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The target laboratory of the Pelletron accelerator at the University of São Paulo
188
Nuclear Instruments and Methods in Physics Research A282 (1989) 188-190 North-Holland, Amsterdam
THE TARGET LABORATORY OF THE PELLETRON ACCELERATOR AT THE UNIVERSITY OF SÂO PAULO N. UETA and W.G.P. ENGEL Departamento de Fisica Nuclear, Universidade de Säo Paulo, CP. 20516, 01498 Säo Paulo, SP, Brasil
A brief description of the facilities installed in the target laboratory will be given. Successes and problems encountered during last year's target making as well as progress made toward the installation of a carbon arc apparatus will be described . 1. Laboratory equipment The target laboratory was installed around 1970 to fabricate targets for all users of the Pelletron accelerator . Equipment being used in our laboratory during this time includes a small evaporator, a roller, a press, two analytical balances, a water-distiller apparatus and a water deionizer . The roller is motor driven (0.75 kW from Brook Motors Ltd .) and has two cylinders (6-cm diameter x 12-cm long). The press is a Beckmann 25-ton ring press (type 00-25) . The diameter of the platens is 3 cm; no heating is possible. The evaporator is an Edwards E12 E3 with a bell jar (30-cm diameter x 36-cm height) mechanical pump, a 10-cm diameter Edwards diffusion pump (600 1/s), and a LN2 trap. The usual vacuum to begin evaporation is between 10-° and 10-3 Pa, reached within about 30 min. In the small evaporator, all evaporated targets are made by Joule heating or by electron bombardment with electrostatic focusing, adjusted mechanically by varying the distance between crucible and filament without breaking the vacuum. The maximum power with electron bombardment is about 300 W. Only relative power measurements are made in the Joule-heating method. 2. General preparation procedure 2.1. Evaporation As a substrate, the usual microscope slides are used after a bath in acetone and a complete polishing with cleaner 1 and 2 from Balzers . A parting agent commonly used is RBS diluted in distilled water (1 part RBS, 3 parts water), spread on the cleaned slides and * Supported by Fundaçâo de Amparo à Pesquisa do Fstado de Sâo Paulo, Brasil . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
then rubbed with a Kleenex until only a very thin layer remains on both sides of the glass slide. The best surface is then chosen. Generally all release agents, e.g., CsI, NaCl, RBS + NaCl, and RBS + CsI are first tried in test evaporations using natural material before the separated-isotope evaporation is attempted . The substrates with only a release agent, or with a thin layer of carbon (= 5 ,u g/cm2 ) evaporated on them, are mounted about 10 cm above the crucible or boat. Cooling of the substrates during evaporation is sometimes necessary. The LN2-filled substrate cooler we have been using works only for a short time; ice builds up and no refilling is possible . A new design will be tested soon. Once evaporated, the targets are cut appropriately, floated in distilled water and fished onto Inox target frames of about 10 mm x 20 mm size, with holes of up to 8 mm diameter . To avoid dust and airborne contamination, the floating and drying of targets are accomplished in a laboratory hood with laminar flow. When water or oxygen has to be avoided, the target material is deposited on a thin vacuum-evaporated carbon backing already mounted on a frame. Usually no mask is used. 2.2. Rolling Pack rolling is usually done by sandwiching the material between sheets of 1-mm thick Inox cut into 3-cm x 10-cm pieces, polished and then cleaned with acetone and isopropyl alcohol inside a laboratory hood used for "dirty" purposes. The material is also cleaned and weighed . The pressure on the cylinders is gradually raised in many steps, until the desired area is obtained . Sometimes it is necessary to cut out some irregular parts and use only the uniform piece in further rolling . Alcohol is sometimes used as a lubricant to prevent adhesion to the Inox rolling pack.
189
N. Ueta, W.G.P. Engel / Target laboratory at Sâo Paulo Table 1 Target preparations of 1988 Element
Backing k)
Thickness
Release agent
Method 1 )
Notes
27AI
S .S .
RBS RBS
W filament Wfilament
a)
27AI 63
S.S.
1 mg/cmZ 200 pg/cmZ 4 mg/cmZ 50 pg/cm Z 800 pg/cm Z
27AI
cu
63C u
ZnO
Sn02 24 MgO 24 MgO 11 11 Ti02 Au Ta Fe Co
S.S . S.S. S.S.
