- Email: [email protected]
Target-making in the Nuclear Physics Department at the Australian National University
Nuclear Instruments and Methods in Physics Research A 397 (1997) 200-203
NW’--iLLnn INSTRl UMENTS
% METHODS IN PHYSICS RESEARCH
SectlonA
ELSEWIER
Target-making in the Nuclear Physics Department at the Australian National University R.B. Turkentine*,
D.C. Weisser
Nuclear Physics Department, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, Australian National University, Canberra, ACT 0200, Australia
Abstract The target-making activities in the Department are presented, including improvements to the modified Saddle Field Ion Source and a summary of carbon stripper foil production. Over the past three years, the installation and commissioning of a superconducting linear accelerator (LINAC) has dominated the activities of the Department and so a brief description of the LINAC is given.
1. Introduction This year, the Australian National University celebrates its 50th anniversary. The Department of Nuclear Physics, there from the beginning, has developed targetmaking facilities throughout its history. In the present day, targets for nuclear physics research are produced for use with the 14UD tandem accelerator (up-graded to 16 MV) and the recently commissioned LINAC booster. The Department also supplies carbon foils to other Australian institutions, where two cyclotrons and an FN accelerator operate. This paper will feature a description of the continuing improvements to the Saddle Field Ion Source subsequent to the work of Muggleton [l]. The target-making activities have been pursued over the past three years, against the background of the installation and commissioning of the LINAC which was transferred from the Daresbury Laboratory in the UK to the ANU. This work will be sketched to establish the context of the target-making efforts.
2. LINAC The arrival of the LINAC in April 1993 has caused target-making activities to be restricted almost entirely to essential carbon stripper foils. An intense work
*Corresponding author. Fax: +61 6 249 0748; e-mail: bob. [email protected].
schedule, required to get the LINAC commissioned by 1996, has meant that all members of the Department have had to re-focus their skills on this new project. The shipping containers that were used for shipping the LINAC to Australia from the UK, were packed in such a way that it resulted in corrosion damage which created extra work in the installation process. The accelerating function of the stage-l LINAC is accomplished by nine split-loop superconducting RF resonators, housed in three cryostats. A single niobiumsputtered quarter wave resonator in a separate cryostat functions as the beam buncher. All cryostats are supplied with liquid helium and liquid nitrogen from the cryogenic distribution system. The beam transport system for the LINAC can operate in several modes. The beam from the 14UD can be accelerated through the LINAC modules and directed to either the new target area or the existing 14UD target area. Alternatively, it can by-pass the modules and be directed into the LINAC target area without further acceleration. This allows for exploitation of all existing experimental equipment by the LINAC beam, as well as the parallel development of the LINAC while the new target facilities are used for the 14UD beam.
3. Stripper
The terminal stripper apparatus of the 14UD has a 260 foil capacity. The carbon stripper foils are produced by carbon arc evaporation onto glass slides coated with a detergent release agent. All foils used in the terminal
016%9002/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII
SO168-9002(97)00586-X
foils
R.B. Turkentine, D.C. Weissevj NwL Ins@. and Meth. in Phys. Rex A 397 (1997) 200-203
stripper are 3-5 &cm2 thick as measured by light attenuation using a calibrated light source. To enable efficient floating and mounting of such thin foils, the carboncoated slide is further coated with collodion. It has been observed that a speck of dust on the beam-exposed area of a foil develops a thickening of the material in the exact shape of the dust speck surrounded by a thinned halo. It is speculated that this leads to weakening of the foil and premature rupture. To avoid such specks, all operations are conducted in a laminar flow bench. The foils are mounted on frames with 9.35 mm diameter holes and then the collodion is removed by baking in air at 673 K. Each foil is individually inspected, using transmitted and scattered light in order to discard any with imperfections. Complete terminal stripper foil replacement is needed two to three times a year. This requires the production of about 400 foils allowing selection of 260 which meet the ANU standard. The second stripper has lo-15 pg/cm2 foils and only needs to be replenished about every two years. A third stripper, at the LINAC entrance is yet to be commissioned. It is expected to require 20 pg/cm* foils.
anode gap to cause electrical failure. Second, break-down through the high-voltage rod insulator causes its destruction. Experience gained in working with high voltages in the 14UD accelerator has been applied to the Saddle Field Ion Source to eliminate insulator failure. To gain an understanding of the magnitudes of electric fields that can be tolerated, gradients that occur in an accelerator have been considered. Nominally, 10 kV is supposed to be applied to the Saddle Field Ion Source across the 1.25 mm gap between the rod and the ground. If this were a uniform field region, it would correspond to E = 10 kV/0.125 cm = 80 kV/cm.
