Optical imaging of muons

The study investigates whether optical imaging can be applied to muon beams for estimating beam range, width, and other properties relevant to beam quality assurance and potential radiotherapy applications. Prior optical imaging work has focused on Cherenkov and luminescence signals from X-rays, electrons, and ion beams in water, but had not been tested for muons. Muons differ from familiar radiations due to their mass (207 times electron), decay characteristics (positive muons decay to a positron and two neutrinos), and mean lifetime (2.2 μs). With the advent of high-intensity muon beams at J-PARC, there is a need for efficient QA methods; existing approaches have only trialed lateral profile measurements with scintillators. The authors previously simulated dose and light distributions of positive muons in water, predicting that dose arises mainly from muons while light in water is dominated by Cherenkov emission from decay positrons. This work experimentally tests optical imaging of positive muons in water and plastic scintillator, aiming to assess feasibility and potential applications such as momentum determination, momentum spread estimation, and positron emission asymmetry measurement.

• Optical imaging has been used for QA of high-energy LINAC X-rays, largely via Cherenkov-light from electrons/secondary electrons in water, and via sub-Cherenkov luminescence for particle ions at lower energies, enabling dose and range estimation (multiple studies cited). Luminescence in water has also been observed for X-rays, alpha, and beta particles, and applied for dose/range estimation.
• For muons, applications include cosmic-muon radiography of large structures (volcanoes, pyramids, reactors), but QA imaging methods for accelerator-produced muon beams are lacking, with only a prior attempt at lateral beam profiling using a scintillator.
• The authors’ prior Monte Carlo work suggested that in water the measured light for positive muons would mainly arise from Cherenkov emission of decay positrons rather than the muons themselves, motivating experimental validation.
• Plastic scintillators are known to exhibit quenching effects that can depress Bragg peak heights; this may impact interpreting scintillation-based images for dose estimation.

Beam and facility: Positive muon beams at J-PARC (D1 area) with momenta 73.9, 84.5, and 95.1 MeV/c (momentum deviation ~4%) and intensity ~5×10^6 muons/s were used. Positive muons were selected due to higher intensity and negligible radionuclide production.

Phantoms and imaging setup: Two 10×10×10 cm^3 targets were used: (1) a water phantom made of 0.5-cm-thick black acrylic with a 5-mm-thick transparent acrylic face (Kuraray PARAGLAS UV00) toward the camera; (2) a plastic scintillator block (EJ-200). The phantom and a cooled CCD camera (BITRAN BU-56DUV) with C-mount F1.4 lens (Computar) were housed in a black box with thin black paper sides to minimize muon energy loss. The camera viewed at 90 degrees to the beam. Distance from phantom surface to lens: 30 cm. Beamline collimator diameter at exit: 40 mm.

Acquisition parameters: Water imaging during irradiation for 300 s (calculated dose ~0.3 Gy to irradiated area). Plastic scintillator imaging for 10 s (dose ~0.01 Gy). CCD image size 680×512 pixels; pixel size 0.34×0.34 mm.

Image processing: Images processed in ImageJ. Noise spots from direct hits (scattered muons, escaped positrons, annihilation photons) were removed using Remove Outlier based on high-intensity/small-pixel features. Background images were subtracted to correct offset and non-uniformity. Depth and lateral profiles were extracted; spatial scaling derived from known phantom dimensions.

Monte Carlo simulation: Geant4 v10.4.2 with G4EmStandardPhysics_option4 for electromagnetic processes (ionization, bremsstrahlung, pair production, multiple scattering, Coulomb scattering). Optical photon processes (G4OpticalPhysics) included Cherenkov (G4Cerenkov) and scintillation (G4Scintillation). Luminescence of water below Cherenkov threshold was modeled via scintillation, following prior methodology.

Application studies (proof-of-concept):

• Momentum determination: Constructed a LUT of muon momentum versus peak depth from simulated water images; validated with measured water images at the three momenta.
• Momentum deviation estimation: For 84.5 MeV/c muons, simulated profiles at different momentum spreads to form a LUT of width versus momentum deviation; compared with measured width.
• Positron emission asymmetry: Acquired five 10-minute images at 84.5 MeV/c (total 50 min) to reduce statistical noise. Measured distances at half maximum from the peak in forward and backward directions and computed the forward/backward ratio to quantify asymmetry due to muon spin polarization.

