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UHV double crystal monochromator design
Nuclear Instruments and Methods in Physics Research A266 (1988) 467-470 North-Holland, Amsterdam
UHV DOUBLE CRYSTAL MONOCHROMATOR
DESIGN
467
*
T.I. M O R R I S O N Physics Department, Illinois Institute of Technology, l i T Center, Chicago, IL 60616, USA
N e i l C. L I E N Baker Manufacturing Company, 133 Enterprise Street, Evansville, WI 53636, USA
S.M. H E A L D , L a r r y S. F A R E R I A
a n d J. S C R O F A N I
Brookhaven National Laboratory, Upton, N Y 11973, USA
An ultrahigh vacuum, double crystal, fixed-exit X-ray monochromator has been designed and built for soft X-ray (0.8-8 keV) spectroscopic studies. This unit, currently installed at the NSLS on beamline X-11B, uses two crystal carousels, each of which can hold four different crystals to allow crystal changes without opening the chamber. The incident crystal carousel, which only rotates, is water cooled to remove the incident beam heat load. The second carousel, which rotates and translates parallel to the beam axis, can be heated to equilibrate crystal temperatures. Design principles and results of preliminary tests will be presented.
The advent of synchrotron radiation spectroscopies (EXAFS and XANES) as structural tools in condensed matter physics, chemistry, biology, and geology brought forth a large n u m b e r of innovative designs for X-ray monochromators. In the early stages, most of these devices were based on a double crystal design that operated best in the hard X-ray regime, from about 5 to 22 keV. There were probably two reasons for this: first, there was and is a lot of interesting physics, chemistry, biology, and geology that can be studied in this spectral region, and secondly, the instrumentation was fairly straightforward. In general, most of these devices could operate without the complications imposed by ultrahigh vacuum techniques for reducing intensity losses due to air or X-ray windows. It has also long been realized, though, that there is just as much interesting physics, chemistry, biology, or geology in the soft X-ray regime, from about 1 to 5 keV. This energy range covers the absorption edges of aluminum, silicon, phosphorus, sulfur, and titanium, all of which are important in a large n u m b e r of advanced materials, heterogeneous catalysis, biophysics and biochemistry, and geochemistry. The situation arose that, while pushing previous designs to their limits (either " u p " for UV based designs or " d o w n " for hard X-ray * This work was performed at Beam Line X-11 at the NSLS and is (partially) supported by the Division of Materials Science under Contract No. DE-AS05-80-ER10742 and (partially) under U.S. Department of Energy BES-Materials Science Contract #W-31-109-ENG-38.
based designs) made studies at these edges possible, it became more and more desirable to have beamlines and optical devices, such as monochromators, optimised for use in this spectral region. Consequently, beamlines specifically dedicated to this spectral region have been designed and implemented [1]. We will present some of the features of just such a facility. Beamline X-11B at the National Synchrotron Light Source has been designed specifically for operation in the 0.8-8.0 keV spectral region; paired with the high energy line X-11A described elsewhere [2], we have an EXAFS facility that will span nearly the entire spectral range of interest for synchrotron radiation absorption studies. The X-11B line has a path length of about 20 m, and an upstream Be window to separate the line from the storage ring would cause excessive intensity losses in the soft X-ray spectrum. Therefore, the fine must share c o m m o n vacuum with the storage ring itself, and consequently the entire line must be ultrahigh vacuum. The line is centered about a U H V compatible monochromator designed specifically for this purpose. The monochromator, known by the trade-name " D C M " (Baker Mfg. Co.), was designed and built to meet a n u m b e r of general and specific requirements. First and most obviously, it must work under ultrahigh vacuum conditions (10-9 Torr range). Secondly, since a single pair of crystals will not span the entire energy range, provisions must be made for a rapid and precise crystal change in the course of an experiment. Thirdly, since there can be no intervening windows between the Ill(e). CRYSTAL MONOCHROMATORS
468
T.L Morrison et al. / U H V double crystal monochrornator design
t ~ °
oC1
[
Fig. 1. External housing showing base, chamber, pump, and slides. Beam comes from left of the side view (right hand figure) at a nominal height of 140 cm.
