Luce di sincrotrone: generazione e proprietà

Transcription

1 Luce di sincrotrone: generazione e proprietà Giorgio Margaritondo Vice-président pour les affaires académiques Ecole Polytechnnique Fédérale de Lausanne (EPFL)

2 Programma: Come costruire un ottima sorgente di raggi x utilizzando la relatività di Einstein Alcuni esempi di applicazioni Raggi x coerenti: una rivoluzione nella radiologia Dai sincrotroni ai laser a elettroni liberi e al loro uso Il futuro: nuove sorgenti, laser a raggi x SASE

3 From ancient fires to synchrotrons and FEL s, the same problems: A fire is not very effective in "illuminating" a specific target: its emitted power is spread in all directions A torchlight is much more effective: it is a smallsize source with emission concentrated within a narrow angular spread -- it is a "bright" source Likewise, we would like to use bright sources for x-rays (and ultraviolet light)

4 Why x-rays and ultraviolet? Chemical bond lengths Wavelength (Å) Photon energy Core electrons Molecules (ev) 100 Proteins Valence electrons

5 The brightness of a light source: Source area, S Angular divergence, Ω Flux, F Brightness = constant x F S x Ω

6 A real synchrotron facility: Swiss Light Source (SLS) VIII Scuola Nazionale di Luce di Sincrotrone - Frascati 2005

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8 Objective: building a very bright x-ray source. Solution: relativity!! Ring under vacuum Circulating electrons (speed c) Undulator Emitted x-rays The undulator (periodic magnet array) period determines the emitted wavelength. This period is shortened by the relativistic Lorentz contraction giving x-ray wavelengths The emitted x-rays are projected ahead by the motion of their sources (the electrons), and therefore collimated. Relativity enhances the effect

9 Heat flux (watt/mm 2 ) Surface of the sun 100 Swiss Light Source (SLS) Interior of rocket nozzle 10 ALS, Elettra, SRRC, PAL Nuclear reactor core 1

10 The historical growth in brightness/brilliance (units: photons/mm 2 /s/mrad 2, 0.1% bandwidth) SLS Wigglers Bending magnets Rotating anode SLS (Swiss Light Source)

11 Objective: building a very bright x-ray source Details of the solution: electron Speed c Undulator (periodic B-field, period L centimeters In the electron reference frame: Periodic B-field periodic B & E-fields moving at speed c, similar to electromagnetic wave Lorentz contraction: L L/γ Undulation of electron trajectory emission of waves with wavelength L/γ In the laboratory frame: Doppler effect wavelength further reduced by a factor of 2γ, changing from L/γ to L/2γ 2 Overall: L L/2γ 2 Centimeters 0.1-1,000 Å (x-rays, UV)

12 What causes the high brightness? Free electrons can emit more light than bound electrons high flux The electron beam control is very sophisticated: small transverse beam cross section small synchrotron source size Relativity collimates the emitted synchrotron radiation: Electron reference frame c y = dy/dt electron velocity θ c x = dx/dt emitted x-ray tn(θ) (c y /c x ) Laboratory frame c y = dy /dt c x = dx /dt Lorentz transformation: γ-factor for x and t but not for y tn(θ) reduced by a factor 1/γ

13 3 types of sources: 1. Undulators: small undulations detector continuously illuminated detector long signal pulse narrow hν-band hν/ hν N time frequency

14 3 types of sources: 2. Bending magnets: short signal pulse broad hν-band time frequency

15 3 types of sources: 3. Wigglers: large undulations Series of short pulses broad hν-band time frequency

16 3 types of sources - summary: Undulators: time frequency Bending magnets: time frequency Wigglers: More intensity than bending magnets time frequency

17 Undulator emission spectrum: Central wavelength: L/2γ 2 L = period First correction: out of axis, the Doppler factor is not 2γ 2 but changes with θ Central wavelength: (L/2γ 2 )/(1+ 2γ 2 θ 2 ) θ Second correction: higher B-field means stronger undulations and less on-axis electron speed. This changes γ so that: Central wavelength: (L/2γ 2 )/(1+ ab 2 )

18 Bending magnet emission spectrum: The (relativistic) rotation frequency of the electron determines the (Dopplershifted) central wavelength: λ o = (1/2γ 2 )(2πcm o /e)(1/b) The sweep time δt of the emitted light cone determines the frequency spread δν and the wavelength bandwidth: λ / λ o = 1 λ 0 λ λ A peak centered at λ c with width λ: is this really the well-known synchrotron spectrum? YES -- see the log-log plot: log(emission) λ o log(λ)

