Monoenergetic Beam Research Articles

Calibration and validation measurements of the Extended-range Bonner sphere spectrometer

An Extended-range Bonner sphere spectrometer (EBSS) has been developed to investigate the neutron spectra of the China initiative Accelerator Driven System. This paper presents the design, calibration, and validation measurements of the EBSS system using a standard 241Am-Be neutron source and the cosmic ray neutrons. The EBSS system was simulated using the PHITS code, and the geometric structures were designed using the response functions obtained by simulations. The EBSS system comprises of seven polyethylene-only spheres and seven extended-range spheres encased in lead, copper, or tungsten shells along with a bare 3He proportional counter. To verify the accuracy of the simulated response functions, the EBSS system was calibrated with monoenergetic neutron beams of 565 keV at the China Institute of Atomic Energy in Beijing. The EBSS system was used to measure neutron count rates of the 241Am-Be neutron source and cosmic ray neutrons. Subsequently, the measured neutron count rates were converted into neutron spectra by the unfolding software. Experimental findings demonstrate the precise capability of the EBSS system to measure neutron spectra across a wide range of neutron radiation fields.

Electron beam response corrections for an ultra-high-dose-rate capable diode dosimeter.

Ultra-high-dose-rate (UHDR) electron beams have been commonly utilized in FLASH studies and the translation of FLASH Radiotherapy (RT) to the clinic. The EDGE diode detector has potential use for UHDR dosimetry albeit with a beam energy dependency observed. The purpose is to present the electron beam response for an EDGE detector in dependence on beam energy, to characterize the EDGE detector's response under UHDR conditions, and to validate correction factors derived from the first detailed Monte Carlo model of the EDGE diode against measurements, particularly under UHDR conditions. Percentage depth doses (PDDs) for the UHDR Mobetron were measured with both EDGE detectors and films. A detailed Monte Carlo (MC) model of the EDGE detector has been configured according to the blueprint provided by the manufacturer under an NDA agreement. Water/silicon dose ratios of EDGE detector for a series of mono-energetic electron beams have been calculated. The dependence of the water/silicon dose ratio on depth for a FLASH relevant electron beam was also studied. An analytical approach for the correction of PDD measured with EDGE detectors was established. Water/silicon dose ratio decreased with decreasing electron beam energy. For the Mobetron 9 MeV UHDR electron beam, the ratio decreased from 1.09 to 1.03 in the build-up region, maintained in range of 0.98-1.02 at the fall-off region and raised to a plateau in value of 1.08 at the tail. By applying the corrections, good agreement between the PDDs measured by the EDGE detector and those measured with film was achieved. Electron beam response of an UHDR capable EDGE detector was derived from first principles utilizing a sophisticated MC model. An analytical approach was validated for the PDDs of UHDR electron beams. The results demonstrated the capability of EDGE detector in measuring PDDs of UHDR electron beams.

Computational evaluation of the bucky components influence on the estimation of normalized glandular dose in digital mammography

The mean glandular dose (MGD) is the most suitable dosimetric quantity used in mammography to describe the absorbed dose by the breast, although it cannot be directly acquired. Studies have provided conversion factors widely implemented in international dosimetry protocols to estimate MGD, such as normalized glandular dose (DgN). Over time, the DgN estimation was refined by considering geometric models that approach a real clinical environment, such as new anode/filter combinations, compression plate and breast models. However, there is no detailed study of how the bucky (support plate, antiscatter grid and detector) can affect the DgN estimation. A modified PENELOPE Monte Carlo code was used for DgN estimation. The irradiation geometric model was built as a complete digital mammography system, considering a homogeneous breast and different typical bucky models in commercial mammography units. Simulations were carried out for mono and polyenergetic beams considering different imaging geometries. Studies with monoenergetic beams showed that the bucky presence affected DgN mainly for higher beam energies and thinner breasts. The breast support plate was the bucky component that most affected the DgN, followed by the antis-scatter grid and finally, the image detector. Studies with polyenergetic conventional (low-energy) spectra showed that the bucky exerted a minimal influence on DgN values (less than 1.0%). For high-energy spectra, mainly employed in modalities such as contrast-enhanced digital mammography, the DgN values were more affected by the bucky, increasing by 4.8% the DgN values for a 2 cm thick breast and a W/Cu 50 kV spectrum. Bucky inclusion in computer simulations is highly recommended mainly for thinner breasts and high-energy spectra. To simplify the simulations, we confirm that a homogeneous carbon fibre block support, with thickness between 3.9 and 4.1 mm, can be used as a good substitute for a complete bucky model.

