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An on-line target transport system for short-lived radioisotopes
Nuclear Instruments and Methods in Physics Research A235 (1985) 431-434 North-Holland. Amsterdam
431
AN O N - L I N E T A R G E T T R A N S P O R T S Y S T E M F O R S H O R T - L I V E D R A D I O I S O T O P E S D.W. H O L D S W O R T H a n d D.A.L. P A U L Physics Department, University o/Toronto, Toronto, Ontario, Canada M5S IA 7 Received 15 October 1984
Equipment has been developed to transport self supporting target films in vacuo at high speeds. The targets may be transferred over 3 m in less than 2 s and are gently accelerated and decelerated. In addition, the targets are accurately positioned at each end to within 25 ~tm. Non-magnetic materials are used throughout and wear is reduced so that 500000 transfers should be possible without loss of accuracy or reliability. Such a system should have many applications for on-line experiments using short-lived isotopes. Prototype construction and testing are described.
1. Introduction We have been involved recently in a collaborative project of precision positron polarimetry with the University of Michigan in Ann Arbor. Skalsey et al. [1,2] propose to compare the polarization of positrons from the decay of different nuclei to an accuracy of 7 × 10 -4. Our principal contribution to this "positron polarization comparator" (PPC) has been the development of a two-lens magnetic spectrometer which transmits a selected fraction of the available positrons and injects them into the polarimeter nearly parallel to the axis [3]. During tests of the combined spectrometer and polarimeter we have been developing the additional equipment which is required before the experiment can go on-line at the cyclotron laboratory in Princeton in collaboration with F. Calaprice. In order to reach the necessary level of precision in this comparison of polarizations, Skalsey has proposed that the measurement be made with two isotopes having nearly identical endpoint energies. One is 26mAl which exhibits pure Fermi positron decay with a half-life of 6.36 s and E 0 = 3.21 MeV, where E 0 is the positron kinetic energy end-point. The other isotope is 30p which exhibits pure Gamow-Teller decay with a half-life of 2.5 min, 99% positron decay and E 0 = 3.24 MeV. These isotopes may be produced on-line in a 10 MeV proton beam from 3°Si and 26Mg respectively. Therefore the experiment requires the production of 3°Si and 26Mg targets which may be cycled from an irradiation position in a proton beam to the object position of the magnetic spectrometer. Many methods of target transport are recorded in the literature but none meets all the requirements of our 0168-9002/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
experiment. Previous researchers have used pneumatic systems to move thin targets [4] but our measurements will all be made in a vacuum system ( < 10 -5 Torr) so another method must be used. Magnetic transport devices have also been used successfully for target exchange [5] but it is important that our mechanism not interfere with the field of the spectrometer, which necessitates the use of weakly magnetic materials throughout. Since we wish to improve the efficiency of our experiment by analyzing one target while another is being irradiated we must be able to shield the detectors from the neutrons produced at the target. This requirement sets a lower limit of about 2 m on the distance over which the transport must take place, eliminating many previous designs which used simple mechanical devices for transport over short distances [6-8]. Since the 26reAl source has such a short half-life it is necessary to transfer the 26Mg within about 2 s in order to provide any advantage. This can be quite difficult since the foils must be thinner than 5 mg cm -2 to avoid errors due to differential energy loss in the source material, and they are therefore quite fragile. In addition, the foils must be re-positioned after each transfer to within 25 /~m in order to further limit systematic errors. The average kinetic energy of the focussed beam will vary with spectrometer object position so this limit in object jitter is quite important. Finally, the experiment will require well over 100000 target transfers in order to achieve the necessary precision. This makes mechanical reliability and resistance to wear important factors in transport mechanism design. All of these requirements have motivated us to develop a target transport system which is significantly different from those previously reported in the literature.
