Experimental Equipment at XMaS
Index
Experimental Hutch
Beam Delivery System
Diffractometer
Detectors
Beamline Control
The Experimental Hutch
The physical layout of the equipment in the experimental hutch is shown below. The hutch is air-conditioned (temperature stability ± 1 deg.) and serviced with cold water, compressed air and nitrogen gas together with a pump exhaust system. Strict atmospheric control is possible in the experimental hutch, utilising a radiation monitored air/ozone extraction system, to allow experiments on actinide materials. To facilitate manipulation of heavy equipment, the hutch is equipped with a 1 ton overhead crane. There is a hinged chicane between the hutch and the main control cabin (CC3) to permit temporary installation of special electrical connections if required. A motorised closed circuit television camera is permanently installed, together with a second tripod mounted camera, both of which are displayed on monitors in the adjacent control cabin.
The Beam Delivery System
Harmonic Rejection Mirrors
A double mirror harmonic rejection is installed in the experimental hutch to overcome harmonic contamination from the monochromator. This is also exagerated by preferential absorption of the Si (111) reflection in the beryllium windows compared to the (333) and (444) reflections. The pyrex mirrors are Rhodium coated over half their width to cover the energy range 6 - 15 KeV, no coating is necessary at the lower energies and the angular range is 4 - 7 mrad.
An image of the mirror system is shown below.
Diamond Phase Plate and Flipper
The incident linear polarised radiation can be converted into circular polarisation (Pc) either from X-rays emitted above/below the electron orbit or using a quarter phase-plate. The latter technique is the only way to look at highly anisotropic magnetic materials for which the direction of the sample magnetisation cannot be reversed. The phase-plate assembly is situated upstream of the diffractometer. The diamond phase-plate crystal is mounted on a Huber goniometer and is driven to the quarter-wave plate conditions, for maximum Pc , with a theta and a chi circle. The former rotation stage, which moves the crystal off the Bragg condition, has an accuracy of 10-4 degrees (0.36 arcseconds). It is orthogonally mounted on the chi circle. The chi circle sets the optical axis of the diamond at 45° to the incident and plane polarization, with an accuracy of ±10 arcseconds. A counter balance is also mounted onto the arm of the chi circle. These two circles are positioned upon two translation stages giving horizontal and vertical adjustment within 10 µm. All the motors are controlled from the SPEC software, which is also used elsewhere on the beamline. The theta circle has an encoder installed.
A full description of the phase plate can be found be clicking here.
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| Phase-plate assembly |
In-vacuum phase-plate for low energy (~3 keV) optimisation |
Currently there are three diamond phase-plates at XMaS, which are described below.
| Diamond Thickness (mm) | Cut | Energy Range (KeV) |
| 0.8 | (110) | 5 - 10 |
| 0.3 | (110) | 3 - 5 |
| 0.1 |
(001) | 3 - 5 |
Specific spectroscopy experiments such as X-ray Magnetic Circular Dichroism (XMCD) require to reverse the degree of circular polarisation (or helicity) at a few tens of hertz in order to improve the signal-to-noise ratio. In that respect, a piezo driven oscillation stage, the so-called flipper, was designed by the National Physical Laboratory (UK). The device uses two pairs of multilayer piezoelectric stacks mounted on opposite sides of, and coupled to, an aluminium plate which is free to rotate via two weak links. They are driven in opposite directions to provide displacement of up to 350 arcseconds. This specification, considerably larger, allows each diamond to be used in a wider energy range. A second advantage is that this flipper is very compact compared to existing oscillation stages and allows easy mounting on the phase-plate Huber 410 circle. The diamond is attached to a small copper goniometer which screws into an aluminium adapter plate. The resulting block is then mounted onto the aluminium plate with weak links. Two manual linear translations allow positioning of the diamond at the centre of rotation of the phase-plate assembly.
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| Diamond crystal mounted on the phase-plate flipper |
Phase-plate assembly with the diamond flipper |
In normal operation, the flipper is driven by a 11.5 Hz trapezoidal wave-form generated by a TGA1240 arbitrary wave form generator. The signal is amplified by a PI LVPZT amplifier unit and then sent across the two piezo pairs.
