Design and Optimisation of Detector Cells for the PoGOLite Polarimeter

Abstract

The field of X-ray polarimetry provides a new way to observe astrophysical objects by measuring the polarisation fraction and angle of emitted radiation flux in the X-ray regime. The PoGOLite (Polarised Gamma-ray Observer) Pathfinder experiment is a balloon-borne Compton scatteringbased X-ray (15-240 keV) polarimeter whose primary target is the Crab Nebula and Pulsar. It consists of an array of 61 detector cells consisting of three types of scintillators.

The PoGOLite Pathfinder had its first successful flight in 2013, where it followed a circumpolar path for 13 days before landing in Russia. For the planned 2016 flight, a number of changes are planned to be made to the detector based on experiences from the 2013 flight. To evaluate which solutions should be used for these changes a number of tests have been performed.

One of the most noticeable issues with the current iteration of the polarimeter is unintended optical cross-talk between detector cells. Scintillation light from a scintillator in a detector cell leaks over to neighbouring cells where it induces a fake signal. These fake signals create fake polarisation events, significantly reducing the performance of the detector. By covering the detector cells with a new type of light absorbing material it was possible to eliminate this issue and significantly increase the performance of the detector.

A significant improvement could also be made to the collection of scintillation light from the ”fast” scintillator. Tests to find the optimal reflective cover for the detector cell parts were performed, and it was found that it was possible to significantly improve the light collection with a change of reflective materials. By eliminating optical cross-talk and improving the light collection of the detector cells the M100 of the polarimeter is expected to be improved by approximately 50%.

The final test performed was a comparison between two types of ”fast” plastic scintillator models. It was thought that the current ”fast” scintillator could be replaced by one which is superior for the polarimeters use. These two types were tested using both waveform analysis and a multichannel analyser but no significant performance improvement was found and parts of the tests were inconclusive. In the end it was decided to use the same scintillator which was used in previous iterations.

With the new design of the detector cells the polarimeter’s performance will be greatly increased. Monte Carlo-based simulations based on a six hour observation of the Crab in conditions taken from the 2013 flight show an improvement in MDP99% from 25.8% to 17.4%. The increased precision will result in more statistically significant observations of the Crab Nebula and Pulsar which will allow us to understand more about the emission mechanisms for high energy radiation.