THESEUS (Transient High Energy Sky & Early Universe Surveyor) is one of the three candidates for the M5 mission of the European Space Agency. The favoured mission will be announced in 2021 for an expected launch in 2032. THESEUS will be equipped with a Soft X-ray Imager (SXI) composed of a set of two telescopes using micro-pore optics offering an overall field of view of 0.5 sr (<2’ accuracy) for X-ray energies between 300 eV and 5 keV. The focal plane of each SXI telescope has a 16 x 16 cm2 cooled detector area. However, the limited radiator accommodation on the spacecraft prohibits the use of CCDs since cooling the focal planes to an optimal temperature for radiation hardness (<-100 ◦C) is not feasible. Therefore, the development of a suitable CMOS Image Sensor (CIS), capable of handling the expected levels of radiation at higher operating temperatures (approximately -30 ◦C) has been proposed. To demonstrate the performance required for the THESEUS SXI detector, a 2 x 2 cm2 prototype is under development using Open University pixel designs in a Teledyne-e2v digital CMOS platform. The pixel design will allow full depletion over silicon thickness of 35 µm for optimal soft X-ray quantum efficiency and instrument background suppression, and will be capable of near-Fano-limited spectral resolution that will also be of prime interest for synchrotron and Free Electron Lasers (FEL) applications. In this paper, we will present the design considerations and simulations leading to the implemented structures complying with THESEUS’ SXI requirements.

/pdf/development-of-a-photon-counting-near-fano-limited-x-ray-1r1tepu5q1.pdf

Development of a photon-counting near-fano-limited x-ray CMOS image sensor for THESEUS' SXI

Within the solar system, X-ray emissions have been detected from every planet except the Ice Giants: Uranus and Neptune. We analyze the three archival Chandra X-ray observations of Uranus (each 24–30 ks duration) to date: a stand-alone Advanced CCD Imaging Spectrometer (ACIS) observation on August 7, 2002 and two High Resolution Camera (HRC) observations on November 11 and 12, 2017 coordinated with optical observations. For the earlier ACIS observation, the Uranus-coincident photons were clustered in the 0.6–1.1 keV spectral range, consistent with emission from Jupiter and Saturn. To test the significance of the detected signal, we distributed a grid of ∼10,000 Uranus-sized regions across the field of view (FoV). The number of Uranus-coincident X-ray photons in the 0.5–1.2 keV range exceeded 99.9% of Uranus-sized regions across the FoV (10.2 standard deviations > FoV mean; probability of chance occurrence ∼10−6–10−7). However, the planetary signal was low with only 5 ± 2.2 X-ray photons against a FoV mean background of 0.16 photons. Without the possibility of energy filtering, the recent HRC observations had a much brighter background (FoV mean ∼10 photons). Consequently, neither of the new observations provided a second unambiguous Uranus detection, although a 40-min interval of brightening on November 12, 2017 did produce a signal above 99.9% of the FoV. The observed Uranus X-ray fluxes of 10−15–10−16 erg/cm2/s are consistent with previous observational limits and modeling predictions. These fluxes exceed expectations from scattered solar emission alone, suggesting either a larger X-ray albedo than Jupiter/Saturn or the possibility of additional X-ray production processes at Uranus. Further observations are needed to test this.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020JA028739

A Low Signal Detection of X‐Rays From Uranus

Solar Wind Charge-Exchange (SWCX) emission is present in every X-ray observation of an astrophysical object. The emission is problematic when one cannot remove the foreground by the simultaneous measurement of a nearby field. SWCX emission is a serious impediment to the study of the diffuse hot ISM, including the Galactic halo, as its contribution to diagnostic emission lines is temporally variable. Modeling the SWCX emission, in order to remove it from our observations, has proven to be more difficult than originally anticipated. This work reviews our current understanding of SWCX emission, with special attention to all of the components required for future modeling tools. Since, in the absence of such a tool, observing programs can still be constructed to minimize the effect of SWCX, mitigation strategies are discussed. Although some aspects of SWCX will be very difficult to characterize, progress continues on many fronts.