However, the geometry of a small diameter rod passing through a plate causes the electric field to be concentrated onto the rod and onto the hole through the plate. The field on the rod would be even larger than that experienced by an infinitely long rod in a coaxial cylinder. The field enhancement in this coaxial cylinder limit is calculated using the formula
with a = 0.125 cm,
4. Saddle Field Ion Source modifications This sputter source has been successfully used for the production of targets since 1977. The typical operating parameters are 6 kV with 26 mA ion current. The operating voltage is restricted to 6 kV to avoid electrical breakdown problems. The present geometry for target production is shown in Fig 1. Two types of such problems have occurred. First, sputtered material from the cathode collimators deposited within the source can be dislodged when the system is vented for sample change, landing across a cathode to
201
b = 0.25 cm.
An enhancement factor of 1.44, l/ln(b/a), is produced at the surface of the rod. Thus, the electric field experienced by an insulator at an infinitely long rod is 115 kV/cm. However, there is no infinitely long outer cylinder, but only a 7 mm thick plate. The 2 mm radius of curvature on the hole in the plate will further increase the field on any insulating material placed in this gap. It is interesting to compare this field of at least 115 kV/cm, to which the insulating material is subjected, to the electric field in regions of electrostatic accelerators.
Fig. 1. Modified Saddle Field Ion Source.
VIII. TARGET LABORATORY REPORT
R.B. Turkentine, D.C. WeisseP/ Nucl. Instv. and Meth. in Phys. Res. A 397 (1997) 200-203
202
,-
chamber, the vacuum at the high-voltage rod is much poorer than the 1 x lo-’ Pa in the chamber. To accommodate this, we find that operating at 6 kV is satisfactory under these vacuum conditions. An additional simplification made to the source was to remove the anode insulator at the front of the source where loose sputtered material gathered causing electrical breakdown problems. The support this insulator gave to the anode can easily be provided by the high-voltage rod. Accurate location of the anode is obtained from the back main insulator. Corona from the connection to the high-voltage rod was eliminated by attaching a 12 mm diameter bronze sphere to the top of the rod. A threaded hole allows for the clamping of the electrical conductor that enters the sphere at right angles.
Gas inlet Connection
sphere
HV Insulator
Anode centering insulator
Anode
L
Cathode
Fig. 2. Schematic diagram of Saddle Field Ion Source showing modified high-voltage insulator and the removal of front anode insulator.
A typical HVEC accelerator uses 2.5 cm thick insulation to separate 50 kV in the column structure and accelerating tube. In both, an effort is made to keep the field across the insulator uniform, i.e., geometry-caused field enhancements are minimised. The design field for these insulators is therefore approximately 20 kV/cm. It is scarcely surprising that the insulator in the Saddle Field device fails when it passes through the hole in the plate. The simplest solution to this overstressing was to remove the insulating material from the gap by counterboring the Macor insulator. This also effectively increases the path length across the insulator from 1.25 to 11 mm as shown in Fig 2. The bore is made slightly larger in the insulator than the end plate hole to shield it from sputtered material that would otherwise make an electrical path to the ground on the insulator inner surface. The field across the vaccum gap of at least 115 kV/cm might be just acceptable for high vacuum applications, Latham [2], but not for the higher pressure regimes used for this source. Since argon gas is introduced into the source head directly and then escapes into the larger
Table 1 A sample Target
of typical
species
Molybdenum Zirconium Molybdenum Platinum Tungsten Iridium Titanium Titanium oxide Gold
targets
made using the saddle field ion source Thickness
5. Saddle Field Ion Source operation Best operation of the source has been accomplished by first conditioning the source to 10 kV without any source gas, i.e. at good vacuum. This moves particulate matter, that may be present in the source, to electrically inactive sites where it should not cause any problem during operation. Conditioning is complete when all electrical activity, displayed by the power supply, ceases at 10 kV operating voltage. The original Saddle Field Source was made almost entirely of aluminium. It was soon discovered that aluminium was present in the sputtered films, that flakes were produced in the source and that the aperture eroded rapidly. The aperture erosion rate and the production of flakes were reduced by using a tantalum aperture and Faraday cup. However, the tantalum contamination, though less in quantity than for aluminium, was unacceptable for some nuclear physics needs. The replacement of tantalum with carbon for the aperture and Faraday cup, solved this problem, especially for targets
