Although radiation is often associated with nuclear and medical technology in people’s minds, it is a general term used to describe any form of energy that can be transmitted over large distances without special conductors or conduits. For example, the sun radiates both its heat and light through space. Microwave towers radiate energy as signals for TV and mobile phone networks. The cathode in a TV tube radiates electrons that generate the picture and in hospitals, X-ray machines radiate energy to image body parts. In addition, certain radioactive materials emit radiation of several types. Many sources of radiation, such as the light bulbs mentioned above, have little or no direct effect on the surfaces they illuminate. However other forms of radiation can penetrate and interact with the materials they strike. The sun can burn the skin as its ultraviolet component actually penetrates and deposits energy in the body cells. Likewise infrared and microwave radiation penetrate food and cook it. X-rays and electron beams are even more penetrating and more dangerous than heat, light and microwaves. Electrons, X-Rays and gamma rays ionize the material they strike by stripping electrons from the atoms of the exposed material. This ionized environment is very damaging to the bacteria, viruses or insects and can also change the chemical structure of materials. Irradiation is simply the act of applying radiation (or radiant energy) to some material. Irradiation by penetrating electrons, X-rays and gamma rays ionizes materials rather than simply heating them. Radioactivity is quite a different matter. The natural elements are composed of a nucleus (of protons and neutrons) surrounded by orbiting electrons. For most elements (the stable elements) the number of nuclear particles is fixed. However, a few, equally “natural” elements such as uranium, radium, and thorium come with an unstable number of atomic particles in their nucleus. Periodically these elements eject energy and nuclear particles to achieve a more stable configuration. The energy that is emitted is radiation and the process itself is called radioactive decay or disintegration. The elements that undergo these disintegrations are called radioactive materials. Stable elements can also be made unstable by exposure to intense nuclear radiation as in nuclear power generating plants, nuclear explosions and high energy particle accelerators. In the public mind there is a unclear association between irradiation and radioactive materials (which if leaked or spread would be a health hazard) leading to the erroneous conclusion that irradiation is dangerous. There are two ways in which radioactive materials are associated with industrial irradiation, and an understanding of the connection shows why such worries are unfounded. The first connection between radioactivity and irradiation is associated with the radiation source. Radioactive materials can and often are used as a source of radiation energy. The most widely used radiation processing source material is a form of Cobalt known as Cobalt 60. This is produced by irradiation of pre-encapsulated stable Cobalt 59 in the intense neutron fields of a nuclear power reactor. After irradiation, the Cobalt 60 decays and the energy released in the decay creates a penetrating X-ray like beam of radiation. If these sources are used it is essential they remain encapsulated and shielded. The second way in which radioactivity is discussed in connection with industrial irradiation is mainly a theoretical concern. Radioactive materials can be created when very high energy particles (as created in nuclear reactors and very high energy electron accelerators) bombard a target. In this case, the radiation energy entering the target material can not only ionize it, it can transform a stable element into an unstable one. This is called induced radioactivity. After extensive research, it has been established and internationally agreed, that keeping the energy of machine sources below certain well defined thresholds will ensure that any such induced radioactivity will be negligible.

Characteristics Of Electrons, X-Rays And Gamma Rays.

There are three forms of ionizing radiation of practical importance in industrial irradiation, electrons, X-rays and gamma rays.

Electrons, the electrically charged particles that flow in wires as electricity, or strike the face of a television set to make the image, may seem different from the X-ray and gamma ray forms of radiation because they are usually thought of as particles. However, at high energies, beams of electrons can penetrate solid materials and they cause the same ionizing effect. Electron particle beams are therefore referred to as radiation beams even though their particulate nature is somewhat different from the wave nature of an X-ray or gamma ray.

