Proton Distribution Research Articles

Solar Wind Protons Forming Partial Ring Distributions at Comet 67P

AbstractWe present partial ring distributions of solar wind protons observed by the Rosetta spacecraft at comet 67P/Churyumov‐Gerasimenko. The formation of ring distributions is usually associated with high activity comets, where the spatial scales are larger than multiple ion gyroradii. Our observations are made at a low‐activity comet at a heliocentric distance of 2.8 AU on 19 April 2016, and the partial rings occur at a spatial scale comparable to the ion gyroradius. We use a new visualization method to simultaneously show the angular distribution of median energy and differential flux. A fitting procedure extracts the bulk speed of the solar wind protons, separated into components parallel and perpendicular to the gyration plane, as well as the gyration velocity. The results are compared with models and put into context of the global comet environment. We find that the formation mechanism of these partial rings of solar wind protons is entirely different from the well‐known partial rings of cometary pickup ions at high‐activity comets. A density enhancement layer of solar wind protons around the comet is a focal point for proton trajectories originating from different regions of the upstream solar wind. If the spacecraft location coincides with this density enhancement layer, the different trajectories are observed as an energy‐angle dispersion and manifest as partial rings in velocity space.

Possible origin of high proton/deuteron solubility at 10GDC grain boundaries

To understand the proton dissolution behavior into 10GDC from water vapor, a deuteron map of polycrystalline 10GDC was collected using nanoscale secondary ion mass spectrometry (NanoSIMS) as an indicator of proton distribution. NanoSIMS imaging clearly revealed that deuterons dissolved mainly into grain boundaries. Because Gd enrichment was observed at grain boundaries, the effect of Gd concentration (10–40 mol%) on deuteron solubility was also investigated using dynamic SIMS. For 10GDC, the bulk deuteron solubility was very low (4.8 × 1017 cm−3) compared to grain boundaries, while the bulk solubility significantly increased with increasing Gd concentration (1.1 × 1019 cm−3 for 30GDC). This indicates that Gd enrichment is at least partly responsible for the high proton/deuteron solubility at grain boundaries. The bulk solubility increase upon high-concentration Gd doping can be attributed to the lower electronegativity of Gd3+ compared to Ce4+, which results in increased oxygen basicity.

Measuring deformed neutron skin with free spectator nucleons in relativistic heavy-ion collisions

The neutron skin in deformed nuclei is generally not uniformly distributed but has an angular distribution, depending on both the spin-dependent nuclear interaction and the nuclear symmetry energy. To extract the information of the deformed neutron skin, we have explored the possibility of using free spectator nucleons in central tip-tip and body-body collisions at top RHIC energy with four typical deformed nuclei. The density distributions of neutrons and protons are consistently obtained from the Skyrme-Hartree-Fock-Bogolyubov calculation, and the angular distribution of the neutron skin can be varied by adjusting the strength of the nuclear spin-orbit coupling. With the information of spectator nucleons obtained based on a Monte-Carlo Glauber model, the free spectator nucleons are generated from a multifragmentation process. By investigating the results from different systems and with different collision configurations, we found that although it is difficult to probe the deformed neutron skin in 96Zr and 238U by their collisions, it is promising to extract the polar angular distributions of the neutron skin in 96Ru and 197Au by comparing the yield ratios of free spectator neutrons to protons in their central tip-tip and body-body collisions. The proposed observables can be measured by dedicated zero-degree calorimeters in heavy-ion collision experiments that have been carried out in recent years by RHIC.

Corrections for regular identification of high energy positive particles in experimental data using lobachevsky space

In this work, high-energy positive charged particles are distinguished using the Lobachevsky space or Hyperbolic space, which is defined as the total rapidity multiplied by hyperbolic cosines of the transverse and longitudinal rapidity of the particles. Experimental data from eight different types of interactions detected in the bubble chambers accumulated in the high-energy sector were used in the calculations. The weights used to construct the proton and positive pion distributions for each of the interacting secondary particles have been eliminated, allowing such studies to be performed such as particle counting and clustering.These weights do not include calculated weights at azimuth angles, near the center of the star, or without momentum measurements. We now have the opportunity to study positive pions and protons. The percentage of confused particles increases with the beam energy.
 After the reconstruction, we conducted a study of the temperature of the charged particles produced by the p + p interaction of 205 GeV, where Tsallis temperatures are close to Hagedorn . On the other hand, Hagedor and temperatures are higher than Tsallis, which means that the unstable states exchange heat as they move to equilibrium.

