XtraMCP: Advanced UV/EUV Detector

• XtraMCP is an advanced microchannel plate detector integrating an open-face MCP with a direct-deposited AlGaN photocathode, extending sensitivity from 10 to 180 nm and enhancing quantum detection efficiency.
• Its novel architecture reduces system size, mass, and power while providing high spatial resolution and rapid single-photon counting through an FPGA-based non-iterative centroiding algorithm.
• AlGaN photocathode engineering via controlled RF-plasma MBE growth offers tunable bandgap properties and improved imaging performance, making it well-suited for UV/EUV astronomy.

XtraMCP is an advanced microchannel plate (MCP) detector technology developed to address the requirements of ultraviolet (UV) and extreme ultraviolet (EUV) astronomy. Designed and prototyped at IAAT, University of Tübingen, XtraMCP integrates an open-face MCP with an opaque aluminum gallium nitride (AlₓGa₁₋ₓN, or AlGaN) photocathode directly deposited on the MCP stack, targeting enhanced quantum detection efficiency (QDE), extended wavelength response, improved spatial and temporal resolution, and significant reductions in system size, mass, and power. The detector leverages a coplanar cross-strip anode (CSA) with onboard FPGA-based non-iterative centroiding for high-rate, high-resolution single-photon counting across the 10–180 nm spectral range (Diebold et al., 4 Dec 2025).

1. Development Goals and System Architecture

XtraMCP’s principal objective is to extend MCP-based ultraviolet detectors beyond the traditional 118 nm cutoff into the full far-ultraviolet (FUV: 90–180 nm) and the extreme ultraviolet (EUV: 10–90 nm). The instrument utilizes an open-face MCP geometry with an AlGaN photocathode deposited directly on the MCP stack, which is expected to surpass the limitations of conventional KBr and CsI photocathodes.

The architecture emphasizes reducing per-count gain and power consumption, increasing detector lifetime, and minimizing overall size and mass. The CSA readout module consists of a 64 × 64 cross-strip configuration implemented on a low-temperature cofired ceramic (LTCC) substrate, achieving an active area of approximately 39 × 39 mm² with a total module diameter near 88 mm. All readout electrodes reside in a single plane, minimizing cross-talk and inter-strip capacitance (order of 1 pF).

2. AlGaN Photocathode Engineering

XtraMCP incorporates AlGaN photocathodes with tunable bandgap properties for optimal FUV/EUV sensitivity. Growth of AlGaN thin films is conducted using RF-plasma molecular beam epitaxy (MBE) at 600 °C on single-crystal MgO(100) substrates (10 × 10 mm²). The Al fraction ($x$) is controlled between 0.00 and 1.00 to tune the bandgap across 3.31 eV to 5.52 eV, enabling visible blindness (bandgap $\geq 4$ eV for $x > 0.25$).

HR-XRD analyses reveal pure cubic crystal phases up to $x = 0.75$, with minimal lattice mismatch at $x = 0.25$ ($a_\text{film} = 0.4208$ nm, $a_\text{MgO} = 0.4212$ nm, mismatch $\approx 0.1\%$). Optical characterization via transmission and reflection spectroscopy (190–1100 nm, $\Delta \lambda=5$ nm) results in uniform bandgap values, as detailed in the following table.

Al fraction x	$a_\text{AlGaN}$ (nm)	$a_\text{MgO}$ (nm)	Bandgap $E_g$ (eV)
0.00	0.4520	0.4212	3.31
0.25	0.4208	0.4212	4.02
0.50	0.4060	0.4212	5.49
0.75	0.4060	0.4212	5.51
1.00	0.4027	0.4212	5.52

Surface morphology studies have identified Ga droplet formation at intermediate Al concentrations, with ongoing mitigation via pulsed growth and III/V ratio adjustments. Prospective enhancements include p-doping and atomic layer deposition (ALD) of MgO buffer layers on MCPs to promote epitaxy and optimize photocathode performance.