Technical note: Radiological clinical equivalence for phantom materials in carbon ion therapy.

As carbon ion radiotherapy increases in use, there are limited phantom materials for heterogeneous or anthropomorphic phantom measurements. This work characterized the radiological clinical equivalence of several phantom materials in a therapeutic carbon ion beam. Eight materials were tested for radiological material-equivalence in a carbon ion beam. The materials were computed tomography (CT)-scanned to obtain Hounsfield unit (HU) values, then irradiated in a monoenergetic carbon ion beam to determine relative linear stopping power (RLSP). The corresponding HU and RLSP for each phantom material were compared to clinical carbon ion calibration curves. For absorbed dose comparison, ion chamber measurements were made in the center of a carbon ion spread-out Bragg peak (SOBP) in water and in the phantom material, evaluating whether the material perturbed the absorbed dose measurement beyond what was predicted by the HU-RLSP relationship. Polyethylene, solid water (Gammex and Sun Nuclear), Blue Water (Standard Imaging), and Techtron HPV had measured RLSP values that agreed within ±4.2% of RLSP values predicted by the clinical calibration curve. Measured RLSP for acrylic was 7.2% different from predicted. The agreement for balsa wood and cork varied between samples. Ion chamber measurements in the phantom materials were within 0.1% of ion chamber measurements in water for most materials (solid water, Blue Water, polyethylene, and acrylic), and within 1.9% for the rest of the materials (balsa wood, cork, and Techtron HPV). Several phantom materials (Blue Water, polyethylene, solid water [Gammex and Sun Nuclear], and Techtron HPV) are suitable for heterogeneous phantom measurements for carbon ion therapy. Low density materials should be carefully characterized due to inconsistencies between samples.

Two-dimensional time-resolved scintillating sheet monitoring of proton pencil beam scanning FLASH mouse irradiations.

Dosimetry in pre-clinical FLASH studies is essential for understanding the beam delivery conditions that trigger the FLASH effect. Resolving the spatial and temporal characteristics of proton pencil beam scanning (PBS) irradiations with ultra-high dose rates (UHDR) requires a detector with high spatial and temporal resolution. To implement a novel camera-based system for time-resolved two-dimensional (2D) monitoring and apply it in vivo during pre-clinical proton PBS mouse irradiations. Time-resolved 2D beam monitoring was performed with a scintillation imaging system consisting of a 1mm thick transparent scintillating sheet, imaged by a CMOS camera. The sheet was placed in a water bath perpendicular to a horizontal PBS proton beam axis. The scintillation light was reflected through a system of mirrors and captured by the camera with 500 frames per second (fps) for UHDR and 4 fps for conventional dose rates. The raw images were background subtracted, geometrically transformed, flat field corrected, and spatially filtered. The system was used for 2D spot and field profile measurements and compared to radiochromic films. Furthermore, spot positions were measured for UHDR irradiations. The measured spot positions were compared to the planned positions and the relative instantaneous dose rate to equivalent fiber-coupled point scintillator measurements. For in vivo application, the scintillating sheet was placed 1cm upstream the right hind leg of non-anaesthetized mice submerged in the water bath. The mouse leg and sheet were both placed in a 5cm wide spread-out Bragg peak formed from the mono-energetic proton beam by a 2D range modulator. The mouse leg position within the field was identified for both conventional and FLASH irradiations. For the conventional irradiations, the mouse foot position was tracked throughout the beam delivery, which took place through repainting. For FLASH irradiations, the delivered spot positions and relative instantaneous dose rate were measured. The pixel size was 0.1mm for all measurements. The spot and field profiles measured with the scintillating sheet agreed with radiochromic films within 0.4mm. The standard deviation between measured and planned spot positions was 0.26mm and 0.35mm in the horizontal and vertical direction, respectively. The measured relative instantaneous dose rate showed a linear relation with the fiber-coupled scintillator measurements. For in vivo use, the leg position within the field varied between mice, and leg movement up to 3mm was detected during the prolonged conventional irradiations. The scintillation imaging system allowed for monitoring of UHDR proton PBS delivery in vivo with 0.1mm pixel size and 2ms temporal resolution. The feasibility of instantaneous dose rate measurements was demonstrated, and the system was used for validation of the mouse leg position within the field.

