Publisher’s Note: This paper, originally published on 20 July 2018, was replaced with a corrected/revised version on 12 October 2018. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
Next generation X-ray mission concepts (e.g. <i>Lynx</i>) call for a wide field X-ray imager with high detection
efficiency from 0.2 keV to 10 keV and fast readout capability (> 10 Hz frame rate). In order to properly sample
the planned fine angular resolution of the optical assembly (0:5’’ HPD), small pixel sizes of less than or equal to
16 microns are required. Hybrid CMOS detectors are a type of active pixel sensor that is well suited to the high
throughput and wide bandpass requirements of such instruments, and the pixel size goals are well within reach.
In collaboration with Teledyne Imaging Sensors, the Penn State X-ray detector lab has developed new small
pixel Hybrid CMOS sensors to meet these needs. These prototype 128 x 128 pixel devices have 12.5 micron pixel
pitch, 100 micron fully depleted depth, and include crosstalk-eliminating CTIA amplifiers and in-pixel correlated
double sampling capability. We report on characteristics of one of these new detectors, including read noise,
energy resolution, and pixel-to-pixel gain variation. The read noise was measured to be as low as 5:54 e<sup>-</sup> ± 0:05
e<sup>-</sup>, while the gain variation was found to be 1:12% ± 0:06%. The energy resolution, including calibration for
gain variation, was measured to be as good as 148 eV (2.5%) at 5.9 keV.
When an X-ray is incident onto the silicon absorber array of a detector, it liberates a large number of electrons, which tend to diffuse outward into what is referred to as the charge cloud. This number can vary from tens to thousands across the soft X-ray bandpass (0.1 - 10 keV). The charge cloud can then be picked up by several pixels, and forms a specific pattern based on the exact incident location of the X-ray. We present experimental results on subpixel resolution for a custom H2RG with 36μm pixels, presented in Bray 2018,<sup>1</sup> and compare the data to simulated images . We then apply the model simulation to a prototype small pixel hybrid CMOS detector (HCD) that would be suitable for the Lynx X-ray surveyor. We also discuss the ability of a small pixel detector to obtain subpixel resolution.
X-ray Hybrid CMOS Detectors (HCDs) have advantages over X-ray CCDs due to their higher readout rate abilities, flexible readout, inherent radiation hardness, and low power, which make them more suitable for the next generation large-area X-ray telescope missions. The Penn State high energy astronomy laboratory has been working on the development and characterization of HCDs in collaboration with Teledyne Imaging Sensors (TIS). A custom-made H2RG detector with 36 μm pixel pitch and 18 μm ROIC shows an improved performance over standard H1RG detectors, primarily due to a reduced level of inter-pixel capacitance crosstalk (IPC). However, the energy resolution and the noise of the detector and readout system are still limited when utilizing a SIDECAR at non-cryogenic temperatures. We characterized an H2RG detector with a Cryo-SIDECAR readout and controller, and we find an improved energy resolution of ∼2.7 % at 5.9 keV and read noise of ∼6.5 e- . Detections of the ∼0.525 keV Oxygen Kα and ∼0.277 keV Carbon Kα lines with this detector display an improved sensitivity level at lower energies. This detector was successfully flown on NASA’s first water recovery sounding rocket flight on April 4th, 2018. We have also been developing several new HCDs with potential applications for future X-ray astronomy missions. We are characterizing the performance of small-pixel HCDs (12.5 μm pitch), which are important for the development of a next-generation high-resolution imager with HCDs. The latest results on these small pixel detectors has shown them to have the best read noise and energy resolution to-date for any X-ray HCD, with a measured 5.5 e- read noise for a detector with in-pixel correlated double sampling. Event recognition in HCDs is another exciting prospect. We characterized a 64 × 64 pixel prototype Speedster-EXD detector that uses comparators in each pixel to read out only those pixels having detectable signal, thereby providing an order of magnitude improvement in the effective readout rate. Currently, we are working on the development of a large area Speedster-EXD with a 550 × 550 pixel array. HCDs can also be utilized as a large FOV instrument to study the prompt and afterglow emissions of GRBs and detect black hole transients. In this context, we are characterizing a Lobster-HCD system for future CubeSat experiments. This paper briefly presents these new developments and experimental results.
X-ray lobster optics provide a unique way to focus X-rays onto a small focal plane imager with wide field of view imaging. Such an instrument with angular resolution of a few arcminutes can be used to study GRB afterglows, as well as the variability and spectroscopic characteristics for other astrophysical objects. At Penn state University, we characterize these lobster optics with an H1RG CMOS sensor (100 μm thick Silicon with 18 μm pixel size), procured from Teledyne Imaging Sensors at its focal plane. The light-weight compact lobster optic with a 25 cm focal length provides two dimensional imaging with ~25 cm^2 effective area at 2 keV. We chose the hybrid CMOS detector (HCD) since X-ray HCDs offer several advantages (e.g. radiation hard, low power, faster and flexible readout rate) over CCDs for future X-ray missions. We utilize 47 m long X-ray beam line at Penn state University to do our experiments where we characterize the overall effective area of the instrument at 1.5 - 8 keV for both on-axis and off-axis angles. In this presentation, we will describe the characterization test stand and methods, as well as the detailed results. We perform ray-tracing simulations to theoretically validate the results which would also be briefly discussed here. While this is simply a proof-of-concept experiment, such an instrument with significant collecting area can be explored for future rocket or CubeSat experiments.
The Water Recovery X-ray Rocket (WRXR) is a sounding rocket payload that will launch from the Kwajalein Atoll in April 2018 and seeks to be the first astrophysics sounding rocket payload to be water recovered by NASA. WRXR's primary instrument is a grating spectrometer that consists of a mechanical collimator, X-ray reflection gratings, grazing-incidence mirrors, and a hybrid CMOS detector. The instrument will obtain a spectrum of the diffuse soft X-ray emission from the northern part of the Vela supernova remnant and is optimized for 3rd and 4th order OVII emission. Utilizing a field of view of 3.25° × 3.25° and resolving power of λ/δλ ≈40-50 in the lines of interest, the WRXR spectrometer aims to achieve the most highly-resolved spectrum of Vela's diffuse soft X-ray emission. This paper presents introductions to the payload and the science target.
The Penn State X-ray detector lab, in collaboration with Teledyne Imaging Sensors (TIS), have progressed their efforts to improve soft X-ray Hybrid CMOS detector (HCD) technology on multiple fronts. Having newly acquired a Teledyne cryogenic SIDECARTM ASIC for use with HxRG devices, measurements were performed with an H2RG HCD and the cooled SIDECARTM. We report new energy resolution and read noise measurements, which show a significant improvement over room temperature SIDECARTM operation. Further, in order to meet the demands of future high-throughput and high spatial resolution X-ray observatories, detectors with fast readout and small pixel sizes are being developed. We report on characteristics of new X-ray HCDs with 12.5 micron pitch that include in-pixel CDS circuitry and crosstalk-eliminating CTIA amplifiers. In addition, PSU and TIS are developing a new large-scale array Speedster-EXD device. The original 64 × 64 pixel Speedster-EXD prototype used comparators in each pixel to enable event driven readout with order of magnitude higher effective readout rates, which will now be implemented in a 550 × 550 pixel device. Finally, the detector lab is involved in a sounding rocket mission that is slated to fly in 2018 with an off-plane reflection grating array and an H2RG X-ray HCD. We report on the planned detector configuration for this mission, which will increase the NASA technology readiness level of X-ray HCDs to TRL 9.