3D range-modulators for proton therapy: near field simulations with FLUKA and comparison with film measurements

The 3D range-modulator is a device used in particle delivery systems that can create a highly conformal and homogeneous dose distribution in the target volume with mono-energetic beams, providing an option for high dose-rate FLASH therapy. In the normal case, the modulators are positioned at a typical distance of 30-50 cm in front of the target in order to avoid the fluence ripples resulting from the periodic structure of the modulators. FLUKA Monte Carlo simulation package was used to investigate the fluence distributions of protons penetrating through the 2D range-modulator, the simplified version of the 3D range-modulator, and to determine the minimum distance at which the fluence is homogeneous enough for the treatment. To implement the complex geometry of the modulator in FLUKA, a dedicated FLUKA user routine was developed for the simulation of the periodic pin structures. The highest fluence ripple occurred at a few centimetres behind the modulators and then faded away as the distance increased, which can be described by the edge-scattering effect and later by the blur-out of the overlapping contributions from the pins. Moreover, the dose distribution in water was investigated, particularly for small distances between the modulators and the water phantom. Furthermore, the Monte Carlo results were compared with radiochromic film measurements irradiated with a 3D-printed range modulator and showed a good qualitative agreement. Prospectively, for low modulator-to-target distances, the strong dose inhomogeneities which appear in the proximal part of the target, could introduce additionally a kind of ‘mini beam’ normal-tissue sparing by the 3D range-modulators.

Charge state and Energy distribution of carbon ions and protons emitted from laser-produced graphite plasma

The energy and charge state distribution of atomic carbon ions, carbon cluster ions, and protons emitted during Q-switched Nd:YAG laser (1064 nm/6 ns) laser irradiation of high purity graphite target was investigated by employing a time of flight ion energy analyzer (TOF-IEA). Laser fluence on the target surface was varied from 2 to 26 J cm−2. At the lower fluence range (2–6 J cm−2), multiply charged low mass odd numbered cluster ions with n = 3, 5, 7 and charge state up to 5 + and carbon atomic ions up to the charge state of 3 + were detected. For the fluence greater than 6.4 J cm−2, carbon atomic ions up to a charge state of 6 + and protons were observed. The energy distribution of various carbon ions and protons was measured at the fluence of 5.1 and 25.5 J cm−2. At 5.1 J cm−2, the measured energy range for carbon cluster ions and carbon was 0.2–3.1 keV and 0.2–1.2 keV, respectively. At 25.5 J cm−2, the energy of carbon ions (q = 2–4) was in the range of 0.4–3.8 keV. Surprisingly, energetic protons with maximum energy of 1.5 keV and considerable intensity were detected at 25.5 J cm−2. The bi-component structure of ion energy distribution suggested a complex ion accelerating mechanism, which is discussed within the framework of the electrostatic model.

Restoration of the focal parameters for an extreme-power laser pulse with ponderomotively scattered proton spectra by using a neural network algorithm

A neural network-based approach is proposed both for reconstructing the focal spot intensity profile and for estimating the peak intensity of a high-power tightly focused laser pulse using the angular energy distributions of protons accelerated by the pulse from rarefied gases. For these purposes, we use a convolutional neural network architecture. Training and testing datasets are calculated using the test particle method, with the laser description in the form of Stratton–Chu integrals, which model laser pulses focused by an off-axis parabolic mirror down to the diffraction limit. To demonstrate the power and robustness of this method, we discuss the reconstruction of axially symmetric intensity profiles for laser pulses with intensities and focal diameters in the ranges of 1021–1023 W cm−2 and ∼(1–4)λ, respectively. This approach has prospects for implementation at higher intensities and with asymmetric laser beams, and it can provide a valuable diagnostic method for emerging extremely intense laser facilities.

