Simulating an X-ray Polarimeter Gas Pixel Detector
2019 July 22
X-ray polarimetry is one of the last remaining largely untouched frontiers in observational astronomy and provides a novel way to probe poorly understood details of high energy emission processes for a variety of astrophysical sources. X-ray polarization is exceedingly difficult to measure, one solution is the Gas Pixel Detector (GPD). In a GPD an astronomical X-ray enters a gas cell, collides with an atom of gas, which emits a high energy electron. Critically, the direction of this electron corresponds to the polarization of the astronomical X-ray. This electron creates an ionization tract whose electrons are drifted by a small electric potential across a gas cell on to a bottom plate consisting of a double layered conductor separated by an insulator with a strong potential difference between them. This bottom plate, called a Gas Electron Multiplier (GEM), has an array of tiny holes and the ionization tract electrons fortunate enough to pass though the holes are strongly accelerated causing them to create secondary cascades in the direction of a pixelated ASIC detector array. The cumulative pattern of secondary shower electrons across the detector pixels thus reflects the direction of the initial electron ejected from the collision with the astronomical X-ray and thus ultimately the polarization of the astrophysical X-ray source.
Therefore, for effective detector operation, it is necessary to characterize the detector's response to varying internal physical properties such as pressure, temperature, and voltage configurations both to determine optimal parameters for launch and to understand changes in post-launch detector performance. Here I will discuss my work simulating the secondary cascade electron behavior using the Garfield++ software package, developed by CERN for modeling particle detectors with gas particle interactions such as drift chambers.