When electron beams strike a target such as a plastic medical device, most of the energy goes towards ionizing atoms and killing micro-organisms. If however, the electron beam is stopped in a very dense material such as tungsten or tantalum, some of the energy stimulates the metal atoms to emit X-rays. These X-rays are more penetrating than their parent electrons but only a portion of the electron power is converted. The effective capacity of the plant with an electron-to-X-ray converter is therefore significantly reduced. X-rays are also spread over a wider angle than an electron beam)

Just as X-rays are formed from atoms excited by the electron beam, gamma rays are formed from atoms that have been excited by nuclear radiation. The physical nature of X-rays and gamma rays is identical, although they may differ slightly in penetration. The different names serve to identify the different origins.

A most important difference between X and gamma rays is that X-rays are only generated when the parent electron beam is “on” so an X-ray source can be switched on and off. On the other hand, once a radioactive gamma ray source has been produced, it will continue to radiate for ever, though with decreasing power over time.

Energy

Electron Accelerators. The energy, in joules, that a particle of charge q (coulombs) acquires when it is accelerated in a field of V volts is
E = V.q

When an electron (whose charge is 1.6 x10-19 coulombs) is accelerated in an electric field of 1 Volt its energy becomes 1.6 x10-19 joules. This is more commonly stated as one electronvolt or 1eV. In electron irradiation processing it is common to quote energies, not in microscopic numbers of joules but simply as electron-volts or millions of electron-volts (MeV) where the “volts” factor is equal to the accelerating potential of the source. Not all electrons in a beam have identical energies and it is common for there to be a spectrum or range of energies. The spectrum shape depends on the type of accelerator, and how it is adjusted.

X-Ray Sources. X-rays produced by bombarding heavy metals with electrons have energies which cover a wide range, but never exceed the maximum energy of the parent electrons. It is common to refer to an X-ray spectrum by the nominal energy of the electron beam which generated it. For example, a 5 MeV electron beam is said to generate 5 MeV X-rays even though the parent electron beam and daughter X-rays include a range of energies. For X-Rays, this “energy” is a label which is useful for comparing the penetration of X-Ray sources and for ensuring that the source will meet the international guidelines for maximum energy

Radioisotope Sources. Gamma ray energies depend on the characteristic energy levels of the nucleus of the source material. Cobalt emits two gamma rays with energies of 1.17 and 1.33 MeV. These are therefore less penetrating than the X-rays from a typical accelerator of 5 MeV. The difference is not however of major importance in the operation of the irradiator as both have quite sufficient penetration for most products and the configurations in which they may be packed.

Power

The rate at which radiation energy is delivered defines the power of the radiation source. For an electron beam source: Power (Watts) = Average beam current(amps) x Average beam energy (Volts). For an X-Ray source the power is given by: Power = I_av.E_av. where is the conversion efficiency from electron beam power to X-Ray power. The conversion factor depends on the converter material, its geometry and the energy of the incident electrons. Values seldom exceed 10%. Low incident electron energies produce much lower conversion efficiencies.

A radioactive source is normally characterized not by its power but by its disintegration rate.

By definition:

Power = number of disintegrations/sec. X average energy released per disintegration

The S.I. unit of radioactivity (1 disintegration/sec.) is the becquerel (Bq). However, the bequerel is so small relative to the size of industrial sources that an old unit, the Curie (Ci) or the mega Curie (MCi), are almost always used. For a cobalt-60 source, 67.578 Ci, (a number derived from adding the energies of each of the gamma ray emissions as known from basic physics) emits one watt of radiation power. Thus

1 kW of electron or X-ray radiation is equivalent to 67,578.00 Ci
1MCi of cobalt 60 is equivalent to 14.8 kW of electrons or Xrays

Note that the above equations relate the power equivalence of the sources only. In practice, the efficiency with which the radiation is absorbed by the product is different for each source Thus the amount of material processed by 1 MCi is not the same as that by 14.8 kW of electrons or 14.8 kW of X-rays.

