BM29 Overview

last modified 17-11-2008 17:17

Introduction

BM29 is the general purpose X-ray absorption spectroscopy beamline at the ESRF. It aims to meet the needs of the member countries in the area of conventional X-ray absorption spectroscopy. Principally, BM29 is designed to perform experiments which have requirements that fall beyond the capabilities of most second generation synchrotron sources, but which do not require the specialist characteristics of the other ESRF X-ray absorption instruments. Those instruments are optimised for X-ray absorption measurements on ultra-dilute systems (ID26), time resolved measurements using a dispersive set-up (ID24) and experiments that have special requirements concerning the polarisation of the photons (ID8, ID12). The strengths to which BM29 operates arise from the intrinsic properties of the ESRF synchrotron, coupled with a bending magnet source and the high quality performance of the beamline's principle optical element - its monochromator. These strengths can be summarised as

a very large operational energy range with reasonable X-ray flux: 4.5 keV to 74 keV.
high energy resolution: typically a factor 3 to 5 better than the intrinsic spectral broadening at any K or L absorption edge.
high spectral signal to noise ratio: in the region of 2 * 10⁴ for well prepared samples.
high beam stability: compatible with the demands of extreme sample environments such as pressure cells, where beam dimensions of 1.0 mm * 0.2 mm are required.

In addition to the above listed fundamental characteristics, an operational philosophy has been adopted at the beamline that promotes the implementation of a high degree of system automation to simplify routine experimental procedures. As a consequence of applying this philosophy to the control of sample environments, it is now a routine matter to monitor in real time, environmental parameters such as temperature and pressure. This has led to the development of a number of novel scanning methodologies that have further developed the practical utility of the X-ray absorption technique as a scientific tool.

The following overwiew we will try to give you an outline of the principle components of this beamline and their relevant characteristics. The parts which are of interest of your experiment will be expained in detail in the later section. Since end 2001, we modified twice the control software in order to clarify the beamline control.

The whole experiment is controller via the SPEC software. There is a single application called EXPERIMENT to command the data acquisition. Two other application used for special purposes.

Information relevant to BM29's SPEC applications are given in green (or just black if it's inside the text and extra color does not really help) as Spec_application > motor_name. In the case of the standard application EXPERIMENT, we will in general omit the name.

Optics

A sketch of the optical hutch is shown in figure [1.1]. The whole optics hutch is equipped with UHV components, with the exception of the monochromator, this contains some UHV incompatible parts. To protect the machine front end from vacuum failures and to facilitate operation, the beamline is equipped with a thick Be window following the front end shutter. Another Be window separates the optics from the experimental setup, limiting the low energy operation to approximately 4.5 keV. The whole optics section is pumped via ion pumps in order to avoid any vibrations. After interventions, the monochromator can be evacuated via a turbo pump, which is switched off when experiments are in progress.

Fig. 1.1: Components of the BM29 optics hutch (drawing with 4 mirror device and without monochromator cooling) Typical pressures during operation are ~ 2 * 10^-8 mbar and ~ 1.5 * 10^-7 mbar at the safety shutter and in the monochromator, respectively (with cooling on). The different elements can be isolated via different vacuum valves. These must of course be open for operation, (i.e. to be able to open the front end). Their state can be checked and eventually changed via the vacuum application xvacuum and are interlocked via a control system to avoid damadge of the storage ring vacuum or beamline components in case of a vacuum failure. xvacuum allows as well to monitor the beamline vacuum. The vacuum system and the xvacuum application will be discribed in a different section. For detailed information (but not BM29 specific) your may have a look on the xvacuum link of the ESRF computing service.

Front end shutter and primary slits

The first element of the beamline after the front end shutter are the standard ESRF primary slits. These are used to define the white-beam profile that is incident on the monochromator crystals. In normal operation, these slits are set to provide a typical beam profile of 10 mm to 20 mm in the horizontal plane and 0.2 mm to 1 mm in the vertical plane. The primary slits vertical aperture is a particularly important parameter, as it largely defines the energy resolution of the instrument for a given set of monochromator crystals.

vertical aperture	pvg
vertical offset	pvo
horizontal aperture	HEAT> phg
horizontal offset	HEAT> pho

Attenuators

Following the primary slits package the beamline is equipped with a series of beam attenuators, consisting of Al and Cu plates of various thickness. Although seldom used in practice, these devices do however serve a purpose during commissioning tasks and are thus maintained as a serviceable unit. They may be moved from an application accesible in the beamline applications menu (right mouse button pressed on the workstation desktop).

