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The production of molybdenum targets for heavy-ion experiments by electron beam evaporation
Nuclear Instruments and Methods in Physics Research A303 (1991) 165-167 North-Holland
165
The production of molybdenum targets for heavy-ion experiments by electron beam evaporation John P. Greene and George E. Thomas
Physics Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA
Self-supporting targets of 92,98Mo, with thicknesses of 100 and 200 FLg/cm2, were prepared by electron beam gun evaporation. Substrate heating proved crucial for the production of these foils. The numerous parting agents explored will be discussed. Targets of 9298 Mo were also prepared on carbon backings of various thicknesses. Problems encountered in the production of 100-200 wg/cm2 thick self-supporting and carbon-backed Mo targets for reactions with heavy ions are discussed. 1. Introduction 92,98Mo Targets of for experiments performed at the Enge split-pole spectrograph of the ATLAS facility have been produced . The experiments used a 400 MeV 86 beam of Kr which was accelerated in the new positiveion injector of the ATLAS heavy-ion accelerator at ANL. 2. Experimental arrangement All of the targets were prepared by electron beam evaporation using a Temescal STIH-270-1 four-pocket electron beam source installed within a cryo-pumped vacuum chamber. This e-gun has been extensively modified [1]. The four-pocket turret has been redesigned, mainly to facilitate cleaning and maintenance, but also to replace the 7 cm3 crucibles with a more shallow "dimple-shaped" hearth. This is an important modification, because only small quantities of isotopic material are used in most vacuum evaporations . The original electron emitter assembly has also been replaced with one of a new design, using filaments with two turns of 0.5 mm diam tungsten wire . These small filaments have a dramatic effect on the beam spot size ; hence, the deposition rates have increased substantially. Electron beam voltage, adjustable from 2 to 10 kV, was obtained using an Eratron EB-8 power supply from Innotec (Innotec Group, 61 W. Moreland Rd ., Semi Valley, CA 93065). The electron gun control module allows adjustment of the beam current as well as the x-y positioning
* Work supported by the U .S . Department of Energy, Nuclear Physics Division, under contract W-31-109-ENG-38.
of the beam on the sample. Filament current is monitored by a digital readout and is internally adjustable . Substrate heating is possible using a quartz lamp with the temperature measured via an iron-constantan thermocouple with digital display. Film thickness was determined with an Inficon XTM digital quartz crystal monitor with deposition rate indicator (Leybold Inficon, Inc., 6500 Fly Rd., East Syracuse, NY 13057) . A rotatable shutter protects the substrates when no evaporation is occurring. Fig. 1 is a photograph showing the evaporator setup. 3. Sample preparation It is known that in arc-melted molybdenum a coarse-grained structure is produced which allows an oxide film coating to form at the grain boundaries . This makes rolling difficult due to the cracks which develop [2]. In prior discussions with F. Karasek [3], regarding rolling of isotopically enriched molybdenum, it was mentioned that one should exercise caution in the use of this material if it is to be rolled without additional chemical reduction. Indeed, as experience has shown, the difference in the behavior of the isotopic material as compared to the natural material can be dramatic. This may be due in part to the method of preparation for the Calutron-enriched material or perhaps the age of the sample. The appearance of the powdered isotopic sample as obtained from Oak Ridge National Laboratory (ORNL), exhibits a bluish coloration . This form of molybdenum is the so-called " molybdenum-blue" or Mo20, - xH20 [4]. A recent sample of the natural metal (purity 99 .95%) obtained from Aldrich Chemical Co . (Milwaukee, WI 53233, USA) shows a more molybdenum-like, light gray appearance. Further chemical reduction of the 92 .98Mo
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B .V. (North-Holland)
IV . VAPOR DEPOSITED TARGETS
166
J. P Greene, G E. Thomas / Production
of Mo
targets by e-beam evaporation
very short lifetime when tested in a 1 pnA beam of 181 MeV of 36S ions at the ATLAS facility . The reason for this behavior is not understood . 4.2 . Carbon-backed molybdenum targets
