Thin Targets Research Articles

Improved energy spread in the radiation pressure acceleration of protons with a linearly polarized laser.

Degradation in the energy spread of accelerated protons due to the transverse instability induced transparency is one of the critical issues in the laser-driven radiation pressure acceleration (RPA) scheme. This issue is more severe for linearly polarized lasers due to enhanced heating of electrons. Therefore, in spite of being experimentally challenging, most of the numerical studies are performed with circularly polarized lasers. In this work, through particle-in-cell simulations, we demonstrate a significant improvement in the energy spread of the accelerated protons when a multilayered target is irradiated by a linearly polarized laser. This multilayered target consists of a near-critical-density (NCD) layer, sandwiched between a thick metallic foil and a thin RPA target. The role of the NCD target is to suppress the laser transparency to increase the coupling of laser momentum to the RPA protons. On the other hand, the metallic foil utilizes the kinetic energy of the escaping fast electrons to form an electrostatic sheath to filter the low-energy RPA protons. This results in significant improvement in the accelerated proton spectrum, even with a linearly polarized laser.

Mechanical properties of refractory HEA WNbTaV and its penetration behavior to aluminum alloy plate

Quasi-static and dynamic compression experiments were conducted on WNbTaV to investigate its mechanical characteristics and its penetration behavior into an aluminum alloy target plate. Additionally, ballistic experiments with WNbTaV projectiles against 2A12 aluminum alloy thin targets were performed. The results indicate that HEA WNbTaV at a strain rate of 10−3 s−1 has a fracture strength of 1340 MPa, a yield strength of around 980 MPa, and a fracture strain of less than 7%. However, its dynamic yield strength is almost twice its quasi-static yield strength, and its dynamic fracture strain is more than twice the quasi-static compression fracture strain.When striking a 2 mm thick 2A12 aluminum alloy target plate with a velocity less than 348.5 m/s, the HEA WNbTaV projectile remains stiff and undeformed. Its maximum velocity before entering the target plate is roughly 273 m/s. Furthermore, within a certain impact velocity range, the residual velocity, initial impact velocity, and ultimate penetration velocity of the HEA WNbTaV projectile penetrating the 2 mm thick 2A12 aluminum alloy target plate meet the following relationships. vr=v02−2.13vb2.

Reference measurements for indirect dark matter searches with p+C collisions at the NA61/SHINE experiment

NA61/SHINE is a large-acceptance fixed-target experiment located at the CERN SPS, which studies final hadronic states in interactions of various particles and nuclei. It is unique in terms of providing data on a variety of collision systems at different collision energies. This allows for wide deuteron, antiproton and antideuteron production cross-section studies. The latter are currently considered a possible dark matter interaction signal with exceptionally small background. The measurements on carbon targets are important to reduce systematic experimental effects due to experiment-internal antideuteron production, as the most abundant element in the path of an incoming particle for the AMS-02 experiment is carbon. This manuscript will focus on analysis of NA61/SHINE data on p+C thin target collisions in the context of light (anti)nuclei production. A preliminary analysis of experimental data and the particle identification method as well as current deuteron and antideuteron yields will be described.

Efficient ion acceleration driven by a Laguerre–Gaussian laser in near-critical-density plasma

Laser-driven ion accelerators have the advantages of compact size, high density, and short bunch duration over conventional accelerators. Nevertheless, it is still challenging to generate ion beams with quasi-monoenergetic peak and low divergence in experiments with the current ultrahigh intensity laser and thin target technologies. Here we propose a scheme that a Laguerre–Gaussian laser irradiates a near-critical-density (NCD) plasma to generate a quasi-monoenergetic and low-divergence proton beam. The Laguerre–Gaussian laser pulse in an NCD plasma excites a moving longitudinal electrostatic field with a large amplitude, and it maintains the inward bowl-shape for dozens of laser durations. This special distribution of the longitudinal electrostatic field can simultaneously accelerate and converge the protons. Our particle-in-cell (PIC) simulation shows that the efficient proton acceleration can be realized with the Laguerre–Gaussian laser intensity ranging from 3.9 × 1021 W⋅cm−2–1.6 × 1022 W⋅cm−2 available in the near future, e.g., a quasi-monoenergetic proton beam with peak energy ∼ 115 MeV and divergence angles less than 5° can be generated by a 5.3 × 1021 W⋅cm−2 pulse. This work could provide a reference for the high-quality ion beam generation with PWclass laser systems available recently.

