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X-ray Astrophysics Probe Technology Landscape

Established: 2026-04-07 (session 71). Updated: 2026-04-07 (session 72) — expanded from detectors-only to full AXIS instrument stack. Updated: 2026-04-07 (session 73) — expanded from AXIS-only to four-probe landscape (AXIS + LEM + STROBE-X + HEX-P). Updated: 2026-04-07 (session 74) — HXPP polarimetry track added; APRA 2023 full cohort breakdown; calibration infrastructure; TX04 mismatch noted for [157554]. Updated: 2026-04-07 (session 75) — HREXI/4piXIO correction (primary mission target is constellation, not HEX-P); full 30/30 APRA confirmed; MANTIS CubeSat noted; REDSoX no-docs confirmed; gap items updated.

Summary

Astro2020 recommended one X-ray probe mission next decade. Four probe concepts each have technology development programs visible in TechPort:

Probe Science Driver Detector Mirror Heritage TRL TechPort Investment
AXIS Sub-arcsecond imaging, AGN/galaxy feedback CCD (SiSeRO) / HCMOS Piezo-adjustable Si / Wolter Si 4-5 HIGHEST — 10+ SAT+APRA grants, all 3 instrument components
LEM Wide-FOV emission spectroscopy, missing baryons/CGM TES microcalorimeter Si shell (Zhang-style) 5-6 (XRISM heritage) MODERATE — 4-5 GSFC IRAD+APRA grants
STROBE-X Large-area timing, compact objects, TDAMM SDD (NICER derivative) X-ray concentrator 7+ (NICER heritage) MINIMAL — 1 APRA detector grant
HEX-P Hard X-ray (10-100 keV), NuSTAR follow-on CdZnTe / TES hard Multilayer mirrors 4-6 (NuSTAR heritage) LOW — 3-4 APRA+IRAD grants

Investment asymmetry explains technology challenges: AXIS requires the most new development (sub-arcsecond optics is unsolved) → gets the most investment. STROBE-X and LEM have mature detector heritage → need less early-stage funding. HEX-P has NuSTAR heritage but still needs coating improvements.

AXIS detail: SAT structural shift visible — the 2023 SAT cohort has essentially no X-ray technology, now almost exclusively HWO/exoplanet. AXIS technology moved to APRA for 2023-2026. Only the MIT CCD detector track retained a SAT grant (via MIT Lincoln Laboratory). Penn State leads 3 simultaneous APRA grants covering all three AXIS instrument components ([157545] detectors, [157551] gratings, [157541] optics).

LEM concept sketch — Si shell mirror + TES calorimeter, f=3.5m LEM concept sketch from [95880] (GSFC IRAD 2020). f=3.5m focal length, TES calorimeter grid array at focal plane.

SECTION 1: DETECTORS

Track 1: MIT CCD (AXIS Baseline Candidate)

Status: Active continuation. The SAT-funded strategic detector bet for AXIS.

This collaboration spans MIT Kavli Institute, MIT Lincoln Laboratory, Stanford, and Brookhaven. PI has evolved: Marshall Bautz (MIT Kavli) through 2025, then Christopher Leitz (MIT Lincoln Lab) from 2023. The organization leading SAT grants shifted from academia to FFRDC/UARC — MIT LL is the device fabrication facility; MIT Kavli is the astrophysics requirements group.

Grant lineage

ID Program Lead Org PI Period TRL Notes
94329 SAT MIT Bautz 2018-2019 3→4 Early CCD development
96368 SAT MIT Bautz 2020-2022 3→3 (target 4) Lynx-era; "Advanced To" [117276]
117276 SAT MIT Bautz 2022-2025 3→3 AXIS-era; "very close to TRL 4"
117287 APRA Stanford Allen 2022-2025 2→3 MCRC readout ASIC; parallel to [117276]
157581 SAT MIT LL Leitz 2023-2026 4→5 Soft X-ray optimization; target TRL 5
  • [96368] outcome: "Advanced To | 2022-12-01 | partner: Other" — explicit lineage link to [117276]
  • [117276] and [117287] share nearly identical co-investigator lists — same collaboration, two program mechanisms
  • [157581] lead is MIT Lincoln Laboratory (FFRDC), not MIT Kavli — shift from astrophysicist-led to device engineer-led. Leitz PI. Focus: soft X-ray (E < 0.5 keV) response optimization.
  • Technology: pJFET CCD amplifiers + SiSeRO (Single-electron Sensitive ReadOut) — genuinely novel, driving slow TRL progression
  • NASA SMD Technology Highlight issued for [117276] (URL-only, not readable via TechPort)
  • [157581] starts at TRL 4 — confirms [117276] achieved TRL 4 before closeout despite the description saying "very close to TRL 4." The TRL recording lag is real.

TRL note: Three consecutive SAT grants hovered at TRL 3-4. [157581] (MIT LL, 2023-26) starts at TRL 4 and targets TRL 5 — the first AXIS-era grant with a realistic shot at TRL 5 by Sep 2026. At TRL 5, a SAT application for the full mission-level validation cycle becomes viable.

Track 2: Penn State HCMOS (Long-Arc Development)

Status: Active ([157545] through 2027). Longest-running detector development arc in TechPort X-ray portfolio.

PI rotation: Abe Falcone and David Burrows at Penn State alternate as PI, with each other as Co-I. This is strategic — prevents eligibility gaps between grant cycles. Different aspects are segmented across different programs.

