Chapter 18. Useful Radiation Effects

Radiation in the form of gamma rays, beta particles, and neutrons is being used in science and industry to achieve desirable changes. Radiation doses control offending organisms, including cancer cells and harmful bacteria, and sterilize insects. Local energy deposition can also stimulate chemical reactions and modify the structure of plastics and semiconductors. Neutrons and X-rays are used to investigate basic physical and biological processes. In this chapter we will briefly describe some of these interesting and important applications of radiation. For additional information on the uses around the world, proceedings of international conferences can be consulted. Thanks are due to Albert L. Wiley, Jr., MD, PhD, for suggestions on the subject of nuclear medicine.

18.1. Medical Treatment

The use of radiation for medical therapy has increased greatly in recent years, with millions of treatments given patients annually. The radiation comes from teletherapy units in which the source is at some distance from the target, from isotopes in sealed containers implanted in the body, or from ingested solutions of radionuclides.

Doses of radiation are found to be effective in the treatment of diseases such as cancer. In early times, X-rays were used, but they were supplanted by cobalt-60 gamma rays, because the high-energy (1.17 and 1.33 MeV) photons penetrated tissue better and could deliver doses deep in the body, with a minimum of skin reaction. In modern nuclear medicine, there is increasing use of accelerator-produced radiation in the range 4 to 35 MeV for cancer treatment.

Treatment of disease by implantation of a radionuclide is called interstitial brachytherapy (“brachys” is Greek for “short”). A small radioactive capsule or “seed” is imbedded in the organ, producing local gamma irradiation. The radionuclides are chosen to provide the correct dose. In earlier times, the only material available for such implantation was α-emitting radium-226 (1599 y). Most frequently used today are iridium-192 (73.8 d), iodine-125 (59.4 d), and palladium-103 (17.0 d). Examples of tumor locations where this method is successful are the head and neck, breast, lung, and prostate gland. Other isotopes sometimes used are cobalt-60, cesium-137, tantalum-182, and gold-198. Intense fast neutron sources are provided by californium-252. For treatment of the prostate, 40 to 100 “rice-sized” seeds, (4.5 mm long and 0.81 mm diameter) containing a soft-gamma emitter, Pd-103, are implanted with thin hollow needles (see References). Computerized tomography (CT) and ultrasound aid in the implantation.

One sophisticated device for administering cancer treatment uses a pneumatically controlled string of cesium-137–impregnated glass beads encapsulated in stainless steel, of only 2.5 mm diameter. Tubes containing the beads are inserted in the bronchus, larynx, and cervix.

Success in treatment of abnormal pituitary glands is obtained by charged particles from an accelerator, and beneficial results have come from slow neutron bombardment of tumors in which a boron solution is injected. Selective absorption of chemicals makes possible the treatment of cancers of certain types by administering the proper radionuclides. Examples are iodine-125 or iodine-131 for the thyroid gland and phosphorus-32 for the bone. However, there is concern in medical circles that use of iodine-131 to treat hyperthyroidism could cause thyroid carcinoma, especially in children.

Relief from rheumatoid arthritis is obtained by irradiation with beta particles. The radionuclide dysprosium-165 (2.33 h) is mixed with ferric hydroxide, which serves as a carrier. The radiation from the injected radionuclide reduces the inflammation of the lining of joints.

Table 18.1 shows some of the radionuclides used in treatment.

Table 18.1. Radionuclides Used in Therapy

Radionuclide	Disease Treated
P-32	Leukemia
Y-90	Cancer
I-131	Hyperthyroidism
Thyroid cancer
Sm-153 and Re-186	Bone cancer pain
Re-188 and Au-198	Ovarian cancer

A great deal of medical research is an outgrowth of radioimmunoassay (see Section 17.5). It involves monoclonal antibodies (MAbs), which are radiolabeled substances that have an affinity for particular types of cancer, such as those of the skin and lymph glands. The diseased cells are irradiated without damage to neighboring normal tissue. The steps in this complex procedure start with the injection into mice of human cancer cells as antigens. The mouse spleen, a part of the immune system, produces antibodies through the lymphocyte cells. These cells are removed and blended with myeloma cancer cells to form new cells called hybridoma. In a culture, the hybridoma clones itself to produce the MAb. Finally, a beta-emitting radionuclide such as yttrium-90 is chemically bonded to the antibody.

A promising treatment for cancer is boron neutron capture therapy (BNCT). A boron compound that has an affinity for diseased tissue is injected, and the patient is irradiated with neutrons from a reactor. Boron-10, with abundance of 20% in natural boron, strongly absorbs thermal neutrons to release lithium-7 and helium-4 ions. An energy of more than 2 MeV is deposited locally because of the short range of the particles. The technique was pioneered in the 1950s by Brookhaven National Laboratory, but the program was suspended from 1961 to 1994 and terminated in 1999. Research is continuing at other locations, however (see References). A compound, Bisphenol A (BPA), was found that localized boron better, and thermal neutrons were replaced by intermediate energy neutrons, with favorable results. A single treatment with BNCT is as effective as many conventional radiation-chemotherapy sessions. The method has been found to be effective in treatment of malignancies such as melanoma (skin) and glioblastoma multiforme (brain). The discovery of monoclonal antibodies opens up new possibilities for large-scale use of BNCT.

The mechanism of the effects of radiation is known qualitatively. Abnormal cells that divide and multiply rapidly are more sensitive to radiation than normal cells. Although both types are damaged by radiation, the abnormal cells recover less effectively. Radiation is more effective if the dosage is fractionated (i.e., split into parts and administered at different times, allowing recovery of normal tissue to proceed).

Use of excess oxygen is helpful. Combinations of radiation, chemotherapy, and surgery are applied as appropriate to the particular organ or system affected. The ability to control cancer has improved over the years, but a cure on the basis of better knowledge of cell biology is yet to come.