and the operating
@g/cm’)
Sputter (hours)
125 40 220 440 200 390 17 190 250
7 3 7 17 26 20 5 18 1.5
time
parameters
used
Collimator material
Backing material
Tantalum Carbon Tantalum Aluminium Aluminium Aluminium Aluminium Aluminium Aluminium
Carbon Carbon Gadolinium Iron Iron Gadolinium Copper Havar Copper
R.B. Turkentine, D.C. Weisser / Nucl. Instr. and Meth. in Phys. Rex A 397 (1997) 200-203
which were deposited onto carbon backing anyway. Similarly, the halo beam from the source would sputter material from whatever supported the sample being sputtered. The halo was first reduced by adding a second aperture 10 mm closer to the sample. This reduced the halo but at the cost of a reduced sputtering rate. Our current practice is to support the sample on carbon and use a single carbon aperture. It may well be that the use of tantalum for both aperture and sample support is more suitable for targets that cannot tolerate carbon but can accept heavy element contamination.
203
commissioning of the LINAC. It is anticipated that the Saddle Field Ion Source will continue to reliably produce, a wide variety of targets whose composition will be controlled through the choice of appropriate materials.
Acknowledgements The authors wish to thank Gavin Gilmour for his artistry in producing the three-dimensional drawings.
References 6. Summary The target-making Department is about
activity in the Nuclear Physics to be reinvigorated following the
[l] A.H.F. Muggleton, Nucl. Instr. and Meth. A 303 (1991) 157. [2] R.V. Latham, High Voltage Vacuum Insulation: The Physical Basis, Academic Press, London, 1981.
VIII. TARGET
LABORATORY
REPORT
NW’--iLLnn INSTRl UMENTS
% METHODS IN PHYSICS RESEARCH
SectlonA
ELSEWIER
Target-making in the Nuclear Physics Department at the Australian National University R.B. Turkentine*,
D.C. Weisser
Nuclear Physics Department, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, Australian National University, Canberra, ACT 0200, Australia
Abstract The target-making activities in the Department are presented, including improvements to the modified Saddle Field Ion Source and a summary of carbon stripper foil production. Over the past three years, the installation and commissioning of a superconducting linear accelerator (LINAC) has dominated the activities of the Department and so a brief description of the LINAC is given.
1. Introduction This year, the Australian National University celebrates its 50th anniversary. The Department of Nuclear Physics, there from the beginning, has developed targetmaking facilities throughout its history. In the present day, targets for nuclear physics research are produced for use with the 14UD tandem accelerator (up-graded to 16 MV) and the recently commissioned LINAC booster. The Department also supplies carbon foils to other Australian institutions, where two cyclotrons and an FN accelerator operate. This paper will feature a description of the continuing improvements to the Saddle Field Ion Source subsequent to the work of Muggleton [l]. The target-making activities have been pursued over the past three years, against the background of the installation and commissioning of the LINAC which was transferred from the Daresbury Laboratory in the UK to the ANU. This work will be sketched to establish the context of the target-making efforts.
2. LINAC The arrival of the LINAC in April 1993 has caused target-making activities to be restricted almost entirely to essential carbon stripper foils. An intense work
*Corresponding author. Fax: +61 6 249 0748; e-mail: bob. [email protected].
schedule, required to get the LINAC commissioned by 1996, has meant that all members of the Department have had to re-focus their skills on this new project. The shipping containers that were used for shipping the LINAC to Australia from the UK, were packed in such a way that it resulted in corrosion damage which created extra work in the installation process. The accelerating function of the stage-l LINAC is accomplished by nine split-loop superconducting RF resonators, housed in three cryostats. A single niobiumsputtered quarter wave resonator in a separate cryostat functions as the beam buncher. All cryostats are supplied with liquid helium and liquid nitrogen from the cryogenic distribution system. The beam transport system for the LINAC can operate in several modes. The beam from the 14UD can be accelerated through the LINAC modules and directed to either the new target area or the existing 14UD target area. Alternatively, it can by-pass the modules and be directed into the LINAC target area without further acceleration. This allows for exploitation of all existing experimental equipment by the LINAC beam, as well as the parallel development of the LINAC while the new target facilities are used for the 14UD beam.
