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Recent advances in electron processing
Radiation Physics Chemistry Vol. 22 No. 1/2, pp. 77-79, 1985
0146-5724/83/07077-03505.00/0 © 1983 Pergamon Press Ltd.
Printed in Great Britain
RECENT ADVANCES IN ELECTRON PROCESSING
Walter J. Chappas Laboratory for Radiation and Polymer Science Institute for Physical Science and Technology University of Maryland College Park, MD 20742 U.S.A.
INTRODUCTION The June edition of the EPRI Journal contains an article which describes the "electrification" of America (Wayne and others, 1982). It carefully outlines the principal power sources for several major industries and describes why many companies are making the move to electricity. It also addresses an important question: How can electricity, an energy source twice the price of oil and four times the price of gas expect to alleviate inflation or improve a company's competitive edge in world markets? One answer, or at least part of the answer, is electricity's unique properties. Perhaps the most unique and versatile form of electricity is high energy electron beams. Competition and Electron Processing Electron beams provide world industry with approximately 15 MW of the most controllable, intense, flexible, and environmentally safe energy now available. Yet, it is naive to believe any new technology, even one with these remarkable characteristics, will avoid competition. It should be expected from both conventional energy sources and established technologies. For example, the inertial problems of existing capital investments, even within successful corporations, is often insurmountable during these times of high and unstable interest rates, a situation that is often further aggravated when the conversion requires major federal regulatory changes. However, the most perplexing roadblock to electron processing's development is industry's resistance to change. There has to be a very good reason for a successful company to abandon a solid manufacturing program. That impetus, the driving force that can push a company over the acceptance activation barrier, is economic. It comes in the form of electron processing's higher production rates, lower operating expenses (usually in the form of less facility space, fewer employees, etc.), and reduced material loss. There is one other source of competition-- gamma radiation from cobalt-60. Yet the controversial issues that have raged between these two opposing camps for almost three decades are almost irrelevant today. For example, applications requiring high dose rates (kGy/s), high processing speeds, small penetration depths, or high powers (>i00 kW), are absolutely dominated by electrons. These include: (i) the curing of wire and cable insulation, (2) the crosslinking of heat shrinkable connectors and packaging, (3) the sulfurless vulcanization of rubber products, and (5) the removal of toxic components of stack gas. Not all processes are so dominated by electrons. The sterilization of medical supplies, the treatment of waste water and sewage, and the preservation of food do not necessarily require high dose rates or electrons. High dose rates can reduce material damage, odor problems, and color changes, but they are not essential for these applications, especially with the development of new plas£ic formulations for packaging and products. For this group of processes electrons and gammas can be competitors and, depending on the thickness Of the product, gammas may dominate. Even though gamma sources provide less than 10% of the world's total radiation processing power they are a valuable high-penetration, reliable, and still indispensable radiation processing complement to electron beams. ~PC 22-I/2-G
77
78
Low Energy Applications:
W. J. CHAPPAS
Floppies
and Warps
Two new and promising applications for low energy accelerators (machines with energies between 150 keV and 300 keV) are: (i) the manufacture of floppy disks and magnetic tapes, and (2) the modification of fabrics. The manufacture of the former is a difficult, carefully controlled, and highly monitored process. It is the kind of process application where electron beams may be perfectly suited. The textile industry, on the other hand, after a decade of hardship, may finally be ready to accept a new technology. Financial World magazine (Taub 1982) has even earmarked the industry as an outstanding investment opportunity giving it high marks on creative management and flexibility toward new technologies. The radiation industry may soon have an opportunity to put to work a range of marvelous radiation techniques for the manufacture of (i) non-wrinkle, (2) permanent press, (3) flame retardant, (4) soll resistant, (5) water repellent, and (6) dyeable textiles and non-woven fabrics. Medium Energy Applications:
Brem and SO 2