18.2. Radiation Preservation of Food

The ability of radiation treatment to eliminate insects and microorganisms from food has been known for many years. Significant benefits to the world's food supply are beginning to be realized, as a number of countries built irradiation facilities.^[†] Such application in the United States has been slow because of fears related to anything involving radiation.

^† Thanks are due Food Technology Service in Mulberry, Florida, for extensive literature on food irradiation.

Spoilage of food before it reaches the table is due to a variety of effects: sprouting as in potatoes, rotting caused by bacteria as in fruit, and insect infestation as in wheat and flour. Certain diseases stem from microorganisms that contaminate food. Examples are the bacteria Salmonella, found in much of poultry products, and the parasite trichinae that infest some pork. The National Centers for Disease Control and Prevention state that foodborne illnesses affect millions of people in the United States each year, with thousands of deaths.

Various treatments are conventionally applied to preserve food, including drying, pickling, salting, freezing, canning, pasteurization, sterilization, the use of food additives such as nitrites, and until they were banned, the application of fumigants such as ethylene dibromide (EDB). Each treatment method has its advantages, but nitrites and EDB are believed to have harmful physiological effects.

On the other hand, research has shown that gamma radiation processing can serve as an economical, safe, and effective substitute and supplement for existing treatments. The shelf life of certain foods can be extended from days to weeks, allowing adequate time for transportation and distribution. It has been estimated that 20% to 50% of the food supplied to certain countries is wasted by spoilage that could be prevented by radiation treatment. The principal sources of ionizing radiation suitable for food processing are X-rays, electrons from an accelerator, and gamma rays from a radionuclide. Much experience has been gained from the use of cobalt-60, half-life 5.27 y, with its two gamma rays of energy 1.17 MeV and 1.33 MeV. The largest supplier of cobalt-60 is a Canadian firm, MDS Nordion, formerly part of Atomic Energy of Canada, Ltd. The isotope is prepared by irradiating pure cobalt-59 target pellets with neutrons in the CANDU reactors of Ontario Hydro. The targets are disassembled and shipped for processing into double-layer capsules of approximately 10 Ci each. Another attractive isotope is cesium-137, gamma ray 0.662 MeV, because of its longer half-life of 30.2 y and its potential availability as a fission product. A considerable amount of cesium-137 has been separated at Hanford, Washington, as a part of the radioactive waste management strategy. Arrangements for loans of capsules from the DOE to industrial firms have been made. Additional cesium-137 could be obtained through limited reprocessing of spent reactor fuel.

Many people are concerned about the use of irradiated products because of the association with nuclear processes. The first worry is that the food might become radioactive. The concern is unfounded, because there is no detectable increase in radioactivity at the dosages and particle energies of the electrons, X-rays, or gamma rays used. Even at higher dosages than are planned, the induced radioactivity would be less than that from natural amounts of potassium-40 or carbon-14 in foods. Another fear is that hazardous chemicals may be produced. Research shows that the amounts of unique radiolytic products (URP) are small, less than those produced by cooking or canning, and similar to natural food constituents. No indication of health hazard has been found, but scientists recommend continuing monitoring of the process. A third concern is that there would be a loss in nutritional value. Some loss in vitamin content occurs, just as it does in ordinary cooking. Research is continuing on the effects of radiation on nutritional value. It seems that the loss is minor at the low dose levels used. On various food products, there are certain organoleptic effects (taste, smell, color, texture), but these are a matter of personal reaction, not of health. Even these effects can be eliminated by operating the targets at reduced temperatures. The astronauts of the Apollo missions and the space shuttle have dined regularly on treated foods while in orbit. They were enthusiastic about the irradiated bread and meats. Many years ago, some scientists in India reported that consumption of irradiated wheat caused polyploidy, an increase in cell chromosomes. Extensive studies elsewhere disproved the finding.

Finally, it has been suggested that radiation might induce resistance of organisms, just as with pesticides and antibiotics, but the effect seems not to occur. The difference is attributed to the fact that there is a broad effect on enzymes and compounds.

The radiation dosages required to achieve certain goals are listed in Table 18.2. Note that 1 gray (Gy) is 100 rads.

Table 18.2. Doses to Achieve Beneficial Effects

Effect	Dose (Gy)
Inhibit sprouting of potatoes and onions	60–150
Eliminate trichinae in pork	200–300
Kill insects and eggs in fruits	200–500
Disinfect grain, prolong berry life	200–1000
Delay ripening of fruit	250–350
Eliminate salmonella from poultry	1000–3000

The main components of a multiproduct irradiation facility that can be used for food irradiation on a commercial basis are shown in Figure 18.1. Important parts are: (a) transfer equipment, involving conveyors for pallets, which are portable platforms on which boxes of food can be loaded; (b) an intense gamma ray source, of approximately 1 million curies strength, consisting of doubly encapsulated pellets of cobalt-60; (c) water tanks for storage of the source, with a cooling and purification system; and (d) a concrete biological shield, approximately 2 meters thick. In the operation of the facility, a rack of cobalt rods is pulled up out of the water pool, and the food boxes are exposed as they pass by the gamma source. Commercial firms providing irradiation equipment and carrying out irradiations, mainly for sterilization of medical supplies, are MDS Nordion of Kanata Ontario, Canada; Food Technology Service, Inc. of Mulberry, FL; Isomedix, Inc. of Whippany, NJ; and SteriGenics International of Oak Brook, IL, which has facilities around the world (see References). Among services provided is mold remediation in books, documents, and records with gamma rays.

Figure 18.1. Gamma irradiation of Sterigenics International, at Haw River, NC. Pallets containing boxes of products move on a computer-controlled conveyor through a concrete maze past a gamma-emitting screen.

