1. May 6, 2005Division Director Seminar - K. Black1 The Prospects for X-ray Polarimetery with Time Projection Chambers Kevin Black Code 662 - Laboratory for X-Ray Astrophysics

2. May 6, 2005Division Director Seminar - K. Black2 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

3. May 6, 2005Division Director Seminar - K. Black3 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

4. May 6, 2005Division Director Seminar - K. Black4 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 Strip number Drift direction X-ray

5. May 6, 2005Division Director Seminar - K. Black5 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 10 3 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

6. May 6, 2005Division Director Seminar - K. Black6 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

7. May 6, 2005Division Director Seminar - K. Black7 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 CO 2

8. May 6, 2005Division Director Seminar - K. Black8 A Sampling of Track Images 6 keV in 0.3 atm CO 2 Time Bin (40 nsec) Strip # (150 microns)

9. May 6, 2005Division Director Seminar - K. Black9 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

10. May 6, 2005Division Director Seminar - K. Black10 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.