Water phantom (Cherenkov-light from decay positrons):

• Images showed elliptical light distributions whose peak depth increased with muon momentum, consistent with simulation.
• Measured peak positions from water surface (Table 1): 32 mm (73.9 MeV/c), 48 mm (84.5 MeV/c), 67 mm (95.1 MeV/c). Simulated: 28, 43, 63 mm. Measured peaks were deeper by ~4–5 mm.
• Lateral widths at maximum intensity (FWHM, Table 2): measured 41, 45, 48 mm for 73.9, 84.5, 95.1 MeV/c, respectively; simulated 37, 38, 40 mm. Measured widths were wider by ~4–8 mm and increased slightly with momentum.

Plastic scintillator block (dose-like scintillation with Bragg peaks):

• Images exhibited Bragg-peak-like light distributions with peak depth increasing with momentum; simulations matched qualitatively.
• Measured peak positions from block surface (Table 3): 30 mm (73.9 MeV/c), 45 mm (84.5 MeV/c), 62 mm (95.1 MeV/c). Simulated: 25, 39, 54 mm. Measured peaks were deeper by ~5–8 mm.
• Lateral widths (FWHM, Table 4): At 10 mm from entrance, measured 37, 37, 39 mm vs simulated 35, 34, 34 mm for 73.9, 84.5, 95.1 MeV/c. At Bragg peak, measured 37, 38, 41 mm vs simulated 33, 33, 32 mm. Widths increased slightly with momentum and depth; measured wider than simulated.

Application demonstrations:

• Momentum determination LUT: Measured peak depths for three momenta lay on the simulated calibration curve, supporting use of peak depth in water images to infer beam momentum.
• Momentum deviation estimation: Simulations showed a linear relation between image width and momentum deviation for 84.5 MeV/c (example linear fit y = 0.2857x − 2.8571); measured width aligned with the curve, supporting deviation estimation from water images.
• Positron emission asymmetry: From the 50-min water image at 84.5 MeV/c, the forward-to-backward half-maximum distance ratio B/A = 1.19, indicating the forward extent was 19% longer than backward, consistent with expected asymmetry due to muon spin polarization.

This work provides the first optical images of accelerator muon beams, demonstrating that optical methods can capture beam range and lateral characteristics in two media. In water, light arises predominantly from Cherenkov emission by decay positrons, producing wider lateral distributions than the underlying muon beam due to high positron energies and long ranges; this matched prior simulations. In plastic scintillator, images resemble dose with Bragg peaks, enabling direct estimation of beam range and width, although scintillator quenching likely reduces observed peak heights versus simulations. Beam broadening with depth in the plastic scintillator increased with momentum, attributable to multiple scattering, and could be advantageous for mini-beam therapy concepts requiring broader distal distributions. Differences between measurements and simulations (peak depths ~4–8 mm deeper, widths larger) may stem from limitations in the optical and scintillation models, or uncorrected optical imaging errors (distortion, parallax). Despite these discrepancies, proof-of-concept applications showed that image-derived peak depth and width can be used to estimate beam momentum and momentum spread, and long acquisitions can quantify positron emission asymmetry, offering a compact, low-cost QA and research tool for muon facilities and prospective muon radiotherapy.

Optical imaging of positive muons is feasible. In water, Cherenkov-light from decay positrons forms elliptical distributions near the range end; peak depths and widths correlate with beam momentum and can be used to determine momentum and its deviation, and to measure positron emission asymmetry. In a plastic scintillator block, images show Bragg-peak-like profiles similar to dose, enabling range and width estimation. Measured ranges in water were 4–5 mm larger than calculated. Overall, optical imaging is a promising approach for QA and research on muon beams and for future muon radiotherapy.

• Systematic differences from simulations: measured peak depths and widths were larger than simulated in both media (water: peaks +4–5 mm, widths +4–8 mm; plastic scintillator: peaks +5–8 mm, widths larger), suggesting model or calibration uncertainties.
• No optical corrections (distortion, parallax) were applied; such corrections may reduce measurement–simulation discrepancies.
• Scintillator quenching likely affected Bragg peak heights, complicating precise dose inference from scintillator images and requiring corrections.
• Only positive muons were studied; results may not directly generalize to negative muons.
• Proof-of-concept LUTs were validated on limited datasets (three momenta); broader validation is needed for robust QA deployment.