A number of experimental facilities and irradiation pilot plants have been built and used in some 70 countries. Some of the items irradiated have been grain, onions, potatoes, fish, fruit, and spices. The most active countries in the development of large-scale irradiators have been the United States, Canada, Japan, and the former U.S.S.R.

Table 18.3 shows the approvals for irradiation as issued by the United States Food and Drug Administration (FDA). Limitations are typically set on dosages to foodstuffs of 1 kilogray (100 kilorad) except for dried spices, not to exceed 30 kGy (3 Mrad).

Table 18.3. Approvals by the Food and Drug Administration for Use of Irradiated Substances

Product	Purpose of Irradiation	Dose (krad)	Date
Wheat and powder	Disinfest insects	20–50	1963
White potatoes	Extend shelf life	5–15	1965
Spices, seasonings	Decontaminate	3000	1983
Food enzymes	Control insects	1000	1985
Pork products	Control trichinae	30–100	1985
Fresh fruits	Delay spoilage	100	1986
Enzymes	Decontaminate	1000	1986
Dried vegetables	Decontaminate	3000	1986
Poultry	Control salmonella	300	1990
Beef, lamb, pork	Control pathogens	450	1997
Shell eggs	Control salmonella	300	2000
Shellfish	Control bacteria	550	2005
Seeds	Control pathogens	800	2005

Note: 1 krad = 10 Gy.

Source: FDA, (see References).

Labeling of the packages to indicate special treatment is required by use of a phrase such as “treated with radiation.” In addition, packages will exhibit the international logo, called a radura, shown in Figure 18.2. The symbol's solid circle represents an energy source; the two petals signify food; the breaks in the outer circle mean rays from the energy source. The radura label is required for shipment to the first purchaser, not for a consumer in a restaurant. Labeling of radiation-treated food is obviously a factor in acceptance by the public. The FDA is planning to allow the word “pasteurized” for products that are unchanged except for shelf life. The FDA also seeks a label other than “irradiated.”

Figure 18.2. International logo to appear on irradiated food.

Final rules on red meat irradiation as a food additive were issued by the FDA in December 1997 and by United States Department of Agriculture (USDA) in December 1999 (see References). The action was prompted in part by the discovery of the bacterium E. coli contamination of hamburger by an Arkansas supplier. Some 25 tons of meat were recalled and destroyed. The new rule cites statistics on outbreaks of disease and numbers of deaths related to beef. Maximum permitted doses for meat are 4.5 kGy (450 krad) as refrigerated and 7.0 kGy (700 krad) as frozen. More than 80 technical references are cited on all aspects of the subject.

Irradiated fresh ground beef prepared by SureBeam Corporation of San Diego is available at thousands of groceries in the United States. Electron beams are used to process beef at prices comparable to those of nonirradiated meat.

Approval to irradiate does not guarantee that it actually will be done, however. Many large food processors and grocery chains tend to shy away from the use of irradiated food products, believing that the public will be afraid of all of their products. Obviously, people will not have much opportunity to find treated foods acceptable if there are few products on the market. Anti-irradiation activists, who claim that the nuclear irradiation process is unsafe, have taken advantage of that reluctance. In contrast, enthusiastic endorsement of food irradiation is provided by organizations such as World Health Organization, American Medical Association, American Dietetic Association, International Atomic Energy Agency, Grocery Manufacturers of America, and many others (see References).

At the First World Congress on Food Irradiation, held in 2003, a number of facts were reported. The 2002 Food Bill specified that irradiated food should be made available to the National School Lunch Program; regulation of facilities was developed by an International Consultative Group on Food Irradiation (ICGFI) and adopted worldwide; consumers are generally aware of beef irradiation (68%) and favor marketing irradiated ground beef (78%).

Plans were laid in 2005 by a fruit company, Pa'ina Hawaii, to install a cobalt-60 irradiator at the Honolulu airport to be used for treatment of exotic fruits such as papayas. Approval by the Nuclear Regulatory Commission (NRC) was received. E. coli contamination in 2006 of spinach and lettuce have stimulated new interest in food irradiation.

18.3. Sterilization of Medical Supplies

Ever since the germ theory of disease was discovered, effective methods of sterilizing medical products have been sought. Example items are medical instruments, plastic gloves, sutures, dressings, needles, and syringes. Methods of killing bacteria in the past include dry heat, steam under pressure, and strong chemicals such as carbolic acid and gaseous ethylene oxide. Some of the chemicals are too harsh for equipment that is to be reused, and often the substances themselves are hazardous. Most of the previous methods are batch processes, difficult to scale up to handle the production needed. More recently, accelerator-produced electron beams have been introduced and preferred for some applications.

The special virtue of cobalt-60 gamma-ray sterilization is that the rays penetrate matter very well. The item can be sealed in plastic and then irradiated, assuring freedom from microbes until the time it is needed in the hospital. Although the radioactive material is expensive, the system is simple and reliable, consisting principally of the source, the shield, and the conveyor. A typical automated plant requires a source of approximately 1 MCi.

18.4. Pathogen Reduction

In the operation of public sewage treatment systems, enormous amounts of solid residues are produced. In the United States alone the sewage sludge amounts to 6 million tons a year. Typical methods of disposal are by incineration, burial at sea, placement in landfills, and application to cropland. In all of these there is some hazard caused by pathogens—disease-causing organisms such a parasites, fungi, bacteria, and viruses. Experimental tests of pathogen reduction by cobalt-60 or cesium-137 gamma irradiation have been made in Germany and in the United States. The program in the United States was part of the Department of Energy's (DOE) studies of beneficial uses of fission product wastes and was carried out at Sandia Laboratories and the University of New Mexico. Tests of the effectiveness of radiation were made, and the treated sludge was found to be suitable as a feed supplement for livestock, with favorable economics. However, no use of those results was made in the United States. Apparently, the only large-scale application of sewage sludge irradiation is in Argentina, in the large city of Tucuman (see References). It is conceivable that the time is not yet ripe in the United States and Europe for such application of radiation. It took a number of years to adopt recycling of household wastes.

