MVD | Compressed Baryonic Matter Experiment at FAIR

Micro Vertex Detector (MVD)

Project leader:
Joachim Stroth

j.stroth(at)gsi.de

Documents and publications:
Publications (Click to show)

Introduction to Physics for MVD

Figure 1. CAD model of the CBM Micro Vertex Detector (MVD).

The CMOS-MAPS-based Micro Vertex Detector (MVD) is the first detector subsystem positioned downstream of the target in the CBM experiment, inside the target vacuum box. It is located between 5 to 20 cm and forms, together with the µ-strip Silicon Tracking System (STS), the CBM Silicon Tracker, positioned inside the CBM magnet. The primary goal of the MVD is to offer high-precision track reconstruction for charged particles, extending the detection range to low momenta down to a few hundreds of MeV/c.

The MVD enables precise secondary-vertex reconstruction for the identification of weakly decaying charged hyperons produced in A+A collisions and open charm mesons in p+A collisions in the SIS-100 energy regime, with a precision better than 100 μm along the beam axis for decay products with laboratory momenta above 1 GeV/c. In addition, it assists in background rejection for dielectron analyses through topological cuts that suppress contributions from π₀-Dalitz decays and photon conversion pairs.

Figure 2. Reconstruction of short-lived strange hyperons with the MVD and STS. Precise secondary-vertex reconstruction enables a clear Σ signal in the invariant-mass spectrum of its decay products, even in the high-multiplicity environment of Au+Au collisions.

Design Criteria

The Micro Vertex detector is designed under rigorous constraints shaped by the need for high-precision vertexing and tracking of particles down to small laboratory momenta in the CBM energy regime. A central design objective is the reduction of the material budget to a limit of 0.3 – 0.5 x/X₀ for each station, together with vacuum-compatible sensor- and detector-integration. In addition, the detector is placed inside the magnet (Integral (𝐵 𝑑𝑧) = 1 Tm). The most irradiated sensors have to withstand intense radiation: After already one CBM year the most exposed sensors can accumulate the end-of-lifetime (EOL) dose (5 MRad ionizing and 7 × 10¹³ n_eq/cm²non-ionizing).

This environment makes radiation hardness a critical requirement for the choice of materials as well as for the sensing element (the MIMOSIS sensor). In order to maintain high detection efficiency, suppress fake hits and control leakage current the sensors demand a stable temperature-controlled environment with a baseline operation temperature of 15 °C. Consequently thermal management poses major technical challenges, addressed by:

Mounting sensors on thin, highly heat-conductive Thermal Pyrolytic Graphite (TPG) carriers to minimize temperature gradients over the sensors
Moving active cooling to the periphery to maintain a low material budget
Low sensor power density of up to 75 mW/cm²

In summary, for CBM high-precision tracking in immediate neighborhood to the target calls for employing irradiation-hard pixel sensors and dedicated high-performance materials, while respecting vacuum operation and minimizing the overall material budget. High-granular CMOS-MAPS (MIMOSIS) thinned to 50 µm and mounted on TPG and read-out by ultra-thin flex cables represent the key design features of the CBM MVD.

Figure 1: CAD model of the CBM Micro Vertex Detector (MVD) station layout.

Figure 2: Mechanical design of an MVD half-station showing sensor placement and support structure.

MVD Technical Details

Positioning & Environment

MVD is positioned inside the CBM dipole magnet, integrated magnetic flux density of O(1 Tm).
Located inside the target vacuum chamber, operating in moderate vacuum of approx. 10^-4 mbar.
During a CBM running year, the most exposed sensors can accumulate:
- Up to 5 MRad ionizing radiation dose.
- Up to 7 × 10¹³ neq/cm² non-ionizing radiation dose.