Fig 1 . Photograph of the electron beam system setup for the evaporation of molybdenum self-supporting films. samples yielded percent weight losses of 7.3% and 4.5% respectively, accompanied also by a change in appearance from a bluish to a grayish color. 4. Target preparation 4.1 . Self-supporting molybdenum targets Self-supporting films of molybdenum have been prepared previously to thicknesses of 200 lag/cmz by rolling [5,6], and to 100 gg/cmz by electron bombardment evaporation [7]. Our initial attempts to produce molybdenum films by electron beam evaporation onto glass slides using 40-60 lag/cm z NaCI as a release agent were unsuccessful . They could not be floated off due to severe residual stresses. This effect has been discussed m the literature [8-11] . In a second attempt, BaC1 2 was tried as a parting agent (melting point 1235 K) and the substrate temperature was kept at 623 K [12] . The high melting point of BaC1 2 makes it better suited to withstand high substrate temperatures . This method proved 92,98Mo . to be successful for 100 wg/cmz targets of Also . 200 wg/cmz thick foils were produced using the same technique. These targets, however, exhibited a
In light of the short in-beam lifetimes encountered for the self-supporting foils, the production of isotopic targets on 15-20 ~Lg/cmz carbon backings was tried. When attempting to prepare these targets using natural material, it was found that these carbon foils, mounted over a 12 .7 mm diam aperture, would not support a deposition of more than 80 Irg/cm z of molybdenum . This might be due to the heat generated during the evaporation process. In instances such as this, requiring high vaporization temperatures, the area radiating the highest heat within the water-cooled hearth can be decreased by reducing the size of the beam spot or by the use of heat shielding. Using a smaller amount of material in the pocket helps, but for these thick films several evaporations would then be necessary. It was next decided to evaporate the molybdenum onto carbon-coated glass slides . Because commercially available carbon foils usually employ organic release agents which would tend to decompose under these elevated temperature conditions, carbon-coated slides using an inorganic salt as a parting agent were produced. The four-pocket capability of our electron beam source is well suited for these types of multiple evaporations. It was found, however, that concurrent evaporation of salt, carbon and molybdenum produced foils which were highly stressed, primarily due to the differing coefficients of thermal expansion of the carbon and molybdenum . In fact, even the uncoated carbon foils showed residual stress which could be removed by post-annealing in a furnace at - 433 K overnight [13] . Carbon foils without stress could also be prepared by preheating the substrate to 523 K for the deposition . Using the reduced material (section 3) and stress-relieved carbon foils of 20 lag/cmz as substrates . films of 92,98Mo 200 wg/cmz were prepared . A number of targets were produced by this method . In some cases, further annealing was necessary to relieve stress in the foils . These targets sometimes appeared brittle and would occasionally break for no apparent reason . Foils prepared using natural molybdenum did not exhibit such behavior. It is known that in the production of ductile molybdenum, the resulting degree of brittleness is highly dependent upon the precise conditions present during the reduction process [2]. In addition, the mechanical strength properties of molybdenum may be drastically altered by a small degree of recrystallization. Further research on these properties is indicated, particularly because these carbon-backed isotopic targets again exhibited short lifetimes when exposed to a 400 MeV 86 Kr beam with a current of 4 pnA.
J. P Greene, G E. Thomas / Production of Mo targets by e-beam evaporation In a later attempt, spot targets of 100 pg/cm2 92 MO on carbon backings were prepared by electron beam gun evaporation using the reduced material . The carbon backings of 40 hg/cm2 thickness were obtained commercially (Arizona Carbon Foil Co ., Inc., 2239 E.
167
Acknowledgement The authors would like to thank Dr . D.S . Gemmell, Dr . I. Ahmad, and Dr . K.E. Rehm for their continued
support of and encouragement in thus project.
Kleindale Rd ., Tuscon, AZ 85719) and floated in the
usual fashion onto standard ATLAS frames having a 12 .7 mm diam aperture . The spot size was 9.5 mm diameter . These targets performed well to the beam. An explanation as to why spot targets with
this
thickness of carbon backing withstand beam while selfsupporting foils fail rather quickly is under study. It appears that depositing the molybdenum in a circular area on the carbon is the crucial aspect of the target lifetime, the strength of the target being derived mainly
from the carbon backing. It is not known whether a comparable foil prepared by depositing the molybdenum
over the full surface of a 40 ~Lg/cm2 carbon-coated slide, which was then floated in the usual manner, would survive the beam as well as the spot targets.
