ImpCresst -- A versatile simulation tool focusing on solid-state detectors at keV energies

ImpCresst is a versatile, Geant4-based Monte Carlo simulation tool developed by the CRESST collaboration to model backgrounds and calibration signals in solid-state detectors at keV energies, featuring dynamic CAD-based geometry, advanced particle generators for radionuclides, and a workflow optimized for HPC environments.

The Gist

Imagine you are trying to find a single, specific grain of sand on a beach, but the beach is covered in millions of other grains that look almost exactly the same. This is the challenge scientists face when searching for Dark Matter or other "rare events" in the universe. They build incredibly sensitive detectors deep underground to catch these elusive particles.

The problem? The detectors are so sensitive that they also pick up "noise" from natural radioactivity in the rocks, the air, and even the detector materials themselves. To find the "needle" (Dark Matter), they first need to perfectly understand and map out the "haystack" (the background noise).

This paper introduces ImpCresst, a sophisticated computer program designed to simulate that noise and the detector's reaction to it. Think of ImpCresst as a hyper-realistic video game engine specifically built for particle physics.

Here is a breakdown of how it works, using simple analogies:

1. The Digital Twin (Geometry)

In a real lab, detectors are complex machines made of different crystals, metals, and shields. Building a simulation of this used to be like trying to build a Lego model by hand, piece by piece, in code. If the real machine changed slightly, the code had to be rewritten.

ImpCresst changes the game by allowing scientists to import 3D CAD drawings (the blueprints engineers use to build the real machines) directly into the simulation.

• Analogy: Instead of manually describing every brick in a castle, you can just upload the architect's blueprint, and the computer instantly builds a perfect digital replica. This lets scientists test new designs instantly without rewriting the code.

2. The "Contaminant" Sprinkler (Radioactive Sources)

One of the biggest headaches is knowing exactly where radioactive atoms are hiding. Are they deep inside a copper block? Are they just a thin layer on the surface?

ImpCresst has a special tool called ContaminantSource.

• Analogy: Imagine you have a digital castle and you want to see what happens if you sprinkle radioactive dust on it. Instead of manually placing dust on every single brick, you tell the computer: "Sprinkle dust on all the copper walls," or "Sprinkle dust on the surface of this crystal." The program then randomly places millions of "dust particles" (radioactive atoms) exactly where you asked, simulating how they decay and shoot out energy.

3. The "Blurry Camera" (Detector Response)

The simulation generates "raw" data: perfect, mathematical energy deposits. But real detectors are like blurry cameras. They don't see the exact energy; they see a slightly fuzzy version because of time delays and measurement limits.

ImpCresst works in two steps:

1. The Simulation (ImpCresst): It calculates the perfect, theoretical physics of what happens when a particle hits the detector.
2. The Filter (CresstDS): This is a second tool that takes that perfect data and runs it through a "blur filter" based on real-world measurements. It adds the fuzziness, the time delays, and the noise that a real detector would see.

• Analogy: ImpCresst takes a high-resolution photo of a bird. CresstDS then applies a filter to make it look like it was taken with an old, slightly shaky camera. This allows scientists to compare their simulation directly with what their real cameras see.

4. The "Black Box" of Reproducibility

In science, if you can't repeat an experiment, you can't trust the result. With complex simulations, it's easy to lose track of which version of the software or which settings were used.

ImpCresst is built like a time capsule.

• Every time a simulation runs, it automatically stamps the file with a "digital ID card." This card records the exact version of the software, the computer it ran on, the random seed used, and the exact settings.
• Analogy: It's like baking a cake and automatically attaching a label that says exactly which brand of flour, which oven temperature, and which minute of the day you baked it. Years later, anyone can look at the label and recreate the exact same cake.

5. The "Assembly Line" (HPC Workflow)

Simulating billions of particle interactions takes a massive amount of computing power. ImpCresst is designed to run on supercomputers (HPC) efficiently.

