Proceedings of IBIC2012, Tsukuba, Japan

MOPB72

FIRST MEASUREMENTS WITH CODED APERTURE X-RAY MONITOR
AT THE ATF2 EXTRACTION LINE
J.W. Flanagan, M. Arinaga, H. Fukuma, H. Ikeda, T. Mitsuhashi, KEK, Tsukuba, Japan
G.S. Varner, U. Hawaii, Honolulu, HI USA
Abstract

INTRODUCTION
Coded aperture imaging is a technique well-developed
among x-ray astronomers, which consists of using apseudorandom array of pinholes, which project a mosaicof pinhole camera images onto a detector[1]. This imageis then decoded using the known mask pattern toreconstruct the original image. One example of such apattern is the Uniformly Redundant Array (URA)[2],
which features an open aperture of 50% with an evensampling of spatial frequencies in the non-diffractive
limit. This provides the spatial resolution somewhat better
than that of a pinhole camera, but with much greater x-ray
photoncollection
efficiency
forsingle-shotmeasurements. Coded aperture x-ray imaging has beentested at CesrTA, with beam sizes down to about ~10
microns[3], and it is planned to use it at SuperKEKB[4].
An x-ray beam line has been constructed in the ATF2
extraction line, with the goal of testing coded aperture
measurements with beam sizes down to ~4-5 m.

Figure 1: Location of x-ray beamline in the ATF2
extraction line.
The extraction chamber for the x-ray line has a 200 m
thick, polished beryllium window, with ATF2 vacuum on
one side and atmosphere on the other side. Just
downstream of the beryllium window is the coded
aperture mask mounted on a rotating stage, which is thenmounted on a 2-axis translational stage. Downstream ofthe mask is a vacuum chamber with 25 m thick kapton
at both the entrance and exit. There is a 6.4 cm air gap
between the beryllium window, and entrance window of
the second vacuum chamber, and a 1.5 cm air gap
between the second chamber and the detector at the far
downstream end, which is, like the mask, mounted on arotating stage and 2-axis translational stage, on an opticaltable. Mask and detector mounts are shown in Fig. 3.
The distance between the source point and the mask is
193.5 cm, and the distance from the mask to the detector
is 835.7 cm.

BEAMLINE LAYOUT
The beamline is located off the last bend (BH3X) after
the extraction point from the ATF ring, at the upstream
end of the ATF2 line, as shown in Fig. 1. The bending
radius of this magnet is 4.3 m, and the beam energy is
1.28 GeV, for a critical energy of 1.1 keV. The beam sizeat this bend is typically around 20 microns, but can be
reduced, by lowering the beta function, to around 4-5
microns.[5] This bend is also used as the source for avisible-light SR monitor.

Figure 2: Views from downstream (left) and upstream(right) during construction of x-ray beamline at ATF2.
The downstream vacuum chamber is only pumped
down to a few tens of Pa with a scroll pump; its function
is to provide a low-loss path for the x-ray beam, and it isseparate from the accelerator beam vacuum. The total
effective air path length, including the air gap around themask and between the downstream kapton window andthe detector, is ~10 cm.

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1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The ATF2 extraction line is used as a test-bed fortechnologies needed for the ILC final-focus region. An xray extraction beam line has been constructed at the final
upstream bend before the extraction line straight section,
for development and testing of optics and readout systems
for a coded aperture-based imaging system. The x-ray
monitor is expected to eventually be able to measuresingle-shot vertical bunch sizes down to a few microns insize at its source location in the ATF2 extraction line.
Preliminary scanned measurements have been made withbeams in the ~15 micron range, and it is planned to makemore measurements with further-tuned beam, and with
fast read-out electronics. The details of the layout,
expected performance, and preliminary measurement
results will be presented.

MOPB72

Proceedings of IBIC2012, Tsukuba, JapanThe calculated image pattern in the detector plane forvertical beam sizes of from 1 to 20 microns is shown inFig. 4 The coded aperture mask is a 47-element arraywith 5-micron minimum feature size with a tantalummask material on a 2.5 micron silicon carbide substrate,
made by NTT-AT Nanofabrication.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 4: Simulation of expected beam patterns for 50micron pitch detector at different beam sizes.

Figure 3: Rotating mask holder (above), and detector on
rotating stage (below). Next to detector are a PMT with
Ce:YAG scintillator and a CCD for initial alignment.

The minimum beam size possible at the x-ray monitor
source point is expected to be around 4.5 m. Figure 4
shows the resolution contours for the 47-element coded
aperture mask, for 1 nC bunches, which produce anaverage of ~250 photons per pixel per shot on theFermionics detector. As can be seen in Figure 5, singleshot statistical resolutions of ~1 m can be expected forminimum beam sizes around 4 m.

The detector is an InGaAs array produced by Fermionics,Inc., which features 25 μm ×500 μm pixels on a 50 μm pitch. The thickness of the InGaAs layer is 3.5 μm,
which captures about 90% of the flux that reaches it after
passing through the diamond window, and other layers on
the chip above the InGaAs layer. For future bunch-bybunch use at the ATF2 and SuperKEKB, a high-speed
readout is being developed, but for preliminary checkingof the x-ray system, a single Fermionics pixel wasmechanically scanned across the imaging plane while
being read out with an oscilloscope.