Complete grant timeline

ID Program PI Period TRL Title
10860 APRA Burrows 2009-2015 1→1 New Detector Development for X-ray Astronomy
89094 APD Burrows 2016-2019 n/a Testing and Characterization of a Next-Gen Event Driven Hybrid CMOS Detector
71967 APRA Falcone 2016-2020 3→4 (target 5) Novel Hybrid CMOS X-ray Detector Developments
91991 APRA Falcone 2017-2021 3→3 (target 4) Novel Hybrid CMOS X-ray Detector Developments
94295 APRA Burrows 2018-2021 n/a Fabrication of Sparse Readout Detectors
96348 APRA Falcone 2020-2024 4→4 (target 4) Fabrication and Testing of Novel Hybrid CMOS X-ray Detectors
118424 STRG Falcone 2020-2024 2→3 Developing Next-Gen X-Ray HCMOS: Low-Energy Response
157545 APRA Falcone 2023-2027 4→5 (target) Event-Driven and Rapid-Readout Hybrid CMOS Detectors

8 grants, 4 programs (APRA, APD, STRG, STMD), ~$10M+ invested over 15 years

Technology evolution

  • 2009-2015: Design phase only. "Current funding allows us to carry this design through CDR, but will not cover fabrication." TRL 1 throughout.
  • 2016-2021: Three parallel grants develop: (a) ROIC readout circuit [71967], (b) silicon detector layer [91991], (c) ROIC integration [94295]. Parallel grants explain the apparent redundancy — each covers a different component.
  • 2020-2024: Fabrication phase [96348] — "a 1k × 1k pixel full-size device" now ready for fabrication after 3-year ROIC design + 3-year ROIC array design.
  • 2020-2024: STRG parallel grant [118424] for low-energy response characterization — cross-directorate (STMD) funding for a specific performance metric.
  • 2023-2027: [157545] "event-driven" architecture — reads only pixels with signal above threshold, enabling fast effective readout without reading every pixel. Target TRL 5.

TRL reality check

Achieved TRL: 4 (confirmed in [96348] and [71967] live API)
Target: 5 by 2027
Gap to AXIS: Mission-level (TRL 7) is still 2-3 program increments away
Key gap: Never had a SAT grant — APRA is the primary mechanism. SAT would require demonstrating AXIS-specific performance requirements, which would be the natural next step if TRL 5 is achieved.

Target missions (as stated in descriptions)

  • [10860] 2009: IXO, Xenia, Gen-X
  • [71967] 2016: "SMART-X," X-ray Surveyor
  • [96348] 2020: Lynx (HDXI instrument), AXIS
  • [157545] 2023: "future X-ray missions with focused optics and/or large effective area"

The mission target has shifted with each Decadal cycle (IXO → Lynx → AXIS) while the technology continued developing. The detector is mission-agnostic at this TRL stage.

Track 3: SAO/JPL Delta-Doped Si Sensors

Status: Completed ([117498], 2022-2025).

ID Program PI Period TRL Title
117498 SAT Kraft (SAO) 2022-2025 4→4 Development of Advanced Pixelated Si Sensors for Next-Gen X-ray Observatories
  • Lead: Smithsonian Astrophysical Observatory (Ralph Kraft), JPL partner (John Hennessy, Shouleh Nikzad — the JPL delta-doping pioneer)
  • Technology: Delta-doped CMOS, JPL's surface passivation process that enables near-100% quantum efficiency for low-energy photons
  • Also targets Lynx/AXIS missions (Astro2020 language)
  • TRL 4→4 (maintaining — likely a performance characterization grant, not a development grant)
  • Program Director/Manager: Mario Perez (same as MIT SAT track)

Significance: Delta-doped Si has applications across UV, optical, and X-ray. The SAO/JPL connection bridges the Nikzad JPL UV detector program (which appears elsewhere in the APRA/SAT portfolio) into the X-ray regime.

Track 4: Iowa Monolithic CMOS (Minor/Emerging)

ID Program PI Period TRL Notes
117482 APRA Kaaret (Iowa) 2022-2025 n/a CMOS Sensors for Soft X-Ray Detection
  • PI: Philip Kaaret (Iowa), Co-I: Casey T DeRoo (Iowa RTF fellow, see programs/rtf.md)
  • Technology: Monolithic CMOS (not hybrid) — room temperature operation, low power
  • No TRL data. Co-investigator: Argonne National Laboratory (McChesney) — likely for X-ray synchrotron characterization
  • No explicit AXIS connection; more modest scope than HCMOS

DeRoo bridges Iowa's RTF grating program ([157605]) and this detector effort — assembling a soft X-ray instrument stack at Iowa.

Track 5: NRL Silicon Drift Detector (STROBE-X / TDAMM-focused)

ID Program PI Period TRL Notes
157538 APRA Ray (NRL) 2023-2026 3→5 Large Area Silicon Drift Detector for X-rays
  • PI: Paul S. Ray (NRL). Co-I: Gianluigi Zampa, Yuri Evangelista (Italian — INFN/INAF heritage)
  • Technology transfer from Italy: INFN/INAF developed 11cm × 7cm large-format SDDs with 1.5D position sensitivity, 10 μs time resolution for the CERN ALICE experiment, then re-optimized for X-ray astronomy
  • Target mission: STROBE-X (not AXIS): 2.1 m² X-ray Concentrator Array + 5.1 m² Large Area Detector + Wide Field Monitor; all three instruments use INFN/INAF SDDs. If STROBE-X is not selected as probe, a descoped MIDEX version is planned for 2026.
  • Keith C. Gendreau (GSFC, NICER PI) in program contact emails — direct NICER heritage connection; STROBE-X XRCA = "NICER ×10"
  • TRL target 5 in 3 years — developing the NSX1 ASIC readout specifically designed for these detectors
  • Not AXIS: STROBE-X is about timing and spectroscopy of compact objects (neutron stars, black holes), not high-angular-resolution imaging

This is the most distinct detector technology of the 2023 cohort: not a CCD or CMOS, but a silicon drift detector (SDD) optimized for timing over imaging. See Section 5 for full STROBE-X context.

