Thin Aluminum Targets Research Articles

Proton beam generation by ultra-high intensity laser–solid interaction

We report on some recent experimental results on proton production from ultra-intense laser pulse interaction with thin aluminium and plastic foil targets. These results were obtained at Laboratoire d'Optique Appliquée with the 100 TW ‘salle jaune’ laser system, delivering 35 fs laser pulses at 0.8 μm, reaching a maximum intensity on target of a few 1019 W/cm2. In such extreme interaction conditions, an intense and collimated relativistic electron current is injected from the plasma created on the laser focal spot into the cold interior of the target. Its transport through dense matter, ruled by both collisions and self-induced (electro-magnetic) field effects, is the driving mechanism for proton acceleration from the rear side of thin foils: when reaching and leaving the foil rear-side, the fast electrons create a large charge separation and a huge electrostatic field with a maximum value of few TV/m, capable of accelerating protons. A parametric study as a function of the laser driver and target parameters indicates an optimal value for target thickness, which strongly depends on the laser prepulse duration. In our experiments, we did irradiate targets of various materials (CH, Al, Au) changing the prepulse duration by using fast Pockels cells in the laser chain. CR-39 nuclear track detectors with Al filters of different thickness and a Thomson parabola were used to detect proton generation. The best results were obtained for 2 μm Al targets, leading to the generation of proton energies with energies up to 12 MeV.

Measurement of laser driven shock wave transit time through thin aluminium targets by optical shadowgraphy

Laser driven shock wave transit time in thin aluminium targets was experimentally estimated by determining the shock emergence time at the rear of thin aluminium foils of varying thickness from 5 to 35 μm. A 20 J, 5 ns Nd:glass laser was focused to produce laser intensity of 1012 to 5 × 1013 W/cm2 on the targets which were placed in vacuum. Target foil movement was measured to an accuracy of 10 μm using optical shadowgraphy technique. This technique was used to accurately measure the shock transit time by recording the optical shadowgrams at various instants of time and thus identify the instant at which the foil is just set into motion. Shock transit time measured in foils of different thickness can give the value of shock velocity at a given laser intensity. Target motion recorded by shadowgraphy can also give the target foil velocity from which shock pressure can be estimated. Experimental values of shock transit time, shock velocity and shock pressure were observed to agree well with the values using one-dimensional multi-group radiation hydrodynamic simulations.

HARP measurements of pion yield for neutrino experiment

The HARP experiment at CERN is performing extensive measurements of hadron production cross section and secondary particle yields in the momentum range of from 1.5 to 15 GeV/ c over full solid angle, using a large set of cryogenic and solid targets. This paper reports the first measurements of the momentum and angular distributions for pion production in the forward region using an 20mm thin aluminum target and a 12.9 GeV/ c proton beam, which are, respectively, the same target material and beam momentum as those used in K2K experiment.

Hypervelocity impact of spaced plates by a mock kill vehicle

In support of the National Missile Defense (NMD) program, a series of Light Gas Gun (LGG) lethality tests were conducted at the Arnold Engineering Development Center (AEDC). A new projectile was designed for this test series that would be representative of aspects of a generic NMD system kill vehicle. A series of projectile development tests were performed during the design phase of the projectile. This paper reports the results from the second development shot, in which the projectile impacted normally against two thin aluminum target plates, spaced approximately 5.5 diameters apart. Results reported include the documentation of the damage to the first and second plates, the debris generated behind the first plate, and correlation of these with analytical and numerical predictions. Hydrocodes used for analyses included ALE3D, run by the Lawrence Livermore National Laboratory, and CTH, run by the Sandia National Laboratory. The purpose of the hydrocode analyses was to help in assessing the ability of these codes to predict the debris formation process and the target damage.

Ultrafast measurement of the optical properties of aluminum during shock-wave breakout

We have used high-resolution frequency domain interferometry to make ultrafast measurements of shock-induced changes in the optical properties of thin aluminum targets. Measurements with an 800 nm probe wavelength found an unexpected phase shift during a 4.65 GPa shock breakout. Further measurements at 400 nm associate this phase shift with the pressure dependence of the 1.5 eV interband transition in aluminum. Data taken at several angles of incidence allowed the separation of optical from material motion effects, yielding an effective complex index for the shocked material.

Doppler broadening of in-flight positron annihilation radiation due to electron momentum.

We report the first observation of electron momentum contributions to the Doppler broadening of radiation produced by in-flight two-photon annihilation in solids. In these experiments an approximately 2.5 MeV positron beam impinged on thin polyethylene, aluminum, and gold targets. Since energetic positrons easily penetrate the nuclear Coulomb potential and do not cause a strong charge polarization, the experimental annihilation line shapes agree well with calculations based on a simple independent-particle model. Moreover, annihilations with the deepest core electrons are greatly enhanced.