18.5. Crop Mutations

Beneficial changes in agricultural products are obtained through mutations caused by radiation. Seeds or cuttings from plants are irradiated with charged particles, X-rays, gamma rays, or neutrons; or chemical mutagens are applied. Genetic effects have been created in a large number of crops in many countries. The science of crop breeding has been practiced for many years. Unusual plants are selected and crossed with others to obtain permanent and reproducible hybrids. However, a wider choice of stock to work with is provided by mutant species. In biological terms, genetic variability is required.

Features that can be enhanced are larger yield, higher nutritional content, better resistance to disease, and adaptability to new environments, including higher or lower temperature of climate. New species can be brought into cultivation, opening up sources of income and improving health.

The leading numbers of mutant varieties of food plants that have been developed are as follows: rice, 28; barley, 25; bread wheat, 12; sugar cane, 8; and soybeans, 6. Many mutations of ornamental plants and flowers have also been produced, improving the income of small farmers and horticulturists in developing countries. For example, there are 98 varieties of chrysanthemum. The International Atomic Energy Agency (IAEA), since its creation in 1957, has fostered mutation breeding through training, research support, and information transfer. The improvement of food is regarded by the IAEA as a high-priority endeavor in light of the expanding population of the world.

More recently, the application of genetic engineering to improve crops and foodstuffs has drawn a great deal of criticism, especially in Europe, and a deep-seated conflict with the United States over use of biotechnology will be difficult to resolve.

18.6. Insect Control

To suppress the population of certain insect pests the sterile insect technique (SIT) has been applied successfully. The standard method is to breed large numbers of male insects in the laboratory, sterilize them with gamma rays, and release them for mating in the infested area. Competition of sterile males with native males results in a rapid reduction in the population. The classic case was the eradication of the screwworm fly from Curaçao, Puerto Rico, and the southwestern United States. The flies lay eggs in wounds of animals and the larvae feed on living flesh and can kill the animal if untreated. After the numbers were reduced in the early 1960s, flies came up from Mexico, requiring a repeat operation. As many as 350 million sterile flies were released each week, bringing the infestations from 100,000 to zero. The annual savings to the livestock industry was approximately $100 million.

The rearing of large numbers of flies is a complex process, involving choice of food, egg treatment, and control of the irradiation process to provide sterilization without causing body damage. Cobalt-60 gamma rays are typically used to give doses that are several times the amounts that would kill a human being.

SIT has been used against several species of mosquito in the United States and India and stopped the infestation of the Mediterranean fruit fly in California in 1980.

The discovery of a screwworm infestation in Libya in 1988 prompted international emergency action by the United Nations Food and Agriculture Organization, the International Atomic Energy Agency, and others (see References). Arrangements were made for the fly factory in Mexico to supply millions of radiation-sterilized males to Libya. There, light aircraft dropped them in a grid pattern, starting in 1990. Within 5 months the screwworm was eradicated, thus protecting Libyan wildlife as a whole.

The technique was effective on the island of Zanzibar, part of Tanzania, in combating the tsetse fly (see References). The insect is a carrier of trypanosomiasis, a livestock disease, and of sleeping sickness, which affects humans. Prior pesticide use made SIT feasible, and within 4 years, by 1996, there were no flies left. Unfortunately, vast areas of Africa are infested with tsetse flies, and the fly-free zones are overloaded.

SIT can potentially control Heliothis (American bollworm, tobacco budworm, and corn earworm) and other pests such as ticks and the gypsy moth. Other related techniques include genetic breeding that will automatically yield sterile males.

Some of the organizations providing gamma-ray insect irradiation are ceasing operations for fear of terrorist action. An alternative is the use of X-rays.

18.7. Applications in Chemistry

Radiation chemistry refers to the effect of high-energy radiation on matter, with particular emphasis on chemical reactions. Examples are ion-molecule reactions, capture of an electron that leads to dissociation, and charge transfer without a chemical reaction when an ion strikes a molecule. Many reactions have been studied in the laboratory, and a few have been used on a commercial scale. For a number of years, Dow Chemical used cobalt-60 radiation in the production of ethyl bromide (CH³CH²Br), a volatile organic liquid used as an intermediate compound in the synthesis of organic materials. The application terminated for reasons of cost and safety. As catalysts, gamma rays have been found to be superior in many cases to chemicals, to the application of ultraviolet light, and to electron bombardment.

Various properties of polymers such as polyethylene are changed by electron or gamma ray irradiation. The original material consists of long parallel chains of molecules, and radiation damage causes chains to be connected in a process called cross-linking. Irradiated polyethylene has better resistance to heat and serves as a good insulating coating for electrical wires. Fabrics can be made soil-resistant by radiation bonding of a suitable polymer to a fiber base.

Highly wear-resistant wood flooring is produced commercially by gamma irradiation (see References). Wood is soaked in a monomer plastic, encased in aluminum, and placed in a water pool containing a cesium-137 source of 661 keV photons. The process of polymerization takes place throughout the wood. The molecular structure is changed so that the surface cannot be scratched or burned.

A related process has been applied in France to the preservation of artistic or historic objects of wood or stone. The artifact is soaked in a liquid monomer and transferred to a cobalt-60 gamma cell where the monomer is polymerized into a solid resin.

18.8. Transmutation Doping of Semiconductors

Semiconductor materials are used in a host of modern electrical and electronic devices. Their functioning depends on the presence of small amounts of impurities such as phosphorus in the basic crystal element silicon. The process of adding impurities is called “doping.” For some semiconductors, impurities can be introduced in the amounts and locations needed by use of neutron irradiation to create an isotope that decays into the desired material.

