Industrial applications of electron accelerators M.R. Cleland Ion Beam Applications, Edgewood, NY 11717, USA Abstract This paper addresses the industrial applications of electron accelerators for modifying the physical, chemical or biological properties of materials and commercial products by treatment with ionizing radiation. Many beneficial effects can be obtained with these methods, which are known as radiation processing. The earliest practical applications occurred during the 1950s, and the business of radiation processing has been expanding since that time. The most prevalent applications are the modification of many different plastic and rubber products and the sterilization of single-use medical devices. Emerging applications are the pasteurization and preservation of foods and the treatment of toxic industrial wastes. Industrial accelerators can now provide electron energies greater than 10 MeV and average beam powers as high as 700 kW. The availability of high-energy, high-power electron beams is stimulating interest in the use of X-rays (bremsstrahlung) as an alternative to gamma rays from radioactive nuclides. 1 Introduction Radiation processing can be defined as the treatment of materials and products with radiation or ionizing energy to change their physical, chemical or biological characteristics, to increase their usefulness and value, or to reduce their impact on the environment. Accelerated electrons, X-rays (bremsstrahlung) emitted by energetic electrons, and gamma rays emitted by radioactive nuclides are suitable energy sources. These are all capable of ejecting atomic electrons, which can then ionize other atoms in a cascade of collisions. So they can produce similar molecular effects. The choice of energy source is usually based on practical considerations, such as absorbed dose, dose uniformity (max/min) ratio, material thickness, density and configuration, processing rate, capital and operating costs. In the case of electron beam (EB) processing, the incident electron energy determines the maximum material thickness, and the electron beam current and power determine the maximum processing rate. In the case of X-ray processing, the emitted power increases with the electron energy and beam power. For high throughput industrial processes, the capital costs and operating costs of an irradiation facility are competitive with more conventional treatment methods. Successful irradiation processes provide significant advantages in comparison to typical thermal and chemical processes, such as higher throughput rates, reduced energy consumption, less environmental pollution, more precise control over the process and the production of products with superior qualities. In some applications, radiation processing can produce unique effects that cannot be duplicated by other means. Radiation processing was introduced more than fifty years ago, and many useful applications have since been developed. The most important commercial applications involve modifying a variety of plastic and rubber products, and sterilizing medical devices and consumer items. Emerging applications are pasteurizing and preserving foods, and reducing environmental pollution. 383 M.R. CLELAND 2 Basic concepts of radiation processing 2.1 Absorbed dose definition The most important specification for any irradiation process is the absorbed dose. The quantitative effects of the process are related to this factor. Absorbed dose is proportional to the ionizing energy delivered per unit mass of material. The international unit of dose is the gray (Gy), which is defined as the absorption of one joule per kilogram (J/kg) [1]. A more convenient unit for most radiation processing applications is the kilogray (kJ/kg or J/g). An older unit is the rad, which is defined as the absorption of 100 ergs per gram or 10–5 joules per gram. So, 100 rads is equivalent to 10–3 joules per gram or 1 joule per kilogram or 1 gray. The rad unit is now obsolete, but many commercial processes are still specified in rads, kilorads or megarads. Absorbed dose requirements for various industrial processes cover a wide range, from 0.1 kGy to more than 1000 kGy, as indicated by the applications listed in Table 1. Most of these processes need less than 100 kGy, some need less than 10 kGy and some need even less than 1 kGy. Table 1: Absorbed dose requirements for various industrial processes Sprout inhibiting 0.1–0.2 kGy Sterilization 10–30 kGy Insect disinfesting 0.3–0.5 kGy Polymerization 20–50 kGy Parasite control 0.3–0.5 kGy Grafting monomers 20–50 kGy Delay of ripening 0.5–1.0 kGy Crosslinking polymers 50–150 kGy Fungi control 1.5–3.0 kGy Degrading polymers 500–1500 kGy Bacteria control 1.5–3.0 kGy Coloring gemstones >>> 1500 kGy 2.2 Temperature rise vs absorbed dose If the