D.W. Holdsworth, D.A.L Paul / On -line target transport system
432
/STOP ARM PUSHROD
2. The apparatus In our system the foils are transported on small target carts (fig. 1). The foils are first affixed with dilute epoxy resin to aluminium target mounts which taper to 0.25 mm in thickness at the inner diameter of 1.1 cm in order to minimize positron backscattering. The mounts are then securely fastened to the brass target carts which are equipped with accurately machined delrin wheels. The carts are then mounted in the transport mechanism as shown in fig. 2. Four strips of extruded aluminium are fastened to the inside faces of an aluminium H-beam to provide two pairs of tracks in which the target carts may travel. At the ends of the tracks specially machined plates provide a semi-circular return path between the top and bottom tracks. This design allows the target carts to complete an entire circuit in the transport mechanism. A thin stainless steel chain loop runs the length of the apparatus, supported at both ends by sprockets. The carts are fastened to this drive chain by two attachment springs and may be positioned anywhere in the system by rotation of the drive sprocket. A stepping motor under microprocessor control is used to rotate the drive shaft. In this manner the acceleration rate and distance travelled may be specified by simple computer programs, making the system very easily adapted to other experiments. In order to ensure that the carts return to the same position after each transfer we have designed an arm which comes into place to locate the cart accurately. The arm is driven into place by a solenoid which has been modified to enable it to operate in vacuo without overheating or outgassing. The solenoid is also controlled by the microprocessor so that the carts may
0
i
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Fig. 1. Target cart with target mount in place. One of the precision stainless steel ball bearings is shown in cross-section.
ill
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Fig. 2. Target cart shown locked in position at one end of the transport mechanism. One of the aluminum end plates and the target mount have been removed in order to illustrate the details of the device. When the cart is to be moved the push rod retracts, causing the stop arm to swing up. Rotation of the drive sprocket then causes the target cart to roll around the corner and return along the upper track. There are two attachment springs on the target cart but in this case the upper spring is hidden by the target cart frame.
be trapped and released in the proper sequence. Any error in the position of the stepping motor is compensated for by the tension in the attachment springs so that the cart is held tightly in place with no jitter in position. The same process occurs at the other end of the system and in this way pairs of targets may be exchanged along the 3 m track and positioned gently within 2 s. The proton beam may enter the target from any angle up to 45 ° from the axis of the transport system and be dumped either in the center of the H-beam or outside it. Vacuum compatible material such as Apiezon W wax may be placed in the central channel on both sides of the H-beam to act as neutron shielding. Since the mechanism must be capable of operating in a vacuum system all components are thoroughly cleaned and lubricated with vacuum compatible silicone lubricants. The drive shaft passes through a Varian N RC 1324 bali-bearing rotary feedthrough capable of operating at speeds up to 3000 rpm with minimal leakage. During tests the feedthrough operated continuously at about 800 rpm for several days with leak rates less than 20 #Torr l s - t . The design of the transport system makes it possible to mount many targets on one chain. For our purposes they are mounted in pairs which are a distance L / 2 apart, where L is the total length of the chain. The final design for our experiment calls for eight carts mounted at intervals of L / 8 such that any of the four pairs may be exchanged while the other three pairs simply ride through the system. Fig. 3 shows the manner in which this is possible. The carts which ride through the system
D. IV. Holdsworth, D.A.L. Paul / On-line target transport system POSITION OF TARGET .... CTROMETER
E TARGET .2HAIN
ARM SWITCH ARGET IN 'ROTON BEAM .DRIVE SHAFT
Fig. 3. Pictorial end view of the transport mechanism. One member of a pair of targets is shown locked into place in the proton beam while the other member is located 3 m away in a magnetic spectrometer. Only one member of an alternate target pair is shown in this diagram but in our experiment four pairs of targets will be attached at all times.
must travel through the semicircular return path in the end plates at considerable speed and thereby experience frequent centripetal accelerations of at least 70 m s -2. It is this centripetal acceleration which limits the speed at which carts may be transferred in our forthcoming experiment. One advantage of this system which should be of particular interest to most investigators is the relatively low cost. Commercially available components were used wherever possible in order to reduce the amount of custom machining required. The aluminum H-beam and cart tracks are both standard extrusions, the latter being available locally as drapery support track. The chain, sprockets, shafts and bearings are all available commercially at low cost [9]. Only the target carts, mounts, aluminum end plates and stopping mechanism require specially machined components. The electronic components necessary to control the apparatus would be the only large expense for investigators who do not have a microprocessor and stepping motor controller already in the laboratory.
