International
Tables for
Crystallography
Volume I
X-ray absorption spectroscopy and related techniques
Edited by C. T. Chantler, F. Boscherini and B. Bunker

International Tables for Crystallography (2022). Vol. I. Early view chapter
https://doi.org/10.1107/S1574870720006321

Beamlines

Steve M. Healda*

aAdvanced Photon Source, Building 435E, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA
Correspondence e-mail: heald@anl.gov

Beamlines for X-ray spectroscopy can have a variety of designs, but there are many common components and requirements. These include handling the heat load from the source, monochromating and focusing the X-rays, protection from radiation, and sample handling. This chapter introduces these requirements in a general sense to motivate more detailed discussion in later chapters.

Keywords: beamlines.

1. Introduction

While there are many different types of beamlines, they all share many similar functions and components. This chapter will look at beamlines in a general sense, highlighting common requirements and components. More detailed descriptions of the components can be found in individual chapters.

All beamlines begin at the source, which is either a bending magnet or a specialized insertion device such as a wiggler or an undulator. The major parameters important from a beamline standpoint are the beam divergence and power distribution. Most sources deflect the beam in the horizontal direction. In this case, the vertical divergence is related to the electron beam energy and is similar to 1/γ, where γ is the beam energy/0.511 MeV. For high-energy rings this is a very narrow collimation. Bending magnets and wiggler sources have a broad horizontal swath of radiation that requires correspondingly larger apertures and optics to handle. Both wigglers and undulators will typically have much larger beam powers that need to be handled by the components. For wigglers the issue is mainly the total power. Undulators can have a similar total power, but much of it can be absorbed early in the beamline since the usable part of the beam (the central cone) contains only a fraction of the total. The issue for undulators is usually the high power density. The first components in the beamline (the front end) are generally inside the accelerator shield wall and include components such as safety shutters, beam-defining apertures, beam-position monitors and vacuum interlocks. Access to these components is limited, and they generally consist of standard components controlled by the facility.

The beamline optics are usually located outside the shield wall, although sometimes the first optical element such as a slit or mirror will be placed inside the wall to minimize the distance to the source point. For an X-ray absorption fine-structure (XAFS) beamline the monochromator is an essential component, and a simple beamline may consist of only a monochromator along with some slits and windows. However, it is often desirable to focus the beam to increase the flux density, especially for the more divergent bending-magnet and wiggler sources. Both focusing mirrors (Susini, 1995[link]) and sagittally bent crystals (Pascarelli et al., 1996[link]; Sparks et al., 1982[link]) are commonly used to collect the divergent radiation from these sources. To maintain energy resolution, focusing mirrors should be placed after the monochromator. The acceptance angle of common silicon and diamond monochromator crystals is usually less than the divergence of the source. Mirrors to collimate the vertical divergence of the source can improve the monochromator throughput. Such mirrors can also remove some of the power incident on the first monochromator crystal. Therefore, a common beamline layout for higher energies consists of a collimating mirror followed by a double-crystal fixed-exit monochromator and a focusing mirror. At lower energies gratings are used in the typical monochromator. These lack the simplicity of a double-crystal monochromator, but often follow a similar layout with a first mirror coupling the beam to the grating followed by additional mirrors to maintain a fixed exit and for possible focusing. Another optical component often found on XAFS beamlines is a harmonic rejection mirror, which is typically located close to the sample. This can be used at lower energies to remove higher order energies (for example the 333 reflection for a Si 111 monchromator) that are not already rejected by the upstream optics.

The final beamline component is the endstation. This consists of a radiation enclosure, sample mounts and alignment stages, detectors and specialized experimental equipment. It also may include optics such as a harmonic rejection mirror or additional focusing to produce microbeams. Depending on the experimental programs to be supported there might also be extensive support equipment such as for hazardous gas handling.

The following sections consider these beamline components as an overview, with further details of the major components available in the following chapters.

2. Heat loads

A major issue at any beamline is handling the heat loads generated from the source. This is especially critical for XAFS beamlines, where the energy is being changed and the measurements are very sensitive to beam motion and energy-calibration changes.

There are two aspects to consider in the heat load: the total power and the power density. For bending-magnet and wiggler sources the total power is a primary consideration since it is spread out over a wide horizontal angle. For undulators the power density becomes an important factor since the power density can exceed the safe limits of most materials. For an undulator typical power densities are 100–200 W mm−2, while for both undulators and wigglers the total power can be several kilowatts or more (Hofmann, 2004[link]).