3. Stripper
The terminal stripper apparatus of the 14UD has a 260 foil capacity. The carbon stripper foils are produced by carbon arc evaporation onto glass slides coated with a detergent release agent. All foils used in the terminal
016%9002/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII
SO168-9002(97)00586-X
foils
R.B. Turkentine, D.C. Weissevj NwL Ins@. and Meth. in Phys. Rex A 397 (1997) 200-203
stripper are 3-5 &cm2 thick as measured by light attenuation using a calibrated light source. To enable efficient floating and mounting of such thin foils, the carboncoated slide is further coated with collodion. It has been observed that a speck of dust on the beam-exposed area of a foil develops a thickening of the material in the exact shape of the dust speck surrounded by a thinned halo. It is speculated that this leads to weakening of the foil and premature rupture. To avoid such specks, all operations are conducted in a laminar flow bench. The foils are mounted on frames with 9.35 mm diameter holes and then the collodion is removed by baking in air at 673 K. Each foil is individually inspected, using transmitted and scattered light in order to discard any with imperfections. Complete terminal stripper foil replacement is needed two to three times a year. This requires the production of about 400 foils allowing selection of 260 which meet the ANU standard. The second stripper has lo-15 pg/cm2 foils and only needs to be replenished about every two years. A third stripper, at the LINAC entrance is yet to be commissioned. It is expected to require 20 pg/cm* foils.
anode gap to cause electrical failure. Second, break-down through the high-voltage rod insulator causes its destruction. Experience gained in working with high voltages in the 14UD accelerator has been applied to the Saddle Field Ion Source to eliminate insulator failure. To gain an understanding of the magnitudes of electric fields that can be tolerated, gradients that occur in an accelerator have been considered. Nominally, 10 kV is supposed to be applied to the Saddle Field Ion Source across the 1.25 mm gap between the rod and the ground. If this were a uniform field region, it would correspond to E = 10 kV/0.125 cm = 80 kV/cm.
However, the geometry of a small diameter rod passing through a plate causes the electric field to be concentrated onto the rod and onto the hole through the plate. The field on the rod would be even larger than that experienced by an infinitely long rod in a coaxial cylinder. The field enhancement in this coaxial cylinder limit is calculated using the formula
with a = 0.125 cm,
4. Saddle Field Ion Source modifications This sputter source has been successfully used for the production of targets since 1977. The typical operating parameters are 6 kV with 26 mA ion current. The operating voltage is restricted to 6 kV to avoid electrical breakdown problems. The present geometry for target production is shown in Fig 1. Two types of such problems have occurred. First, sputtered material from the cathode collimators deposited within the source can be dislodged when the system is vented for sample change, landing across a cathode to
201
b = 0.25 cm.
An enhancement factor of 1.44, l/ln(b/a), is produced at the surface of the rod. Thus, the electric field experienced by an insulator at an infinitely long rod is 115 kV/cm. However, there is no infinitely long outer cylinder, but only a 7 mm thick plate. The 2 mm radius of curvature on the hole in the plate will further increase the field on any insulating material placed in this gap. It is interesting to compare this field of at least 115 kV/cm, to which the insulating material is subjected, to the electric field in regions of electrostatic accelerators.
Fig. 1. Modified Saddle Field Ion Source.
VIII. TARGET LABORATORY REPORT
R.B. Turkentine, D.C. WeisseP/ Nucl. Instv. and Meth. in Phys. Res. A 397 (1997) 200-203
202
,-
chamber, the vacuum at the high-voltage rod is much poorer than the 1 x lo-’ Pa in the chamber. To accommodate this, we find that operating at 6 kV is satisfactory under these vacuum conditions. An additional simplification made to the source was to remove the anode insulator at the front of the source where loose sputtered material gathered causing electrical breakdown problems. The support this insulator gave to the anode can easily be provided by the high-voltage rod. Accurate location of the anode is obtained from the back main insulator. Corona from the connection to the high-voltage rod was eliminated by attaching a 12 mm diameter bronze sphere to the top of the rod. A threaded hole allows for the clamping of the electrical conductor that enters the sphere at right angles.
