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

TL;DR

This work repurposes the Manitoba II mass spectrometer into a steerable, low-energy proton beam facility for detector characterization, targeting Nab silicon detectors and similar DUTs. A Penning Ion Generator, followed by an Electro-static Analyzer and a Magneto-static Analyzer, produces monoenergetic protons in the $25$–$35~\mathrm{keV}$ range and delivers them through a four-plate Proton Steerer to the detector with a Gaussian spot profile of a few millimeters. The system achieves an energy resolution around $3\times 10^{2}$ eV FWHM and verified beam-spot control down to $3.1\pm0.2~\mathrm{mm}$ diameter, demonstrated by phosphor-screen imaging and a segmented silicon-diode test, enabling per-pixel, single-pixel calibration across a $117~\mathrm{mm}$ Nab detector face. This setup provides a practical, low-rate proton source for precise detector calibration and characterization, with direct impact on BSM detector performance studies and Nab-like experiments.

Abstract

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.

Figures (8)

• Figure 1: Top-down schematic of the Manitoba II proton source. Starting counter-clockwise from top-left, ions are born in the the gas discharge ion source (or penning ion generator (PIG), see Section \ref{['sec:penning_ion_generator']}), are selected based on energy in the electro static analyzer (ESA, see Section \ref{['sec:ESA']}), selected based on momentum in the magneto-static analyzer (MSA see Section \ref{['sec:MSA']}), pass through the steerer (see Section. \ref{['sec:Steerer']}), before being deposited into the detector-under-test (DUT) (see Section \ref{['fig:detection_region']}).
• Figure 2: Cross-section of the penning ion generator used to create positive ions, including protons. A hydrogen-argon gas mixture is fed through the gas feed-through into the discharge region. A plasma is created between the extraction plate and the anode ring, "cracking" the hydrogen in to positive ions. The ions are then accelerated through a potential of 30kV and electro-static forming plates (not shown) focus the ions into a beam.
• Figure 3: Model of the proton steerer assembly, showing the four plate configuration. The proton beam trajectory can be seen in red. The copper electrodes are 47.63 mm wide x 193.68 mm long. These are encased in a PTFE housing for electrical isolation. Voltages of up to ±2kV may be individually applied to the plates to steer the beam in the desired direction.
• Figure 4: Top-down view of the detection region in the Manitoba II proton source. From left to right, one can see the position of the phosphor screen and supporting vacuum extension (cyan and yellow, respectively) - this configuration was only installed for a short time to study the beam movement (see Sec. \ref{['subsubsec:phosphor_screen']}). After this was removed, the detection chamber was installed. This included the silicon detector, and calibration sources. During this campaign, a pencil beam of protons (emerging from the collimation slit) could be used to probe individual segments of the silicon detector, via the electro-static steerer (Sec. \ref{['sec:Steerer']}).
• Figure 5: A Micro-channel plate (MCP) detector measures counts per minute with pencil beams of various hydrogen species passing through both analysis sectors while varying the magnetic field in the MSA. The solid vertical lines are calculated using a 27.245kV acceleration voltage and Equation \ref{['MSA_eq1']}. This acceleration voltage can be varied within a range of stability --- for this test 27.245kV was the highest stable voltage, however improvements were made to achieve up to 35kV. Also note that the count rate is highly dependent on the PIG configuration, and the noise in the detector used Harrison2013.