The process is relatively simple. A pure silicon monocrystal is placed in a research or experimental reactor of several megawatts power level. The sample is irradiated with a previously calibrated thermal neutron flux for a specified time. This converts one of the silicon isotopes into a stable phosphorus isotope by the reactionswhere the abundance of Si-30 is 3.1% and the half-life of Si-31 is 2.62 h. After irradiation, the silicon resistivity is too high because of radiation damage caused by the fast neutron component of the flux. Heat treatment is required before fabrication to anneal out the defects.

The principal application of neutron transmutation doping (NTD) has been in the manufacture of power thyristors, which are high-voltage, high-current semiconductor rectifiers (see References), so named because they replaced the thyratron, a vacuum tube. The virtue of NTD compared with other methods is that it provides a uniform resistivity over the large area of the device. Annual yields of the product material are more than 50 tons, with a considerable income to the reactor facilities involved in the work. NTD is expected to become even more important in the future for household and automotive devices. The doping method is also applicable to other substances besides silicon (e.g., germanium and gallium arsenide).

18.9. Neutrons in Fundamental Physics

Intense neutron beams produced in a research reactor serve as powerful tools for investigation in physics. Three properties of the neutron are important in this work: (a) the lack of electrical charge, which allows a neutron to penetrate atomic matter readily until it collides with a nucleus; (b) a magnetic moment, resulting in special interaction with magnetic materials; and (c) its wave character, causing beams to exhibit diffraction and interference effects.

Measurements of neutron cross section of nuclei for scattering, capture, and fission are necessary for reactor analysis, design, and operation. An area of study that goes beyond those needs is called inelastic neutron scattering. It is based on the fact that the energy of thermal neutrons, 0.0253 eV, is comparable to the energy of lattice vibrations in a solid or liquid. Observations of changes in the energy of bombarding neutrons provide information on the interatomic forces in materials, including the effects of impurities in a crystal, of interest in semiconductor research. Also, inelastic scattering yields understanding of microscopic magnetic phenomena and the properties of molecular gases.

We recall that the magnetic moment of a bar magnet is the product of its length s and the pole strength p. For charges moving in a circle of radius r, the magnetic moment is the product of the area πr² and the current i. Circulating and spinning electrons in atoms and molecules also give rise to magnetic moments. Even though the neutron is uncharged, it has an intrinsic magnetic moment. Thus the neutron interacts differently with materials according to their magnetic properties. If the materials are paramagnetic, with randomly oriented atomic moments, no special effect occurs. Ferromagnetic materials such as iron and manganese have unpaired electrons, and moments are all aligned in one direction. Antiferromagnetic materials have aligned moments in each of two directions. Observations of scattered neutrons lead to understanding of the microscopic structure of such materials.

The wavelength of a particle of mass m and speed υ according to the theory of wave mechanics iswhere h is Planck's constant, 6.64 × 10⁻³⁴ J−s. For neutrons of mass 1.67 × 10⁻²⁷ kg, at the thermal energy, 0.0253 eV, speed 2200 m/s, the wavelength is readily calculated to be λ = 1.8 × 10⁻¹⁰ m. This is fairly close to d, the spacing of atoms in a lattice; for example, in silicon d is 3.135 × 10⁻¹⁰ m. The wave property is involved in the process of neutron diffraction, in analogy to X-ray and optical diffraction, but the properties of the materials that are seen by the rays differ considerably. Whereas X-rays interact with atomic electrons and thus diffraction depends strongly on atomic number Z, neutrons interact with nuclei according to their scattering lengths, which are unique to the isotope, and are rather independent of Z. Scattering lengths, labeled a, resemble radii of nuclei but have both magnitude and sign. For nearby isotopes, a values and the corresponding cross sections σ = πr² differ greatly. For example, the approximate σ values of three nickel isotopes are: Ni-58, 26; Ni-59, 1; Ni-60, 10. In neutron diffraction one applies the Bragg formula λ = 2d sin θ, where d is the lattice spacing and θ is the scattering angle. A host of isotopes, elements, and compounds have been investigated by neutron diffraction, as discussed by Bacon (see References).

A still more modern and sophisticated application of neutrons is interferometry in which neutron waves from a nuclear reactor source are split and then recombined. We can describe the essential equipment needed. A perfect silicon crystal is machined very accurately in the form of the letter E, making sure the planes are parallel. A neutron beam entering the splitter passes through a mirror plate and analyzer. Reflection, refraction, and interference take place, giving rise to a periodic variation of observed intensity. Insertion of a test sample causes changes in the pattern. The method has been used to measure accurately the scattering lengths of many materials. Images of objects are obtained in phase topography, so named because the introduction of the sample causes a change in phase in the neutron waves in an amount dependent on thickness, allowing observation of surface features. Interference fringes have been observed for neutrons passing through slightly different paths in the Earth's magnetic field. This suggests the possibility of studying the relationship of gravity, relativity, and cosmology.

In the Spallation Neutron Source (SNS) (in Section 8.6), wavelengths and energies of neutrons produced match the size and energy scales of many materials of interest. Its enhanced neutron beams allow higher resolution images of biological materials. Of special benefit in the SNS is the ability to locate hydrogen atoms in complex molecules. Crystallography studies will lead to more effective drugs.

The broad scope of research with SNS can be appreciated by a listing of areas adapted from the Oak Ridge National Laboratory (ORNL) Web site (see References):

• Chemistry. Use of neutron scattering to study microstructures in chemical products.
• Complex fluids. Investigation of new time-release drug-delivery systems targeting specific parts of the human body.
• Crystalline materials. Research on ways to tailor structures and properties of new materials.
• Disordered materials. Study of proteins of interest to biological industries.
• Engineering. Knowledge on material failures and substitutes.
• Magnetism and superconductivity. Understanding leading to improved devices.
• Polymers. Small-angle scattering to reveal behavior of molecular chains.
• Structural biology. Neutrons as complement to X-rays for studying vitally important chemicals.

