In-situ high voltage generation with Cockcroft-Walton multiplier for xenon gas time projection chamber

The authors successfully demonstrated a new in-situ Cockcroft-Walton high-voltage generation system integrated into the AXEL xenon gas time projection chamber, which eliminated the need for external high-voltage feedthroughs and achieved an energy resolution of 0.67% at 2615 keV during a 40-day operation at 6.8 bar.

The Gist

The Big Picture: Catching Ghosts in a Gas Giant

Imagine scientists are trying to catch a very rare, invisible ghost called a "neutrino." Specifically, they are looking for a ghostly event called "neutrinoless double beta decay." To do this, they built a giant, high-pressure balloon filled with Xenon gas (a heavy, noble gas). This balloon is called a Time Projection Chamber (TPC).

When a particle passes through this gas, it leaves a trail of ionized electrons, like a boat leaving a wake in water. The goal is to track this wake perfectly to measure the energy of the particle.

The Problem: The High-Voltage Wall

To make those electron "wakes" move toward the detectors, you need a strong electric field. This requires a massive amount of electricity—over 40,000 volts (40 kV).

Usually, you would plug a giant high-voltage cable into the side of the balloon. But here's the problem:

1. The Balloon is Pressurized: The gas inside is squeezed tight (like a deep-sea diver's suit).
2. The Leak Risk: Drilling a hole for a thick, high-voltage cable is dangerous. It could leak gas or cause a spark (arc) that ruins the experiment.

The Solution: Instead of bringing the high voltage in, the scientists decided to build a small power plant inside the balloon. They wanted to bring in a low, safe voltage from the outside and boost it up to the dangerous levels right where it's needed.

The Hero: The Cockcroft-Walton Multiplier

To solve this, they invented a new type of voltage booster called a Cockcroft-Walton (CW) multiplier.

Think of this device like a staircase of elevators.

• You step onto the first elevator (low voltage).
• It lifts you up a bit.
• You step onto the next elevator, which lifts you higher.
• You keep stepping up until you reach the top floor (high voltage).

In this experiment, the "elevators" are tiny electronic components (capacitors and diodes) arranged in a chain. They take a gentle AC (alternating current) wave from the outside and pump it up step-by-step until it becomes a massive DC (direct current) voltage inside the chamber.

The Engineering Challenge: Fitting a Elephant in a Teacup

The inside of the detector is incredibly cramped. The scientists had to fit this "power plant" into a space no bigger than a large pizza box (about 20cm wide and 3cm high).

To make it fit and work safely, they used some clever tricks:

1. Flexible Circuit Boards: Instead of a bulky metal box, they built the multiplier on a flexible circuit board (like a high-tech, bendy ribbon). This allowed them to wrap it around the inside of the detector.
2. The "Bubble" Problem: Electronics often release tiny amounts of gas (outgassing) when they get warm. In a pure Xenon gas chamber, even a tiny bit of "dirty" gas can eat up the electron signals, ruining the data. The team had to ensure their new device was so clean it wouldn't pollute the gas. They tested it and found it was clean enough.
3. The "Spark" Problem: High voltage loves to jump across gaps (sparks). To stop this, they coated the entire circuit in a special silicone resin (like a waterproof, insulating varnish) and added tiny grooves to the plastic housing to force any potential sparks to take a long, difficult path, preventing them from jumping.

The Experiment: The 40-Day Marathon

They installed this new device into a 180-liter prototype detector (the "180 L prototype"). They filled it with Xenon gas at high pressure and ran it for 40 days straight.

What happened?

• It Worked: The device successfully generated the high voltage needed to drift electrons across the chamber.
• No Noise: Usually, when you run high-voltage AC power near sensitive electronics, it creates static noise (like a radio picking up a station you don't want). The team was worried the "staircase" would buzz and ruin their signal. They found that the noise was so quiet it was barely noticeable—less than one tiny step on their measurement scale.
• Clear Pictures: They used a radioactive source (Thorium-doped tungsten rods) to shoot gamma rays into the chamber. The detector successfully tracked the paths of the electrons.
  • They could see a single electron track (one long line).
  • They could see a pair of tracks (an electron and a positron) coming from a single point.
  • This is crucial because the "ghost" event they are hunting (neutrinoless double beta decay) looks like two tracks, while background noise usually looks like one.

The Result: Crystal Clear Vision

The most important number they got was the Energy Resolution. Think of this as the sharpness of a camera lens.

• If the lens is blurry, you can't tell if two objects are close together.
• If the lens is sharp, you can see fine details.

Their new setup produced a "lens" so sharp that at an energy level of 2615 keV, the blur was only 0.67%. This is an incredibly high level of precision.

Summary

The paper describes a successful engineering feat where scientists built a tiny, high-voltage power plant inside a pressurized gas tank. By using flexible circuits and special coatings, they managed to generate the massive electricity needed to track subatomic particles without causing leaks, sparks, or electrical noise. They proved this system can run stably for weeks, paving the way for larger, more sensitive detectors to hunt for the rarest events in the universe.

