---
title: IAXO-D1 Micromegas Detector
url: https://www.emergentmind.com/topics/iaxo-d1-micromegas-detector
type: topic
---

# IAXO-D1 Micromegas Detector

The IAXO-D1 Micromegas detector is a low-background gaseous X-ray detector prototype developed for BabyIAXO, the intermediate helioscope stage toward the International AXion Observatory. It is a microbulk Micromegas time projection chamber derived from the last generation of CAST Micromegas detectors and adapted to the BabyIAXO detection concept, in which solar-axion-induced X rays are focused by optics onto a small spot on the detector plane. Within that program, IAXO-D1 functions both as a detector prototype for BabyIAXO deployment and as an instrument for dedicated studies of spatial resolution, intrinsic background, radiopurity-oriented construction, shielding, and gas operation [2509.08138] [2303.15028].

## 1. Lineage, naming, and experimental role

BabyIAXO is described as the intermediate experimental stage toward IAXO and as a fully fledged helioscope with discovery potential. Its architecture comprises a 10 m long dipole magnet with two 70 cm diameter bores, two detection lines, X-ray focusing optics, and low-background X-ray detectors, on a rotating platform enabling 12 hours per day of solar tracking [2303.15028]. In the broader IAXO conceptual design, ultra-low-background Micromegas X-ray detectors are the baseline focal-plane detectors, imaging focused signal spots of about \(0.2\ \mathrm{cm^2}\) at the ends of magnet bores [1401.3233].

The designation “IAXO-D1” belongs to the BabyIAXO Micromegas prototype line. The same development program also includes IAXO-D0, an earlier prototype used for background-discrimination studies and surface-background reduction. In this sequence, D1 is presented as a new optimized BabyIAXO-relevant Micromegas prototype with improved radiopurity, optimized shielding, active veto integration, and new radiopure electronics [2303.15028]. A later status review characterizes IAXO-D1 more specifically as an underground, radiopure, shielded, low-background BabyIAXO/IAXO Micromegas prototype derived from the CAST microbulk detector concept and installed at the Laboratorio Subterráneo de Canfranc to validate intrinsic-background performance independently of surface cosmic rays [2404.09727].

The detector’s physics role follows the standard helioscope chain. Solar axions convert into X rays in the magnetic field, the optics focus the emerging photons into a small focal spot, and the detector must record those few-keV events with high efficiency and very strong background rejection. Earlier CAST systems already combined low-background microbulk Micromegas with dedicated X-ray optics as a technological pathfinder for IAXO, but those papers did not use the name IAXO-D1; they are best regarded as direct precursors rather than the D1 prototype itself [1509.06190].

## 2. Detector architecture and materials

IAXO-D1 is a small time projection chamber with a \(3\,\mathrm{cm}\) conversion/drift volume. In the dedicated spatial-resolution campaign it was filled with Ar + \(5\%\) isobutane at atmospheric pressure and operated in open flow at \(5\,\mathrm{L/h}\). An alternative BabyIAXO mixture is under consideration at \(500\,\mathrm{mbar}\): \(50\%\) Xe, \(48\%\) Ne, \(2\%\) isobutane [2509.08138].

X rays enter through a gas-tight \(4\,\mu\mathrm{m}\) aluminized Mylar window, which also serves as the TPC cathode and is supported by a metallic strong-back. The chamber walls are \(18\,\mathrm{mm}\)-thick radiopure Cu-ETP copper, all gaskets are radiopure PTFE, and a Kapton field shaper is included to improve drift-field uniformity and reduce border effects; in the D1 design it is externally covered with \(1.5\,\mathrm{mm}\) PTFE to suppress copper fluorescence from the chamber body [2509.08138]. More generally, the BabyIAXO Micromegas construction line is described as microbulk Micromegas fabricated from radiopure copper and Kapton, reflecting the CAST-derived emphasis on radiopurity and low internal background [2404.09727].

