How are X-rays obtained from an accelerator?

Electromagnetic waves are emitted when a charged particle is submitted to an acceleration. In a circular accelerator such as a synchrotron or a storage ring, electrons are deviated by magnetic fields. This deviation is due to the radial force (i.e. a radial acceleration) which attracts the electrons towards the centre or the ring. The light emitted by electrons in a synchrotron or a storage ring is called “synchrotron radiation”. Its wide spectrum reaches the X-ray range only when the energy of the electrons is high enough (of the order of several billion electronvolts - GeV).

Could you please explain how the electric field in the synchrotron accelerates the charged particles?

The electrons in a synchrotron complex are accelerated at several different stages in a linear accelerator and also in special cavities in the circular rings (booster or storage ring).  To accelerate, you need electric fields parallel to the motion of the charged particle. To get strong fields, it's much easier to use oscillating fields, rather than static fields. The trick then, is to use a conducting metal structure to reshape the mode patterns of the oscillating electric and magnetic fields so that there is a component parallel to the motion of the wave.  This is what is done in both the acceleration structures for the linear accelerator, and in the cavities that accelerate in the booster and storage ring.  Radio frequency waves are sent into the cavities via wave guides, and the conducting walls of the cavities reshape the modes so that there is a component of the electric field in the parallel direction.  Then it's just a matter of timing to send the particles in with the proper phase so that the field will cause acceleration, and not deceleration. In the booster and storage ring, the frequence is chosen very carefully, so that each time the electron returns to the cavity after circulating in the ring, the phase of the field will be such that it will be accelerated.  In the linac, the phase of the different acceleration structures are synchronised so that the particle is accelerated the whole time.

What are the most import advantages/characteristics of synchrotron radiation compared to lab X-ray sources?

The most important advantage of synchrotron radiation over a laboratory X-ray source is its brilliance. A synchrotron source like the ESRF has a brilliance that is more than a billion times higher than a laboratory source. Brilliance is a term that describes both the brightness and the angular spread of the beam. The difference between the two sources can be likened to the difference between a laser beam and a light bulb. Higher brilliance lets us see more detail in the material under study e.g. there is a greater precision in the diffraction of light from a crystal where both the angle and the intensity is significant and recorded by a detector.

Other useful properties of synchrotron light are:
- high energy beams to penetrate deeper into matter
- small wavelengths permit the studying of tiny features, e.g. bonds in molecules; nanoscale objects
- synchrotron beams can be coherent and/or polarised, permitting specific experiments
- the synchrotron beam can be made to flash at a very high frequency, giving the light a time structure. This lets us follow chemical reactions on a very short time scale.

Which monochromators are used with synchrotron radiation and how do they work? Do they work with diffraction gratings or prisms? Do they have to be cooled and how is this accomplished?

The monochromators used are typically periodic, diffracting elements to select given energies of radiation.
- Gratings are used for energies (roughly) below 1 keV.
- For higher energies crystals are used which are natural three-dimensional "gratings", mainly high quality silicon crystals. For the ESRF, typical energies are between 5 and 60 keV, but there exists beamlines using lower and higher energies.
- A further possibility of X-ray optical elements exists that allows the scientist to select a narrow energy band. These elements are multilayers, a periodic arrangement of rather thin double layers, usually a metal and a lower density material.

Prisms could be used in the visible range, however synchrotrons are mainly dedicated to producing much higher photon energies, that is radiation with much shorter wavelengths. So prisms are not used in synchrotrons like the ESRF.

Cooling is achieved with water or liquid nitrogen. The cooling circuit typically cools the faces of the monochromator crystals.

When does one use platinum coated reflecting mirrors and when does one use crystal monochromators in a beamline?

Mirrors, including platinum coated ones, are used mainly for two applications: either as 1D or 2D focussing elements (then the surface is not plain) or as elements to cut off higher energies (then it is used with flat surface). Whereas a crystal monochromator is used to extract a narrow energy band from a polychromatic or even a "white" beam.

Why is the X-ray diffraction pattern a reciprocal picture instead of a real one, like in electron microscopy for example?

In an electron microscope, the scattered electrons are refocussed to form an image by the lenses of the microscope's optics. The electron is a charged particle so it can be focussed via electromagnets. The focussing is the equivalent of converting the reciprocal to the real. In general, X-rays cannot be focussed, so stay as the reciprocal.   (Note that lenses do now exist for X-rays, but with very small apertures and very small focussing power so this is not yet practicable for X-rays scattered by diffraction from a crystal).