SECTION 2: DISPERSIVE GRATINGS (for X-ray spectroscopy)

Two parallel grating technologies are being developed for AXIS-class soft X-ray spectroscopy. They compete architecturally: MIT's CAT gratings use critical-angle transmission through deep Si membranes; Penn State's off-plane reflection gratings use a different geometry.

Grating Track 1: MIT CAT-XGS (Critical Angle Transmission)

Status: Completed [117320] (2022-25). Active sounding rocket flight [117485] (2022-27).

PI: Mark Schattenburg (MIT). Co-PI/co-I: Ralf Heilmann (MIT). 10+ year SAT lineage targeting AXIS and Explorer-class missions.

Grant lineage

ID Program Period TRL Notes
18556 SAT 2015-2016 3→5 Advanced packaging for CAT gratings
92150 SAT 2017-2019 3→4 (target 6) CAT spectrometer development; significant miss
95127 SAT 2019-2022 4→4 (target 5) High resolution + high efficiency XGS; partial miss
117320 SAT 2022-2025 3→4 AXIS-era; Enhanced SOI wafers for taller grating bars
117485 APRA 2022-2027 5→9 REDSoX Polarimeter sounding rocket — TRL 9 by 2027!

Technology: CAT gratings are nanofabricated deep-etch Si membranes. Tall, thin, vertically aligned bars enable simultaneous high diffraction efficiency and high resolving power. The current bottleneck is grating bar aspect ratio — addressed in [117320] via new Enhanced Silicon-on-Insulator (E-SOI) wafers. Also mentioned for Arcus MidEx mission (Explorer class).

REDSoX is a polarimeter, not an AXIS precursor: [117485] (Herman Marshall PI, MIT APRA) is the Rocket Experiment Demonstration of a Soft X-ray Polarimeter — measuring first linear X-ray polarization in E < 1 keV band. Technology: laterally-graded multilayer mirrors (LGMLs) as Bragg reflectors at Brewster angle + CAT gratings + CCD detectors. Target sources: Mk 421 blazar (20%+ polarization expected) and isolated neutron stars (80-100% via vacuum birefringence). This is NOT an AXIS spectral component — it's an independent polarimetry program that shares grating fabrication heritage. Targets TRL 5→9 by September 2027 via sounding rocket flight. Partners: MSFC (mirror fabrication), Boston University (science), LBNL.

TRL reset pattern: [92150] targeted TRL 6, achieved 4. [95127] targeted 5, achieved 4. [117320] reset to TRL 3→4 — reflecting the more stringent AXIS probe requirements vs Explorer class. The pattern: TRL advance is real but slower than proposals claim.

Collaborator network: Schattenburg/Heilmann (MIT) are Co-I on MIT optics grant [157540]. The MIT X-ray instrumentation group is vertically integrated: grating fabrication + optics development + sounding rocket flight test.

Grating Track 2: Penn State Off-Plane Reflection Gratings

Status: Active [157551] (2023-2026).

ID Program PI Period TRL Notes
157551 APRA McEntaffer (Penn State) 2023-2026 4→5 X-ray Reflection Gratings: Key Developments
  • PI: Randall McEntaffer (Penn State), Co-I: Casey T DeRoo (Iowa) — same Iowa connection
  • Co-I: James Tutt (Penn State)
  • States include IA, PA, CA — broad collaboration
  • Technology: Off-plane reflection gratings — different geometry from CAT. Ruled grating surface in reflection mode vs transmission mode. Competitive with CAT for resolving power at comparable efficiency.
  • TRL starts at 4 (ahead of CAT grating [117320] which started at 3) — already proven at component level

Iowa connection: McEntaffer founded the Iowa grating program before moving to Penn State. DeRoo (Iowa, RTF) continues the Iowa grating work as McEntaffer's former student. The Penn State reflection grating grant and Iowa RTF grating grant ([157605]) are essentially the same research group split across two institutions.

SECTION 3: OPTICS (Sub-arcsecond X-ray Mirrors)

The most challenging component. AXIS requires ~0.5 arcsecond angular resolution with >2 m² effective area. No X-ray optics program currently in SAT (2023 cohort has none). All 2023+ X-ray optics grants are APRA or CIF.

Key network node: William W. Zhang (GSFC) — appears as Co-I on BOTH the Penn State adjustable mirrors grant ([157541]) and the MIT Wolter optics grant ([157540]). Was PI on the best-performing SAT optics grant ([92158], TRL 3→5, 2016-18). He provides GSFC's X-ray optical figure metrology and testing infrastructure for both groups.

Optics Track 1: SAO/PSU Adjustable Mirrors (Piezo-Actuated Figure Correction)

ID Program Lead/PI Period TRL Notes
18579 SAT SAO/Reid 2015-2016 3→3 (target 4) SMART-X era; 0.5" adjustable optics
92140 SAT SAO/Reid 2016-2018 3→3 (target 4) Hybrid approach; X-ray Surveyor era
95122 SAT SAO/Reid 2018-2020 3→4 Lynx era; full Wolter-I pair X-ray test
157541 APRA PSU/Trolier-McKinstry 2023-2026 3→4 AXIS era; APRA (not SAT)

Technology: Thin piezoelectric film actuators bonded to grazing-incidence mirror shells. Applying voltage to individual actuators reshapes the mirror surface to correct figure errors post-mounting. Goal: correct launch distortion and achieve 0.5" HPD on-orbit.

