Experimental verification of the PRIORA Monte Carlo code for proton radiography

Introduction to the show: ident: Physics. The day in general physics: instruments, methods and results across the field.

Kai: We're looking at the paper: "Experimental verification of the PRIORA Monte Carlo code for proton radiography".

Mira: Here's the gist of the paper. Experimental verification of the PRIORA Monte Carlo code for proton radiography is presented to validate its performance against experimental data obtained with GSI data obtained with the Ta bench target The single reconstructed instrument state must reproduce all these measurements simultaneously Model Architecture The PRIORA model is a “train” of a random beam generator, matter targets, ion-optical transfer maps of arbitrary order and an image saver For each material and reference energy, PRIORA uses functions of path length L and relative energy deviation k: the rms scattering angle σa(L, k), the mean kinetic-energy loss ∆K(L, k) and the energy-loss straggling σK(L, k) These functions are obtained from full LADA matter-transport calculations

5: Thick material slices are automatically divided into thinner sub-slices before transport Within each sub-slice of thickness l, a scattering kick θ at longitudinal coordinate z is accompanied by the correlated transverse displacement θ(z − l/2) relative to the sub-slice mid-plane The ion-optical system is represented by transfer maps of arbitrary order, generated by LADA from measured magnet-gradient profiles

6: or imported from COSY

4: Matter and maps alternate in the train, so energy loss inside an extended object changes the subsequent ion-optical transport automatically Experimental Observables and Analysis The Ta bench target provides six observables in one exposure Slit positions give scale and magnification Slit ESF widths in the PMMA bands give resolution versus controlled energy loss The pre/post asymmetry gives signed focus in the slit plane: upstream scattering acts with the lever θdz, downstream scattering with θ(dz+z), where z is the distance from the plate to the PMMA scatterer The sign of the blur asymmetry is therefore the sign of the longitudinal focus error These observables constrain magnification, focus in both transverse planes, chromatic blur and collimator acceptance Reconstruction Results Here the physics is fixed and the instrument state is fitted from one radiograph The design state gives ≈26 µm ESF widths; the data give 121–189 µm The PMMAband dependence of the ESF widths fixes the longitudinal focus sign The joint fit of the profile shapes and ESF widths gives dzx ≈ +15 mm and an effective Gaussian detectorresponse FWHM of 115 ± 6 µm in the target plane (0.40 ± 0.02 mm at the detector) The reconstructed state is astigmatic Cross-Check and Energy Dependence The independent 4 GeV experiment shows a central Ta/open transmission of 0.743 and a spatial resolution of 69 ± 8 µm The scattering angle decreases from θ0 ≈ 3.5 mrad at 2 GeV to θ0 ≈ 1.9 mrad at 4 GeV At 4 GeV, the effective 3.75 mrad acceptance is larger than θ0, giving weaker contrast (Ta/open ≈ 0.74) but a sharper measured edge The PRIORA forward model simulates proton transport through matter and a complete ion-optical system by combining Monte Carlo matter physics with arbitraryorder transfer maps; third-order energy-dependent maps are used in this study The agreement in absolute transmission, spatial resolution and chromatic-load dependence verifies the matter model and its coupling to the ion optics PRIORA recovers a non-nominal experimental state instead of assuming the design tune The comparison also resolves the expected energy trade-off for the same Ta plate: increasing the proton energy reduces scattering and material contrast while improving spatial resolution PRIORA/Ta bench target pair provides a baseline for later designs and simulations and permits rapid tuning during experiments Table I Summary The table shows the band-wise slit ESF widths (FWHM, µm) of the 2 GeV experiment and of the forward model in the reconstructed state The reconstruction is insensitive to the tested analysis and beam-distribution variations The remaining residuals are the outer post-side 65 mm band (+15%) and the absolute pre-side PMMA-step width in the orthogonal plane, likely caused by the mechanical staircase joints The same records give a 54 × 55 mm FWHM object illumination and matched correlation slopes The reconstruction is insensitive to the tested analysis and beam-distribution variations The fitted Ta-edge positions vary by only ≈ 1 µm along each ROI Increasing the initial momentum spread to 5 × 10−3 or vary- Figure 4 Details The synthetic radiograph of the reconstructed state (left) and the measured 2 GeV image (right), in target-plane coordinates Both panels use the same target-plane grid and limits Coloured rectangles mark the regions averaged to obtain the profiles in Fig.