Technical Summary

Technical Summary: In-situ High Voltage Generation with Cockcroft-Walton Multiplier for Xenon Gas Time Projection Chamber

Problem Statement
High-pressure xenon gas Time Projection Chambers (TPCs) are critical for searching for neutrinoless double beta decay ($0\nu\beta\beta$) of $^{136}$Xe, offering the potential for ton-scale detectors with superior energy resolution and background rejection. A significant technical challenge in scaling these detectors is the generation of the high voltage (HV) required to create the drift electric field. For detectors ranging from 100 kg to 1000 kg, voltages exceeding 100 kV are necessary. Supplying such high voltages from outside the pressure vessel requires specialized, high-pressure-compatible feedthroughs, which introduce complexity and potential failure points. An alternative approach involves introducing a lower voltage from outside and boosting it internally. While Cockcroft-Walton (CW) multipliers can convert low AC input to high DC output, their implementation in TPCs has been hindered by issues such as large baseline shifts caused by AC input, which complicate signal readout, and the difficulty of maintaining voltage stability if the AC input is interrupted. Furthermore, previous proposals for liquid argon TPCs utilizing this method had not been realized in actual operation.

Methodology
The authors developed and integrated a custom Cockcroft-Walton multiplier into the AXEL (A Xenon ElectroLuminescence detector) 180 L prototype, a high-pressure xenon gas TPC designed to search for $0\nu\beta\beta$.

• Detector Configuration: The AXEL detector utilizes electroluminescence (EL) to detect ionization electrons. Ionization electrons drift toward an Electroluminescence Light Collection Cell (ELCC) plane, where they are accelerated to produce VUV photons detected by Silicon Photomultipliers (SiPMs). This optical readout method provides high resistance to electronic noise.
• CW Multiplier Design: To fit the narrow space (approx. 20 cm width, 3 cm height) between the field cage and the chamber wall, the CW multiplier was constructed using flexible printed circuit (FPC) boards with surface-mount devices. Each board contains 10 stages of the basic CW circuit (capacitors and diodes) and a resistor chain for voltage distribution.
  • Components: The design employs Knowles Syfer ceramic capacitors (0.1 $\mu$F, 2 kV), Micro Commercial Components fast switching diodes (FM2000GP), and Bourns resistors (100 M$\Omega$).
  • Insulation and Outgassing: To prevent electrical discharges and minimize gas contamination, the circuit was coated with methyl silicone resin (KR-251) after ultrasonic cleaning and degassing. The design avoids polyimide sheets in the two-layer structure to prevent surface discharges, and cell holes were tapped to prevent discharge along the holes.
  • Voltage Generation: The system was designed to generate a target voltage of -44.8 kV (operating at 34.3 kV during the test) to establish a drift field of -800 V/cm at 8 bar. The AC input was supplied via a sine wave generator and amplifier.
• Operational Test: The prototype was operated at 6.8 bar xenon pressure for 40 days. The CW multiplier supplied the cathode voltage and field cage stages. Data was acquired using thorium-doped tungsten rods as a gamma-ray source, providing 2615 keV gamma rays from $^{208}$Tl.

Key Contributions

1. In-situ HV Generation: The successful integration and operation of a CW multiplier inside a high-pressure xenon gas TPC, demonstrating a viable alternative to external HV feedthroughs.
2. Compact and Low-Outgassing Design: The development of a compact, FPC-based CW multiplier with an outgassing rate ($1.4 \times 10^{-5}$ Pa m$^3$/s for the multiplier assembly) well below the operational limit of the prototype detector, ensuring gas purity is maintained.
3. Signal Stability: Demonstration that the AC input required for the CW multiplier does not introduce significant noise or baseline shifts to the ELCC signal, with fluctuations remaining within one ADC count.
4. Discharge Mitigation: Implementation of specific mechanical and coating modifications (tapped holes, resin coating, modified PTFE structure) to suppress electrical discharges in the high-voltage environment.

Results

• Stable Operation: The CW multiplier operated stably for 40 days at a cathode voltage of 34.3 kV (90% of the design value) and 6.8 bar pressure.
• Energy Resolution: The detector achieved an energy resolution of $(0.67 \pm 0.08)\%$ (FWHM) at the 2615 keV peak from $^{208}$Tl. Interpolation suggests a resolution of $(0.678 \pm 0.010)\%$ at the $0\nu\beta\beta$ Q-value of 2458 keV (assuming statistical fluctuation dominance).
• Signal Quality: The baseline standard deviation of the ELCC channels remained stable (mean $\sigma_{bl} \approx 0.46$), confirming that the AC pickup from the CW multiplier did not degrade the energy resolution.
• Track Reconstruction: The system successfully reconstructed 3D tracks. Events from 2615 keV gamma rays were identified as single electron tracks, while 1593 keV events (double escape peaks) showed two distinct tracks (electron and positron) originating from a single vertex, demonstrating the detector's capability to distinguish topologies.
• Gas Purity: The electron lifetime was measured to be $(25.0 \pm 0.9)$ ms, corresponding to an attenuation length of $(27,500 \pm 1020)$ mm, indicating sufficient gas purity was maintained despite the presence of the multiplier.

Significance
The paper claims that this work demonstrates the feasibility of using a Cockcroft-Walton multiplier for in-situ high voltage generation in high-pressure xenon gas TPCs. By successfully operating the multiplier for an extended period (40 days) without compromising gas purity or signal resolution, the authors validate a design that could eliminate the need for complex high-voltage feedthroughs in future ton-scale detectors. This approach addresses a key technical barrier in scaling up xenon-based experiments for neutrinoless double beta decay searches, offering a path toward detectors with better energy resolution and lower backgrounds. The successful mitigation of AC-induced noise and electrical discharges provides a practical blueprint for future implementations.