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Particle-induced X-ray emission

Particle-induced X-ray emission (PIXE) is a non-destructive analytical technique that employs a beam of high-energy charged particles, most commonly protons, to bombard a sample and induce the emission of characteristic X-rays from its constituent elements, allowing for the identification and quantification of elemental composition with sensitivities down to parts per million.^[1]^[2] The principle of PIXE relies on the interaction between the incident particles and the inner-shell electrons of target atoms: the particles eject these electrons through Coulombic repulsion, creating vacancies that are subsequently filled by outer-shell electrons, releasing X-rays with energies unique to each element, which are then detected and analyzed using energy-dispersive spectrometers such as silicon detectors.^[3]^[2] This process enables multielemental analysis in a single measurement, with detection limits typically in the 1–100 ppm range or better depending on the matrix, element, and experimental conditions, and is particularly effective for elements with atomic numbers above 11 (sodium), though lighter elements can be analyzed when combined with complementary techniques like particle-induced gamma-ray emission (PIGE).^[1]^[2]^[3] Historically, PIXE emerged from research on ion-atom collisions in the 1950s, with practical development accelerating in the late 1960s and early 1970s, culminating in its first international conference in 1970 organized by S.A.E. Johansson at Lund University, Sweden, and widespread adoption by the 1980s due to advances in detector technology like Si(Li) systems.^[3] Key advantages include its non-destructive nature, minimal sample preparation requirements, ability to analyze insulating or heterogeneous materials without conductivity issues, and superior signal-to-background ratios compared to electron-based X-ray methods, making it suitable for probing surface layers up to about 10–20 micrometers deep.^[1]^[2] PIXE finds broad applications across diverse fields, including materials science for thin-film characterization, environmental monitoring for trace pollutant detection, archaeology for artifact composition analysis, biology and medicine for studying elemental distributions in tissues, and geology for mineral identification, often integrated with other ion-beam techniques like Rutherford backscattering spectrometry (RBS) for comprehensive profiling.^[3]^[1] Typical setups use proton beams of 1–4 MeV energy, with analysis areas of 1–2 mm diameter, achieving relative accuracies of 2–3% and absolute accuracies of 5–10% through software like GUPIX for spectrum deconvolution and quantification.^[2]

Introduction

Definition and Basic Principles

Particle-induced X-ray emission (PIXE) is a nuclear analytical technique employed for the non-destructive elemental analysis of materials, where samples are bombarded with charged particles, typically protons in the MeV energy range, to induce the emission of characteristic X-rays from the sample's constituent atoms.^[4] This method leverages ion-atom collisions to eject inner-shell electrons, creating vacancies that are subsequently filled by electrons from higher energy levels, resulting in the emission of X-rays with energies specific to each element.^[5] The process primarily targets the K, L, or M electron shells, producing distinct spectral lines such as Kα and Kβ for lighter elements, which allow for unambiguous identification of elements based on their atomic number.^[6] The basic principle of PIXE relies on the interaction of fast-moving charged particles with target atoms, where the particles' high energy facilitates inner-shell ionization through Coulombic interactions, far exceeding the binding energies of these electrons (typically a few keV).^[4] As outer-shell electrons cascade to fill the resulting vacancies, the energy differences are released as characteristic X-rays, whose wavelengths follow Moseley's law, providing a direct correlation to atomic number Z.^[5] Protons are the most commonly used particles due to their favorable ionization cross-sections and low nuclear stopping power, which minimize sample damage; energies of 1-4 MeV are typically selected to optimize X-ray production while ensuring sufficient beam penetration (on the order of microns to tens of microns in solids).^[7] This setup enables the simultaneous detection of multiple elements in a single measurement without requiring chemical preparation.^[6] PIXE offers high sensitivity for trace element detection, achieving limits from parts per million (ppm) to parts per billion (ppb) for elements with Z > 11 (sodium and heavier), depending on beam current, measurement time, and matrix effects.^[8] For instance, under standard conditions with a 3 MeV proton beam, detection limits around 1-10 ppm are routine for mid-Z elements like iron or copper, while optimized setups can reach sub-ppm or ppb levels for environmentally relevant traces.^[6] The technique's multi-element capability stems from the broad excitation range, allowing 10-20 elements to be analyzed concurrently from a single spectrum, making it efficient for complex samples such as aerosols, biological tissues, or geological materials.^[4] In contrast to X-ray fluorescence (XRF), which excites samples using photon beams and relies on photoelectric absorption, PIXE employs charged particle beams that produce significantly higher inner-shell ionization cross-sections—often 10-100 times greater for light elements—resulting in superior sensitivity and the ability to detect lighter elements (Z down to 11) with minimal background interference.^[4] While XRF is more portable and does not require an accelerator, PIXE's particle-induced process enables deeper probing and better quantification in thick samples, though it demands specialized facilities for beam generation.^[9]