Key personnel consolidation: [157541] moves leadership from SAO's Paul Reid (Co-I) to Penn State's Susan Trolier-McKinstry (PI) — she's the piezo materials expert. Reid remains as Co-I. Zhang (GSFC) also Co-I. States: IA, MD, MA, PA — DeRoo from Iowa may be involved in X-ray testing.

SAT→APRA shift: This program was in SAT from 2015-2020 (3 grants). In 2023, it moved to APRA. The SAT 2023 cohort has no X-ray optics grants at all. Possible explanations: (1) the technology needs further foundational work before another SAT cycle, or (2) the SAT program pivoted focus to HWO.

Optics Track 2: MIT/GSFC Wolter Optics (Silicon Mandrel + Differential Deposition)

ID Program Lead/PI Period TRL Notes
10833 APRA GSFC/Zhang 2010-2014 1→1 Monocrystalline silicon mirror fabrication
92158 SAT GSFC/Zhang 2016-2018 3→5 X-ray Surveyor; best TRL performance in optics category
96334 APRA MIT/Schattenburg 2020-2022 3→3 AXIS/TAP/HEX-P sub-arcsecond tech
157540 APRA MIT/Schattenburg 2023-2026 2→3 Diffraction-limited Wolter optics; TRL 2

Technology: Thin silicon mirror shells fabricated by slumping/depositing on mandrels, then figure-corrected via differential deposition (selective coating to smooth figure errors). Goal: push below 0.1" (diffraction limit at 1 keV). "Surface figure errors now trending into sub-1 arcsec domain, exceeding Chandra mirror quality."

TRL regression in latest grant: [157540] starts at TRL 2 — a step back from [96334]'s TRL 3. The description claims diffraction-limited performance (0.1") is achievable within a decade, which would be transformative. But the TRL 2 starting point reflects that this is a new performance regime, not incremental progress.

GSFC achievement: [92158] (Zhang PI, SAT 2016-18) reached TRL 5 — the highest TRL ever achieved for X-ray optics in this program lineage. No follow-on SAT grant was funded for Zhang's approach after 2018. The SAT bet shifted to SAO's adjustable optics ([95122]).

Optics Track 3: MSFC RECXO (Replicated Epoxy Ceramic Shells)

ID Program Lead/PI Period TRL Notes
146969 MSFC CIF MSFC/Bongiorno 2023-2024 2→3 Replicated Epoxy Ceramic X-ray Optics
  • Technology: Ceramic mandrel blank + epoxy replication (pour liquid epoxy over mandrel, cure, release mirror)
  • Has "Advanced From | 2024-10-01" and "Advanced To | 2024-10-01" outcomes — there's a follow-on project
  • Hardware built: A ceramic mandrel blank ~15cm diameter was fabricated and CMM-measured
  • CMM metrology result: radius error of ~35 µm peak-to-valley (parabolic + hyperbolic segments — full Wolter-I pair) — far above sub-arcsecond requirements. This is the starting point, not the corrected result.
  • Saves as: RECXO ceramic mandrel blank

This is early-stage (CIF seed, 1-year, TRL 2→3) — the ceramic formulation choice is the innovation. Ceramic has lower thermal expansion than nickel/aluminum mandrels traditionally used. If the replication fidelity is sufficient, figure errors can be corrected post-replication.

Optics Manufacturing Infrastructure: OptiPro SBIR

ID Program Company Period TRL Notes
113442 SBIR/STTR OptiPro Systems (Ontario NY) 2021 1→4 Phase I: X-ray mirror polishing
125349 SBIR/STTR OptiPro Systems 2022-2024 2→7 Phase II: Advanced mandrel polishing platform

OptiPro developed a specialized polishing platform for X-ray mirror mandrels with 5 innovations: dual-tool machining head, rotisserie A-axis, force/torque feedback, adaptive tool-path algorithms, polishing software. Technology explicitly transfers to GSFC and MSFC polishing operations. TRL 7 achieved — a commercial tool capability for the mirror fabrication supply chain.

Dual-use application: missile dome polishing (mentioned in SBIR briefing). The same precision polishing economics serve both NASA X-ray astronomy and defense optics.

SECTION 4: PROGRAM STRUCTURE AND THE SAT→APRA SHIFT

The 2023 bifurcation

SAT 2023 cohort (all active): [157581] MIT LL CCD (X-ray), plus ~9 HWO/exoplanet grants (coronagraphs, UV detectors, microshutters, IR sensors). The only X-ray technology in 2023 SAT is the MIT LL CCD detector. No gratings, no optics.

APRA 2023 cohort — full breakdown (30 active, confirmed via portfolio_aggregate query 2026-04-07):

Domain Count Key grants
X-ray (probe technology) 8 (27%) [157541],[157540],[157551],[157545],[157538],[157615],[157553],[117485]
Far-IR / sub-mm detectors 5 (17%) [157547],[157546],[157536],[157557],[157544]
UV / optical 3 (10%) [157560] MANTIS,[157534] UV SNSPDs,[157539] FUV metamaterials
Gamma-ray / MeV 3 (10%) [157559] diamond Compton,[157535] ComPair,[157562] GRAMS
HWO / exoplanet 5 (17%) [157537] astrocomb,[157548] mid-IR SNSPDs,[157550] dark hole SCC,[157558] segmented telescope testbed,[117300] PICTURE-D balloon
Calibration / software 2 (7%) [117307] CANDLE (NIST),[157606] XSTAR
Other (radio, materials, photonics) 4 (13%) [157542] LCRT 21-cm,[157561] Al MMC mirrors,[157543] photonic NIR spectrograph,[157554] low-TCR MCP UV

TX mismatch discovery: [157554] Argonne MCP (microchannel plates for UV detectors) is classified TX04 (Robotics/Autonomy) — clearly wrong. ML presumably predicts TX08. This is documented in field-completeness.md as Issue 21.

