Measurement of ionization yield of low energy ions in low pressure $\mathrm{CF}_{4}$ gas for dark matter searches

This study establishes a low-energy ion injection scheme into a gaseous detector and measures the ionization yield of fluorine ions in low-pressure $\mathrm{CF}_{4}$ gas, finding a value of 0.45 at 30 keV to improve the accuracy of direction-sensitive dark matter search experiments.

The Gist

Imagine you are trying to find a ghost. But this isn't a spooky ghost; it's a Dark Matter particle (specifically a WIMP) that is so shy and light that it barely interacts with anything in the universe.

To catch this ghost, scientists use giant, invisible nets made of gas. When a dark matter ghost bumps into an atom in the gas, it knocks the atom backward. This is called a "nuclear recoil." The problem? These recoils are tiny, fast, and happen at very low energies.

The big question scientists have is: When that tiny atom gets knocked backward, how much of a "spark" (electric charge) does it actually create?

If you know exactly how much spark a 30 keV (kilo-electronvolt) hit makes, you can build a better detector to find dark matter. If you guess wrong, you might miss the ghost entirely.

This paper is about a team of scientists in Japan who decided to stop guessing and start testing. Here is the story of what they did, explained simply.

1. The Setup: A "Bullet" Range for Atoms

Usually, to test detectors, scientists use radioactive sources (like X-rays). But X-rays are like throwing a tennis ball at a wall; they interact differently than a dark matter particle would.

To get the real answer, these scientists needed to shoot actual atoms at their detector, just like a dark matter particle would.

• The Gun: They used a massive machine at Kanagawa University that acts like a super-precise particle cannon. It can shoot Fluorine ions (atoms of Fluorine that have been stripped of an electron) at speeds ranging from very slow to moderately fast (5 to 50 keV).
• The Target: Their target was a special "wire chamber" filled with CF4 gas (Carbon Tetrafluoride). Think of this gas as a room full of invisible, fluffy pillows.
• The Challenge: The gun needs to be in a vacuum (empty space) to work, but the target needs to be full of gas. How do you shoot a bullet from a vacuum room into a gas room without letting all the gas escape?
  • The Solution: They built a microscopic "airlock." It's a tiny, tapered hole in a super-thin metal film (only 10 micrometers thick—thinner than a human hair). It's like shooting a needle through a piece of tissue paper without tearing the whole sheet.

2. The Experiment: Shooting and Listening

They fired Fluorine ions into the gas chamber at different speeds.

• The Goal: Measure the "Ionization Yield."
• The Analogy: Imagine you drop a pebble into a pond.
  • The Beam Energy is how hard you throw the pebble.
  • The Ionization Yield is how big the splash is.
  • Scientists want to know: "If I throw a pebble with 30 units of force, how big is the splash?"

They measured the electrical "splash" (charge) created when the Fluorine ions hit the gas molecules.

3. The Calibration: Tuning the Microphone

Before they could trust their measurements, they had to make sure their "microphone" (the detector electronics) was tuned correctly.

• They used an electron gun (shooting tiny electrons) and a known X-ray source (like a standard flashlight) to check if their ruler was straight.
• They found a small discrepancy (about 7.7%) between the electron measurements and the X-ray measurements. It's like finding that your ruler says 10 inches, but a standard tape measure says 10.7 inches. They had to adjust their math to account for this "ruler error."

4. The Results: The "Splash" Size

After all the shooting and measuring, they found the answer they were looking for:

• The Big Number: At an energy of 30 keV, the ionization yield was 0.45.
• What does that mean? It means that when a Fluorine atom gets hit with 30 keV of energy, only about 45% of that energy turns into a detectable electric spark. The rest is lost to other things, like heat or shaking the gas molecules (vibrations).
• The Trend: As the energy of the ion increased, the yield went up slightly, but not by much. It's a "moderate dependence."

They compared their results to two other things:

1. Computer Simulations (SRIM): A digital model of what should happen.
2. Previous Experiments (COMIMAC): Another lab that tried something similar.

The Verdict: Their results matched the computer simulations and the other lab's data pretty well! This gives scientists confidence that their models for dark matter detectors are accurate.

Why Does This Matter?

Dark matter experiments are like trying to hear a whisper in a hurricane. If you don't know exactly how loud the whisper is (the ionization yield), you can't build a microphone sensitive enough to hear it.

By proving that they can shoot ions into gas and measure the exact "spark" they create, this team has:

1. Validated their equipment: They showed their "microscopic airlock" works perfectly.
2. Improved the map: They gave the dark matter hunting community a more accurate map of how energy turns into signals in gas detectors.
3. Opened the door: Now, they can use this setup to test other gases and other types of ions to make future dark matter detectors even better.

In short: They built a tiny, high-tech shooting range to see exactly how much "electric noise" a single atom makes when it gets bumped. This helps us build better tools to catch the universe's most elusive ghost.

Technical Summary

1. Problem Statement

Direct dark matter search experiments, particularly those aiming to detect Weakly Interacting Massive Particles (WIMPs) with directionality, rely on gaseous Time Projection Chambers (TPCs) filled with fluorine-containing gases (e.g., CF4). These detectors reconstruct the short tracks of nuclear recoils caused by WIMP interactions.