The readout is a microbulk Micromegas with a two-dimensional X-Y strip pattern covering \(6\times 6\,\mathrm{cm^2}\), with 120 strips per axis at a pitch of \(500\,\mu\mathrm{m}\). The microbulk mesh has \(40\,\mu\mathrm{m}\)-diameter holes. The detector is connected to the front-end electronics by a custom solder-less “face-to-face connector,” in which a flat Kapton cable with copper pads is mechanically compressed between screwed copper parts to contact the readout pads; this avoids soldering at the detector interface and is relevant to both reliability and radiopurity-oriented implementation [2509.08138].

| Element | IAXO-D1 value or feature | Context |
|---|---|---|
| Drift volume | \(3\,\mathrm{cm}\) deep | BabyIAXO spatial-resolution study |
| Entrance window | \(4\,\mu\mathrm{m}\) aluminized Mylar | Cathode, gas-tight, strong-back supported |
| Chamber body | \(18\,\mathrm{mm}\) Cu-ETP copper | Radiopure construction |
| Gaskets | Radiopure PTFE | Low-background design |
| Field shaper | Kapton, \(1.5\,\mathrm{mm}\) PTFE cover | Uniformity and fluorescence suppression |
| Readout area | \(6\times 6\,\mathrm{cm^2}\) | Two-dimensional X-Y strips |
| Segmentation | 120 strips/axis, \(500\,\mu\mathrm{m}\) pitch | High granularity |
| Mesh holes | \(40\,\mu\mathrm{m}\) diameter | Microbulk mesh |
| Test gas | Ar + \(5\%\) isobutane | Atmospheric pressure, \(5\,\mathrm{L/h}\) |
| Alternative gas | \(50\%\) Xe, \(48\%\) Ne, \(2\%\) isobutane | \(500\,\mathrm{mbar}\), under consideration |

## 3. Electronics, calibration, and reconstruction

In laboratory characterization, different IAXO-D1 prototypes were assembled and operated with AGET electronics in an Argon–5% Isobutane mixture. A \(^{55}\)Fe source was used for characterization, and a representative event showed five triggered strips; the reconstructed spectrum resolved the \(^{55}\)Fe \(K_\alpha\) line, the \(^{55}\)Fe \(K_\beta\) line, and the low-energy argon escape peak [2303.15028]. In the SOLEIL synchrotron campaign, the detector was read out with AGET electronics interfaced through a FEMINOS card, using a \(45\,\mathrm{cm}\) flat Kapton cable. The strip capacitance, including the cable contribution, was measured to be about \(20\,\mathrm{pF}\) [2509.08138].

The spatial-resolution study used a self-triggered acquisition, and because of the high rate only hit channels were recorded rather than the full channel set. This differed from the intended BabyIAXO low-rate regime, where event rates are expected below \(1\,\mathrm{Hz}\), and limited detailed studies of diffusion and reconstruction [2509.08138]. Related IAXO/BabyIAXO developments emphasize the move from CAST AFTER-based electronics to AGET-type autotrigger strip readout, since strip-triggering can reduce threshold relative to mesh-triggering because the mesh capacitance is much higher than that of individual strips [2404.09727].

Event reconstruction in the D1 spatial study was performed with the REST-for-Physics framework. The chain had three stages: raw signal analysis, detector-hits analysis, and track reconstruction. Raw signal analysis removed noise events and isolated pulses; detector-hits analysis converted channel signals into energy deposits with physical \((X,Y)\) coordinates and inferred \(Z\) from charge-collection time using a Garfield++ drift velocity; track reconstruction connected hits into 3D tracks through shortest-path algorithms and produced topological observables [2509.08138]. For that specific analysis, event selection was intentionally simple: events were required to have at least one hit in both X and Y, and only single-track events were kept, reflecting the point-like nature of photoabsorbed X rays in the relevant energy range [2509.08138].

The key spatial metric was empirical. The spatial resolution was taken as the standard deviation \(\sigma\) of the event mean-position distribution, extracted from a Gaussian fit. An alternative fit model, a step function convolved with a Gaussian, was tested to account for the square beam profile and produced no significant difference in the extracted \(\sigma\) [2509.08138]. This suggests that, in the reported configuration, the measured widths were dominated by detector-plus-beam response rather than fit-model choice.

