Direct in-chamber radon-220 (thoron) emanation measurements for rare-event physics experiments

This study presents a direct in-chamber measurement method for 220Rn (thoron) emanation that minimizes transfer losses and achieves a fivefold sensitivity increase over conventional flowthrough setups, offering an effective tool for background control in rare-event physics experiments.

The Gist

Imagine you are trying to catch a very specific type of ghost that haunts your house. This ghost is called Radon-220 (or "thoron"). It's a problem for scientists building ultra-sensitive machines to detect dark matter or neutrinos because even a tiny amount of this ghostly gas can create "noise" that hides the real signals they are looking for.

The problem with this specific ghost is that it is incredibly shy and fleeting. It has a half-life of only 55 seconds. This means if you catch it in a room, half of it disappears into thin air every minute and a half. By the time you try to move it to your detector, most of it is already gone.

The Old Way: The "Long Walk"

Traditionally, scientists measured radon by putting a sample in a sealed box, letting the gas build up, and then pumping it through a long tube into a detector.

Think of this like trying to catch a butterfly (the radon) in a jar, then running a long distance to a butterfly net (the detector) to show it off. Because the butterfly is so fast and the run is so long, most of them escape or die before you get to the net. For the super-fast 55-second radon, this "long walk" method is very inefficient. You lose most of your catch before you even start counting.

The New Way: The "Living Room" Method

In this paper, the researchers came up with a clever solution: Stop the long walk.

Instead of pumping the gas from a box to a detector, they put the sample directly inside the detector itself.

• The Analogy: Imagine you are trying to catch a firefly that only lives for a minute. Instead of catching it in a jar and running to the porch to look at it, you simply sit on the porch with the jar open. The firefly is already right where you are looking.
• The Result: Because the sample is right inside the "net," the radon doesn't have to travel. It decays right where the detector can see it immediately. This method (called the "in-chamber" method) made the scientists 3 times more sensitive than the old way.

The Super-Charge: The "Helium Highway"

The researchers didn't stop there. They realized that the type of air they used to carry the gas mattered. They switched from regular air to Helium.

• The Analogy: Imagine the radon atoms are runners. In regular air, they are running through thick mud. In Helium, they are running on a smooth, frictionless ice track. The helium helps the charged particles (the "offspring" of the radon) fly straight to the detector much faster and more efficiently.
• The Result: Using helium as the "carrier gas" boosted their sensitivity even more. Combined with the "living room" method, they became 5 times more sensitive than the old standard.

Why Does This Matter?

1. Speed: Because this new method is so much faster and more sensitive, scientists can test materials in hours instead of weeks.
2. The Proxy Trick: Radon-222 (the more common, longer-lived version) takes weeks to build up enough to measure. But Radon-220 is its "fast cousin." By testing the fast cousin (Radon-220) quickly, scientists can predict how the slow cousin (Radon-222) will behave. It's like testing a car's engine at high speed for a few minutes to know how it will perform on a long road trip.
3. Cleaner Experiments: This helps scientists find the cleanest, most "ghost-free" materials for their dark matter detectors, ensuring that when they finally catch a real dark matter particle, they know it's not just background noise.

In summary: The paper describes a new, faster, and smarter way to catch a fleeting radioactive gas by bringing the sample directly to the detector and using helium to speed things up. This allows scientists to screen materials much faster, helping them build better machines to explore the secrets of the universe.

Technical Summary

1. Problem Statement

Radon contamination is a critical background source for rare-event physics experiments, such as dark matter searches and neutrinoless double beta decay. While Radon-222 ($^{222}$Rn) is the primary concern due to its long half-life (3.8 days), Radon-220 ($^{220}$Rn or Thoron) is the second most significant isotope.

• The Challenge: $^{220}$Rn has a very short half-life of 55.6 seconds. Conventional emanation measurement techniques involve accumulating gas in a sealed chamber and then transferring it to a detector. This transfer process introduces significant delays, causing most of the $^{220}$Rn to decay before detection, thereby severely limiting sensitivity.
• The Gap: Existing industrial thoron monitors are designed for high-activity environments and lack the sensitivity required for low-background physics. Furthermore, standard flow-through methods are inefficient for the ultra-low activity levels typical of rare-event detector materials.
• The Opportunity: Despite its short half-life, $^{220}$Rn emanating from detector surfaces can contribute measurable backgrounds. Additionally, because $^{220}$Rn emanates rapidly from surfaces (unlike $^{222}$Rn which requires weeks to reach equilibrium), it could serve as a rapid proxy for studying surface treatments and coatings designed to suppress radon.