• The Core Issue: While the energy deposition of charged particles in materials is a long-studied topic, there is a lack of full agreement between experimental measurements and theoretical predictions (such as Lindhard theory or SRIM simulations), especially at low energy scales (tens of keV).
• Specific Challenge: Nuclear recoils in gas involve complex processes (nuclear stopping, ionization, excitation, recombination) that differ significantly from electron or gamma-ray interactions. Standard energy calibrations using mono-energy radioisotopes (gamma/alpha) or X-rays do not accurately reflect the ionization yield of nuclear recoils due to differences in particle type.
• Goal: To experimentally measure the ionization yield (the ratio of detected ionization charge to the true kinetic energy) of low-energy fluorine ions in low-pressure CF4 gas to improve the energy reconstruction accuracy for dark matter experiments.

2. Methodology

The study utilized a dedicated low-energy ion beamline at Kanagawa University to inject mono-energetic ion beams directly into a gaseous detector.

A. Experimental Setup

• Ion Source & Accelerator: A Freeman-type ion source generated F+ ions from SiF4 gas. The ions were extracted at +30 kV, analyzed by a magnet, and further accelerated using a Cockcroft-Walton generator. The beam energy was tunable from 5 keV to 200 keV.
• Beam Injection Interface: To bridge the high-vacuum beamline ($\sim 10^{-4}$ Pa) and the gas detector (0.06 atm), a specialized interface was designed. It featured a 10 $\mu$m-thick stainless steel film with a tapered injection hole (2.9 $\mu$m upstream, 1.2 $\mu$m downstream). Geant4 simulations confirmed that energy loss and scattering at the hole edges were negligible.
• Detector: A custom proportional wire chamber was used.
  • Geometry: A cross-shaped stainless steel pipe (15.8 mm inner diameter) with a central 30 $\mu$m tungsten anode wire.
  • Gas: Filled with CF4 at 0.06 atm.
  • Readout: Signals were amplified by a charge-sensitive amplifier (Cremat CR-110), shaped, and digitized by an ADALM2000 (100 MHz sampling, 12-bit ADC).
  • Field Shaping: A copper mesh cylinder surrounded the anode to ensure a stable electric field, with a specific hole aligned for beam injection.

B. Calibration and Measurement Procedure

• Energy Calibration: Performed using two mono-energetic sources to establish the relationship between signal charge and energy (in "electron-equivalent" or keVee):
  1. Electron Gun: Tuned to 2.0, 2.5, and 3.0 keV.
  2. X-ray Source: $^{55}$Fe source (5.9 keV).
  • Note: A systematic discrepancy of ~7.7% was observed between the electron-gun and X-ray calibrations, attributed to potential collection efficiency drops near the beam injection hole. This was treated as a systematic uncertainty.
• Beam Scans: F+ beams were injected at energies ranging from 5 keV to 50 keV (steps of 5 keV up to 30 keV, then 10 keV steps).
• Gas Gain Adjustment: To accommodate the dynamic range of the readout system, the anode voltage was adjusted (1085 V for 5–15 keV; 1000 V for 20–50 keV), resulting in gas gains of 3760 and 1170, respectively.

3. Key Contributions

1. Establishment of Injection Scheme: Successfully developed and validated a method to inject low-energy ion beams (5–50 keV) from a high-vacuum accelerator into a low-pressure gas detector through a micro-tapered hole without significant beam degradation.
2. First Direct Measurement: Provided the first direct measurement of the ionization yield for fluorine ions in low-pressure CF4 gas using a dedicated beam facility in Japan.
3. Validation of Models: Directly compared experimental data against SRIM simulations and previous measurements from the COMIMAC facility, offering empirical data to refine theoretical models for dark matter recoil detection.

4. Results

• Ionization Yield: The ionization yield ($Y$) was defined as $Y = E_{measured} [keV_{ee}] / E_{beam} [keV]$.
  • At 30 keV, the measured ionization yield was 0.45.
  • The yield showed a moderate dependence on ion energy, slightly increasing as the beam energy increased within the 5–50 keV range.
• Energy Resolution:
  • At 10 keV: Resolution was 29% ($\sigma$).
  • At 40 keV: Resolution improved to 12.5% ($\sigma$).
• Comparison: The results were consistent with data from the COMIMAC facility (measured at 0.05 atm) and SRIM simulations (calculated for 0.06 atm), despite the assigned systematic uncertainty arising from the calibration discrepancy.

5. Significance

• Dark Matter Search Optimization: Accurate ionization yield data is critical for converting the measured charge in gaseous TPCs into the true nuclear recoil energy. This study provides essential calibration parameters for experiments like NEWAGE, DM-TPC, and MIMAC, which use fluorine-based gases to search for spin-dependent dark matter.
• Detector Response Understanding: The findings help resolve discrepancies between theoretical predictions and experimental reality at low energy scales, where nuclear stopping power dominates over ionization.
• Future Capabilities: The successful establishment of this low-energy ion injection facility at Kanagawa University opens the door for future studies using different ion species, gases, and detector geometries to further map the complex energy deposition processes in gaseous detectors.