In the 300 keV to 5 MeV region simple accelerators give way to a variety of more sophisticated systems. Only two, the ICT and Dynamitron, provide the medium energy, high power, efficient, simple, and durable electron machines required by industry. The Dynamitron has historically led the radiation processing industry by relentlessly pushing to higher energies and power levels. Today's Dynamitron has been manufactured with energies as high as 4.5 MeV and power levels to 200 kW, and a 6 MeV - 300 KW machine may very soon become a reality. This device is particularly interesting as it might provide an economical means of producing bremsstrahlung. The conversion of low penetration electrons to high penetration bremsstrahlung or X-rays is an old concept that has been used in medical and dental offices for many years. Yet its use as an industrial tool has, up to recently, been economically prchibitive. The reason is simple -- the conversion efficiency. As an electron slows a portion of its energy is converted to high energy photons. The fraction of energy converted depends on: (i) the energy of the electron, and (2) the atomic number of the target. A 3 MeV electron striking a medium Z target would have perhaps 20% of its energy converted to bremsstrahlung, and a smaller portion of that "brem" radiation would be usable. A $1.5 million 3 MeV - 300 kW machine that can provide only 5 or lOkW of usable X-rays (approximately $2/Ci equivalent) cannot compete with cobalt-60. The 6 MeV device can, however, achieve three times this conversion and might drop the price to less than $1/Ci equivalent. A prime candidate for one of the first converters is the 4 MeV - 200 kW Dynamitron recently purchased by Becton-Dickinson. This Becton-Dickinson purchase is important for another reason. It is confirmation that 4 MeV electrons can, with the proper choice of packaging and product geometry, sterilize disposable medical supplies in a reliable and uniform manner. Becton-Dickinson's acquisition of this machine is confirmation that shadow effects, caused by density changes through a package, can be satisfactorily reduced or even eliminated. This facility may become a prototype operation for the medical supply and food industry, providing low cost 4 MeV electrons at high dose rates for those packages where color, odor, and material damage are critical, and bremsstrahlung for that fraction of the product line where high penetration is essential. The ICT, in contrast, cannot achieve the highest energy and power levels of the Dynamitron. It is, however, superior to the Dynamitron in one very important way -- its line power to beam efficiency. This may be particularly valuable for the processing of sewage, waste water, and the removal of toxic effluent from stack gas. All of these applications have been pioneered by the Japanese. Ebara, for example, has developed methods for the radiation removal of NO and SO from stack gas, and with Nippon Steel has built a pilot operation at a sinter x x plant. Work is now underway to develop a system for coal combustion flue gas containing even higher concentrations of SO . It is in these large scale applications, where one facility may x have several megawatts of electron processing power, that the ICT with its higher efficiency may very well dominate. High Energy Applications:
SW and ~
There are several types of high energy accelerators, yet only one, the electron linear accelerator (linac), currently offers the energy, power, and reliability needed for modern industrial processing. Microwave powered linacs are available from several vendors with energies to i0 MeV (the maximum useful energy for most processing applications) and powers of
Recent advances
in electron processing
79
50 kW. The unreliable R.F. amplifiers of the past have been replaced by a new generation of klystrons with operating lives of 20,000 hours, the power supplies have been redesigned, and of course everything is solid state and computerized. Some small linacs simply require the push of a button for routine use and the flip of a switch to change energies. Yet with all of these recent advances only a few units are in relatively routine industrial use. One of these, an old Hughes linac at Becton-Dickinson, was recently decommissioned. The three contract linac facilities, IRT in the United States, Raychem ApS in Denmark, and CARIC in France continue to operate successfully, even with antedated accelerators. Also, linac vendors, such as RDI Ltd, RPC Industries, Varian Associates, and CGR MeV are maintaining solid linac manufacturing programs with the expectation of a major linac market. The linac, in addition to increased penetration, has some other advantages over lower energy D.C. machines. For example it can be used both as an electron device and a bremsstrahlung generator. A linac would provide X-rays with more than three times the efficiency of a 4 MeV device and would still give the user the option of using the high-dose-rate low-cost electron mode on a much larger fraction of their product line. A second advantage is the linac's pulsed characteristics. Unlike a direct current machine, the linac uses a few microseconds pulse followed by milliseconds of dead time. This thousand-fold increase in