source and the crystal optics that would eliminate power deposition due to low energy light, the monochromator must be able to handle the heat loads of the incident radiation on the first crystal. Finally, of course the monochromator must provide a clean, fixed-height beam of good energy resolution ( - 10 4) and have the capability of detuning to eliminate or at least minimize higher harmonics. All this must be accomplished within the safety [3,4] and vacuum [5] guidelines provided by the NSLS. Fig. 1 is an external view drawing of the D C M , which can fulfill all these requirements. Shown are the view looking downstream and side view. The vacuum chamber sits on a granite bed for mechanical stability and vibration isolation, and is equipped with top and side access flanges. It is pumped by a 220 1/s ion pump mounted on the top access flange, and there are ports for low and high vacuum pressure gauges, roughing, and "up-to-atmosphere". In addition there are internally mounted viewports for use with a fiber optic scope, allowing inspection of interior components without breaking vacuum. Provision is made for bremsstrahlung
shielding about the beam exit port. The entire unit is bakeable to 105 ° C. The internal mechanisms to provide crystal positioning are based on the use of bandwheels (fig. 2). The angular range necessary to cover the desired energy range spans from 8 ° to 70 o. In this arrangement, linear motions brought into the chamber by means of rods pushing and pulling long paired bellows (to balance vacuum forces) are converted to circular motions by the band that drives a wheel. This arrangement gives an angular positioning accuracy of at least + 5 arc sec. The alloys from which the bands are made are specifically chosen to minimize any thermal expansion in the temperature range in which they will be used. The initial linear motions are provided by stepping motors driving lead screws which in turn push rods mounted on air bearings and MicroSlides TM. Thus, the motions brought into the mechanism are precise, essentially frictionless, and subject to minimal unbalanced forces. Two carousels have been installed, each of which can hold four different crystals. This allows changes of crystals to be made without opening the vacuum cham-
T.L Morrison et a L / UHV double crystal monochromator design
\
",
\
~
!
~
£ "\
i//
1
9
. . . .
469
/ j
~
Fig. 2. Internal mechanisms showing bandwheels, drive bars, and carousels. Upper left; beam comes from right hand side. Upper right: Geneva mechanism antibacklash device. Lower left: beam comes from left hand side. Lower right: view looking upstream.
ber; this point is especially important, since breaking vacuum can cause long downtimes due to the need for pumpdown and bakeout. Both carousels are driven by Geneva-type mechanisms and are positioned by springloads against stops, assuring reproducibility of position; the carousels allow the crystals to be registered to better that 0.1 ° on the axis perpendicular to the dispersive axis. The axes of rotation of the two carousels are parallel to the axes of rotation of the crystals, but are not, of course, coaxial. The incident crystal carousel, which only rotates, is positioned such that the axis of rotation is coaxial with the bandwheel driving it. This carousel is water cooled, as indicated in fig. 3. In this arrangement, cooling water is brought in through the shaft of the carousel and circulates between the copper carousel body and the internal " c a n " before returning through a separate line, also in the shaft of the carousel. The cooling water is recirculated through a cooling unit
Fig. 3. Section through incident beam carousel showing carousel body ( \ \ \ \ ) , internal "can" ( / / / / ) , water cooling tubes, and gimbal mount. III(e). CRYSTAL MONOCHROMATORS
470
T.L Morrison et al. / UHVdouble crystal monochromator design
@ Fig. 4. UHV compatible piezoelectric stack showing contacts, piezo material, and flexural hinge.
