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Testing and Characterization of a Next-Generation Event Driven Hybrid CMOS X-ray Detector

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Testing and Characterization of a Next-Generation Event Driven Hybrid CMOS X-ray Detector
X-ray astronomy excels at observing some of the most energetic objects in the universe, such as supernovae and black hole accretion disks. These immense energies have adverse effects on the surrounding universe. Supernovae introduce pressure waves that are needed to form the next generation of stars in molecular clouds, while accretion disks give rise to powerful jets that allow observations of the universe as it existed billions of years ago. Because the Earth's atmosphere is very effective at shielding us from cosmic x-rays, it is necessary to do our observing from orbit. By using charge coupled devices (CCDs), current X-ray space telescopes have provided insights into the universe, and have opened the doors for the next-generation of x-ray satellites. These future missions will have a greater throughput of incoming photons; surpassing the capabilities of the fastest CCDs. These missions will be observing gamma ray bursts to glean information about large-scale structure of space, and studying the environment around blazars and supermassive black holes, among other things. Current x-ray missions suffer from photon pileup when observing moderately bright sources, resulting in a loss of energy and flux information. This demonstrates a need for more advanced hardware in order investigate the universe around us. The replacement for CCDs on future missions will come in the form of hybrid CMOS detectors (HCDs). HCDs offer many benefits over CCDs, and are already utilized in infrared and optical astronomy. Their foremost advantage is their ability to read individual pixels out as opposed to the “bucket-brigade” style associated with CCDs. This allows for low noise levels while maintaining a significantly faster readout rate that avoids pileup issues from high-intensity sources. Because of this novel readout style, HCDs are much more radiation hard than CCDs; resulting in a longer useful lifetime. Lastly, HCDs are more energy efficient and require an order of magnitude less power than current counterparts. A new hybrid CMOS detector called the Speedster-EXD, produced by the x-ray instrumentation lab at Penn State, has been proven to offer all of these benefits. It is a 64x64 pixel detector that was done as a proof-of-concept to show that such technology is a realistic option in x-ray astronomy, and has made clear the techniques that will be employed as we move closer to a space-ready setup. The next step in the development process is to move to a larger format array that will be an equivalent to what one might expect to find on a next-generation x-ray telescope. We are currently designing and building a comparable 550x550 pixel detector using the scaled up techniques learned from the prototype. My research will focus on calibrating and characterizing its performance. Along with using standard techniques to model noise levels and gain variations, I will perform new and rigorous tests to describe the energy resolution, quantum efficiency, and effects of proton damage. These tests span an energy range of 0.5 to 14 keV through controlled exposures to a variety of x-ray sources. I will calculate the quantum efficiency by designing and building a unique test stand that is compatible with the new detector. By utilizing a well-characterized proportional counter, I will accurately determine the limits of detection for this new detector. It is also imperative to define how such detectors will fare in the high-energy proton winds found in outer space. Because charge is transferred through several centimeters of material in a CCD, charge-transfer abilities are hindered by proton damage. This reduces image quality due to degradation in the energy resolution. On an HCD the charge is transferred through a few hundred microns, so trauma is restricted to the local area. The use of the Relativistic Heavy Ion Collider at Brookhaven National Labs will allow me to gather vital data on how these detectors will behave once in orbit. More »

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