Gas inlet Connection
sphere
HV Insulator
Anode centering insulator
Anode
L
Cathode
Fig. 2. Schematic diagram of Saddle Field Ion Source showing modified high-voltage insulator and the removal of front anode insulator.
A typical HVEC accelerator uses 2.5 cm thick insulation to separate 50 kV in the column structure and accelerating tube. In both, an effort is made to keep the field across the insulator uniform, i.e., geometry-caused field enhancements are minimised. The design field for these insulators is therefore approximately 20 kV/cm. It is scarcely surprising that the insulator in the Saddle Field device fails when it passes through the hole in the plate. The simplest solution to this overstressing was to remove the insulating material from the gap by counterboring the Macor insulator. This also effectively increases the path length across the insulator from 1.25 to 11 mm as shown in Fig 2. The bore is made slightly larger in the insulator than the end plate hole to shield it from sputtered material that would otherwise make an electrical path to the ground on the insulator inner surface. The field across the vaccum gap of at least 115 kV/cm might be just acceptable for high vacuum applications, Latham [2], but not for the higher pressure regimes used for this source. Since argon gas is introduced into the source head directly and then escapes into the larger
Table 1 A sample Target
of typical
species
Molybdenum Zirconium Molybdenum Platinum Tungsten Iridium Titanium Titanium oxide Gold
targets
made using the saddle field ion source Thickness
5. Saddle Field Ion Source operation Best operation of the source has been accomplished by first conditioning the source to 10 kV without any source gas, i.e. at good vacuum. This moves particulate matter, that may be present in the source, to electrically inactive sites where it should not cause any problem during operation. Conditioning is complete when all electrical activity, displayed by the power supply, ceases at 10 kV operating voltage. The original Saddle Field Source was made almost entirely of aluminium. It was soon discovered that aluminium was present in the sputtered films, that flakes were produced in the source and that the aperture eroded rapidly. The aperture erosion rate and the production of flakes were reduced by using a tantalum aperture and Faraday cup. However, the tantalum contamination, though less in quantity than for aluminium, was unacceptable for some nuclear physics needs. The replacement of tantalum with carbon for the aperture and Faraday cup, solved this problem, especially for targets
and the operating
@g/cm’)
Sputter (hours)
125 40 220 440 200 390 17 190 250
7 3 7 17 26 20 5 18 1.5
time
parameters
used
Collimator material
Backing material
Tantalum Carbon Tantalum Aluminium Aluminium Aluminium Aluminium Aluminium Aluminium
Carbon Carbon Gadolinium Iron Iron Gadolinium Copper Havar Copper
R.B. Turkentine, D.C. Weisser / Nucl. Instr. and Meth. in Phys. Rex A 397 (1997) 200-203
which were deposited onto carbon backing anyway. Similarly, the halo beam from the source would sputter material from whatever supported the sample being sputtered. The halo was first reduced by adding a second aperture 10 mm closer to the sample. This reduced the halo but at the cost of a reduced sputtering rate. Our current practice is to support the sample on carbon and use a single carbon aperture. It may well be that the use of tantalum for both aperture and sample support is more suitable for targets that cannot tolerate carbon but can accept heavy element contamination.
203
commissioning of the LINAC. It is anticipated that the Saddle Field Ion Source will continue to reliably produce, a wide variety of targets whose composition will be controlled through the choice of appropriate materials.
Acknowledgements The authors wish to thank Gavin Gilmour for his artistry in producing the three-dimensional drawings.
References 6. Summary The target-making Department is about
activity in the Nuclear Physics to be reinvigorated following the
[l] A.H.F. Muggleton, Nucl. Instr. and Meth. A 303 (1991) 157. [2] R.V. Latham, High Voltage Vacuum Insulation: The Physical Basis, Academic Press, London, 1981.
VIII. TARGET
LABORATORY
REPORT