1987, underwent initial tests at BNL just before the Phase II expansion shutdown. While there was not sufficient time to run all the desired "shakedown" tests, we were able to ascertain the vacuum integrity of the monochromator and the proper mechanical functioning of the unit under synchrotron running conditions. A preliminary single crystal scan indicated that all internal components were working correctly and within specifications; unfortunately, complete tests will have to wait until the ring is back up. This line, and others like it, will play an increasingly valuable role in the application of E X A F S spectroscopy to problems in advanced materials, catalysis, biology, and geology. The new generation of optical components, such as the monochromator described here, represents a significant step in the sophistication and consequent versatility of synchrotron instrumentation. This sophistication will become more and more important as new, brighter synchrotron facilities are brought on line.
Acknowledgements to minimize thermal fluctuations and maximize control over variables in the water such as p H and conductivity. The entire water cooling system is designed to be in accord with NSLS vacuum guidelines [5]. The second crystal carousel both rotates and translates in order to give a fixed position output beam. Since the second crystal only sees a monochromatic beam there is no need for water cooling. However, in order to match d-spacings between the two crystals used for a given energy range, provisions have been made for electrically heating the second carousel. The temperature is controlled by a feedback mechanism to minimize temperature differences. In addition, it is possible to detune the second crystal to minimize harmonic content by means of an U H V compatible piezo drive, shown in fig. 4. This piezoelectric stack can provide up to 40 arcsec of adjustment or be used to dither the second crystal up to 2 arc sec at greater than 30 Hz. The unit, which was aligned and installed in February
The authors wish to thank Prof. Dale Sayers for his continued support and contributions and Mr. John Stolz for assistance in project engineering. We also wish to thank the entire N S L S staff, with special thanks to Dr. Roger Klaffky.
References [1] A.A. MacDowell, J.B. West, G.N. Greaves and G. van der Laan, to be submitted to Rev. Sci. Instr. [2] Steve M. Heald, Michael A. Pick, John M. Tranquada, Dale E. Sayers, Joseph I. Budnick, Edward A. Stern, Joe Wong, Galen Stucky, Art Chester, Geoff Woolery and Tim Morrison, Nucl. Instr. and Meth. A246 (1986) 120. [3] NSLS Bulletin 82-7. [4] NSLS Safety Analysis Report BNL 51584. [5] Requirements and Guidelines for NSLS Beamline Vacuum Systems, BNL 28073.
UHV DOUBLE CRYSTAL MONOCHROMATOR
DESIGN
467
*
T.I. M O R R I S O N Physics Department, Illinois Institute of Technology, l i T Center, Chicago, IL 60616, USA
N e i l C. L I E N Baker Manufacturing Company, 133 Enterprise Street, Evansville, WI 53636, USA
S.M. H E A L D , L a r r y S. F A R E R I A
a n d J. S C R O F A N I
Brookhaven National Laboratory, Upton, N Y 11973, USA
An ultrahigh vacuum, double crystal, fixed-exit X-ray monochromator has been designed and built for soft X-ray (0.8-8 keV) spectroscopic studies. This unit, currently installed at the NSLS on beamline X-11B, uses two crystal carousels, each of which can hold four different crystals to allow crystal changes without opening the chamber. The incident crystal carousel, which only rotates, is water cooled to remove the incident beam heat load. The second carousel, which rotates and translates parallel to the beam axis, can be heated to equilibrate crystal temperatures. Design principles and results of preliminary tests will be presented.
The advent of synchrotron radiation spectroscopies (EXAFS and XANES) as structural tools in condensed matter physics, chemistry, biology, and geology brought forth a large n u m b e r of innovative designs for X-ray monochromators. In the early stages, most of these devices were based on a double crystal design that operated best in the hard X-ray regime, from about 5 to 22 keV. There were probably two reasons for this: first, there was and is a lot of interesting physics, chemistry, biology, and geology that can be studied in this spectral region, and secondly, the instrumentation was fairly straightforward. In general, most of these devices could operate without the complications imposed by ultrahigh vacuum techniques for reducing intensity losses due to air or X-ray windows. It has also long been realized, though, that there is just as much interesting physics, chemistry, biology, or geology in the soft X-ray regime, from about 1 to 5 keV. This energy range covers the absorption edges of aluminum, silicon, phosphorus, sulfur, and titanium, all of which are important in a large n u m b e r of advanced materials, heterogeneous catalysis, biophysics and biochemistry, and geochemistry. The situation arose that, while pushing previous designs to their limits (either " u p " for UV based designs or " d o w n " for hard X-ray * This work was performed at Beam Line X-11 at the NSLS and is (partially) supported by the Division of Materials Science under Contract No. DE-AS05-80-ER10742 and (partially) under U.S. Department of Energy BES-Materials Science Contract #W-31-109-ENG-38.
