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Modification of a medical linac to a polymer irradiation facility
Nuclear Instruments and Methods in Physics Research B79 (1993) 871-874 North-Holland
Modification
of a medical linac to a polymer irradiation
W. van Duijneveldt, Eindhoven University
Beam Interactions with Materials % Atoms
J.I.M. Botman, C.J. Timmermans
of Technology,
facili~
and R.W. de Leeuw
Department of Physics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
A linear accelerator for X-ray therapy has been modified to generate a 5 MeV pulsed electron beam. The main objective is to irradiate polymeric materials in open air in order to alter their chemical and mechanical properties. To meet the radiation protection standards a shielding has been built round the target. Safety is guaranteed by a fail-safe secured progra~able logic controller (PLC) monitored by a personal computer. The accelerator can be monitored and controlled by a graphics oriented and menu driven program running on the personal computer. In addition, a control panel has been designed and built to show warning signals and to set various linac parameters. A description of the accelerator modification and of the new control system is presented.
1. Introduction
Nowadays, the polymer indust~ uses electron beams or X-rays to study or to develop new polymer materials. Irradiation is used to develop new plastics with a high resistance to radiation, or as a technique to alter the chemical and mechanical properties of polymers [l]. For example, the degree of hardness can be improved or a mechanical memory effect such as heat shrinkability can be established. For polymers, the most important chemical processes involved are Gross-linking and degradation. Cross-linking implies glueing together large organic molecules by knocking off molecular hydrogen and by forming intermolecular bonds. A few chemical changes per molecule is sufficient to alter the mechanical properties drastically. Degradation means scission of the polymer main chain as a result of irradiation. Several sources of high energy radiation are available: radioactive substances which produce beta-, or gamma-radiation, and accelerators which produce electron beams or X-rays. Accelerators have the advantage of high intensity, availability, control, and safety. Various small electron beam machines were made especially to cure coatings on substrates e.g. the so-called Electrocurtain and the RPC Broadbeam [l]. The electron penetration depth is rather small, < 0.3 rmn, due to the low accelerating voltage of 100-500 kV. As a consequence, these machines are self shielded; the metal skin and lead shielding of the maehine itself are sufficient. When bulks (tubes, pellets, etc.) need to be modified, a higher penetration depth is required. The depth R of electrons of energy 500 keV or higher can be expressed as pR =s OSE - 0.1, where E is in MeV, p in gcmp3, and R in cm. Consequently, a higher electron energy has to be chosen. Electron beams of 0168-583X/93/$~.~
0.5-10 MeV have a penetration depth of 0.3-30 mm. Because the secondary X-ray yield increases, an additional shielding is required typically extending to 150 cm of concrete. At the Eindhoven University, Laboratory of Nuclear Physics Techniques, a 5 MeV linac has been installed, primarily for studies in polymer technology. Since the linac is from a hospital and was used for X-ray therapy, it had to be modified. In addition, the necessary shielding of concrete and lead, and a new safety and control system have been built.
2. The linear acceIerator A block diagram of the modified linac is shown in fig. 1, and some machine parameters are shown in table 1. Irradiation of polymer targets takes place in atmospheric air. Therefore a new magnet bends the beam by 8” upwards to the irradiation exit, or altematively 90” downwards towards a Faraday cup for emittance and energy spread measurements. A 100 urn thin Al-foil is the partition between vacuum in the accelerator and atmospheric air in the target room. The electron beam penetrates through the foil and becomes slightly diverged. Multiple scattering of medium energy electrons through thin foils has been investigated theoretically by Moliere [Z] and by Snyder and Scott [3]. Evaluating Molieres expression leads to a mean scattering angle of 8.2”. The half-thickness, i.e. the distance for which half the number of electrons are absorbed, is approximately L&,~= 0.03E’.‘/p, where E is expressed in MeV, p in gcms3, and Q2 in cm. For Al (p = 2.7 gcmm3) and 5 MeV electrons this results in a half-thickness of 0.77 mm. Only 10% of the incident electrons is lost in a 100 pm Al-foil.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
XVI. RADIATION PROCESSING
872
W. van Duijneveldt et al. / Modification of a medical linac
Table 1 Machine parameters
cal scan is sufficient to uniformly irradiate the whole target.