energy transfers from chemical reactions are negligible, then the adiabatic temperature rise (ΔT) from the absorption of thermal energy per unit mass (H) is given by the following equation: ΔT = H/c (1) where ΔT is in ºC, H is in J/g and c is the thermal capacity in J/g ºC. Similarly, the adiabatic temperature rise from the absorption of ionizing energy is given by: ΔT = D(ave)/c (2) where D(ave) is the average dose in kGy (kJ/kg or J/g), and ΔT and c are the same as in Equation (1). The thermal capacity of water is 4.19 J/g ºC, so the adiabatic temperature rise would be 0.24 ºC with an average absorbed dose of 1.0 kGy. Most other materials have lower thermal capacities and higher rises in temperature with the same dose. For example, the thermal capacity of polyethylene is 2.3, polytetrafluoroethylene is 1.05, aluminum is 0.90, copper is 0.38 and tantalum is 0.15 J/g ºC. Typical doses for pasteurizing fresh meat are in the range of 2 to 3 kGy. Since this material is about 80% water, the adiabatic temperature rise would be in the range of 0.5 to 0.7 ºC. On the other hand, when electrical wire receives a typical dose of 100 kGy to crosslink the insulation, the temperature rise of the copper conductor could be as high as 260 ºC. This excessive temperature rise can be reduced by passing the wire many times back and forth through the electron beam to allow most of the heat to dissipate in the air and in the underbeam wire handling fixture. 384 INDUSTRIAL APPLICATIONS OF ELECTRON ACCELERATORS 2.3 Absorbed dose vs molecular weight and G value The yields of radiation-induced chemical reactions are indicated by their G values. This is the number of molecules or ions produced or destroyed per 100 eV of absorbed ionizing energy. Typical G values are in the range of 1 to 10, i.e. the energy consumption is in the range of 100 eV to 10 eV per molecule. Since the ionization potentials for the light elements (H, C, N, O) in most polymeric materials are in the range of 10 eV to 15 eV, any energy in excess of the ionization potential must be dissipated by atomic and molecular excitations and ultimately degraded to thermal energy. The absorbed dose D is related to the G value and the relative molecular mass Mr (which is commonly called the molecular weight) as follows: D = NA(100/G)e/Mr (3) where NA is the Avogadro constant (number of molecules per mole), 100/G is the number of electron volts absorbed per reactive molecule, e is the electron charge in coulombs, which is also the conversion factor from electron volts to joules, and Mr represents the mass per mole in grams. This 23 equation gives the dose in J/g or kJ/kg, therefore in kGy. By substituting NA = 6.022 × 10 and e = 1.602 × 10-19, Equation (3) can be written as follows [2–4]: 6 D = 9.65 × 10 /(MrG) (4) Equation (4) indicates that polymeric materials with high relative molecular masses will be attractive candidates for radiation processing because the dose will be acceptable. For example, if the Mr value is 100,000 and the G value is about 1, which would be typical for crosslinking common plastics, then the dose required to convert all of the molecules in the irradiated material would be about 100 kGy. Industrial irradiation processes for modifying plastics often use doses in the range of 50 kGy to 150 kGy. By the same consideration, inorganic compounds with much lower relative molecular masses would not be suitable because the dose would be excessive. By combining Equation (4) with Equation (12) from Section 4.2 below, the mass throughput equation can be expressed as follows: 6 M/T = FpPMrG/(9.65 × 10 ) (5) where M/T is in kg/s, P is the emitted power in kW and Fp is the fraction of power absorbed by the material. Equation (5) also indicates the advantage of applying radiation processing to materials with high values of the relative molecular mass Mr and high G values [5–7]. Even so, the treatment of dilute solutions can present exceptions to this rule. Examples of such applications are the extraction of sulfur and nitrogen oxides from combustion gases to reduce the effects of acid rain, and the decomposition of toxic chemicals in industrial waste water. In these cases, the relative molecular masses are low, but the concentrations of the polluting molecules are also very low, and they represent only a small fraction of the total mass. Most of the radiation energy will be absorbed by the solvent material, but only a small fraction of the solvent must be ionized to modify or decompose the pollutants by secondary reactions. 3 Applications of radiation processing 3.1 Modifying polymeric materials Many papers on radiation effects in polymeric materials have been published in the thirteen proceedings of the International Meetings on Radiation Processing [8], the five International 385 M.R.