MANTIS [157560] — notable CubeSat program: 12U CubeSat (2023-2027, TRL 4→9), PI Briana Indahl (CU LASP, early career). First EUV astrophysics capability in 25+ years (EUV last active with EUVE, shut down 2001). INAF (4 Italian institutes) contributes the grazing incidence EUV telescope. Key connectivity: Randall McEntaffer (PSU, X-ray gratings PI [157551]) and Fabien Grise (PSU, zone plate calibration PI [117487]) are Co-Is — the same two PSU researchers who build X-ray calibration infrastructure for probe missions are also working on a UV CubeSat. Heritage: CUTE + SPRITE CubeSats (LASP). Science: UV monitoring of exoplanet host stars contemporaneous with JWST transiting planet observations (JUMP survey). MANTIS = the only EUV capability on the horizon for the next decade.

X-ray dominates the 2023 cohort (27%) — more than any single science domain. The concentration reflects both the number of probe concepts competing and the breadth of instrument components each requires.

APRA 2023 cohort (X-ray portion): [157541] Penn State adjustable optics, [157551] Penn State reflection gratings, [157545] Penn State HCMOS detectors, [157540] MIT Wolter optics, [157538] NRL SDD detectors, [157615] LLNL LEM calibration, [157553] GSFC HXPP additive mfg, [117485] MIT REDSoX [2022].

Interpretation: After the 2020 Decadal Survey endorsed HWO as the flagship priority, SAT pivoted almost entirely to HWO technology (coronagraphs, UV coatings, WFC). X-ray probe technology continues to mature, but through APRA (exploratory, competitive) rather than SAT (strategic, mission-connected). This is not a programmatic death — it reflects X-ray probe being 1-2 mission cycles behind HWO in the priority queue.

Penn State as the most active 2023-2026 institution

Penn State leads 3 simultaneous APRA grants in the AXIS technology space (2023-2026): - [157541] Adjustable X-ray mirror optics (Trolier-McKinstry PI) - [157551] Off-plane reflection gratings (McEntaffer PI) - [157545] Hybrid CMOS detectors (Falcone PI)

No other institution leads more than one. Penn State effectively covers the entire AXIS instrument stack except the SAT-funded CCD detector track (MIT LL) and the Wolter optics track (MIT APRA).

The Casey DeRoo network

Casey T. DeRoo (University of Iowa) appears as Co-I on: - [95122] SAO adjustable optics SAT (2018-2020) - [157551] Penn State reflection gratings APRA (2023-2026) - [117482] Iowa CMOS detector APRA (2022-2025)

And as PI on [157605] Iowa RTF grating (2023-2025). DeRoo spans optics + gratings + detectors across three institutions. His role is X-ray instrument systems engineer — the person who understands all three components and their integration.

Detector Program Structure

Program Mechanism Track TRL Range Notes
SAT (SMD) Strategic, competed MIT CCD (MIT LL) 4→5 [157581] Active 2023-26; Mario Perez PD
SAT (SMD) Strategic, competed SAO/JPL delta-doped Si 4→4 [117498] Completed; maintenance grant
APRA (SMD) Exploratory, competed Penn State HCMOS 4→5 [157545] Active 2023-27; 15-year lineage
APRA (SMD) Exploratory, competed NRL SDD 3→5 [157538] Active 2023-26; Italian tech transfer
APRA (SMD) Exploratory, competed Iowa monolithic CMOS n/a [117482] Completed 2025

Penn State HCMOS has never entered SAT despite 15 years and ~8 grants. Either the technology hasn't met SAT performance thresholds, or NASA's strategic bet for AXIS has already settled on CCDs (MIT). [157545] targeting TRL 5 by 2027 would be the natural trigger for a SAT application.

Dual-mechanism strategy (MIT/Stanford, 2022-25)

MIT and Stanford ran parallel grants for the same system: - [117276] SAT: The detector sensor (Bautz MIT PI) - [117287] APRA: The readout ASIC (Allen Stanford PI)

Same co-investigator list, two programs, two grant mechanisms. This efficiently funds both components without forcing one into the scope of a single grant.

TRL Summary Across All Tracks

Detectors (where technology stands now, April 2026)

Technology Best Achieved TRL Key Grant Status
MIT CCD (SiSeRO) 4 (entering 5) [157581] Active SAT-funded; target TRL 5 by Sep 2026
Penn State HCMOS 4 [157545] Active APRA-funded; target TRL 5 by 2027
SAO/JPL delta-Si 4 [117498] Completed No continuation in TechPort
NRL SDD (Italian) ~3 (TRL 5 target) [157538] Active New; TDAMM science focus

Gratings

Technology Best Achieved TRL Key Grant Status
MIT CAT-XGS 4-5 (component) [117320] Completed Sounding rocket path to TRL 9 via [117485]
Penn State off-plane reflection 4 [157551] Active APRA-funded; target TRL 5

Optics

Technology Best Achieved TRL Key Grant Status
SAO/PSU piezo adjustable 4 (prior cycle [95122]) [157541] Active APRA-funded; TRL 3→4 target
GSFC slumped Si (Zhang) 5 [92158] Completed 2018 No continuation in TechPort — gap
MIT Wolter diffraction-limited 2 [157540] Active TRL 2→3 target; very early
MSFC RECXO ceramic 2→3 [146969] Completed 1-year CIF; follow-on exists but unresolved

The optics track is furthest from mission-ready. Despite the GSFC approach ([92158]) achieving TRL 5 in 2018 — the best result — it received no SAT follow-on. Zhang's subsequent contribution is as Co-I on other groups' grants, not as PI. The optics gap (TRL 5 maximum, no funded path to TRL 6+) is the most significant bottleneck in the AXIS instrument stack.

