high resolution uv, alpha and neutron imaging with the timepix cmos readout
DESCRIPTION
High resolution UV, Alpha and Neutron Imaging with the Timepix CMOS readout. J. Vallerga, J. McPhate, A. Tremsin and O. Siegmund Space Sciences Laboratory University of California, Berkeley. Desire for better detector spatial resolution. Optics cannot always solve problem - PowerPoint PPT PresentationTRANSCRIPT
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9th IWoRiD: Erlangen,July 2007, John Vallerga
High resolution UV, Alpha and Neutron Imaging with the Timepix CMOS readout
J. Vallerga, J. McPhate, A. Tremsin and O. Siegmund
Space Sciences Laboratory
University of California, Berkeley
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Desire for better detector spatial resolution
• Optics cannot always solve problem
– Limits on physical size of detector is often fixed e.g. Space-based imaging (mass, optical focal
length)– Projection length where optics don’t work
• Make pixels smaller rather than detector bigger
• Ultimate limit is size of interaction region of radiation of interest
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Imaging, Microchannel Plate Detectors
Charge distribution on stripsCharge CloudMCP stackTube Window withphotocathodeγ
Photocathode converts photon to electron
MCP(s) amplify electron by 104 to 108
Rear field accelerates electrons to anode
Patterned anode measures charge centroid
MCP neutronelectron alpha
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Medipix/Timepix ASIC readout
• 256 x 256 array of 55 µm pixels
• 100 kHz/pxl
• Frame rate: 1 kHz
• Low noise (<100e-) = low gain operation (10 ke-)
• ~1 W watt/chip, abuttable
• Developed at CERN
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9th IWoRiD: Erlangen,July 2007, John Vallerga
“Time over Threshold” = ADC
Clock
Thresh.
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Timepix version of Medipix
Amplitude rather than counts using “time over threshold’ technique
If charge clouds are large, can determine centroid to sub-pixel accuracy
Tradeoff is count rate as event collisions in frame can destroy centroid information
Single UV photon events
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Centroiding AlgorithmFor each Frame
Find local event peaks in 3x3 “boxcar” smoothed image
For each event Calculate simple 5x5 “center of gravity” Threshold on event sum and size Apply distortion correction Histogram centroids into high resolution 2D image
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Original Medipix mode readout
256 x 256
(14 mm)
UV image limited by 55 m pixel
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Zoomed UV image of pattern in Medipix mode
Pattern 3-2 (= 9 lp/mm) barely resolved
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Timepix centroided mode
Factor of 8 improved resolution!
256 x 256 converted to 8192x8192 pixels (1.7µm pixels)
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Current Implementation
• Installed as a plugin in Pixelman software
• Designed to be computationally fast for later use in FPGA at kHz frame rates
• 30-60 events per frame
• Muros board at ~20 frames/sec
• 50k to 200k frames into 1 image (4096x4096, 1.7µm pixels)
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Zoomed
5-6 pattern resolved = 57 lp/mmLinewidth = 8.8 µm
5-6
The MCP pore spacing of 8µm limits further improvement
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9th IWoRiD: Erlangen,July 2007, John Vallerga
MCP pores (10 on 12 micron)
For better resolution see R. Bellazzini’s poster 11.16
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Sub-pixel distortion
• There is a well understood distortion in the calculated centroid vs. the true centroid due to windowed sampling– Function of distribution size and sampling
parameters – “Pulls” events towards center of 55 µm pixel– Distortion repeats for every pixel
• If event sizes are uniform, it can be corrected by a histogram equalization technique and applied as a simple look-up table
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Modeled distortion for Gaussian
4x4 COG non-linearity for Gaussian input
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
Centroid true position
Calculated position(center of gravity )
Sigma = 0.4Sigma = 0.8Sigma = 1.2
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Distortion (cont.)
0
50
100
150
200
250
300
1 37 73 109 145 181 217 253 289 325 361 397 433 469 505
X-AxisY-Axis
Distribution of sub-pixel locations across
a single pixel
Use a histogram equalization technique
to redistribute events uniformly across
pixel
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Distortion corrected
0
10000
20000
30000
40000
50000
60000
70000
1 38 75 112 149 186 223 260 297 334 371 408 445 482
Series1Series2
Distribution of corrected sub-pixel
locations across a single pixel
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9th IWoRiD: Erlangen,July 2007, John Vallerga
High resolution particle imaging
• This technique can improve particle imaging as long as the initial interaction length is smaller than a pixel and there is a mechanism to spread the collected charge to many pixels
– Neutrons in 10B or Gd MCPs– Alphas in Si or MCPs– Electrons (< 50 keV?) in MCPs– Soft x-rays in MCPs (< 10 keV?)
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Neutron MCPs (10B or Gd doped)
Absorption of Neutron
Secondary(s) reach surface of pore
Emission of photoelectron
Electron gain above electronic threshold
Similar to UV resolution
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Portable MCP vacuum housing
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Neutron detected with 10B doped MCP
McClellan nuclear rad. facility
Thermal neutrons
Very high gamma bkgd.
Laser drilled Gd. mask
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Zoom - Neutron events
Holes approx. 60 microns
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Zoom - UV through same mask
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Best resolution of neutrons
0
0 . 5
1
1 . 5
2
2 . 5
3
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0
X (
m )
Counts
N e u t r o n s + G a m m a
U V p h o t o n s
G a m m a r a y
b a c k g r o u n d
3 5
m F W H M
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Alpha imaging using Timepix/Si
Ni mask
Si
Diffuse 241Am
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Centroiding large events with Silicon/Timepix
241Am Alphas (5MeV) through 10m holes on 500m centers
< 18 micron FWHM
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Future work
• Integrate centroiding into PRIAM board (ESRF Grenoble) to get kHz framerates - factor of 50!
• Collision detection algorithm to reject miss-analyzed events
• Correct for pixel to pixel gain variations in TOT mode (necessary?)
• Optimize charge cloud size for resolution vs. rate
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Summary
• Timepix TOT mode allows us to improve spatial resolution of MCP readout by order of magnitude
• Very robust to variations of Timepix settings, non-linearities, and non-uniformities
• Global ct. rate limited by event collision avoidance.– Counts/frame x Frames/sec ~ 200 kHz– Local rate < 10% of frame rate. ~ 100 Hz
• Excellent for photon-starved astronomical applications (UV to x-ray)
• Biological Imaging? Neutron imaging? Electron imaging?
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9th IWoRiD: Erlangen,July 2007, John Vallerga
Acknowledgements
This work was funded by an AODP grant managed by NOAO and funded by the NSF
The boron sensitive MCPs were loaned to us by Bruce Feller at Nova Scientific (Sturbridge Mass.)
We would also like to thank the Medipix2 Collaboration
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9th IWoRiD: Erlangen,July 2007, John Vallerga
X-ray QE of CsI photocathode