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

We report on the conversion of the Manitoba II mass spectrometer into a versatile low-energy proton beam facility. This infrastructure is adaptable to any detector-under-test (DUT), and has proven itself effective with the characterization of silicon detectors used in subatomic beyond-the-StandardModel (BSM) searches, namely the Nab experiment. A pencil beam of monoenergetic protons can be produced in a range from 25 keV to 35 keV, achieving a beamcurrent of ~1x10-18 A. Electrostatic steering plates were constructed to direct the Gaussian-profile proton beam over a 117mm diameter areaof-interest with full-width at half-maxima (FWHM) ranging from 0.6 mm to 1.26 mm. This work discusses the modifications and subsequent tests to confirm the beam specifications met the demands of the aforementioned detectors.

💡 Research Summary

The authors describe the conversion of the historic Manitoba II double‑focusing mass spectrometer into a low‑energy, steerable proton source suitable for detector characterization, with a particular focus on the silicon detectors used in the Nab experiment. A Penning ion generator (PIG) filled with a hydrogen‑argon mixture creates a stable plasma that, when accelerated through a 30 kV potential, produces singly‑charged protons (and minor H₂⁺, H₃⁺ components) at energies selectable between 25 keV and 35 keV. The ion beam first passes through an electrostatic analyzer (ESA) that uses a ±600 V potential across a 2 cm gap to select a narrow kinetic‑energy band, followed by a magnetostatic analyzer (MSA) that applies a uniform 3.989 × 10⁻² T magnetic field to select particles of the correct momentum (p = e B r). Both analyzers together ensure a mono‑energetic beam of ~30 keV protons with an energy spread of roughly 300 eV (FWHM), as demonstrated by scanning the MSA magnetic field and detecting the H⁺ peak with a micro‑channel plate (MCP). This resolution is well below the intrinsic energy resolution of the Nab silicon detectors, allowing the beam to serve as a precise calibration tool.

A custom electrostatic steerer, consisting of four independent copper plates housed in a PTFE‑lined 10‑inch flange, can apply up to ±2 kV to each plate. SIMION simulations guided the plate dimensions (≈48 mm × 194 mm) and voltage ranges, enabling beam deflection up to 75 mm at the detector plane, which is 0.48 m downstream of the steerer. This steering capability allows the narrow Gaussian beam (FWHM 0.6–1.26 mm) to be positioned anywhere within the 117 mm diameter active area of the Nab detector, which contains 127 hexagonal pixels each about 10 mm high.

Beam current is on the order of 1 × 10⁻¹⁸ A, corresponding to roughly 10 counts s⁻¹ when the source is tuned for low noise operation. The physical beam diameter is less than 5 mm, justifying a Gaussian approximation for profile analysis. Beam spot size was measured in two complementary ways. First, a phosphor screen (P43, Gd₂O₂S:Tb) coupled with a digital SLR camera captured spot images at various steering voltages; composite images showed spot areas ranging from 0.9 mm² to 15.4 mm², all comfortably smaller than a single Nab pixel (≈70 mm²). Second, direct measurements with the Nab silicon detector confirmed that individual pixel responses could be isolated without significant charge sharing, validating the spot size estimates derived from the phosphor screen.

The detector region is housed in a vacuum vessel maintained below 5 × 10⁻⁷ Torr using a turbomolecular pump backed by a diaphragm pump, minimizing background gas interactions and protecting sensitive detector electronics. The system also includes provisions for temperature sensors, residual gas analysis, and retractable radioactive sources for additional calibration.

Overall, the paper demonstrates that a repurposed high‑precision mass spectrometer can be transformed into a versatile, low‑energy proton source that delivers a well‑characterized, steerable beam with sub‑keV energy resolution and sub‑millimeter spatial precision. This capability enables detailed, pixel‑by‑pixel studies of silicon detectors and can be extended to other low‑energy detector technologies relevant to neutron, beta‑decay, and beyond‑Standard‑Model experiments.