• Analogy: Instead of one person trying to count every grain of sand on the beach alone, ImpCresst organizes a fleet of thousands of robots. Each robot works on a tiny patch of the beach independently. Because they don't need to talk to each other (lazy parallelism), they can work incredibly fast. The program uses "containers" (like shipping crates) to ensure every robot has the exact same tools and instructions, no matter which computer it's running on.

Why Does This Matter?

The CRESST collaboration (and others like Nucleus and Cosinus) uses this tool to hunt for Dark Matter. By using ImpCresst, they can:

• Predict the noise: Know exactly how much background radiation to expect.
• Design better detectors: Test new shapes and materials in the computer before building them in real life.
• Spot the signal: When they see a blip in their real data, they can check their simulation to see if it's just background noise or a potential discovery.

In short, ImpCresst is the ultimate "flight simulator" for particle physicists, allowing them to navigate the dangerous waters of background noise to find the hidden treasure of new physics.

Technical Summary

1. Problem Statement

Rare-event searches (e.g., Dark Matter, Coherent Elastic Neutrino-Nucleus Scattering, Neutrinoless Double Beta Decay) using cryogenic solid-state detectors operate in an environment where the signal rate is significantly lower than the background rate. The dominant background sources are radioactive contaminations (bulk and surface) and cosmogenic radionuclides.

• The Challenge: Extracting new physics signatures requires precise modeling of background sources and detector responses in the keV to 100 eV energy range.
• Limitations of Existing Tools: While Geant4-based tools exist, many are not open-source, not tailored for solid-state detectors, or lack the flexibility to handle the rapidly evolving, heterogeneous geometries of modern experiments like CRESST. Furthermore, simulating low-energy interactions (down to 100 eV) and complex detector responses (scintillation quenching, time resolution) requires specific tuning not found in generic packages.

2. Methodology

The authors present ImpCresst, a Geant4-based Monte Carlo simulation package designed specifically for cryogenic solid-state detectors. The methodology is structured into a two-step workflow:

A. Microscopic Simulation (ImpCresst)

• Geometry Handling:
  • Supports modular geometry definitions via C++ classes derived from DetectorPart.
  • CAD Integration: Directly imports Wavefront OBJ files using CADMesh, allowing for high-fidelity modeling of complex detector modules without manual Constructive Solid Geometry (CSG) coding.
  • Dynamic Configuration: Allows runtime modification of shielding, detector placement, and material properties via macro commands without recompilation.
• Physics Modeling:
  • Uses the Geant4 "Shielding" physics list (v10.6.3) with specific adjustments for low-energy precision.
  • Electromagnetic Interactions: Employs G4EmStandardPhysics_option4 (or Livermore models) with production cuts lowered to 250 eV (or 1 nm in detector volumes) to accurately simulate low-energy leakage and surface effects.
  • Radioactivity: Uses G4Radioactivation (instead of standard decay) to support biasing techniques. Handles atomic de-excitation (fluorescence, Auger electrons, PIXE) by ignoring production cuts for these processes.
  • Neutron Physics: Accurately models thermal neutron capture using G4NeutronHPThermalScattering and the fifrelin4geant4 library for precise nuclear de-excitation gamma emission probabilities.
• Particle Generation:
  • Interfaces with CRY (cosmic rays), MUSUN (deep-underground muons), and SOURCES4 (neutron yields).
  • ContaminantSource: A novel, independent particle generator that simulates bulk and surface radioactive contamination on arbitrary geometric volumes without requiring user-defined confinement volumes. It supports statistical weighting based on volume/surface area and implantation depth models (exponential, Gaussian, uniform).
• Data Persistence:
  • Stores the full event topology (tracks, hits, parent-child relationships) in ROOT files.
  • Uses a Bridge Pattern to decouple Geant4 dependencies from the data storage layer, ensuring data can be processed without the Geant4 library.
  • Includes automatic metadata annotation (Git commit hash, macro file used, RNG seed) for full reproducibility.