based designs) made studies at these edges possible, it became more and more desirable to have beamlines and optical devices, such as monochromators, optimised for use in this spectral region. Consequently, beamlines specifically dedicated to this spectral region have been designed and implemented [1]. We will present some of the features of just such a facility. Beamline X-11B at the National Synchrotron Light Source has been designed specifically for operation in the 0.8-8.0 keV spectral region; paired with the high energy line X-11A described elsewhere [2], we have an EXAFS facility that will span nearly the entire spectral range of interest for synchrotron radiation absorption studies. The X-11B line has a path length of about 20 m, and an upstream Be window to separate the line from the storage ring would cause excessive intensity losses in the soft X-ray spectrum. Therefore, the fine must share c o m m o n vacuum with the storage ring itself, and consequently the entire line must be ultrahigh vacuum. The line is centered about a U H V compatible monochromator designed specifically for this purpose. The monochromator, known by the trade-name " D C M " (Baker Mfg. Co.), was designed and built to meet a n u m b e r of general and specific requirements. First and most obviously, it must work under ultrahigh vacuum conditions (10-9 Torr range). Secondly, since a single pair of crystals will not span the entire energy range, provisions must be made for a rapid and precise crystal change in the course of an experiment. Thirdly, since there can be no intervening windows between the Ill(e). CRYSTAL MONOCHROMATORS
468
T.L Morrison et al. / U H V double crystal monochrornator design
t ~ °
oC1
[
Fig. 1. External housing showing base, chamber, pump, and slides. Beam comes from left of the side view (right hand figure) at a nominal height of 140 cm.
source and the crystal optics that would eliminate power deposition due to low energy light, the monochromator must be able to handle the heat loads of the incident radiation on the first crystal. Finally, of course the monochromator must provide a clean, fixed-height beam of good energy resolution ( - 10 4) and have the capability of detuning to eliminate or at least minimize higher harmonics. All this must be accomplished within the safety [3,4] and vacuum [5] guidelines provided by the NSLS. Fig. 1 is an external view drawing of the D C M , which can fulfill all these requirements. Shown are the view looking downstream and side view. The vacuum chamber sits on a granite bed for mechanical stability and vibration isolation, and is equipped with top and side access flanges. It is pumped by a 220 1/s ion pump mounted on the top access flange, and there are ports for low and high vacuum pressure gauges, roughing, and "up-to-atmosphere". In addition there are internally mounted viewports for use with a fiber optic scope, allowing inspection of interior components without breaking vacuum. Provision is made for bremsstrahlung
shielding about the beam exit port. The entire unit is bakeable to 105 ° C. The internal mechanisms to provide crystal positioning are based on the use of bandwheels (fig. 2). The angular range necessary to cover the desired energy range spans from 8 ° to 70 o. In this arrangement, linear motions brought into the chamber by means of rods pushing and pulling long paired bellows (to balance vacuum forces) are converted to circular motions by the band that drives a wheel. This arrangement gives an angular positioning accuracy of at least + 5 arc sec. The alloys from which the bands are made are specifically chosen to minimize any thermal expansion in the temperature range in which they will be used. The initial linear motions are provided by stepping motors driving lead screws which in turn push rods mounted on air bearings and MicroSlides TM. Thus, the motions brought into the mechanism are precise, essentially frictionless, and subject to minimal unbalanced forces. Two carousels have been installed, each of which can hold four different crystals. This allows changes of crystals to be made without opening the vacuum cham-
T.L Morrison et a L / UHV double crystal monochromator design
\
",
\
~
!