Modular Sections
A flexible modular beam delivery system is installed in the experimental hutch allowing the continuation of an air-free path from the beamline final slit vessel to the diffractometer. It is comprised of two tubular assemblies vertically displaced, the lower tube encloses the unfocused monochromatic or white beams and the upper one the focused monochromatic beam. The main stand can accommodate up to eight modular vessels, four above and four below, mounted on X95-compatible carriages. Each vessel is 150 mm long and interconnects through DN100 vacuum flanges making the relative positioning of the vessels flexible. Modules are provided that contain: beam intensity monitor; motorised anti-scatter slits; pneumatically operated attenuator foils. A typical arrangement is illustrated below. The system is terminated using adjustable length telescopic tubes that allow continuation of vacuum close to the sample position and the vacuum path is closed with a kapton or mylar widowed flange.
Tube Slits
These slits were developed in order to define the incident or outgoing beam at a position very close to the sample. The design adopted satisfies three basic requirements, which are to define a footprint on a sample, to reduce background scatter and to enable isolation of small regions of interest on the sample. The assembly may equally well be mounted on the incident or diffracted beam path. In order to avoid possible collisions with the diffractometer all of the actuation mechanics are spatially separated from the sample position, with the four independent slit jaws positioned by a system of levers as shown below. The maximum overall opening aperture of the slit assembly is 4 mm x 4 mm and each jaw can be independently positioned to micron precision. The slit assembly may be mounted on X-95 compatible optical rail and is therefore interchangeable with other beam-line modular components. Motion is provided to the levers through in-house designed miniature linear vacuum feed-through with limit switches fitted on all translations.
These slits are now Huber products (Vakuum Tube Cross Slit Screen 3002.60M
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| 3D Model |
Finite slits |
The Diffractometer
The 11-axis Huber diffractometer was installed in 1997. The vertical (z) translation exists to permit alignment of the sample to the three possible beam positions: white unfocused (z = 0 mm); monochromatic unfocused (z = -20 mm); monochromatic focused (z = 225 mm). Since the mirror deflects the monochromatic beam upward through 9 mrad, the facility to tilt (t ±1 deg) is provided in order to maintain perpendicularity between the focused beam and all horizontal circle axes. A horizontal translation (x) is present to centre the diffractometer in the beam. With the use of the SPEC software, orientation matrices may be defined in either vertical four-circle (phi, chi, omega, 2-theta) or horizontal four-circle (phi, chi, omega (horizontal), 2-theta (horizontal)) scattering geometries. In either of these geometries one may still drive/scan the two out-of scattering-plane circles.
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The physical layout of the diffractometer is shown above together with a picture of the device. Two multi-position plates at the front and rear of the base can be used to mount user equipment. All metallic parts around the sample circles are of non-magnetic material. The Huber 512 Eulerian cradle carries a 1003 sample goniometer head on the phi circle. There is a 10 mm diameter hole in the head and the distance from the base of the hole to the centre of rotation is 43.5 mm. Although not shown on the drawing, the detector arm is equipped with an X-95 compatible mounting rail able to carry detectors and any of the modules, as described for the beam delivery system. This permits the minimisation of air in the beam path.
Polarisation Analyser
The three-axis XMaS polarisation analyser, shown below, has been designed to facilitate the study of changes in the polarisation of the x-ray beam after diffraction from the sample. The in-vacuum design allows the device to be used at low energies (~3 keV) which is particularly useful, for example, in experiments performed at uranium M-edges. The theta axis allows alignment of the analyser crystal to a diffracting condition, with theta Bragg at, or close to, 45°. The second rotation axis allows rotation of the diffracting plane of the analyser crystal about the scattered beam. Any component of the incident polarisation lying in the diffracting plane will go to zero on charge scattering for theta Bragg = 45°. The third linear axis allows translation of the detector in a 2-theta geometry in order to track the diffracted beam. Conventionally, for vertical scattering experiments, the incident polarisation is referred to as sigma polarised and any component orthogonal to it (i.e. vertically polarised) as pi polarised. Thus, by positioning the rotation about the beam such that the diffracting plane of the analyser crystal is vertical, one is sensitive only to the sigma polarised component and conversely a horizontal analyser crystal diffraction plane measures the pi component.