5. Conclusion Self-supporting
foils
of
isotopically
enriched
molybdenum can be produced routinely down to 100 !rg/cm2 using substrate heating and BaC1 2 as a release agent . Spot targets of 92 Mo on relatively thick carbon backings were also prepared . These targets exhibited substantially longer
lifetimes when
exposed
to
the
heavy-ion beam used in the experiment . Further work
suggested by our experience will include exploring the limits of survivability of thin carbon foils used as backings in heavy-ion beams.
References [1] G.E . Thomas, J.P . Greene, P. Maier-Komor and R.H . Leonard, to be published. [2] W. Espe, Materials of High Vacuum Technology, (Pergamon, Oxford, 1966) pp . 82-99. [31 F.J . Karasek, Argonne National Laboratory, private communication . [4] R.C . Weast (ed.), Handbook of Chemistry and Physics (CRC press, Cleveland, OH, 1969) p. B-131. [5] F.J . Karasek, Nucl . Instr. and Meth . 102 (1972) 457. [6] H. Folger, W. Hartmann, J. Klemm and W. Thalheimer, Proc . 1983 Workshop of the Int. Nuclear Target Development Society, Argonne National Laboratory, Argonne, IL, ANL/PHY-84-2, p. 15 . F. Nickel, W. Hartmann and D. Marx, Nucl. Instr. and Meth . 167 (1979) 175. [8] K.L Chopra, Thin Film Phenomena (McGraw-Hill, New York, 1969) pp . 274-280. D. Ramsay, Proc . 1974 Ann. Conf. of the Int. Nuclear Target Development Society, Chalk River, Ontario, Canada, Report AECL-5503, p 151 . [10] J.C. Gursky, Proc . 5th Ann. Conf. of the INTDS, Los Alamos, NM, Los Alamos Scientific Laboratory Report LA-6850C (1976) p. 198. [11] E. Klokholm and B.S . Berry, J. Electrochem. Soc. 115 (1968) 823. [12] P. Maier-Komor, Technische Universität München, private communication . [13] J.0 . Stoner, Jr ., Arizona Carbon Foil Co, Inc., private communication .
IV . VAPOR DEPOSITED TARGETS
165
The production of molybdenum targets for heavy-ion experiments by electron beam evaporation John P. Greene and George E. Thomas
Physics Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA
Self-supporting targets of 92,98Mo, with thicknesses of 100 and 200 FLg/cm2, were prepared by electron beam gun evaporation. Substrate heating proved crucial for the production of these foils. The numerous parting agents explored will be discussed. Targets of 9298 Mo were also prepared on carbon backings of various thicknesses. Problems encountered in the production of 100-200 wg/cm2 thick self-supporting and carbon-backed Mo targets for reactions with heavy ions are discussed. 1. Introduction 92,98Mo Targets of for experiments performed at the Enge split-pole spectrograph of the ATLAS facility have been produced . The experiments used a 400 MeV 86 beam of Kr which was accelerated in the new positiveion injector of the ATLAS heavy-ion accelerator at ANL. 2. Experimental arrangement All of the targets were prepared by electron beam evaporation using a Temescal STIH-270-1 four-pocket electron beam source installed within a cryo-pumped vacuum chamber. This e-gun has been extensively modified [1]. The four-pocket turret has been redesigned, mainly to facilitate cleaning and maintenance, but also to replace the 7 cm3 crucibles with a more shallow "dimple-shaped" hearth. This is an important modification, because only small quantities of isotopic material are used in most vacuum evaporations . The original electron emitter assembly has also been replaced with one of a new design, using filaments with two turns of 0.5 mm diam tungsten wire . These small filaments have a dramatic effect on the beam spot size ; hence, the deposition rates have increased substantially. Electron beam voltage, adjustable from 2 to 10 kV, was obtained using an Eratron EB-8 power supply from Innotec (Innotec Group, 61 W. Moreland Rd ., Semi Valley, CA 93065). The electron gun control module allows adjustment of the beam current as well as the x-y positioning
* Work supported by the U .S . Department of Energy, Nuclear Physics Division, under contract W-31-109-ENG-38.