Crescendo: an on-the-fly Fokker–Planck solver for spectral cosmic rays in cosmological simulations

ABSTRACT Non-thermal emission from relativistic cosmic ray (CR) electrons gives insight into the strength and morphology of intra-cluster magnetic fields, as well as providing powerful tracers of structure formation shocks. Emission caused by CR protons on the other hand still challenges current observations and is therefore testing models of proton acceleration at intra-cluster shocks. Large-scale simulations including the effects of CRs have been difficult to achieve and have been mainly reduced to simulating an overall energy budget, or tracing CR populations in post-processing of simulation output and has often been done for either protons or electrons. We introduce crescendo: Cosmic Ray Evolution with SpeCtral Electrons aND prOtons, an efficient on-the-fly Fokker–Planck solver to evolve distributions of CR protons and electrons within every resolution element of our simulation. The solver accounts for CR (re-)acceleration at intra-cluster shocks, based on results of recent particle-in-cell simulations, adiabatic changes, and radiative losses of electrons. We show its performance in test cases as well as idealized galaxy cluster (GC) simulations. We apply the model to an idealized GC merger following best-fitting parameters for CIZA J2242.4 + 5301-1 and study CR injection, radio relic morphology, spectral steepening, and synchrotron emission.

Aqueous Proton Transportation in Graphene-Based Nanochannels.

Graphene oxide (GO) has been unveiled to exhibit high proton conductivity in a humidified or aqueous environment, making it a promising candidate to construct proton conduction nanochannels. In this work, we systematically investigate how the confinement effect and surface chemistry influence the proton transportation behavior in graphene-based nanochannels via extensive ReaxFF MD simulations. Graphene (GE), graphane (GA), and hydroxygraphane (HG) sheets were employed to mimic the graphitic and functionalized region of GO and construct nanochannels with different interlayer distances. We find that confined water molecules are stratified and their orientation is influenced by the surface chemistry, thus impacting the distribution of protons. Surface chemistry makes the compression of the hydrogen-bond network induced by the confinement effect more variable. The hydrogen-bond network between GE slabs is crushed by extreme confinement and ultrafast proton transportation behavior mainly achieved via vehicle mechanism. Meanwhile, the hydrogen-bond network and solvation structure can be kept more complete with the existence of functional groups. The hydrogen bonds formed with surface functional groups impede the transportation of water molecules but allow more Grotthuss hopping of protons to different extents. Our work clarified the proton transportation mechanism in graphene-based nanochannels with different interlayer distances and surface chemistry and can guide the future design of proton conduction devices such as proton exchange membranes.

Surprising Solid-State ESIPT Emission from Apparently Ordinary Salicyliden Glycinates Schiff Bases

Excited-State Intramolecular Photon Transfer (ESIPT) is known for the geometry-related phenolic and imine groups. The Schiff bases formed upon condensation of salicyl aldehyde and glycine led to the formation of ESIPT models. A series of alkali metal salicyliden glycinates were analyzed by X-ray diffraction of their monocrystals and spectroscopy measurements. The X-ray analysis revealed varied hydration levels between the salts. They adapted trans geometry on the imine groups and mostly anticlinal conformation with the neighboring atoms, which is different from the other structurally-related compounds in literature. Fluorescence of these compounds was found for the crystalline forms only. Protonation of the imine nitrogen atom and further proton distribution was consistent with the ESIPT theory, which also explained the observed fluorescence with the highest Stokes shift of 10,181 cm-1 and 10.1% of fluorescence quantum yield for the sodium salt.

Nuclear charge radii of Cr46–62 isotopes and reaction cross sections for p−Cr

The nuclear charge radii and density distributions for $^{46--62}\mathrm{Cr}$ have been calculated in the frameworks of both relativistic and nonrelativistic self-consistent mean-field models. The relativistic Hartree-Bogoliubov model with density-dependent meson-exchange functional DD-ME2 and the nonrelativistic Hartree-Fock-Bogoliubov model with the Gogny D1S interaction, hereafter referred to as DIRHB and HFB, respectively, are used to compute ground-state properties of even $^{46--62}\mathrm{Cr}$ isotopes. Both calculations for the root-mean-squared charge radii reveal the characteristic kink at the $N$ = 28 shell closure in accordance with the corresponding experimental radii. The point proton and neutron density distributions calculated from both DIRHB and HFB are used to obtain the optical potentials for $p$-Cr at an incident proton energy of 65 MeV. The elastic scattering differential and total reaction cross sections, computed from the semimicroscopic proton optical potentials, have been derived by folding each of the target matter densities with the Jeukenne-Lejeune-Mahaux-Bruye\ifmmode \acute{r}\else \'{r}\fi{}es (JLMB) energy- and density-dependent internucleon interaction. The calculated elastic scattering differential cross sections for stable even isotopes, $^{50,52,54}\mathrm{Cr}$, using the respective DIRHB and HFB densities in the folding model reproduce the corresponding cross-section data well. The correlation between root-mean-squared charge radii and nuclear reaction observables obtained from the DIRHB and HFB calculations have been used in the folding model approach to predict the reaction cross sections for $p\text{\ensuremath{-}}^{46--62}\mathrm{Cr}$ at an incident proton energy of 65 MeV.