C on frame C C+Bi C+Au S.S .
C on frame S .S . S .S . S .S . S .S .
RBS RBS RBS
50 ,/cmZ 30-50 pg/cmZ
30-50 lLg/cmZ 20 pg/cmZ 1-4 mg/cmZ 1-4 mg/cmZ 1-4 mg/cmZ 10 mg/cmZ
BO RBS
a) b)
rolling Ta boat Ta boat Ta boat Ta, e.b . Ta, e.b . Ta, e.b . Cermet, e.b . Ta, e.b . rolling
e) c) d) e)
0 s) n)
rolling rolling rolling
O
a) A tungsten helical-coil filament from Balzers, 15-mm long and 4-mm diameter was used. The evaporated foil was about 250
pg/cmZ. Two double foils were folded together which gave the required thickness.
b) Rolling was started using a wire of 1.6 mm diameter and 5 mm long, 99.9% pure Al .
63 C A Ta boat of 6 cm x 1 cm with 1 cm diameter depression, without cover, was used as the heater of metallic
U.
d) The Ta boat is similar to the one described in') . No mask was used in front of the carbon . e) A home-made cylindrical Ta crucible of 15 mm total height, 3 mm outer diameter, 1.5 mm inner diameter, and a well of 10 mm
was used . r) This Ta crucible was similar to the previous one (e)) . To get better adhesion, a very thin layer of Bi in one case and Au in the other one was evaporated on the carbon film, previously mounted on a target frame. No thickness measurement was made (Bi) gave better adhesion, but Au was preferred) . s) Initially, a thin layer of B was evaporated on a clean and polished glass slide, then air was introduced into the evaporator to get the BO layer. After a good vacuum was again attained, further evaporations with 150-W power were carried out. h) A power range of 80-100 W is sufficient to evaporate the Ti0 inside a Ta crucible similar to that described in e). ') The usual rolling procedure already described. j) This 59 Co target was made by rolling the Co between Pb foils.
2
k) s.s. means self-supporting. 1) e.b . means electron bombardment.
2.3. Thickness determination The target thicknesses are determined by gravimetry . For evaporated films, a small piece of thin plain glass is used as a monitor. The absolute thicknesses are ob-
tained also by Rutherford-scattering measurements. The targets are generally stored in small containers (diameter of about 8 cm) in an argon atmosphere, at room temperature. Some are stored in a freezer within plastic boxes.
3. Specific preparations
1.
Targets made during the last year are shown in table
4. Troublesome preparations a) Ni + Pb : for a plunger used in y experiments, Pb must be evaporated to a Ni foil. The double target must
be stretched to obtain a flat surface. When the Pb is thicker than 20 mg/cm2, the Pb separates from the Ni during the stretching .
b) Sb + Au : Au foil was rolled to = 2 mg/cm2 and Sb was evaporated onto it . 5 mg of Sb was used in the crucible but the target was only 200 lrg/cm2. The efficiency is therefore too low.
Table 2 Stripper foil results Type Evaporated, slackened Evaporated, selected with microscope ( = 5 p g/cmZ) Evaporated, selected with microscope ( >_ 7ltg/cm2) Cracking
Transmission 160 beam
Lifetime [nA h]
3
20000±12000
4
5000± 3000
=2
15000±10000 11000±11000
1 .3-2 .5
VI . ACTIVITY REPORTS
190
N. Ueta, W.G.P. Engel / Target laboratory at Sdo Paulo
c) F: LiF was used in attempts to make F targets of about 50 wg/cm2. No target was obtained . d) Si : Self-supporting targets of > 200 wg/cm2 were needed . Natural Si in metallic form was evaporated on NaCl, floated well but was not stable for a sufficiently long time . Evaporated Si02 targets of about 50 wg/cm2 were made on carbon backing. e) Fe : on carbon backing. The high temperature damaged the carbon backing. 5. Stripper foils Usually the stripper foils used in the accelerator are changed two or three times per year . So, during the course of a year about 200-300 stripper foils must be made . An attempt was made some years ago to fabricate good-quality C stripper foils by cracking ethylene gas. The apparatus used was similar to those used at Daresbury and Heidelberg [1, 2] . Unfortunately, the cracked foils were not reliable . We suspect the key variable to be the quality of the gas, because gas from different bottles gave different results. Most stripper foils are made by electron bombardment. Some foils evaporated using electron bombardment were slackened by a method similar to that used in Miinchen [3]. Other stripper foils are chosen, inspecting each foil carefully through a microscope . Only foils with neither wrinkles nor pinholes are chosen . Typical results obtained with stripper foils over the last several years are shown in table 2. We are now attempting to produce stripper foils by the carbon-arc method with heated substrates, similar to Jaeri's method [4]. The prepared substrates are mounted on another evaporator from NRC with a 45-cm diameter x 75-cm height bell jar, a 10 cm NRC diffusion
pump, a LN2 trap, and an Edwards E2M18 mechanical pump. The pumping speed of this system is 8001/s . The maximum power attainable in the evaporation is about 300 W. The substrate heater used with this system consists of one 500-W halogen lamp, positioned over the glass substrates and under an Al reflector. The temperature of the substrates is measured using 6 Zener diodes (Phillips 4148) in series with a current source fixed at 1 mA. The voltage across the diodes is linearly dependent on their temperature. Good thermal contact between diodes and glass substrates must be guaranteed. The first batch of these foils is now in the accelerator.