In-situ positron annihilation spectroscopy during tensile tests on Al alloys

We present a materials analysis method to enable in-situ Doppler broadening spectroscopy (DBS) of the 511 keV annihilation line in matter during tensile tests. This technique allows the correlation between the formation of lattice defects on an atomic scale and the macroscopic physical materials properties stress and strain. By implanting a monoenergetic positron beam into samples of AlMg3 (3.3535) and AlMgSi (3.3206) the onset of plastic deformation was clearly observed in the recorded annihilation spectra during the measurement of the corresponding stress–strain curves. The changes of the DBS spectra are attributed to positron annihilation in (open-volume) defects—predominantly vacancies elastically bound to dislocation lines—formed during plastic deformation. The elastic strain (Hook’s region), however, does not lead to changes in DBS spectra.

X-ray production cross sections for Ir and Bi M-subshells induced by electron impact

M-subshell X-ray production cross sections were indirectly measured for Ir and Bi targets irradiated with monoenergetic electron beams. The projectile energy range ran from 2.2 to 28 keV, impinging on Ir and Bi pure bulk targets in a scanning electron microscope. The resulting X-ray emission spectra were acquired with an energy dispersive spectrometer, and processed afterwards by means of a robust parameter optimization procedure developed previously. X-ray production cross sections were finally obtained through an approach involving an analytical prediction for the emission spectra, which relies on the ionization depth distribution function. The values obtained by this approach were compared with empirical and theoretical predictions, appealing to different relaxation data taken from the literature.

The contribution of high-LET track to DNA damage formation and cell death for Monoenergy and SOBP carbon ion irradiation

Carbon ion radiotherapy (CIRT) is a heavy ion charge particle therapy with 29 years of prominent use. Despite advantages like high relative biological effectiveness (RBE), improved quality of life, and reduced treatment time, challenges persist, especially regarding heavy nuclear fragments. Our research addresses these challenges in horizontal irradiation, aiming to comprehend Monoenergetic and Spread-Out Bragg peak (SOBP) carbon ion beam trajectories using cell survival analysis and visualizing biological effects through DNA damage (γ-H2AX). This reveals repair-related protein foci near the Bragg peak. CR-39, a plastic nuclear track detector, was explored to understand high-linear energy transfer (LET) tracks and radiation quality near the Bragg peak. Findings unveil high-LET DNA damage signatures through aligned γ-H2AX foci, correlating with LET values in SOBP. CR-39 visualized high-LET particle exposure, indicating comet-type etch-pits at the Bragg peak and suggesting carbon ion fragmentation. Unexpectedly, dot-type etch-pits in irradiated and post-Bragg peak regions indicated high-LET neutron production. This investigation highlights the intricate interplay of carbon ion beams, stressing the importance of understanding LET variations, DNA damage patterns, and undesired secondary exposure.