19 Synchrotron light polarization: Electron in a storage ring: TOP VIEW TIL TED VIEW SIDE VIEW Polarization: Linear in the plane of the ring, elliptical out of the plane Special (elliptical) wigglers and undulators can provide ellipticaly polarized light with high intensity

20 synchrotron radiation molecular atom or fragments molecule fragmentation spectroscopy scattered photons, fluorescence photoelectrons, Auger electrons small-angle scattering fluorescence spectroscopy photoelectron/auger spectroscopy transmitted absorption spectroscopy photons EXAFS Synchrotron x-rays: Many different interactions scattered photons scattering solid Many different fluorescence applications fluorescence spectroscopy photoelectrons, Auger electrons photoelectron/auger spectroscopy diffracted photons X-graphy transmitted absorption spectroscopy photons EXAFS Atoms & molecules desorption spectroscopy

21 European Integrating Initiative on Synchrotron Radiation and Free Electron Laser Science VIII Scuola Nazionale di Luce di Sincrotrone - Frascati 2005

22 Synchrotron Facilities in the World (2005): TOTAL: 72 in 24 Countries 62 Operating 10 under construction

23 Historical Growth Worldwide ISI data , Keyword: synchrotron VIII Scuola Nazionale di Luce di Sincrotrone - Frascati

24 and, for a broader picture: Hits from a Google search: Synchrotron 5,540,000 Free electron laser 3,020,000 Cyclotron 1,850,000 LINAC 1,860,000 Hadron collider 730,000 Protein crystallography 1,890,000 Synchrotron photoemission Viagra Pamela Anderson 14,

25 Photoelectron spectroscopy: basic ideas Formation of chemical bonds: atom solid electron energy The photon absorption increases the electron energy by hν before ejection of the electron from the solid Photoelectric effect: Photon (hν) electron energy hν Photoelectron

26 Photoelectron spectroscopy of high-temperature superconductivity: electrons superconducting state normal state energy The limited energy resolution of conventional photoemission makes it impossible to observe the phenomenon High-resolution spectra taken with ultrabright synchrotron radiation Energy (ev)

27 Superconducting gap spectroscopy: Different gaps in different directions (d-wave superconductivity) (Kelly, Onellion et al.

28 From spectroscopy to spectromicroscopy: Spectroscopy (energy and momentum resolution) Microscopy (spatial resolution) Chemical information Spectromicroscopy

29 The two modes of photoemission spectromicroscopy: SCANNING ELECTRON IMAGING x-y scanning stage sample X-ray lens sample X-rays e X-rays e Electron optics Electron analyzer

30 The ESCA Microscopy Beamline at ELETTRA VIII Scuola Nazionale di Luce di Sincrotrone - Frascati 2005

31 Photoelectron spectromicroscopy (on untreated specimens) beats optical microscopy + staining in revealing cell nuclei (B. Gilbert, M. Neumann, S. Steen, D. Gabel, R. Andres, P. Perfetti, G. Margaritondo and Gelsomina De Stasio) 20µm The distribution of nuclei in human glioblastoma tissue, revealed (left) by staining for optical microscopy and (right) by a MEPHISTO phosphorus map on ashed tissue (phosphorus is shown dark). The MEPHISTO section on gold had no treatment other than ashing. The imaged areas are in adjacent tissue sections, so the exact pattern of nuclei distributions is not identical.

32 Coherence: the property that enables a wave to produce visible diffraction and interference effects Example: screen with pinhole ξ source ( λ) θ fluorescent screen The diffraction pattern may or may not be visible on the fluorescent screen depending on the source size ξ, on its angular divergence θ and on its wavelength bandwidth λ

33 Longitudinal (time) coherence: source ( λ) Condition to see the pattern: λ/λ < 1 Parameter characterizing the longitudinal coherence: coherence length : L c = λ 2 / λ Condition of longitudinal coherence: L c > λ

34 Lateral (space) coherence analyzed with a source formed by two point sources: Two point sources produce overlapping patterns: diffraction effects are no longer visible. However, if the two source are close to each other an overall diffraction pattern may still be visible: the condition is to have a large coherent power (2λ/ξθ) 2

35 Coherence summary: Large coherence length L c = λ 2 / λ Large coherent power (2λ/ξθ) 2 Both difficult to achieve for small wavelengths (x-rays) The conditions for large coherent power are equivalent to the geometric conditions for high brightness

36 Some Problems in Conventional Radiology: Low absorption Limited contrast, may require a high x- ray dose Low-intensity, divergent beam

37 Light-matter Interactions: Absorption -- described by the absorption coefficient α Refraction (and diffraction/interference) -- described by the refractive index n For over one century, radiology was based on absorption: why not on refraction /diffraction?