## 4. Low-background strategy and operating environments

The low-background program around IAXO-D1 combines radiopure detector construction, passive shielding, active vetoing, underground operation, and topology-based rejection. BabyIAXO requires a low-background X-ray detector with high efficiency in the 1–10 keV energy range and stringent background rejection capabilities, and the detector line is explicitly aimed at background levels of the order of \(10^{-7}\) in the standard helioscope normalization [2509.08138] [2303.15028].

The final BabyIAXO implementation is intended to surround the detector with a \(20\,\mathrm{cm}\) passive lead shield and an active muon veto with nearly \(4\pi\) coverage and target efficiency of \(99\%\), using \(5\,\mathrm{cm}\)-thick plastic scintillators about \(20\,\mathrm{cm}\) wide and \(1\,\mathrm{m}\) long. A neutron tagger is also under development because cosmic-ray-induced neutrons may mimic X-ray signatures through nuclear recoils in the gas [2509.08138]. Earlier background-discrimination work with IAXO-D0 already implemented a \(20\,\mathrm{cm}\) lead castle and a \(4\pi\) triple-layer plastic-scintillator veto with cadmium between layers, specifically to tag cosmogenic-neutron background [2403.06316].

On the surface, IAXO-D0 reached a final background of \((8.56 \pm 1.22)\times 10^{-7}\,\text{counts keV}^{-1}\,\text{cm}^{-2}\,\text{s}^{-1}\) in the 2–7 keV ROI and central 9 mm spot over 52.1 days, after Micromegas topology cuts, a muon-veto coincidence cut, and advanced veto cuts targeting delayed neutron-like signatures. The same work states that this is the lowest background level achieved at surface level for this detector class [2403.06316]. A later overview reports the same surface program as the IAXO-D0 stage, with \(9.8 \times 10^{-7}\ \text{counts/keV/cm}^2/\text{s}\) after the central-region and discrimination analysis and \(8.6 \times 10^{-7}\ \text{counts/keV/cm}^2/\text{s}\) after the additional neutron-tagging handle, with \(79\%\) efficiency for calibration events [2404.09727].

IAXO-D1 serves the underground complement to that surface program. Preliminary results at LSC with passive shielding of \(20\,\mathrm{cm}\) lead and \(1\)–\(2.5\,\mathrm{cm}\) internal copper gave \((5.5 \pm 1.0)\times 10^{-7}\ \text{counts/keV/cm}^2/\text{s}\) with a Xe-Ne mixture and \((1.7 \pm 0.5)\times 10^{-7}\ \text{counts/keV/cm}^2/\text{s}\) with Ar in the \([2\text{–}7]\ \mathrm{keV}\) range. The better Ar performance led to the suspicion of radon contamination in the Xe-Ne gas mixture [2404.09727]. In this respect, D1 is the intrinsic-background reference of the BabyIAXO Micromegas line.

## 5. Spatial-resolution measurements

The first dedicated spatial-resolution characterization of IAXO-D1 was carried out at the SOLEIL Metrology beamline with monochromatic low-energy X rays. The detector was scanned under various beam energies, positions, and drift field configurations, with beam energies of 5, 6, 7, 8.5, and 10 keV; drift field from \(50\,\mathrm{V/cm}\) to \(400\,\mathrm{V/cm}\); mesh voltage fixed at \(360\,\mathrm{V}\); and beam sizes between about \(90\times 90\,\mu\mathrm{m}^2\) and \(1.3\times 1.3\,\mathrm{mm}^2\) [2509.08138].

At that operating point, the detector yielded a gain of about \(10^4\) and an energy resolution of about \(18\%\) at 6 keV. A practical complication was elevated electronic noise at SOLEIL, which required a threshold of about 120 ADC counts above baseline, compared with a typical laboratory threshold of 40 ADC. In the same campaign, the field shaper was left disconnected because connecting it introduced substantial DAQ noise; this omission became central to the interpretation of off-center and edge results [2509.08138].

The central quantitative result was a spatial resolution of approximately \(100\,\mu\mathrm{m}\) at 6 keV. More precisely, the text states that the minimum is “slightly under \(100\)” at 6 keV, while at 10 keV the resolution worsens to about \(260\,\mu\mathrm{m}\). In the conclusion, the central performance over 5–10 keV is summarized as “100–300” \(\mu\mathrm{m}\) for operation at drift fields of at least \(100\,\mathrm{V/cm}\) [2509.08138]. This comfortably exceeds the BabyIAXO requirement of a spatial resolution on the order of, or better than, \(1\,\mathrm{mm}\).

