A radon emanation measurement system at the Carleton Noble Liquid Detector Laboratory

The Carleton Noble Liquid Detector Laboratory (COLD Lab) developed and calibrated a comprehensive radon emanation measurement system, comprising a stainless steel chamber, ZnS(Ag) cell, and charcoal trap, to characterize radon contamination from materials and gases in order to reduce backgrounds in rare event search experiments like DEAP-3600.

The Gist

The Problem: The Invisible "Ghost" in the Machine

Imagine you are trying to hear a single, tiny whisper in a very quiet room. Now, imagine someone is constantly dropping marbles on the floor nearby. The sound of the marbles (the noise) drowns out the whisper (the signal you want).

In the world of physics, scientists are looking for "whispers" from the universe, like Dark Matter or rare neutrinos. These events are incredibly rare—sometimes happening only once a year. The biggest problem they face is Radon.

Radon is a radioactive gas that is naturally produced by tiny amounts of uranium found in almost everything: rocks, concrete, and even the plastic and metal parts of the detectors themselves. Because Radon is a gas, it can sneak out of materials, float around, and stick to sensitive equipment. When it decays, it creates a "clatter" (background noise) that looks exactly like the "whisper" the scientists are trying to find.

The Solution: Building a "Radon Sniffer"

To fix this, the team at Carleton University built a specialized machine called a Radon Emanation System. Think of this system as a high-tech "sniffer dog" or a vacuum cleaner designed specifically to suck up and count invisible radioactive gas coming off materials.

Here is how their machine works, step-by-step:

1. The Vacuum Chamber (The "Scent Trap")

First, they put a piece of material (like a rubber gasket or a metal part) inside a shiny, stainless steel box. They pump all the air out of the box to create a vacuum.

• The Analogy: Imagine putting a smelly sock in a sealed, empty room. If the sock is smelly, the smell will eventually fill the room. Here, the "smell" is Radon gas escaping from the material.

2. The Cold Trap (The "Ice Cube Filter")

Once the Radon has built up in the box, they need to catch it. They use a special filter made of activated charcoal (the same stuff used in water filters, but much more powerful) that is frozen to a temperature of about -122°C using a mix of liquid nitrogen and ethanol.

• The Analogy: Imagine the Radon gas is a moth flying around a room. The frozen charcoal trap is like a giant, sticky ice cube. When the air flows over it, the moth (Radon) freezes and sticks to the ice, while the clean air (Nitrogen gas) flies right past.

3. The Lucas Cell (The "Light Bulb Counter")

Once the Radon is caught on the ice, they warm it up so it turns back into a gas and move it into a special glass bowl called a Lucas cell. The inside of this bowl is coated with a glowing powder called Zinc Sulfide.

• The Analogy: When the Radon decays, it shoots out tiny particles (alpha particles). When these particles hit the glowing powder on the wall of the bowl, they make tiny flashes of light, like fireflies blinking. A sensitive camera (a photomultiplier tube) counts these flashes. By counting the flashes, the scientists know exactly how much Radon was in the material.

What They Found

The team used this machine to test materials they planned to use in a massive experiment called DEAP-3600 (a giant tank of liquid argon looking for Dark Matter).

• The Results: They tested things like rubber seals, gloves, and O-rings.
  • Some materials, like a specific type of rubber called Buna-N, were "loud" (emitting a lot of Radon).
  • Others, like Butyl gloves, were "quiet" (emitting very little).
• The Glove Box: They also calculated how much Radon was floating inside the clean room (glove box) where they built the detector parts. They found that even the air used to purge the box and the gloves worn by the workers contributed to the Radon levels.

Why This Matters

This machine is now a standard tool in their lab. Before they build a new, ultra-sensitive detector, they can test every single screw, gasket, and piece of plastic to make sure it isn't a "noise maker."

By swapping out the "loud" materials for "quiet" ones, they can silence the background noise. This makes the detector much more sensitive, giving it a better chance to hear the faint "whisper" of Dark Matter from the rest of the universe.

In short: They built a super-sensitive counter to find and measure invisible radioactive gas leaking out of building materials, ensuring their future experiments aren't fooled by false alarms.

Technical Summary

1. Problem Statement

Radon ($^{222}$Rn) is a critical source of background noise in rare-event search experiments, such as Dark Matter detection (e.g., DEAP-3600) and neutrinoless double beta decay (e.g., nEXO).

• Source: Radon is produced by the decay of natural uranium and thorium trace impurities in all materials. It can emanate from deep within materials via outgassing or be trapped on surfaces from the atmosphere.
• Impact: Radon daughters (specifically $^{210}$Pb, $^{210}$Po, and $^{214}$Bi) can deposit on sensitive detector components.
  • In Dark Matter experiments, alpha decays from progeny can mimic WIMP signals by losing energy in the detector material.
  • In neutrinoless double beta decay experiments, the high Q-value of $^{214}$Bi (3.27 MeV) exceeds the signal region (2.46 MeV for $^{136}$Xe), creating a significant background.
• Challenge: Accurately quantifying radon emanation rates from detector materials and characterizing radon levels in clean environments (like glove boxes) is essential for material screening. However, commercial counters often lack the sensitivity required for ultra-low-background applications.