Conventional Analyser
This is a conventional Huber analyser (415, two circle goniometer) which carries a Huber goniometer head (1001, with two translations and two arcs ). Both Si (111) and Ge (111) crystals are available for use with this analyser.
Cryostat mounts
Both manual and motorised mounting system for the APD cryostat fit into the Huber 410 phi circle. The manual Huber 5012 mount utilises dovetail linear slides for the X and Y translations, driven by manually adjusted screw threads. The Z translation consists of a sleeve and cylinder linear guide adjusted by a large threaded ring. These slides and the way they are adjusted makes precise and reproducible movements very difficult to obtain. Since these adjustments are usually not motorised, many hours of useful beamtime can be lost trying to find a "sweet spot" on a crystalline sample. Also, problems are encountered with this type of mount whilst tightening the screws to lock these translations in place. During cooling the cryostat contracts, sometimes by more than 0.5 mm and the sample alignment thus becomes very difficult. However, once the cryostat is fixed in place it is very stable.
The motoried Huber 5012.12M mount has been designed to facilitate and thereby save time during sample align. It fixes to the diffractometer in exactly the same manner as the manual version and has the same translation limits. The three linear translations (X, Y and Z) are mounted onto high precision linear bearings. The independent X and Y movements are obtained by mounting a cam onto the shaft of a stepper motor with a 100:1 harmonic drive gearbox to minimise backlash. This cam is mounted within two linear bearings, thus allowing movement perpendicular to the axis of the motor (X and Y) with a minimum of backlash. The Z translation is guided by high precision linear bearings and driven by a stepper motor with a 100:1 harmonic drive gearbox via a large toothed ring cam. The cryostat and Z stage are mounted upon cam followers and as the large toothed cam turns this provides the Z translation. This stage is held in place on the cam by eight strong springs.
Technical details can be found here.
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3D model of the motorised XYZ mount (to the left). The motorised XYZ mount allowing precise alignment of a ARS (APD) cryostat (to the right).
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Detectors
The following detectors are available for use on the beamline:
Bicron NaI Scintillator
This X-ray detector consists of a cleaved NaI crystal with a 125 micron thick beryllium window mounted into a housing behind a removable brass collimator. A low noise photomultiplier tube, optically coupled within the housing connects a low noise voltage divider with a FET preamplifier. The output signal is fed into an ORTEC amplifier, onto a single channel analyser (SCA) and then into the VTC6 VME counter card, which in turn is read by SPEC.
It is important to note that the bias voltage for the Bicron detectors is positive and for the solid state detector, negative.
Cyberstar Scintillator
The Cyberstar detector uses a NaI(Tl) scintillator crystal providing a working energy range up to 50 keV. The detector aperture is 32 mm in diameter with a Beryllium window of 200 micron thickness. After the scintillator there is a photo-multiplier tube with the photo-cathode adapted to the NaI(Tl) scintillation wavelength. The photo-multiplier has a typical gain of 106 providing an output pulse with rise time of 2.8 ns. Directly attached to the dynode chain is a fast preamplifier.
In the back part of the detector housing are a Lemo ERA size zero connector for the high voltage and an Anphenol 17DMW connector for the preamplifier power and signal output.
The control electronics Cyberstar Model CS93PPU. This electronics integrates the high voltage power supply, the shaping amplifier and the single channel analyser. In the rear panel is a DB9 female connector for the preamplifier power, a BNC for the signal input and a high voltage connector for the photo-multiplier high voltage output. The unit is equipped with a serial line communication port configurable between RS232 and RS485. The DB9 male serial line connector is located in the rear panel.
The photo-multiplier voltage is adjustable between 250 V and 1250 V by means of a 10-turn potentiometer in the front panel. A 3 1/2 digit led display provides the reading of this voltage.