In flight positron annihilation in thin targets

We report on the first direct energy resolved measurements of two quantum annihilation in flight (TQAF) using monoenergetic positrons below 100 keV. A monoenergetic positron beam at Washington State University with variable kinetic energy up to 70 keV was guided onto thin aluminum targets. To clearly identify in flight positron annihilation a two dimensional Doppler broadening technique was employed utilizing two HPGe detectors. The method uniquely identifies TQAF by providing the total energy of the annihilating electron-positron pair. A basic understanding of the fundamental TQAF process has been obtained using this high resolution technique. With this fundamental understanding we believe several new avenues of experimental exploration are possible.

A Rutherford scattering diagnostic for intense pulsed proton beams transported in low-pressure gases

Rutherford scattering of pulsed 1 MeV proton beams is developed as a time-resolved fluence diagnostic for proton transport experiments. The incident beam is scattered from a thin aluminum target and recorded with a silicon PIN detector. For a pinched-beam diode, fluences of protons transported in 1 Torr air are consistent with fluences determined with calorimeters, provided the proton energy is smaller than expected from the measured ion current and voltage. Beam bunching is observed as the flight path increases, consistent with reduced proton energy at the head of the beam.

An analytical model for tumbling projectile perforation of thin aluminum plates

An analytical model for the perforation of thin aluminum targets by tumbling cylindrical projectiles was developed. The target material was considered to be rigid—perfectly plastic without strain hardening, while the projectile was treated as undeformable. The perforation process was experimentally found to consist of three stages: plugging, hole enlargement, and front petaling. Both conservation of energy and conservation of momentum laws were used for modeling the plugging stage, while a lower bound method was employed during the hole enlargement stage. The energy dissipated during the petaling stage consists of shearing fracture of the petal, localized plastic shear in a zone contiguous with the edges, the momentum of the petal and the bending energy of the petal. The analytical results provided generally good agreement with the corresponding experimental data in terms of the final velocity and final oblique angle of the projectile as well as the crater length of the target.

Normal impact of blunt projectiles on moving targets: experimental study

An experimental investigation was performed to study impact on targets moving at right angles to the striker trajectory with a velocity of 40 m/s. Tests were executed with a compressed gas gun using 6.35 mm diameter, 38.1 mm long hard steel blunt-nosed cylindrical strikers in the velcity range 49–213 m/s and with a powder gun using a blunt 9.53 mm diameter cylindrical projectile with an aspect ratio of 2 up to velocities of 492 m/s. The post-impact projectile trajectory and attitute as well as the configuration of the craters produced in thin targets of cold-rolled steel and soft aluminum were observed. The impact process produced asymmetric plugging followed by either front or side petaling. The latter phenomenon was found to be controlled by the ultimate tensile strain of the target and the ratio of the projectile to target speed. The test parameters and resulting data were primarily selected to permit comparison with the predictions of an analytical model of the process described in a companion paper.

The determination of electron momentum densities by inelastic scattering gamma ray-electron coincidence measurements: The (γ, eγ) experiment

Measurements have been made of the recoiling electron in 320 keV gamma ray inelastic scattering collisions in thin aluminium targets. The angular correlation of these electrons detected in coincidence with the scattered photon is in agreement with the kinematic requirements of the Compton effect and is correctly predicted by Monte Carlo simulations based on the impulse approximation. Further simulations of ideal-geometry experiments indicate that information about the initial electron momenta is available from an examination of those electron-photon events originating in a surface layer of one electronic mean free path depth and that elastic scattering of the recoil electrons from greater depths produces a nearly flat background to this signal. The results clearly demonstrate the feasibility of the (γ, eγ) experiment for studying electron momentum densities with synchrotron radiation.

Calorimetric study of laser-irradiated thin foil targets

Hydrodynmaic efficiency of laser-irradiated thin aluminum and gold-coated aluminum targets was experimentally determined using a specially designed cone calorimeter. Velocity of the accelerated target and ablation pressure were also estimated from the experimental data. The laser irradiance range used in the experiments was between 1012 and 1013 watts/cm2. Experiments indicate that the fall in the hydrodynamic efficiency due to gold coating on aluminum target is about 12% at an irradiance of 8 × 1012 W/cm2.

Inelastic scattering of 120 keV electrons from carbon, aluminum, vanadium and silver foils

Data are presented for absolute inelastic electron scattering cross sections from thin targets of carbon, aluminum, vanadium and silver at 45° and 60° for an incident electron energy of 120 keV. The data are compared with the theory of Cooper and Kolbenstvedt and with previous experimental data.