SECTION 5: OTHER PROBE TECHNOLOGY STACKS

LEM (Line Emission Mapper)

Science: Map emission lines 0.1–2 keV at 1–2 eV FWHM energy resolution over 30' × 30' FOV. Primary science: intergalactic medium (missing baryons, CGM feedback, galaxy formation). GSFC-originated concept.

Architecture: Silicon shell mirror (f=3.5m focal length) + TES microcalorimeter focal plane (see sketch above from [95880]).

Technology advantage over AXIS: TES microcalorimeters are well-proven via XRISM/Resolve (launched 2023, operational). The GSFC TES dynasty (Kelley, Bandler, Smith, Kilbourne, Leutenegger, Markevitch) has continuous development since the Astro-H era. LEM's main R&D challenge is scaling to a larger-format array and improving the blocking filter infrastructure.

LEM TechPort investment

ID Program Lead Period TRL Role
95880 GSFC IRAD GSFC/Markevitch 2019-2020 1→2 Mission concept study; Si shell mirror + TES array
117122 GSFC IRAD GSFC/Mateo 2022-2023 3→5 Anti-coincidence detector (LEM focal plane larger than XRISM/Athena)
183273 ISFM NASA HQ/Smith 2022-2025 n/a Advanced microcalorimeter arrays (TES + MMC); GSFC-based
158140 SBIR PhotonFoils/Lairson 2024-2025 4→5 SiC blocking filter grids — NASA Tier 1 tech gap for LEM and newAthena
157615 APRA LLNL/Hell 2023-2026 n/a Microcalorimeter calibration — parallelized approach for LEM FOV

Key personnel: Natalie H. Hell (LLNL, PI on [157615]), Frederick Porter (GSFC), Stephen Smith (GSFC), Caroline Kilbourne (GSFC), Simon Bandler (GSFC) — the same team that flew the XRISM/Resolve instrument. Maxim Markevitch (GSFC) originated the LEM mission concept.

TX mismatch in [157615]: Primary TX = TX08.3 (In Situ Instruments) — clearly wrong (ML predicts TX08.1.1 Detectors correctly). Another case of the calibration/lab work falling in an in-situ TX bin.

Mirror gap: LEM's Si shell mirror (larger FOV, relaxed angular resolution vs. AXIS) last appeared in TechPort as [95880] concept sketch (2019-2020). William Zhang (GSFC) was a Co-I. No dedicated LEM optics development grant found in 2023 cycle. This may reflect that silicon shell optics for moderate resolution (arcminute-class) are already at TRL 5 from the Zhang [92158] lineage — below the AO threshold, not requiring new TechPort investment.

HEX-P (High Energy X-ray Probe)

Science: Hard X-ray imaging spectroscopy 2-80+ keV (beyond NuSTAR's 3-79 keV), improved sensitivity and angular resolution for compact objects, stellar coronae, diffuse hard X-ray emission. NuSTAR successor.

Architecture: Multilayer-coated focusing mirror + CdZnTe detector (NuSTAR-heritage) OR TES microcalorimeter (for very high resolution).

Technology advantage: Fiona Harrison (Caltech, NuSTAR PI) is Co-I on the HREXI detector program. NuSTAR ASICs (NuASICs) are already flight-proven. The main R&D is: (a) extending multilayer mirror coatings to higher energies/larger shells, and (b) improving detector packaging with TSV instead of wire bonds.

HEX-P TechPort investment

ID Program Lead Period TRL Role
95084 APRA (unknown) 2019-2021 3→4 Hard X-ray multilayer coatings (Co/Ni/Pt up to ~200 keV) — explicitly names HEX-P
157924 MSFC IRAD MSFC 2024 3→6 Large-shell hard X-ray multilayer deposition chamber — major TRL jump
117256 APRA Harvard/Grindlay 2022-2025 4→6 HREXI CdZnTe with TSV connectivity (NuASIC); Completed June 2025. See 4piXIO note below.
117226 APRA UNH/Kislat 2022-2025 3→5 TES microcalorimeter hard X-ray (55 eV FWHM at 100 keV — 20× better than CdZnTe)

Two detector philosophies with NuASIC heritage: 1. CdZnTe ([117256]): Better proven (NuSTAR heritage), TRL 4→6, larger format achievable. PI Grindlay (Harvard), Co-I Harrison (NuSTAR PI) + Miyasaka (JPL). TSV replaces fragile wire bonds. 2. TES hard X-ray ([117226]): Much better energy resolution (55 eV vs ~700 eV for CdZnTe), but lower TRL (3→5), requires cryogenic operation. PI Kislat (UNH), detector fabrication by NIST (Becker/Weber/Schmidt).

CORRECTION — HREXI primary target is 4piXIO, not HEX-P: The full HREXI project description (confirmed session 75) names 4piXIO (4π X-ray Imaging Observatory) as the mission target — not HEX-P. 4piXIO is a 8-12 SmallSat constellation with coded aperture wide-field detectors (0.87 sr FoV), targeting GRBs, transients, and multi-messenger astrophysics. This is architecturally different from HEX-P's focusing telescope + small focal plane.