of the beam on the sample. Filament current is monitored by a digital readout and is internally adjustable . Substrate heating is possible using a quartz lamp with the temperature measured via an iron-constantan thermocouple with digital display. Film thickness was determined with an Inficon XTM digital quartz crystal monitor with deposition rate indicator (Leybold Inficon, Inc., 6500 Fly Rd., East Syracuse, NY 13057) . A rotatable shutter protects the substrates when no evaporation is occurring. Fig. 1 is a photograph showing the evaporator setup. 3. Sample preparation It is known that in arc-melted molybdenum a coarse-grained structure is produced which allows an oxide film coating to form at the grain boundaries . This makes rolling difficult due to the cracks which develop [2]. In prior discussions with F. Karasek [3], regarding rolling of isotopically enriched molybdenum, it was mentioned that one should exercise caution in the use of this material if it is to be rolled without additional chemical reduction. Indeed, as experience has shown, the difference in the behavior of the isotopic material as compared to the natural material can be dramatic. This may be due in part to the method of preparation for the Calutron-enriched material or perhaps the age of the sample. The appearance of the powdered isotopic sample as obtained from Oak Ridge National Laboratory (ORNL), exhibits a bluish coloration . This form of molybdenum is the so-called " molybdenum-blue" or Mo20, - xH20 [4]. A recent sample of the natural metal (purity 99 .95%) obtained from Aldrich Chemical Co . (Milwaukee, WI 53233, USA) shows a more molybdenum-like, light gray appearance. Further chemical reduction of the 92 .98Mo
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B .V. (North-Holland)
IV . VAPOR DEPOSITED TARGETS
166
J. P Greene, G E. Thomas / Production
of Mo
targets by e-beam evaporation
very short lifetime when tested in a 1 pnA beam of 181 MeV of 36S ions at the ATLAS facility . The reason for this behavior is not understood . 4.2 . Carbon-backed molybdenum targets
Fig 1 . Photograph of the electron beam system setup for the evaporation of molybdenum self-supporting films. samples yielded percent weight losses of 7.3% and 4.5% respectively, accompanied also by a change in appearance from a bluish to a grayish color. 4. Target preparation 4.1 . Self-supporting molybdenum targets Self-supporting films of molybdenum have been prepared previously to thicknesses of 200 lag/cmz by rolling [5,6], and to 100 gg/cmz by electron bombardment evaporation [7]. Our initial attempts to produce molybdenum films by electron beam evaporation onto glass slides using 40-60 lag/cm z NaCI as a release agent were unsuccessful . They could not be floated off due to severe residual stresses. This effect has been discussed m the literature [8-11] . In a second attempt, BaC1 2 was tried as a parting agent (melting point 1235 K) and the substrate temperature was kept at 623 K [12] . The high melting point of BaC1 2 makes it better suited to withstand high substrate temperatures . This method proved 92,98Mo . to be successful for 100 wg/cmz targets of Also . 200 wg/cmz thick foils were produced using the same technique. These targets, however, exhibited a
In light of the short in-beam lifetimes encountered for the self-supporting foils, the production of isotopic targets on 15-20 ~Lg/cmz carbon backings was tried. When attempting to prepare these targets using natural material, it was found that these carbon foils, mounted over a 12 .7 mm diam aperture, would not support a deposition of more than 80 Irg/cm z of molybdenum . This might be due to the heat generated during the evaporation process. In instances such as this, requiring high vaporization temperatures, the area radiating the highest heat within the water-cooled hearth can be decreased by reducing the size of the beam spot or by the use of heat shielding. Using a smaller amount of material in the pocket helps, but for these thick films several evaporations would then be necessary. It was next decided to evaporate the molybdenum onto carbon-coated glass slides . Because commercially available carbon foils usually employ organic release agents which would tend to decompose under these elevated temperature conditions, carbon-coated slides using an inorganic salt as a parting agent were produced. The four-pocket capability of our electron beam source is well suited for these types of multiple evaporations. It was found, however, that concurrent evaporation of salt, carbon and molybdenum produced foils which were highly stressed, primarily due to the differing coefficients of thermal expansion of the carbon and molybdenum . In fact, even the uncoated carbon foils showed residual stress which could be removed by post-annealing in a furnace at - 433 K overnight [13] . Carbon foils without stress could also be prepared by preheating the substrate to 523 K for the deposition . Using the reduced material (section 3) and stress-relieved carbon foils of 20 lag/cmz as substrates . films of 92,98Mo 200 wg/cmz were prepared . A number of targets were produced by this method . In some cases, further annealing was necessary to relieve stress in the foils . These targets sometimes appeared brittle and would occasionally break for no apparent reason . Foils prepared using natural molybdenum did not exhibit such behavior. It is known that in the production of ductile molybdenum, the resulting degree of brittleness is highly dependent upon the precise conditions present during the reduction process [2]. In addition, the mechanical strength properties of molybdenum may be drastically altered by a small degree of recrystallization. Further research on these properties is indicated, particularly because these carbon-backed isotopic targets again exhibited short lifetimes when exposed to a 400 MeV 86 Kr beam with a current of 4 pnA.