Historical Development

Particle-induced X-ray emission (PIXE) traces its origins to earlier ion-beam experiments in the 1950s and 1960s, which explored atomic collisions and X-ray production in solids, laying the groundwork for charged-particle interactions with matter.^[10] The technique was formally proposed in 1970 by Sven A. E. Johansson at Lund University in Sweden, who demonstrated that MeV protons could induce characteristic X-rays with high intensity for multi-elemental analysis, building on these prior studies.^[4] This proposal, detailed in a seminal paper, highlighted PIXE's potential as a sensitive, non-destructive method for trace element detection using silicon-lithium detectors.^[11] The first experimental demonstrations followed shortly, with Johansson and Folkmann publishing results in 1972 that validated PIXE's analytical capabilities through proton bombardment of samples, achieving detection limits down to 10^{-11} g for elements.90251-5) Rapid adoption occurred in the 1970s and 1980s, as PIXE facilities proliferated worldwide; key sites included the original setup at Lund University, the Oxford scanning proton microprobe established in the mid-1970s for focused beam applications, and IAEA-supported laboratories in developing regions that integrated PIXE for environmental and materials research.^[4] The first International PIXE Conference in 1970, held in Lund, marked a pivotal moment, fostering global collaboration and leading to triennial conferences that documented the technique's growth.^[3] Expansion continued in the 1990s and 2000s with advancements like micro-PIXE, which achieved sub-micrometer beam focusing for spatially resolved analysis, enabling intracellular mapping in biological samples.^[3] Concurrently, external beam configurations emerged, allowing non-vacuum analysis of delicate artifacts and aerosols by extracting the beam through thin foils, as implemented at facilities like the AGLAE laboratory.^[4] Over its more than 50 years of development, PIXE has evolved from primarily geological applications to diverse biomedical and environmental uses, supported by over 10,000 publications that underscore its enduring impact.^[12] Recent milestones from 2020 to 2025 include integrations with laser-driven proton sources, where Geant4 simulations validated PIXE yields for compact accelerator alternatives.^[13] Additionally, innovations like the STRAS aerosol sampler, introduced in 2025, enable high-time-resolution (hourly) particle collection for PIXE-based environmental monitoring, enhancing urban air quality assessments.^[14]

Theoretical Basis

Inner-Shell Ionization Processes

In particle-induced X-ray emission (PIXE), inner-shell ionization arises primarily from Coulomb interactions between the incident projectile ion, such as a proton, and the tightly bound electrons in the target atom's inner shells, resulting in the ejection of these electrons and the creation of atomic vacancies. This direct ionization process dominates for swift projectiles, where the projectile's electric field perturbs the target electron orbits, transferring sufficient energy to overcome the binding energy of the inner-shell electron. Excitation, where the electron is promoted to a higher unoccupied state without immediate ejection, can also occur but typically leads to ionization via subsequent Auger processes. These interactions are treated perturbatively, with the projectile modeled as a point charge moving through the atomic potential.^[15]^[3] Theoretical descriptions of these processes rely on quantum mechanical approximations to compute ionization cross sections, which quantify the probability of vacancy creation. The Plane Wave Born Approximation (PWBA), developed by Merzbacher and Lewis, provides a foundational model for high-velocity ions by assuming the projectile follows a straight-line trajectory and is described by plane-wave wavefunctions, neglecting binding effects in the initial and final states. This first-order perturbation theory yields analytical expressions suitable for relativistic velocities but overestimates cross sections at lower speeds due to its neglect of projectile-target binding and trajectory curvature. For improved accuracy in intermediate velocity regimes typical of PIXE (e.g., 1-5 MeV protons), the Energy-loss Coulomb-repulsion Perturbed-Stationary-State Relativistic (ECPSSR) theory, formulated by Brandt and Lapicki, incorporates corrections for energy loss during the collision (E), Coulomb deflection of the projectile trajectory (C), perturbed stationary states of the target electron (P), increased binding energy due to multiple ionization (S), and relativistic effects (R). The ECPSSR model reduces discrepancies with experimental data to within 10-20% for light ions impacting medium-Z targets, making it a standard for PIXE simulations.^[16] The PWBA cross section for K-shell ionization is obtained by integrating the doubly differential cross section over energy and momentum transfers, assuming hydrogenic orbitals for the target electron, and is typically expressed in scaled variables such as the reduced velocity \xi_K and binding parameter \theta_K. The ECPSSR modifies the PWBA result through multiplicative correction factors: \sigma_K^{ECPSSR} = f_E f_C f_P f_S f_R \sigma_K^{PWBA}, where each f adjusts for the respective effects, enhancing agreement with data for non-equilibrium conditions.^[17] Cross sections depend strongly on projectile energy, peaking when the projectile velocity matches or slightly exceeds the target inner-shell orbital velocity (\xi_K \approx 1-2), which for protons corresponds to energies of approximately 10-20 MeV for K-shells in mid-Z elements (e.g., iron or copper, where U_K \approx 7-10 keV). At lower energies, such as the 2-3 MeV typical in PIXE setups, the cross sections rise steeply from threshold but remain below the maximum, balancing ionization efficiency with minimal sample damage from stopping power. Heavier ions (e.g., alpha particles or carbon) exhibit higher cross sections per unit charge due to stronger Coulomb fields (\propto Z_1^2), but their greater mass shifts the velocity matching to higher energies, often requiring adjustments in ECPSSR for multiple electron promotions. For protons at 2-3 MeV, K-shell cross sections are typically 10-100 times larger than those induced by electrons in energy-dispersive X-ray (EDX) spectroscopy at equivalent effective velocities, owing to better kinematic matching for deeper shells and reduced multiple scattering.^[16]^[3]

