Generating X-rays and EUV radiation at ASTA Tanaji Sen March 5, 2015
Outline • Channeling Radiation • Parametric X-rays (PXR) • PXR while channeling • Phase contrast imaging • X-rays and EUV with nanostructures T. Sen X-arys and EUV at ASTA 2
Goals of X-ray generation • Create a source of brilliant, monochromatic and tunable X-rays. Increase spectral brightness by orders of magnitude • Use the X-rays for applications; especially phase contrast imaging • Establish ASTA as a model for a compact X-ray source with X-band linacs • Establish ASTA as a user facility based on applications T. Sen X-arys and EUV at ASTA 3
Radiation sources for X-rays • Channeling radiation • Parametric radiation • Compton Scattering • Transition radiation • Synchrotron radiation • Coherent Bremsstrahlung, ---- Source Beam Energy 10 keV X-rays Synchrotron Radiation 3 GeV Transition Radiation 300 MeV Compton Scattering 22 MeV Channeling Radiation ~ 10 MeV Parametric Radiation 5.7 MeV T. Sen X-arys and EUV at ASTA 4
Channeling Radiation Particles are channeled within a critical angle Energy of X-rays E X ~ 2 γ 2 ( 𝜁 𝑗 − 𝜁 𝑔 )/(1+ γ 2 θ 2 ) QM needed for 𝐹 𝑓 < 100 MeV For planar channeling, eigenvalues 𝜁 found by solving 1D Schrodinger’s equation using the Doyle-Turner form of the atomic potential T. Sen X-arys and EUV at ASTA 5
Crystal planes (111) plane (100) plane (110) plane • (110) plane has a deeper potential than (100), higher energy X-rays • (111) plane has more bound states, broader X-ray spectrum • Preferred plane is (110) T. Sen X-arys and EUV at ASTA 6
Photon Yields • Radiative transitions : apply Fermi’s golden rule Selection rule: transitions only between states of opposite parity, |m-n| = Odd • Non-radiative transitions, mainly inelastic thermal scattering off lattice vibrations, determine population P n (z) • Approximate selection rule: |m-n| = Even T. Sen X-arys and EUV at ASTA 7
Photon Angular Distribution Dipole moment Lorentzian line shape J.U. Andersen et al, (1983) Population in nth state Photon absorption T. Sen X-arys and EUV at ASTA 8
Line Width • Intrinsic line width from finite lifetime of channeling states • Bloch wave broadening from finite width of energy bands, • Multiple scattering: scattering in planar channels weaker than in amorphous media • Energy spread of electron beam • Detector resolution T. Sen X-arys and EUV at ASTA 9
Experiment at ELBE (2007) FZDR, Rossendorf, Germany • Observed CR at beam energies from 14.6 MeV to 30 MeV • Beam current ~ 100 nA • Transverse emittance = 3 μ m Now in ASTA beamline Wagner et al (2007) T. Sen X-arys and EUV at ASTA 10
ELBE Measurements & Simulations Energy Thickness Exp yield Theor. Yield Exp/Theor [MeV] [ μ m] [phot/sr-e] [phot/sr-e] 14.6 42.5 0.048 0.11 0.45 168 0.090 0.22 0.41 17 42.5 0.059 0.15 0.39 168 0.13 0.29 0.45 25 42.5 0.16 0.32 0.50 30 42.5 0.23 0.45 0.51 168 0.52 0.89 0.59 Azadegan (2007) T. Sen X-arys and EUV at ASTA 11
Dechanneling T. Sen X-arys and EUV at ASTA 12
Dechanneling Model T. Sen X-arys and EUV at ASTA 13
Rechanneling, Yield Saturation Yield saturates beyond ~7 L occ T. Sen X-arys and EUV at ASTA 14
Improvements to Model • Introduced heuristic dechanneling model • Corrected potential V I for thermal scattering; affects transition rate between states • Include effects of a finite beam divergence • Inclusion of linewidth contributions from multiple scattering, Bloch wave broadening • Correction to the line shape in the intensity spectrum • Not included: electron-atomic electron scattering contribution to linewidth T. Sen X-arys and EUV at ASTA 15
Updated ELBE calculations • Theory bounds for 42.5/168/500 μ m • Lower line : dechanneling from 1 st /7 th /14 th free state and above. • Upper line: dechanneling from 3 rd /9 th / 15 th free state and above • Importance of rechanneling (free state to bound state) for thicker crystals T. Sen X-arys and EUV at ASTA 16
ASTA parameters Parameter Value Beam energy 20 – 50 MeV Bunch charge ~ 20 pC (low charge operation) Bunch frequency 3 MHz Average beam current 300 nA Transverse emittance < 100 nm Bunch length 3 ps Energy spread < 1% Crystal, plane Diamond, (110) Critical angle 1.5 mrad(20 MeV), 1mrad(50MeV) T. Sen X-arys and EUV at ASTA 17