Impact of supporting substrate on the measured neutron yields from natLi (p, n) system: A case study with natC and natTa stopping targets

Abstract The high energy neutron generation facilities worldwide uses the Li(p,n) system for production of the quasi mono-energetic neutrons from epithermal to few hundreds of MeV. Among the facilities either self-standing thick Li target or Li foil supported on a target assembly is used. In case of a supported Li target, either low or high mass supporting substrates like Al, C, Au, Ta etc. can be used. However, the neutron emission from the supporting substrate can interfere with the Li(p,n) reaction neutrons. In the present study neutron yields from a thin natural Li target supported on the thick natural Ta (high Z) and C (low Z) targets were measured at incident proton energies between 8-20 MeV. The proton irradiation of targets were performed at the Pelletron accelerator Facility, Mumbai and the emitted neutrons were measures at both forward (0°) and lateral (90°) directions with respect to the incident beam. The neutron yield was estimated using the CR-39 detectors and dual scintillator based active neutron spectrometer. A neutron dose equivalent (NDE) meter was also used to measure the neutron ambient dose equivalent at the measurement point. Experimentally measured neutron yields and ambient neutron dose equivalent estimates indicated higher Li(p,n) neutron yield with carbon as the supporting substrate. This is due to fewer self-emissions from natC compared to natTa target. Carbon supporting substrate also ensures improved spectral features of the Li(p,n) emission neutrons and lesser neutron fluence around the source location. This ensures efficient implementation of radiation protection practices with carbon target due to lower neutron fluence around the source compared to tantalum target. Carbon is a favorable supporting substrate compared to high Z- metal targets like Ta for emission neutron studies on Li target at proton energies between 8-20 MeV.

Extended characterization of alpha particles via laser-induced p-11B fusion reaction in silicon hydrogenated boron-doped thin targets

The nuclear fusion channel of the p-11B reaction producing α particles with multi-MeV kinetic energies was induced by a sub-nanosecond laser pulse focused onto 10 μm thick boron-doped thin targets at intensities of ∼ 1016 W/cm2. A full characterization in terms of α particle flux and angular distribution was performed thanks to the simultaneous use of several diagnostics (time-of-flight detectors, nuclear track detectors, and Thomson Parabola spectrometers), which enabled to measure key features of particles produced both in the backward (target front side) and forward (target rear side) directions. Maximum α particle flux and cut-off energy were recorded at small detection angles with respect to the target normal in the backward direction. The maximum kinetic energy shown by the α-particles produced in the nuclear fusion reaction was ascribable to a post-accelerating transient electric field present in the laser-generated plasma, in agreement with our previous preliminary results.

Novel approach to TNSA enhancement using multi-layered targets—a numerical study

In the context of ion acceleration driven by ultra-high contrast lasers using thin foils, there is a clear trend towards increasing ion energy when the target thickness is reduced. However when the target is too thin and the prepulse strength is not negligible, this trend is reversed due to degradation of the target mainly caused by prepulse-induced shocks, among other effects (thermal plasma expansion, early onset of transparency, etc). In this paper, we propose and motivate the use of multi-layered targets for the purpose of enhancing the target normal sheath acceleration mechanism by means of attenuating the shock waves inside the target. It is demonstrated through hydrodynamic simulations that multi-layered targets, composed of alternating layers of plastic and gold, can significantly delay the time of shock wave breakout, reducing the shock energy that breaks out of the target and shortening the plasma scale-length. This approach paves the way for enhanced laser-driven ion acceleration using thinner targets even for relatively low contrast lasers.

Overview of nuclear data production system at RAON

Nuclear Data Production System (NDPS), a fast neutron facility for nuclear science and applications, was constructed at the Rare Isotope Accelerator complex for ON-line experiments (RAON) in Korea. NDPS is designed to provide both white and quasi-monoenergetic neutrons using 98 MeV deuteron and 20 – 83 MeV proton beams with thick graphite and thin lithium targets, respectively. Neutron energy is determined by employing the Time-Of-Flight (TOF) technique, along with a pulsed deuteron (or proton) beam with a repetition rate of less than 200 kHz. Fast neutrons are produced in the target room and are guided to the TOF room through a 4 m long neutron collimator consisting of iron and 5% borated polyethylene. The neutron beam is monitored using a parallel plate avalanche counter (PPAC) and a micro-mesh gaseous (MICROMEGAS) detector installed in the TOF room, so as to measure the energy and the position of neutrons.

