A Novel, Steerable, Low-Energy Proton Source for Detector Characterization

This paper reports on the successful conversion of the Manitoba II mass spectrometer into a versatile, steerable low-energy proton beam facility (25–35 keV) capable of characterizing silicon detectors for BSM searches like the Nab experiment by delivering a monoenergetic pencil beam with a 0.6–1.26 mm spot size over a 117 mm area.

The Gist

Imagine you have a very delicate, high-tech camera sensor (specifically, a silicon detector used in the "Nab" experiment) that is trying to take pictures of the tiniest particles in the universe. Before scientists can trust this camera to capture real data, they need to test it thoroughly. They need to know: Does every little pixel on this sensor work? Can it tell exactly where a particle hit?

To do this, the team at the University of Manitoba built a special "proton flashlight."

Here is the story of how they turned an old, heavy-duty scientific instrument into a precise tool for testing these detectors, explained simply.

The Old Machine Gets a Makeover

The team started with a giant, vintage machine called the Manitoba II mass spectrometer. Think of this like a very old, very precise car that was originally built in 1967 to weigh tiny ions (charged atoms) with extreme accuracy. It was like a high-end scale for atoms.

Instead of letting this old machine retire, they gave it a "second lease on life." They modified it to stop weighing things and start acting as a steerable proton beam. Imagine taking a massive, industrial laser cutter and re-tooling it so it can gently paint tiny dots on a canvas. That's what they did.

How the "Proton Flashlight" Works

The machine creates a beam of protons (hydrogen nuclei) and shoots them at the detector. Here is the journey of a single proton, step-by-step:

1. The Birth (The Ion Source):
   Inside a vacuum chamber, they mix hydrogen and argon gas. Think of this like a foggy room. They zap this gas with electricity to create a plasma (a soup of charged particles). A special magnet acts like a "traffic cop," keeping the particles spinning in circles so they bump into each other enough to turn into protons. This creates a steady stream of protons.

2. The Speed Trap (The Electro-static Analyzer):
   The protons fly out, but they might be going at slightly different speeds. The machine has a curved path with electric plates on the sides. Only protons with the exact right speed can make it through the curve without hitting the walls. It's like a turnstile that only lets people of a specific height pass through. This ensures all the protons have the same energy (about 30,000 electron volts).

3. The Sorting Hat (The Magneto-static Analyzer):
   Next, the protons enter a magnetic field. This field bends their path. Since the protons are all the same speed, the magnetic field acts like a filter, ensuring only the specific type of particle (protons) makes it through, while other heavier or lighter particles get bent the wrong way and get stuck.

4. The Steering Wheel (The Electro-static Steerer):
   This is the most important part for the test. The machine has four metal plates that can be charged with electricity. By turning the voltage up or down on these plates, the scientists can push the beam left, right, up, or down.

   • The Goal: They needed to paint a tiny dot (a "spot") on the detector.
   • The Challenge: The detector is a large circle (117 mm wide) covered in 127 tiny hexagonal tiles (pixels). The beam had to be small enough to hit just one tile without accidentally hitting its neighbors.

The Results: Did It Work?

The team ran several tests to see if their "flashlight" was good enough:

• Energy Precision: They checked how "pure" the beam was. They found the energy was incredibly consistent, with a tiny variation of only 300 electron volts. This is much sharper than the detector itself, meaning the test tool is more precise than the thing being tested.
• The "Spot Size" Test: They needed to know how big the dot was.
  • First, they used a phosphor screen (like a glow-in-the-dark board). When the protons hit it, it lit up green. They took photos of the glowing dots. The dots were tiny—about the size of a pinhead (roughly 1 mm²).
  • Second, they used the actual silicon detector. They moved the beam across the boundary between two tiles and counted how many protons hit each side. This confirmed the beam was small enough to stay inside a single tile (about 3.1 mm in diameter).

Why This Matters

The Nab experiment is looking for clues about physics "beyond the Standard Model" (new, weird physics we haven't discovered yet). To do this, they need silicon detectors that are perfectly calibrated.

This new facility proved that they could:

1. Shoot a beam of protons at a specific energy.
2. Steer that beam to hit any specific spot on a large detector.
3. Keep the beam so small that it only tests one tiny pixel at a time.

In short, they built a custom, low-energy proton "paintbrush" that allowed them to carefully check every single pixel of a giant, sensitive detector to make sure it was ready for the big science experiments. The paper concludes that this facility successfully met all the requirements to characterize the Nab detectors.