Real and Imaginary Potentials 20 MeV • Depth of real potential about 24 eV 50 MeV • Energy bands differ by few eV in rest frame, transform to 10’s of keV in lab frame • Corrected imaginary potential is weaker, reduces non-radiative transition rates, reduces intrinsic line widths T. Sen X-arys and EUV at ASTA 18
Intensity spectra 20 MeV, 168 μ m 20 MeV, 42.5 μ m 50 MeV, 42.5 μ m 50 MeV, 168 μ m T. Sen X-arys and EUV at ASTA 19
Brilliance at ASTA 𝐅𝐬𝐠[ 𝜄 𝐷 𝐂 𝐎 = 𝐞 𝟑 𝐉 𝐛𝐰 𝐟 𝛅 𝟑 (𝝉′ 𝒇 ) 𝟑 𝐅 𝑌 ] 𝟐𝟏 −𝟒 𝛝 𝐎𝟑 𝐎 𝐞𝛛𝐞𝛁 𝟑𝝉 ′𝒇 𝑡 − mm − mrad 2 − 0.1% BW photons T. Sen X-arys and EUV at ASTA 20
Increasing the brilliance • Expected brilliance with 100nm about 10 4 x ELBE • Field emitter nanotip cathode : emittance ~ 10nm • Tested at A0-HBESL • Brilliance would increase by ~100 • Thicker crystals may help P. Piot et al (2014) T. Sen X-arys and EUV at ASTA 21
ASTA Channeling Experiments • Initial studies will look at CR dependence on electron beam parameters: energy, bunch charge, emittance, spot size, energy spread. Operate initially with low bunch charge (~ 20 pC), fewer bunches. T. Sen X-arys and EUV at ASTA 22
Channeling Summary • Updated model results in better agreement with measurements at ELBE. • Yields agree within 15%, linewidth to a factor of 2 because of neglecting e- <-> atomic e- scattering • Occupation length & rechanneling increase with thickness, but insensitive to particle energy. • Yield saturates at ~ 7x Occupation length for 1-> 0 transition. • ASTA 50 MeV beam: 142 keV from 1-> 0, 89 keV from 2-> 1 transition. Linewidth around 14% • Expected brilliance ~ 3x10 10 phot/(s-(mm-mrad) 2 -0.1% BW) with transverse emittance 100nm, crystal t=168 μ m • Field emitter cathodes could increase brilliance another 100 times T. Sen X-arys and EUV at ASTA 23
Parametric X-Rays 𝑒( 1 𝛾 −𝑜 𝑑𝑝𝑡𝜄 𝐸 ) 𝜕 = 2𝜌𝑛 • Phase difference = sin𝜄 𝐶 𝑑 • Constructive interference of virtual photons around 𝜄 𝐸 = 2𝜄 𝐶 • PXR photon energy at this angle 𝐹 = 𝑛ℏ𝑑τ/(2𝑡𝑗𝑜𝜄 𝐶 ) • Independent of electron beam energy M.L. Ter-Mikaelian, V. Baryshevsky (1970s) T. Sen X-arys and EUV at ASTA 24
PXR Features • Emitted at large angles from the beam direction. Differentiates it from channeling, bremsstrahlung • X-ray energy is independent of beam energy • Tunable by changing crystal orientation • Narrow line width. Typically ~ 1% • Photon energy depends on detection angle (spatial dispersion); collimation further reduces energy spread • Lower intensity than channeling radiation • Lower background from other sources, especially at large angles. • Similar angular spectrum as transition radiation T. Sen X-arys and EUV at ASTA 25
Crystal Geometries T. Sen X-arys and EUV at ASTA 26
Spectral Angular Distribution T. Sen X-arys and EUV at ASTA 27
Spectral angular distribution - 2 T. Sen X-arys and EUV at ASTA 28
PXR with 8 MeV beam Freudenberger et al, Phys. Rev Lett 74, 248 (1995) • Angular distribution similar to transition radiation. Minimum at the Bragg angle, maxima at (1/ γ )on either side of 2𝜄 𝐶 . • Intensity scales ~ quadratically with beam energy in this range. Saturates at higher beam energy 𝛿 > 𝜕/𝜕 𝑞 T. Sen X-arys and EUV at ASTA 29
Linewidth: geometric contributions Significan t T. Sen X-arys and EUV at ASTA 30
Linewidth: multiple scattering T. Sen X-arys and EUV at ASTA 31
Comparisons with experiments - 1 T. Sen X-arys and EUV at ASTA 32
Comparisons with experiments - 2 Not included: detector efficiency, multiple scattering before crystal T. Sen X-arys and EUV at ASTA 33
ASTA: PXR T. Sen X-arys and EUV at ASTA 34
ASTA – PXR while channeling Does not require rotations T. Sen X-arys and EUV at ASTA 35
ASTA: New goniometer Available ports determine X-ray energies Yield in photons/(el-sr) T. Sen X-arys and EUV at ASTA 36
Spectral Brilliance Comparison Y. Hayakawa et al, J. Inst. (2013) Absorption Phase Contrast LEBRA, Nihon University, Japan LEBRA (2013) ASTA Beam energy 100 MeV 50 MeV Average beam current 1-5 μ A ~ 200 nA Normalized emittance 15 μ m 100 nm Crystal Silicon Diamond X ray energy 6.5 – 34 keV (220 ) 9.8 keV (400 plane) 1.6x10 -6 2.3x10 -6 Photon yield/el 1.5x10 5 1.9x10 9 Spectral brilliance T. Sen X-arys and EUV at ASTA 37
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