The amplifier gain can be continuously adjusted between 0 and 10 by a 10-turn potentiometer located in the front panel. The shaping constant can be changed by a 4-position rotary switch.
The low level discriminator threshold and the upper level threshold are adjustable between 50 mV and 10 V by two 10-turn potentiometer in the front panel. A 4-positions rotary switch is used to change the operation mode of the window discriminator:
| n (normal) | Upper and lower level are separately adjustable between 0 and 10V. |
| i (integral) | Upper level disabled |
| a (asymmetric) | Upper level adjust between 0V and 1V the window above the lower level. |
| s (symmetric) | Upper level adjust symmetrically between 0.5 V and -0.5 V above and below the lower level. |
The amplified signal is present at the signal out BNC connector in the front panel. The discriminator output is a TTL or negative current fast Nim logic level jumped selected in the printed circuit board. The output signal is accessible at the window out BNC output in the front panel.
The control unit can be remote controlled through the communication port. To use this feature the local-remote switch in the front panel must be set to remote (red light on). The SPEC macro to control the cyberstar is called "CYBERMENU"
For an energy of ~ 8.5 KeV the following settings have been used:
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Gain: 30
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High Voltage: 1000 V
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Peak Time: 300 nano seconds
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Levels: 0.5 - 10 V
Vortex Si Drift Diode
Vortex® multi-cathode X-ray detectors feature the largest active area single element detector (~50 mm2, 8 mm diameter) available of its kind. Vortex® detectors are produced from high purity silicon using state-of-the-art CMOS production technology. They feature excellent energy resolution (<136 eV FWHM at Mn Kα is typical) and a high count rate capability (input rate >1 Mcps). At a very short peaking time of 0.25 µs, an output count rate of 600 kcps is achieved. A unique feature of these detectors is their ability to process high count rates with virtually zero loss in energy resolution and no peak shift with count rate. The detector at XMaS features a 25 µm thick Be window mounted onto a KF-40 vacuum flange, reducing air absorption.
Avalanche Photodiodes
Avalanche Photodiode detectors (APD) are available and offer a very high dynamic range and linearity. They consist of PerkinElmer silicon avalanche photodiode chips with an integrated preamplifier. Two types of APD are available on the beamline: one with a chip with a 10 mm x 10 mm active area shielded by a kapton window and another one with a 5 mm x 5 mm active area protected by a 80 µm thick Be window which is mounted onto a KF-16 vacuum flange.
Each APD is controlled by an ACE unit through SPEC. This module includes a user interface in local and remote control mode. SPEC communicates to the module through a RS232 serial line port.
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| APD with KF-16 vacuum flange (5 mm x mm PerkinElmer chip) |
Standard APD (10 mm x mm PerkinElmer chip) | ACE unit |
MAR165 CCD Camera
The MAR165 CCD camera is a 2D detector with a round, 165 mm diameter active area. It uses one 4k x 4k chip, permanently bonded to a single fiber-optic taper. The camera reads the chip from four channels (quadrants) simultaneously, resulting in a 2048 x 2048 pixel image in 2.5 seconds. The pixel size is 80 µm x 80 µm.
The MAR camera is controlled from SPEC, by specific macros, through a Linux device server running on xmas6 holding the camera controller card.
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| Dim. (WxHxD): 220 x 220 x 340 Weight: ~20 kg |
The MAR165 Technical Sheet is available here.
PILATUS 300K-W
Only available on request !!! The Dectris Pilatus 300K-W is based on the newly developed CMOS hybrid-pixel technology and operates in single-photon counting mode. It reaches a framing rate up to 200 Hz with a 2.7 ms readout time in fast mode (100Hz and 3.6 ms in standard operation) and has a pixel size of 172 µm x 172 µm. The detector features 1475 x 195 pixels (3x1 chips).
The Pilatus 300K-W consists of a detector head connected to a Linux acquisition workstation by a fiberoptic link. The detector head includes the detector module, the interface board with connection to the acquisition workstation, and power supplies. The acquisition workstation is based on the "LIMA" software library developed by ESRF for 2D detectors control and 2D data acquisition.
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Pilatus 300K-W Technical Sheet available here.