3. Results Initial results with our first prototype indicated several areas where improvements were necessary. Tests
433
revealed that if lucite rollers are used to support the drive chain they wear down at an unacceptable rate. At the same time it was found that target cart wheels constructed from teflon wear out after less than 10000 transfers. Teflon was originally chosen because of its low coefficient of friction but it is too soft to stand up to continuous wear. Both the rollers and the target cart wheels were replaced with similar components manufactured from delrin, an acetal resin, and have since completed over 300000 transfers with no appreciable wear. Testing also revealed that adequate in-vacuo lubrication of mechanical components is very important. The system was operated beiefly without lubricants in an effort to obtain a better vacuum but the drive chain and main bearings failed to perform adequately for more than a few thousand transfers in this case. The chain is now lubricated with DC704 silicone oil and the bearings have been packed with high vacuum silicone grease. There has been one failure of a main bearing lubricated in this manner after 250000 transfers and this may indicate the period for which maintenance should be scheduled. Other problems encountered during wear tests were not as easily anticipated. Initially we experienced regular breakage of the target cart attachment springs after less than 10000 transfers. It was determined that this failure was caused by the shape of the attachment post which was causing a sharp bend in the spring and when this post was properly machined the springs were able to provide over 200000 transfers without failure. Another important design consideration was the choice of wire for use in the vacuum system. Adequate insulation is essential because any electrical short circuit could cause a control error with potentially catastrophic results. For example, if a target cart travelling at full speed were to be accidentally intercepted by the stop arm due to such a short circuit a very expensive target could easily be destroyed. The insulation problem has been solved by the use of a poly-imide coated magnet wire designed for service at 220°C. The low vapor pressure of the insulation allows us to use this wire in the vacuum system and still maintain an adequate vacuum. A version of the transport system which incorporates all of the improvements mentioned above has been constructed and rigorously tested, both in air and in vacuo. The drive system (chain, sprockets, and bearings) has provided over 520000 transfers with wear on all components being less than 10 1am. Our latest design of target cart has been transferred over 400000 times without failure. The stop arm mechanism has been tested 200000 times with no appreciable wear. A mounted, self-supporting, 0.5 mg c m - : Mg target 1 cm in diameter has been subjected to 80000 transfers in vacuo without breakage. Self-supporting silicon targets have proven to be
434
D.W. Holdsworth, D.A.L. Paul / On -line target transport system
more difficult to transport as they are very brittle when laid down by vacuum evaporation and separated by floatation [10]. Due to the centripetal acceleration previously described the Si targets tested thus far have broken after less than 10000 transfers. The development of more robust silicon targets, either sputtered or backed by thin Be foils, is being pursued by us at the present time. However, in its present form the transport system is capable of transferring nearly any self-supporting films if the transfer involves only one pair, in which case neither need experience a large centripetal acceleration.
4. Conclusions We have shown that it is possible to provide in-vacuo exchange of fragile metal target foils with a mechanical system using components which are not significantly magnetic. This system is easily adaptable to any configuration of on-line experiment where moderately fragile foils must be transferred quickly and located precisely. Average transfer speeds of over 1 m s-1 are possible and the system may be constructed at relatively low cost.
References [1] M. Skalsey, T.A. Girard, D. Newman and A. Rich, Phys. Rev. Lett. 49 (1982) 708. [2] M. Skalsey, Ph.D. Thesis, University of Michigan (1982). [31 David Holdsworth and Derek Paul, Nucl. Instr. and Meth. 223 (1984) 79. [4] R.D. Macfarlane and W.C. McHarris, Nuclear spectroscopy and reactions (Academic Press, New York, 1974) p. 243. [5] R.M. Delvecchio, W.L. Bacco, W.L. McNamee and W.W. Daehnick, Nucl. Instr. and Meth. 144 (1977) 429. [6] J.T. Heinrich, Nucl. Instr. and Meth. 31 (1964) 337. [7] W.C. Turkenburg, E. de Haas, A.F. Neuteboom, J. Ladru and H.H. Kersten, Nucl. Instr. and Meth. 126 (1975) 241. [81 D. Marx, F. Nickel and G. Muzenberg, K. Guttner, H. Ewald, W. Faust, S. Hoffman, H.J. Schott and W. Thalheimer, Nucl. Instr. and Meth. 163 (1979) 15. {91 Nordex Incorporated, 226 White Street, Danbury, Connecticut 06810, USA. [10] A.M. Sandorfi, L.R. Kilius and J.L. Gallant, Nucl. Instr. and Meth. 136 (1976) 395.