peak dose rate may, in some instances, reduce material damage or improve biological kill. Thirdly, the higher penetration reduces the chance of charge build-up and subsequent dielectric breakdown. This of particular importance to the manufacturers of thick insulation wire and cable, especially as the need for heavy gauge transmission lines becomes increasingly important for the economical distribution of electric power. Finally, the increase in energy and penetration will open the door for a wide variety of new products and processes now unobtainable. One problem which should be of concern to linac vendors is the Codex Alimentarius Commission's recommendation that the irradiation of food be held to X-rays of 5 MeV or less. Modification of this recommendation is likely, however, since there is no significant difference between the activation crosssections for i0 MeV electrons, already accepted, and i0 MeV photons. I find it difficult to even imagine an application or geometry that might result in an unacceptable situation. Finally, AECL has an interesting program to develop a continuous wave linac. This program, headed by Joseph McKeown, would produce a high-powered high-conversion linac that could compete with the D.C. machines, especially in the 3-6 MeV region and at power levels in excess of i00 kW. CONCLUSION In response to international and industrial needs, the electrification of the world is taking place, and electron processing is playing an increasingly important role. Its fine process control, high efficiencies, and greater potential for automation have made it a basic industrial processing tool. The questions no longer lie in the relative advantages or disadvantages of electrons or cobalt-60, or in the respective merits of the ICT or Dynamitron. There is a more important problem, one concerning industrial operations, energy, and productivity; how can electron processing expect to alleviate the problems of land, labor, capital, energy, raw materials, and environmental protection? These are the problems that confront the world and control the standard of living, and the remarkable properties of electron beams are an important part of the solution. Aqknowledgement s The author gratefully acknowledges the counsel and support of Dr. V. Markovi~, Mrs. W. Stevens' keen editing eye, and the insight and depth Professor J. Silverman has given to this paper. REFERENCES Taub, S. (1982). Why the Interest in Textile. Financial World, June, 44~ Wayne, M., R. Mauro, and P. Schmidt (1982). Electroteehnology: Sparking Production dustry. EPRI Journal, June, 6.
in In-
0146-5724/83/07077-03505.00/0 © 1983 Pergamon Press Ltd.
Printed in Great Britain
RECENT ADVANCES IN ELECTRON PROCESSING
Walter J. Chappas Laboratory for Radiation and Polymer Science Institute for Physical Science and Technology University of Maryland College Park, MD 20742 U.S.A.
INTRODUCTION The June edition of the EPRI Journal contains an article which describes the "electrification" of America (Wayne and others, 1982). It carefully outlines the principal power sources for several major industries and describes why many companies are making the move to electricity. It also addresses an important question: How can electricity, an energy source twice the price of oil and four times the price of gas expect to alleviate inflation or improve a company's competitive edge in world markets? One answer, or at least part of the answer, is electricity's unique properties. Perhaps the most unique and versatile form of electricity is high energy electron beams. Competition and Electron Processing Electron beams provide world industry with approximately 15 MW of the most controllable, intense, flexible, and environmentally safe energy now available. Yet, it is naive to believe any new technology, even one with these remarkable characteristics, will avoid competition. It should be expected from both conventional energy sources and established technologies. For example, the inertial problems of existing capital investments, even within successful corporations, is often insurmountable during these times of high and unstable interest rates, a situation that is often further aggravated when the conversion requires major federal regulatory changes. However, the most perplexing roadblock to electron processing's development is industry's resistance to change. There has to be a very good reason for a successful company to abandon a solid manufacturing program. That impetus, the driving force that can push a company over the acceptance activation barrier, is economic. It comes in the form of electron processing's higher production rates, lower operating expenses (usually in the form of less facility space, fewer employees, etc.), and reduced material loss. There is one other source of competition-- gamma radiation from cobalt-60. Yet the controversial issues that have raged between these two opposing camps for almost three decades are almost irrelevant today. For example, applications requiring high dose rates (kGy/s), high