Electron energy Pulse repetition frequency Pulse duration Nominal radio-frequency Peak current during pulse Average current (300 pulses s-l) Average power (300 pulses s-l)
5 MeV f 10% single shot/ 50/100/150/300 4 CLs 3GHz 80 mA 100 PA 500w
s-1
The proposed polymer target may be as large as 10 x 10 cm2 and about 1 cm thick. It is positioned near the Al-foil (5 cm) to limit ozone production in air. It is fixed on a water cooled (movable) target table since the beam power may exceed a few hundred watt. Electrons which are not absorbed by the synthetic target are stopped by a 1.5 cm thick aluminium plate. The target is irradiated with a beam spot of about 1.6 cm2. Moreover it will be scanned vertically and horizontally. Vertically a separate magnet coil sweeps the beam, and horizontally an electric motor moves the target forward and backward. The sweeping frequency of the magnet is adjustable from 1 to 20 Hz, and the sweeping angle is adjustable from 0 to f5”. The horizontal and verti-
3. Radiation level control The yield of secondary X-rays from electron deceleration in the target material is strongly peaked in the forward direction. Low-Z target materials have a restricted X-ray yield: in our case it is calculated on the basis of using Al, with an average beam current of 100 uA at 5 MeV, and a maximum work load of 1000 p,A h/week. Allowed radiation levels are determined from the general radiation protection requirements [4,5]: 10 @/h and 100 p&/week just outside the linac vault in a controlled radiological area accessible for radiological workers, and negligible levels elsewhere in the building. This requires an attenuation factor of 107-lo5 in the horizontal plane from the forward to the backward direction, and lo4 upward in the vertical direction. It is realized by a lead and concrete shielding. A lead pit with walls of lo-15 cm has been constructed directly around the target, with a 10 cm thick removable cover, and coated with 3 g/cm2-Al for stopping scattered electrons. Concrete shielding, 1.50 m thick, has been applied directly around the lead pit. Polymer
trigger
rllPub=
modulator
I
PRP generator single shot/ 50/100/150/300 pulses per scum horizontal and vertical centring dipoles movable target table bendinggnet
focus solenoids
1-1 magnetron Fig. 1. Block diagram of the modified linac.
Al -,foil
I+? van Duijneveldt
et al. / Modifiation
irradiation gives reduced radiation levels; measurements will be carried out to optimize the shielding.
of a medical lime
Q 7 0
WbXOll screen
printer
4. Active safety and control system An active safety system is needed: to prevent people from entering the radiation facility during operation; to prevent damage due to improper operator actions; and to secure the machine against damage caused by linac parts not working properly. The safety system responds to signals representing the linac status. Aside from the safety system, a control system is needed to monitor and control various irradiation parameters like beam current, beam, energy, pulse repetition frequency, and Jbsorbed dose. Since both systems make use of the same control signals, it is obvious to combine them. The original safety and control system proved to be outdated and needed to be replaced by a more sophisticated and flexible programmable logic controller (PLC). It is a Simatic S5-115U, which is an independent control unit, however equipped with a communication processor connected to a PC (fig. 2). The operator is able to survey and control the linac performance and all related systems entirely by means of a graphics oriented program run on the PC (Wizcon, PC Soft Int. Ltd.); power circuits, pressurized air circuits, valves, temperatures, levels, etc. are all shown graphically on the screen. The system is menu driven. Pressing function keys displays screens with specific information, and physical parameters can be changed. A separate control panel is used to display and set most important data. The linac operation is divided into a number of stages, each stage representing a linac activity (fig. 3). These stages have to be passed through sequentially for irradiation to take place. A successive stage is only possible if a number of conditions is fulfilled. A rough subdivision into stages coincides with the room classifi-
873
wor
indication and control panel
P1
communication processor
central processor unit
L
digital and analoque input/output modules linear accelerator
Fig. 2. Block diagram data flow.