The energy dependence is interpreted as a track-size effect: at higher X-ray energies, the photoelectron track becomes longer and spreads the deposited charge over a larger region, broadening the strip cluster and degrading the event centroid precision [2509.08138]. The drift-field scan showed a clear optimum near \(100\,\mathrm{V/cm}\) for all tested energies. Below that field, resolution deteriorated quickly; above it, degradation was more gradual. The paper attributes the low-field data–simulation mismatch to gas impurities such as \(\mathrm{O_2}\) or \(\mathrm{H_2O}\), which were not included in the simulation, and the higher-field deterioration to increased transverse diffusion [2509.08138].

Position scans exposed field non-uniformities. At 6 keV and \(100\,\mathrm{V/cm}\), the resolution along X reached a minimum of about \(100\,\mu\mathrm{m}\) at the center, while along Y the minimum occurred around \(Y \sim 2\,\mathrm{mm}\). The paper attributes the Y asymmetry to the high-voltage connections located on one side of the readout and oriented perpendicular to the Y axis, which distort the drift field. More generally, moving the beam only a few millimeters away from the center degrades the resolution by a factor of 3–4; even so, the worst measured resolution remains well below the \(1\,\mathrm{mm}\) BabyIAXO requirement [2509.08138]. This indicates that the intrinsic 2D imaging capability is strong, while field uniformity and low-noise integration of the field shaper are decisive engineering details.

## 6. Relation to CAST heritage and BabyIAXO deployment

IAXO-D1 is explicitly based on the last generation of CAST Micromegas detectors, but it is embedded in the BabyIAXO low-background concept rather than being a simple carry-over of CAST hardware [2509.08138]. CAST established the microbulk Micromegas line as a rare-event X-ray technology with radiopure copper-and-Kapton construction, high granularity, topology-based rejection, and background levels that were progressively reduced below \(10^{-6}\ \mathrm{counts\,keV^{-1}\,cm^{-2}\,s^{-1}}\) at surface and to the \(10^{-7}\) scale underground [1501.01466]. The BabyIAXO/IAXO program inherits that platform and re-optimizes it for optics-coupled focal-plane operation, stringent shielding, and improved electronics [2303.15028].

A parallel strand of development came from the CAST sunrise IAXO pathfinder detector, an optics-coupled microbulk Micromegas line installed at CAST and later operated in an extended run with a Xe-based gas mixture. In that pathfinder detector, the active conversion/drift volume had a 3 cm drift distance, a \(4\,\mu\mathrm{m}\) aluminized Mylar X-ray window, and a \(6\times 6\ \mathrm{cm^2}\) readout with \(120\times 120\) channels at 0.5 mm pitch, all behind 10–15 cm lead shielding and a plastic-scintillator muon veto. The xenon-based mixture \((48.85\%\ \mathrm{Xe} + 48.85\%\ \mathrm{Ne} + 2.3\%\ \mathrm{isobutane})\) at 1.05 bar was used as an IAXO-relevant pathfinder exercise because it removed the argon fluorescence feature near 3 keV and provided higher X-ray detection efficiency [2406.16840]. This pathfinder is not IAXO-D1 in nomenclature, but it is part of the same detector lineage.

Within BabyIAXO, the most important implication of the D1 measurements is that the detector already fulfills and surpasses the spatial-performance specification, while the underground results indicate that the intrinsic-background scale required by IAXO is credible in a CAST-derived microbulk platform [2509.08138] [2404.09727]. The studies also identify concrete optimization priorities: integrate the field shaper without introducing electronic noise, maintain laboratory-like thresholds rather than the elevated synchrotron threshold, preserve gas purity to avoid low-field collection losses, and complete the combined passive-shielding, active-veto, and radiopure-electronics program foreseen for BabyIAXO [2509.08138]. A plausible implication is that IAXO-D1 occupies a transitional position between CAST pathfinder operation and the mature BabyIAXO focal-plane detector architecture.

Source: https://www.emergentmind.com/topics/iaxo-d1-micromegas-detector