38 Conventional radiology Refractive-index radiology

39 Refraction x-ray imaging: Edge between regions with different n-values detector Detected intensity Idealized edge image Real example (leaf)

40 Refraction x-ray imaging -- potential advantages over absorption: Differences between object and vacuum: small in both cases, but larger for n than for α This advantages increases as the wavelength decreases Better edge visibility, better contrast, smaller dose

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42 Examples of refraction radiology: X-rays images of a 0.5 mm live microfish

43 Building on bubbles (zinc electrodeposition): QuickTime et un décompresseur H.264 sont requis pour visionner cette image.

44 Localized Electrochemical Deposition (LECD): a novel technique for growing high-aspect-ratio nanostructures: Below a critical value of the microelectrode-structure distance, the growth rate increases dramatically but the grown microstructure becomes porous This effect depends on the applied voltage

45 Coherent x-ray micrographs of cells Cells in a leaf skin Neurons from a mouse brain Cultured fibroblast cells from a rabbit bone

46 Opening of a stoma QuickTime et un décompresseur H.264 sont requis pour visionner cette image.

47 A bit more sophisticated description Detector Coherent* source * Small & collimated Object In the actual image, each edge is marked by fringes produced by Fresnel edge diffraction. The fringes enhance the edge and carry holographic information

48 Modeling: interplay of refraction and diffraction Refraction radiographs Diffraction radiographs Note: with bending-magnet emission, the effects are only in the vertical direction (no space coherence in the horizontal direction)

49 Coherent x-ray tomography:

50 Phase contrast microtomography: microfossil Li Chai Wei, Yeukuang Hwu, Jung Ho Je et al.

51 Phase contrast micro-tomography: rat kidney QuickTime et un décompresseur H.264 sont requis pour visionner cette image. Yeukuang Hwu, Jung Ho Je et al.

52 Phase contrast micro-tomography: rat aorta QuickTime et un décompresseur H.264 sont requis pour visionner cette image. Yeukuang Hwu, Jung Ho Je et al.

53 New types of sources: Ultrabright storage rings (SLS, new Grenoble project) approaching the diffraction limit Self-amplified spontaneous emission (SASE) X-ray free electron lasers VUV FEL s (such as CLIO) Energy-recovery machines Inverse-Compton-scattering table-top sources

54 Take a standard photon source with limited brightness and no lateral coherence ξ with a pinhole (size ξ), we can extract coherent light with good geometrical characteristics (at the cost of losing most of the emission) However, if the pinhole size is too small diffraction effects increase the beam divergence so that: θ ξθ > λ No source geometry beats this diffraction limit

55 Infrared Electron, photon hν energy = γ X-ray photon hν γ γ Doppler effect: in the electron beam frame, the photon energy 2γ hν. This is also the energy of the backscattered photon in the electronbeam frame. In the laboratory frame, there is again a Doppler shift with a 2γ factor, thus: hν' 4γ 2 hν

56 Energy-recovery LINAC sources The brightness depends on the geometry of the source, i.e., of the electron beam In a storage ring, the electrons continuously emit photons. This warms up the electron beam and negatively affects its geometry Controlling the electron beam geometry is much easier in a linear accelerator (LINAC). Thus, LINAC sources can reach higher brightness levels

57 Energy-recovery LINAC sources However, contrary to the electrons in a storage ring, the electrons in a LINAC produce photons only once: the power cost is too high Solution: recovering energy Accelerating section Energyrecovery section

58 Example: Kulipanov s super-microtron ER LINAC Wiggler

59 THE 4 GLS CONCEPT AT DARESBURY VIII Scuola Nazionale di Luce di Sincrotrone - Frascati 2005

60 Free-electron lasers: VIII Scuola Nazionale di Luce di Sincrotrone - Frascati 2005

61 Free-electron laser surgery: Wavelength selection much less collateral damage:

62 The scanning near-field optical microscope (SNOM): like the stethoscope Heart: Frequency Hz Wavelength λ 102 m Accuracy in localization 10 cm λ /1000 Small aperture Coated small-tip optics fiber Small distance Microscopic lightemitting object SNOM resolution: well below the diffraction limit of standard microscopy ( λ)