Acknowledgements Some targets were cooperatively developed with the users. The financial support of the target laboratory comes from the University of Sâo Paulo, from the Conselho Nacional de Pesquisas (CNPq), and from the Fundaçâo de Amparo à Pesquisa do Estado de Sâo Paulo. References [1] D.S . Whitmell, B.H . Armitage, D.W .L . Tolfree, and N.R.S . Tait, Nucl . Instr. and Meth . 159 (1979) 605. [2] B. Huck, E. Jaeschke, W. Kratschmer, R. Repnow, and H . Wirth, Nucl . Instr. and Meth . 184 (1981) 215. [3] P. Maier-Komor, E. Ranzinger, and H . Münzer, Proc . 3rd Int. Conf . on Electrostatic Accelerator Technology, 1981, p. 163. [4] S. Takeuchi, C. Kobayashi, Y. Satoh, T. Yoshida, E. Takekoshi, and M. Maruyama, Nucl . Instr. and Meth . 158 (1979) 333.
Nuclear Instruments and Methods in Physics Research A282 (1989) 188-190 North-Holland, Amsterdam
THE TARGET LABORATORY OF THE PELLETRON ACCELERATOR AT THE UNIVERSITY OF SÂO PAULO N. UETA and W.G.P. ENGEL Departamento de Fisica Nuclear, Universidade de Säo Paulo, CP. 20516, 01498 Säo Paulo, SP, Brasil
A brief description of the facilities installed in the target laboratory will be given. Successes and problems encountered during last year's target making as well as progress made toward the installation of a carbon arc apparatus will be described . 1. Laboratory equipment The target laboratory was installed around 1970 to fabricate targets for all users of the Pelletron accelerator . Equipment being used in our laboratory during this time includes a small evaporator, a roller, a press, two analytical balances, a water-distiller apparatus and a water deionizer . The roller is motor driven (0.75 kW from Brook Motors Ltd .) and has two cylinders (6-cm diameter x 12-cm long). The press is a Beckmann 25-ton ring press (type 00-25) . The diameter of the platens is 3 cm; no heating is possible. The evaporator is an Edwards E12 E3 with a bell jar (30-cm diameter x 36-cm height) mechanical pump, a 10-cm diameter Edwards diffusion pump (600 1/s), and a LN2 trap. The usual vacuum to begin evaporation is between 10-° and 10-3 Pa, reached within about 30 min. In the small evaporator, all evaporated targets are made by Joule heating or by electron bombardment with electrostatic focusing, adjusted mechanically by varying the distance between crucible and filament without breaking the vacuum. The maximum power with electron bombardment is about 300 W. Only relative power measurements are made in the Joule-heating method. 2. General preparation procedure 2.1. Evaporation As a substrate, the usual microscope slides are used after a bath in acetone and a complete polishing with cleaner 1 and 2 from Balzers . A parting agent commonly used is RBS diluted in distilled water (1 part RBS, 3 parts water), spread on the cleaned slides and * Supported by Fundaçâo de Amparo à Pesquisa do Fstado de Sâo Paulo, Brasil . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
