Proton Tracks Research Articles

Distortion of Thomson Parabolic-Like Proton Patterns Due to Electromagnetic Interference

Intense electromagnetic pulses (EMPs) accompany the production of plasma when a high-intensity laser irradiates a solid target. The EMP occurs both during and long after the end of the laser pulse (up to hundreds of nanoseconds) within and outside the interaction chamber, and interferes with nearby electronics, which may lead to the disruption or malfunction of plasma diagnostic devices. This contribution reports a correlation between the frequency spectrum of the EMP and the distortion of Thomson parabola tracks of protons observed at the kJ-class PALS laser facility in Prague. EMP emission was recorded using a simple flat antenna. Ions accelerated from the front side of the target were simultaneously detected by a Thomson parabola ion spectrometer. The comparison of the two signals suggests that the EMP may be considered to be the source of parabolic track distortion.

Machine learning for proton path tracking in proton computed tomography

A Machine Learning approach to the problem of calculating the proton paths inside a scanned object in proton Computed Tomography is presented. The method is developed in order to mitigate the loss in both spatial resolution and quantitative integrity of the reconstructed images caused by multiple Coulomb scattering of protons traversing the matter. Two Machine Learning models were used: a forward neural network (NN) and the XGBoost method. A heuristic approach, based on track averaging was also implemented in order to evaluate the accuracy limits on track calculation, imposed by the statistical nature of the scattering. Synthetic data from anthropomorphic voxelized phantoms, generated by the Monte Carlo (MC) Geant4 code, were utilized to train the models and evaluate their accuracy, in comparison to a widely used analytical method that is based on likelihood maximization and Fermi−Eyges scattering model. Both NN and XGBoost model were found to perform very close or at the accuracy limit, further improving the accuracy of the analytical method (by 12% in the typical case of 200 MeV protons on 20 cm of water object), especially for protons scattered at large angles. Inclusion of the material information along the path in terms of radiation length did not show improvement in accuracy for the phantoms simulated in the study. A NN was also constructed to predict the error in path calculation, thus enabling a criterion to filter out proton events that may have a negative effect on the quality of the reconstructed image. By parametrizing a large set of synthetic data, the Machine Learning models were proved capable to bring—in an indirect and time efficient way—the accuracy of the MC method into the problem of proton tracking.

Development of a new tracking detector with fine-grained nuclear emulsion for sub-MeV neutron measurement

Abstract We have developed a new sub-MeV neutron detector with high position resolution, energy resolution, and directional sensitivity, and low background. The detector is based on a super-fine-grained nuclear emulsion, called a nano-imaging tracker (NIT), and it is capable of detecting neutron-induced proton recoils as tracks through topological analysis with sub-micrometric accuracy. We used a type of NIT with AgBr:I crystals of ($98 \pm 10$) nm size dispersed in gelatin. First, we calibrated the performance of the NIT device for detecting monochromatic neutrons with sub-MeV energy generated by nuclear fusion reactions, and the detection efficiency for recoil proton tracks of more than 2 $\mu$m range was consistently 100% (the 1 $\sigma$ lower limit was 83%) in accordance with expectations from manual analysis. In addition, the recoil energy and angle distribution obtained good agreement with kinematical expectations. The primary neutron energy was reconstructed using these, and it was evaluated as 42% full width at half maximum at 540 keV. Furthermore, we demonstrated a newly developed automatic track recognition system dedicated to track ranges of more than a few micrometers. It achieved a recognition efficiency of ($74 \pm 4$)%, and the recoil energy and angle distribution obtained good agreement with manual analysis. Finally, it indicated a very high rejection power for $\gamma$-rays.

A static beam delivery device for fast scanning proton arc-therapy

Arc-therapy is a dose delivery technique regularly applied in photon radiation therapy, and is currently subject of great interest for proton therapy as well. In this technique, proton beams are aimed at a tumor from different continuous ranges of incident directions (so called ‘arcs’). This technique can potentially yield a better dose conformity around the tumor and a very low dose in the surrounding healthy tissue. Currently, proton-arc therapy is performed by rotating a proton gantry around the patient, adapting the normally used dose-delivery method to the arc-specific motion of the gantry. Here we present first results from a feasibility study of the conceptual design of a new static fast beam delivery device/system for proton-arc therapy, which could be used instead of a gantry. In this novel concept, the incident angle of proton beams can be set rapidly by only changing field strengths of small magnets. This device eliminates the motion of the heavy gantry and related hardware. Therefore, a reduction of the total treatment time is expected. In the feasibility study presented here, we concentrate on the concept of the beam transport. Based on several simple, but realistic assumptions and approximations, proton tracking calculations were performed in a 3D magnetic field map, to calculate the beam transport in this device and to investigate and address several beam-optics challenges. We propose and simulate corresponding solutions and discuss their outcomes. To enable the implementation of some usually applied techniques in proton therapy, such as pencil beam scanning, energy modulation and beam shaping, we present and discuss our proposals. Here we present the concept of a new idea to perform fast proton arc-scanning and we report on first results of a feasibility study. Based on these results, we propose several options and next steps in the design.