Study of transfer-like fragments for 136Xe+209Bi/176Yb: Prospect for multinucleon transfer reactions at IGISOL

Multinucleon transfer (MNT) reactions have been demonstrated as a promising pathway to produce and study very neutron-rich heavy nuclei, which can enhance our understanding of the nuclear structural features relevant to the r-process. Translead fragments were produced from the MNT reaction approach using 136Xe+209Bi at IGISOL utilizing an MNT gas cell. The 211Bi, 211mPo, 211Po, and 212mPo nuclei have been observed prominently in the α-decay spectrum. The Geant4 simulation, which played a crucial role in optimizing the experimental parameters, reveals broader angular-energy distributions of MNT fragments released from a thick target compared to those observed from the GRAZING and Langevin models for a mono-energetic beam. Comparative yield analyses of MNT fragments for 136Xe+209Bi and 136Xe+176Yb reactions estimated using Geant4 Simulation as well as analytical formula support the production of many neutron-rich unknown mass nuclei in the rare-earth region.

Improving input contrast estimation to an x-ray imaging system

Objective. An appropriate parameter to study the performance of an x-ray imaging system is contrast transfer, or the system’s ability to capture contrast in the radiation beam and bring it to the image. However, determining the input contrast to the system is difficult, as it is heavily affected by secondary radiation, which in turn depends on a multitude of factors. This work presents a method to improve the calculation of input contrast to the imaging system when PMMA phantoms are used. Approach. An expression to obtain input contrast from primary radiation attenuation and scatter-to-primary ratio is shown, and the approximation upon which it is based is discussed. Primary and secondary radiation emerging from the phantoms are calculated for monoenergetic pencil beams impinging on planar PMMA phantoms of different thicknesses. Monte Carlo simulations of two types of anti-scatter grids are also incorporated into the calculations. Main results. The primary and secondary components of radiant energy and grid transmission factors are presented for monoenergetic beams with energies from 10 to 150 keV. These results are then used to calculate input contrast for polyenergetic beams when using a commercial image quality phantom combined with different thicknesses of PMMA and anti-scatter grids. Significance. The information of the object contrast carried by the beam constitutes the input to the imaging system. An accurate determination of this input contrast can be carried out in a wide variety of situations from the study of a reduced number of cases, as those presented in this work for monoenergetic beams, PMMA phantoms of different thicknesses and anti-scatter grids. The relationship between the input contrast and the contrast due to primary radiation used in this work provides a good approximation for the different combinations of inserts, phantoms, grids, and energy spectra analyzed here.

The influence of Monte Carlo radiation transport codes and hadron physics models on stray neutron dose estimations in proton therapy: An extended intercomparison

PurposeSelecting a Monte Carlo (MC) radiation transport code and nuclear physics model affects the simulation of secondary neutron fluence energy spectra and the estimation of the neutron dose equivalent in proton therapy. This study compares the MC codes FLUKA, MCNPX and GEANT4 from previous work with two additional MC codes, TOPAS (with either the Binary Intra-nuclear Cascade (BIC) or the Bertini intra-nuclear cascade model (Bert)) and PHITS (with either the Intra-Nuclear Cascade of Liege (INCL) or the Bert model), to facilitate a more comprehensive comparison. Material and methodsSecondary neutron fluence energy spectra were scored at different locations inside a water phantom, exposed to a 10 × 10 cm2 parallel mono-energetic proton beam with energies of 110 MeV, 150 MeV, 180 MeV and 210 MeV. The neutron dose equivalent was estimated by applying fluence-to-dose equivalent conversion coefficients. ResultsDiscrepancies between MC codes and physics models reach 90 % for the total neutron fluence. Differences are most pronounced in the forward direction of the beam due to a notable presence of high-energy neutrons and the higher uncertainty on the neutron cross-section data for these neutrons. Nuclear model selection within PHITS (INCL or Bert) and TOPAS (BIC or Bert) shows a worse agreement for the Bert model than for the other MC codes and nuclear models. The discrepancies in the neutron spectrum simulations propagate into differences in the estimated neutron dose equivalents. They are more prominent for distal positions (up to a factor of 3) than for lateral positions (up to a factor of 2) and decrease at higher proton energies. ConclusionsThis study shows considerable differences in the out-of-field neutron fluence energy spectra calculated by different MC codes and nuclear physics models. Differences in the neutron dose equivalent reach a factor of 3. The discrepancies are most apparent in distal positions and at lower proton energies.