X-ray Emission and Characteristic Spectra

When an inner-shell vacancy is created in an atom during particle bombardment, the atom relaxes through either radiative or non-radiative de-excitation processes. In radiative de-excitation, an electron from a higher shell fills the vacancy, emitting a characteristic X-ray photon whose energy corresponds to the difference between the two shell binding energies. The probability of radiative de-excitation, known as the fluorescence yield \omega, represents the fraction of vacancies that result in X-ray emission rather than non-radiative Auger electron emission. This yield increases with atomic number Z; for the K-shell, \omega_K \approx 0.3 for Z=30 (zinc) and rises to \approx 0.9 for Z=50 (tin), reflecting stronger Coulomb interactions in heavier atoms that favor photon emission over electron ejection.^[18] The production cross-section for a specific X-ray line, \sigma_X, quantifies the likelihood of generating that photon and is given by

\sigma_X = \sigma_{\text{vacancy}} \cdot \omega \cdot f,

where \sigma_{\text{vacancy}} is the inner-shell ionization cross-section from the incident particle, \omega is the fluorescence yield for the relevant shell, and f is the branching ratio representing the probability that the de-excitation populates the specific line among possible transitions. For K-shell emissions, the branching ratio for the K\alpha lines (L to K transition) versus K\beta (M to K) is typically around 8:1, meaning K\alpha dominates the spectrum due to the higher probability of L-shell involvement. This formula underpins quantitative PIXE analysis by linking ionization efficiency to observable X-ray intensities.^[19]^[20]^[21] Characteristic X-ray spectra in PIXE consist of discrete emission lines superimposed on a continuous background. The lines arise from specific atomic transitions, such as the K\alpha_1 and K\alpha_2 doublet from the $2p_{3/2} \to 1s and $2p_{1/2} \to 1s processes, respectively; these doublets, separated by 10–20 eV, are generally not resolved in standard PIXE detectors for lines above 1 keV due to typical energy resolutions of 130–180 eV FWHM but contribute to the overall peak shape. The continuum background primarily stems from bremsstrahlung radiation produced when target electrons, ejected by the incident particles, are decelerated in the Coulomb fields of nearby nuclei, creating a broad, falling spectrum that complicates detection of weak characteristic lines from minor elements.^[22]^[23] The observed intensity of characteristic X-rays is modulated by several factors beyond production cross-sections. Self-absorption within the sample attenuates emitted photons, particularly for lower-energy lines, as they are reabsorbed by the matrix before escaping; this effect is more pronounced in thick or high-density samples and requires corrections based on sample composition and thickness to avoid underestimation of concentrations. Additionally, detector geometry influences the collected intensity, which is proportional to the solid angle subtended by the detector relative to the emission point; optimizing this angle (often 20–45° to the beam) maximizes signal while minimizing contributions from scattered particles.^[24]^[25] For elements with Z < 30, L-shell emissions often dominate the spectrum due to low K-shell fluorescence yields (\omega_K < 0.4), necessitating higher detector resolution to distinguish overlapping L lines (e.g., L\alpha, L\beta) separated by tens of eV. In contrast, for high-Z elements (Z > 70), M-shell emissions become prominent, contributing complex multiplets in the 2–5 keV range that require careful deconvolution for accurate identification.^[18]^[26]

Instrumentation and Setup

Particle Beam Sources

Particle beam sources for particle-induced X-ray emission (PIXE) primarily consist of electrostatic accelerators such as Van de Graaff or tandem configurations, which generate proton beams in the 1-5 MeV energy range suitable for inducing inner-shell ionization in target atoms.^[4] These accelerators operate by injecting ions from an external source into a high-voltage terminal, where electrostatic fields accelerate the particles to the desired energy, providing stable beams with low energy spread essential for precise PIXE measurements. For applications requiring higher beam currents, cyclotrons are employed, as they can deliver intense proton fluxes while maintaining energies around 3-4 MeV, though they are less common due to their size and complexity compared to electrostatic systems. Optimal beam parameters for PIXE balance ionization cross-sections with minimal sample damage and background radiation; proton energies typically range from 0.5 to 4 MeV, where the cross-section for K-shell ionization peaks while keeping the beam's stopping power low to ensure penetration into the sample.^[27] Beam currents are controlled between 1 and 100 nA to deliver sufficient ion flux for detectable X-ray yields without excessive heating or charging of the sample, which could alter its composition or induce unwanted secondary effects.^[28] Proton beams are preferred over heavier ions like alpha particles or helium ions because they exhibit lower multiple scattering in the sample, allowing deeper penetration and sharper depth resolution, while providing higher X-ray production yields per unit charge due to more efficient energy transfer to inner-shell electrons.^[29] Beam delivery systems focus on transporting the accelerated ions to the sample with minimal divergence and contamination. Collimators, often made of tantalum or graphite, reduce the beam spot size to less than 1 mm, enabling localized analysis while shielding against stray particles that could increase background noise.^[30] Experiments are typically conducted in high-vacuum chambers maintained at pressures around 10^{-6} Torr to prevent ion beam scattering by residual gas, though external beam setups in air or helium atmospheres allow non-destructive analysis of sensitive samples by exiting the vacuum through thin windows like Si_3N_4 or Kapton.^[4] Recent advancements in the 2020s include compact laser-driven ion sources, which use ultraintense laser pulses to accelerate protons to MeV energies in tabletop setups, offering potential for portable PIXE systems with fluxes suitable for in-air applications.^[31] Setup considerations emphasize precise beam alignment and monitoring to ensure reproducible results. Optical cameras or scintillator screens are used for initial alignment, directing the beam onto the sample position within the chamber, while Faraday cups—electron-suppressed collectors—measure and integrate the beam current in real-time, compensating for fluctuations and providing accurate charge normalization for quantitative PIXE.^[32] These components are often integrated with beam deflectors to sample current without interrupting analysis, maintaining stability across extended irradiation periods.^[4]