MAXIPIX
Only available on request !!! Maxipix is a fast readout, photon-counting pixel detector system developed by ESRF and based on the Medipix2 readout chip developed by CERN and the Medipix2 collaboration. The system achieves up to 1.4 kHz frame rate with 0.29 ms readout dead time and has a pixel size of 55 µm x 55 µm. The available detector features 1280 x 256 pixels (5x1 chips).
MAXIPIX consists of a detector head connected to a Linux acquisition workstation by a fiberoptic link. The detector head includes the detector module, the interface board with connection to the acquisition workstation, and power supplies. The interface board can drive detector modules implementing up to five Medipix2 or Timepix chips with simultaneous readout of each chip. The acquisition workstation is based on the "LIMA" software library developed by ESRF for 2D detectors control and 2D data acquisition.
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| Dim. (WxHxD): 145 x 140 x 222 Weight: ~4.7 Kg |
Maxipix Technical Sheet available here.
Canberra Ge Solid State Detector and MCA
Germanium detectors are semiconductor diodes having a P-I-N structure in which the Intrinsic (I) region is sensitive to ionizing radiation, particularly X-rays and gamma rays. Under reverse bias, an electric field extends across the intrinsic or depleted region. When photons interact with the material within the depleted volume of a detector, charge carriers (holes and electrons) are produced and are swept by the electric field to the P and N electrodes. This charge, which is in proportion to the energy deposited in the detector by the incoming photon, is converted into a voltage pulse by an integral charge sensitive preamplifier.
Because germanium has a relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers (thus reverse leakage current) to an acceptable level. Otherwise, leakage current induced noise destroys the energy resolution of the detector. Liquid nitrogen, which has a temperature of 77 K is the common cooling medium for such detectors. The detector is mounted in a vacuum chamber which is attached to or inserted into an LN2 dewar or an electrically powered peltier cooler. The sensitive detector surfaces are thus protected from moisture and condensable contaminants.
The solid state detectors on XMaS are connected as shown below:
Beamline Control
User Interface
The XMaS beam-line is controlled through a Linux workstation (xmas2 running Red Hat). The user interface is achieved with use of SPEC software that includes the standard macro FOURC. In order to run FOURC it is necessary to open an xterm window and then simply type fourc. It is then possible to drive slits and optical elements as well as the diffractometer by using standard SPEC commands. FOURC also allows the definition of an orientation matrix and subsequent reciprocal space scans.
Software Interface
The beam-line is controlled through three VME crates (d281, d282 and d283) situated in CC1, CC3 and EH1. The crate d281 in CC1 controls all optical elements, including slit-sets s1 to s5. The crate d282 in CC3 controls the diffractometer and all other experimental equipment. The crates employ a diskless VME system and when booted loads all OS9 device servers into RAM through the Ethernet link from the hard disc of the workstation. Device servers are controlled either through SPEC or through other ESRF clients, via the Ethernet.
Hardware Interface
All hardware is driven through VME cards such as VCT6 for detectors, CC133 for encoders, VPAP for stepper motors, etc. These cards are hard-wired either to the specific hardware item, such as an encoder for the CC133, or to interface electronics such as DPAP current pulse generators for stepper motors or NIM electronics for the detectors.
Vacuum and Personal Safety systems
Both the vacuum control and personal safety (PSS) systems are hard wired, stand-alone systems controlled by PLC's in the CC1 racks. The vacuum control system automatically closes valves in the event of high pressure, from a leak for example, in a given vacuum section in order to isolate that section. The PSS system enables or disables the opening of the front-end shutter, depending on the status of optics hutch PSS interlocks while the station-shutter is similarly regulated by the status of experimental hutch interlocks. The vacuum control system similarly influences the state of the front-end shutter – i.e. if any valve is closed the shutter will not open.
The workstation (xmas2) also runs several GUI applications that communicate with the vacuum and PSS PLC's. The station-shutter can thus be opened and closed by clicking with the mouse the appropriate icon. Valves can be opened or closed and vacuum gauges read in the same manner.
email:pthompso@esrf.fr
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