cation code displayed with three warning lights near the entry and indicating the facility accessibility. Green indicates a free entrance for authorized workers. Dur-
morn classification code orange
5 mains
Fig. 3. The switch-on sequence. XVI. RADIATION PROCESSING
874
W. van Duijneveldt et al. / Modification of a medical linac
ing this stage the power is switched off. Yellow indicates the linac is being serviced or being prepared for irradiation. Power is fed to various linac parts, but the high voltage to accelerate electrons is switched off. Access is only allowed with permission of the operator. Red indicates the linac is irradiating a target and entrance is forbidden. An additional warning light indicates a possibly dangerous concentration of ozone. Because of the intended experimental use of the linac, the program objectives may change in the future. Also minor changes in the linac and irradiation room can be expected, with consequences for the safety and control system. These are easily accommodated by reprogramming the PLC.
monitored and controlled from the control panel, and with the display and control program run on the personal computer.
Acknowledgements The authors wish to thank the Health Physics Group and especially Mrs. T. Klaver for the radiation protection advices and calculations.
References [l] J. van
5. Conclusion
A medical linac has been modified for polymer irradiation with a 5 MeV, 100 PA, pulsed electron beam. Shielding has been built of 150 cm concrete and 15 cm lead walls in accordance with radiation protection standards. A fail-safe secured PLC is the central unit of a safety and control system. The linac can be
Gisbergen, Ph.D. Thesis, Technical University of Eindhoven, The Netherlands (1991). [2] G. Moliere, Z. Naturforsch. 3a (1948) 78. [3] H.S. Snyder and W.T. Scott, Phys. Rev. 76 (1949) 220. [4] Radiation Protection Design Guidelines for 0.1-100 MeV Particle Accelerators Facilities, NCRP Report no. 51 (1977). [S] Radiological Safety Aspects of the Operation of Electron Linear Accelerators, IAEA Technical Report Series no. 188 (1979).
Modification
of a medical linac to a polymer irradiation
W. van Duijneveldt, Eindhoven University
Beam Interactions with Materials % Atoms
J.I.M. Botman, C.J. Timmermans
of Technology,
facili~
and R.W. de Leeuw
Department of Physics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
A linear accelerator for X-ray therapy has been modified to generate a 5 MeV pulsed electron beam. The main objective is to irradiate polymeric materials in open air in order to alter their chemical and mechanical properties. To meet the radiation protection standards a shielding has been built round the target. Safety is guaranteed by a fail-safe secured progra~able logic controller (PLC) monitored by a personal computer. The accelerator can be monitored and controlled by a graphics oriented and menu driven program running on the personal computer. In addition, a control panel has been designed and built to show warning signals and to set various linac parameters. A description of the accelerator modification and of the new control system is presented.
1. Introduction
Nowadays, the polymer indust~ uses electron beams or X-rays to study or to develop new polymer materials. Irradiation is used to develop new plastics with a high resistance to radiation, or as a technique to alter the chemical and mechanical properties of polymers [l]. For example, the degree of hardness can be improved or a mechanical memory effect such as heat shrinkability can be established. For polymers, the most important chemical processes involved are Gross-linking and degradation. Cross-linking implies glueing together large organic molecules by knocking off molecular hydrogen and by forming intermolecular bonds. A few chemical changes per molecule is sufficient to alter the mechanical properties drastically. Degradation means scission of the polymer main chain as a result of irradiation. Several sources of high energy radiation are available: radioactive substances which produce beta-, or gamma-radiation, and accelerators which produce electron beams or X-rays. Accelerators have the advantage of high intensity, availability, control, and safety. Various small electron beam machines were made especially to cure coatings on substrates e.g. the so-called Electrocurtain and the RPC Broadbeam [l]. The electron penetration depth is rather small, < 0.3 rmn, due to the low accelerating voltage of 100-500 kV. As a consequence, these machines are self shielded; the metal skin and lead shielding of the maehine itself are sufficient. When bulks (tubes, pellets, etc.) need to be modified, a higher penetration depth is required. The depth R of electrons of energy 500 keV or higher can be expressed as pR =s OSE - 0.1, where E is in MeV, p in gcmp3, and R in cm. Consequently, a higher electron energy has to be chosen. Electron beams of 0168-583X/93/$~.~
0.5-10 MeV have a penetration depth of 0.3-30 mm. Because the secondary X-ray yield increases, an additional shielding is required typically extending to 150 cm of concrete. At the Eindhoven University, Laboratory of Nuclear Physics Techniques, a 5 MeV linac has been installed, primarily for studies in polymer technology. Since the linac is from a hospital and was used for X-ray therapy, it had to be modified. In addition, the necessary shielding of concrete and lead, and a new safety and control system have been built.