B. Detector Response Simulation (CresstDS)

• Post-Processing: A separate tool (CresstDS) applies empirical detector response models to the raw simulation data.
• Time Resolution: Integrates raw hits over a finite time window ($T \approx 2$ ms) using a state machine to simulate the detector's integration period.
• Energy & Light Resolution: Applies Gaussian smearing to integrated energy and scintillation light yield based on empirical parametrizations derived from calibration data.
• Quenching: Calculates quenched light yield using Birk's law or empirical parametrizations specific to particle types (e.g., $\alpha$, $\beta$, nuclear recoils) and materials (e.g., $CaWO_4$, $Al_2O_3$).

C. Workflow & Reproducibility

• Designed for High-Performance Computing (HPC) using lazy parallelism (independent jobs).
• Utilizes Apptainer (formerly Singularity) containers and Nextflow for workflow management, ensuring bitwise-identical environments across different hardware and operating systems.

3. Key Contributions

1. Open-Source Specialization: ImpCresst is the only publicly available, open-source Geant4 tool specifically tuned for solid-state detectors in the 100 eV–10 MeV range.
2. CAD-Driven Geometry: The ability to import CAD files directly accelerates the prototyping of new detector designs and ensures mechanical fidelity between simulation and reality.
3. ContaminantSource: A unique generator that decouples contamination simulation from specific volume definitions, allowing flexible "contamination" of any selected geometry component with bulk or surface radioactivity.
4. Two-Step Workflow: Separation of microscopic physics simulation (ImpCresst) from detector response modeling (CresstDS) allows for efficient re-analysis of raw data with updated resolution parameters without re-running expensive simulations.
5. Full Event Topology Storage: Unlike many tools that only store final energy deposits, ImpCresst stores the complete particle track history, enabling detailed decomposition of background sources (e.g., identifying specific decay chains).
6. Reproducibility Framework: Integration of semantic versioning, Git-based metadata, and containerized HPC workflows ensures that any simulation result can be exactly reproduced.

4. Results & Validation

• Validation: The tool has been validated and used extensively by the CRESST collaboration for publications on Dark Matter limits and background studies. It has also been adopted by the Nucleus and Cosinus collaborations.
• Example 1 (Extrinsic Background): Simulated $^{234}\text{Th}$ contamination in copper shielding. The tool successfully mapped the range of decay radiation, showing that 80% of hits in the detector originated from within 3 cm of the detector surface.
• Example 2 (Intrinsic Background): Simulated $^{234}\text{Th}$ contamination within a $CaWO_4$ crystal. The workflow successfully decomposed the energy deposition into specific nuclear decay pathways (prompt vs. delayed hits due to the $^{234}\text{Pa}^m$ isomeric state), matching the observed spectral features in real data.
• Performance: The two-step approach (simulation + post-processing) significantly reduces storage requirements. In one test case, 683 GB of raw data was reduced to 3.46 GB of processed data (a factor of 197 reduction) while retaining the ability to re-apply different resolution models.

5. Significance

ImpCresst addresses a critical bottleneck in rare-event physics: the need for highly accurate, flexible, and reproducible background modeling in the low-energy regime.

• For the CRESST Experiment: It is a cornerstone of their roadmap for next-generation Dark Matter searches, enabling the design of detectors with thresholds as low as 6.7 eV.
• For the Broader Community: By releasing the tool as open-source (GPLv3), it lowers the barrier to entry for other experiments using cryogenic solid-state detectors (e.g., Nucleus, Cosinus, SuperCDMS).
• Future Outlook: The tool is designed for extensibility, with planned upgrades to Geant4 v11, inclusion of coherent scattering processes, and improved surface roughness modeling for contaminants.

In summary, ImpCresst provides a robust, modular, and scientifically rigorous framework for simulating the complex interplay between radioactive backgrounds and cryogenic detector responses, essential for pushing the boundaries of sensitivity in rare-event searches.