Magnesium Target Research Articles

Emission spectra of a magnesium plasma created in vacuum by short laser pulses

An investigation was made of the emission spectra of a plasma formed on the surface of a magnesium barrier in a vacuum by nanosecond laser pulses of wavelengths 1.06 and 0.532 μm. The intensity of the radiation incident on the magnesium target was (1–8)×1012 W cm-2. Photographic and photoelectric methods were used to record the emission spectra in the visible and near-ultraviolet parts of the spectrum. The ratio of the intensities of the ionic magnesium lines at λ= 448.1, 292.8, and 293.6 nm was used to determine the electron temperature of the plasma during the cooling stage (20–50 ns after the beginning of irradiation). The measurements yielded kTe > 2 eV near the target, but at distances exceeding 3 μm from the target the corresponding value was kTe ≳ 1 eV. Broadening of the ionic line of magnesium with λ = 448.1 nm was used to determine he maximum electron density (during the existence of the plasma); at 0.5–1 mm from the target it was Ne ≈ 1018 cm-3. Closer to the target only the continuum was emitted. The results were compared with the parameters of a magnesium plasma determined earlier on irradiation of a target with quasi-cw neodymium laser radiation of relatively low intensity of 106—107 W cm-2.

Gas dynamics of impulsive jets and pressure oscillations of a laser-irradiated target

GAS DYNAMICS OF IMPULSIVE JETS AND PRESSURE OSCILLATIONS OF A LASER-IRRADIATED TARGET N. M. Bulgakova and L. I. Kuznetsov UDC 533.6.011:535.211 I. Introduction. The interaction of an intense millisecond pulse of laser radiation (LR) with a solid target can be accompanied by pressure oscillations, recorded by a piezoelec- tric transducer on the backside of the irradiated target, with frequencies of the order of i0 kHz [i]. This phenomenon occurs on a lead target with LR intensity of about 2 MW/cm 2. In [2] it is shown that pressure oscillations are observed on different dielectric and metal- lic targets in a wide range of LR energies and is accompanied by intense vaporization of the material in the irradiation spot. Possible mechanisms for the appearance of these oscilla- tions are suggested in [3, 4]: bursts of absorption in the plasma in an unstable vaporiza- tion regime or auto-oscillatory regime of self-screening of the LR by the products of erosion. Probe measurements through a narrow channel in the target under high pressures of the sur- rounding medium have shown that oscillations of absorption of the LR in the photoerosion flame are observed in the tail part of the oscillograms in some irradiation regimes [5]. These absorption oscillations can be interpreted in a manner supporting the above-mentioned mechanism of the pressure oscillations on iradiated targets [3]. However, investigations of the attenuation of radiation in the photoerosion flame of ebonite and magnesium targets in air with LR intensities such that pressure oscillations should be observed indicate that there are no pronounced pulsations of the absorption coeffi- cient for laser radiation through the photoerosion flame [6]. Finally, simultaneous measure- ments of the pressure on the target and LR attenuation by the photoerosion flame [7] have shown that the LR transmission curves are quite smooth, and the small nonmonotonic variations on these curves are not related with the pressure oscillations on the target. These investi- gations, as well as measurements performed with different irradiation configurations [8] and change in sign of the pressure vector [2] have made it possible to propose a gas-dynamic mechanism for the appearance of the pressure oscillations on irradiated targets. In the present work, in order to check this mechanism we performed numerical modeling of the gas dynamics of an erosion flame and compared the computational and experimental re- suits in a wide range of irradiation intensities. 2. Experimental Procedure. Detailed experimental investigations of the pressure on irradiated targets were performed on a VIKA vacuum chamber [9]. The 1.06 ~m laser pulses with a width of 0.3 msec at half-height was focused on the target through a long focal length lens. The diameter of the irradiation spot ranged from 5 to 20 mm, and the target had a diam- eter of 20 man. The target was placed on a piezoelectric transducer [i0], which measured the pressure on the irradiated target. A coaxial FK-19 photocell was used to record the initial laser radiation from a beam splitter placed at the output of the laser system. The signals from the piezoelectric trans- ducer and from the FK-19 photocell were fed into an $9-8 two-channel digital oscillograph. The data were transmitted from the oscillograph to an IVK-6 measurement-computing system through a KOP interface. Processing of the signals from the piezoelectric transducer on the IVK-6 system consisted of dividing the area under the oscillogram into 10-~sec long elemen- tary sections and normalizing these elementary areas Pi twice - with respect to both the total area EP i under the oscillogram and the energy E 0, normalized in the same manner, of the initial laser radiation: ~ =

Cross sections of neon isotopes induced by 5, 7, 16, and 19 MeV neutrons

Neutron-irradiated targets of natural magnesium have been analyzed by mass spectrometry in order to determine the excitation functions of neon from the energy threshold up to 20 MeV. Production cross sections of {sup 20,21,22}Ne isotopes were measured at 5.20, 7.00, 16.20, and 19.05 MeV.