2. Methodology

The authors developed and tested a direct in-chamber measurement technique using a DURRIDGE RAD8 electrostatic radon detector. This method eliminates the gas transfer step entirely by placing the sample directly inside the active detector volume.

• Experimental Setup:

  • Source: A low-activity thoron source consisting of 31 g of 2% thoriated tungsten welding rods (total activity $\approx$ 76 mBq) housed in a 3D-printed holder.
  • Baseline (Flow-through): The conventional method was used first. The source was placed in an external acrylic emanation chamber, and gas was pumped through a drying tube and filter into the RAD8 detector at 0.6 L/min.
  • Proposed (In-chamber): The source assembly was inserted directly into the RAD8's internal electrostatic detector chamber (0.6 L volume). The holder was designed to provide electrical isolation and align with the Passivated Implanted Planar Silicon (PIPS) detector.
  • Carrier Gas: Experiments were conducted using both ambient air and 99.99% Helium to test for enhanced collection efficiency.
  • Measurement Protocol: Runs lasted 3 hours. Humidity was strictly controlled (<15%) to maintain electrostatic collection efficiency. For the in-chamber method, the source was allowed to emanate for 12 hours prior to measurement to ensure equilibrium of short-lived progeny.

• Detection Mechanism:

  • The RAD8 uses electrostatic collection to attract positively charged radon progeny to the PIPS detector.
  • Analysis focused on the $^{216}$Po alpha peak (6.9 MeV) in "Window B," which reaches equilibrium quickly (within minutes) and provides a direct measure of $^{220}$Rn activity.
  • Background subtraction was performed using empty chamber runs.

3. Key Contributions

1. Direct In-Chamber Technique: The primary innovation is placing the sample directly within the detector chamber, eliminating transfer losses that plague $^{220}$Rn measurements due to its short half-life.
2. Helium Carrier Gas Optimization: The study demonstrated that using Helium as the carrier gas further enhances sensitivity compared to air, likely due to improved ion drift and collection efficiency in the electrostatic field.
3. Absolute Calibration Method: The authors developed a method to calibrate the in-chamber setup for absolute activity measurements. By measuring the suppression of the $^{222}$Rn signal (Window C) caused by the physical presence of the sample holder, they derived a correction factor to calculate absolute $^{220}$Rn activity without needing an external reference source.
4. Rapid Surface Screening: The paper proposes using $^{220}$Rn as a "fast proxy" for $^{222}$Rn surface behavior, allowing researchers to screen surface treatments and coatings in hours rather than the weeks required for $^{222}$Rn equilibrium.

4. Results

The study compared three configurations using the same thoriated source ($A_{220} \approx 76$ mBq):

Method	Carrier Gas	Window B Count Rate (cpm)	Sensitivity Gain (Relative to Flow-through)
Flow-through	Air	$0.32 \pm 0.09$	1.0 (Baseline)
In-chamber	Air	$0.98 \pm 0.15$	$3.1 \pm 1.0$
In-chamber	Helium	$1.71 \pm 0.19$	$5.3 \pm 1.6$

• Sensitivity Improvement: The in-chamber method in air improved sensitivity by a factor of ~3. Using Helium increased this to a factor of ~5.
• Activity Verification:
  • Using the flow-through method, the calculated activity was $76 \pm 20$ mBq.
  • Using the in-chamber method with the new absolute calibration (correcting for electric field distortion), the calculated activity was $69 \pm 19$ mBq.
  • These values are consistent within uncertainties, validating the accuracy of the in-chamber technique.
• Contamination Check: Post-measurement background checks confirmed no measurable residual contamination in the detector chamber after removing the thoron source, as $^{216}$Po decays rapidly (0.145 s half-life).

5. Significance

• Low-Background Physics: This technique provides a highly sensitive tool for screening detector materials for $^{220}$Rn emanation, a critical step in reducing backgrounds for next-generation dark matter and neutrino experiments.
• Efficiency and Speed: By leveraging the short half-life of $^{220}$Rn, researchers can perform rapid pre-screening of surface treatments. Treatments that show promise in reducing $^{220}$Rn can be quickly identified and then subjected to long-term $^{222}$Rn testing, significantly accelerating the material selection process.
• Scalability: The concept of "in-chamber" measurement could be adapted for $^{222}$Rn as well, potentially reducing intrinsic backgrounds from large external emanation chambers and complex transfer lines in future high-sensitivity facilities.

In conclusion, the paper demonstrates that direct in-chamber measurement, particularly when combined with Helium carrier gas, is a robust, sensitive, and accurate method for quantifying Thoron emanation, offering a practical solution to a long-standing challenge in rare-event physics.