Monochromator

The monochromator is a double crystal, fixed exit double cam type from Kohzu-Seiki Corporation, Japan. Its operating geometry is in the vertical plane with the first crystal mounted so as to be able to rotate and translate along a mechanical cam, and directing the Bragg reflected beam (Sir William...) upwards. The second crystal in the monochromator is free to rotate about a single axis, and thus in combination with the translation and rotation of the first crystal, Bragg diffracts (Sir Lawrence...) the beam through a fixed exit point - parallel with the horizontal plane of the incident white beam, though vertically offset by 25 mm. The stability of the fixed exit of this monochromator is characterised as 2 mu m vertical offset at 2 m distance, between any specified energies defined by the crystal angles.

The monochromator is equipped since 1998 with a helium gas cooling system to minimise thermal perturbations to the energy of the monochromatic beam. This cooling system (an ESRF in house development) utilises two compressors. The first compressor is used to power a closed cycle refrigerator fitted with a heat exchanger in a vacuum chamber. Through this heat heat exchanger, a second compressor is used to flow helium gas, which then passes through the copper crystal supports of the two monochromator crystals. By these means, and with the assistance of a resistive heating element, the operating temperature of monochromator can now be kept within the temperature range of the minimum in the thermal expansivity of silicon (T ~ 125 K).

The monochromator design allows for simple changes of the crystal pairs in use. This facilitates the possibility to offer the user community a choice of Si(111), Si(220), Si(311) and Si(511) crystal pairs, depending upon their requirements for X-ray flux and operational energy range. The principle pairs of crystals that are most often requested are Si(111) and Si(311), which have corresponding operational energy ranges of 4.5 keV to 24 keV and 5 keV to 50 keV, respectively. Recently, we enlarged the accessible energy range by commissioning Si(511) crystals (10 keV to 74 keV operation). The intrinsic energy resolutions range from approximately 1 * 10^-4 for the Si(111) to 1 * 10^-5 for the Si(511). This is well inside the core hole lifetime broadening of the absorption edges within their operational energy ranges.

A typical crystal change takes 1 day to perform. The actual change of crystals takes little more than one or two hours, but the remaining time is required to evacuate the monochromator. In the recent operation using the cooling system, this time is prolongated by the heating up and cooling down cycle of approximately 3 hours each. It is recommended to wait for thermal stabilisation after having reached the operating temperature. This may take some 6 hours. Therefore, we will not change crystals during a user experiment and try in general to regroup similar experiments in order to minimise interventions on the monochromator.

One important operational characteristic of the double crystal monochromator design is the possibility of harmonic rejection by means of detuning the parallelism of the two crystals. The first monochromator crystal is mounted such that a piezo electric piston can slightly change its angle with respect to the second, fixed axis crystal. The detuning of the monochromator is performed in units of the full-width at half maximum of the rocking curve, measured by scanning the beam intensity as a function of the crystal parallelism. Typically, when two Si(111) crystals are detuned by 50 %, the intensity of the 3rd harmonic is reduced by a factor of 10³.

energy	Emono
angle	mono
queensgate piezo	qg1

Secondary slits

Following the monochromator the standard ESRF secondary slit package is installed, to control the energy resolution and to eliminate possible parasitic reflections from the monochromator. In most operating modes these slits are used to define the horizontal beam profile, since it is often more convenient to work with the known energy resolution provided by the primary slits vertical aperture.

vertical aperture	svg
vertical offset	svo
horizontal aperture	shg
horizontal offset	sho

In Figure [1.1], the next optical element is the 4 mirror multilayer device. The idea of this instrument is to use non grazing incidence operation to reduce higher harmonics with very high efficiency via Bragg reflection on W/Si multilayers. The angle of this multilayer has to be scanned in parallel with the monochromator in order to stay on the multilayer reflection. Mainly due to problemes related to the initial control of this unit, the development and use of this device had been stopped in the early stage of beamline operation and crystal detuning has been used instead as the standard harmonic suppression method. The instrument is current dismounted to allow a revision of the control electronics in order to improve its operation. It is planned to equip the optics hutch in 2001 with 2 mirrors working in specular reflection mode in order to improve the rejenction of harmonics and the work specially below 10 keV.