Influence of the attitude of cylindrical tungsten alloy fragment on the ability of penetrating thin metal target plate

Influence of the attitude of cylindrical tungsten alloy fragment on the ability of penetrating thin metal target plate

Dose voxel resolution determination for Monte Carlo electron beam simulation in thin targets.

Monte Carlo particle simulation has become the primary tool for designing low-energy miniature x-ray tubes due to the difficulties of physically prototyping these devices and characterizing their radiation fields. Accurate simulation of electronic interactions within their targets is necessary for modelling both photon production and heat transfer. Voxel-averaging can conceal hot spots in the target heat deposition profile that can threaten the integrity of the tube. This research seeks a computationally-efficient method of estimating voxel-averaging error in energy deposition simulations of electron beams penetrating thin targets to inform the appropriate scoring resolution for a desired accuracy level. An analytical model to estimate voxel-averaging along the target depth was developed and compared to results from Geant4 via its wrapper, TOPAS. A 200 keV planar electron beam was simulated to impinge tungsten targets of thicknesses between 1.5- and 12.5- . For each target, the model was used to calculate the energy deposition ratio between voxels of varying sizes centered on the longitudinal midpoint of the target. Model-calculated ratios were compared to simulation outputs to gauge the model's accuracy. Then, the model was used to approximate the error between the point value of electron energy deposition and a voxel-based measurement. The model underestimates error to within 5% for targets less than 7.5- in thickness with increasing error for greater thicknesses. For the 1.5- target, calculations of the point-vs.-voxel energy deposition show an 11% averaging effect between the midpoint and a 1.5- voxel. Energy deposition profiles along the target depth were also calculated in the Monte Carlo for reference. A simple analytical model was developed with reasonable accuracy to guide Monte Carlo users in estimating the appropriate depth-voxel size for thin-target x-ray tube simulations. This methodology can be adapted for other radiological contexts to increase robustness in point-value estimations. This article is protected by copyright. All rights reserved.

Optical Channeling of Low Energy Antiprotons in Thin Crystal Targets

A relevant aspect of the interactions between charged fermions and crystal targets is coherence, which can exist at both classical and quantum levels. In the case of antiprotons crossing crystal targets, there are theories and measurements of classical-level coherence effects, in particular, channeling effects. For the present study, we assume the existence of a low-energy regime where the electrostatic interactions between an antiproton and the crystal atoms lead to a local loss in the beam flux as their leading effect. We expect this assumption to be well-justified for antiproton (p¯) energies below 100 eV, with a progressive transition to a standard “Rutherford regime” in the energy range 100–1000 eV. Under these conditions, the target can be treated as an optical absorber with a periodical structure, which can be simplified by considering a multi-layer planar structure only. As in standard optics, wave absorption is accompanied by interference and diffraction. Assuming sub-nanometer ranges for the relevant parameters and a realistic angular spread for the antiproton beam, we find narrow-angle focusing effects that reproduce the classical channeling effect at a qualitative level. We also find that diffraction dominates over interference, although this may strongly depend on the target details.

Mechanics of ballistic impact with non-axisymmetric projectiles on thin aluminium targets. Part I: Failure mechanisms

The ballistic performance of thin aluminium targets under normal and oblique impacts with plate-like projectiles is investigated with emphasis on the local projectile-induced target response. Three projectiles of 5 mm thickness with decreasing bluntness were considered, with the ratio of the projectile’s equivalent diameter to the target’s thickness (deq/hT) within the range of 0 to 4.89. The obtained results suggest that defeat mechanisms, and target resistance measures were different from those observed in impact with equivalent axisymmetric projectiles, as a result of a high projectile’s cross-sectional aspect ratio. Projectiles with inclined nose sections inflicted a shear-dominant failure that was locally energetically expensive and depended on the product of length (LN) and half-angle (α) of the projectile’s nose. On the other hand, projectiles with blunt sections were associated with retardation of their penetration capacity due to dynamic effects, followed by a low-energy mechanism associated with membrane stretching and tensile failure of targets. A total of 48 experiments were performed at normal-impact conditions, to estimate the critical velocity of perforation/penetration and examine the real-time deformation patterns at sub-critical velocities, by employing the digital image correlation technique. Overall, the critical velocity showed a quadratic dependence on deq/hT, where the benefit of ballistic performance decreased with an increase in this ratio. The observed non-monotonic behaviour of the critical velocity with increasing impact obliquity in some cases and the distinction in failure mechanisms highlight the importance of the projectile’s geometrical parameters for the energy transfer mechanisms. Also, the lack of correlation between the critical velocity and the local work in the target defeat term suggests that the resultant energy transfer mechanisms considerably contributed to the dissipation of the projectile’s kinetic energy. Experimental data were utilised for calibrating the material model and separate experimental results for validating the numerical (finite-element) model. A total of 119 simulations were carried out for normal and oblique impact incidences to examine the role of the projectile’s (i) rotation, (ii) geometrical features, and (iii) obliquity on the target’s dynamic performance and induced defeat mechanisms. The conclusions of this study form the basis for considering the role of the projectile’s geometrical parameters on the energy transfer mechanisms in the target through statistical and semi-analytical approaches presented in the second part of this work.