~
£ "\
i//
1
9
. . . .
469
/ j
~
Fig. 2. Internal mechanisms showing bandwheels, drive bars, and carousels. Upper left; beam comes from right hand side. Upper right: Geneva mechanism antibacklash device. Lower left: beam comes from left hand side. Lower right: view looking upstream.
ber; this point is especially important, since breaking vacuum can cause long downtimes due to the need for pumpdown and bakeout. Both carousels are driven by Geneva-type mechanisms and are positioned by springloads against stops, assuring reproducibility of position; the carousels allow the crystals to be registered to better that 0.1 ° on the axis perpendicular to the dispersive axis. The axes of rotation of the two carousels are parallel to the axes of rotation of the crystals, but are not, of course, coaxial. The incident crystal carousel, which only rotates, is positioned such that the axis of rotation is coaxial with the bandwheel driving it. This carousel is water cooled, as indicated in fig. 3. In this arrangement, cooling water is brought in through the shaft of the carousel and circulates between the copper carousel body and the internal " c a n " before returning through a separate line, also in the shaft of the carousel. The cooling water is recirculated through a cooling unit
Fig. 3. Section through incident beam carousel showing carousel body ( \ \ \ \ ) , internal "can" ( / / / / ) , water cooling tubes, and gimbal mount. III(e). CRYSTAL MONOCHROMATORS
470
T.L Morrison et al. / UHVdouble crystal monochromator design
@ Fig. 4. UHV compatible piezoelectric stack showing contacts, piezo material, and flexural hinge.
1987, underwent initial tests at BNL just before the Phase II expansion shutdown. While there was not sufficient time to run all the desired "shakedown" tests, we were able to ascertain the vacuum integrity of the monochromator and the proper mechanical functioning of the unit under synchrotron running conditions. A preliminary single crystal scan indicated that all internal components were working correctly and within specifications; unfortunately, complete tests will have to wait until the ring is back up. This line, and others like it, will play an increasingly valuable role in the application of E X A F S spectroscopy to problems in advanced materials, catalysis, biology, and geology. The new generation of optical components, such as the monochromator described here, represents a significant step in the sophistication and consequent versatility of synchrotron instrumentation. This sophistication will become more and more important as new, brighter synchrotron facilities are brought on line.
Acknowledgements to minimize thermal fluctuations and maximize control over variables in the water such as p H and conductivity. The entire water cooling system is designed to be in accord with NSLS vacuum guidelines [5]. The second crystal carousel both rotates and translates in order to give a fixed position output beam. Since the second crystal only sees a monochromatic beam there is no need for water cooling. However, in order to match d-spacings between the two crystals used for a given energy range, provisions have been made for electrically heating the second carousel. The temperature is controlled by a feedback mechanism to minimize temperature differences. In addition, it is possible to detune the second crystal to minimize harmonic content by means of an U H V compatible piezo drive, shown in fig. 4. This piezoelectric stack can provide up to 40 arcsec of adjustment or be used to dither the second crystal up to 2 arc sec at greater than 30 Hz. The unit, which was aligned and installed in February
The authors wish to thank Prof. Dale Sayers for his continued support and contributions and Mr. John Stolz for assistance in project engineering. We also wish to thank the entire N S L S staff, with special thanks to Dr. Roger Klaffky.
References [1] A.A. MacDowell, J.B. West, G.N. Greaves and G. van der Laan, to be submitted to Rev. Sci. Instr. [2] Steve M. Heald, Michael A. Pick, John M. Tranquada, Dale E. Sayers, Joseph I. Budnick, Edward A. Stern, Joe Wong, Galen Stucky, Art Chester, Geoff Woolery and Tim Morrison, Nucl. Instr. and Meth. A246 (1986) 120. [3] NSLS Bulletin 82-7. [4] NSLS Safety Analysis Report BNL 51584. [5] Requirements and Guidelines for NSLS Beamline Vacuum Systems, BNL 28073.