Proton uptake and proton distribution in perovskite materials for protonic ceramic fuel cell applications

Protonic ceramic fuel cells can generate electric power directly by converting the chemical energy stored in fuels through electrochemical reactions, offering a great potential for practical applications due to their high efficiency, low emissions and fuel flexibility. Lower and intermediate working temperatures (400–700 °C) are prerequisites for the commercialization, but inefficient proton uptake and the conduction ability of electrolyte and cathode materials limits the output performance. In this review, we summarize the common methods used to detect the proton concentration and distribution in some typical proton-conducting perovskites. The infrared absorption and Raman spectra combined with the first-principle calculations could provide the most information about hydrogen bond types with vibrational frequencies at 1000–4500 cm<sup>-1</sup>, the local proton environment and interactions between proton and crystal defects. The protons in a symmetric environment are easier to transport in the structure compared with that in an asymmetrical and trapped environment. A good understanding of proton uptake and proton distribution features in perovskite materials is necessary to design suitable proton-conducting materials.<br> The bibliography includes 167 references.

Extensive/Nonextensive Statistics for pT Distributions of Various Charged Particles Produced in p + p and A + A Collisions in a Wide Range of Energies

A comprehensive review on various experimental parametrizations proposed to fit the transverse momentum distributions of charged pions, kaons, and protons produced at energies ranging between 7.7 GeV and 2.76 TeV is introduced. We present a systematic study for their statistical fits to the extensive Maxwell–Boltzmann (MB) and nonextensive statistics (generic axiomatic statistics and the Tsallis one as a special case). The inconsistency that the MB approach is to be utilized in characterizing the chemical freezeout, while the Tsallis approach determining the kinetic freezeout is discussed. The resulting energy dependence of the different fit parameters largely varies with the particle species and the degree of (non)extensivity. This manifests itself in that the Tsallis nonextensive approach seems to work well for p + p, rather than for A + A collisions. Nevertheless, discussing the deeper physical insights of nonextensive statistical approaches is not targeted, drawing a complete picture of the utilization of the Tsallis statistics in modeling the transverse momentum distributions of several charged particles produced at a wide range of energies and, accordingly, presenting a criticism or a support of the relevant works. This may be considered as the main advantage of this review.

Prediction of Hepatic Toxicity after Radiotherapy Using a Neural Network Including Blood Biomarkers and Liver Dose Distributions

<h3>Purpose/Objective(s)</h3> ASTRO recently published practical guidelines for treating unresectable hepatocellular carcinoma with radiotherapy, recommending hypofractionated dose escalated regimens when tolerated by the liver. This highlights the need for patient-specific prediction models of radiation-induced hepatic toxicities, which necessitates inclusion not only of radiation dose, but also baseline liver function biomarkers. <h3>Materials/Methods</h3> We developed a shallow neural network to predict declines in Child-Pugh score (CP) of 2 or more, with a focus on robustness, interpretability and ability to include both proton and photon radiation dose distributions. The network uses 1 convolution layer to extract 8 dosimetric features that are representative of the differential DVH (dDVH) and interactions of different dose levels within, which are then combined in a fully connected layer with baseline platelet counts and liver function biomarkers—CP and albumin-bilirubin grade. After registering a study analysis plan, we trained the model on 117 patients from a single institution and independently validated it on 88 patients from another institution, where 47%/36% of the training/validation cohorts received proton radiotherapy. We compared the model to logistic regression to evaluate the importance of feature interactions on predictive power, measured by the area under the receiver operating characteristics (AUC). <h3>Results</h3> The model prediction of CP decline had a cross-validation AUC = 0.86; 95%CI: 0.69-0.97 and independent validation AUC = 0.74 with 89% accuracy in 10% high-risk group. In contrast, logistic regression showed cross-validation AUC 0.73; 95%CI: 0.56-0.88 and validation AUC 0.56, demonstrating a significant importance of feature interactions. Sensitivity analysis of the neural network revealed that patients with low baseline platelet counts were most sensitive to high doses to the normal liver compared to other patient groups, indicating a potential benefit of liver-sparing therapies in these patients. <h3>Conclusion</h3> We developed a patient-specific hepatic toxicity model that can take into account both proton and photon dose distributions and the interaction of radiation dose to normal liver with baseline liver function. Robust model performance across institutions and particularly in high-risk patients could allow stratification of patients to standard, hypofractionated regimens, or proton therapy based on a patient's risk score.