3. Centroiding Algorithm and Anode Readout

Photon event localization in XtraMCP is accomplished through a non-iterative centroiding algorithm implemented on an FPGA, enabling fixed-latency, parallel X/Y event processing. Each event’s charge cloud, assumed Gaussian in both dimensions, is modeled by $z(x) = A \exp[-(x - \bar{x})^2 / (2\sigma^2)]$, which is reformulated for weighted least-squares (Guo 2012 weighting). The system solves for coefficients $(a, b, c)$ such that $\ln z = a + b x + c x^2$ over all strips exceeding threshold.

Position extraction follows:

• $\bar{x} = -b / (2c)$,
• $\bar{y} = -b_y / (2c_y)$.

FPGA implementation leverages look-up tables (LUTs) for integer $\ln(z) \cdot z^2$ computation, with no iterative loops, ensuring deterministic throughput. Nonlinearity correction is achieved by LUT remapping (1k × 5-bit per axis, converting 10-bit fractional input to corrected output).

4. Detector Performance Metrics

Spatial and temporal resolution, count rate, power, mass, and operating gain are key performance figures for XtraMCP:

• Spatial resolution: Software Gaussian fit achieves $\Delta x_\text{FWHM} = 11\,\mu$m and $\Delta y_\text{FWHM} = 12\,\mu$m; FPGA realization currently delivers $\Delta x_\text{FWHM} = 32\,\mu$m and $\Delta y_\text{FWHM} = 33\,\mu$m, with further refinement underway.
• Maximum count rate: Pre-optimization FPGA pipeline supports sustained $\approx 28$ kcps, expected to exceed $100$ kcps with ongoing improvements.
• Timing accuracy: Event timestamping limited by MCP/transimpedance amplifier shaping, sub-10 ns resolution (not yet precisely quantified).
• Linearity: After LUT correction, positional distortion across the strip is $<1\%$.
• Size and power: Detector package occupies $<10$ cm³, draws $<3$ W, and has a total mass $<1$ kg (including housing and optics flange), representing a $\approx 50\%$ mass reduction compared to traditional MCP/CCD.
• Operating gain: $10^6$ (versus $10^7$ heritage systems), extending the extracted charge budget by a tenfold factor.

Dark count rates are measured at $<0.05$ events cm⁻² s⁻¹ with wide-bandgap AlGaN ($E_g > 5$ eV) and low-outgassing MCP glass. The extracted charge budget is cited at $\sim 5$ C cm⁻² versus 0.2–1 C cm⁻² for conventional sealed tubes.

5. Comparative Assessment and Technical Challenges

XtraMCP is designed to outperform detectors based on KBr or CsI photocathodes, which display QDE peaks below 60% in FUV; AlGaN aims for QDE $>70\%$ (pending measurement). The system delivers comparable or better spatial resolution ($<15\,\mu$m, software; $\leq 30\,\mu$m, FPGA) than delay-line or wedge-strip readout approaches ($\sim 50$–100 $\mu$m FWHM).

This suggests XtraMCP will enable higher-fidelity imaging for astrophysical UV/EUV applications while maintaining single-photon sensitivity and superior timing resolution. However, challenges persist in MBE growth directly on MCP substrates due to lattice mismatch and Ga droplet control. FPGA implementation requires optimization of bit depth and pipeline architecture to reach software-equivalent resolution and count rate.

6. Current Status and Prospects

AlGaN films on MgO substrates demonstrate tunable, wide bandgaps and predominantly cubic crystal phases, with minimal mismatch at $x \approx 0.25$. Suppression of Ga droplets, implementation of p-doping, and direct growth onto ALD-MgO-coated MCP stacks remain in development. Initial FPGA centroiding approaches closely match software benchmark resolution; further improvement targets flight-ready operation with FWHM $\leq 15\,\mu$m and count rates $>100$ kcps.

Planned work includes:

• Measurement of photocathode QDE across 10–180 nm using dedicated VUV/EUV facilities,
• Activation (Cs/O NEA layers) and absolute QDE quantification,
• FPGA centroiding refinement (bit depth, nonlinearity LUT),
• Environmental qualification (vibration, thermal vacuum, radiation) of the assembled detector.

Once these milestones are met, XtraMCP is poised to deliver a compact, efficient, high-resolution imaging solution for next-generation ultraviolet and extreme-ultraviolet astronomical missions (Diebold et al., 4 Dec 2025).