Measuring the cross section of the N15(α,γ)19F reaction using a single-fluid bubble chamber

$^{15}$N($\alpha$,$\gamma$)$^{19}$F is believed to be the primary means of stellar nucleosynthesis of fluorine. Here we present the use of a single-fluid bubble chamber to measure the cross section of the time-inverse photo-dissociation reaction. The method benefits from a luminosity increase of several orders of magnitude due to the use of a thicker liquid target -- when compared to thin films or gas targets -- and from the reciprocity theorem. We discuss the results of an experiment at the Thomas Jefferson National Accelerator Facility, where the cross section of the photodisintegration process $^{19}$F($\gamma$, $\alpha$)$^{15}$N was measured by bombarding a superheated fluid of C$_3$F$_8$ with bremsstrahlung $\gamma$-rays produced by impinging a 4 - 5.5 MeV electron beam on a Cu radiator. From the photodissociation yield the cross section was extracted by performing a convolution with a Monte Carlo-generated $\gamma$-ray beam spectrum. The measurement produced a cross section that was then time inverted using the reciprocity theorem. The cross section for the $^{15}$N($\alpha$,$\gamma$)$^{19}$F reaction was determined down to a value in the range of hundreds of picobarns. With further improvements of the experimental setup the technique could potentially push cross section measurements down to the single picobarn range.

Bimetallic BaTiO3 and Cu nanoparticle synthesis via pulsed laser ablation in liquid of powder targets

Laser processing has become a viable method of synthesising bimetallic and biomolecule-free nanoparticles. Laser processing enable for different materials to be processed via the same laser system with ease, unlike chemical and biological methods that require special equipment/organisms for each material. The objective of this paper is to study the laser processing of powders for the fabrication of nanoparticles of various types, shapes, sizes and concentration. A Nd:YAG 1064 nm laser system was used to perform ablation processes of Cu and BaTiO3 powders under isopropyl alcohol, hydrochloric acid and ammonium buffer solution. This produced nanoparticles of various shapes including rods, spheres and pyramids and various sizes (14–280 nm). The effect of the laser processing parameters such as hatch spacing (10 or 30 μm) and scan speed (2 or 3 m/s) were examined. These processing parameters are seldom studied in the literature. Furthermore, this paper describes a way to produce nanoparticles from powder targets instead of the conventional rod, plate or thin film targets that are difficult to reuse. However, powders are compatible with a limited number of liquids than rods, plates and thin films and this issue is addressed herein.

Probing bulk electron temperature via x-ray emission in a solid density plasma

Bulk electron temperatures are calculated for thin Cu targets irradiated by the petawatt class Vulcan laser, from the Kα yield obtained using highly oriented pyrolytic graphite crystals. Cu-Kα emission studies have been used to probe the bulk electron temperature. A 30–80 eV core temperature extends homogeneously over distances up to ten times the laser focal spot size. Energy shifting has been observed due to different ionization states produced for different temperatures in the plasma. Polarization dependencies of plasma temperature are observed through the production of x-rays in different targets. 2D PIC simulations were performed to measure the polarization dependency of bulk electron temperature, which supports our experimental results. This paper could be of importance in understanding the different behavior of laser coupling at different polarizations and their role in x-ray production.

