Large Format Cross Strip Readout Image Sensors for High Temporal Resolution Astronomy and Remote Sensing
Abstract
Photon counting detectors with high time resolution ( < 1 ns) have been developed to cover a wide range of optical/UV sensing applications. Specifically, electronic readout microchannel plate (MCP) based sensors have been used as photon counting, imaging, event time tagging detectors for astronomical, remote sensing, time resolved biological imaging, imaging LIDAR and night time sensing applications. They also have applications in detection of Cherenkov light (RICH), scintillation detection, and neutron imaging applications.
Most of the high time resolution devices to date have been relatively small format low spatial resolution devices. However, cross strip readout anodes can provide a novel high spatial resolution image encoding system for microchannel plate sensors. Devices we have made in formats of 18 mm have shown good spatial resolution and imaging with < 100 ps timing resolution for astronomical observations and imaging 3D LIDAR. For future applications we have recently been developing and implementing cross strip readouts with metal & ceramic layers for larger format 50 mm sealed tubes with selected photocathodes covering the UV and optical regimes. These larger image format devices will be important for the next generation of moderate and large NASA astrophysics instruments under study (eg. LUVOIR, HabEx, CETUS), ground based focal plane instruments for fast timing / imaging in astronomical and remote sensing applications. To realize these larger format devices we have focused on the critical elements of the sensors, high detection efficiency from the UV to visible/red, high spatial resolution and large event rate limits, with long lifetimes and high time resolution. Our photocathode work includes investigations of specialized photocathodes with high efficiency down to 200 nm. We have also achieved optimized photocathodes with a 3 order of magnitude "solar blind" cutoff at 400 nm to reduce noise and longer wavelength background for blue end observations. Continuing evaluations of combinations of these semitransparent photocathodes with opaque photocathodes in the same device are also underway. Microchannel plates are used as electron multipliers in these devices. They amplify the detected photon signal to a charge cloud of about a million elelctrons, which is then deposited onto an imaging readout anode. An recent enhancement comes through incorporation of new microchannel plate electron multipliers utilizing atomic layer deposition of resistive and secondary emissive layers on borosilicate micro-capillary arrays. The borosilicate substrates are more robust than traditional MCPs, allowing them to be produced in large formats (20 x 20 cm) even with 10 micron pores (capillaries). We have successfully integrated this type of MCP into 50 mm aperture sealed tubes for the first time. These devices show stable, uniform gain, and can provide very good event timing accuracy (sub 100 ps). Spatial fidelity of better than 20 microns can be achieved with this new type of MCP providing more than 2k x 2k resolution elements for a 50mm device. Compared with the current generation of MCPs, the ALD-borosilicate MCPs have shown lifetime stability increase an order of magnitude improvement in vacuum sealed device gain retention and long term preservation of the photocathode efficiency. Cross strip electronic readouts are of particular interest since they have the capability to be scaled for the largest format detectors and generally have best spatial resolution and highest event rate handling capability. Cross strip anodes are an array of closely spaced charge collection strips to collect charge from the events detected by the MCPs, one set for each orthogonal axis. The cross strip anodes require specialized readout electronics. Fast low noise amplifiers on each strip line (72 x 2 for a 50 mm device) amplify the MCP signals, and these signal amplitudes must be digitized, and a centroid algorithm in an FPGA implemented to provide the final X, Y and timing for individual photon events. Standard electronics components have been used to implement a baseline amplifier, ADC, FPGA event encoding system. For the 50 mm cross strip anode designs we have achieved < 2 0 µm spatial resolution at photon counting rates up to 5 MHz. Timing accuracies for single photon events are 100ps for fast laser pulses. Since the standard electronics components are not optimal for a high fidelity system that is also spaceflight capable we have developed a customized ASIC encoding electronics system at the University of Hawaii. This employs a faster front end amplifier and an integrated ADC/FPGA package that reduces the front end noise and increases the overall event throughput rate while reducing the power and weight of the electronics for compact focal planes and spaceflight applications. Cross strip readout sealed tube microchannel plate sensors working at the blue end of the spectrum do not need cooling, and red optimized versions operate well with a modestly cooled (0 C) device to reduce photocathode single electron noise. The background rates of a few hundred events/cm2/sec (one background event per resolution element every 1000 sec) permit observations of very faint signals in practical observation times. Events are also stored as an event list tagged by X and Y position and time of arrival. So rather than an integrated "frame" image these devices provide post facto image reconstruction possibilities that can accommodate issues such as seeing conditions, instrument pointing drift, and reconstruction of LIDAR 3D images. These features are also useful for Cherenkov radiation detection and optical SETI (implemented with the 10m SALT telescope) by discrimination of fast multiphoton event detection. Current, and future, uses of these devices include applications in ground based astronomy for observations of transient sources, imaging and spectroscopic instruments on satellite platforms and larger, more stable detectors for remote sensing.- Publication:
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Advanced Maui Optical and Space Surveillance Technologies Conference
- Pub Date:
- September 2019
- Bibcode:
- 2019amos.confE..84S
- Keywords:
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- Imaging;
- photon counting;
- microchannel plate;
- high time resolution