1. 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

2. 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

3. 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

4. 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

5. 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

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

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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

14. No foil
Iron foil
Nickel foil
Silver foil

15. 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

16. 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?

17. 72 MeV
70 MeV
68 MeV

18. 66 MeV
65 MeV
64 MeV

19. At what observation angle do we see 7.11 keV photons?

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

21. 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

22. Pinhole flux and BW
Quantitative value of angular smearing due to scattering through two, 250 μm Be-windows and 50 μm Fe-foil???
Need 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

23. 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???

24. 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

25. 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!