Solar Wind Charge Exchange: An Astrophysical Nuisance

The progress of X-ray Optics joint research activity of the European Union Horizon 2020 AHEAD project is presented here covering the X-ray optic technologies that are currently being worked on in Europe. These are the Kirkpatrick Baez, lobster eye micropore (SVOM, SMILE), slumped glass, and silicon pore (ATHENA, ARCUS) optics technologies. In this activity detailed comparisons of the measurements, of the different optics produced by the participating optics groups, obtained mainly at the MPEs PANTER X-ray test facility, are compared with simulations. In preparation for the ATHENA mission a study has been made to design the BEaTRiX X-ray test facility for testing individual silicon pore optics mirror modules, and the realization of the facility is now on going. A zone plate collimating optics developed for PANTER is being studied, optimized, and tested at PANTER. This zone plate will be used for characterising a high quality optics module in a parallel beam to verify the BEaTriX performance. Several of the measurements and selected results are presented here.

AHEAD joint research activity on x-ray optics

A prototype CMOS Image Sensor optimised for soft X-ray applications has been designed by the Centre for Electronic Imaging in partnership with Teledyne-e2v. The device features four different pixel variants (three variants of 40 μm pitch pixels, and one variant of 10 μm pixels) each covering a quarter of the 2 × 2 cm2 image area. The pixel designs feature fully depleted pinned photodiodes using reverse substrate bias and have been optimised for low noise operation, high responsivity and low image lag. The fabricated front-illuminated devices have been tested in a custom-built vacuum test setup at operating temperatures between -30°C and -40°C. The sensors feature less than 5 e- RMS readout noise and energy resolution of 142 eV at Mn-Kα (5.9 keV). The response to soft X-ray with different sensor parameters (e.g. pixel pitch, deep-depletion extension implant depth, and back-bias voltage) is also studied.

Characterisation of a soft X-ray optimised CMOS Image Sensor

SMILE, the Solar wind Magnetosphere Ionosphere Link Explorer, is a joint science mission between the European Space Agency and the Chinese Academy of Sciences. The spacecraft will be uniquely equipped to study the interaction between the Earth's magnetosphere-ionosphere system and the solar wind on a global scale. SMILE's instruments will explore this science through imaging of the solar wind charge exchange soft X-ray emission from the dayside magnetosheath, simultaneous imaging of the UV northern aurora and in-situ monitoring of the solar wind and magnetosheath plasma and magnetic field conditions. The Soft X-ray Imager (SXI) is the instrument being designed to observe X-ray photons emitted by the solar wind charge exchange process at photon energies between 200 eV and 2000 eV . X-rays will be collected using a focal plane array of two custom-designed CCDs, each consisting of 18 μm square pixels in a 4510 by 4510 array. SMILE will be placed in a highly elliptical polar orbit, passing in and out of the Earth's radiation belts every 48 hours. Radiation damage accumulated in the CCDs during the mission's nominal 3-year lifetime will degrade their performance (such as through decreases in charge transfer efficiency), negatively impacting the instrument's ability to detect low energy X-rays incident on the regions of the CCD image area furthest from the detector outputs. The design of the SMILE-SXI CCDs is presented here, including features and operating methods for mitigating the effects of radiation damage and expected end of life CCD performance. Measurements with a PLATO device that has not been designed for soft X-ray signal levels indicate a temperature-dependent transfer efficiency performance varying between 5×10−5 and 9×10−4 at expected End of Life for 5.9 keV photons, giving an initial set of measurements from which to extrapolate the performance of the SXI CCDs.

/pdf/the-smile-soft-x-ray-imager-sxi-ccd-design-and-development-3yyp48mmql.pdf

W. Raab

Papers

The SMILE Soft X-ray Imager (SXI) CCD design and development

The Soft X-ray Imager (SXI) on the SMILE Mission