processing speeds, small penetration depths, or high powers (>i00 kW), are absolutely dominated by electrons. These include: (i) the curing of wire and cable insulation, (2) the crosslinking of heat shrinkable connectors and packaging, (3) the sulfurless vulcanization of rubber products, and (5) the removal of toxic components of stack gas. Not all processes are so dominated by electrons. The sterilization of medical supplies, the treatment of waste water and sewage, and the preservation of food do not necessarily require high dose rates or electrons. High dose rates can reduce material damage, odor problems, and color changes, but they are not essential for these applications, especially with the development of new plas£ic formulations for packaging and products. For this group of processes electrons and gammas can be competitors and, depending on the thickness Of the product, gammas may dominate. Even though gamma sources provide less than 10% of the world's total radiation processing power they are a valuable high-penetration, reliable, and still indispensable radiation processing complement to electron beams. ~PC 22-I/2-G
77
78
Low Energy Applications:
W. J. CHAPPAS
Floppies
and Warps
Two new and promising applications for low energy accelerators (machines with energies between 150 keV and 300 keV) are: (i) the manufacture of floppy disks and magnetic tapes, and (2) the modification of fabrics. The manufacture of the former is a difficult, carefully controlled, and highly monitored process. It is the kind of process application where electron beams may be perfectly suited. The textile industry, on the other hand, after a decade of hardship, may finally be ready to accept a new technology. Financial World magazine (Taub 1982) has even earmarked the industry as an outstanding investment opportunity giving it high marks on creative management and flexibility toward new technologies. The radiation industry may soon have an opportunity to put to work a range of marvelous radiation techniques for the manufacture of (i) non-wrinkle, (2) permanent press, (3) flame retardant, (4) soll resistant, (5) water repellent, and (6) dyeable textiles and non-woven fabrics. Medium Energy Applications:
Brem and SO 2
In the 300 keV to 5 MeV region simple accelerators give way to a variety of more sophisticated systems. Only two, the ICT and Dynamitron, provide the medium energy, high power, efficient, simple, and durable electron machines required by industry. The Dynamitron has historically led the radiation processing industry by relentlessly pushing to higher energies and power levels. Today's Dynamitron has been manufactured with energies as high as 4.5 MeV and power levels to 200 kW, and a 6 MeV - 300 KW machine may very soon become a reality. This device is particularly interesting as it might provide an economical means of producing bremsstrahlung. The conversion of low penetration electrons to high penetration bremsstrahlung or X-rays is an old concept that has been used in medical and dental offices for many years. Yet its use as an industrial tool has, up to recently, been economically prchibitive. The reason is simple -- the conversion efficiency. As an electron slows a portion of its energy is converted to high energy photons. The fraction of energy converted depends on: (i) the energy of the electron, and (2) the atomic number of the target. A 3 MeV electron striking a medium Z target would have perhaps 20% of its energy converted to bremsstrahlung, and a smaller portion of that "brem" radiation would be usable. A $1.5 million 3 MeV - 300 kW machine that can provide only 5 or lOkW of usable X-rays (approximately $2/Ci equivalent) cannot compete with cobalt-60. The 6 MeV device can, however, achieve three times this conversion and might drop the price to less than $1/Ci equivalent. A prime candidate for one of the first converters is the 4 MeV - 200 kW Dynamitron recently purchased by Becton-Dickinson. This Becton-Dickinson purchase is important for another reason. It is confirmation that 4 MeV electrons can, with the proper choice of packaging and product geometry, sterilize disposable medical supplies in a reliable and uniform manner. Becton-Dickinson's acquisition of this machine is confirmation that shadow effects, caused by density changes through a package, can be satisfactorily reduced or even eliminated. This facility may become a prototype operation for the medical supply and food industry, providing low cost 4 MeV electrons at high dose rates for those packages where color, odor, and material damage are critical, and bremsstrahlung for that fraction of the product line where high penetration is essential. The ICT, in contrast, cannot achieve the highest energy and power levels of the Dynamitron. It is, however, superior to the Dynamitron in one very important way -- its line power to beam efficiency. This may be particularly valuable for the processing of sewage, waste water, and the removal of toxic effluent from stack gas. All of these applications have been pioneered by the Japanese. Ebara, for example, has developed methods for the radiation removal of NO and SO from stack gas, and with Nippon Steel has built a pilot operation at a sinter x x plant. Work is now underway to develop a system for coal combustion flue gas containing even higher concentrations of SO . It is in these large scale applications, where one facility may x have several megawatts of electron processing power, that the ICT with its higher efficiency may very well dominate. High Energy Applications:
SW and ~
There are several types of high energy accelerators, yet only one, the electron linear accelerator (linac), currently offers the energy, power, and reliability needed for modern industrial processing. Microwave powered linacs are available from several vendors with energies to i0 MeV (the maximum useful energy for most processing applications) and powers of
Recent advances
in electron processing
79
50 kW. The unreliable R.F. amplifiers of the past have been replaced by a new generation of klystrons with operating lives of 20,000 hours, the power supplies have been redesigned, and of course everything is solid state and computerized. Some small linacs simply require the push of a button for routine use and the flip of a switch to change energies. Yet with all of these recent advances only a few units are in relatively routine industrial use. One of these, an old Hughes linac at Becton-Dickinson, was recently decommissioned. The three contract linac facilities, IRT in the United States, Raychem ApS in Denmark, and CARIC in France continue to operate successfully, even with antedated accelerators. Also, linac vendors, such as RDI Ltd, RPC Industries, Varian Associates, and CGR MeV are maintaining solid linac manufacturing programs with the expectation of a major linac market. The linac, in addition to increased penetration, has some other advantages over lower energy D.C. machines. For example it can be used both as an electron device and a bremsstrahlung generator. A linac would provide X-rays with more than three times the efficiency of a 4 MeV device and would still give the user the option of using the high-dose-rate low-cost electron mode on a much larger fraction of their product line. A second advantage is the linac's pulsed characteristics. Unlike a direct current machine, the linac uses a few microseconds pulse followed by milliseconds of dead time. This thousand-fold increase in peak dose rate may, in some instances, reduce material damage or improve biological kill. Thirdly, the higher penetration reduces the chance of charge build-up and subsequent dielectric breakdown. This of particular importance to the manufacturers of thick insulation wire and cable, especially as the need for heavy gauge transmission lines becomes increasingly important for the economical distribution of electric power. Finally, the increase in energy and penetration will open the door for a wide variety of new products and processes now unobtainable. One problem which should be of concern to linac vendors is the Codex Alimentarius Commission's recommendation that the irradiation of food be held to X-rays of 5 MeV or less. Modification of this recommendation is likely, however, since there is no significant difference between the activation crosssections for i0 MeV electrons, already accepted, and i0 MeV photons. I find it difficult to even imagine an application or geometry that might result in an unacceptable situation. Finally, AECL has an interesting program to develop a continuous wave linac. This program, headed by Joseph McKeown, would produce a high-powered high-conversion linac that could compete with the D.C. machines, especially in the 3-6 MeV region and at power levels in excess of i00 kW. CONCLUSION In response to international and industrial needs, the electrification of the world is taking place, and electron processing is playing an increasingly important role. Its fine process control, high efficiencies, and greater potential for automation have made it a basic industrial processing tool. The questions no longer lie in the relative advantages or disadvantages of electrons or cobalt-60, or in the respective merits of the ICT or Dynamitron. There is a more important problem, one concerning industrial operations, energy, and productivity; how can electron processing expect to alleviate the problems of land, labor, capital, energy, raw materials, and environmental protection? These are the problems that confront the world and control the standard of living, and the remarkable properties of electron beams are an important part of the solution. Aqknowledgement s The author gratefully acknowledges the counsel and support of Dr. V. Markovi~, Mrs. W. Stevens' keen editing eye, and the insight and depth Professor J. Silverman has given to this paper. REFERENCES Taub, S. (1982). Why the Interest in Textile. Financial World, June, 44~ Wayne, M., R. Mauro, and P. Schmidt (1982). Electroteehnology: Sparking Production dustry. EPRI Journal, June, 6.
in In-