18.10. Neutrons in Biological Studies

One of the purposes of research in molecular biology is to describe living organisms by physical and chemical laws. Thus, finding sizes, shapes and locations of components of biological structures is the first step in understanding. Neutron scattering provides a useful tool for this purpose. The radiation does not destroy the specimen; cross sections of materials of interest are of the same order for all nuclei so that heavier elements are not favored as in the case of X-rays; long wavelength neutrons needed to study the large biological entities are readily obtained from a reactor. Of special importance is the fact that scattering lengths for hydrogen (3.8 × 10⁻¹⁵ m) and deuterium (6.5 × 10⁻¹⁵ m), are quite different, so that the neutron scattering patterns from the two isotopes can be readily distinguished.

An example is the investigation of the ribosome. It is a particle approximately 25 nanometers in diameter that is part of a cell and helps manufacture proteins. The E. coli ribosome is composed of two subunits, one with 34 protein molecules and two RNA molecules, the other with 21 proteins and one RNA. The proteins are quite large, with molecular weight as high as 65,000. Study with X-rays or an electron microscope is difficult because of the size of the ribosome. For the neutron experiment, two of the 21 proteins are “stained” with deuterium (i.e., they are prepared by growing bacteria in D²O rather than H²O).

Early research on ribosomes was performed at the Brookhaven National Laboratory High Flux Beam Reactor (now shut down). A beam of neutrons was scattered from a graphite crystal that selected neutrons of a narrow energy range at wavelength 2.37 × 10⁻¹⁰ m. The specimen to be studied was placed in the beam in front of a helium-3 detector, which counted the number of neutrons as a function of scattering angle. The neutron wave, when scattered by a protein molecule, exhibited interference patterns similar to those of ordinary light. A distinct difference in pattern would be expected depending on whether the two molecules are touching or separated, as shown in Figure 18.3. For the ribosome, the distance between centers of molecules was deduced to be 35 × 10⁻¹⁰ m. Tentative “maps” of the ribosome subunit were developed, as well.

Figure 18.3. Interference patterns for the ribosome, a particle in the cell. Estimates of size and spacing are a start toward understanding biological structures.

18.11. Research with Synchrotron X-rays

Knowledge of the structure of molecules is made possible by the use of synchrotron X-rays because of their high intensity and sharp focus. Studies are faster and less damaging than those with conventional X-rays (see References). Materials in crystalline form are bombarded with photons, and the diffraction patterns are produced on a sensitive screen. The patterns are analyzed by computer by use of the Fourier transform to determine electron densities and thus atom locations. Suitable manipulations yield 3D data. Knowing molecular structure provides information on how chemicals work and helps find better drugs and treatment for disease. A classic example of a synchrotron X-ray study result was the determination of the structure of the rhinovirus HRV14, the cause of the common cold. The crystals were very sensitive to radiation and would cease to diffract before data were obtained by ordinary X-rays. Many other macromolecular proteins, enzymes, hormones, and viruses have been investigated. It is possible to observe chemical processes as they occur (e.g., photodissociation of hydrocarbons and of ozone). Information for improvement of industrial processes and products is also made available.

18.12. Summary

Many examples of the use of radiation for beneficial purposes can be cited. Diseases such as cancer can be treated by gamma rays. Food spoilage is reduced greatly by irradiation. Medical supplies are rendered sterile within plastic containers. Sewage sludge can be disinfected by irradiation. New and improved crops are produced by radiation mutations. The sterile insect technique has controlled insect pests in many areas of the world. Radiation serves as a catalyst in the production of certain chemicals. Properties of fibers and wood are enhanced by radiation treatment. Desirable impurities can be induced in semiconductor materials by neutron bombardment. The scattering by neutrons provides information on magnetic materials, and interference of neutron beams is used to examine surfaces. Scattered neutrons yield estimations of location and size of minute biological structures. Synchrotron X-rays are required for detailed study of biological molecules.

18.13. Exercises

1. Thyroid cancer is treated successfully by the use of iodine-131, half-life 8.04 d, energy release approximately 0.5 MeV. The biological half-life of I-131 for the thyroid is 4 d. Estimate the number of millicuries of the isotope that should be administered to obtain a dose of 25,000 rads to the 20-grams thyroid gland.
2. The disease polycythemia vera (PV) is characterized by an excess of red blood cells. Treatment by chemotherapy and radiation is often successful. In the latter, the patient is injected with a solution of sodium phosphate containing phosphorus-32, half-life 14.28 d, average beta energy 0.69 MeV. Estimate the dose in rads resulting from the administration of an initial 10 mCi of P-32, of which 10% goes to the 3-kg bone marrow. Recall 1 rad = 10⁻⁵ J/g and 1 mCi = 3.7 × 10⁷ dps. Suggestion: Neglect biological elimination of the isotope.
3. A company supplying cobalt-60 to build and replenish radiation sources for food processing uses a reactor with thermal flux 10¹⁴/cm²−s. To meet the demand of a megacurie a month, how many kilograms of cobalt-59 must be inserted in the reactor? Note that the density of Co-59 is 8.9 g/cm³ and the neutron cross section is 37 barns.
4. A cobalt source is to be used for irradiation of potatoes to inhibit sprouting. What strength in curies is needed to process 250,000 kg of potatoes per day, providing a dose of 10,000 rads? Note that the two gammas from Co-60 total approximately 2.5 MeV energy. What is the amount of isotopic power? Discuss the practicality of absorbing all of the gamma energy in the potatoes.
5. Transmutation of silicon to phosphorus is to be achieved in a research reactor. The capture cross section of silicon-30, abundance 3.1%, is 0.108 barns. How large must the thermal flux be to produce an impurity content of 10 parts per billion in a day's irradiation?