A novel proton counting detector and method for the validation of tissue and implant material maps for Monte Carlo dose calculation

The presence of artificial implants complicates the delivery of proton therapy due to inaccurate characterization of both the implant and the surrounding tissues. In this work, we describe a method to characterize implant and human tissue mimicking materials in terms of relative stopping power (RSP) using a novel proton counting detector. Each proton is tracked by directly measuring the deposited energy along the proton track using a fast, pixelated spectral detector AdvaPIX-TPX3 (TPX3). We considered three scenarios to characterize the RSPs. First, in-air measurements were made in the presence of metal rods (Al, Ti and CoCr) and bone. Then, measurements of energy perturbations in the presence of metal implants and bone in an anthropomorphic phantom were performed. Finally, sampling of cumulative stopping power (CSP) of the phantom were made at different locations of the anthropomorphic phantom. CSP and RSP information were extracted from energy spectra at each beam path. To quantify the RSP of metal rods we used the shift in the most probable energy (MPE) of CSP from the reference CSP without a rod. Overall, the RSPs were determined as 1.48, 2.06, 3.08, and 5.53 from in-air measurements; 1.44, 1.97, 2.98, and 5.44 from in-phantom measurements, for bone, Al, Ti and CoCr, respectively. Additionally, we sampled CSP for multiple paths of the anthropomorphic phantom ranging from 18.63 to 25.23 cm deriving RSP of soft tissues and bones in agreement within 1.6% of TOPAS simulations. Using minimum error of these multiple CSP, optimal mass densities were derived for soft tissue and bone and they are within 1% of vendor-provided nominal densities. The preliminary data obtained indicates the proposed novel method can be used for the validation of material and density maps, required by proton Monte Carlo Dose calculation, provided by competing multi-energy computed tomography and metal artifact reduction techniques.

Response of CR-39 nuclear track detectors to protons with non-normal incidence

This paper presents data from experiments with protons at non-normal incidence to CR-39 nuclear track detectors, analyzing the properties of detection efficiency, proton track diameter, track contrast, and track eccentricity. Understanding the CR-39 response to protons incident at an angle is important for designing charged particle detectors for inertial confinement fusion (ICF) applications. This study considers protons with incident energies less than 3 MeV. In this regime, an incident angle of 10° has no effect on CR-39 detection efficiency, and >85% detection efficiency is preserved up through 25° in the range of 1.0 MeV-2.1 MeV. For ICF applications, incident angles above 30° are deemed impractical for detector design due to significant drops in proton detection at all energies. We observe significant reductions in detection efficiency compared to theoretical predictions, particularly at low energies where proton tracks are etched away. The proton track diameter measured by the scan system is observed to decrease with higher incident angles. The track diameters are analyzed with two fitting models, and it is shown that the diameter-energy relation can be fit with the existing models at angles up to 30°. The optical contrast of the tracks tends to increase with the angle, meaning that the tracks are fainter, and a larger increase is observed for higher energies. Eccentricity, a measure of how elongated proton tracks are, increases with the incident angle and drops after the critical angle. The lowest energy tracks remain nearly circular even at higher angles.

A Computer Modeling Study of Water Radiolysis at High Dose Rates. Relevance to FLASH Radiotherapy.