431
AN O N - L I N E T A R G E T T R A N S P O R T S Y S T E M F O R S H O R T - L I V E D R A D I O I S O T O P E S D.W. H O L D S W O R T H a n d D.A.L. P A U L Physics Department, University o/Toronto, Toronto, Ontario, Canada M5S IA 7 Received 15 October 1984
Equipment has been developed to transport self supporting target films in vacuo at high speeds. The targets may be transferred over 3 m in less than 2 s and are gently accelerated and decelerated. In addition, the targets are accurately positioned at each end to within 25 ~tm. Non-magnetic materials are used throughout and wear is reduced so that 500000 transfers should be possible without loss of accuracy or reliability. Such a system should have many applications for on-line experiments using short-lived isotopes. Prototype construction and testing are described.
1. Introduction We have been involved recently in a collaborative project of precision positron polarimetry with the University of Michigan in Ann Arbor. Skalsey et al. [1,2] propose to compare the polarization of positrons from the decay of different nuclei to an accuracy of 7 × 10 -4. Our principal contribution to this "positron polarization comparator" (PPC) has been the development of a two-lens magnetic spectrometer which transmits a selected fraction of the available positrons and injects them into the polarimeter nearly parallel to the axis [3]. During tests of the combined spectrometer and polarimeter we have been developing the additional equipment which is required before the experiment can go on-line at the cyclotron laboratory in Princeton in collaboration with F. Calaprice. In order to reach the necessary level of precision in this comparison of polarizations, Skalsey has proposed that the measurement be made with two isotopes having nearly identical endpoint energies. One is 26mAl which exhibits pure Fermi positron decay with a half-life of 6.36 s and E 0 = 3.21 MeV, where E 0 is the positron kinetic energy end-point. The other isotope is 30p which exhibits pure Gamow-Teller decay with a half-life of 2.5 min, 99% positron decay and E 0 = 3.24 MeV. These isotopes may be produced on-line in a 10 MeV proton beam from 3°Si and 26Mg respectively. Therefore the experiment requires the production of 3°Si and 26Mg targets which may be cycled from an irradiation position in a proton beam to the object position of the magnetic spectrometer. Many methods of target transport are recorded in the literature but none meets all the requirements of our 0168-9002/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
experiment. Previous researchers have used pneumatic systems to move thin targets [4] but our measurements will all be made in a vacuum system ( < 10 -5 Torr) so another method must be used. Magnetic transport devices have also been used successfully for target exchange [5] but it is important that our mechanism not interfere with the field of the spectrometer, which necessitates the use of weakly magnetic materials throughout. Since we wish to improve the efficiency of our experiment by analyzing one target while another is being irradiated we must be able to shield the detectors from the neutrons produced at the target. This requirement sets a lower limit of about 2 m on the distance over which the transport must take place, eliminating many previous designs which used simple mechanical devices for transport over short distances [6-8]. Since the 26reAl source has such a short half-life it is necessary to transfer the 26Mg within about 2 s in order to provide any advantage. This can be quite difficult since the foils must be thinner than 5 mg cm -2 to avoid errors due to differential energy loss in the source material, and they are therefore quite fragile. In addition, the foils must be re-positioned after each transfer to within 25 /~m in order to further limit systematic errors. The average kinetic energy of the focussed beam will vary with spectrometer object position so this limit in object jitter is quite important. Finally, the experiment will require well over 100000 target transfers in order to achieve the necessary precision. This makes mechanical reliability and resistance to wear important factors in transport mechanism design. All of these requirements have motivated us to develop a target transport system which is significantly different from those previously reported in the literature.
