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Register a new userThe Speedster-EXD- A New Event-Driven Hybrid CMOS X-ray Detector
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Christopher Griffith
Publication date
2016
fields
Physics
and research's language is
English
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The Speedster-EXD is a new 64x64 pixel, 40 $mu$m pixel pitch, 100 $mu$m depletion depth hybrid CMOS X-ray detector (HCD) with the capability of reading out only those pixels containing event charge, thus enabling fast effective frame rates. A global charge threshold can be specified, and pixels containing charge above this threshold are flagged and read out. The Speedster detector has also been designed with other advanced in-pixel features to improve performance, including a low-noise, high-gain CTIA amplifier that eliminates interpixel capacitance crosstalk (IPC), and in-pixel Correlated Double Sampling (CDS) subtraction to reduce reset noise. We measure the best energy resolution on the Speedster-EXD detector to be 206 eV (3.5 %) at 5.89 keV and 172 eV (10.0 %) at 1.49 keV. The average IPC to the four adjacent pixels is measured to be 0.25 $pm$ 0.2 % (i.e. consistent with zero). The pixel-to-pixel gain variation is measured to be 0.80 $pm$ 0.03 %, and a Monte Carlo simulation is applied to better characterize the contributions to the energy resolution.
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We present preliminary characterization of the Speedster-EXD, a new event driven hybrid CMOS detector (HCD) developed in collaboration with Penn State University and Teledyne Imaging Systems. HCDs have advantages over CCDs including lower susceptibility to radiation damage, lower power consumption, and faster read-out time to avoid pile-up. They are deeply depleted and able to detect x-rays down to approximately 0.1 keV. The Speedster-EXD has additional in-pixel features compared to previously published HCDs including: (1) an in-pixel comparator that enables read out of only the pixels with signal from an x-ray event, (2) four different gain modes to optimize either full well capacity or energy resolution, (3) in-pixel CDS subtraction to reduce read noise, and (4) a low-noise, high-gain CTIA amplifier to eliminate interpixel capacitance crosstalk. When using the comparator feature, the user can set a comparator threshold and only pixels above the threshold will be read out. This feature can be run in two modes including single pixel readout in which only pixels above the threshold are read out and 3x3 readout where a 3x3 region centered on the central pixel of the x-ray event is read out. The comparator feature of the Speedster-EXD increases the detector array effective frame rate by orders of magnitude. The new features of the Speedster-EXD hybrid CMOS x-ray detector are particularly relevant to future high throughput x-ray missions requiring large-format silicon imagers.
We have been developing monolithic active pixel sensors, X-ray Astronomy SOI pixel sensors, XRPIXs, based on a Silicon-On-Insulator (SOI) CMOS technology as soft X-ray sensors for a future Japanese mission, FORCE (Focusing On Relativistic universe and Cosmic Evolution). The mission is characterized by broadband (1-80 keV) X-ray imaging spectroscopy with high angular resolution ($<15$~arcsec), with which we can achieve about ten times higher sensitivity in comparison to the previous missions above 10~keV. Immediate readout of only those pixels hit by an X-ray is available by an event trigger output function implemented in each pixel with the time resolution higher than $10~{rm mu sec}$ (Event-Driven readout mode). It allows us to do fast timing observation and also reduces non-X-ray background dominating at a high X-ray energy band above 5--10~keV by adopting an anti-coincidence technique. In this paper, we introduce our latest results from the developments of the XRPIXs. (1) We successfully developed a 3-side buttable back-side illumination device with an imaging area size of 21.9~mm$times$13.8~mm and an pixel size of $36~{rm mu m} times 36~{rm mu m}$. The X-ray throughput with the device reaches higher than 0.57~kHz in the Event-Driven readout mode. (2) We developed a device using the double SOI structure and found that the structure improves the spectral performance in the Event-Driven readout mode by suppressing the capacitive coupling interference between the sensor and circuit layers. (3) We also developed a new device equipped with the Pinned Depleted Diode structure and confirmed that the structure reduces the dark current generated at the interface region between the sensor and the SiO$_2$ insulator layers. The device shows an energy resolution of 216~eV in FWHM at 6.4~keV in the Event-Driven readout mode.
FORCE is a Japan-US space-based astronomy mission for an X-ray imaging spectroscopy in an energy range of 1--80 keV. The Wideband Hybrid X-ray Imager (WHXI), which is the main focal plane detector, will use a hybrid semiconductor imager stack composed of silicon and cadmium telluride (CdTe). The silicon imager will be a certain type of the silicon-on-insulator (SOI) pixel sensor, named the X-ray pixel (XRPIX) series. Since the sensor has a small pixel size (30--36 $mu$m) and a thick sensitive region (300--500 $mu$m), understanding the detector response is not trivial and is important in order to optimize the camera design and to evaluate the scientific capabilities. We have developed a framework to simulate observations of celestial sources with semiconductor sensors. Our simulation framework was tested and validated by comparing our simulation results to laboratory measurements using the XRPIX 6H sensor. The simulator well reproduced the measurement results with reasonable physical parameters of the sensor including an electric field structure, a Coulomb repulsion effect on the carrier diffusion, and arrangement of the degraded regions. This framework is also applicable to future XRPIX updates including the one which will be part of the WHXI, as well as various types of semiconductor sensors.
The Sun is the nearest astrophysical source with a very intense emission in the X-ray band. The study of energetic events, such as solar flares, can help us to understand the behaviour of the magnetic field of our star. There are in the literature numerous studies published about polarization predictions, for a wide range of solar flares models involving the emission from thermal and/or non-thermal processes, but observations in the X-ray band have never been exhaustive. The gas pixel detector (GPD) was designed to achieve X-ray polarimetric measurements as well as X-ray images for far astrophysical sources. Here we present the possibility to employ this instrument for the observation of our Sun in the X-ray band.
With the observation of the gravitational wave event of August 17th 2017 the multi-messenger astronomy era has definitely begun. With the opening of this new panorama, it is necessary to have new instruments and a perfect coordination of the existing observatories. Crystal Eye is a detector aimed at the exploration of the electromagnetic counterpart of the gravitational waves. Such events generated by neutron stars mergers are associated with gamma-ray bursts (GRB). At present, there are few instruments in orbit able to detect photons in the energy range going from tens of keV up to few MeV. These instruments belong to two different old observation concepts: the all sky monitors (ASM) and the telescopes. The detector we propose is a crossover technology, the Crystal Eye: a wide field of view observatory in the energy range from 10 keV to 10 MeV with a pixelated structure. A pathfinder will be launched with Space RIDER in 2022. We here present the preliminary results of the characterization of the first pixel.
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