Detection Systems and Sample Handling

In particle-induced X-ray emission (PIXE) analysis, detection systems primarily employ solid-state detectors for energy-dispersive X-ray spectroscopy (EDS), with silicon lithium-drifted detectors (Si(Li)) and silicon drift detectors (SDDs) being the most common choices due to their high efficiency and energy resolution. Si(Li) detectors typically achieve resolutions of 160–180 eV full width at half maximum (FWHM) at the Mn Kα line (5.9 keV), though optimized systems can reach ~130 eV, while SDDs offer improved performance with resolutions of 130–160 eV FWHM at the same energy, enabling better separation of closely spaced spectral lines from mid-Z elements. SDDs also support higher counting rates, up to 100–200 kHz, compared to ~5 kHz for Si(Li), reducing the need for extensive dead-time corrections in high-flux environments, where dead times can exceed 5% at rates above 1000 counts per second (cps). Multi-detector arrays, such as configurations with multiple SDDs, are increasingly used to enhance throughput by increasing the effective solid angle and minimizing sample exposure time, particularly in external beam setups for sensitive artifacts. Detector geometry is optimized to maximize X-ray collection while minimizing background from scattered particles and low-energy photons. Detectors are positioned at angles of 45° to 135° relative to the incident beam axis, with common setups at 45° to the sample normal to balance solid angle coverage (typically 8–180 millisteradians) and reduce proton backscattering interference. Thin beryllium windows (e.g., 8–25 μm) are used on the detector entrance to transmit X-rays above ~1 keV while attenuating low-energy photons and protons; these windows, often made of beryllium foil, are essential for filtering helium or air in external beam lines. Absorbers such as aluminum foil or Mylar (e.g., 150–450 μm thick) are placed between the sample and detector to suppress pile-up from intense low-Z lines (e.g., from argon or silicon) and improve signal-to-noise for higher-Z elements, though they slightly reduce overall efficiency for lighter elements. Sample handling in PIXE requires compatibility with high-vacuum environments (~10^{-3} to 10^{-4} Pa) to prevent beam scattering, with solids mounted on conductive, planar holders using spring-loaded clamps or carbon paste to ensure electrical grounding and minimize charging effects. Vacuum-compatible multi-sample changers with load-lock systems allow sequential analysis without repeated venting, supporting thin targets (e.g., aerosols on polycarbonate or Teflon filters, <50 μg/cm²) where the sample thickness is less than the ion range, and thick targets (>100 μg/cm²) that require matrix corrections for attenuation. For liquids, specialized cells with thin windows (e.g., Mylar or Kapton) enable analysis without evaporation, though they are less common than solid mounts. Aerosol collectors, such as the Size- and Time-Resolved Aerosol Sampler (STRAS), facilitate high-resolution sampling by depositing PM1, PM2.5, or PM10 particles onto small areas (0.9–1.5 cm²) of polycarbonate membranes at hourly intervals for up to 168 samples, enhancing PIXE sensitivity through concentrated deposits without pre-treatment. Non-destructive external beam configurations, using helium-flushed exit foils (e.g., Kapton), allow in-situ analysis of fragile artifacts like paintings or jewelry by extracting the proton beam from the vacuum chamber, avoiding direct sample exposure to vacuum or high currents. Counting rates in PIXE setups are typically limited to 10^3–10^4 cps to maintain spectral integrity, with dead-time corrections applied via pulse pile-up rejection or software modeling to account for losses at higher fluxes, particularly for thick targets where continuous X-ray production leads to elevated backgrounds. Sample thickness significantly affects detection: thin targets yield uniform excitation with minimal self-absorption, ideal for trace elements, whereas thick targets necessitate corrections for X-ray attenuation and secondary fluorescence from the matrix. Calibration of detection efficiency is performed using standards like NIST SRM 2783 (air particulates on filter media) or MicroMatter thin films, generating curves that account for detector response, geometry, and absorbers across the 1–20 keV range to ensure quantitative accuracy.

Data Analysis

Spectrum Processing Techniques

In particle-induced X-ray emission (PIXE) analysis, raw spectra are acquired through pulse-height analysis, where the energy of detected X-rays is measured and binned into channels using multi-channel analyzers (MCAs). These devices convert analog signals from detectors, such as silicon drift detectors (SDDs) or Si(Li) detectors, into digital histograms representing the X-ray energy distribution. Digital signal processing techniques enable real-time corrections during acquisition, including baseline restoration and energy calibration to improve spectral accuracy at high count rates.^[33] Preprocessing of PIXE spectra begins with dead-time correction to account for the detector's inability to register events during signal processing. The non-paralyzable model is commonly applied, where the true count rate r_{\text{true}} is calculated as r_{\text{true}} = \frac{r}{1 - \tau \cdot r}, with \tau as the dead time per event and r as the observed count rate; this model assumes lost events do not extend the dead time. Pile-up rejection is another critical step, rejecting events where multiple X-rays arrive nearly simultaneously, distorting the energy measurement; hardware or software filters, such as those based on pulse shape analysis, identify and discard these coincident pulses to minimize spectral artifacts.^[34] Background subtraction is essential to isolate characteristic X-ray peaks from the underlying continuum, primarily arising from bremsstrahlung radiation produced by secondary electrons in the sample. Common methods include polynomial fitting for empirical modeling of the smooth continuum or physical models based on electron energy loss; for instance, a low-order polynomial (degree 2–4) is fitted to regions between peaks to estimate and subtract the background.^[35] Escape peaks, resulting from the escape of silicon Kα X-rays (at ~1.74 keV) from the detector, are removed by subtracting a scaled fraction (typically ~10–15%) of the parent peak intensity at the escape energy offset.^[4] For thick targets, the background intensity B(E) can be approximated as B(E) \approx A \cdot E^{-m} \cdot \exp(-\mu \rho x), where A is an amplitude factor, m (typically 1.5–3) represents the power-law slope of the bremsstrahlung spectrum, \mu is the mass absorption coefficient, \rho is the sample density, and x is the effective thickness; this accounts for both the continuum generation and self-absorption effects.^[36] Automated software packages like GUPIX and GeoPIXE facilitate these processing steps through integrated fitting routines that handle pile-up, sum peaks from coincident X-ray events, and overall spectral deconvolution. GUPIX, developed at the University of Guelph, employs Bayesian-like methods for robust background estimation and peak fitting, supporting batch processing for high-throughput analysis. GeoPIXE, from CSIRO, extends this to imaging data with dynamic analysis for real-time corrections and quantitative yield modeling. These tools ensure reproducible extraction of peak areas while minimizing user bias in preprocessing.^[37]^[33]^[38]