Fluorescence Resonance Energy Transfer (FRET) as a Spectroscopic Ruler for the Investigation of Protein Induced Lipid Membrane Curvature: Bacteriorhodopsin and Bacteriorhodopsin Analogs in Model Lipid Membranes.

Bacteriorhodopsin (bR) is a light-driven proton pump existing in the purple membranes (PM) of Halobacterium salinarum. The effects associated with changes in proton distribution (proton gradient, membrane electric potential) play a key role in ATPase stimulation. However, how the bioenergetic modulus (bR-PM-ATPase) functions remains unclear. One can find indications that hydrophobic matching and the curvature of the lipid membrane may form a functional link between bR and ATPase. To verify whether an interaction between bR and lipids can lead to curvature of the lipid membrane, a spectroscopic ruler, that is, a fluorescence resonance energy transfer (FRET) tool, was used. The distances from fluorescent lipid probes [octadecyl rhodamine B chloride (RhB), 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), 16-(9-anthroyloxy) palmitic acid (16AP), and hydrophobic probe 1,6-diphenyl-1,3,5-hexatriene (DPH), to the retinal chromophore of bR incorporated into phospholipid vesicles, were measured. The incorporation of retinal analogues with changed shape and/or altered electronic properties into the binding site of a bR or bR mutant were used to strengthen the feedback between the protein surrounding and chromophore. The experiments were performed with wild-type and D96N-mutated bR carrying retinal or 14-(12-,10-, 13,14-bi-) fluororetinal. As far as it is known, this is the first time that results obtained by the FRET method show that bR can induce a change in lipid structure interpreted as hydrophobically induced curving of the lipid membrane. Evidence was provided that the chromophore contributed to this effect. The extent of contribution was dependent on the chromophore structure in close vicinity to the place of its link with opsin. The implications of these findings for bR-PM-ATPase module functioning are also discussed.

An optimized microchannel Ta target for high-current accelerator-driven neutron sources

An optimized neutron producing tantalum target with an optimized internal microchannel cooling was developed for a 70 MeV proton beam with a peak current of 100 mA, a duty cycle of 1.43% and an average power of 100 kW on a target surface area of 100 cm 2. In this work a target with microchannel cooling structure is described which matches with the proton’s energy to minimize hydrogen implantation and to produce energy deposition with optimum homogeneity inside the target to minimize the thermal stresses. For the purpose of getting an optimal target design, the investigations of energy deposition, proton fluence, the spatial distribution of (p, n) reactions and the spatial distribution of stopping protons of the target with different microchannel geometries were performed with the particle transport code FLUKA. The resulting design produces a homogeneous proton fluence and energy deposition without hot spots. Furthermore, only 4.4% of the impinging protons accumulate in the metal target, which significantly decreases the risk of hydrogen embrittlement and blistering.

Regulating proton distribution by ion exchange resin to achieve long lifespan aqueous Zn-MnO2 battery

Despite progress in aqueous rechargeable Zn/MnO 2 batteries, mechanism of the complicated cathode reactions is not fully understood. Therefore, the battery system still suffers from some operation issues, such as unsatisfactory cyclability, especially at small current densities (<1C). In this work, we report that the side product from pH fluctuation, Zn 4 (OH) 6 SO 4 ∙xH 2 O (ZHS), can react with Mn 2+ during charge process at over 1.55 V (vs. Zn/Zn 2+ ) to form a lowly crystallized Woodruffite ((Zn,Mn) 2 Mn 5 O 12 ∙4H 2 O, or ZMO), which severely hinders the electrochemical performance of MnO 2 cathode and causes rapid capacity fading. To tackle this issue, we demonstrate that the introduction of ion exchange resin (IER) can regulate proton distribution in the electrolyte and eliminate the ZHS at 1.55 V during charge process, thus effectively inhibit the generation of ZMO. In Mn 2+ -additive free electrolyte, the proposed battery with IER provides a reversible specific capacity of 170 mAhg −1 over 200 cycles (>1450 h continuous discharging/charging) at 50 mAg −1 . When Mn 2+ is pre-added in the electrolyte, a specific capacity of 384 mAhg −1 and a capacity retention of 77.7% after 900 cycles can be obtained at 100 mAg −1 and 5 Ag −1 , respectively. This study shows a general approach to the design of long lifespan aqueous Zn/MnO 2 batteries.

