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The Prospects for X-ray Polarimetery with Time Projection Chambers. Kevin Black Code 662 - Laboratory for X-Ray Astrophysics. Gas pixel detector polarimeter concept. Track images. Modulation. Photoelectric polarimetery with a pixelized micropattern gas detector. - PowerPoint PPT Presentation
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May 6, 2005 Division Director Seminar - K. Black 1
The Prospects for X-ray Polarimetery
with Time Projection Chambers
Kevin BlackCode 662 - Laboratory for X-Ray Astrophysics
May 6, 2005 Division Director Seminar - K. Black 2
Gas pixel detector polarimeter concept
Photoelectric polarimetery with a pixelized micropattern gas detector
• Highly sensitive technique first demonstrated by Bellazzini et al (2001)• First practical device demonstrated by GSFC (2003)• Basis of the AXP SMEX proposal (awarded new technology funding)
Track images Modulation
May 6, 2005 Division Director Seminar - K. Black 3
Limits of the pixelized detector technique
• Electron diffusion in the drift region creates a tradeoff between quantum efficiency, modulation
Polarimeter figure-of-merit vs energy
May 6, 2005 Division Director Seminar - K. Black 4
The time projection chamber (TPC) as a photoelectric polarimeter
• Concept: create a virtual pixel detector from a strip detector by using time to derive the second coordinate
• Construct pixels by digitizing the pulse-train waveform on each strip
Time of arrival
Str
ip
num
ber
Drift direction
X-ray
May 6, 2005 Division Director Seminar - K. Black 5
Tradeoffs in a TPC polarimeter
1. Much greater quantum efficiency without loss of modulation
1. Not imaging
Pros Cons
2. Fundamentally asymmetric: will require careful calibration and/or rotation
3. Factor of 103 fewer electronics channels than a pixel polarimeter
2. Geometry accommodates an imager below the TPC
TPC polarimeter
Pixel polarimeter
Our proposed rocket instrument provides simultaneous observations with a time-projection and a pixel polarimeter
May 6, 2005 Division Director Seminar - K. Black 6
A demonstration TPC polarimeter
• Constructed in 2 months from stuff we found along the side of the road• Thanks to Bob Baker, Ken Sims, Norman Dobson, Richard Koenecke • Components:
• GEMs: off-the-shelf etched stainless steel foils (150 micron hex pitch)• Strip anode: standard printed circuit (150 micron pitch)• Commercial preamps• 24-channel digitizer based on 8-channel, 40 MHz ADC
Stainless steel GEM 24-channel waveform digitizer
The “roadkill” polarimeter
May 6, 2005 Division Director Seminar - K. Black 7
Demo TPC polarimeter operation
• Goal: photoelectron track images using this TPC technique• Digitize raw charge-sensitive preamp signals at 25 MSPS• Drift velocity = 3.75 cm/microsecond (= 150 microns bins)
Derived image
Interaction point / Auger electron
End point / Bragg peak
Digitized waveforms
6 keV photon in 0.3 atm CO2
May 6, 2005 Division Director Seminar - K. Black 8
A Sampling of Track Images6 keV in 0.3 atm CO2
Time Bin (40 nsec)
Str
ip #
(15
0
mic
ron
s)
May 6, 2005 Division Director Seminar - K. Black 9
Next Steps
• Quantitative demonstration of TPC as a polarimeter
• Measure modulation with Roadkill II with:• 130 micron pitch micromegas using etched stainless steel foils• 24 channel, 50 MHz ADCs – operate near diffusion minimum• Polarized and unpolarized 6 keV• Rotation mechanism or drift velocity calibration
May 6, 2005 Division Director Seminar - K. Black 10
Conclusion
• Images of photoelectron tracks from a TPC are qualitatively comparable to those from a pixel detector.
• A TPC-based photoelectric polarimeter is a promising complement to a pixel polarimeter or other focal plane instrument.
• Further work is required to quantify these results and to understand and control systematic errors.