Keith Jahoda/Goddard Space Flight Center
X-ray Polarimetry with Time Projection Chambers

The objective of this proposed effort is to bring to maturity a new instrument concept for astronomical X-ray polarimetry that we demonstrated very recently at GSFC. This new technique, based on the photoelectric effect and using gas micropattern Time Projection Chambers (TPCs) promises more than an order-of-magnitude greater quantum efficiency than the best previously demonstrated X-ray polarimeters, while using simpler technology. The improved polarization sensitivity would enable compelling scientific investigations in sub-orbital or Small Explorer class missions that were previously only possible with large advanced mission concepts. Photoelectric X-ray polarimetry with finely spaced, pixelized gas detectors has matured into a powerful and practical technique for astronomical observations that has revived interest in this as-yet unexploited field. In 2003, we developed the first practical gas pixel polarimeter suitable for use at the focus of a conical foil mirror. Based on that technology, we proposed an X-ray polarimetry mission, the Advanced X-ray Polarimeter (AXP), to NASA's Small Explorer program. The AXP mission would be able to make sensitive polarimetry measurements of about a hundred X-ray sources. AXP received the highest science rating, category one, and was awarded technology development funding to bring gas pixel polarimeters to greater flight readiness. As a result of those efforts and others, gas pixel polarimeters have now reached a mature level of development, including prototype flight units and detailed investigations of the instrument performance. Nevertheless, the gas pixel polarimeters are fundamentally limited to quantum efficiencies of about ten percent. Almost all the AGN in the AXP observing plan required observations of one million seconds, so improved quantum efficiency is highly desirable. We very recently demonstrated the TPC as a photoelectric polarimeter that overcomes this limitation. Like the gas pixel detector, this new polarimeter forms images of photoelectron tracks to extract the polarization information. We propose in this effort to develop the TPC polarimeter to the same level of maturity as the gas pixel detector. The TPC polarimeter would open interesting new possibilities for astronomical X-ray polarimetry across a range of mission concepts, including sub-orbital investigations and modest satellite missions. The TPC polarimeter can be tuned to different energy bands from 1-30 keV.