D.W. Holdsworth, D.A.L Paul / On -line target transport system
432
/STOP ARM PUSHROD
2. The apparatus In our system the foils are transported on small target carts (fig. 1). The foils are first affixed with dilute epoxy resin to aluminium target mounts which taper to 0.25 mm in thickness at the inner diameter of 1.1 cm in order to minimize positron backscattering. The mounts are then securely fastened to the brass target carts which are equipped with accurately machined delrin wheels. The carts are then mounted in the transport mechanism as shown in fig. 2. Four strips of extruded aluminium are fastened to the inside faces of an aluminium H-beam to provide two pairs of tracks in which the target carts may travel. At the ends of the tracks specially machined plates provide a semi-circular return path between the top and bottom tracks. This design allows the target carts to complete an entire circuit in the transport mechanism. A thin stainless steel chain loop runs the length of the apparatus, supported at both ends by sprockets. The carts are fastened to this drive chain by two attachment springs and may be positioned anywhere in the system by rotation of the drive sprocket. A stepping motor under microprocessor control is used to rotate the drive shaft. In this manner the acceleration rate and distance travelled may be specified by simple computer programs, making the system very easily adapted to other experiments. In order to ensure that the carts return to the same position after each transfer we have designed an arm which comes into place to locate the cart accurately. The arm is driven into place by a solenoid which has been modified to enable it to operate in vacuo without overheating or outgassing. The solenoid is also controlled by the microprocessor so that the carts may
0
i
I
I
2
3
I
t
cm
4
Fig. 1. Target cart with target mount in place. One of the precision stainless steel ball bearings is shown in cross-section.
ill
~R:~m,~l ~
!
1
I
~
1
\
I
I ~/j
D~IVESPROCKET 7TAcCHAIMIENN T
!
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Fig. 2. Target cart shown locked in position at one end of the transport mechanism. One of the aluminum end plates and the target mount have been removed in order to illustrate the details of the device. When the cart is to be moved the push rod retracts, causing the stop arm to swing up. Rotation of the drive sprocket then causes the target cart to roll around the corner and return along the upper track. There are two attachment springs on the target cart but in this case the upper spring is hidden by the target cart frame.
be trapped and released in the proper sequence. Any error in the position of the stepping motor is compensated for by the tension in the attachment springs so that the cart is held tightly in place with no jitter in position. The same process occurs at the other end of the system and in this way pairs of targets may be exchanged along the 3 m track and positioned gently within 2 s. The proton beam may enter the target from any angle up to 45 ° from the axis of the transport system and be dumped either in the center of the H-beam or outside it. Vacuum compatible material such as Apiezon W wax may be placed in the central channel on both sides of the H-beam to act as neutron shielding. Since the mechanism must be capable of operating in a vacuum system all components are thoroughly cleaned and lubricated with vacuum compatible silicone lubricants. The drive shaft passes through a Varian N RC 1324 bali-bearing rotary feedthrough capable of operating at speeds up to 3000 rpm with minimal leakage. During tests the feedthrough operated continuously at about 800 rpm for several days with leak rates less than 20 #Torr l s - t . The design of the transport system makes it possible to mount many targets on one chain. For our purposes they are mounted in pairs which are a distance L / 2 apart, where L is the total length of the chain. The final design for our experiment calls for eight carts mounted at intervals of L / 8 such that any of the four pairs may be exchanged while the other three pairs simply ride through the system. Fig. 3 shows the manner in which this is possible. The carts which ride through the system
D. IV. Holdsworth, D.A.L. Paul / On-line target transport system POSITION OF TARGET .... CTROMETER
E TARGET .2HAIN
ARM SWITCH ARGET IN 'ROTON BEAM .DRIVE SHAFT
Fig. 3. Pictorial end view of the transport mechanism. One member of a pair of targets is shown locked into place in the proton beam while the other member is located 3 m away in a magnetic spectrometer. Only one member of an alternate target pair is shown in this diagram but in our experiment four pairs of targets will be attached at all times.
must travel through the semicircular return path in the end plates at considerable speed and thereby experience frequent centripetal accelerations of at least 70 m s -2. It is this centripetal acceleration which limits the speed at which carts may be transferred in our forthcoming experiment. One advantage of this system which should be of particular interest to most investigators is the relatively low cost. Commercially available components were used wherever possible in order to reduce the amount of custom machining required. The aluminum H-beam and cart tracks are both standard extrusions, the latter being available locally as drapery support track. The chain, sprockets, shafts and bearings are all available commercially at low cost [9]. Only the target carts, mounts, aluminum end plates and stopping mechanism require specially machined components. The electronic components necessary to control the apparatus would be the only large expense for investigators who do not have a microprocessor and stepping motor controller already in the laboratory.
