A Suborbital Payload for Soft X-ray Spectroscopy of Extended Sources

We present a suborbital rocket payload capable of performing soft X-ray spectroscopy on extended sources. The payload can reach resolutions of ~100(lambda/dlambda) over sources as large as 3.25 degrees in diameter in the 17-107 angstrom bandpass. This permits analysis of the overall energy balance of nearby supernova remnants and the detailed nature of the diffuse soft X-ray background. The main components of the instrument are: wire grid collimators, off-plane grating arrays and gaseous electron multiplier detectors. This payload is adaptable to longer duration orbital rockets given its comparatively simple pointing and telemetry requirements and an abundance of potential science targets.

💡 Research Summary

The paper presents a novel suborbital rocket payload designed to perform soft X‑ray spectroscopy of extended astronomical sources with a spectral resolving power of λ/Δλ ≈ 100 over a 17–107 Å (0.12–0.73 keV) bandpass. The instrument is capable of handling sources up to 3.25° in diameter, enabling comprehensive studies of objects such as nearby supernova remnants, the Galactic halo, the Local Hot Bubble, and the diffuse soft X‑ray background—targets that are difficult to analyze with current X‑ray observatories because of limited energy resolution and the angular extent of the sources.

The payload consists of two identical modules, each comprising three main components: a wire‑grid collimator, an off‑plane grating array, and a Gaseous Electron Multiplier (GEM) detector. The wire‑grid collimator uses 24 stacked plates with progressively narrower slits (from 725 µm down to 500 µm) and wire bars (166 µm to 114 µm). This “converging collimator” allows only photons that will intersect a predefined focal line to pass, effectively sculpting a one‑dimensional beam with a full‑width‑half‑maximum of 1.6 mm and a total field of view of 3.25° × 3.25°. The design is inexpensive, mechanically robust, and avoids the complexity of grazing‑incidence mirror assemblies.

After the collimator, the beam encounters an off‑plane grating array. Each module contains 67 thin nickel‑coated gratings, fabricated by electroforming nickel on a substrate that is only 5 µm thick. The gratings have a groove density of 5670 grooves mm⁻¹ with a sinusoidal profile, and are cut to a 20 mm active length using femtosecond laser pulses to preserve the groove shape. The off‑plane geometry (graze angle 4.4°) disperses the light conically, minimizing shadowing and allowing all diffracted orders to be captured. Laboratory measurements with a 0.28 keV carbon K‑α line show a first‑order diffraction efficiency of 22% and a second‑order efficiency of 5%, matching theoretical predictions. The authors note that further efficiency gains could be achieved by blazing the grooves or adjusting the graze angle.

The dispersed spectrum is recorded by a large‑area GEM detector (10 cm × 10 cm). X‑ray photons pass through a 5000 Å polyimide‑carbon window into an argon‑CO₂ gas volume. The initial ionization creates 5–30 electron‑ion pairs, which are amplified through four GEM foils, each providing a ~400 V drop across microscopic pores. The cascade yields a total gain of 10⁴–10⁵, sufficient to detect the weak soft X‑ray signals. Early versions of the detector suffered from manufacturing defects that caused shorted pores, hot spots, and reduced gain. The team solved these problems by switching to laser‑cut GEM foils from SciEnergy, which exhibit higher pore fidelity and lower breakdown rates, and by reinforcing the detector windows (now 5000 Å thick with a 300 Å carbon coating supported by a stainless‑steel mesh). These upgrades eliminated the need for long warm‑up periods and improved overall detector stability.

Overall, the instrument provides a factor of ten improvement in spectral resolution compared with CCD‑based instruments on current X‑ray satellites, while maintaining a wide field of view suitable for extended sources. Its simple pointing and telemetry requirements make it well suited for short suborbital flights, and the design is readily adaptable to longer‑duration orbital missions. The authors describe the successful 2006 CyXESS flight, the 2009 EXOS flight (which suffered landing damage), and the subsequent rebuild into the CODEX payload, which incorporates the described enhancements. Future flight plans aim to re‑observe the Cygnus Loop and other supernova remnants, leveraging the high‑resolution, large‑area spectroscopic capability to investigate plasma composition, charge‑exchange processes, and the energy budget of the diffuse soft X‑ray background.