A diffraction peak observed from a solid is a straight narrow line, whereas a peak from a liquid crystal looks like a Lorentzian peak, what is the difference between these structures?

A perfectly crystalline material gives very sharp diffraction peaks because the crystal acts as a perfect diffraction grating. If the level of crystal perfection in reduced, then the peaks are broadened because the diffraction condition is less stringently imposed. Crystal imperfection arises from various effects, such as strain in the crystal, chemical inhomogeneity, defects, dislocations, disorder, etc. Also, if the crystals are very tiny, then there are fewer diffraction planes to impose the diffraction condition so stringently, and broadening of the diffraction peaks can also occur.  It is possible to assess the level of perfection or imperfection of a crystalline material and to assess the type and number of defects from a detailed analysis of the shape of the diffraction peak. Thus the simple answer is that your liquid crystal is less perfect than the solid with which you are comparing it (which is not so surprising as a liquid crystal is not a perfect crystalline solid).

I would like to know whether you can determine if a drawing by Picasso is authentic based on paper quality and age as well as on scientific proof of signature. Drawing dates circa 1911?

The ESRF's precise and intense synchrotron X-ray beams can be used to look in detail at the chemical composition of a painting, both for the surface layer and within the depth of the painting and its support, using non-destructive techniques. However, the results of such analyses cannot be interpreted on their own and knowledge of other paintings, and art history would be required. For the case of a Picasso, this analysis would be extremely challenging because Picasso used a broad variety of styles, pencils/painting techniques, and substrates. Therefore, synchrotron radiation techniques on their own are unlikely to be able to elucidate the problem, and an expert on Picasso's art should be consulted in the first instance.

For information about studies that have already taken place by teams of researchers using the ESRF's X-ray beams, we have just published an article on the investigation of a Van Gogh:
http://www.esrf.fr/news/general/vangogh/
There was also an edition of the ESRF Newsletter that featured artwork studies:

I was wondering what is the speed of an electron in space? Is it constant and approximately the speed of light or does it change according to its energy and/or the media? Which speed do electrons reach in the storage ring of the synchrotron?

An electron in space may have any speed between zero and a value close to (but not reaching) the speed of light. In order to "speed up" an electron, it has to be accelerated by an electric field which in space is possible for example in the plasma corona around a star.

When you accelerate an electron in a very strong electric field, rather than giving it an ever increasing speed the energy acquired is converted into an increase of mass so that the electron will never reach a speed higher than the speed of light but rather will becomes heavier and heavier (thus needing more and more energy to be accelerated to higher speeds).

In the electron storage ring at the ESRF, the electrons are roughly 10000 times heavier than at rest. This is why their speed is very, very close to the speed of light: v = 0.999999995c - to illustrate this, take the distance incredibly long between the Sun and Earth: light takes 8 minutes to travel this distance of 150 million kilometres, and an electron travelling at the speed of the electrons in the ESRF storage ring, will arrive "only" a quarter of a microsecond "later".

Is it possible to study the composition of the bulk of terfenol-D to analyse its behaviour?

Experiments on such alloys are possible at ESRF beamlines ID15 and ID11 (and others...).  We would need more information to be able to give you a precise answer about your experimental requirements.

I'd like to know if the ESRF cooperates with CERN and how? In particular do you collaborate on quantum mechanics?

The ESRF does indeed cooperate with CERN.
1) There is general exchange of experience with accelerator operations, control etc.
2) ESRF investigates the superconducting wires used in the LHC for mechanical stability after quenches.
3) Both are members of EIROforum where we have joint activities, see the website: www.eiroforum.org.

We do not have any examples of collaboration on quantum mechanics.

What might X-ray science provide in terms of content, age, condition or the like for the recent fine art discovery of early Chinese silk?

Archeological silk (e.g. from the Famen-si site) shows different degrees of degradation. Synchrotron radiation techniques can contribute to understanding the principal mechanisms behind silk degradation and the role of heavier elements in the degradation process (either from additives during textile processing or from environmental factors). The main synchrotron radiation techniques are structural such as wide-angle scattering (variation of crystalline structure and texture of fabric), small-angle scattering (variation of morphology of crystalline and amorphous domains, nanofibrillar structure), X-ray fluorescence (elemental distribution), X-ray microscopy (imaging of fibrillar structure). Synchrotron radiation scattering techniques are also used to study the fine structure of silk after conservation.