Kai: 3 The lines inside them mark the profile centres The remaining residuals are the outer post-side 65 mm band (+15%) and the absolute pre-side PMMA-step width in the orthogonal plane, likely caused by the mechanical staircase joints The same records give a 54 × 55 mm FWHM object illumination and matched correlation slopes The reconstruction is insensitive to the tested analysis and beam-distribution variations The fitted Ta-edge positions vary by only ≈ 1 µm along each ROI Increasing the initial momentum spread to 5 × 10−3 or vary- Figure 5 Details Profiles of the four central 0.5 mm Ta slits are translated to their fitted centres and averaged without rescaling; transmission is normalized to the openslit level The measured profile (points) is compared with PRIORA (red): 3.75 mrad effective acceptance, 69 µm targetplane Gaussian exposure response and N = 1010 protons The dashed curves show perfect-focus calculations for the nominal 3.0 and 5.0 mrad apertures, which bracket the measured Ta/open transmission PRIORA gives 0.746 for the Ta/open transmission and 69 µm for the spatial resolution (Fig.

Paper summary: Kai: So, to recap, this paper introduces PRIORA as a fast Monte Carlo model for proton radiography that combines fitted interaction tables with energy-dependent transfer maps of arbitrary order. The core idea is that you can simulate high-energy proton transport through a complete ion-optical system using this setup on an eight-core laptop.

Mira: The authors verify this by testing it against GSI data from the Ta bench target, which involves fifteen precision slits in a two millimeter tantalum plate between symmetric PMMA staircases. They are demonstrating that one experimental image provides the scale, resolution as a function of chromatic load, longitudinal defocus in both transverse planes, effective angular acceptance and multiple-scattering contrast.

Lev: The central claim is that identical processing of the experiment and simulation yields results with no physics adjustment needed for PRIORA to reproduce data from two GeV and four GeV protons. That's a big deal because it validates the model’s coupling between matter transport and ion optics.

Kai: It means if you use this model, you can rapidly tune your system during experiments without having to guess every single parameter based on prior knowledge of the design state.

Mira: But they also show that the experimental observables—like slit positions and ESF widths in PMMA bands—constrain magnification, focus in both transverse planes, chromatic blur, and collimator acceptance all at once.

Lev: So it’s not just a simulation; it’s a reconstruction tool based on fitting the observed data to find the instrument state. That's how you get those quantitative answers from the measurement.

Kai: The paper shows that when they perform this reconstruction, they fit an actual non-nominal experimental state instead of assuming the design tune for everything.

Mira: This implies that we don't have to rely solely on perfect knowledge of our design parameters to interpret experimental data; the experiment itself provides enough information to constrain them.

Conclusion: Kai: So, looking at the title, "Experimental verification of the PRIORA Monte Carlo code for proton radiography," it’s clear this work is about proving that this specific simulation tool works when tested against real hardware measurements. It’s a check on the model's reliability.

Mira: The implications are that we have a validated way to use PRIORA to rapidly tune our instruments during experiments by reconstructing the state from a single radiograph, rather than relying only on initial design parameters.

Lev: For error correction, this means if you get an image, you can get a high-fidelity estimate of the actual focusing and acceptance profile without needing an impossibly detailed pre-run simulation for every possible tuning.

Kai: It also gives us a way to understand the energy trade-off: increasing proton energy reduces scattering and material contrast, but it also changes the angular acceptance, which affects resolution. PRIORA helps quantify that relationship between those two things using concrete numbers like the one hundred fifteen micrometers resolution at two GeV <ref:2610.26121#pg1>.

Mira: Ultimately, this paper provides a baseline for future designs and simulations because it shows how to map experimental observables directly onto instrument parameters in a way that is consistent across different proton energies.

Lev: It sets up a baseline where we can test new error correction strategies against the actual measured performance metrics from the Ta bench target data, which is much more useful than just theoretical predictions.