The 13-year HREXI development arc: 13966 (2012-2016) → 91919 (2017-2020, TRL 2→4, explicitly names 4piXIO + 12U CubeSat test flight) → 117256 (2022-2025, TRL 4→6, TSV module). Grindlay has been building toward a 4piXIO SMEX/MIDEX/Probe since at least 2017. The NuASIC heritage is shared with HEX-P (Fiona Harrison's group at Caltech supplies the 300 µm pixel ASIC upgrade), but the detector plane architecture (1024 cm² coded aperture) is specific to 4piXIO's large-area, wide-field requirement.

MSFC IRAD [157924] significance: TRL 3→6 in one year (MSFC CIF-type internal grant) for multilayer deposition — a major manufacturing capability achievement if valid. The MSFC hard X-ray mirror team has been developing shell X-ray optics for decades (heritage: HEROES/NuSTAR testing).

STROBE-X (Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays)

Science: Time-domain and multi-messenger (TDAMM) astrophysics. Neutron stars, black holes, gamma-ray bursts, gravitational wave counterparts. ~10× NICER's collecting area + all-sky wide-field monitor.

Architecture: Three instruments: (1) 2.1 m² X-ray Concentrator Array (XRCA — NICER concentrators stacked) at 0.2-12 keV, (2) 5.1 m² Large Area Detector (LAD) at 2-30 keV using SDDs, (3) Wide Field Monitor (WFM) using SDDs for all-sky coverage.

Technology advantage: NICER (launched 2017, operational at ISS) is STROBE-X XRCA at smaller scale. The concentrator + SDD technology is flight-proven at TRL 9. Main new development is the Italian INFN/INAF SDD for the LAD and WFM components.

STROBE-X TechPort investment

ID Program Lead Period TRL Role
157538 APRA NRL/Ray 2023-2026 3→5 Large-area SDD with NSX1 ASIC readout (Italian INFN/INAF technology transfer)

Only one TechPort grant. The rest of STROBE-X technology is either: (a) already at TRL 9 via NICER, or (b) being developed outside TechPort (international collaboration with Italy). The Italian INFN/INAF SDD was originally for CERN ALICE experiment — the ASIC development is the missing link for US flight use.

PI Paul Ray (NRL): Key bridge between US and Italian teams. Keith Gendreau (NICER PI, GSFC) as Co-I confirms heritage path.

Hard X-ray Photoelectric Polarimeter (HXPP) — Independent Polarimetry Track

Science: Linear X-ray polarimetry at 10-70 keV — energy range above IXPE's 2-8 keV band. Primary targets: accretion-powered pulsars with cyclotron absorption features at ~20 keV. Polarization measurements constrain accretion geometry models that spectral/timing data cannot distinguish.

Architecture: Time Projection Chamber (TPC) gas detector — photoelectric effect is the dominant interaction mechanism in the 10-70 keV range, enabling high-sensitivity, low-systematic polarimetry. Platform: InFOCμS hard X-ray focusing optics on a long-duration balloon flight, then potential space mission.

Why this is distinct from the four probes: HXPP is NOT a probe precursor. It is an independent science program targeting a specific measurement that a probe would not prioritize. IXPE (launched Dec 2021) proved soft X-ray polarimetry works. HXPP extends it to hard X-rays where no mission has measured.

Heritage: GEMS (Gravity and Extreme Magnetism Small Explorer, proposed SMEX) → PRAXyS (Polarimeters for Relativistic Astrophysical X-ray Sources, proposed SMEX) → HXPP balloon. Neither GEMS nor PRAXyS was selected for flight. The TPC technology from both programs is being recycled for the balloon program.

HXPP TechPort investment

ID Program Lead PI Period TRL Role
94322 APRA GSFC Hill-Kittle 2017-2019 n/a First APRA development cycle — gas mixture optimization
93229 GSFC IRAD GSFC Leutenegger 2017-2023 4→4 Parallel IRAD — gas mixtures and pressures for flight components
117144 GSFC IRAD GSFC (Paquette) 2022-2023 2→3 Aerosol jet printing software for detector readout boards
157553 APRA GSFC Paquette 2023-2026 3→4 Additive manufacturing for TPC assembly — references parent "Zajczyk APRA"

Institutional team (GSFC-dominated): - Beth M. Paquette (PI, [157553]) - Keith M. Jahoda (GSFC) — Co-I on [94322] and [157553]; RXTE/NICER heritage, X-ray timing community - Maurice Leutenegger (GSFC) — PI on IRAD [93229]; also Co-I on XRISM Resolve analysis - Anna Zajczyk (GSFC) — Co-I on IRAD; PI on referenced parent APRA "Continued Development" grant (not separately found in TechPort) - Rock Creek Scientific (Reston VA industry partner) and University of Maryland on [157553]

TX mismatch in [157553]: Listed as TX08.3 (In Situ Instruments) — clearly wrong. ML correctly predicts TX08.1.1 (Detectors/Focal Planes).

Technology note — additive manufacturing innovation: The key advance in [157553] is aerosol jet printing (AJP) of the Readout Board (ROB) strip patterns directly onto 3D surfaces. GEMS/PRAXyS used flat strip-on-surface techniques. AJP on curved surfaces enables a more robust TPC geometry. GSFC IRAD [117144] proved AJP patterns survive X-ray environment; [157553] extends to attaching readout electronics and performing environmental tests.

Probe Selection Context

All four probe concepts were response to the 2020 Decadal Survey recommendation for one probe-class X-ray mission this decade (~$1B scale). As of April 2026, no probe AO has been issued for X-ray. The technology investment pattern visible in TechPort suggests:

  • AXIS is the most "investment-hungry" probe (hardest technology, most active investment). The AXIS sub-arcsecond optics bottleneck (best TRL = 5 in 2018, no clear path to 7) is the main barrier.
  • LEM has the most mature technology but serves a narrower science case (emission spectroscopy). XRISM is already doing some of this science — LEM would be a factor of 100+ improvement.
  • STROBE-X has the most mature technology (NICER heritage). If mission cost is the constraint, STROBE-X may have the best technology readiness vs cost ratio.
  • HEX-P addresses a clear NuSTAR science gap. CdZnTe technology at TRL 6 makes it relatively low-risk on the detector side; mirrors are the remaining gap.