J. P Greene, G E. Thomas / Production of Mo targets by e-beam evaporation In a later attempt, spot targets of 100 pg/cm2 92 MO on carbon backings were prepared by electron beam gun evaporation using the reduced material . The carbon backings of 40 hg/cm2 thickness were obtained commercially (Arizona Carbon Foil Co ., Inc., 2239 E.
167
Acknowledgement The authors would like to thank Dr . D.S . Gemmell, Dr . I. Ahmad, and Dr . K.E. Rehm for their continued
support of and encouragement in thus project.
Kleindale Rd ., Tuscon, AZ 85719) and floated in the
usual fashion onto standard ATLAS frames having a 12 .7 mm diam aperture . The spot size was 9.5 mm diameter . These targets performed well to the beam. An explanation as to why spot targets with
this
thickness of carbon backing withstand beam while selfsupporting foils fail rather quickly is under study. It appears that depositing the molybdenum in a circular area on the carbon is the crucial aspect of the target lifetime, the strength of the target being derived mainly
from the carbon backing. It is not known whether a comparable foil prepared by depositing the molybdenum
over the full surface of a 40 ~Lg/cm2 carbon-coated slide, which was then floated in the usual manner, would survive the beam as well as the spot targets.
5. Conclusion Self-supporting
foils
of
isotopically
enriched
molybdenum can be produced routinely down to 100 !rg/cm2 using substrate heating and BaC1 2 as a release agent . Spot targets of 92 Mo on relatively thick carbon backings were also prepared . These targets exhibited substantially longer
lifetimes when
exposed
to
the
heavy-ion beam used in the experiment . Further work
suggested by our experience will include exploring the limits of survivability of thin carbon foils used as backings in heavy-ion beams.
References [1] G.E . Thomas, J.P . Greene, P. Maier-Komor and R.H . Leonard, to be published. [2] W. Espe, Materials of High Vacuum Technology, (Pergamon, Oxford, 1966) pp . 82-99. [31 F.J . Karasek, Argonne National Laboratory, private communication . [4] R.C . Weast (ed.), Handbook of Chemistry and Physics (CRC press, Cleveland, OH, 1969) p. B-131. [5] F.J . Karasek, Nucl . Instr. and Meth . 102 (1972) 457. [6] H. Folger, W. Hartmann, J. Klemm and W. Thalheimer, Proc . 1983 Workshop of the Int. Nuclear Target Development Society, Argonne National Laboratory, Argonne, IL, ANL/PHY-84-2, p. 15 . F. Nickel, W. Hartmann and D. Marx, Nucl. Instr. and Meth . 167 (1979) 175. [8] K.L Chopra, Thin Film Phenomena (McGraw-Hill, New York, 1969) pp . 274-280. D. Ramsay, Proc . 1974 Ann. Conf. of the Int. Nuclear Target Development Society, Chalk River, Ontario, Canada, Report AECL-5503, p 151 . [10] J.C. Gursky, Proc . 5th Ann. Conf. of the INTDS, Los Alamos, NM, Los Alamos Scientific Laboratory Report LA-6850C (1976) p. 198. [11] E. Klokholm and B.S . Berry, J. Electrochem. Soc. 115 (1968) 823. [12] P. Maier-Komor, Technische Universität München, private communication . [13] J.0 . Stoner, Jr ., Arizona Carbon Foil Co, Inc., private communication .
IV . VAPOR DEPOSITED TARGETS