Quantitative Determination Methods

Quantitative determination in particle-induced X-ray emission (PIXE) relies on converting measured X-ray peak intensities from processed spectra into elemental concentrations, primarily through the fundamental parameters method (FPM) or standard-based calibration. The FPM uses theoretically calculated physical parameters such as ionization cross-sections, fluorescence yields, detector efficiencies, and beam charge to derive concentrations without requiring reference samples, enabling reference-free analysis. Standard-based calibration, in contrast, employs known elemental concentrations in reference materials to establish empirical sensitivity factors, offering simplicity for routine analyses but necessitating well-characterized standards matched to the sample matrix.^[4]^[39] In the FPM, the concentration c_i of element i is determined by relating the measured X-ray intensity I_i to the production yield, which integrates the ionization cross-section \sigma_i, fluorescence yield \omega_i, detector efficiency \varepsilon_i, and beam charge Q over the particle path in the sample, accounting for energy loss, X-ray self-absorption, and geometric factors such as detector solid angle. For thin targets, where energy loss and absorption are negligible, the relation simplifies to c_i \propto \frac{I_i}{Q \cdot \sigma_i \cdot \omega_i \cdot \varepsilon_i}, with proportionality involving sample density and geometry. For thick targets, full numerical integration is required, often combining PIXE with complementary techniques like Rutherford backscattering spectrometry (RBS) to resolve matrix composition and depth profiling.^[4] Error sources in PIXE quantification include statistical uncertainties from Poisson statistics in peak counting, typically dominating for trace elements, and systematic errors from beam charge measurement, detector efficiency, and matrix assumptions, yielding overall uncertainties of 5-10% for mid-Z elements. Minimum detection limits (MDL) are calculated using the Currie formula, L_D = 2.71 + 4.65 \sqrt{B}, where B is the background counts under the peak, converted to concentration via FPM factors to achieve sensitivities down to parts per million.^[4]^[37] The GUPIXWIN software implements FPM for PIXE spectrum analysis, incorporating updated databases for cross-sections and stopping powers in the 2020s, achieving accuracies of ±5% for major elements and ±20% for traces in validated geological and environmental samples.^[33]^[40]

Applications

Environmental and Geological Analysis

Particle-induced X-ray emission (PIXE) has become a vital tool in environmental and geological analysis, enabling non-destructive, multi-elemental characterization of inorganic matrices such as atmospheric aerosols, soils, and rocks. In environmental monitoring, PIXE facilitates the detection of trace elements at parts-per-million levels, supporting pollution assessment and source identification without sample digestion.^[41] In geological contexts, it provides insights into mineral compositions and sediment origins, aiding geochemical modeling and provenance studies.^[42] A primary application of PIXE lies in aerosol studies, where it profiles elemental content in fine particulate matter like PM_2.5 and PM₁₀, identifying key pollutants such as sulfur (S), potassium (K), calcium (Ca), and heavy metals including lead (Pb) and arsenic (As). These analyses reveal concentrations of up to several micrograms per cubic meter for crustal elements and nanograms per cubic meter for anthropogenic tracers, allowing differentiation between natural and human-induced sources.^[41] Source apportionment often employs enrichment factors (EF), calculated as EF = \frac{(X/Ref)_{\text{sample}}}{(X/Ref)_{\text{crust}}}, where X is the element of interest and Ref is a reference crustal element like aluminum (Al); EFs >10 indicate enrichment from non-crustal sources such as traffic or industry.^[43] In geological applications, PIXE excels at quantifying trace elements in minerals.^[44] Sediment provenance studies utilize PIXE to compare trace element compositions across samples to link depositional sources. Recent advancements include the 2025 STRAS (Size- and Time-Resolved Aerosol Sampler), which enables hourly PIXE analysis for urban air quality monitoring by sequentially collecting up to 168 PM₁₀, PM_2.5, or PM₁ samples on a single filter strip. This integration supports time-resolved data, capturing diurnal variations in pollutants, with PIXE detection limits of approximately 1–10 ng/m³ for Pb and As under typical urban conditions.^[45] Case studies highlight PIXE's utility in tracking long-range transport, such as Saharan dust events, where non-destructive analysis of filter-collected aerosols identifies elevated Al, Fe, and Ca levels, contributing 20–50% to PM₁₀ mass during intrusions via positive matrix factorization of PIXE spectra.^[46] Similarly, PIXE has characterized volcanic ash compositions in sediments, revealing trace element signatures like high Ti that aid in eruption reconstruction and dispersal modeling without altering fragile samples.^[47]