"FLASH radiotherapy" is a new method of radiation treatment by which large doses of radiation are delivered at high dose rates to tumors almost instantaneously (a few milliseconds), paradoxically sparing healthy tissue while preserving anti-tumor activity. To date, no definitive mechanism has been proposed to explain the different responses of the tumor and normal tissue to radiation. As a first step, and given that living cells and tissues consist mainly of water, we studied the effects of high dose rates on the transient yields (G values) of the radical and molecular species formed in the radiolysis of deaerated/aerated water by irradiating protons, using Monte Carlo simulations. Our simulation model consisted of two steps: 1. The random irradiation of a right circular cylindrical volume of water, embedded in nonirradiated bulk water, with single and instantaneous pulses of N 300-MeV incident protons ("linear energy transfer" or LET ∼ 0.3 keV/µm) traveling along the axis of the cylinder; and 2. The development of these N proton tracks, which were initially contained in the irradiated cylinder, throughout the solution over time. The effect of dose rate was studied by varying N, which was calibrated in terms of dose rate. For this, experimental data on the yield G(Fe3+) of the super-Fricke dosimeter as a function of dose rate up to ∼1010 Gy/s were used. Confirming previous experimental and theoretical studies, significant changes in product yields were found to occur with increasing dose rate, with lower radical and higher molecular yields, which result from an increase in the radical density in the bulk of the solution. Using the kinetics of the decay of hydrated electrons, a critical time (τc), which corresponds to the "onset" of dose-rate effects, was determined for each value of N. For the cylindrical irradiation model, τc was inversely proportional to the dose rate. Moreover, the comparison with experiments with pulsed electrons underlined the importance of the geometry of the irradiation volume for the estimation of τc. Finally, in the case of aerated water radiolysis, we calculated the yield of oxygen consumption and estimated the corresponding concentration of consumed (depleted) oxygen as a function of time and dose rate. It was shown that this concentration increases substantially with increasing dose rate in the time window ∼1 ns-10 µs, with a very pronounced maximum around 0.2 µs. For high-dose-rate irradiations (>109 Gy/s), a large part of the available oxygen (∼0.25 mM for an air-saturated solution) was found to be consumed. This result, which was obtained on a purely water radiation chemistry basis, strongly supports the hypothesis that the normal tissue-sparing effect of FLASH stems from temporary hypoxia due to oxygen depletion induced by high-dose-rate irradiation.

First Results From All-Digital PET Dual Heads for In-Beam Beam-On Proton Therapy Monitoring

Novel technological solutions for the design of positron emission tomography (PET) systems to be used in the in-beam beam-on monitoring of proton tracks within biological tissues represent the frontier of the instrumentation in proton therapy. We report a novel all-digital PET dual heads prototype based on the multivoltage threshold (MVT) technology, able to operate in beam-on modality. We demonstrate in phantom and animal experiments that, by using an event selection based on the energy of singles and on the delay with respect to the radio-frequency period of the proton beam bunches, the monitoring PET system is able to image the induced activity by both pristine and pencil proton beam with energy ranging between 70 and 150 MeV during irradiation.

Can We Assess Early DNA Damage at the Molecular Scale by Radiation Track Structure Simulations? A Tetranucleosome Scenario in Geant4-DNA

Advanced track structure codes excel as state-of-the-art tools to low scale dosimetric models: the rational evolution for a cell-like scenario is detailed within a microsecond of an ion collision, that is the standard timescale for critical DNA modifications. The in vitro DNA double strand breaks (DSB) yield is matched indirectly by nanodosimetric track structure assessments. However, the score to specific DSB motifs (i.e. the yield associated to each DSB distance between DNA cuts) is mostly overlooked. In this work, we extend the PDB4DNA example of the Geant4-DNA toolkit, to briefly assess the hit and double strand break scores over a nucleosome tetramer framework (Protein Data Bank entry: 1zbb). We describe a critical scenario that biases the statistical significance for an event-by-event track structure assessment at the nanometric scale, based on a Shannon’s entropy estimate of the volumetric hit score; finally, we draw a tentative correlation between the mean DSB quality and a proton track features, and conclude that short-distanced DSBs by direct effect are favored within a Bragg peak-relevant energy range.

Evaluation of GATE-RTion (GATE/Geant4) Monte Carlo simulation settings for proton pencil beam scanning quality assurance.