Evaluation of Absorbed Energy in High-Velocity Particle Impact for Aero-Engine Materials

A single particle impact testing system was developed to estimate an absorbed energy in particle perforation process of aero-engine materials. Thin target plates of 0.1-1 mm thickness were impacted by a 3 mm diameter spherical particle. A wide range of impact velocities from 50-375 m/s was covered using a compressed gas gun. First, particle impact tests were conducted on aluminum and copper plates to evaluate the validity of the experimental system. The experimental results show that there is no significant difference in the effect of the particle impact velocity on the absorbed energy of aluminum and copper, and the ballistic limit can be reasonably estimated based on a simple energy balance equation. Second, impact tests were conducted on TiAl alloys and Ni-based alloys. There was no clear ballistic limit for the TiAl alloys, and the absorbed energy tended to increase as the particle impact velocity increased. The absorbed energy of the TiAl alloy was half that of the Ni-based alloy for specimens of the same thickness. Radial and petal-shaped cracks were observed in the Ni-based alloys after impact tests, while brittle cracks, fractures, and scattering of debris were observed at the periphery of the penetration hole in the TiAl alloys.

Generation of gold and silver nanoparticles using laser ablation of thin bimetallic films and bulk targets in water

Generation of gold and silver nanoparticles by laser ablation in water without the addition of surfactants is very attractive due to the purity of such nanoparticles. However, such nanoparticles face the problem of long-term stability, which severely limits their applicability. Here, we demonstrate the generation of hybrid gold-silver nanoparticles using the laser ablation of thin-film composites on a glass substrate in water, and compare with the laser fabrication of nanoparticles by using bulk targets of silver and gold. Thin-film method allows the formation of stable hybrid Au–Ag nanoparticles in distilled water without any stabilizers and additional ligands with an opportunity to regulate the nanoparticle composition using different metal ratios, layer order, and thickness of films. The stability and optical properties of the hybrid nanoparticles depend on the used laser pulse energy. This study demonstrates that the thin film method produces nanoparticles with higher stability compared to the bulk target method. These outcomes are encouraging for the creation of better-performing, stable, and clean nanoparticles that can be used in a variety of applications in biomedicine, sensors, integrated circuits, filters, batteries, covid-19 tests, solar cells, dual-energy mammography (DEM), computed tomography (CT), and other components of optical and electronic devices.

Diagnosis of ultrafast surface dynamics of thin foil targets irradiated by intense laser pulses.

The temporal modulation of an electron bunch train accelerated from a foil target irradiated by an intense laser pulse is studied by measuring the coherent transition radiation (CTR) from the rear surface of a target. We experimentally obtained CTR spectra from a 1 µm thick foil target irradiated at a maximum intensity of 6.5 × 1019 W/cm2. Spectral redshifts of the emitted radiation corresponding to increases in laser intensity were observed. These measurements were compared with the theoretical calculation of CTR spectra considering ultrafast surface dynamics, such as plasma surface oscillation and relativistically induced transparency. Plasma surface oscillations induce a spectral redshift, while relativistic transparency causes a spectral blueshift. Both effects are required to find reasonable agreement with the experiment over the entire range of laser intensities.

Addressing key aspects of J × B driven MeV fast electron generation in ultra-short ultra-intense laser foil interaction

Applicability of J × B mechanism of MeV fast electron generation is clearly demonstrated through observation of directed electron beam along laser propagation direction (simultaneous emission also along laser polarization/transverse directions) in the interaction of ∼25 fs laser pulse with thin foil target at an intensity of 1–7 × 1019 W/cm2. Fast electron temperature is found to be lower (higher) than ponderomotive for shorter (longer) preplasma scale lengths. Role of pre-acceleration of electrons in the rising part of the laser pulse is suggested for observed efficient J × B acceleration for p-polarization case. 2D particle in cell simulation also supports the above-mentioned observations.

Evidence for WZW anomaly in the coherent reaction K + Cu → K + π 0 Cu

On the statistics of ∼ 8·109 positively charged kaons passing through a thin copper target a coherent production of the K + π 0 - pair is studied. Contributions of Coulomb, strong interactions and their interference to the total cross section are determined in the region of K*(892). In the distribution over K + π 0 invariant mass, an effect is seen, which can be explained by the interference of the chiral anomaly and the K* (892) amplitudes. From that, the ratio of the measured chiral anomaly amplitude and the theoretical one is obtained: Aexp/Ath = 0.9 ± 0.24 (stat.)±0.3 (syst.).