Biological and Medical Applications

Particle-induced X-ray emission (PIXE) has emerged as a valuable technique for trace element analysis in biological systems, enabling the mapping of essential and toxic metals at concentrations as low as 1-10 ppm in dry samples. In protein studies, micro-PIXE facilitates the identification and quantification of metal-binding sites, such as iron in hemoglobin and zinc in enzymes like glucose dehydrogenase, by depositing small volumes (0.1-0.3 μl) of protein solutions on thin Mylar films and drying them for analysis. This approach achieves detection limits of approximately 1 μg/g for elements like Fe, Zn, Ca, Mn, and Cu, with accuracies of 10-20%, allowing researchers to confirm stoichiometric ratios, for instance, 0.97 Fe atoms per molecule in cytochrome c. Buffers without sulfur, such as ammonium acetate, are preferred to avoid spectral interferences during proton bombardment at 2-4 MeV. For cellular and tissue analysis, PIXE, particularly in micro-PIXE mode, reveals elemental distributions in cryo-preserved sections, preserving spatial integrity for studying diseases like Alzheimer's. In brain tissues, micro-PIXE has detected elevated iron and altered calcium levels in affected regions, correlating with plaque formation and neurodegeneration, with spatial resolutions down to 1-10 μm. Sample preparation typically involves freeze-drying or cutting thin (10-20 μm) frozen-hydrated sections to minimize element redistribution, enabling non-destructive imaging that serves as a proxy for live-cell studies. Quantitative data processing, as outlined in spectrum analysis methods, supports these mappings by accounting for matrix effects in organic tissues. In medical applications, PIXE assesses trace metals in fluids like blood serum and urine for toxicology, detecting cadmium and mercury exposures at trace levels to monitor occupational or environmental risks. For example, PIXE analysis of blood serum from healthy donors has quantified elements like Fe, Zn, and Cu, establishing baseline concentrations for comparison in toxicological cases. In cancer research, PIXE compares elemental profiles between malignant and normal tissues, revealing anomalies such as increased zinc and copper in tumor samples from the intestine or breast, which inform metal uptake mechanisms and therapeutic targeting. Recent 2020s advancements, including hybrid imaging, continue to explore iron dysregulation in lung tissues, though specific PIXE applications remain focused on broader trace element roles in pathogenesis.

Cultural Heritage and Archaeological Studies

Particle-induced X-ray emission (PIXE) has become a vital tool in the non-destructive analysis of cultural heritage artifacts, enabling the identification of elemental compositions in pigments, metals, and corrosion products without significant sample preparation. For instance, in ancient Egyptian wall paintings, PIXE has confirmed the presence of copper in Egyptian blue pigments, a synthetic calcium copper silicate used extensively from the predynastic period onward, distinguishing it from natural blue minerals like azurite or lapis lazuli.^[48] Similarly, PIXE analysis of ancient coins, such as those from the Hindu Shahis Dynasty (990–1015 AD), reveals gold and silver contents along with trace impurities like copper and lead, providing insights into minting techniques and economic practices.^[49] In metallic artifacts, external beam PIXE characterizes corrosion layers, identifying elements such as copper, zinc, and lead in patinas on bronze statuettes, which helps reconstruct environmental exposure histories and guide restoration efforts.^[50] In archaeological contexts, PIXE facilitates provenance studies of ceramics and pottery by quantifying trace elements, such as ratios of sodium, potassium, and iron, which reflect clay sources and firing conditions unique to specific regions or workshops. For example, analyses of ancient Iranian luster pottery have used PIXE to group sherds based on these elemental signatures, tracing trade routes and production centers across the Islamic world.^[51] Impurity profiles detected via PIXE also serve as proxies for dating, as variations in trace metals like chromium or nickel correlate with chronological changes in raw material sourcing or technological evolution, aiding in the authentication of artifacts without invasive methods.^[52] In conservation, PIXE monitors degradation in stone artifacts by detecting sulfur enrichment indicative of sulfation processes, such as gypsum formation from atmospheric pollution, which informs protective interventions for marble sculptures and architectural elements. When combined with micro-PIXE scanning, it generates 2D elemental maps to visualize degradation patterns across surfaces.^[53] An exemplary case is the non-destructive PIXE and X-ray-induced PIXE analysis of parchment fragments from the Dead Sea Scrolls, which revealed that the parchment was produced using water from the Dead Sea, suggesting local manufacture near Qumran and preserving the 2nd-century BCE manuscripts without damage.^[54]