Role of particles of target material in the dynamics of plasma formation

An automated system was used to study the optical characteristics of erosion jets formed by the interaction of neodymium laser radiation of 100–1500 J pulse energy with targets of zinc, lead, magnesium, cadmium, and bismuth. Low-threshold bursts of absorption were initiated by liquid droplet particles of the target material. The range of neodymium laser energies in which these particles exerted a strong influence on the optical characteristics of the erosion jets was determined.

Oxygen-rich polycrystalline magnesium oxide—A high quality thin-film dielectric

Sputtering of a magnesium target with a beam of argon ions in the presence of a partial pressure of reactive oxygen gas has been found to yield smooth MgOx thin-film dielectrics with low electrical loss, good mechanical stability, and excellent reproducibility. The films consist of polycrystalline MgO with a grain size ≲50 Å and an OH-containing component. The thickness dependence of the areal capacitance is discussed in the context of a two-layer model in which this oxygen-rich phase (x≂1.4) overlays a thin (≲40 Å) stoichiometric phase (x≂1.0). Observation of superconducting tunneling characteristics in trilayer Au-MgOx-Pb structures confirms pinhole-free coverage which may have possible application as artificial tunnel barriers.

Electron capture by protons and helium ions from lithium, sodium, and magnesium.

Cross sections for single-electron capture by protons and single- and double-electron capture by helium ions from atomic lithium, sodium, and magnesium targets are presented. The absolute cross sections were measured by using the growth-curve method. Impact energies are in the range 2--100 kV q, where q is the projectile charge state. The present data are compared with previous experimental and theoretical work.

The 26Mg( 18O, 17F) 27Na and 26Mg( 18O, 15O) 29Mg reactions and the level schemes of 27Na and 29Mg

The 26Mg( 18O, 17F) 27Na and 26Mg( 18O, 15O) 29Mg reactions have been used to determine the ground-state masses and partial level schemes of 27Na and 29Mg. Values of −5514 ± 60 keV and −10600 ± 45 keV were determined for the mass excesses of 27Na and 29Mg, respectively. The probable structure of some of the levels observed was deduced from a comparison with shell-model calculations and with previous experimental information. A number of other 18O- and 16O-induced reactions on magnesium targets were also studied in order to obtain information on the reaction mechanisms of the ( 18O, 17F) and ( 18O, 15O) reactions.

Spectroscopic-factor discrepancies in ( 9Be, 10B) for different ejectile excitations

Angular distributions have been measured for the ( 9Be, 10B[3 +, 0.0]) and ( 9Be, 10B[1 +, 0.72]) reactions on 63Cu, 54Fe, 26, 24Mg and 16O at 43 MeV and on 40Ca at 45 and 30 MeV. EFR-DWBA calculations which used optical potentials obtained from the measured elastic scattering of 9Be from 26Mg, 16O and 40Ca and 10B from 25Mg and 39K have generally reproduced well the shape of the cross sections. Initially, a standard prescription was used in the form-factor calculation and Cohen and Kurath spectroscopic factors were assumed for 9Be- 10B. Inconsistencies, concerning whether 10B was left in its ground or first excited state, of up to 60% were found between the extracted target spectroscopic factors. This anomaly was insensitive to changes in either optical-potential or bound-state parameters. A simple calculation of a two-step process could not account for the anomaly. Use of modified spectroscopic factors for the 9Be- 10B system essentially removed the discrepancies except for the magnesium targets, and an improved form-factor description further alleviated the anomaly. This form-factor method uses a shell-model potential in conjunction with a surface-peaked potential, the depth of which is adjusted to give the correct asymptotic form to the wave function.

Study of atomic excitations in sputtering with targets partially covered with oxygen

We bombarded pure elemental targets of beryllium, boron, magnesium, aluminium, silicon, titanium and gold with 80 keV Ar + ions and studied the excitation of sputtered atoms or ions under ultrahigh vacuum conditions as well as with oxygen present at the target surface. The measurements on magnesium, aluminium, silicon and titanium were made at projectile incidence angles from 0° to 85°. Excitation probabilities for gold were found to be very little influenced by oxygen, but for beryllium, boron, magnesium, aluminium, silicon and titanium the excitation probabilities were in many, but not all, cases found to depend strongly on the oxygen pressure as well as on the beam current density. This indicates that the excitation mechanism is markedly dependent on the initial electronic conditions in the solid.

Study of atomic excitation in sputtering as a function of the projectile incidence angle

Solid targets of beryllium, boron and magnesium have been bombarded with 80 keV Ar + ions. Intensities of photons emitted from sputtered particles have been measured as functions of the angle of incidence of the projectile. Generally, the excitation intensities increase with increasing angle of incidence, but a few levels show much stronger angular variations than the majority of levels do. The results are discussed. It is concluded that whereas the common increase in excitation is caused by the general increase in the number of swift, sputtered atoms, a few levels are in addition selectively excited through molecular-orbital electron-promotion processes taking place in binary collisions.