Energy limits for sources

Many materials are naturally radioactive: part of the unavoidable radiation environment in which we have evolved. Bombarding materials with energy can induce further radioactivity. However extensive theoretical and practical research has shown that below certain energy thresholds, any induced radioactivity is insignificant compared with that which is naturally present. These limits have been agreed on by the Joint Expert Committee on Irradiated Foods of the UN Food and Agriculture Organization, the World Health Organization and the International Atomic Energy Agency and are published in the Codex Alimentarius . They have also been accepted by the USFDA and other national bodies.

The energy maxima for food are generally applied as safe limits for the irradiation of all other items. However, higher energies may be used when the material contained in the product is rigorously controlled and can be shown not to contain any element that can be activated at the proposed beam energy.

Penetration

The different forms of radiation penetrate items to quite different degrees. Electrons are much less penetrating than X-Rays and gamma rays. Most electrons collide with the product irradiated within a few atomic distances of the surface but each collision creates secondary electrons under the surface. These continue to create more electrons in a shower effect. Radiation is scattered forward and the peak dose actually lies a short distance below the surface. Thereafter it diminishes quite quickly. Electrons from electron accelerators have a usable penetration of about 3 mm in water for each million volts of accelerating potential. A 10 million volt (MeV) beam will therefore penetrate about 3 cm. In lower density materials, the penetration will be correspondingly higher.

If the electrons are converted to X-Rays, the penetration of the X-Rays is an order of magnitude higher but there is a considerable loss of useful radiation power (see

Dose

The dose of radiation is a measure of the radiation energy deposited in unit mass of the material. It is measured in Gray (Gy) and a dose of 1 Gy means 1 joule of radiation energy has been deposited in each kilogram of material. To achieve a specific radiation effect it is necessary to apply a specific dose. For example, to sterilize medical devices doses of the order of 25k Gy are required. Satisfactory control of pathogens such as salmonella and e-coli can be achieved with doses of 1.5 to 3.0 kGy.

Max-Min ratios

The energy deposited in the product by irradiation (the dose) will cause the temperature of the product to rise. Temperature rise is about 0.3 °C for each kilogray of dose when irradiating medical products or food. Temperature increases are important for other uses of radiation such as the degradation of Teflon or the curing of composites where doses required are much higher.

Power-Throughput Formula

The power of the radiation source determines the rate at which product can be processed and hence the maximum total capacity of the plant. In any practical situation, power is wasted in its application to the product. For example, some electrons spill over the edges of the packages in order that the edges be fully treated. Others passe right through the product and emerges on the other side to ensure that the material at the far side gets an adequate dose. Yet other “dose” is wasted because an excess must be provided in some regions to ensure all area are adequately exposed. The processing efficiency factor ? takes into account all losses and thus:

Throughput (T) = Power (W)/Dose (D) . Energy absorption efficiency(a)

where throughput is in kg/sec, power is in watts, efficiency of absorption (a) is in % and dose is in Grays.

Once the beam power needed to treat the plant capacity has been determined, the line speed can be calculated from the dimensions and unit weight of the product.

Line Speed (L) = W.a./(D.d.s)

where d = density and s is the cross sectional area of the product irradiated in the direction of travel of the conveyor.

Electron Accelerators

All electron accelerators include a source (of electrons), an evacuated accelerating chamber, and a system for extraction from the vacuum and distribution over the product surface. Most obtain their electrons from a heated filament source (similar to that of a TV tube) called the electron gun. The energy of these electrons is then increased in one or more stages as they pass through a vacuum with an applied electric field. There are numerous ways to generate this electric field. DC accelerators generate and maintain the full accelerating voltage between just two electrodes. As the voltage is raised to millions of volts, electrical insulation becomes a major engineering problem. Even at low powers, these accelerators can have dimensions exceeding 15m (45 feet). For systems with both the energy and the power needed for industrial irradiation, the most common commercially available models are the Dynamitron®, and Insulated Core Transformer (ICT). The maximum voltage available is usually less than 5 MeV.