2. The linear acceIerator A block diagram of the modified linac is shown in fig. 1, and some machine parameters are shown in table 1. Irradiation of polymer targets takes place in atmospheric air. Therefore a new magnet bends the beam by 8” upwards to the irradiation exit, or altematively 90” downwards towards a Faraday cup for emittance and energy spread measurements. A 100 urn thin Al-foil is the partition between vacuum in the accelerator and atmospheric air in the target room. The electron beam penetrates through the foil and becomes slightly diverged. Multiple scattering of medium energy electrons through thin foils has been investigated theoretically by Moliere [Z] and by Snyder and Scott [3]. Evaluating Molieres expression leads to a mean scattering angle of 8.2”. The half-thickness, i.e. the distance for which half the number of electrons are absorbed, is approximately L&,~= 0.03E’.‘/p, where E is expressed in MeV, p in gcms3, and Q2 in cm. For Al (p = 2.7 gcmm3) and 5 MeV electrons this results in a half-thickness of 0.77 mm. Only 10% of the incident electrons is lost in a 100 pm Al-foil.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
XVI. RADIATION PROCESSING
872
W. van Duijneveldt et al. / Modification of a medical linac
Table 1 Machine parameters
cal scan is sufficient to uniformly irradiate the whole target.
Electron energy Pulse repetition frequency Pulse duration Nominal radio-frequency Peak current during pulse Average current (300 pulses s-l) Average power (300 pulses s-l)
5 MeV f 10% single shot/ 50/100/150/300 4 CLs 3GHz 80 mA 100 PA 500w
s-1
The proposed polymer target may be as large as 10 x 10 cm2 and about 1 cm thick. It is positioned near the Al-foil (5 cm) to limit ozone production in air. It is fixed on a water cooled (movable) target table since the beam power may exceed a few hundred watt. Electrons which are not absorbed by the synthetic target are stopped by a 1.5 cm thick aluminium plate. The target is irradiated with a beam spot of about 1.6 cm2. Moreover it will be scanned vertically and horizontally. Vertically a separate magnet coil sweeps the beam, and horizontally an electric motor moves the target forward and backward. The sweeping frequency of the magnet is adjustable from 1 to 20 Hz, and the sweeping angle is adjustable from 0 to f5”. The horizontal and verti-
3. Radiation level control The yield of secondary X-rays from electron deceleration in the target material is strongly peaked in the forward direction. Low-Z target materials have a restricted X-ray yield: in our case it is calculated on the basis of using Al, with an average beam current of 100 uA at 5 MeV, and a maximum work load of 1000 p,A h/week. Allowed radiation levels are determined from the general radiation protection requirements [4,5]: 10 @/h and 100 p&/week just outside the linac vault in a controlled radiological area accessible for radiological workers, and negligible levels elsewhere in the building. This requires an attenuation factor of 107-lo5 in the horizontal plane from the forward to the backward direction, and lo4 upward in the vertical direction. It is realized by a lead and concrete shielding. A lead pit with walls of lo-15 cm has been constructed directly around the target, with a 10 cm thick removable cover, and coated with 3 g/cm2-Al for stopping scattered electrons. Concrete shielding, 1.50 m thick, has been applied directly around the lead pit. Polymer
trigger
rllPub=
modulator
I
PRP generator single shot/ 50/100/150/300 pulses per scum horizontal and vertical centring dipoles movable target table bendinggnet
focus solenoids
1-1 magnetron Fig. 1. Block diagram of the modified linac.