Geometry & Structure

Geometrical acceptance: 2.5° ≤ θ ≤ 25° in the full azimuth.
Consists of four equidistant thin planar detector stations, featuring a material budget per station of 0.3–0.5 x/X₀.
Can be arranged in two detector geometries:
- Tracking Geometry (TR): focuses on track reconstruction of low-momentum particles; stations positioned 8–20 cm downstream of the target. TR will be the day-1 geometry.
- Vertex Geometry (VX): focuses on the identification of secondary vertices of decaying open charm D mesons; stations positioned 5–20 cm downstream of the target.
Equipped with 288 CMOS Monolithic Active Pixel Sensors (MIMOSIS) in TR detector geometry.

Performance

Achieves approx 5 μm spatial precision per measurement point.
Operates in free-streaming readout mode with a 5 μs time frame.
Enables secondary vertex precision of <100 μm along the beam axis for 1 GeV/c tracks.

Positioning of MVD at the center of the magnet.

Figure 1. Positioning of MVD inside the CBM dipole magnet.

Figure 2. Pointing precision σ vs. the pion (kaon) momentum, for two detector geometries VX and TR, respectively, shown as insert. The calculation is based on assuming two planar detector stations with a given distances to the target, spatial precisions and material budgets.

MVD MIMOSIS

Overview

- MIMOSIS: a Monolithic Active Pixel Sensor (MAPS), primary detection element of the MVD.

- Developed by IPHC Strasbourg, IKF Goethe University Frankfurt and GSI, manufactured with 180 nm CMOS imaging process from TowerJazz.

- Sensing element inspired by ALPIDE (ALICE experiment).

Key Features

- Fully depleted sensing node.

- Integrated analog front-end in each pixel (amplification, signal shaping, and hit discrimination).

- Includes on-matrix hit clustering and priority encoder.

- Enhanced digital readout system (front-end and back-end) with increased bandwidth and temporary data storage via elastic buffer.

Physical & Pixel Specifications

- Chip size: 31.15 × 17.25 mm², of which 31 × 15 mm² consist the active are.

- Pixel matrix: 1024 × 504 pixels.

- Pixel size: ~27 × 30 μm².

- 3.6 mm non-sensitive region for on-chip readout circuitry.

Pixel architecture evaluation ongoing, final design submission planned for 2026.

Performance & Radiation Tolerance

- Designed for interaction rates: Au+Au: up to 0.1 MHz, p+Au: up to 10 MHz.

- Handles average hit rates of 20 MHz/cm² and peak rates of 80 MHz/cm² (beam intensity fluctuations).

- Designed radiation tolerance: 5 MRad ionizing, 7 × 10¹³ neq/cm² non-ionizing

- Greater than 99.9% det. efficiency after irradiation

Most exposed sensor reaches radiation tolerance after 1 CBM data-taking year.

Figure 1. Cross section of the MIMOSIS pixel design, illustrating the sensing diode and in-pixel electronics architecture.

Figure 2. The MIMOSIS sensor prototype developed at IPHC Strasbourg.

MVD Integration

mMVD

In May 2025, the mMVD(mini MVD) detector module was successfully integrated for the first time into the mCBM setup. The mMVD comprises two MIMOSIS-2.1 sensors (25 and 50 µm epitaxial layer thickness) mounted back-to-back, featuring sensor-to-sensor tracking.

The goal of this integration was to:

Validate the readout concept of the MVD by relying on a CROB (GBTx, GBTSCA) and CRI readout.
Demonstrate synchronization within the mMVD detector system and with other subsystems (mSTS).

Figure 1. Spatial correlation between the mMVD and mSTS subsystems, demonstrating proper spatial matching and synchronization after integration.	Figure 2. Spatial correlation between the two MIMOSIS sensors of the mMVD module, confirming sensor-to-sensor tracking performance.
Figure 3. Online monitoring of the accumulated pixel matrix for front (25 µm epi) and back (50 µm epi) sensors. First beam hits observed during commissioning.	Figure 4. The mMVD detector module installed in the mCBM setup during the May 2025 integration.