then rubbed with a Kleenex until only a very thin layer remains on both sides of the glass slide. The best surface is then chosen. Generally all release agents, e.g., CsI, NaCl, RBS + NaCl, and RBS + CsI are first tried in test evaporations using natural material before the separated-isotope evaporation is attempted . The substrates with only a release agent, or with a thin layer of carbon (= 5 ,u g/cm2 ) evaporated on them, are mounted about 10 cm above the crucible or boat. Cooling of the substrates during evaporation is sometimes necessary. The LN2-filled substrate cooler we have been using works only for a short time; ice builds up and no refilling is possible . A new design will be tested soon. Once evaporated, the targets are cut appropriately, floated in distilled water and fished onto Inox target frames of about 10 mm x 20 mm size, with holes of up to 8 mm diameter . To avoid dust and airborne contamination, the floating and drying of targets are accomplished in a laboratory hood with laminar flow. When water or oxygen has to be avoided, the target material is deposited on a thin vacuum-evaporated carbon backing already mounted on a frame. Usually no mask is used. 2.2. Rolling Pack rolling is usually done by sandwiching the material between sheets of 1-mm thick Inox cut into 3-cm x 10-cm pieces, polished and then cleaned with acetone and isopropyl alcohol inside a laboratory hood used for "dirty" purposes. The material is also cleaned and weighed . The pressure on the cylinders is gradually raised in many steps, until the desired area is obtained . Sometimes it is necessary to cut out some irregular parts and use only the uniform piece in further rolling . Alcohol is sometimes used as a lubricant to prevent adhesion to the Inox rolling pack.
189
N. Ueta, W.G.P. Engel / Target laboratory at Sâo Paulo Table 1 Target preparations of 1988 Element
Backing k)
Thickness
Release agent
Method 1 )
Notes
27AI
S .S .
RBS RBS
W filament Wfilament
a)
27AI 63
S.S.
1 mg/cmZ 200 pg/cmZ 4 mg/cmZ 50 pg/cm Z 800 pg/cm Z
27AI
cu
63C u
ZnO
Sn02 24 MgO 24 MgO 11 11 Ti02 Au Ta Fe Co
S.S . S.S. S.S.
C on frame C C+Bi C+Au S.S .
C on frame S .S . S .S . S .S . S .S .
RBS RBS RBS
50 ,/cmZ 30-50 pg/cmZ
30-50 lLg/cmZ 20 pg/cmZ 1-4 mg/cmZ 1-4 mg/cmZ 1-4 mg/cmZ 10 mg/cmZ
BO RBS
a) b)
rolling Ta boat Ta boat Ta boat Ta, e.b . Ta, e.b . Ta, e.b . Cermet, e.b . Ta, e.b . rolling
e) c) d) e)
0 s) n)
rolling rolling rolling
O
a) A tungsten helical-coil filament from Balzers, 15-mm long and 4-mm diameter was used. The evaporated foil was about 250
pg/cmZ. Two double foils were folded together which gave the required thickness.
b) Rolling was started using a wire of 1.6 mm diameter and 5 mm long, 99.9% pure Al .
63 C A Ta boat of 6 cm x 1 cm with 1 cm diameter depression, without cover, was used as the heater of metallic
U.
d) The Ta boat is similar to the one described in') . No mask was used in front of the carbon . e) A home-made cylindrical Ta crucible of 15 mm total height, 3 mm outer diameter, 1.5 mm inner diameter, and a well of 10 mm
was used . r) This Ta crucible was similar to the previous one (e)) . To get better adhesion, a very thin layer of Bi in one case and Au in the other one was evaporated on the carbon film, previously mounted on a target frame. No thickness measurement was made (Bi) gave better adhesion, but Au was preferred) . s) Initially, a thin layer of B was evaporated on a clean and polished glass slide, then air was introduced into the evaporator to get the BO layer. After a good vacuum was again attained, further evaporations with 150-W power were carried out. h) A power range of 80-100 W is sufficient to evaporate the Ti0 inside a Ta crucible similar to that described in e). ') The usual rolling procedure already described. j) This 59 Co target was made by rolling the Co between Pb foils.
2
k) s.s. means self-supporting. 1) e.b . means electron bombardment.