Optimized plasma waveguides for single-stage bubble acceleration of monoenergetic electron beams with enhanced quality

The laser wakefield acceleration in bubble regime is now a promising method for producing high-quality electron beams, which is crucial to the development of the next-generation compact and low-cost particle accelerators. In this paper, we demonstrate the possibility of controlling particle injection and electron beam quality in sub-petawatt laser pulse interaction with simple plasma waveguides with a radially step-like density profile. Fully relativistic particle-in-cell calculations for different laser intensities indicate that by considering a sufficiently low electron density for a central plasma channel to ensure both increased acceleration length and formation of a well-structured bubble, one can optimize the surrounding plasma density and central channel diameter to reach the best injection conditions and produce a high-quality monoenergetic electron beam with improved peak energy and reduced energy spread. Calculations are performed for 33 fs, 140–315 TW laser pulses interacting with a ∼2 cm long plasma waveguide. It is demonstrated that by using the proposed scheme in optimized conditions, monoenergetic electron beams with peak energies 3–4 GeV and energy spreads less than 1.5% can be obtained in a single acceleration length of 1.85 cm. The beam charge and conversion efficiency of laser energy into the beam energy were also enhanced to values around 195 pC and 14.7%, respectively.

Experimental validation in a neutron exposure frame of the MINAS TIRITH for cell damage simulation

In the domains of medicine and space exploration, refining risk assessment models for protecting healthy tissue from ionizing radiation is crucial. Understanding radiation-induced effects requires biological experimentations at the cellular population level and the cellular scale modeling using Monte Carlo track structure codes. We present MINAS TIRITH, a tool using Geant4-DNA Monte Carlo-generated databases to study DNA damage distribution at the cell population scale. It introduces a DNA damage location module and proposes a method to convert double-strand breaks (DSB) into DNA Damage Response foci. We evaluate damage location precision and DSB-foci conversion parameters. MINAS TIRITH’s accuracy is validated against γ-H2AX foci distribution from cell population exposed to monoenergetic neutron beams (2.5 or 15.1 MeV) under different configurations, yielding mixed radiation fields. Strong agreement between simulation and experimental results was found demonstrating MINAS TIRITH’s predictive precision in radiation-induced DNA damage topology. Additionally, modeling intercellular damage variability within a population subjected to a specific macroscopic dose identifies subpopulations, enhancing realistic fate models. This approach advances our understanding of radiation-induced effects on cellular systems for risk assessment improvement.

Ground-based passive generation of Solar Particle Event spectra: Planning and manufacturing of a 3D-printed modulator

The generation of space radiation on Earth is essential to study and predict the effects of radiation on space travelers, electronics, or materials during future long-term space missions. Next to the heavy ions of the galactic cosmic rays, solar particle events play a major role concerning the radiation risk in space, which consist of intermediate-energy protons with broad spectra and energies up to a few hundred MeV. This work describes an approach for the ground-based generation of solar particle events. As a proof of principle, a passive beam modulator with a specific funnel-shaped periodic structure was designed and is used to convert a monoenergetic proton beam into a spectral proton energy distribution, mimicking a solar particle event from August 1972, which is known as one of the strongest recorded SPE events. The required proton beam of 220 MeV can be generated at many existing particle accelerators at research or particle therapy facilities. The planning, manufacturing and testing of the modulator is described step by step. Its correct manufacturing and the characteristics of the solar particle event simulator are tested experimentally and by means of Monte Carlo simulations. Future modulators will follow the same concept with minor adjustments such as a larger lateral extension. As of now, the presented beam modulator is available to the research community to conduct experiments at GSI for exposure under solar particle event conditions. In addition, researchers can use and apply the described concept to design and print their individualized modulator to reproduce any desired solar particle event spectrum or request the presented modulator geometry from the authors.