2. Methodology

The authors developed a custom, low-background radon emanation system at the Carleton nOble Liquid Detector (COLD) Lab. The system comprises three main subsystems:

A. Radon Emanation Apparatus

• Chamber: A stainless steel cylinder (40 cm length, 20 cm diameter) sealed with a ConFlat flange and copper gasket to maintain vacuum integrity.
• Extraction Line: Includes a primary and secondary cryogenic trap, a vacuum pump, and a gas handling system.
• Gas Adapter: An optional module for measuring radon in compressed gas (e.g., Nitrogen from a Dewar). It utilizes a charcoal trap submerged in a liquid nitrogen-ethanol slurry ($\sim -122^\circ$C) to adsorb radon while allowing nitrogen to pass.

B. Detection System (Lucas Cell)

• Cell: A 2-inch hemispherical acrylic Lucas cell coated internally with Zinc-Sulfide (ZnS) phosphor.
• Operation: The cell is assembled in a nitrogen-filled glovebox to minimize surface contamination.
• Detection: Alpha particles from radon decay excite the ZnS, producing scintillation light ($\sim 450$ nm). This is detected by a Hamamatsu R1828-01 Photomultiplier Tube (PMT).
• DAQ: Signals are amplified (CAEN A1424), digitized (CAEN DT5780SCM), and analyzed using ComPASS software. Pulse-height analysis distinguishes alpha events from electronic noise.

C. Measurement Procedure

1. Emanation: Samples are placed in the chamber, pumped down for 24 hours, and sealed.
2. Extraction: Radon is cryopumped into a primary trap, transferred to a secondary trap, and finally moved to the Lucas cell via volume sharing.
3. Counting: The Lucas cell is counted for at least 24 hours to accumulate statistics.

3. Key Contributions & System Characterization

• System Calibration: The system was calibrated using a Buna rubber gasket (high emanation rate $>30,000$ atoms m$^{-2}$h$^{-1}$). The dominant $^{214}$Po alpha peak (7.69 MeV) served as the energy reference.
• Background Determination:
  • Intrinsic Background: $\sim 5$ events/day (from ZnS powder and $^{210}$Po on acrylic).
  • System Background: $\sim 7$ events/day (including the chamber and board).
• Efficiency Calculation:
  • Collection Efficiency: Determined to be 89%. The 11% loss is attributed to the volume sharing between the secondary trap and the Lucas cell (secondary trap is 10% of the cell volume).
  • Detection Efficiency: Adopted from previous measurements at Queen's University as 63%.
  • Total System Efficiency: $0.89 \times 0.63 = \mathbf{56\%}$.
• Steady-State Modeling: The authors developed a numerical model (Equations 4–9) to predict radon concentration in the glove box. This model accounts for:
  • Radon emanation from internal components.
  • Residual radon from room air/impure nitrogen.
  • Intrinsic radon in liquid nitrogen boil-off gas.
  • Flow dynamics (turnover rate of $\sim 1$ volume/hour).

4. Results

• Material Assays: The system successfully measured radon emanation rates for various materials intended for the DEAP-3600 upgrade.
  • Buna-N Gasket: Highest emitter at $31,714.6 \pm 1,400$ atoms m$^{-2}$h$^{-1}$.
  • Butyl Gasket: $186.5 \pm 37$ atoms m$^{-2}$h$^{-1}$.
  • Silicone Gasket: $141.2 \pm 28$ atoms m$^{-2}$h$^{-1}$.
  • Butyl Gloves: $10 \pm 2$ atoms per glove.
  • EPDM O-rings: $11 \pm 2$ atoms m$^{-1}$.
• Gas and Glove Box Analysis:
  • Measurements of Nitrogen gas from the Dewar showed low radon levels ($\sim 744 - 1264$ $\mu$Bq/m$^3$).
  • When circulating gas through the glove box, the total radon emanation rate was calculated to be 178–203 atoms/h.
  • Validation: The gas-flow sampling results (via charcoal trap) were consistent with the sum of individual component measurements taken in the vacuum chamber (Table 2 vs. Table 3), validating the accuracy of the system.

5. Significance

• Diagnostic Tool: The COLD Lab system is now a fully operational, calibrated diagnostic tool essential for reducing backgrounds in future rare-event experiments.
• Material Screening: It enables the rigorous screening of materials for the DEAP-3600 upgrade and the DarkSide experiment (specifically for TPB coating chambers).
• Process Optimization: By quantifying radon loads in glove boxes and on specific components, the system allows researchers to optimize fabrication and assembly procedures to minimize radon contamination.
• Future Upgrades: The authors plan to add temperature regulation to the vacuum chamber to study the temperature dependence of emanation rates, further aiding the design of ultra-low-background detectors.

In summary, this paper presents a robust, low-background radon measurement system that bridges the gap between material characterization and environmental monitoring, providing critical data to enhance the sensitivity of next-generation Dark Matter and neutrino experiments.