The Prospects for X-ray Polarimetery with Time Projection Chambers

1. The Prospects for X-ray Polarimetery with Time Projection Chambers Kevin Black Code 662 - Laboratory for X-Ray Astrophysics Division Director Seminar - K. Black

2. Gas pixel detector polarimeter concept Track images Modulation 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) 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 Division Director Seminar - K. Black

4. X-ray Strip number Time of arrival Drift direction 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 Division Director Seminar - K. Black

5. 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 Tradeoffs in a TPC polarimeter • ProsCons 1. Much greater quantum efficiency without loss of modulation • 1. Not imaging • 2. Fundamentally asymmetric: will require careful calibration and/or rotation • 3. Factor of 103 fewer electronics channels than a pixel polarimeter 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 24-channel waveform digitizer Stainless steel GEM The “roadkill” polarimeter Division Director Seminar - K. Black

7. Interaction point / Auger electron 6 keV photon in 0.3 atm CO2 End point / Bragg peak Digitized waveforms Derived image 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) Division Director Seminar - K. Black

8. Strip # (150 microns) Time Bin (40 nsec) A Sampling of Track Images 6 keV in 0.3 atm CO2 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 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. Division Director Seminar - K. Black