A second family of particle accelerators is based on radio frequency (rf) power technology. These high frequency waves generate very intense electrical and magnetic fields in suitably shaped conducting cavities. By matching the field oscillations with the injection of charged particles, the rf fields can drive the particles to high energies without having to create the full final potential at any one instant. They do not therefore require the large scale insulation of DC units and are more compact. They can easily reach energies of 10 MeV, and currently are available in models reaching up to 200 kW of electron beam power.

The rf accelerators that employ a linear series of rf cavities are called linacs. The assembly of copper cavities is referred to variously as a structure, a guide or a waveguide. They operate either at an rf frequency of 3 GHz (known as S-band; more compact but generally limited to beam powers below 20 kW) or 1 GHz (L-band; physically larger but capable of beam powers considerably above 20 kW). The accelerating structure of a typical 10 MeV industrial linac is 2-4m long. The main power tube for linacs is the klystron. These tubes are expensive and have limited operating life. Their replacement expense can become a significant fraction of the operating cost.

A more compact rf accelerator (the Rhodotron®) has been developed. This device generates radial accelerating fields in a single cavity and uses an array of magnets to pass the beam through this cavity zone on repeated orbits. The accelerating structure for the higher power models is a tank about 1.5 m diameter systems).

Differences in physical size between dc and rf accelerators are significant because dc accelerators require costly larger buildings and shields. Differences between the rf models are less important once the total cost of the shielded facility has been included..

Production staff must be protected from the radiation source. This protection or shielding is usually provided by placing the source inside a concrete vault and passing the product into the radiation zone through a maze. Commonly used material for radiation shields are concrete, steel and lead. The thickness of a lead shield will be about 1/5th that of concrete. The higher cost of lead and steel however generally make them uneconomic for all but in-line sterilizers.

A reduction of the level of radiation to less than 1/1,000,000 of the source level is necessary and this requires about 2.5 m of concrete for a 10 MeV beam or 2.0 m for 5 MeV accelerators.

Radiation itself causes degradation in the product conveyor components, particularly where any plastics or lubricants are used. In addition, radiation beams form high concentrations of ozone in air and this is both corrosive and hazardous. To combat the corrosion, extensive use is made of stainless steel and a regular inspection and replacement schedule for sensitive parts has to be established. All radiation plants need powerful ventilation systems to remove the ozone.

Equipment Suppliers
Significant Recent Developments.

The industrial uses of accelerated electrons, and gamma rays from cobalt 60, had both been identified well before 1980. Industries based on low energy electrons (<750 keV) and medium energy electrons (< 5 MeV) had been established for coatings, tires, cable and heat-shrink tubes. The DC technology is very mature and reliable. It dominates in the coatings, wire, cable and tire industries. The use of cobalt-60 for sterilization is routine and several hundreds of plants have been installed.

Prior to 1990, a number of S-band 10 MeV rf linear accelerators were installed in Europe for medical device sterilization. These 10 MeV machines performed satisfactorily up to about 10 kW.

Accelerators with energies near 5 MeV DC accelerators fitted with X-ray converters were commissioned in Japan to compete with cobalt in that market. The conversion technology was demonstrated to be satisfactory.

In the 1990’s the power available from accelerators jumped significantly with AECL’s 10 MeV, 50 kW linac followed quickly by IBA’s Rhodotrons at 100 kW.

The linear rf technologies are fully reliable at 10 MeV and up to 75kW, but despite the efforts of four manufacturers it seems that compact S-band units cannot reliably provide more than 30 kW.

When Russian technology became accessible to western buyers several different types of accelerators were offered for sale. Of these, perhaps the most interesting is the single resonant cavity ILU from the Budker Institute in Novosibirsk. The ILU’s provide energies to 5 MeV at modest powers compared with the Dynamitron and Insulated Core Transformers but are much small in physical size.