Free spectator nucleons in ultracentral relativistic heavy-ion collisions as a probe of neutron skin

Besides the yield ratio of free spectator neutrons produced in ultracentral $^{96}\mathrm{Zr}$+$^{96}\mathrm{Zr}$ to $^{96}\mathrm{Ru}$+$^{96}\mathrm{Ru}$ collisions [Liu et al., Phys. Lett. B 834, 137441 (2022)], we propose that the yield ratio ${N}_{n}/{N}_{p}$ of free spectator neutrons to protons in a single collision system at the BNL Relativistic Heavy Ion Collider or the CERN Large Hadron Collider can be a more sensitive probe of the neutron-skin thickness $\mathrm{\ensuremath{\Delta}}{r}_{\mathrm{np}}$ and the slope parameter $L$ of the symmetry energy. The idea is demonstrated based on the proton and neutron density distributions of colliding nuclei obtained from Skyrme-Hartree-Fock-Bogolyubov calculations, and on a Glauber model that provides information of spectator matter. The final spectator particles are produced from direct emission, clusterization by a minimum spanning tree algorithm or a Wigner function approach, and deexcitation of heavy clusters by gemini. A larger $\mathrm{\ensuremath{\Delta}}{r}_{\mathrm{np}}$ associated with a larger $L$ value increases the isospin asymmetry of spectator matter and thus leads to a larger ${N}_{n}/{N}_{p}$, especially in ultracentral collisions where the multiplicity of free nucleons are free from the uncertainties of cluster formation and deexcitation. We have further shown that the double ratio of ${N}_{n}/{N}_{p}$ in isobaric collision systems or in collisions by isotopes helps to cancel the detecting efficiency for protons. Effects from nuclear deformation and electromagnetic excitation are studied, and they are found to be subdominant compared to the expected sensitivity to $\mathrm{\ensuremath{\Delta}}{r}_{\mathrm{np}}$.

Peeling away neutron skin in ultracentral collisions of relativistic nuclei

In heavy nuclei the ratio between local densities of neutrons and protons increases towards the nuclear periphery. The excess of neutrons is known as the neutron skin (NS) with a subtle difference ($<0.5$~fm) between the r.m.s radii of the distributions of neutrons and protons. We show that the presence of NS in $^{208}$Pb leads to extra spectator neutrons in ultracentral $^{208}$Pb--$^{208}$Pb collisions at the CERN SPS and LHC. The yields of spectator neutrons and protons were calculated within a new version of Abrasion-Ablation Monte Carlo for Colliders model (AAMCC-MST) taking into account NS and pre-equilibrium clustering of spectator matter. While the average numbers of spectator neutrons and protons in ultracentral collisions vary insignificantly, the cross sections of emission of certain numbers of spectator neutrons in events with 0,1,...5 spectator protons are changed by 50--250\%, depending on the thickness of NS. These cross sections are less sensitive to other parameters in calculations, and their measurements in the ALICE experiment at the LHC will make it possible to restrict the existing variety of neutron density parameterizations in $^{208}$Pb.

Probing neutron-skin thickness with free spectator neutrons in ultracentral high-energy isobaric collisions

We show that the yield ratio of free spectator neutrons produced in high-energy 96Zr+96Zr to 96Ru+96Ru collisions is a clean probe of the neutron-skin thickness of colliding nuclei and the slope parameter L of the symmetry energy. The idea is demonstrated based on the proton and neutron density distributions via a state-of-the-art Skyrme-Hartree-Fock-Bogolyubov calculation. Among spectator nucleons given by the Glauber model, free spectator neutrons include those from direct production that survive from clusterization as well as those from deexcitation of heavy clusters described by the popular GEMINI model. More free neutrons are produced in collisions of 96Zr nucleus due to its larger neutron skin, compared to those produced in collisions of 96Ru nucleus with a smaller neutron skin. The difference of the free spectator neutron yield is further increased with the increasing difference of the neutron-skin thickness between 96Zr and 96Ru with a larger L value, and the increase in ultracentral collisions is particularly insensitive to model details and experimental uncertainties. Since the production of free spectator neutrons is not affected by the complicated dynamics in the mid-rapidity region, the ratio of their multiplicities in ultracentral isobaric collisions is a robust observable for constraining the neutron skin and L value.