Computer Exercises

1. The classic “predator–prey” balance equations simulate interacting populations such as foxes and rabbits. Run the program PREDPREY to see trends with time.
2. An adaptation of the predator–prey equations can be used to analyze the control of the screwworm fly by the sterile male technique. Study the trend in population under different assumptions and initial conditions with the program ERADIC (eradicate/irradiate). In particular, find the time required to reduce the population to less than one female fly.

18.14 References

Interstitial Collaborative Working Group Interstitial Collaborative Working Group Lowell L. Anderson, ... Interstitial Brachytherapy: Physical, Biological, and Clinical Considerations 1990 Raven Press New York

IAEA Bulletin IAEA Bulletin Vol. 33 No. 1 1991 The issue features nuclear medicine

Nuclear Medicine Resource Manual Nuclear Medicine Resource Manual

http://www-pub.iaea.org/MTCD/publications/PDF/Pub1198_web.pdf http://www-pub.iaea.org/MTCD/publications/PDF/Pub1198_web.pdf

An IAEA guide for establishing nuclear medicine service. 532 pages An IAEA guide for establishing nuclear medicine service. 532 pages.

Major Advances in Nuclear Medicine, Diagnosis and Treatment (American Nuclear Society) Major Advances in Nuclear Medicine, Diagnosis and Treatment (American Nuclear Society)

http://www.ans.org/pi/np/diagnosis http://www.ans.org/pi/np/diagnosis

Data on remission rates with cell-targeted therapy Data on remission rates with cell-targeted therapy

The Basis of Boron Neutron Capture Therapy The Basis of Boron Neutron Capture Therapy

http://web.mit.edu/nrl/www/bnct/info/description/description.html http://web.mit.edu/nrl/www/bnct/info/description/description.html

Explanation of effect on tumor Explanation of effect on tumor

Boron Neutron Capture Therapy of Cancer: Current Status and Future Prospects Boron Neutron Capture Therapy of Cancer: Current Status and Future Prospects

Reprint of paper by Rolf F. Barth, et al., 2005 Reprint of paper by Rolf F. Barth, et al., 2005

Search on Rolf F. Barth in Google and Select title of paper Search on Rolf F. Barth in Google and Select title of paper.

“The Present Status of Boron-neutron Capture Therapy for Tumors,” “The Present Status of Boron-neutron Capture Therapy for Tumors,”

Hatanaka et al., 1991 H. Hatanaka, ... Pure & Appl. Chem. 63 1991373-

http://www.iupac.org/publications/pac/1991/pdf/6303x0373.pdf http://www.iupac.org/publications/pac/1991/pdf/6303x0373.pdf

Nanotech News, National Cancer Institute Nanotech News, National Cancer Institute

Research in Denmark using boron carbide nanoparticles Research in Denmark using boron carbide nanoparticles

http://nano.cancer.gov/news_center/nanotech_news_2006-03-06e.asp http://nano.cancer.gov/news_center/nanotech_news_2006-03-06e.asp

United States-Argentina BNCT Program United States-Argentina BNCT Program

http://www.inl.gov/featurestories/2007-02-13.shmtl http://www.inl.gov/featurestories/2007-02-13.shmtl

Collaboration on research Collaboration on research

Garnick and Fair, 1998 Mark B. Garnick, William R. Fair, Combatting Prostate Cancer Scientific American December 199874 ff-

Prostate Cancer Treatment Prostate Cancer Treatment

http://www.theragenics.com http://www.theragenics.com

A commercial supplier of radioactive particles. Select TheraSeed A commercial supplier of radioactive particles. Select TheraSeed.

Foods permitted to be irradiated Foods permitted to be irradiated

http://www.cfsan.fda.gov/~dms/irrafood.html http://www.cfsan.fda.gov/~dms/irrafood.html

FDA approval history FDA approval history.

Henkel, 1998 John Henkel, Irradiation: A Safe Measure for Safer Food FDA Consumer May–June 1998 Cambridge Press

http://www.fda.gov/fdac/features/1998/398_rad.html http://www.fda.gov/fdac/features/1998/398_rad.html

Excellent article, not being updated Excellent article, not being updated.

Background and Status of Labeling of Irradiated Food Background and Status of Labeling of Irradiated Food

http://www.organicconsumers.org/Irrad/LabelingStatus.cfm http://www.organicconsumers.org/Irrad/LabelingStatus.cfm

“FDA seeks to ease labeling requirements,”, May 2007FDA seeks to ease labeling requirements Nuclear News May 200761-

“Rick Michael Irradiated food, good; foodborne pathogens, bad,”, July 2003Rick Michael Irradiated food, good; foodborne pathogens, bad Nuclear News July 200362-

Irradiation of Food and Packaging: An Overview Irradiation of Food and Packaging: An Overview

http://www.cfan.gov/~dms/irraover.html http://www.cfan.gov/~dms/irraover.html

Article by Morehouse and Komolprasert, 2004 Article by Morehouse and Komolprasert, 2004.

Iowa State Food Safety Project Iowa State Food Safety Project

http://www.extension.iastate.edu/foodsafety http://www.extension.iastate.edu/foodsafety

Information and links. Select Food Irradiation Information and links. Select Food Irradiation.

Food Irradiation Information from Food Safety and Inspection Service Food Irradiation Information from Food Safety and Inspection Service

http://www.fsis.usda.gov/Fact_Sheets/Irradiation_Resources/index.asp http://www.fsis.usda.gov/Fact_Sheets/Irradiation_Resources/index.asp

Links to many documents Links to many documents.

IAEA Report IAEA Report.

