Preparation and measurement of an $\rm ^{37}$Ar source for liquid xenon detector calibration

This paper details the production of the radioactive isotope $^{37}$Ar via thermal neutron irradiation of $^{36}$Ar and validates its effectiveness as a precise, low-energy calibration source for liquid xenon time projection chambers used in dark matter experiments.

The Gist

The Big Picture: Calibrating a Cosmic Net

Imagine scientists are building a giant, ultra-sensitive fishing net made of liquid xenon (a heavy, rare gas) to catch "dark matter"—the invisible stuff that makes up most of the universe. But there's a problem: this net is so sensitive that it might get confused by tiny, harmless ripples in the water, mistaking them for a giant fish.

To make sure the net is working correctly, the scientists need to throw a "test fish" into the water. They need something that:

1. Mixes perfectly with the water (xenon).
2. Makes a very specific, known splash (a precise energy signal).
3. Disappears after a while so it doesn't clutter the net forever.

That "test fish" is Argon-37 ($^{37}$Ar). This paper describes how the team cooked up this radioactive isotope and tested it to prove it works perfectly for calibrating these dark matter detectors.

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Part 1: Cooking the "Test Fish" (Preparation)

The Recipe:
To make Argon-37, the scientists didn't just buy it; they had to cook it up in a nuclear reactor.

• The Ingredients: They started with a very pure version of Argon gas (specifically the isotope Argon-36). Think of this as having a bag of only red marbles, with no blue or green ones mixed in.
• The Oven: They sealed this gas in a tiny glass tube (a quartz ampoule) and put it inside a nuclear reactor.
• The Heat: The reactor shot "thermal neutrons" (tiny, slow-moving particles) at the gas. When a neutron hit an Argon-36 atom, it got absorbed, turning it into Argon-37.

The "Burn" on the Glass:
You might notice in the photos that the glass tube turned a dark purple color after the process. The scientists explain this like a "sunburn" for glass. The radiation created tiny defects (called "color centers") inside the glass structure, changing how it absorbs light, much like how a suntan changes your skin color.

Avoiding the "Bad Apples":
The biggest fear was making the wrong kind of Argon. If they accidentally made Argon-39, it would be a disaster. Argon-39 is a "bad apple" that stays radioactive for hundreds of years and creates a lot of background noise, making it impossible to see the dark matter.

• The Simulation: Before they even started, they used a supercomputer (Geant4) to simulate the recipe. They checked the math to ensure that with their specific "oven" settings, they would get plenty of Argon-37 but almost zero Argon-39. The simulation confirmed their recipe was safe.

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Part 2: The Test Drive (Measurement)

Before putting this precious gas into the massive, ton-sized detectors used for real dark matter hunting, they had to test it on a smaller, safer model.

The Test Tank:
They used a Gaseous Xenon Time Projection Chamber (GXe TPC).

• The Analogy: Imagine a giant, transparent 3D room filled with xenon gas. Inside, there are cameras (photomultiplier tubes) on the ceiling and floor.
• How it works: When an Argon-37 atom decays, it releases a tiny burst of energy. This creates two things:
  1. A Flash of Light (S1): Like a camera flash.
  2. A Trail of Electrons (S2): Like a trail of glowing breadcrumbs that drifts upward and creates a second, bigger flash when it hits the ceiling.

The Injection:
They carefully injected a tiny amount of the Argon-37 gas into this test tank. Because Argon and Xenon are chemical cousins (both noble gases), the Argon mixed in perfectly, like adding a drop of food coloring to a glass of water.

The Results:
The detectors "saw" the Argon-37 exactly where they expected it to be.

• They saw a specific signal (around 2000 "photo-electrons") that matched the energy of the K-shell decay (the main type of decay for Argon-37).
• They used clever math to filter out the "noise" (random background signals), isolating the clean signal from their test fish.
• The Verdict: They measured the activity to be about 15 Becquerels (a unit of radioactivity). This is the "Goldilocks" amount: strong enough to be seen clearly, but weak enough not to overwhelm the detector.

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Why This Matters

This paper is like a quality control report for a new tool.

1. It proves the recipe works: They showed they can make Argon-37 without creating the dangerous, long-lived Argon-39 contaminant.
2. It proves the tool is ready: They demonstrated that this gas mixes well and gives a clear, precise signal in a xenon detector.
3. The Future: Now that they have this "calibration source," big experiments like PandaX-4T and XENONnT can use it. They can inject this gas into their massive detectors to check if their "net" is measuring energy correctly. If the net knows exactly how big a "test fish" is, it can be much more confident when it claims to have caught a "dark matter fish."

In short: They successfully baked a radioactive "test fish," checked that it wasn't spoiled, and proved it swims perfectly in the xenon pool. This ensures the world's best dark matter detectors are calibrated and ready to catch the invisible.

Technical Summary

1. Problem Statement

Liquid Xenon (LXe) Time Projection Chambers (TPCs) are the leading technology for dark matter and neutrino detection. However, these detectors suffer from position-dependent signal variations due to electric field inhomogeneities and light collection inefficiencies. To correct these effects and ensure precise energy reconstruction, a calibration source is required that:

• Can be uniformly mixed within the xenon volume.
• Produces monoenergetic signals in the low-energy region (near the detector's energy threshold).
• Does not introduce long-lived radioactive backgrounds that are difficult to remove.

While $^{37}$Ar is an ideal candidate (decaying via electron capture to $^{37}$Cl with K-shell and L-shell energies of ~2.82 keV and ~0.27 keV), its production is challenging. Existing methods (e.g., irradiating $^{40}$Ca) often produce unwanted by-products like long-lived $^{39}$Ar (269-year half-life), which creates a permanent background in LXe detectors. Furthermore, high-temperature distillation methods used in previous approaches risk contaminating the detector with particulates.

2. Methodology

A. Production of $^{37}$Ar

• Target Material: High-purity (99.935%) $^{36}$Ar gas was sealed in a quartz ampoule (1 cm diameter, 4 cm length, 1 mm wall thickness).
• Irradiation: The ampoule was irradiated with thermal neutrons from a reactor (flux: $1.5 \times 10^{13}$ n/cm²/s) for 2.17 hours. The reaction used was $^{36}\text{Ar}(n, \gamma)^{37}\text{Ar}$.
• Containment: A melt-seal technique using liquid nitrogen and a hydrogen torch ensured the ampoule remained airtight and prevented contamination. Post-irradiation, the ampoule turned dark purple due to radiation-induced color centers.
• Extraction: The gas was transferred to a pressure-transfer apparatus under vacuum to ensure quantitative recovery without contamination.

B. Simulation and Feasibility (Geant4)

• A detailed Geant4 simulation was performed to predict nuclide yields and assess the "burn-up" effect (secondary reactions).
• Key Constraint: The simulation confirmed that using thermal neutrons on $^{36}$Ar minimizes the production of $^{39}$Ar (a major background contaminant) compared to fast-neutron methods on $^{40}$Ca.
• By-products: The simulation showed that other potential by-products (e.g., $^{41}$Ar, $^{35}$S) have very short half-lives or negligible yields, ensuring the final source is clean for low-background experiments.

C. Measurement Setup (GXe TPC)

• Before injecting the source into a ton-scale detector, it was tested in a Gaseous Xenon Time Projection Chamber (GXe TPC) (the RELICS demonstrator).
• Detector Specs: Operated at room temperature (~170 kPa) with 14 Hamamatsu R8520-406 PMTs. It utilized a drift region with a gate, cathode, and anode to separate S1 (scintillation) and S2 (ionization) signals.
• Injection: The $^{37}$Ar source was diluted into the GXe TPC system (total volume ~28 L) via a controlled pipeline. Only ~0.07% of the total source activity was introduced into the active drift region (181 mL) for measurement.

D. Data Analysis

• Signal Processing: Waveforms were digitized using CAEN V1725. Events were identified by peak area (in Photoelectrons, PE) and leading time.
• Background Rejection: A data-driven approach was used to distinguish $^{37}$Ar signals from background.
  • S2 Signals: K-shell events were expected at ~2000 PE; L-shell at ~200 PE.
  • Selection Cuts: Events were filtered based on the "Area Fraction of Top" (AFT) and leading time to exclude events occurring outside the drift region (e.g., near the anode or cathode) and to remove electronic noise.
• Fitting: The S2 spectrum was fitted using a Crystal Ball function (combining a Gaussian core with a power-law tail) to account for low-energy detection inefficiencies.

3. Key Contributions

• Novel Production Method: Demonstrated a clean production route for $^{37}$Ar using thermal neutron irradiation of $^{36}$Ar, successfully avoiding the generation of long-lived $^{39}$Ar.
• Validation of Source Purity: Provided experimental verification that the produced source contains negligible amounts of interfering isotopes, making it safe for use in ultra-low-background dark matter experiments.
• Calibration Protocol: Established a rigorous injection and measurement protocol for gaseous calibration sources in TPCs, including specific data-driven background rejection techniques (AFT and leading time cuts).
• Quantitative Activity Measurement: Successfully measured the activity of the $^{37}$Ar source within the detector volume.

4. Results

• Source Activity: The measured activity of the $^{37}$Ar K-shell decay within the drift region was 14.96 Bq.
• Total Estimated Activity: Accounting for the K-shell branching ratio (90.2%) and selection efficiency (94.0%), the total activity in the drift region was estimated at 17.646 ± 0.025 (stat) ± 0.007 (sys) Bq.
• Signal Identification: Clear peaks were observed at 2000 PE (K-shell) and ~200 PE (L-shell), matching theoretical predictions based on the W-value of gaseous xenon (22 eV) and single-electron gain.
• Background Suppression: The data-driven selection criteria effectively removed background events from the anode/gate regions and cosmic muons, isolating the $^{37}$Ar signal with high purity.

5. Significance

• Dark Matter Calibration: This work confirms that reactor-produced $^{37}$Ar is a viable, high-purity calibration source for LXe detectors (such as PandaX-4T and XENONnT). Its low-energy monoenergetic lines are critical for calibrating the detector response near the energy threshold, essential for distinguishing dark matter signals from background.
• Safety for Low-Background Experiments: By proving that the thermal neutron method on $^{36}$Ar does not produce significant $^{39}$Ar, this study removes a major barrier to using $^{37}$Ar in next-generation dark matter searches, where even trace amounts of $^{39}$Ar are unacceptable.
• Future Applications: The successful preparation and measurement pave the way for routine calibration of large-scale xenon detectors, improving the precision of energy reconstruction and position correction in future dark matter and neutrino experiments.