Oliver Williams University of California, Los Angeles Characterization of the BNL ATF Compton X-ray Source via K-edge Absorbing Foils Oliver Williams University of California, Los Angeles

ICS Propaganda Laser wavelength ~ undulator period; allows for very high energy photons with a “low” energy e-beam Easy to change source polarization (linear or circular) Gamma rays for polarized positrons Sub-picosecond pulses Energy tuning Real estate and cost far below 3rd generation sources

ICS Biography at the ATF The Accelerator Test Facility at Brookhaven has been in the ICS business for a decade First photons were in 1999 First observation of the 2nd harmonic was in 2005 (PRL in 2006) ICS had been on vacation for 3 years, until a few months ago But the expertise remains…after re-establishing the Compton beamline, first shots produced many photons

Motivation for Experiments at the ATF Attempting to do initial characterization of source to verify: 1) Photon energy 2) Angular distribution 3) Flux 4) Bandwidth (without spectrometer, if possible!) 5) Change in conditions for sub-ps pulses 6) Shot-to-shot source stability Effects on bandwidth are interesting: long/short laser pulses, laser energy, and beam spot sizes at the interaction point (IP) Initial applications: K-edge contrast imaging, polarization dependent material studies, and dynamic diffraction/pump-probe experiments

Energy/Angular distribution without a spectrometer? Strong photon absorption near element K-edge Foil thickness + K-edge = low-pass filter Above K-edge, photons absorbed, others pass

Photon Energy (eV) Transmission 7108 0.13329 7109.5 0.13345 7111 0.1336 7112.5 1.04E-07 7114 7115.5 1.05E-07 7117 1.06E-07

How does this affect the source transverse profile? Like undulator radiation, red-shift off-axis for an observation angle, θ Produces a “donut” pattern: on-axis photons lie above the K-edge and are strongly attenuated Photons off-axis are below the K-edge and are transmitted through foil Higher γ => Higher Ex => More photons off-axis above K-edge => Bigger donut hole

Experimental Parameters Electron Beam Parameter Value Energy 64-72 MeV Beam size (RMS) 50 μm Bunch length (FWHM) 4 ps Emittance 2 mm-mrad Charge 200 pC Laser Beam Parameter Value Laser energy 2 J Waist size (RMS) 120 μm Pulse length (FWHM) 6 ps Wavelength 10.6 μm Laser potential (aL) 0.33 X-rays Parameter Value Total Photons (per shot) 2 x 108 Energy (on-axis) 7-8.9 keV Pulse length 4 ps Opening angle 8-7 mrad

Making the photons Alignment probe now replaced with retractable 120 μm pinhole and Ge-wafer on micro-positioner

Alignment and synchronization at the IP Pinhole aligned to e-beam HeNe CO2 laser aligned to pinhole e-beam transmission maximized through pinhole After moving Ge-wafer in e-beam path, 10.6 μm absorbing semiconductor plasma created with formation length approx. e-beam bunch length If laser arrives before e-beam, transmitted through Ge Change CO2 delay line to overlap beams in time Arrival of e-beam No e-beam

Analyzing the photons 250 μm Be-window Insertable Ni and Fe foils MCP image intensifier 1 mrad pinhole on remote 2-axis control Remotely insertable Si-diode detector 250 μm Be-window

Microchannel Plate (MCP) Image Intensifier 8 μm pore/pixel size 40 mm detection area ~few % detection efficiency at keV photon energies Very high gain: 107 (allows “single photon detection”) Voltage required for us to see an image after passing through foil: Phosphor screen = 6 kV MCP stack = 1.5 kV Our MCP accepts: Phosphor screen = 6.5 kV MCP stack = 2.4 kV Lots of potential gain remaining, needed for viewing scattered/diffracted x-rays in application experiments

First Run Start with an e-beam energy of 72 MeV (~8.9 keV ICS photons) Guarantees being above K-edge of both iron (7.11 keV) and nickel (8.33 keV) for red-shifted off-axis photons Laser is linearly polarized

No foil Iron foil Nickel foil Silver foil

What’s going on? See asymmetry in angular and energy distribution Ellipticity due to fairly high “K” of linearly polarized laser, fanning out emission along polarization axis “Hot spots” show photons just below K-edge for chosen foil Null shows angular/energy correlation: these photons are K-edge energy or higher See preferential attenuation of low energy photons off-axis through Ag foil

Second run Scan the electron beam energy down from 72 MeV until no donut is formed Does this correspond to expected position of “hot spots”, just below the K-edge of iron, given the (γθ)2 red-shift dependence?

72 MeV 70 MeV 68 MeV

66 MeV 65 MeV 64 MeV

At what observation angle do we see 7.11 keV photons?

K-edge transition across pinhole placed off-axis 65.4 MeV 66.1 MeV 66.4 MeV

Most recent run-circular polarization/compression **Disclaimer: Driver was less experienced beam operator unfamiliar with experiment, could have caused “dirty” beam due to much smaller spot at IP and therefore larger angles** 68 MeV, 4 ps FWHM e-beam (2Δγ/γ=ΔE/E=1%) 68 MeV, 150 fs FWHM e-beam (2Δγ/γ=ΔE/E=2%) X-rays clipped passing through hole in laser mirror

Pinhole flux and BW Quantitative value of angular smearing due to scattering through two, 250 μm Be-windows and 50 μm Fe-foil??? Need 65-66 MeV e-beam to create 1 mrad donut hole (with few photons visibly on-axis) 66 MeV = 7490 eV photons => 5% Measured ~1x106 photons through 1 mrad pinhole placed on-axis Gives 106 photons/pulse (ΔE/E ≤ 5%) over 1 mrad

Future Plans Map bandwidth by small interval energy scan over pinhole and foil, recording flux Differences in source using smaller aL, changing angles in e-beam? K-edge contrast imaging using phantoms: iron wires embedded in material…letter written in blood???

Summary Resurrected ICS from its grave on Long Island Demonstrated production of Compton photons of known energy, from 7 keV to over 8.3 keV Mapped the angular and energy distribution of ICS photons at BNL ATF Familiarization with source running conditions in preparation for K-edge imaging experiments Quantified 106 usable photons over 1 mrad on-axis with ~5% BW

Thanks! UCLA guys: James Rosenzweig Gerard Andonian Erik Hemsing Brookhaven guys: Vitality Yakimenko Igor Pogorelsky Karl Kusche Jiangho Park Don Davis And the rest of the staff!