Geant4 is a multi-purpose Monte Carlo simulation tool for modeling particle transport in matter. It provides a wide range of settings, which the user may optimize for their specific application. This study investigates GATE/Geant4 parameter settings for proton pencil beam scanning therapy. GATE8.1/Geant4.10.3.p03 (matching the versions used in GATE-RTion1.0) simulations were performed with a set of prebuilt Geant4 physics lists (QGSP_BIC, QGSP_BIC_EMY, QGSP_BIC_EMZ, QGSP_BIC_HP_EMZ), using 0.1mm-10mm as production cuts on secondary particles (electrons, photons, positrons) and varying the maximum step size of protons (0.1mm, 1mm, none). The results of the simulations were compared to measurement data taken during clinical patient specific quality assurance at The Christie NHS Foundation Trust pencil beam scanning proton therapy facility. Additionally, the influence of simulation settings was quantified in a realistic patient anatomy based on computer tomography (CT) scans. When comparing the different physics lists, only the results (ranges in water) obtained with QGSP_BIC (G4EMStandardPhysics_Option0) depend on the maximum step size. There is clinically negligible difference in the target region when using High Precision neutron models (HP) for dose calculations. The EMZ electromagnetic constructor provides a closer agreement (within 0.35mm) to measured beam sizes in air, but yields up to 20% longer execution times compared to the EMY electromagnetic constructor (maximum beam size difference 0.79mm). The impact of this on patient-specific quality assurance simulations is clinically negligible, with a 97% average 2%/2mm gamma pass rate for both physics lists. However, when considering the CT-based patient model, dose deviations up to 2.4% are observed. Production cuts do not substantially influence dosimetric results in solid water, but lead to dose differences of up to 4.1% in the patient CT. Small (compared to voxel size) production cuts increase execution times by factors of 5 (solid water) and 2 (patient CT). Taking both efficiency and dose accuracy into account and considering voxel sizes with 2mm linear size, the authors recommend the following Geant4 settings to simulate patient specific quality assurance measurements: No step limiter on proton tracks; production cuts of 1mm for electrons, photons and positrons (in the phantom and range-shifter) and 10mm (world); best agreement to measurement data was found for QGSP_BIC_EMZ reference physics list at the cost of 20% increased execution times compared to QGSP_BIC_EMY. For simulations considering the patient CT model, the following settings are recommended: No step limiter on proton tracks; production cuts of 1mm for electrons, photons and positrons (phantom/range-shifter) and 10mm (world) if the goal is to achieve sufficient dosimetric accuracy to ensure that a plan is clinically safe; or 0.1mm (phantom/range-shifter) and 1mm (world) if higher dosimetric accuracy is needed (increasing execution times by a factor of 2); most accurate results expected for QGSP_BIC_EMZ reference physics list, at the cost of 10-20% increased execution times compared to QGSP_BIC_EMY.

Neutrino-hydrogen interactions with a high-pressure time projection chamber

We investigate the idea of detecting pure neutrino-hydrogen interactions in a multinuclear target using the transverse kinematic imbalance technique [Lu et al., Phys. Rev. D 92, 051302 (2015)] in a high-pressure time projection chamber (HPTPC). With full solid-angle acceptance, MeV-level proton tracking threshold, state-of-the-art tracking resolution, and an O(100 m^3) gas volume at 10 bar, an HPTPC could provide an opportunity to realize this technique. We propose the use of hydrogen-rich gases in the TPC to achieve high detection purity with large hydrogen mass. With the projected neutrino beam exposure at the DUNE experiment, neutrino-hydrogen events of the order of 10^4 per year with purity above 90% could be achieved with such an HPTPC using methane gas. In this paper, we present a systematic study of the event rate and purity for a variety of argon-alkane mixtures, and examine these gas candidates for the TPC tracking-related properties.

Molière maximum likelihood proton path estimation approximated by cubic Bézier curve for scatter corrected proton CT reconstruction

A maximum likelihood approach to the problem of calculating the proton paths inside the scanned object in proton computed tomography is presented. Molière theory is used for the first time to derive a physical model that describes proton multiple Coulomb scattering, avoiding the need for the Gaussian approximation currently used. To enable this, the proposed method approximates proton paths with cubic Bézier curves and subsequently maximizes the path likelihood through parametric optimization, based on the Molière model. Results from the Highland formula-based Gaussian approximation are also presented for comparison. The simplex method is utilized for optimisation. The scattering properties of the material(s) of the scanned object are taken into account by appropriately calculating the scattering parameters from the stopping power map that is calculated/updated at every iteration of the algebraic reconstruction process. Proton track length constraint imposed by the proton energy loss is accounted for. The method is also applied in the case that no exit angle data are measured. Geant4 Monte Carlo simulations were performed for model validation. Our results show that use of Molière probability density function for modelling the multiple Coulomb scattering presents a modest 2% accuracy improvement over the Gaussian approximation and most-likely-path method. Simulations of voxelized phantom showed no essential benefit from the inclusion of the material information into the optimization, while path optimization with energy constraint slightly increased path resolution in a bone/water interface phantom. Method error was found to depend on energy, proton track-length within the medium, and proportion of data filtering.