There is another important difference for undulator sources. While the total power from an undulator may be similar to that of a wiggler, the usable photons are concentrated in a narrow central cone of radiation. Photons outside the central cone are at energies that will not be passed by the monochromator. Fig. 1[link] shows the power distribution from an undulator compared with the distribution of photons in the first harmonic peak. It is clear that much of the power can be rejected by an aperture around the central peak. A suitable aperture can reduce the total power to a few hundred watts. However, this central region will still have a very high power density incident on the optical components, much higher than the maximum power density from a wiggler or a bending magnet. Another aspect of undulators is the narrow energy band of the harmonic peaks. The energy of the peaks can be changed by changing the magnetic field of the undulator, either by controlling the gap in a permanent-magnet device or the current in an electromagnetic undulator. This gap or current must be synchronized with the monochromator energy to maintain maximum flux.

[Figure 1]

Figure 1

Distribution of the total power (dashed lines) and the first harmonic flux (solid lines) at a distance of 30 m from a typical undulator at the Advanced Photon Source. For comparison the distributions are normalized to their maximum values.

Since the power in a high-power wiggler source accompanies the usable photons, spatial filtering of the power is less effective. These beamlines must rely on other forms of power reduction, such as mirrors, which act as low-pass filters, and X-ray absorbers, which act as high-pass filters.

Traditionally, storage rings have been filled with an initial maximum current which is allowed to decay to some level before refilling. Concerns about beamline stability under large changes in heat loads has led to the implementation of `top-off' operation at some facilities. In this mode the beam current is periodically topped off to maintain a nearly constant heat load on the optics.

There are number of approaches for handling the large power densities and total powers of insertion devices. For the front-end components high thermal conductivity materials with optimized cooling must be used. High-conductivity copper or dispersion-strengthened copper such as Glidcop are often used. Even these materials cannot directly handle the power densities of many sources, and the components must be designed to spread out the heat load by intercepting the beam using glancing angles as small as 1–2°. For optics. where it is important to maintain accurate figures as well as withstanding the heat, a useful figure of merit is the thermal conductivity/thermal expansion. High conductivity and low thermal expansion will minimize the thermally induced distortions. For monochromator crystals the best material at room temperature is diamond. However, the small available sizes can make it hard to use. Silicon can provide a similar performance at cryogenic temperatures, where its thermal conductivity is enhanced and its thermal expansion is small (Bilderback et al., 2000[link]). Since it is available in large sizes, most X-ray monochromators on ID beamlines use single-crystal silicon cooled to temperatures near 77 K (liquid nitrogen). For large grazing-incidence mirrors in a white or pink beam the heat load is sufficiently spread out that that water-cooled single-crystal silicon at room temperature will often work. Since the technology for polishing silicon is mature, it has become the preferred material for large mirrors even when its thermal properties are not needed.

There are similar issues for low-energy beamlines that employ grating monochromators, although the source characteristics and pre-monochromator optics for such beamlines can often be designed to minimize the heat load on the monochromator. Since the gratings for soft X-rays operate at small incident angles, water-cooled silicon substrates are again a popular choice.

3. Front-end components

The primary function of the front-end components is to interface the beamline to the ring, filter out some of the unwanted power and provide safe operation. The major components include safety shutters to block the beam when access to the hutches is needed, collimators to remove unwanted radiation from travelling down the beam pipe, X-ray transparent windows or differential pumping stages to separate the storage-ring ultrahigh vacuum (UHV) from the beamline vacuum, valves to isolate the beamline from the ring when work is needed and beam-position monitors (BPMs) to monitor the beam.

Safety shutters along with suitable collimators are needed to stop two kinds of radiation: the synchrotron radiation and the Bremsstrahlung generated when the electron beam interacts with residual gas in the ring or if a mis-steered electron beam strikes a solid object such as the beam pipe. Since the Bremsstrahlung energy can be as high as the electron beam energy it dominates the shielding requirements, and the required shielding thickness can be substantial (20–30 cm of lead or tungsten; Liu et al., 1995[link]). For this reason, multiple Bremsstrahlung collimators are usually placed before and after beamline optics such as mirrors and the monochromator that deflect the monochromatic beam. The line of sight for the Bremsstrahlung sources can then be completely blocked while letting the deflected monochromatic beam pass. Shutters for the monochromatic beam can be much lighter and do not need to be cooled. Also, this has the advantage of allowing access to the experiment while maintaining the full heat load on the optics, increasing the stability of the beam.

To maintain the beam lifetime, storage rings need to be operated under UHV conditions. There are also serious concerns about a beamline vacuum accident contaminating the ring vacuum, affecting all of the facility users. For this reason, it is important to provide separation of the ring vacuum from the beamline. The simplest method is to use an X-ray transparent window. The difficulty for windows in a white beam is proper cooling. For this reason, windows need to be transmitting for X-rays, but also have good thermal and mechanical properties. Typically, beryllium or diamond is used. Beryllium has better transmittance, but the thermal properties of diamond are much better. To avoid windows in the white beam, some beamlines have differential pumping sections with small apertures surrounded by sufficient pumping to allow a significant pressure difference across the aperture. As a backup to the windows or differential pumps, many front ends also incorporate fast valves that can rapidly close to protect the ring if the beamline vacuum is too high.