Google “IAEA Thematic Planning Food Irradiation” Google “IAEA Thematic Planning Food Irradiation”

Select Summary Report Select Summary Report

Radiation Information Network: Food Irradiation Radiation Information Network: Food Irradiation

http://www.physics.isu.edu/radinf/food.htm http://www.physics.isu.edu/radinf/food.htm

Extensive discussion, references, and links by Idaho State University Extensive discussion, references, and links by Idaho State University.

Wilkinson and Gould, 1996 V.M. Wilkinson, G.W. Gould, Food Irradiation: A Reference Guide 1996 Butterworth-Heinemann Oxford Topics, definitions, and discussion with references to all relevant terms

Murano, 1995 E.A. Murano, Food Irradiation: A Sourcebook 1995 Iowa State University Press Ames, IA Processing, microbiology, food quality, consumer acceptance, and economics

Argentina Irradiates Urban Sludge Argentina Irradiates Urban Sludge

http://www.iaea.org/Publications/Magazines/Bulletin/Bull391/argentina.html http://www.iaea.org/Publications/Magazines/Bulletin/Bull391/argentina.html

IAEA publication, March 1997IAEA publication INSIDE Technical Cooperation March 1997

Lindquist and Abusowa, 1992 D.A. Lindquist, M. Abusowa, Eradicating the New World Screwworm from the Libyan Arab Jamahiriya IAEA Bulletin Vol. 34 No. 4 19929- Describes the success of the international program. With 12,000 cases in 1990, the number dropped to zero by May 1991

History of Screwworm Eradication History of Screwworm Eradication

http://www.nal.usda.gov/speccoll/screwworm/history.htm http://www.nal.usda.gov/speccoll/screwworm/history.htm

Discovery and application of Sterile Male Technique. From National Agricultural Library Discovery and application of Sterile Male Technique. From National Agricultural Library.

New World Screwworm Eradication New World Screwworm Eradication

http://www-tc.iaea.org/tcweb/publications/factsheets/jamaica.pdf http://www-tc.iaea.org/tcweb/publications/factsheets/jamaica.pdf

IAEA plans for Jamaica project, still underway IAEA plans for Jamaica project, still underway

Eradication of Tsetse Fly in Zanzibar Eradication of Tsetse Fly in Zanzibar

http://www.fao.org/NEWS/1998/980505-e.htm http://www.fao.org/NEWS/1998/980505-e.htm

http://www-tc.iaea.org/tcweb/Publications/factsheets/tsetse2.pdf http://www-tc.iaea.org/tcweb/Publications/factsheets/tsetse2.pdf

News items from sponsoring agencies News items from sponsoring agencies.

Simulation Exercise on SIT Simulation Exercise on SIT

http://ipmworld.umn.edu/chapters/SirSimul.htm http://ipmworld.umn.edu/chapters/SirSimul.htm

Program “Curaçao” by Phil A. Arneson Program “Curaçao” by Phil A. Arneson.

Gammapar Impregnated Flooring Gammapar Impregnated Flooring

http://www.gammapar.com/faqs.jsp">http://www.gammapar.com/faqs.jsp http://www.gammapar.com/faqs.jsp

Commercial production of irradiated acrylic impregnated hardwood Commercial production of irradiated acrylic impregnated hardwood.

Bacon, 1975 G.E. Bacon, Neutron Diffraction 3rd Ed. 1975 Clarendon Press Oxford A classical reference on the subject

Foldiak, 1986 G. Foldiak, Industrial Applications of Radioisotopes 1986 Elsevier Amsterdam

SNS and Biological Research SNS and Biological Research

http://www.ornl.gov/info/ornlreview/v34_1_01/sns.htm http://www.ornl.gov/info/ornlreview/v34_1_01/sns.htm

Drug studies Drug studies.

Ehmann and Vance, 1991 William D. Ehmann, Diane E. Vance, Radiochemistry and Nuclear Methods of Analysis 1991 John Wiley & Sons New York Covers many of the topics of this chapter

Woods and Pikaev, 1994 Robert J. Woods, Alexei K. Pikaev, Applied Radiation Chemistry: Radiation Processing 1994 John Wiley & Sons New York Includes synthesis, polymerization, sterilization, and food irradiation

pallation Neutron Source Spallation Neutron Source

http://neutrons.ornl.gov http://neutrons.ornl.gov

T.E. Mason, et al., “The Spallation Neutron Source: A Powerful Tool for Materials Research.” T.E. Mason, et al., “The Spallation Neutron Source: A Powerful Tool for Materials Research.”

http://arxiv.org/abs/physics/0007068 http://arxiv.org/abs/physics/0007068

A frequently cited article describing the equipment used in research. Select Download PDF A frequently cited article describing the equipment used in research. Select Download PDF.

The World of Synchrotron Radiation The World of Synchrotron Radiation

http://www.srs.dl.ac.uk/SRWORLD">http://www.srs.dl.ac.uk/SRWORLD http://www.srs.dl.ac.uk/SRWORLD

Locations of facilities Locations of facilities.

Altarelli et al., 1998 Massimo Altarelli, Fred Schlacter, Jane Cross, Ultrabright X-ray Machines Scientific American December 199866 ff-

D'Amico et al., 1996 Kevin L. D'Amico, Louis J. Terminello, David K. Shuh, Synchrotron Radiation Techniques in Industrial, Chemical, and Materials Science 1996 Plenum Press New York Emphasis on structural biology and environmental science

Hofmann, 2004 Albert Hofmann, The Physics of Synchrotron Radiation 2004 Cambridge University Press New York

Helliwell and Rentzepis, 1997 J.R. Helliwell, P.M. Rentzepis, Time-resolved Diffraction 1997 Oxford University Press Oxford, UK Research on time-dependent structural changes by use of X-rays (including synchrotron radiation), electrons, and neutrons