Linear energy transfer dependence of transient yields in water irradiated by 150 keV – 500 MeV protons in the limit of low dose rates

FLASH radiotherapy is a new irradiation method in which large doses of ionizing radiation are delivered to tumors almost instantly (a few milliseconds), paradoxically sparing healthy tissue while preserving anti-tumor activity. Although this technique is primarily studied in the context of electron and photon therapies, proton delivery at high dose rates can also reduce the adverse side effects on normal cells. So far, no definitive mechanism has been proposed to explain the differences in the responses to radiation between tumor and normal tissues. Given that living cells and tissues consist mainly of water, we set out to study the effects of high dose rates on the radiolysis of water by protons in the energy range of 150 keV – 500 MeV (i.e., for linear energy transfer (LET) values between ∼72.2 and 0.23 keV/μm, respectively) using Monte Carlo simulations. To validate our methodology, however, we, first, report here the results of our calculations of the yields (G values) of the radiolytically produced species, namely the hydrated electron ([Formula: see text]), •OH, H•, H2, and H2O2, for low dose rates. Overall, our simulations agree very well with the experiment. In the presence of oxygen, [Formula: see text] and H• atoms are rapidly converted into superoxide anion or hydroperoxyl radicals, with a well-defined maximum of [Formula: see text] at ∼1 μs. This maximum decreases substantially when going from low-LET 500 MeV to high-LET 150 keV irradiating protons. Differences in the geometry of the proton track structure with increasing LET readily explain this diminution in [Formula: see text] radicals.

Geometry optimisation of graphite energy degrader for proton therapy

Introduction: Cyclotron-based proton therapy facilities use an energy degrader of variable thickness to deliver beams of the different energies required by a patient treatment plan; scattering and straggling in the degrader give rise to an inherent emittance increase and subsequent particle loss in the downstream energy-selection system (ESS). Here we study alternative graphite degrader geometries and examine with Monte-Carlo simulations the induced emittance growth and consequent particle transmission.Methods: We examined the conventional multiple-wedge degrader used in the Paul Scherrer Institute PROSCAN proton therapy system, the equivalent parallel-sided degrader, and a single block degrader of equivalent thickness. G4Beamline Monte-Carlo tracking of protons was benchmarked against measurements of the existing degrader for proton energies from 75 to 230 MeV, and used to validate simulations of the alternative geometries.Results: Using a careful calculation of the beam emittance growth, we determined that a single-block degrader placed close to the collimators of the ESS is expected to deliver significantly larger transmission, up to 17% larger at 150 MeV. At the lowest deliverable of 75 MeV there is still a clear improvement in beam transmission.Conclusions: Whilst dose rates are not presently limited on the PROSCAN system at higher energies, a single-block degrader offers the ability to access either lower energies for treatment or a larger dose rate at 75 MeV in case transmission optimisation is desired. Single-block degraders should be considered for the delivery of low-energy protons from a cyclotron-based particle therapy system.

Three-dimensional nanodosimetric characterisation of proton track structure

This work presents a detailed investigation into the nanodosimetric properties of the track structure of protons in water at energies relevant for proton radiotherapy. The ionization component of the tracks had been simulated in previous work using Geant4-DNA for proton start energies between 1 MeV and 100 MeV. From the simulation results, the frequency distribution of ionization clusters formed in nanometric target volumes was obtained in dependence of the impact parameter of the proton trajectory with respect to the target center. In the track core, targets of cylindrical shape and a size comparable to a short segment of DNA were used for scoring ionization cluster size distributions. For the penumbra region, three different options for defining the cylinder shell sectors were investigated, with each cylinder shell sector volume the same as the cylinder target volume. The radial distributions were numerically integrated to obtain the effective track cross sections with respect to different nanodosimetric parameters. Graphically displaying the radial dependence in a similar way as microdosimetric distributions allowed elucidating the contribution of different radial distances to the overall radial integral of the quantities under consideration. Furthermore, it was tested how well the radial dependence of the nanodosimetric parameters could be fitted with a model function derived in literature for the radial energy deposition in proton tracks. It was found that this model function allows describing only the radial dependence of the total frequency of nanometric targets where ionizations occur. The deviation from a 1/r2-dependence of the radial dependence of the frequency of targets receiving more than a minimum number of ionizations (exceeding one) includes a second peak centred around 10 nm radial distance from the proton trajectory.

