Improvement and assessment of the radiopurity of Micromegas readout planes

This paper presents new radioassay results from the Canfranc Underground Laboratory demonstrating that dedicated development has significantly reduced the radiopurity levels of Micromegas readout planes, confirming their suitability for ultra-low background rare event searches.

The Gist

Imagine you are trying to hear a single, tiny whisper in a room that is absolutely silent. That is what scientists do when they search for "rare events" in physics, like dark matter particles or mysterious nuclear decays. These events happen so rarely that if there is even a tiny bit of background noise—like a ticking clock or a buzzing lightbulb—it will drown out the whisper.

In the world of particle physics, that "noise" comes from natural radioactivity. Even the materials we build our detectors with (like copper, plastic, and glue) contain tiny amounts of radioactive atoms that constantly "crackle" and interfere with the experiment.

This paper is about a team of scientists who went on a mission to make their listening device as quiet as possible. Here is the story of how they did it, explained simply.

The Listening Device: The Micromegas

The scientists use a giant, high-tech "cloud chamber" called a Time Projection Chamber (TPC) to catch these rare particles. To read the signals, they use a special component called a Micromegas.

Think of the Micromegas as a super-sensitive microphone. It's a thin sheet of metal mesh suspended over a board. When a particle flies through, it creates a tiny electrical spark that the mesh catches. The problem? If the microphone itself is made of slightly "noisy" materials (materials with hidden radioactivity), it will create its own static, making it impossible to hear the real signal.

The Problem: The "Static" in the System

In a previous study, the scientists found that their Micromegas microphones were already pretty quiet. But for the most sensitive experiments in the universe, "pretty quiet" isn't good enough. They needed "ultra-quiet."

They suspected the main source of the noise was Potassium-40 (a naturally radioactive isotope of potassium). Potassium is often found in cleaning agents and water. It turns out that during the manufacturing process, they might have been using tap water or chemicals containing potassium, which left a radioactive "stain" on their delicate microphones.

The Mission: A Deep Clean

The team went to a secret underground laboratory in Spain (Canfranc) and a research center in Switzerland (CERN) to perform a "deep clean" of their manufacturing process.

1. The Old Way: They looked at old samples. Some were made with standard methods, others were just raw materials.
2. The New Strategy: They started making new batches of Micromegas, but this time they changed the recipe:
   • No Tap Water: They realized tap water was the culprit. They switched to ultra-pure, deionized water for all cleaning steps.
   • No Potassium: They avoided any chemicals containing potassium.
   • Massive Samples: To measure the radioactivity accurately, they couldn't just test a tiny scrap. They had to make huge piles of these microphones (like stacking 70 large sheets) to get a big enough sample to measure the tiny whispers of radioactivity.

The Detective Work: Two Types of "Ears"

To check if their cleaning worked, they used two different types of "ears" (detectors) in the underground lab:

• The Germanium Detector (The Sensitive Microphone): This is a giant, super-cooled crystal that listens for gamma rays (a type of radiation). It's great at identifying what kind of radioactive element is present (like Potassium or Uranium).
• The BiPo-3 Detector (The Specialized Sniffer): This is a unique machine designed specifically to catch a very specific type of radioactive decay chain (Uranium and Thorium). It's like a bloodhound that can smell a specific scent that other dogs miss. It is incredibly sensitive to the "lower parts" of the radioactive chains.

The Results: Silence Achieved!

The results were a massive success. Here is what they found:

• The Potassium Problem Solved: By switching to pure water and avoiding potassium chemicals, they reduced the radioactive potassium content by a factor of 34.
  • Analogy: Imagine your old microphone had a background hiss that sounded like a fan running. After the cleaning, that fan was turned off, leaving only the sound of a gentle breeze.
• The Ultimate Silence: They managed to lower the radioactivity to levels so low they are almost impossible to measure.
  • For the Uranium and Thorium chains, they set limits so low that if there was any noise, it was less than 0.064 and 0.016 units per square centimeter.
  • Analogy: This is like measuring the weight of a single grain of sand on a beach the size of a football field.

Why Does This Matter?

This paper proves that we can build "ultra-quiet" microphones for the universe. By refining how we make these Micromegas and being extremely careful about what touches them (no tap water, no dirty chemicals), the scientists have created a tool that is clean enough to listen for the faintest whispers of dark matter or the decay of atoms that haven't happened in billions of years.

In short: They took a noisy room, scrubbed the walls, changed the water, and now the room is so silent that they can finally hear the secrets of the universe.

Technical Summary

1. Problem Statement

Experiments searching for rare phenomena (e.g., neutrinoless double beta decay, dark matter detection, and solar axion searches) require detectors with ultra-low background radiation. MicroPattern Gas Detectors (MPGDs), specifically Micromegas (Micro-Mesh Gas Structures), are increasingly used as readout planes for Time Projection Chambers (TPCs) due to their excellent spatial/energy resolution and stability.