EXPECTED PERFORMANCEAt low intensities, the resolution of the system islimited by statistical fluctuations in the number ofdetected photons. To estimate the resolution of thesystem as a function of beam size, simulated images arecalculated for Gaussian beams of various sizes. The
simulated detector images for different-sized beams arethen compared pair-wise against each other using residual
weighting function for each channel proportional to thesquare of the signal height in that channel. The 2/ valuethat corresponds to a confidence interval of 68% is chosen
to represent the 1-s confidence interval.[6]Figure 5: Calculated 1- statistical resolution contours for
47-element CA with 5 m elements at the ATF2 with 1
nC bunches.
The single-shot readout electronics for the 64-channel
detector are under development. For the present time, asingle pixel is scanned across the beam to make a slowscan image. The shot-to-shot vertical orbit jitter of theATF2 beam is about 10% of the beam size[7], which issufficient for basic beam size measurements. When thesingle-shot readout system is ready, we can then proceed
to measurements of the single-shot resolution, as
determined by photon statistics and readout noise.

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Proceedings of IBIC2012, Tsukuba, JapanPRELIMINARY MEASUREMENTS
OpticsMOPB72

measurements taken during the coupling scan are shownin Fig. 9.

T. Okugi of the ATF optics group designed a set ofoptics for the ATF2 extraction line which minimized thebeam size at the x-ray source bend, BH3X, shown in Fig.
6. The vertical beta function at the source point is 1 m,
which for a vertical emittance of 20 pm-rad (1% of thehorizontal emittance) gives a vertical beam size of 4.5 m.
(The horizontal beam size is 260 m.) The set of optics
required to reach these parameters is incompatible withregular ATF2 operations, which are targeted atminimizing the beam size at the end of the ATF2 line, so
dedicated machine time is needed to carry out thismeasurement. One shift was allotted for this purpose on
23 Feb., 2012.

Figure 6: Small beam-size optics at BH3X (Courtesy ofT. Okugi).
Figure 8: Dispersion knob scan.

Measurements
A dispersion knob and rotation knob were available totune the beam at the source point, along with an x-y
coupling knob. After setting up the initial optics, andbefore tuning the knobs, the coded aperture mask wasaligned with the x-ray beam, and a detector scan wastaken to verify the coded aperture pattern was detected, asshown in Fig. 7. The initial, unturned beam size at thispoint was about 15 microns (hand fit, based on peakvalley modulation).
Next, a dispersion knob scan was carried out,
maximizing the peak-valley ratios using 50-micron-stepscans. The dispersion knob was set to the value which
maximized the peak-valley ratio (+0.25 machine units).
The measurements taken during the dispersion scan are
shown in Fig. 8.
Following this, the coupling knob was scanned, and set
to the optimum value (0 machine units).
TheFigure 9: Coupling knob scan.
ISBN 978-3-95450-119-9

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3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 7: Pre-tuning scan, corresponding to about 15
micron beam size (hand fit).

MOPB72

Proceedings of IBIC2012, Tsukuba, Japan

Finally, a mask angle scan was performed, and themask angle set to the optimum angle, which was found tobe at 1 degree from the initial orientation. The mask scanmeasurements are shown in Fig. 10.

SUMMARY AND PLANS FOR THE
FUTURE
Preliminary measurements have been taken with acoded aperture mask at the ATF2 in slow-scanning mode.
Beam sizes of around 10-15 microns were measured.
Further improvements to the optics are considered
possible, and will be tried in future studies in order to try
to reach and demonstrate measurements of ~4-5 micron
beams. In addition, a 64-channel fast-readout is being
developed, which will enable the quantitative
measurement of the predicted ~1 m single-shot
statistical resolution at such small beam sizes. The beamline will also continue to be used for testing detector andreadout systems for SuperKEKB.

ACKNOWLEDGMENTS

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 10: Mask angle scan.
At the end of the knob scans and mask angle scan, adetector slow-scan data set was taken using 25-micron
steps (Fig. 11). The final beam size was around 10
microns, based on maximum peak-valley ratio (hand fit).

Figure 11: Final scan, using 25-micron
corresponding to about 10 microns (hand fit).

We would like to thank the ATF/ATF2 group forallowing us to set up a beamline there, and for allowing a
dedicated run to take the preliminary data. In particular,
we would like to thank T. Naitoh for his help with set-up
issues, and T. Okugi for his optics calculations and tuning
efforts during the study.

REFERENCES[1] R.H. Dicke, Astrophys. Journ., 153, L101, (1968).
[2] E.E. Fenimore and T.M. Cannon, Appl. Optics,
V17,No. 3, p. 337 (1978).
[3] N. Rider, et al., these proceedings.
[4] J.W. Flanagan, et al., Proc. 7th Meeting of Particle
Accelerator Society of Japan, Himeji, p. 618 (2010).
[5] T. Okugi, private communication.
[6] J.W. Flanagan,et al., Proc. DIPAC2011.
[7] T. Okugi, private communication.

steps,

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