Al -,foil
I+? van Duijneveldt
et al. / Modifiation
irradiation gives reduced radiation levels; measurements will be carried out to optimize the shielding.
of a medical lime
Q 7 0
WbXOll screen
printer
4. Active safety and control system An active safety system is needed: to prevent people from entering the radiation facility during operation; to prevent damage due to improper operator actions; and to secure the machine against damage caused by linac parts not working properly. The safety system responds to signals representing the linac status. Aside from the safety system, a control system is needed to monitor and control various irradiation parameters like beam current, beam, energy, pulse repetition frequency, and Jbsorbed dose. Since both systems make use of the same control signals, it is obvious to combine them. The original safety and control system proved to be outdated and needed to be replaced by a more sophisticated and flexible programmable logic controller (PLC). It is a Simatic S5-115U, which is an independent control unit, however equipped with a communication processor connected to a PC (fig. 2). The operator is able to survey and control the linac performance and all related systems entirely by means of a graphics oriented program run on the PC (Wizcon, PC Soft Int. Ltd.); power circuits, pressurized air circuits, valves, temperatures, levels, etc. are all shown graphically on the screen. The system is menu driven. Pressing function keys displays screens with specific information, and physical parameters can be changed. A separate control panel is used to display and set most important data. The linac operation is divided into a number of stages, each stage representing a linac activity (fig. 3). These stages have to be passed through sequentially for irradiation to take place. A successive stage is only possible if a number of conditions is fulfilled. A rough subdivision into stages coincides with the room classifi-
873
wor
indication and control panel
P1
communication processor
central processor unit
L
digital and analoque input/output modules linear accelerator
Fig. 2. Block diagram data flow.
cation code displayed with three warning lights near the entry and indicating the facility accessibility. Green indicates a free entrance for authorized workers. Dur-
morn classification code orange
5 mains
Fig. 3. The switch-on sequence. XVI. RADIATION PROCESSING
874
W. van Duijneveldt et al. / Modification of a medical linac
ing this stage the power is switched off. Yellow indicates the linac is being serviced or being prepared for irradiation. Power is fed to various linac parts, but the high voltage to accelerate electrons is switched off. Access is only allowed with permission of the operator. Red indicates the linac is irradiating a target and entrance is forbidden. An additional warning light indicates a possibly dangerous concentration of ozone. Because of the intended experimental use of the linac, the program objectives may change in the future. Also minor changes in the linac and irradiation room can be expected, with consequences for the safety and control system. These are easily accommodated by reprogramming the PLC.
monitored and controlled from the control panel, and with the display and control program run on the personal computer.
Acknowledgements The authors wish to thank the Health Physics Group and especially Mrs. T. Klaver for the radiation protection advices and calculations.
References [l] J. van
5. Conclusion
A medical linac has been modified for polymer irradiation with a 5 MeV, 100 PA, pulsed electron beam. Shielding has been built of 150 cm concrete and 15 cm lead walls in accordance with radiation protection standards. A fail-safe secured PLC is the central unit of a safety and control system. The linac can be
Gisbergen, Ph.D. Thesis, Technical University of Eindhoven, The Netherlands (1991). [2] G. Moliere, Z. Naturforsch. 3a (1948) 78. [3] H.S. Snyder and W.T. Scott, Phys. Rev. 76 (1949) 220. [4] Radiation Protection Design Guidelines for 0.1-100 MeV Particle Accelerators Facilities, NCRP Report no. 51 (1977). [S] Radiological Safety Aspects of the Operation of Electron Linear Accelerators, IAEA Technical Report Series no. 188 (1979).