Silicon Carbide Timepix3 detector for quantum-imaging detection and spectral tracking of charged particles in wide range of energy and field-of-view

The hybrid architecture of the Timepix (TPX) family of detectors enables the use of different semiconductor sensors, most commonly silicon (Si), as well as high-density materials such as Cadmium Telluride (CdTe) or Gallium Arsenide (GaAs). For this purpose, we explore the potential of a silicon carbide (SiC) sensor bump-bonded on a Timepix3 detector as a radiation imaging and particle tracking detector. SiC stands as a radiation-hard material also with the ability to operate at elevated temperatures up to several hundreds of degrees Celsius. As a result, this sensor material is more suitable for radiation harsh environments compared to conventional e.g., Si sensors. In this work, we evaluate the response for precise radiation spectrometry and high-resolution particle tracking of newly developed SiC Timepix3 detector which is built and operated as a compact radiation camera MiniPIX-Timepix3 with integrated readout electronics. Calibration measurements were conducted with mono-energetic proton beams with energies of 13, 22, and 31 MeV at the U-120M cyclotron at the Nuclear Physics Institute Czech Academy of Science (NPI CAS), Prague, as well as 100 and 226 MeV at the Proton Therapy Center Czech (PTC) in Prague. High-resolution pattern recognition analysis and single-particle spectral tracking are used for detailed inspection and understanding of the sensor response. Results include distributions of deposited energy and linear energy transfer (LET) spectra. The spatial uniformity of the pixelated detector response is examined in terms of homogeneously distributed deposited energy.

Proton acceleration with multi-peak energy spectra tailored by vortex laser

A novel flying cascaded acceleration mechanism is proposed to generate energetic proton beams with multi-peak energy spectra using a circularly polarized (CP) Laguerre–Gaussian (LG) laser pulse in three-dimensional particle-in-cell simulations. Simulations show that the protons are initially accelerated and compressed into the beam center via the radiation pressure of the CP LG (σz = −1) laser pulse. Then, they are tailored by flying dipolar electric fields in this LG laser, resulting in a multi-peak energy spectrum. Each shaped proton peak exhibits a narrow energy spread of ∼5% and high flux of ∼2 × 108 protons/MeV at giga-electron volts energy. Such a flying cascaded acceleration mechanism extends the energy spectra of proton beams from monoenergetic to multi-peak structure, thereby potentially enhancing the generation efficiency of monoenergetic proton beams for various applications, such as proton-induced spallation reactions, proton radiography, and proton therapy.

Kinetic Energy of Secondary Particles Produced from Various Electromagnetic Interactions in Water: A Monte Carlo Study

Advancements in imaging systems including Electronic Portal Imaging Devices (EPIDs) play a great role in radiotherapy treatment. It was developed as a verification tool for patient setup during radiotherapy sessions and also become a promising tool for the determination of the accurate placement of radiation beams. However, as part of quality assurance, individual patient treatments are often verified by patient-specific quality control measurements such as before treatment (pretreatment) or during treatment (in vivo). It has been shown that in vivo dosimetry using an electronic portal imaging device (EPID) is an effective QC tool to detect errors and this method has been clinically applied to various treatments. The introduction of advanced EPID technology has led to an interest in its application for dose conformation and dose deposition. Moreover, dose deposition is subject to uncertainties due to several factors, including the presence of secondary particles. Thus, knowing the physical processes that produced the secondary particles as well as their average kinetic energy will help to provide valuable information about the effective filtering of these particles or the possible use of these particles for other applications. In this study, Monte Carlo simulations are performed to determine the average kinetic energy of detected secondary particles, specifically photons, electrons, and positrons produced by each particular physical interaction as a function of detector position using GATE v9.0. The virtual radiotherapy set-up is composed of the box water phantom, which is the target in the simulations with a dimension of 20 cm × 20 cm × 20 cm, an EPID system (detector), and a beam source in which it uses three (3) beams situated at varying positions with an energy of 6 MeV. The monoenergetic pencil beam source is placed 90 cm away from the center of the target and is directed toward the target (+x-axis) while the EPID (detector) is set as 120 cm SDD (source-to-detector distance). Moreover, the photon beam with 10 million primaries is set with varying field sizes of 1 cm × 1 cm, 3 cm × 3 cm, 6 cm × 6 cm, and 9 cm × 9 cm. Overall, the results show that the highest average kinetic energy among secondary particles produced by each physical interaction are electrons coming from Compton scattering (∼ 3 MeV), followed by positrons and electrons from pair production (∼ 2.4 MeV), photons from annihilation and bremsstrahlung (∼ 0.5 MeV), and electrons from ionization (∼ 0.13 MeV).