A simulation study of a high-resolution fast neutron imaging detector based on liquid scintillator loaded capillaries

At present, the highest spatial resolution of a fast neutron imaging detector, mainly determined by the range of secondary particles generated by fast neutrons, is about hundreds of microns. In view of the above inherent spatial resolution limitation, a capillary-based scintillation detector that can improve the spatial resolution of fast neutron imaging by recording and reconstructing the recoil proton track was developed. The purpose of this paper is to develop a detector for recognizing recoil proton events, reconstructing particle track and improving the position resolution with track reconstruction method to reconstruct the position of interaction. The proposed detector consists of a 1000 × 1000 array of glass capillaries loaded with a high refractive index liquid scintillator. Each glass capillary was 10 μm in diameter and 5 cm in length. The recoil protons generated by the incident neutrons move within the detector and produce scintillation light within each capillary that they traverse. The light emitted from the capillary array can be recorded by employing an intensified CCD camera. We used Geant4 to simulate the detector performance and CERN ROOT analysis framework to record physical information of recoil proton, including position, energy deposition in each capillary and track length. Based on Hough transform, a rapid, computerized and efficient proton track reconstruction procedure was developed. The recoil proton events display a continuous extended structure. The track reconstruction algorithms can reconstruct individual track precisely, and when the counting rate was relatively low, the track reconstruction results were in good agreement with simulation data. Moreover, for intensive overlap conditions, this algorithm also reconstructs periphery tracks with high rate of accuracy.

Detection of alpha and 7Li particles from 10B(n, α)7Li reactions using a combination of CR-39 nuclear track detector and potassium hydroxide-ethanol-water solution in accelerator-based neutron fields

We investigated the optimum etching condition for potassium hydroxide-ethanol–water solution (PEW) to detect only alpha and 7Li particles from 10B(n, α)7Li reactions registered in a CR-39 plastic nuclear track detector. The background recoil protons, which are induced by fast neutrons with energies of <2 MeV produced by 9Be(p, n)9B reactions in accelerator-based neutron sources, were discriminated by the desensitization effect with PEW. We found that an ethanol concentration of ≥15 wt% in a PEW solution discriminated recoil proton tracks with energies of <2 MeV. The optimized conditions will be applicable for evaluating dose distribution in tissue in accelerator-based boron neutron capture therapy.

Study of Charged-Current neutrino interactions on water with nuclear emulsion in the NINJA experiment

Neutrino-nucleus interaction is one of the major sources of the uncertainty for neutrino oscillation experiments. The NINJA experiment aims to measure neutrino-water interactions containing low-momentum secondary particles using a nuclear emulsion detector. Since the nuclear emulsion has sub-micron position resolution, it allows us to observe the interaction vertices clearly. Short proton tracks down to 200 MeV/c momentum are expected to be observed, which are hardly reconstructed in plastic scintillator based detectors. A series of test experiments has been carried out with prototype detectors at J-PARC. In a test run in 2017-2018, a 3 kg water target detector was exposed to anti-neutrino beam corresponding to 0.7 × 1021 POT (protons on target). We have successfully detected low momentum protons above 200 MeV/c threshold from neutrino-water interactions.

Study of neutrino charged current interactions on iron in the NINJA experiment

Emulsion Cloud Chamber with 65kg iron target was exposed to the neutrino beam at J-PARC neutrino beam line in 2016. It is capable of detecting short proton tracks down to 200 MeV/c. Data presented in this document corresponds to 0.40×1020 POT. The 194 neutrino-iron charged current interactions were successfully reconstructed in the target. Numbers of protons and charged pions in the final state of each event were measured and compared with those of a Monte Carlo simulation.

Proton deflectometry of a capacitor coil target along two axes

A developing application of laser-driven currents is the generation of magnetic fields of picosecond–nanosecond duration with magnitudes exceeding $B=10~\text{T}$ . Single-loop and helical coil targets can direct laser-driven discharge currents along wires to generate spatially uniform, quasi-static magnetic fields on the millimetre scale. Here, we present proton deflectometry across two axes of a single-loop coil ranging from 1 to 2 mm in diameter. Comparison with proton tracking simulations shows that measured magnetic fields are the result of kiloampere currents in the coil and electric charges distributed around the coil target. Using this dual-axis platform for proton deflectometry, robust measurements can be made of the evolution of magnetic fields in a capacitor coil target.