Synchrotron radiation: A charged particle constrained to move in curved path experiences a centripetal acceleration. Due to it, the particle radiates

Transcription

1 Synchrotron radiation: A charged particle constrained to move in curved path experiences a centripetal acceleration. Due to it, the particle radiates energy according to Maxwell equations. A non-relativistic particle emits radiation primarily at its frequency of revolution, with the characteristic pattern shown in figure 1(a). However, as the speed of the particle approaches the speed of light, the radiation pattern is distorted by relativistic effects and shrinks to a narrow cone of with angular spread φ (see b). The latter type of radiation is called synchrotron radiation a b

2 The relativistic electron has an energy o 1 With: γ = 2 v 1 2 c Synchrotron radiation is emitted in a cone of width θ=γ -1 For an orbital radius R b, emission occurs up to an energy Dipole radiation of accelerated charge E =γm c 2 E c = 3cγ 2R b 3 Frame of reference moving with the electrons k Θ k z Laboratory frame of reference k k x =k x k' = 2π λ' θ = k k x z = k x k z =2γk z k' x 2γk tanθ ' 2γ = = ' z 1 2γ

3 Lay-out of Synchrotron light source Light is extracted with insertion devices bending magnet A wiggler consists of a periodic series of magnets, placed in a ring section where the electron path would otherwise be straight; because of its action, the electrons are forced to oscillate around the straight path. The undulator result is a very high flux of X-rays along the wiggler beamline. An undulator is similar to a wiggler except that it forces the electrons into a much weaker zig-zag, so that during the entire zig-zag motion synchrotron light continues to illuminate the undulator beamline. The result is a longer pulse of light rather than a series of short bursts. Without short pulses, there is no wide band of wavelengths, thus the undulator emission is not spread in a wide band but concentrated, producing high levels of flux and brightness. The technical specification of undulators and wigglers make it possible to perform record levels of flux and brightness.

4 Three possibilities: Bending magnet, wiggler, undulator Synchrotron Radiation Emitted by a Dipole Bending Magnet Synchrotron Radiation Emitted by a Wiggler Array of Magnets

5 Double relativistic effect Note the K 2 term, determined by the magnetic field intensity,b 0, which allows to tune the undulator frequency

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10 Wiggler Undulators and Wigglers Energy (ev) Light pulses Dipole Wiggler Undulator Continuous Light

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12 Monochromators

13 Focussing X-rays: Mirrors

14 Fresnel Zone Plate

15 Focussing X-rays: Fresnel zone plates

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17 Brillance: definition Brightness is defined as radiated power per unit area and per unit solid angle at the source P B= A Ω Brightness is a conserved quantity in perfect optical systems, and thus is useful in designing beamlines and synchrotron radiation experiments which involve focussing to small areas.. B ω / ω = P A Ω ω / ω. Spectral brightness is that portion of the brightness ω/ω lying within a relative spectral bandwidth

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19 Time evolution of achieved brightness Note that Avogadro s number is 10 23

20 Spectral Intensity Distributions for a Variety of Standard Photon Sources and Synchrotron Radiation Sources

21 SPring 8 Japan

22 High resolution beamline Beamlines

23 Injector Lay-out of ELETTRA (Trieste, Italy)

24 ELETTRA is an example of synchrotron radiation source of third-generation, i.e. explicitly dedicated to produce light characterized by high brilliance

25 Synchrotron Radiation (SR): Basic Features Bending Magnets vs. Insertion Devices (Undulators, Wigglers) Full Exploitation of Selection Rules Photon Energy (hν) Tunability: Surface vs. Bulk hν-dependent Cross Sections (PDOS) ARPES Band Mapping Resonant Photoemission Cooper Minimum Photoemission SR Polarization: Linear, Circular, Elliptical Time Structure: Pulse Width ps Bunch-to-Bunch 1-1,000 ns High Brilliance: High Photon Fluxes Low Concentration Systems Tiny Specimens Fast Photoemission High E-Resolution Spectromicroscopy

26 Focalization spatial resolution Microscopy and spectromicroscopy with X Rays nm

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30 Ad Elettra: Lunghezza di coerenza 250 micron a E=17 kev