The Major Suppliers Of Electron Beam Accelerators

Accelerator Type	Supplier	Country
DC (Dynamitron)	RDI or Radiation Dynamics (a division of IBA)	USA
DC Machine of the ICT Type	Nissin High Voltage	Japan
S Band Linac	L3-PSD (formerly Titan)	USA
S Band Linac	Linac Systems	France
S Band Linac	Mevex	Canada
S Band Linac	Mitsubishi Heavy Industries	Japan
L Band Linac	Iotron Industries	Canada
Rhodotron	IBA (Belgium)	Belgium
ILU	Budker Laboratories	Russia

Electron Applications
Effect Of Radiation On Materials

High energy electrons react with materials by stripping loosely bound electrons to create a positively charged ion (ionization) with a free electron. An ionized atom (otherwise called a free radical) can either collide with a free electron and become neutral again (de-ionize) or react chemically. Free radicals are extremely chemically active and may either degrade or enhance products depending on the reactions that dominate. These chemical reactions have led to the use of radiation for a number of industrial purposes.

The Radiation Processing Industry

The irradiation industry is broadly defined to include the processing of materials by gamma radiation and industrial accelerators. Materials which use irradiation as part of their manufacturing process today include medical products, plastics, rubber, wire and cable, some spices, and a small volume of certain food products.

Service centres currently handle medical sterilization, specialty polymers, degradation of Teflon®, specialty wire and cable, gemstones, spices, and laboratory animal feed.

The sterilization of medical products such as surgical gowns, gloves, and syringes composes the bulk of the market for materials processed by highly penetrating radiation today. The size of the market is estimated at 325 million cubic feet of product sterilized annually in North America, and at least 650 million cubic feet globally. There are currently over 50 irradiation facilities in North America, with about half being operated in house by manufacturers and the remainder operated by contract service firms who process products manufactured by other companies for a fee.

As of 2002 there were 29 contract irradiation plants (20 cobalt 60 and 9 electron beam) in North America. There are a number of companies currently operating service centres, the largest being Steris (Isomedix) and Sterigenics International in the U.S. and Isotron plc in Europe.

Sterilization

Sterilization of medical products is presently served by three main technologies - ethylene oxide, gamma radiation (cobalt 60), and electron accelerators. Ethylene oxide sterilisation is an effective and long established method which accounts for more than 50% of the medical sterilization market in the U.S. It is used whenever the product properties would be adversely affected by radiation. It cannot be used on hermetically sealed packaging, on items with sealed inner cavities and where residues are of clinical importance. Cobalt 60, a commercially made isotope, is the industry standard for radiation sterilization and gained market share from ethylene oxide in the 1970’s to 1990’s. The technology is well established and is cost effective at medium volume levels. However, electron accelerators can be more cost effective at higher throughputs. Electron accelerator technology has been in existence for some decades though only in recent years have higher energy and higher power electron beam accelerators been developed for sterilisation plants. The effect of radiation on the medical products is important as this energy may cause crosslinking or degradation of plastics. Special, radiation resistant materials have been formulated for a large number of products.

Typical Electron Beam Plant

Inline sterilization

Curing and cross-linking of plastics are the largest existing industrial applications of radiation processing as there are more than 500 low-energy accelerators currently in use for these purposes. Manufacturing companies use accelerators to cross-link polymers to enhance their physical properties in producing a wide range of industrial and consumer products.

The market for high-power electron beam treatment includes the molecular weight tailoring of plastics in pellet form and the cross-linking of post formed products such as cables and pipes. Post-formed cross-linking improves the performance of traditionally made plastics by changing its properties to provide scuff resistance, fire retardance, and other benefits. Because volumes have not yet justified dedicated plants, polymer cross-linking is primarily a service centre business.