However, the intrinsic radioactivity of the materials used in Micromegas construction (primarily copper and Kapton) can introduce background noise that masks rare signals. Previous studies indicated that while raw materials were radiopure, the fabrication process (specifically the use of potassium compounds to create holes in the mesh) introduced significant contamination, particularly from Potassium-40 ($^{40}$K). The challenge was to optimize fabrication techniques to minimize radioactive impurities and rigorously quantify the resulting radiopurity levels using highly sensitive detection methods.

2. Methodology

The study involved a dedicated development phase at CERN followed by comprehensive radioassays at the Canfranc Underground Laboratory (LSC) in Spain.

• Sample Preparation & Fabrication Optimization:

  • Multiple batches of microbulk Micromegas were produced at CERN.
  • Iterative Cleaning: The team tested various cleaning protocols to remove surface contaminants. This included water baths (tap water vs. deionized water) and chemical treatments.
  • Process Modification: Specific attention was paid to the production steps involving potassium compounds (used for etching holes). Samples were prepared with and without these steps, and cleaning procedures were refined to avoid tap water (which contains natural uranium) and minimize potassium residue.
  • Massive Samples: To achieve statistical significance for ultra-low activity measurements, "massive" samples were created by cutting large foil sheets into hundreds of pieces to fit into Marinelli containers for gamma spectroscopy.

• Detection Techniques:
  Two complementary techniques were employed to measure different isotopes and chain segments:

  1. High-Purity Germanium (HPGe) Gamma Spectroscopy: Performed using ultra-low background detectors (e.g., "Paquito," "GeOroel," "GeAnayet") inside heavy copper/lead shielding. This method was primarily used to quantify $^{40}$K, $^{235}$U, and the upper parts of the $^{238}$U and $^{232}$Th decay chains.
  2. BiPo-3 Detector Analysis: A specialized detector designed by the SuperNEMO collaboration to detect BiPo events (delayed coincidences between a beta particle and an alpha particle). This method is highly sensitive to the lower parts of the natural radioactive chains, specifically $^{214}$Bi (from $^{238}$U) and $^{208}$Tl (from $^{232}$Th), providing superior sensitivity for these specific isotopes compared to standard gamma spectroscopy.

3. Key Contributions

• Fabrication Process Optimization: The paper demonstrates that specific cleaning protocols (using heated deionized water) and the avoidance of tap water significantly reduce contamination from Uranium and Potassium.
• Methodological Validation: It validates the use of the BiPo-3 detector for screening thin foil readouts, showing it offers sensitivity limits an order of magnitude better than HPGe detectors for the lower segments of the uranium and thorium chains.
• Comprehensive Material Screening: Beyond the readouts themselves, the study assessed related materials (Vacrel films, Kapton-Carbon, adhesives, and epoxy) to ensure the entire detector assembly meets ultra-low background standards.

4. Key Results

The study achieved significant reductions in radioactivity through iterative processing:

• $^{40}$K Reduction:

  • Initial samples showed $^{40}$K activity around 3.45 µBq/cm² (attributed to hole-making processes).
  • Through optimized cleaning and massive sample preparation, the lowest measured activity was $0.102 \pm 0.030$ µBq/cm².
  • This represents a reduction factor of ~34 compared to the initial analyzed samples.

• $^{238}$U and $^{232}$Th Chain Limits:

  • Using the BiPo-3 detector, the study set stringent upper limits on the lower parts of the decay chains:
    • $^{238}$U (via $^{214}$Bi): < 0.064 µBq/cm² (64 nBq/cm²).
    • $^{232}$Th (via $^{208}$Tl): < 0.016 µBq/cm² (16 nBq/cm²).
  • These limits are significantly lower than those achievable with standard gamma spectroscopy alone.

• Contamination Sources Identified:

  • Tap Water: Initial cleaning with tap water introduced Uranium contamination (approx. 45.8 ppb), which was eliminated by switching to deionized water.
  • Potassium Compounds: The presence of $^{40}$K was directly linked to the chemical processes used to create holes in the Micromegas mesh.

5. Significance

• Validation for Rare Event Searches: The results confirm that microbulk Micromegas readouts can be manufactured with extremely low radiopurity, making them suitable for next-generation experiments requiring ultra-low backgrounds, such as the International Axion Observatory (IAXO), TREX-DM, and PANDAX-III.
• Benchmarking: The study establishes new benchmarks for the radiopurity of gas detector components, providing a roadmap for future fabrication processes to minimize cosmogenic and anthropogenic contamination.
• Technique Synergy: The work highlights the necessity of combining BiPo analysis (for deep chain sensitivity) with HPGe spectroscopy (for $^{40}$K and upper chain analysis) to fully characterize the radiopurity of complex detector components.

In conclusion, the paper proves that with dedicated material selection and refined fabrication/cleaning protocols, Micromegas readout planes can achieve the radiopurity levels required for the most sensitive rare-event physics experiments currently being planned.