Advanced Techniques and Variants

Micro-PIXE and Scanning Modes

Micro-PIXE, or microbeam particle-induced X-ray emission, extends conventional PIXE by employing a focused ion beam to achieve spatial resolutions typically in the range of 1-10 μm, enabling detailed elemental mapping of heterogeneous samples.^[55] The beam is focused using magnetic quadrupole triplet lenses, which provide precise control over proton trajectories to produce spot sizes as small as 2 μm, as demonstrated in vertical microbeam systems for biological irradiation.^[56] Alternatively, tapered glass capillaries serve as focusing optics, guiding MeV ion beams to sub-micrometer dimensions while minimizing scattering, particularly useful for in-air analyses.^[57] These focusing techniques allow for raster scanning, where the beam is deflected electromagnetically across the sample surface to generate two-dimensional (2D) elemental distribution images or, through depth profiling, three-dimensional (3D) reconstructions.^[58] Scanning modes in micro-PIXE incorporate advanced software for efficient data acquisition, such as the Dynamic Analysis (DA) method implemented in GeoPIXE, which enables real-time quantitative elemental imaging by processing spectra on-the-fly during beam scanning.^[59] This approach supports dwell times of 10-100 ms per pixel, balancing spectral statistics with scan speed to map areas up to several square millimeters without excessive sample damage.^[60] Raster patterns are commonly used, with pixel sizes matching the beam spot to produce high-fidelity maps, and beam currents adjusted from nanoamperes for high-resolution work to microamperes for faster overviews.^[61] High-resolution micro-PIXE finds applications in biology for cellular-level elemental mapping, revealing trace metal distributions in organelles or tissues, such as iron accumulation in single cells.^[62] In materials science, it identifies inclusions and impurities at micrometer scales, aiding analysis of alloys or composites where elemental heterogeneity affects properties.^[63] The Oxford Microbeams system, utilizing quadrupole triplets, routinely achieves resolutions below 1 μm, supporting detailed studies of sub-micrometer features.^[55] Advancements in the 2020s have pushed toward sub-micron capabilities for nanoparticle characterization, enhancing detection of engineered nanomaterials in environmental or biological matrices through improved beam optics and detection sensitivity.^[63] Data from micro-PIXE scans are typically visualized as false-color maps, where intensity gradients represent elemental concentrations across the scanned area; for instance, iron (Fe) distributions in pollen grains can be overlaid to highlight accumulation patterns in exine layers versus cytoplasm.^[64] These maps, generated via software like GeoPIXE, facilitate quantitative interpretation by correlating pixel intensities with calibrated standards, providing insights into spatial correlations between elements.^[61]

Complementary Ion Beam Methods

Particle-induced X-ray emission (PIXE) is often integrated with Rutherford backscattering spectrometry (RBS) to provide complementary depth profiling and matrix composition information. RBS measures the energy of protons scattered by the Coulomb field of target nuclei, enabling non-destructive analysis of elemental composition and thickness through energy loss mechanisms. The scattered proton energy shift corresponds to the depth of interaction, allowing resolution of layered structures up to several micrometers, particularly effective for heavier elements in lighter matrices. Proton transmission techniques, such as scanning transmission ion microscopy (STIM), complement PIXE by assessing the areal density or mass thickness of thin foils and samples via the energy loss of transmitted ions. In STIM, protons passing through the sample lose energy proportional to the integrated density along their path, providing quantitative thickness maps without requiring standards. This is particularly useful for biological or thin-film samples where PIXE alone lacks direct thickness information, enabling normalized elemental concentrations.^[65] Simultaneous PIXE-RBS setups employ dual detectors to collect X-ray and backscattered proton spectra concurrently, yielding both trace elemental data from PIXE and structural depth profiles from RBS. This integration is advantageous for analyzing light elements like carbon and oxygen, where PIXE sensitivity is low, as RBS detects them through matrix effects. Combined systems have been standard in ion beam analysis (IBA) facilities since the 1980s, facilitating self-consistent quantification in applications such as thin-film characterization.^[66] RBS can incorporate ion channeling to probe sample crystallinity, where aligned crystal lattices steer incident protons, reducing backscattering yield from deeper layers. This reveals defect densities and lattice quality, complementing PIXE's compositional focus in crystalline materials. The RBS yield is approximated by the formula

Y \approx N t \Omega \frac{d\sigma}{d\Omega},

where N is the atomic density, t the thickness, \Omega the solid angle, and \frac{d\sigma}{d\Omega} the non-relativistic Rutherford differential cross-section, \left( \frac{Z_1 Z_2 e^2}{8\pi \epsilon_0 E_0} \right)^2 \frac{1}{\sin^4(\theta/2)}.^[67]

Emerging and Hybrid Techniques

Particle Desorption Ionization coupled with Particle-Induced X-ray Emission Mass Spectrometry (PDI-PIXE-MS) represents a hybrid analytical approach that integrates elemental quantification via PIXE with molecular speciation through mass spectrometry, enabling simultaneous analysis of inorganic and organic components in complex samples.^[68] This technique employs MeV particle beams to induce both X-ray emission for PIXE and desorption ionization for mass spectral detection, allowing for multiplexed data collection from millimeter-sized beams.^[69] Originally developed as a proof-of-concept instrument, PDI-PIXE-MS desorbs organic molecules while quantifying inorganic elements, providing complementary insights into sample composition without requiring sample preparation.^[70] Applications of PDI-PIXE-MS have extended to biomolecules since the 2010s, facilitating studies of protein films and adsorbed layers by linking elemental distributions to molecular identities. Proton Beam Writing (PBW) extends PIXE-related ion beam capabilities into high-resolution lithography, utilizing focused MeV proton beams to pattern microstructures directly in resist materials such as PMMA, achieving aspect ratios exceeding 10:1 and depths greater than 50 μm for three-dimensional fabrication.^[71] Unlike electron beam lithography, PBW's heavier protons enable straight-line propagation with minimal scattering, supporting the creation of smooth, high-fidelity features down to sub-micrometer scales.^[72] In biotechnology, PBW has been applied to microfluidics, enabling the production of lab-on-a-chip devices for single-cell analysis and elastomeric structures that integrate fluidic channels with biological assays.^[73] These capabilities allow for rapid prototyping of complex 3D architectures, such as nanofluidic channels, enhancing applications in biomolecular manipulation and diagnostic platforms.^[74] Laser-driven PIXE leverages ultraintense lasers to generate compact proton accelerators, producing MeV ions for X-ray emission analysis without large-scale facilities, marking significant advancements from 2020 onward.^[75] These systems accelerate protons via plasma interactions, enabling external beam configurations for non-vacuum analysis, such as quantitative elemental mapping of specimens in air.^[31] A notable 2024 development demonstrated a compact laser-proton source achieving PIXE sensitivities comparable to conventional setups, with applications in real-time environmental monitoring.^[31] Hybrid variants combining laser-induced X-ray fluorescence with PIXE further improve quantification by mitigating matrix effects in diverse samples.^[76] Looking ahead, emerging trends in PIXE hybrids emphasize AI-enhanced data analysis to automate spectrum deconvolution and anomaly detection, improving accuracy in multi-elemental datasets from complex matrices. Portable PIXE systems, including alpha-particle and combined PIXE-XRF setups, are advancing field-deployable analysis for cultural heritage and environmental monitoring, reducing reliance on laboratory infrastructure.^[77] These innovations promise broader accessibility and integration with on-site hybrid techniques for rapid, in-situ assessments.^[78]

