter 4, which deals with interactions with matter, covers the theoretical aspects of the topic.

Turner, James E. ATOMS, RADIATION AND RADIATION PROTECTION. New York: Pergamon Press, 1986. This book has several chapters on radiation interactions, including one on the chemical and biological effects of radiation.

Urbain, Walter M. FOOD IRRADIATION. New York: Academic Press, 1986. This useful book contains information on radiation effects in food and their applications in food technology.

Materials Analysis with Nuclear Reactions, and Scattering

Defects in Solids