3. Results Initial results with our first prototype indicated several areas where improvements were necessary. Tests
433
revealed that if lucite rollers are used to support the drive chain they wear down at an unacceptable rate. At the same time it was found that target cart wheels constructed from teflon wear out after less than 10000 transfers. Teflon was originally chosen because of its low coefficient of friction but it is too soft to stand up to continuous wear. Both the rollers and the target cart wheels were replaced with similar components manufactured from delrin, an acetal resin, and have since completed over 300000 transfers with no appreciable wear. Testing also revealed that adequate in-vacuo lubrication of mechanical components is very important. The system was operated beiefly without lubricants in an effort to obtain a better vacuum but the drive chain and main bearings failed to perform adequately for more than a few thousand transfers in this case. The chain is now lubricated with DC704 silicone oil and the bearings have been packed with high vacuum silicone grease. There has been one failure of a main bearing lubricated in this manner after 250000 transfers and this may indicate the period for which maintenance should be scheduled. Other problems encountered during wear tests were not as easily anticipated. Initially we experienced regular breakage of the target cart attachment springs after less than 10000 transfers. It was determined that this failure was caused by the shape of the attachment post which was causing a sharp bend in the spring and when this post was properly machined the springs were able to provide over 200000 transfers without failure. Another important design consideration was the choice of wire for use in the vacuum system. Adequate insulation is essential because any electrical short circuit could cause a control error with potentially catastrophic results. For example, if a target cart travelling at full speed were to be accidentally intercepted by the stop arm due to such a short circuit a very expensive target could easily be destroyed. The insulation problem has been solved by the use of a poly-imide coated magnet wire designed for service at 220°C. The low vapor pressure of the insulation allows us to use this wire in the vacuum system and still maintain an adequate vacuum. A version of the transport system which incorporates all of the improvements mentioned above has been constructed and rigorously tested, both in air and in vacuo. The drive system (chain, sprockets, and bearings) has provided over 520000 transfers with wear on all components being less than 10 1am. Our latest design of target cart has been transferred over 400000 times without failure. The stop arm mechanism has been tested 200000 times with no appreciable wear. A mounted, self-supporting, 0.5 mg c m - : Mg target 1 cm in diameter has been subjected to 80000 transfers in vacuo without breakage. Self-supporting silicon targets have proven to be
434
D.W. Holdsworth, D.A.L. Paul / On -line target transport system
more difficult to transport as they are very brittle when laid down by vacuum evaporation and separated by floatation [10]. Due to the centripetal acceleration previously described the Si targets tested thus far have broken after less than 10000 transfers. The development of more robust silicon targets, either sputtered or backed by thin Be foils, is being pursued by us at the present time. However, in its present form the transport system is capable of transferring nearly any self-supporting films if the transfer involves only one pair, in which case neither need experience a large centripetal acceleration.
4. Conclusions We have shown that it is possible to provide in-vacuo exchange of fragile metal target foils with a mechanical system using components which are not significantly magnetic. This system is easily adaptable to any configuration of on-line experiment where moderately fragile foils must be transferred quickly and located precisely. Average transfer speeds of over 1 m s-1 are possible and the system may be constructed at relatively low cost.
References [1] M. Skalsey, T.A. Girard, D. Newman and A. Rich, Phys. Rev. Lett. 49 (1982) 708. [2] M. Skalsey, Ph.D. Thesis, University of Michigan (1982). [31 David Holdsworth and Derek Paul, Nucl. Instr. and Meth. 223 (1984) 79. [4] R.D. Macfarlane and W.C. McHarris, Nuclear spectroscopy and reactions (Academic Press, New York, 1974) p. 243. [5] R.M. Delvecchio, W.L. Bacco, W.L. McNamee and W.W. Daehnick, Nucl. Instr. and Meth. 144 (1977) 429. [6] J.T. Heinrich, Nucl. Instr. and Meth. 31 (1964) 337. [7] W.C. Turkenburg, E. de Haas, A.F. Neuteboom, J. Ladru and H.H. Kersten, Nucl. Instr. and Meth. 126 (1975) 241. [81 D. Marx, F. Nickel and G. Muzenberg, K. Guttner, H. Ewald, W. Faust, S. Hoffman, H.J. Schott and W. Thalheimer, Nucl. Instr. and Meth. 163 (1979) 15. {91 Nordex Incorporated, 226 White Street, Danbury, Connecticut 06810, USA. [10] A.M. Sandorfi, L.R. Kilius and J.L. Gallant, Nucl. Instr. and Meth. 136 (1976) 395.