For further information:

Characterising the decay of ancient silk fabrics by microbeam synchrotron radiation diffraction

Is it possible with synchrotron X-rays to detect particulate matter in the nano-range in human fluids (such as blood) and tell at the same time the composition of the particles (heavy metals, carbon etc) ?

X-ray fluorescence imaging can be used to detect trace levels of heavy elements such as metals (but not carbon). If the nanoparticles contain elements such as Pt, Au, Cd, Ag or lanthanides that are not generally present in body fluids then trace quantities of these nanoparticles can be detected. The ESRF X-ray nanoprobe can be used to visualise nanoparticles or clusters of nanoparticles if they are 100 nm or larger.

Does the ESRF have a synchrotron radiation circular dichroism facility?

For magnetism measurements, there are a number of suitable beamlines: Soft (0.4 - 1.5 keV) and hard (2 - 20 keV) X-ray facilities for magnetic circular dichroism experiments are available at beamlines ID08 and ID12 respectively. In addition dichroism experiments using dispersive EXAFS can be carried out at beamline ID24.

We would like to test mm-sized diffractive-refractive X-ray lenses with a focal length of up to 100 m. Is there an X-ray beamline at ESRF, where they can be tested? If a length of 100 m is not available, what is the maximum operation distance?

At the ESRF we have a dedicated test facility, the so called "Micro-Optics Test Bench" (MOTB), situated in one of the hutches of the beamline ID06. There all our lenses are tested and calibrated (thousands of them). At this test bench, the working distances may be from a few millimeters to about 7 metres. For much larger distances, the situation is difficult. Of course there are beamlines (or will be) with the very popular lense optics installed and using distances of 20 m, 30 m, or more than 40 m, but they are, in principle, not foreseen to make experiments of the type you probably intend to do.

Is a large diameter of storage ring necessary for a synchrotron facility/source? Can they be built on a small scale, only few metres in diameter?

Historically, the larger the storage ring, the higher the electron energy that can be stored. This gives access to brighter X-ray beams with an electromagnetic spectrum reaching higher energy/shorter wavelengths. The ESRF has beamlines that take advantage of the higher energy X-rays, for example to look deep inside metallic samples. We also make use of shorter wavelengths to study certain elements in the periodic table.

There are X-ray sources based on Compton backscattering that have spectral characteristic similar to the ones from the large facilities, but with lower X-ray flux/power.  "ThomX" is one such machine, which has just been funded and will be built in the next few years. We might expect that with more R&D, such a machine will improve its parameters, but it is hard at the moment to predict where their ultimate performances will stand.  One could imagine such a machine being installed in the space of "a couple of rooms" at a university or hospital.

Description of ThomX in English: http://hal.archives-ouvertes.fr/docs/00/44/82/78/PDF/ThomXCDRAV.pdf; Website: www.thomx.fr

What is the shortest X-ray wavelength/highest X-ray photon energy that ESRF can provide for experiments? Can this exceed 1 MeV? What brilliance is available for very hard X-rays?

The majority of our beamlines are optimised for X-rays below 50 KeV. A few routinely use higher energies to gain greater penetration depth, for example in metal samples. Beamline ID15 routinely offers the highest energies, which, for diffraction experiments, are about 400 KeV (8x1011 phs/sec/mrad2/0.1%BW/200 mA). For direct beam experiments such as detector tests we can reach 950 keV (2x107 phs/sec/mrad2/0.1%BW/200 mA).

Further details about ID15 can be found on the beamline home page ID15 - High Energy Diffraction and Scattering Beamlines.

Why does the electron beam become defocussed in one direction when it passes through a quadrupole focussing magnet?

A quadrupole magnet acts as a lens that focusses the beam in one direction and defocusses it in the other. Maxwell's equations tell us that the divergence of the magnetic field must be zero. At the centre of the quadrupole the field is zero, and changes when you move away from the centre. But due to the divergence condition, if the field increases in one direction, it must decrease in the other. The focussing or defocussing comes from this linear variation of the magnetic field (see the Lorentz force equation). So the net effect is that if the quadrupole focuses in one direction, it must defocus in the other. This is different to light optics where the lens focusses (or defocusses) in both directions.