Composites

Cellulose/Viscose

Industries making products from cellulose could use electron beams but have as yet not put the technology into commercial production. Viscose producers transform cellulose pulp into a syrupy liquid called viscose through a reaction with carbon disulphide followed by dissolution in alkali. The viscose is then extruded and spun to produce filaments and fibres for textiles, diapers, cellophane, tapes etc. The use of electron-treated cellulose in the viscose process offers significant advantages. Electron processing renders the cellulose more accessible to chemicals and reduces the amount of alkali, carbon disulphide, and acid used in the process. In addition to potential savings of about U.S. $3 million per annum for a typical plant (a short pay-back period of 2 to 3 years on the initial investment), the lower chemical demand translates into reduced emissions of polluting chemicals, a key factor as viscose producers are facing increasingly strict environmental regulations. Electron processing may allow a plant to stay in operation under current emission standards, or expand its operation without the need for further pollution control. The economic and environmental benefits offered by electron processing present a very attractive opportunity in this industry.

The electron beam accelerator can also be applied to thermomechanical pulp production. Pre irradiating the chips leads to reduces the energy needed for pulping, however at the cost of some paper strength. The principles have been demonstrated but never implemented commercially.

Quarantine

Quarantine treatments are differentiated from food irradiation treatments for two reasons. Some quarantine treatments are used on non-food items. These include tatami mats, bark products, bird seed talcum powder &c. . There is no limit to the dose that can be applied for these products. For food items (e.g. lettuces exported from Mexico to the USA) the need is to eliminate insects being. It is necessary to select a dose high enough to kill or prevent the target insect from reproducing while sufficiently low that it does not damage the food.

Opportunities for electron processing exist in the agricultural sector in the treatment of grain and specialized seed, high grade hay, lumber and wood chips, to eliminate insects and open new export markets. Animal feed and fish meal are also products that would benefit directly from bulk low cost sterilization.

Food

Given the heightened concerns in the U.S. over bacteria in meat and poultry products and the large volume of hamburger and chicken consumed, the food irradiation market has tremendous potential. There are a number of applications within the food area where irradiation offers important benefits, the most important being the elimination of pests and microbes in agricultural commodities and the elimination of food borne disease primarily in meat and poultry. The market for eliminating pests and microbes in agricultural commodities as a quarantine measure has been small historically because of the use of methyl bromide fumigation which is a very low cost treatment. However, the reduction in use or outright ban is currently being considered.

Environmental

Treatment of waste and toxic materials through electron beam processing has long term potential. The primary obstacle to the development of these markets is cheaper alternative disposal methods such as dumping. There have been demonstrations of sewage sludge processing, in Germany, Japan and Florida. None has moved from the demonstration phase to commercial use. For example, a major US civil engineering firm submitted a proposal to a mid sized city several years ago for dewatering and disinfecting sewage sludge. The city's independent review showed that irradiation was equal to the lowest cost biological technique and only marginally more expensive than the cost to spread non-sterile sewage on farmland. Despite this, a decision was made to continue to spread the sewage on farmland.

Degradation

All polymers degrade if irradiated to a high enough dose. The level at which the polymer loses its function differs greatly between polymers. An important material that loses function at relatively low doses is PTFE (Teflon?).

Electron accelerators are used to degrade materials which are not readily broken down by other means. The most widespread use is in the degradation of both scrap and virgin Teflon® to enable it to be ground to a fine powder. The opportunity is also available to degrade toxic chemicals and some of these applications have been demonstrated in laboratory tests and in plants. For example, pollutants from ground water near a synthetic rubber plant in Russia were removed using electron irradiation.

Gemstones

At high doses, certain gemstones change colors when irradiated. This is widely used in the production of topaz for jewellery.

Conferences, Journals And Websites.

The radiation processing industry holds a biennial conference under the title IMRP (International Meeting on Radiation Processing) under the umbrella of an industry association called IIA (International Irradiation Association or double-eye-A). Proceedings of papers presented are normally published in the Journal, Radiation Physics and Chemistry.

APPENDIX

The following table and chart summarize results of a study carried out by I-Ax Technologies Inc several years ago.

Together they represent a power of 2,984 kilowatts or the equivalent of 199 Megacuries of cobalt. Installations by year show no systematic trend (see Figure 1). They have been built by 24 different manufacturers, of which only 12 are currently active.

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