Technical Summary

Technical Summary: A Novel, Steerable, Low-Energy Proton Source for Detector Characterization

Problem Statement
Low-energy, high-precision experiments probing Beyond-the-Standard-Model (BSM) physics, such as the Nab, pNAB, and Katrin experiments, rely heavily on silicon-based detector technologies. These detectors require high energy resolution, fast timing, and position sensitivity. However, characterizing these detectors—specifically the 117 mm diameter, 127-pixel silicon detectors used in the Nab experiment—presents a unique challenge. The testing infrastructure must be capable of delivering a mono-energetic proton beam that can traverse the entire active area of the detector while maintaining a "spot size" small enough (less than 6.5 mm²) to remain within the envelope of a single hexagonal pixel. This capability is essential for studying individual pixels without significant charge sharing between adjacent segments. Prior to this work, a dedicated, versatile, and steerable low-energy proton source adapted for such detector-under-test (DUT) requirements was not available.

Methodology
The authors repurposed the University of Manitoba's "Manitoba II" mass spectrometer, a double-focusing instrument originally built in 1967 for atomic physics, into a steerable low-energy proton beam facility. The modification process involved several key components:

1. Ion Source Replacement: The original tantalum oven source was replaced with a Penning Ion Generator (PIG). A hydrogen-argon gas mixture is introduced into a high-vacuum discharge region. Argon acts as a buffer gas to stabilize the discharge current, while a high-temperature permanent magnet (42% Neodymium) confines charged particles to helical trajectories, increasing ionization efficiency. Protons are extracted and accelerated through a 30 kV potential.
2. Energy and Momentum Selection: The beam passes through an unmodified Electro-static Analyzer (ESA), which selects ions based on kinetic energy using a radial electric field between concentric arc plates. It then traverses a Magneto-static Analyzer (MSA), where a homogeneous magnetic field selects ions based on momentum. The combination of ESA and MSA ensures a mono-energetic beam of protons (specifically isolating H⁺ from H₂⁺ and H₃⁺).
3. Beam Steering: An electro-static steerer, consisting of four independent rectangular copper electrodes, was installed downstream of the MSA. By applying static potentials up to ±2 kV, the Gaussian-profile proton beam can be deflected radially to cover a 117 mm diameter area-of-interest.
4. Characterization Studies: The beam's performance was validated through three primary studies:
   • Energy Resolution: Measured using a Micro-channel Plate (MCP) detector to analyze the separation of hydrogen isotopes in the magnetic field.
   • Phosphor Screen Imaging: A phosphor screen (P43 coating) was used to photograph beam spots at various deflection voltages to visualize the beam profile and spot size across the detector plane.
   • Silicon Detector Scanning: The beam was scanned across the boundary of two segments of a Nab silicon diode detector. Count rates were recorded as a function of beam position to determine the physical beam diameter via error function fitting.

Key Contributions

• Infrastructure Adaptation: Successfully converted a legacy high-precision mass spectrometer into a versatile, steerable low-energy proton source (25–35 keV) specifically tailored for detector characterization.
• Beam Control: Developed an electro-static steering system capable of directing a pencil beam over a large area (117 mm diameter) while maintaining a small spot size.
• Validation Protocol: Established a rigorous testing methodology combining optical (phosphor screen) and electronic (silicon detector) measurements to confirm beam parameters against the strict requirements of the Nab experiment.

Results

• Beam Current and Energy: The facility produces a continuous stream of protons with a current of approximately $1 \times 10^{-18}$ A. The energy resolution of the beam was determined to be $\sim$300 eV Full-Width at Half-Maximum (FWHM), which is significantly lower than the energy resolution of the Nab silicon detectors, ensuring precise energy selection.
• Spot Size:
  • Phosphor screen measurements indicated beam spot areas ranging from 0.9 mm² to 15.36 mm² depending on deflection position, with the majority of spots in the area of interest being approximately 1 mm².
  • Direct scanning of the Nab silicon detector yielded an average beam spot diameter corresponding to an area of 3.1(2) mm².
• Pixel Confinement: The measured spot sizes are well within the footprint of a single Nab detector pixel (70 mm²), confirming the facility's ability to perform single-pixel studies without significant inter-pixel charge sharing.

Significance
This work demonstrates the successful development and commissioning of a novel proton source facility that meets the specific, demanding requirements for characterizing large-area, segmented silicon detectors used in BSM physics searches. The facility proved effective in the characterization of the Nab silicon detectors, enabling segment-by-segment calibration and study. The paper emphasizes that this infrastructure is adaptable to any detector-under-test, providing a unique test station that fills a critical gap in the testing requirements for precision neutron and neutrino experiments. The modifications to the Manitoba II spectrometer extend its utility beyond atomic physics into the realm of detector development and validation.