Advantages and Limitations

Key Advantages

Particle-induced X-ray emission (PIXE) is a non-destructive analytical technique that enables the simultaneous detection of multiple elements with atomic numbers greater than 10 (from sodium to uranium) without requiring sample preparation or chemical treatment, preserving the original sample for further analysis.^[3] This minimal interaction results in negligible mass loss due to low sputtering yields, making it ideal for valuable or irreplaceable specimens.^[1] In contrast to methods like inductively coupled plasma mass spectrometry (ICP-MS), which necessitate sample digestion, PIXE allows direct analysis of solids, liquids, and aerosols, enhancing its practicality for diverse sample types.^[79] PIXE offers high sensitivity for trace element detection, achieving limits as low as parts per billion (ppb) in optimized setups, such as for water samples, due to its superior signal-to-background ratio compared to electron-based methods.^[80] Quantitative accuracy is typically within ±5-10%, facilitated by the fundamental parameter method (FPM), which relies on physical models rather than extensive standards for precise elemental quantification.^[3] This precision holds across a wide range of concentrations, providing reliable results for both major and minor constituents. Analysis with PIXE is rapid, yielding a complete multi-elemental spectrum in as little as 30 seconds to 5 minutes, depending on beam current and sample thickness, which supports high-throughput applications.^[81] The technique is inherently surface-sensitive, probing depths of approximately 5-20 μm with micrometer-scale resolution in micro-PIXE modes, and exhibits no significant matrix effects for thin targets where proton stopping is uniform.^[1] External beam configurations extend PIXE's versatility to in-situ analysis under atmospheric conditions, enabling non-invasive examination of objects without vacuum chambers.^[3] Compared to scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), PIXE provides better sensitivity for trace elements owing to reduced bremsstrahlung background from proton excitation, complementing SEM-EDX in hybrid setups for enhanced trace detection.^[82] Overall, these attributes make PIXE a cost-effective alternative to ICP-MS, avoiding digestion costs and complexities while delivering robust multi-elemental insights.^[83]

Principal Limitations

Particle-induced X-ray emission (PIXE) exhibits limited sensitivity to light elements with atomic numbers Z < 11, such as hydrogen, lithium, and beryllium, primarily due to their low X-ray fluorescence yields and significant absorption of the resulting low-energy X-rays by the detector's beryllium window; sensitivity can be improved with windowless or ultralow-energy detectors for elements Z ≥ 5.^[3]^[84] This constraint necessitates complementary techniques like particle-induced gamma-ray emission (PIGE) or nuclear reaction analysis (NRA) for accurate detection of these elements.^[4] In analyses of thick samples, matrix effects pose substantial challenges, as the absorption of low-energy X-rays within the sample material reduces signal intensity and complicates quantification, often requiring Rutherford backscattering spectrometry (RBS) to determine the matrix composition for corrections.^[3]^[85] PIXE setups demand specialized infrastructure, including access to particle accelerators such as Van de Graaff or tandem accelerators, rendering the technique non-portable and confined to laboratory environments with high initial costs for dedicated systems.^[86] Quantification in organic samples can lead to overestimation of low-Z elements without appropriate standards, owing to background contributions and non-homogeneous target effects that skew X-ray yields.^[12] Additionally, radiation damage from beam deposition affects beam-sensitive materials like organics, causing structural deterioration at fluences exceeding 10^{12} ions/cm² due to heating and ionization.^[12]^[87] The energy resolution of typical silicon drift or Si(Li) detectors in PIXE systems, around 150 eV full width at half maximum, limits the separation of overlapping spectral lines, such as titanium Kβ (4.964 keV) and vanadium Kα (4.952 keV), which differ by only about 12 eV and thus require deconvolution for accurate analysis.^[88]^[89]

References

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### Summary of Ion-Atom Collision Dynamics in PIXE from the Document
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PIXE is one form of X-ray Emission Spectrometry. • Create an inner shell vacancy by exciting the electron shells of the atom.
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[PDF] Electron probe microanalysis_ A review of recent developments and ...
May 25, 2020 · EDS has less energy resolution than WDS which means that overlaps between some X-ray lines are more common (e.g., V Kα by Ti. Kβ, Mn Kα by Cr Kβ) ...