2.3. Thickness determination The target thicknesses are determined by gravimetry . For evaporated films, a small piece of thin plain glass is used as a monitor. The absolute thicknesses are ob-
tained also by Rutherford-scattering measurements. The targets are generally stored in small containers (diameter of about 8 cm) in an argon atmosphere, at room temperature. Some are stored in a freezer within plastic boxes.
3. Specific preparations
1.
Targets made during the last year are shown in table
4. Troublesome preparations a) Ni + Pb : for a plunger used in y experiments, Pb must be evaporated to a Ni foil. The double target must
be stretched to obtain a flat surface. When the Pb is thicker than 20 mg/cm2, the Pb separates from the Ni during the stretching .
b) Sb + Au : Au foil was rolled to = 2 mg/cm2 and Sb was evaporated onto it . 5 mg of Sb was used in the crucible but the target was only 200 lrg/cm2. The efficiency is therefore too low.
Table 2 Stripper foil results Type Evaporated, slackened Evaporated, selected with microscope ( = 5 p g/cmZ) Evaporated, selected with microscope ( >_ 7ltg/cm2) Cracking
Transmission 160 beam
Lifetime [nA h]
3
20000±12000
4
5000± 3000
=2
15000±10000 11000±11000
1 .3-2 .5
VI . ACTIVITY REPORTS
190
N. Ueta, W.G.P. Engel / Target laboratory at Sdo Paulo
c) F: LiF was used in attempts to make F targets of about 50 wg/cm2. No target was obtained . d) Si : Self-supporting targets of > 200 wg/cm2 were needed . Natural Si in metallic form was evaporated on NaCl, floated well but was not stable for a sufficiently long time . Evaporated Si02 targets of about 50 wg/cm2 were made on carbon backing. e) Fe : on carbon backing. The high temperature damaged the carbon backing. 5. Stripper foils Usually the stripper foils used in the accelerator are changed two or three times per year . So, during the course of a year about 200-300 stripper foils must be made . An attempt was made some years ago to fabricate good-quality C stripper foils by cracking ethylene gas. The apparatus used was similar to those used at Daresbury and Heidelberg [1, 2] . Unfortunately, the cracked foils were not reliable . We suspect the key variable to be the quality of the gas, because gas from different bottles gave different results. Most stripper foils are made by electron bombardment. Some foils evaporated using electron bombardment were slackened by a method similar to that used in Miinchen [3]. Other stripper foils are chosen, inspecting each foil carefully through a microscope . Only foils with neither wrinkles nor pinholes are chosen . Typical results obtained with stripper foils over the last several years are shown in table 2. We are now attempting to produce stripper foils by the carbon-arc method with heated substrates, similar to Jaeri's method [4]. The prepared substrates are mounted on another evaporator from NRC with a 45-cm diameter x 75-cm height bell jar, a 10 cm NRC diffusion
pump, a LN2 trap, and an Edwards E2M18 mechanical pump. The pumping speed of this system is 8001/s . The maximum power attainable in the evaporation is about 300 W. The substrate heater used with this system consists of one 500-W halogen lamp, positioned over the glass substrates and under an Al reflector. The temperature of the substrates is measured using 6 Zener diodes (Phillips 4148) in series with a current source fixed at 1 mA. The voltage across the diodes is linearly dependent on their temperature. Good thermal contact between diodes and glass substrates must be guaranteed. The first batch of these foils is now in the accelerator.
Acknowledgements Some targets were cooperatively developed with the users. The financial support of the target laboratory comes from the University of Sâo Paulo, from the Conselho Nacional de Pesquisas (CNPq), and from the Fundaçâo de Amparo à Pesquisa do Estado de Sâo Paulo. References [1] D.S . Whitmell, B.H . Armitage, D.W .L . Tolfree, and N.R.S . Tait, Nucl . Instr. and Meth . 159 (1979) 605. [2] B. Huck, E. Jaeschke, W. Kratschmer, R. Repnow, and H . Wirth, Nucl . Instr. and Meth . 184 (1981) 215. [3] P. Maier-Komor, E. Ranzinger, and H . Münzer, Proc . 3rd Int. Conf . on Electrostatic Accelerator Technology, 1981, p. 163. [4] S. Takeuchi, C. Kobayashi, Y. Satoh, T. Yoshida, E. Takekoshi, and M. Maruyama, Nucl . Instr. and Meth . 158 (1979) 333.