Safty shutter

The final element in the beamline optics hutch is the beam shutter that isolates the experimental hutch from the now monochromatic source. This shutter allows work to be performed at the experimental end stations of the beamline while beam is taken in the optics hutch. The heatload on the monochromator can therefore be kept constant during experimental procedures, which is important for maintaining the stability offered by the beamline.

open safety shutter	shopen
close safety shutter	shclose

If you want to learn more on BM29's front end, please check out the Storage Ring Status. The parameters of the machine can be found here.

Experimental hutch

An overview over the experimental hutch is shown in Figure [1.2]. The experiment is separated from the optics section by a Be window for ease of operation. This is the principal reason for the low energy cut-off around 4.5keV. For low energy operation, all parts of the experimental section can be connected with vacuum pipes to reduce air absorption.

Fluorescence screens

To facilitate the alignment, the first element is an extractable fluorescent screen (fl1, not shown on Figure [1.2]) monitored by a CCD camera. A simplified design of this fluorescence screen beam monitor is installed on the cryostat table and following the reflectometer (fl2 and fl3, respectively).

check state of fluorescence screens	flshow
insert n-th fluorescence screen	flin n
extract n-th fluorescence screen	flout n
n = 1, 2, or 3 for 1st, 2nd, or 3rd fluorescences screen respectively.

Fig. 1.2: Sketch of the BM29 experimental hutch.

Experimental slits

An experimental slits package following the first fluorescence screen allows for an accurate definition of the incident beam dimensions at the sample, and to form in combination with the secondary slits a tube spectrometer configuration.

vertical aperture	evg
vertical offset	evo
horizontal aperture	ehg
horizontal offset	eho

Detectors

The first beam intensity monitor on the beamline, is installed after these slits. In standard operation, we now use ionization chambers [31]. The use of photodiode detectors as intensity monitors in combination with a beam chopper to allow automatic dark current subtraction via an electronic lock-in is possible but not currently used anymore. Using Ionization chambers, detection efficiency is controlled by the gas species, gas pressure and applied voltage of the chamber. Typically for an EXAFS experiment, the efficiency of this first chamber is selected to be operational with 20 to 30% absorption of the fundamental energy and 50 to 80 % in the rear chamber. The absorption is adapted via the partial the partial pressure of the ionization gas, whilst the chamber is backfilled with a helium gas buffer to 2 atm. Depending on the operational energy range, we provide nitrogen (low energy operation), argon (12 keV to 30 keV) and krypton (higher energies) and operate with 1000 V potential across the 3 cm ionization gap. A manual explain how to calculate the optimal gas pressure and fill the chambers.

Other detectors available are

Photo-diodes for transmission experiments or fluorescence not requiring energy resolution
NaI scintillator detector for combined XRD or mounted for example on the reflectometer
13-element fluorescence detector (in collaboration with the ESRF detector pool)
Total electron yield detection in the cryostat
Gas filled position sensitive detector

Experimental station 1

The experimental end-station 1 is equipped with a large motorised table on which various sample environment equipment can be built. Some examples of experimental equipment that is often mounted at this point are:

The L'Aquila-Camerino high temperature oven (T : RT - 3000 K)
The Paris-Edinburgh large volume pressure cell (p : up to 7 GPa,T : RT - 1000 K)
The BM29 liquid pressure cell (p : 1 - 120 bar,T : 263 - 373 K)
The BM29 tubular oven (T : RT - 1000 K)
Catalytic reaction vessels

The advantage of this experimental station is that there is sufficient room to construct large sample environments and even additional detection and sample monitoring systems e.g. diffraction equipment, either with a position sensitive detector or a sodium-iodide photomultiplier tube, or temperature monitoring equipment such as pyrometers etc. The motor to move the sample or sample chamber will depend on the specific set-up, for example the l'Aquila-Camerino oven will be in general mounted on a small table with t3y and t3z movements.

example for horizontal sample movement	t3y
example for vertical sample movement	t3z

When not in use, experimental end-station 1 is bypassed by vacuum tubing and the next component of the beamline is the room temperature automatic sample holder. This is a motorised stage on which can be mounted standard transmission XAS samples such as 13 mm pressed pellets or metal foils. The advantage of this sample stage is that it is easily calibrated and facilitates the performance of series of room temperature scans on different samples. It is generally possible to mount ten to fifteen samples, which may then be measured in an automatic fashion.

room temperature sample holder	rtsam
There is no direct motor for horizontal movements.

