Solar soft X-ray spectrometer: New Instrument on space for solar emissions

Researchers from Beijing Institute of Control Engineering, Northwestern Polytechnical University, and Macau University of Science and Technology have developed a solar X-ray detector (SXD) that features a wide energy range and a high count rate. The main goal of the instrument is to study spectral characteristics of X-ray solar flare eruption with high accuracy. These measurements require high throughput and excellent spectral resolution. The flight model integrated on the MSS-1B satellite has demonstrated remarkable in-orbit performance, achieving an energy resolution of 138 eV for solar X-ray detection. The detector's novel electronic architecture, meticulously engineered to meet stringent space mission requirements, enables reliable operation even during extreme solar events up to X-class flare intensities.  Comparative analysis with existing solar X-ray instruments reveals that this new detector offers significant advancements, including enhanced energy resolution and reduced signal peaking time, which indicates a higher measurable solar flare level. 

Solar X-ray detectors with wide dynamic range and high time cadence

The solar X-ray detector (SXD) on Macao Science Satellite-1B (MSS-1B) measures solar irradiance at soft X-ray band where the solar eruption varies the most. The solar flares are classified according to their X-ray brightness. It is in logarithmic scale. We enhanced the time cadence to 1 second using a dedicated X-ray signal processing design. The peaking times of the shaping amplifers were optimized to enhance the energy resolution and achieve higher count rates. In particular, two soft X-ray detector units with different aperture areas were integrated to detect solar irradiation, targeting both the quiet Sun and solar fares up to the X-class level.

Low noise and high count-rate X-ray detection electronics

Silicon drift detector (SDD) is selected as the X-ray sensor. Its unique electrode structure provides an ultralow capacitance, enabling operation at a short peaking time, which increases the count rate. The energy resolution is further improved using a thermoelectric cooler (TEC). The preamplifier output is differentiated to measure the step voltage. The shaping amplifier, designed using two-pole multiple-feedback active low-pass filters enhances the signal-to-noise ratio. Two parallel shaping amplifiers with different time constants are designed. The shaping amplifiers, with peaking times of 315 ns and 65 ns, are referred to as “slow shaping” and “fast shaping”, respectively.  The differential of the shaping amplifier generates a bipolar output. The pulse width of the differential signal is significantly smaller than that of the shaping amplifier. Therefore, the differential signals of the fast channel are selected for pile-up rejection. The maximum input count rate is measured to more than 1×10⁶ count/s. The energy resolution is calibrated using various X-ray sources. The measured energy resolution is 138 eV at 5.90 keV.

On-orbit detection of solar flares up to the X-class level

The Macau Science Satellite-1 (MSS-1) was successfully launched on 21 May 2023. Since then, the SXD captures solar flare up to X-level, the most powerful type of solar flare, at Earth. The solar X-ray spectrum typically exhibits a distinct profile, with a steeply decreasing continuum as energy increases, resulting from free-free and free-bound thermal radiation, as well as emission lines from different ionization states of elements such as Ca, Fe, S, and Ar. The SXD observations provide solar X-ray spectra with high temporal and spectral resolutions, allowing for precise spectral fits to the measured count-rate spectrum. This enables the estimate of the abundances of various elements and the detailed analysis of the temporal evolution of plasma diagnostics, including temperature, emission measure, and elemental abundances of thermal plasma during flares.

This research developed a solar X-ray detector (SXD) that features a wide energy range and a high count rate. The main goal of the instrument is to study spectral characteristics of X-ray solar flare eruption with high accuracy. These measurements require high throughput and excellent spectral resolution. The instrument on-board MSS-1B enabled the solar X-ray detection in orbit with an resolution of 138 eV. The novel electronics based on space requirements were designed to measure X-ray spectra with a high resolution up to a maximum of X-level solar flare eruption.

The complete study is accessible via DOI: 10.1007/s41365-025-01699-z.