Multisignal ionization chamber with a B4C coating as an active neutron beam spectrometer: Monte Carlo simulations.

In this study, the influence of additional 10B4C polarizing electrodes coating on KW-1 multisignal ionization chamber performance was investigated. Using the FLUKA Monte Carlo code energy deposition, neutron fluence and neutron track-length distributions were simulated. Further calculations allow us to obtain response functions for monoenergetic neutron beams in the energy range from 1 meV to 30MeV. Results indicate further upgrades that can be adapted for moderator-based gaseous detectors for the neutron beam spectrometry approach.

Lossless Monochromator in an Ultrafast Electron Microscope Using Near-Field THz Radiation.

The ability to form monoenergetic electron beams is vital for high-resolution electron spectroscopy and imaging. Such capabilities are commonly achieved using an electron monochromator, which energy filters a dispersed electron beam, thus reducing the electron flux to yield down to meV energy resolution. This reduction in flux hinders the use of monochromators in many applications, such as ultrafast transmission electron microscopes (UTEMs). Here, we develop and demonstrate a mechanism for electron energy monochromation that does not reduce the flux-a lossless monochromator. The mechanism is based on the interaction of free-electron pulses with single-cycle THz near fields, created by nonlinear conversion of an optical laser pulse near the electron beam path inside a UTEM. Our experiment reduces the electron energy spread by a factor of up to 2.9 without compromising the beam flux. Moreover, as the electron-THz interaction takes place over an extended region of many tens of microns in free space, the realized technique is highly robust-granting uniform monochromation over a wide area, larger than the electron beam diameter. We further demonstrate the wide tunability of our method by monochromating the electron beam at multiple primary electron energies from 60 to 200keV, studying the effect of various electron and THz parameters on its performance. Our findings have direct applications in the fast-growing field of ultrafast electron microscopy, allowing time- and energy-resolved studies of exciton physics, phononic vibrational resonances, charge transport effects, and optical excitations in the mid IR to the far IR.

Impact of nanosecond laser annealing on vacancies in electroplated Cu films studied by monoenergetic positron beams

Positron annihilation was used to probe vacancy-type defects in electrodeposited Cu films after nanosecond pulse laser annealing. For the as-deposited Cu film, we identified the coexistence of two different vacancy-type defects, vacancy clusters (such as V16) and monovacancy-type defects, coupled with impurities. An enlargement in the vacancy size was observed after the laser annealing process. The size of these defects was estimated to be close to V30, and such defects could not be formed by conventional furnace annealing. After furnace annealing at 400 °C, the size of the larger vacancy clusters decreased, but that of the smaller vacancies increased. The observed change in the sizes of vacancies is considered to be related to interactions between vacancies and impurities. The depth profile of the defects varied by changing the laser energy density and the number of laser shots. The impact of laser annealing on the vacancy-type defects was observed even after furnace annealing at 800 °C. Because the presence of point defects in electroplated Cu directly correlates with electromigration and grain growth, the ability of laser annealing to introduce large vacancy clusters in the localized region shows the potential of nanosecond laser annealing as a low-thermal budget process tool for back-end-of-line materials.