Experimental station 2: Cryostat

The second experimental end-station of the beamline is a closed cycled helium cryostat that operates in the 20K to 450K range. This cryostat is designed so that the sample is mounted from a vertically suspended sample stick, vibrationally decoupled from the closed cycle refrigerator cold head that is just below the sample and beam position. Apart from the standard 13mm pellet sample holder, different sample holders have been developed to allow the measurement of up to three samples or shock frozen solutions. A total electron yield sample holder has been designed and is still under development. The sample chamber is usually filled with 0.1bar of helium gas so as to act as a thermal transfer medium with minimal convection losses. As with the other elements of the beamline, this operation of the cryostat is automatic with the sample environment monitored and controlled by a temperature sensor/heater on the stick and on the cold head. A detailed manual explains the use of the cryostat. We only mention here the motors associated.

cryostat sample holder	crysam
Altough the cryostat can be moved horizontal and vertical, this should only be used for alignment of the croystat itself by the beamline staff. If necessary, the horizontal slit offset can be used to optimize the sample ilumination.
You as a user should not use cy or cz.

After the cryostat is the second beam intensity monitor, ionization chamber I1 (or diode detector if desired). This detector is followed by the reference sample holder and a third intensity detector, in order to allow a reference spectra to be recorded in the same data file as the sample to be measured on either one of the first two experimental stations.

room temperature sample holder	refsam
no motor for horizontal movements.

Experimental station 3: Reflectometer

The third and final experimental end-station is a Huber, three circle diffractometer. This piece of equipment is primarily designed for the development of EXAFS in reflection mode at BM29. It currently is the subject of an in-house research development project. We just give some examples for the motors here:

Theta1	ref_th1
Theta2	ref_th2
gonio x	ref_x
gonio y	ref_y
gonio z	ref_z

A device for absolute energy calibration is following the diffractometer. This design uses backreflections from a Si crystal and allows absolute energy calibration of the measurements and characterization of the monochromator performance. It is only in use commissioning purposes and not available to the user.

Other rooms

Control room

There are 3 other hutches at BM29: the control room is equipped with a HP workstation (UNIX) from which the experiment is conducted using SPEC control software. A set on-line analysis programs is available, as well as highly perfomend analysis packages like GNXAS, ExCurve, and FEFF. A personal PC with direct data access is available for additional data treatment/visualisation. Portable PC's may be connected. The electronic cabinet attached to the control room houses the control crates, motor power supplies, vacuum controllers, the workstation itself, control electronics like electrometers, counters, etc.

Sample preparation room

A second room offers basic sample handling infrastructure to complement the ESRF Chemistry Laboratory and other support laboratories. Equipment includes a fume cupboard, pellet press, cold storage, balances, drying oven, filtration equipment and ball mill. This room should be refurbished in 2001 (should have been... hope it comes in 2002:-( ).

Storage room

The last room serves mainly for storage of BM29's in-house equipment, as well as equipment for preparation of high-pressure experiments. The user has normally no acces to this areas.

Scanning modes

Extended x-ray absorption spectra with an optimized energy mesh with a constant spacing in the photoelectron wavevector scale, k-space.
Continuousscan EXAFS (QEXAFS) operation (minimum useful acquisition time ~0.2 s)
Sequence of fast scans to monitor possible sample changes during heat treatments (Both in normal and QEXAFS modes).
Single energy temperature scans to monitor phase transitions and sample transformations.
x-ray powder diffraction to check samples measured by EXAFS (available of the reflectometer station).

As a general policy at BM29, we have tried to automize as much of the sample environment and control as possible. The aim is to maximize the reliability of the acquired data and to improve the user operation. For an experiment using the cryostat for example, the user has the choice to record within the experimental data file the temperature of the cold head and sample holder, or the crystal temperature, etc. This results not only in a maximized control of the sample environment and instrumental parameters, but also opens the way for new applications of X-ray absorption spectroscopy.