Application du modèle théorique à l'étude des pertes d'énergie caractéristiques des électrons transmis et rétrodiffusés dans le cas de l'aluminium

This paper deals with a theoretical model concerning transmission and backscattering of electrons by thin self-supported aluminium targets. This model is based on the Boltzmann equation, and our studies are limited to low energy losses; we show the existence of plasmon peaks in the energy distributions of electrons. The effects of physical parameters appearing during interactions with the jellium are analysed, and the sensitivity of our model is checked.

Alpha‐particle‐induced, inner‐shell ionization measurements for the undergraduate laboratory

The interaction of α particles with low atomic number elements has been studied in an attempt to determine the inner‐shell ionization cross section. These cross sections were determined by measuring the characteristic x rays that were produced when a thin aluminum target was bombarded by α particles from a 48‐MBq 224Cm α source. The x rays were detected with a Si(Li) x‐ray detector whose resolution was 190 eV at 5.9 keV. The efficiency of the detector was determined from standard radioactive sources. The energies of the α particles were varied by placing thin Mylar absorbers between the α source and the target. The results are favorably compared to a current ionization theory given in such a form that an undergraduate physics major can do the calculations. This experiment is intended as an undergraduate laboratory experiment in the atomic or nuclear area.

Evidence for plasma jet formation in long-pulse laser irradiation of thin planar Al targets

Thin planar aluminum targets were irradiated with 8-nsec-duration low-intensity (∼3×1013 W/cm2) glass laser pulses in order to analyze the plasma expansion from both the front and back-target surfaces. An array of vacuum-ion calorimeters and Faraday cup charge collectors was used to determine the conversion from incident laser energy to plasma kinetic energy and to determine the ion mass balance of the laser-produced plasma. Our experimental results do not agree with the ablation structure predicted by simple models or 1-D numerical simulations. The experimental results and 2-D calculations indicate that a jet of plasma forms on the back target surface and ablative acceleration of these thin-foil targets does not occur.

Atomic-field bremsstrahlung from 50-140-keV electrons

Experimental data are presented for the absolute cross section for atomic-field bremsstrahlung produced by 50-, 100-, and 140-keV electron bombardment of thin targets of aluminum, copper, silver, and gold at photon angles of 30\ifmmode^\circ\else\textdegree\fi{}, 45\ifmmode^\circ\else\textdegree\fi{}, 60\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, and 135\ifmmode^\circ\else\textdegree\fi{}. The data are compared with the theory of Pratt et al. and generally found to be in very good agreement, although there are some small discrepancies between theory and experiment.

Effective beam energy spreads with molecular ion beams

For several reasons it is sometimes desirable or necessary to use molecular hydrogen beams to produce nuclear reactions for materials analysis purposes. This paper presents the results of a study of the detailed characteristics of (p, γ) resonance yield curves obtained with molecular beams. Experimental gamma-ray yield curves have been measured (1) for the H 2 + beam on a thick aluminum target (992 keV resonance) and (2) for the H 1 0 beam (stripped from the H 2 + beam in a gas cell) on a thin aluminum target. Each of these yield curves has been calculated in detail from first principles and with no free parameters. The agreement between theoretical and experimental curves is excellent, leading to confidence that one can correctly interpret the peculiar features of yield curves from the H 2 + beam and from its stripped components.

Bremsstrahlung induced by proton andHe3-ion bombardments in the 1-4-MeV/amu energy range

Continuous x rays from a thin aluminum target produced by proton and $sup 3$He-ion bombardments have been measured with a Si(Li) detector over the impact-energy ranges 1--4 MeV and 3--9 MeV, respectively. The experimental results obtained were compared with theoretical predictions based on the binary- encounter approximation. Agreements are obtained both in the shape of spectra and absolute cross sections without any adjustable parameter over the entire energy range measured, but generally the theoretical calculation predicts smaller cross sections than the experimental results as the photon energy becomes higher and the bombarding energy gets lower. Energy-dependent directional anisotropy of the bremsstrahlung was theoretically estimated and compared with the experimental results measured at directions 45degree, 90degree, and 135degree with respect to the incident beam, showing qualitative agreement. (AIP)

K- andL-shell ionization by energetic lithium beams

We have measured target $K$ x-ray production cross sections for lithium projectiles incident on thin aluminum, scandium, copper, and tin targets at energies from 4 to 34 MeV. The Sn $L$ x-ray cross sections for those energies have also been obtained. In addition, similar measurements for protons incident at velocities matching those of the lithium projectiles have been made. The absolute magnitudes of the lithium ionization cross sections as deduced from the above results are well predicted by the binary-encounter approximation. Substantial deviations downward occur in the Sc and Cu cases for projectile velocities below one-half of the $L$-shell electron orbital velocities. These decreases in the cross sections, which are also evident in the data when scaled by the corresponding proton values, are examined in light of a previous treatment of increased binding effects.