A negative ion time projection chamber (NI-TPC) was developed that is suitable for measuring the polarization of bright transient events. The polarization of the astrophysical source is determined by imaging photoelectron tracks created by incident x-rays interacting in a gas. The tracks drift through the gas under a uniform electric field to the readout electronics. The initial photoelectron direction is correlated with the polarization of the incident x-ray, and therefore by imaging many tracks, one can determine the polarization magnitude and direction of the incident x-rays.
Whereas instruments for measuring persistent sources can use mirrors to gain enough sensitivity, a wide field of view is required to detect the transient events and precludes the use of focusing optics. To have sufficient sensitivity to measure polarization the detector must therefore have a large area. The NI-TPC drifts tracks of negative ions rather than electrons, minimizing diffusion and enabling large drift distances and hence large collecting areas. In addition, the slower drift velocity of the negative ions allows the electronics to be lower power. The NI-TPC polarimeter operates across the 2 to 10 keV energy band and demonstrates low systematics and high sensitivity with an average modulation factor of 0.37 across the energy band.
An instrument consisting of four or more 6 × 6 × 6 cm3 modules could measure the polarization of 3 to 5 GRBs (gamma ray bursts) in a one-year mission.
This work was done by Joanne Hill-Kittle of Goddard Space Flight Center, J. Kevin Black of Rock Creek Scientific, and Philip Kaaret of University of Iowa. GSC-16893-1