Concern for the beamline vacuum also dictates another requirement for front-end or beamline components exposed to the ring; that is, the avoidance of water in vacuum joints. Policies vary, but generally it is forbidden to have direct joints whose failure could allow water to reach the ring vacuum. This can complicate the design of water-cooled components, since they will generally need to have a separate guard vacuum around water lines and their connections.

4. X-ray mirrors

The basic limitation of mirrors for X-rays is their need to operate at extreme glancing angles. While this makes their use as an optic more difficult, it also provides two important advantages. The use of glancing angles relaxes the requirement for surface smoothness to achievable levels (Church & Takacs, 1995[link]) and the heat load is spread out, making cooling simpler.

As mentioned, there are three important applications of X-ray mirrors: collimation, focusing and harmonic rejection. Collimating mirrors are used to improve the monochromator throughput by matching the divergence of the X-rays to the acceptance of the monochromator crystals. Typically, the throughput can be improved several times depending on the source divergence and crystal reflection used. This increase in throughput does not degrade the energy resolution, as would be the case by simply opening up the entrance slit to the monochromator. Because they precede the monochromator, collimating mirrors must handle a significant portion of the heat load of the source. To date, water cooling of single-crystal silicon mirror substrates has proven to be capable of handling the necessary heat loads, although sometimes the cooling geometry needs to be optimized using sophisticated thermal analysis (Zhang et al., 2015[link]). Collimating mirrors need to collimate the dispersion direction of the monochromator. This is usually the vertical direction for wigglers and bending magnets, but could be in the horizontal for an undulator source. To collimate a single direction of the beam, a singly bent or parabolic shaped mirror is used. It is also possible to collimate both the horizontal and vertical directions using a paraboloidal mirror (Nomura & Koyama, 2001[link]). This has some advantages, but the difficulty in manufacturing such mirrors has limited their use.

Focusing mirrors generally follow the monochromator since they can degrade the natural collimation of the beam, which would reduce the energy resolution of a following monochromator. Therefore, heat loads are low and cooling is not needed. To focus both directions a focusing mirror needs to be ellipsoidal or a bent cylindrical approximation to an ellipsoid. However, if proceeded by a collimating mirror the ideal focusing element would be paraboloid if the beam is collimated in both directions or a more complex shape if collimated in a single direction. For the second case a bent cylindrical approximation can often suffice, and in fact provides a quite good focus when located to demagnify the beam by a factor of two, i.e. its distance to the focus is one half of the distance to the source point (MacDowell et al., 2004[link]).

Harmonic rejection can be provided by the collimating or focusing mirrors. A common situation is for the collimating mirror to have multiple reflecting stripes with different energy cutoffs. A simple translation of a singly bent mirror can then be used to vary the low-pass energy. However, for maximum harmonic rejection and flexibility some beamlines will have separate harmonic rejection mirror(s). These will be located close to the sample and could consist of two mirrors to eliminate beam motion as the mirror angle and harmonic rejection are varied. The energy cutoff or critical energy of a mirror depends on the coating and is inversely proportional to the mirror angle. Table 1[link] gives parameters for calculating the cutoff angle for common mirror materials.

Table 1
Parameters to calculate the critical angle for common reflecting materials

The angle is in milliradians and the energy is in keV. Calculations are based on the formula θc = (28.8/E)(Zρ/A)1/2, where Z is the atomic number, A is the atomic mass and ρ is the density in g cm−3.

MaterialCritical angle
Si 31/E
Ni 59/E
Rh 67/E
Pt 84/E

5. X-ray monochromator

As discussed, for heat-load considerations most monochromators for hard X-rays use silicon crystals, either at room temperature with water cooling or at low temperature with liquid-nitrogen cooling. The minimum energy for silicon is approximately 2.1 keV. For lower energies most monochromators are grating-based, although crystals with large d-spacings are sometimes used. These crystals typically have lesser thermal properties and are restricted to lower power beamlines such as bending magnets or lower energy rings. Details of both crystal and grating monochromators are given in the following chapters.

For spectroscopic applications crystal monochromators generally employ an even number of reflections (two or four; Beaumont & Hart, 1974[link]; Greaves et al., 1983[link]) to avoid large deflections of the outgoing beam as the crystal angles are changed. The exceptions are dispersive XAFS beamlines, which do not need to scan and can employ a fixed-angle polychromator (Pellicer-Porres et al., 1998[link]; Pascarelli et al., 2006[link]). For the most common double-crystal case the beam from two fixed parallel crystals (a so-called channel-cut crystal) will have a vertical displacement as the angle is changed (see Fig. 2[link]). For a simple unfocused beamline this displacement is sometimes accommodated by moving the table supporting the experiment to track the beam. This allows the use of a very stable monolithic channel-cut crystal. However, for beamlines with optics following the monochromator or complicated experimental setups it is usually necessary to use a fixed-offset monochromator where one or both crystals are moved during angle changes to maintain a constant beam height.