63 SNOM: why does it work? Consider two slits: y k y > 2π y > 2π/ k y k y < k y = (k 2 -k x2 ) Wave, k = 2π/λ y x k x real k y < k = 2π/λ y > λ (diffraction limit) However, for k x imaginary the condition does not apply and y < λ becomes possible After a narrow optics fiber tip, there is an evanescent wave with imaginary in the x-direction k x

64 20x20 µm 2 SNOM image of growth medium (A. Cricenti et al.): Intensity line scan λ = 6.6 µm S-O & N-O vibrations (λ = 6.95 µm) SNOM topography Resolution 0.15 µm << λ

65 Self-amplified spontaneous emission x-ray free-electron lasers (SASE X-FEL s) Normal (visible, IR, UV) lasers: optical amplification in amplifying medium plus optical cavity (two mirrors) X-ray lasers: no mirrors no optical cavity need for one-pass high optical amplification SASE strategy: electron bunch Wiggler LINAC (linear accelerator) The microbunching increases the electron density and the amplification and creates very short pulses

66 Seeding-Amplifier X-FELs Electron Beam Bypass Electron Beam First Wiggler (SASE Emitter) Monochromator Second Wiggler (Amplifier) Electron Dump Photon Beam

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68 First Real Experiments at the TESLA X-FEL s VIII Scuola Nazionale di Luce di Sincrotrone - Frascati 2005

69 SASE X-FEL s: superbright (orders of magnitude more than present sources) femtosecond pulses: New chemistry? One-shot crystallography (no crystals)? Total coherence Unprecedented electromagnetic energy density Is this vacuum?

70 New physics? Consider the parameters of the Swiss Light Source: Circumference: 288 m Single Bunch Current: 10-4 A Bunch Length: 4 x 10-3 m Electron speed c 3.3 x m/s The charge per bunch is 10-4 x 288/(3 x 10 8 ) 10-8 coulomb, corresponding to electrons.the horizontal bunch size is < 2 x Assuming 0.1% coupling, the bunch volume is < 1.6 x m 3. Thus, the electron density exceeds 4 x cm -3. What is this: a gas of independent electrons? Or a correlated multiparticle system? What kind of thermodynamics should we use? The covalent form of thermodynamics is still an open issue! For example: T can be defined using the entropy law or the equipartition principle. The two definitions are equivalent in classical physics, but in relativity they lead to different Lorentz transformations of T!

71 Conclusions: 1. The technology of storage rings and FEL's solved the ancient problem of brightness. 2. The brightness increase was so rapid that applications are still trailing behind. 3. Nevertheless, many exciting results were obtained, for example in spectromicroscopy and high resolution spectroscopy. 4. The most important new achievements will be linked to the interdisciplinary use of photon sources exporting physics and chemistry techniques to medical research and the life sciences in general. 5. Coherence-based applications will play a special role. 6. New FEL's like the SASE machines are beyond imagination: towards one-shot crystallography?

72 Thanks: The EPFL colleagues (Marco Grioni, Davor Pavuna, Laszlo Forro Mike Abrecht, Amela Groso, Luca Perfetti, Eva Stefanekova, Slobodan Mitrovic, Dusan Vobornik, Helmuth Berger, Daniel Ariosa...). The POSTECH colleagues (group of Jung Ho Je). The Academia Sinica Taiwan colleagues (group of Yeukuang Hwu). The Vanderbilt colleagues (group of Norman Tolk). The ISM-Frascati colleagues (groups of Antonio Cricenti and Paolo Perfetti) The facilities: PAL-Korea, Elettra-Trieste. Vanderbilt FEL, SRRC-Taiwan, APS-Argonne, SLS- Villigen, LURE-Orsay

73 In 1905, Albert Einstein published his landmark articles about relativity, the photon (and the photoelectric effect) and the Brownian motion (demonstration of the existence of atoms and molecules) In our research, we use relativity to produce x-rays (photons) and use them, for example performing experiments with the photoelectric effect, to study solids, atoms and molecules HAPPY 2005!!! A final remark: