A practical approach of measuring $^{238}$U and $^{232}$Th in liquid scintillator to sub-ppq level using ICP-MS

This study presents a practical method using acid extraction and ICP-MS to measure $^{238}$U and $^{232}$Th in liquid scintillator at sub-ppq levels with nearly 100% recovery efficiency and a detection limit of 0.2–0.3 ppq, achieved through rigorous cleanliness control and validated by three standard addition techniques.

The Gist

Imagine you are trying to find a single, tiny grain of sand hidden inside a massive swimming pool filled with clear blue water. That is essentially what scientists are doing when they look for radioactive impurities like Uranium-238 and Thorium-232 in the liquid used for giant particle detectors.

This paper describes a clever new "fishing" technique developed by researchers at the Institute of High Energy Physics in China to catch these invisible grains of sand (impurities) from a huge pool of liquid scintillator (the detector fluid) so they can be counted.

Here is the story of how they did it, broken down into simple steps:

1. The Problem: The "Needle in a Haystack"

Liquid scintillators are special fluids that glow when hit by particles. They are used in massive underground experiments to catch rare cosmic events. But these fluids must be extremely pure. If there is even a tiny bit of natural Uranium or Thorium in the fluid, it will glow on its own, creating "noise" that hides the rare signals scientists are trying to find.

The goal is to keep the fluid so clean that the amount of Uranium is less than one part per quadrillion. To put that in perspective: if you had a quadrillion grains of sand, you are only allowed to have one or two grains of "bad sand" (Uranium/Thorium) mixed in.

2. The Challenge: The Liquid is "Greasy"

The scientists wanted to test a small sample (about 2 kilograms, or 4.4 lbs) of this liquid to see if it was clean enough before filling up a massive 20-ton detector.

However, there's a problem: The liquid scintillator is like oil. The machine used to detect the impurities (called an ICP-MS) is like a very sensitive scale that can only weigh things dissolved in water. You can't just pour oil into this machine; it would clog it up.

3. The Solution: The "Acid Sponge" Trick

The researchers invented a process to separate the "bad sand" (Uranium/Thorium) from the "oil" (the scintillator) and concentrate it. Think of it like this:

• The Setup: They took a huge bucket of the oily liquid and added a specific amount of strong acid (nitric acid).
• The Mix: They stirred it vigorously with a giant magnetic spoon. Imagine shaking a bottle of salad dressing (oil and vinegar) really hard. The acid acts like a magnet that grabs the Uranium and Thorium atoms, pulling them out of the oil and into the acid layer.
• The Separation: Once the mixing is done, they let it sit in an ice bath. The oil and acid separate again, like oil floating on top of vinegar. The scientists carefully pour off the top oil layer and keep the bottom acid layer, which now holds all the captured impurities.
• The Concentration: They boil the acid down until it's just a tiny drop. Now, instead of looking for a needle in a swimming pool, they are looking for that needle in a single drop of water. This makes it much easier for the machine to find it.

4. Proving It Works: The "Test Run"

How do you know you didn't lose the Uranium during this messy process? The scientists used three different "tracers" (test subjects) to make sure their method was perfect:

1. The "Ghost" Tracers: They added a type of Uranium and Thorium that doesn't exist in nature (like a fake ID card). Since it wasn't there before, any they found at the end proved they successfully moved it from the oil to the acid.
2. The "Dye" Tracer: They added a common chemical (PPO) that acts like the liquid scintillator but has a known amount of Uranium in it. They checked if they could pull that specific amount out.
3. The "Radioactive Balloon": They bubbled a gas containing a radioactive cousin of Thorium (Radon) through the liquid. This is like inflating a balloon inside the pool. They checked if the balloon stayed intact or popped (was lost) during the acid extraction.

The Result: All three tests showed that they recovered nearly 100% of the impurities. They didn't lose any of the "bad sand" during the process.

5. The Final Score: How Clean is Clean?

After testing their method with 2 kilograms of liquid, they calculated the "detection limit." This is the smallest amount of impurity they can reliably spot.

• Old Limit: They could only see impurities if there were about 1 part per trillion.
• New Limit: With this new acid-extraction method, they can now see down to 0.3 parts per quadrillion.

This is a massive improvement. It means they can now test small samples in the lab with extreme confidence, ensuring that the massive 20-ton detectors (like the JUNO experiment) will be pure enough to catch the faintest whispers of the universe.

Summary Analogy

Imagine you are a chef trying to make the purest soup in the world. You suspect there is a single grain of salt hiding in a giant pot of oil.

1. You can't taste the oil directly to find the salt.
2. So, you add a special vinegar that loves salt but hates oil.
3. You shake the pot, and the salt jumps into the vinegar.
4. You pour off the oil and boil the vinegar down to a single drop.
5. Now, you can easily taste that single drop and say, "Yes, there is salt here," or "No, it's pure."

This paper is the recipe for that perfect vinegar-and-oil separation, allowing scientists to ensure their "soup" (the particle detector) is pure enough to taste the secrets of the universe.

Technical Summary

Here is a detailed technical summary of the paper "A practical approach of measuring 238U and 232Th in liquid scintillator to sub-ppq level using ICP-MS."

1. Problem Statement

Liquid scintillators (LS) are critical for rare-event physics experiments (e.g., neutrino detection) due to their high light yield and transparency. However, these experiments require extreme radiopurity, specifically demanding that Uranium-238 ($^{238}$U) and Thorium-232 ($^{232}$Th) concentrations remain below the sub-ppq level ($10^{-15}$ g/g).

Key Challenges:

• Current Limitations: Existing screening methods typically rely on measuring LS after detectors are filled (masses of tens to hundreds of tons). There is a lack of validated methods to measure sub-ppq levels in laboratory-scale samples (kilogram range) prior to detector construction.
• Sensitivity Gap: While Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is highly sensitive, the direct analysis of organic LS is impossible. The organic matrix must be removed, and the trace U/Th must be concentrated without introducing external contamination, which is difficult at the ppq level.

2. Methodology

The study developed a robust pre-treatment protocol to concentrate $^{238}$U and $^{232}$Th from liquid scintillator samples for ICP-MS analysis.

A. Experimental Setup & Cleanliness

• Facility: Class 100 sample pre-treatment room and Class 1000 ICP-MS measurement room.
• Instrumentation: ThermoFisher iCAP-Qc Quadrupole ICP-MS with a PFA concentric nebulizer.
• Cleaning Protocol: Rigorous multi-stage cleaning of all labware (PFA bottles, vessels) involving:
  • Ultrasonic cleaning with Alconox.
  • High-temperature (220°C) acid washing with TraceCLEAN equipment.
  • Multiple acid baths (soaking and boiling) using high-purity GENIUS and Fisher nitric acids.
  • Final verification via ICP-MS (counts < 10 cps).

B. Pre-treatment Process (Acid Extraction)

Since LS is organic, it cannot be directly analyzed. The process involves:

1. Acid Extraction: Mixing LS with nitric acid to transfer U/Th from the organic phase to the aqueous phase.
2. Phase Separation: Allowing the mixture to settle (accelerated by ice bath) to separate the oil and acid layers.
3. Enrichment: Evaporating the acid phase to concentrate the U/Th into a small volume for ICP-MS injection.

C. Parameter Optimization

The authors optimized several variables using a PPO (2,5-diphenyloxazole) master solution spiked with U/Th:

• Stirring: 1300 rpm for 2 hours to ensure thorough mixing of oil and water phases.
• Acid Concentration: 10% Nitric Acid was selected. Higher concentrations (30%) caused PPO precipitation; 10% achieved >90% extraction efficiency in the first stage.
• Acid-to-Oil Ratio: 1:5 was chosen to minimize reagent usage (and thus background contamination) while maintaining efficiency.
• Temperature: 80°C provided the best extraction efficiency, particularly for $^{232}$Th.
• Reusability: The 10% nitric acid was reused three times to reduce reagent background.

D. Recovery Efficiency Validation

Three distinct standard addition methods were employed to verify that U/Th was not lost during the process:

1. Inorganic Standards: Adding $^{233}$U and $^{229}$Th (ion form) to the acid phase.
2. Organic Standards: Adding PPO powder with known U/Th content to the LS.
3. Radon Daughter Loading: Introducing $^{220}$Rn gas into the LS to generate $^{212}$Pb (a decay product), simulating natural impurities in the organic phase.

3. Key Results

A. Recovery Efficiency

All three validation methods confirmed nearly 100% recovery efficiency with approximately 10% uncertainty:

• $^{233}$U: $108 \pm 11%$
• $^{229}$Th: $115 \pm 7%$
• PPO-spiked LS ($^{238}$U): $106 \pm 31%$
• PPO-spiked LS ($^{232}$Th): $123 \pm 43%$
• $^{212}$Pb (Radon method): $98.2 \pm 0.3%$(stat)$\pm 2%$ (sys)

B. Blank Tests and Detection Limits

• Contamination Control: The average background contamination from the entire process (reagents + containers + environment) was measured at 0.41 pg for $^{238}$U and 0.32 pg for $^{232}$Th.
• Method Detection Limit (MDL): For a 2 kg liquid scintillator sample, the MDL at a 99% confidence level was calculated as:
  • $^{238}$U: 0.30 ppq ($3.0 \times 10^{-16}$ g/g)
  • $^{232}$Th: 0.24 ppq ($2.4 \times 10^{-16}$ g/g)

C. Scalability

The study demonstrated that the blank contamination does not increase linearly with sample mass because the acid reagent can be reused and container contamination remains constant. The authors estimate that scaling the sample size to 20 kg could improve the detection limit to 0.03 ppq.

4. Key Contributions

1. First Kilogram-Scale Sub-ppq Measurement: Established the first practical method to screen LS at the sub-ppq level using kilogram-scale laboratory samples, bridging the gap between small-scale material screening and full-detector measurements.
2. Validated Recovery: Proved that acid extraction does not result in significant loss of U/Th, even when impurities are in organic forms (simulated by PPO) or radon decay products.
3. Optimized Protocol: Defined a specific, reproducible protocol (10% HNO$_3$, 1:5 ratio, 80°C, 2h stirring) that maximizes extraction while minimizing reagent background.
4. Scalability Roadmap: Provided a theoretical and experimental basis for further lowering detection limits by increasing sample mass without a proportional increase in background.

5. Significance

This work is critical for the next generation of large-scale neutrino experiments (such as JUNO, which targets 20,000 tons of LS).

• Quality Control: It allows for the rigorous screening of LS materials before they are poured into massive detectors, preventing the construction of detectors that fail radiopurity standards.
• Cost and Time Efficiency: By validating screening on small (kg) samples, the need for expensive, time-consuming measurements on full-scale (hundreds of tons) detectors is reduced.
• Feasibility: It confirms that the stringent radiopurity requirements ($<10^{-15}$ g/g) for future rare-event searches are achievable with current ICP-MS technology and chemical pre-treatment methods.