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.