Calibration Infrastructure (Cross-Probe)

Two grants address calibration challenges that apply to ALL X-ray probe concepts:

[117487] Large-Format X-ray Zone Plates (PSU, Completed 2022-2024, TRL 3→6) - PI: Fabien Grise (PSU). Co-I: Casey T. DeRoo (Iowa), Randall McEntaffer (PSU), Wayne Baumgartner (MSFC) - Problem: Ground calibration beamlines have finite source distances → wavefront curvature → best focus shifts → calibration fidelity compromised - Solution: Large-format zone plates as X-ray collimators to generate plane-wave illumination at beamlines - Deployed one zone plate at MSFC Stray Light Facility. TRL 3→6 in 2 years = significant calibration capability achievement - Baumgartner (MSFC) Co-I bridges this to MSFC's existing X-ray test infrastructure

[157606] XSTAR: Spectral Modeling for the XRISM Era (GSFC, Active 2023-2026) - PI: Timothy Kallman (GSFC, X-ray atomic data specialist). Co-I: Claudio Mendoza, David Leisawitz (GSFC), Javier Garcia (Caltech) - Purpose: Pythonize the xstar atomic physics package to expose internal modules — allow users to "get under the hood" and access intermediate calculations for validation and atomic data inspection - Supports XRISM/Resolve science now; pre-building interpretive infrastructure for LEM's high-spectral-resolution data - Not hardware TRL work — pure software/atomic physics. Classified as TX11. - Leisawitz (GSFC) appears as Co-I here AND on HXPP [157553] — suggesting Leisawitz is a broader GSFC X-ray program coordinator.

Gaps and Open Questions

  1. Why did GSFC Zhang optics ([92158], TRL 5) get no SAT continuation? The best-performing optics technology stopped progressing after 2018. Zhang continues as Co-I on MIT and PSU grants — his infrastructure absorbed into collaborations, not a GSFC-led program.
  2. Does Penn State HCMOS ([157545]) achieve TRL 5? Closes Sep 2027. A TRL 5 result would be the natural trigger for their first SAT application.
  3. REDSoX TRL 5→9 by 2027: [117485] TRL range is unusually ambitious (5→9). TRL 5 reflects lab-verified components; TRL 9 represents a fully-flown sounding rocket system. No library documents in TechPort as of session 75 (mid-program, consistent with 2027 flight target). PI Herman Marshall (MIT). If flight succeeds, soft X-ray polarimetry and CAT gratings achieve flight heritage. Science: first soft X-ray polarization measurement of Mk 421 blazar (~20% expected) and future isolated neutron star (80-100% via vacuum birefringence). Watch Sep 2027.
  4. AXIS probe AO: Not in TechPort. Decadal recommended it. No AO as of April 2026. Technology bottleneck is optics (TRL 5 max, no funded path to 7).
  5. Penn State coordination: Three active APRA grants ([157541]/[157551]/[157545]) share only a program manager. Independent PIs, no evidence of a coordinated instrument team. But DeRoo (Iowa) bridges gratings + detectors.
  6. SAT→APRA shift permanence: Was the 2023 SAT pivot to HWO a temporary adjustment? The 2025+ SAT cohort (not yet visible in TechPort) will answer this.
  7. Zajczyk parent APRA grant: [157553] references "Continued Development of a Hard X-Ray Polarimeter, Zajczyk PI" — this grant is not separately findable in TechPort under GSFC or standard search. Zajczyk is Co-I on GSFC IRAD [93229] (Leutenegger PI). The parent APRA may be under a different lead organization or title.
  8. HXPP path to space: Balloon program targets InFOCμS optics. No space mission planned yet. If the balloon flight is successful, a SMEX proposal would be the natural follow-on — continuing the GEMS/PRAXyS lineage.
  9. HREXI/4piXIO SAT status: HREXI reached TRL 6 June 2025. The mission target is 4piXIO SmallSat constellation, not a SAT. As of April 2026, no SAT or MIDEX application visible in TechPort for 4piXIO. A third APRA cycle (post-[117256]) would be the expected next step; absent such a grant, 4piXIO may be in proposal stage outside TechPort scope.
  10. Full APRA 2023 cohort now confirmed (30/30): All 30 active APRA projects identified (session 75). The "missing" projects not found by initial X-ray keyword search: [157542] LCRT 21-cm (JPL), [157561] Al MMC mirrors (Outpost), [117307] CANDLE calibration (NIST), [117300] PICTURE-D coronagraph balloon (UMass), [157543] photonic NIR spectrograph (JPL), [157537] astrocomb (JPL), [157558] segmented telescope testbed (STScI). All confirmed in the 30-result API page.
  11. LEM optics gap: No active TechPort mirror development for LEM's Si shell approach. May be post-[92158] TRL 5 considered sufficient for the next program step.
  12. TES vs CdZnTe for HEX-P: Both at TRL 5-6. CdZnTe is cryogen-free (advantage for cost/mass); TES has 10× better energy resolution. Selection will drive detector community investment.
  13. STROBE-X MIDEX fallback: If STROBE-X isn't selected as the probe, PI Ray plans a descoped MIDEX version. This is a planned hedge — SDD development for STROBE-X has intrinsic value regardless of probe selection outcome.