[Figure 2]

Figure 2

The beam offset H in a channel-cut crystal varies with the angle. s is the crystal separation.

In the soft X-ray range, gratings are the most common energy-dispersive optic. Natural large d-spacing crystals have poor radiation resistance and thermal properties, while artificial multilayers have insufficient energy resolution. Since the simple two-crystal geometry used for crystals is impractical for gratings, monochromator designs can be complex (Petersen et al., 1995[link]). Also, since gratings are manufactured optics, there many more options available to the beamline designer. Gratings can have variable line spacing and be produced on figured substrates for combined monochromatization and focusing. Energy changes require coordinated scanning of the grating along with one or more mirrors and slits. For further details, see Sutter (2021[link]).

6. Radiation enclosures

All beamlines require shielding from radiation hazards. For most hard X-ray beamlines this means radiation enclosures, often referred to as hutches. A typical XAFS beamline will have an optics enclosure that needs to shield the white beam from the source and one or more experimental enclosures that need to protect against the monochromatic beam that exits the optics enclosure. For low-energy lines the beam pipes and experimental chambers may be sufficient to block the monochromatic beam and the experimental enclosures are absent.

The shielding requirements vary according to the ring energy, ring vacuum and the operational energy of the beamline, but typically the optics or white-beam hutch will require some amount of lead in the walls. Another hazard from the white beam is the generation of ozone if it contacts the air, and its contact with air should be minimized. The monochromatic beam enclosure may also need lead shielding, but sometimes a simple steel construction is sufficient. When considering the shielding it is important to realize that monochromators may pass harmonic energies that are multiples of the operational energy.

Modern-day enclosures tend to be large rooms that are capable of housing complex experimental setups. This means that sophisticated interlocks and controls are needed to ensure that no one is inside when the beam is on. A common practice is a set of search buttons that must be pressed in sequence to ensure that the enclosure is vacated prior to closing the door.

7. Endstation

The endstation is where the experiment is carried out. An experimental enclosure can house more than one end­station. The simplest XAFS endstation is a table with slits for defining the beam and ion chambers for measuring the incident and transmitted beams. Samples would generally be mounted on a motorized sample mount allowing alignment and automated sample exchange. There are many possible additions to this scenario. There may be fluorescence detectors for samples that are unsuitable for transmission measurements. Various types of in situ cells are possible, allowing controlled environments such as low-temperature and high-temperature treatments, high pressures or UHV for surface science. The penetrating nature of X-rays allows a wide range of sample cells.

When operating at lower energies where air absorption is important the beam path must be totally enclosed and operated in vacuum or helium. In this case, the windows needed for a gas-filled ion chamber may be too absorbing. An alternative for I0 is to measure the electron yield or fluorescence from a partially transmitting mesh or thin foil.

Optics for microfocusing need to be close to the sample and are usually integrated into an endstation that includes the sample mounts and detectors.

8. Other components

There are a number of other components that will be found in most beamlines. These might be incorporated into individual endstations or might be part of the main beamline. All beamlines will have slits to define the beam size and potentially the energy resolution. For small samples it is important that the slits are set such that the beam size is smaller than the sample. Otherwise the I0 detector will not be measuring the same part of the beam as is actually being used. For most monochromators, slits can be used to define the energy resolution and must be set correctly. Slits in the white beam can also play an important role in managing the power incident on the downstream optics.

Another common component on high-flux beamlines is a set of filters. These can be used to reduce radiation damage when the full flux is not needed or to reduce the count rates on detectors to acceptable values. Maintaining acceptable count rates for detectors is especially important for XAFS since nonlinearity can significantly affect XAFS amplitudes.

Finally, the beamline may be equipped with detectors for monitoring the beam size and position. These can be inline position monitors or removable screens that block the beam. Inline detectors include split ion chambers, partially transmitting quad diodes and scattering-based beam monitors. Split ion chambers have two collecting plates and their differential signal depends on the position of the beam. Quad diodes can be made of thin silicon or diamond, and again the signals on the four collecting quadrants can be converted into a position. Since these materials can be single crystals, there is possible interference from Bragg reflections in the detector, but they can provide a highly accurate position. Scattering-based detectors utilize a thin scattering foil in the beam. The scattering is monitored by four diodes whose signal will depend on the position of the beam on the foil (Alkire et al., 2000[link]). Since the foil does not need to be a single crystal, Bragg reflections are not a concern. Some beamlines will incorporate these monitors into a feedback system to stabilize the beam position and/or intensity.

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