Study of secondary electron emission from Be, B, Mg, and Au as a function of the projectile incidence angle

Solid targets of elemental boron, beryllium, magnesium and gold have been bombarded with 80 keV Ar + ions. In addition, gold was also bombarded with 80 keV Xe + ions. The emission of secondary electrons has been measured as a function of the incidence angle of the projectile. The results are discussed.

Study of atomic excitation in sputtering as a function of the projectile incidence angle

Solid targets of elemental beryllium, boron, magnesium, and gold have been bombarded with 80-keV ${\mathrm{Ar}}^{+}$ ions. Intensities of photons emitted from sputtered particles have been measured as functions of the angle of incidence of the projectile. Generally, the excitation intensities increase with increasing angle of incidence, but a few levels show much stronger angular variations than the majority of levels do. The results are discussed. It is concluded that whereas the common increase in excitation is caused by the general increase in the number of swift, sputtered atoms, a few levels are in addition selectively excited through molecular-orbital electron-promotion processes taking place in binary collisions.

Evidence for the existence of a low-mass pseudoscalar meson

The existence of a low-mass pseudoscalar meson,phion (φ), is indirectly confirmed by the observation of the 15 hour24Na activity from irradiating a magnesium target by 11.5 MeV electrons. It is suggested the24Na activity results from the reaction24Mg(e, e’φ+)24Na, the cross-section for which is measured to be 4.1× l0-37cm2 at 11.5 MeV. The phion had previously been hypothesized as a constituent of the neutron, subsequently shown to be involved in the np interaction to bind the deuteron and in explaining the reason for the missing solar neutrinos. The results of Reines, Sobel and Pasierb in their recent reactor neutrino experiment, which they interpreted as evidence for neutrino oscillation, can also be explained by considering the effects of phions. The phion has existed only in theory, but if it does exist in reality, as the work described here indicates, many long-standing, and even recent problems in physios will now be explained.

The fabrication of both n-type and p-type GaAs thin films deposited by troide sputtering

Thin films of GaAs were deposited with a triode sputtering system onto single-crystal GaAs semi-insulating substrates. The films were doped to various densities by co-sputtering a silicon or magnesium target with an adjustable duty cycle. The electrical resistivity and Hall effect were measured by the Van der Pauw technique to obtain the carrier concentration and mobility as functions of temperature. It was determined that the dopant density was controlled by the duty cycle of the bias voltage on the dopant target. The resistivity of the films at room temperature ranged from 0.036 to 20.8 × 10 3 ohmcm. The variation of the carrier concentration as a function of temperature was found to be well described by a merged conduction-dopant band model. The electron mobility of the n-type films at room temperature ranged from 81 to 261 cm 2 V −1 s −1. The most important mobility limitations were found to be grain boundary scattering in all films and ionized impurity scattering in the most highly doped films. The average grain size was found from the grain boundary scattering analysis to be about 50–100 Å and this size was confirmed by reflection electron diffraction and electron-beam-induced current analysis.

Determination of the electron temperature of a laser plasma from continuous recombination radiation of multiply charged ions

A description is given of a high-luminosity x-ray spectrograph with vertical focusing. This instrument was used to record the continuous recombination spectrum of the hydrogen-like Mg XII ion. The main advantage of the system employed was a combination of a high luminosity and a wide viewing angle. The continuous spectrum of a laser plasma was used to determine its electron temperature. When the power density of the laser flux reaching a flat magnesium target was q ~ 5×1014 W/cm2, the temperature of the laser plasma was found to be 330 ± 60 eV.

The Electron Spectrum at Equilibrium Depth from a Narrow Beam of 60 Co Photons

Electron spectra from 40 to 1100 keV for Plexiglas, Bakelite, graphite, Teflon, magnesium, and aluminum targets were measured with a lithium-drifted silicon detector and multichannel pulse-height analyzer. The electrons were produced by a narrow beam of60 Co photons and sampled at equilibrium depth. The materials all produced similar spectral shapes which peak at about 700 keV and show secondary electron contributions below 100 keV. The spectral shapes are quite different from those calculated using Spencer-Attix techniques.

Nuclear Thomson scattering of 5.5–7.2 MeV photons

The elastic differential scattering cross section from carbon and magnesium targets was measured at θ = 140° using monoenergetic photons in the range 5.5–7.2 MeV. The contribution of nuclear Thomson scattering is shown to be dominant. The results are in good agreement with predicted values.

Investigation of the Early Phases of Plasma Production by Laser Irradiation of Plane Solid Targets

The time development of line radiation emitted from a plasma produced by focusing a Q-switched ruby laser onto a solid magnesium target has been investigated in the vacuum-ultraviolet wavelength region using a 2-m grazing incidence monochromator. The electron temperature was obtained using the foil-absorption method. The time of appearance of MgIX and MgX lines at the target was measured as a function of the laser power. For the lowest laser power at which both ionic lines appeared, MgIX radiated before MgX. The appearance time of each line decreased as laser power was increased until at the higher powers they appeared at about the same time-2 nsec after the beginning of the laser pulse. The ionization processes occurring in front of the target are discussed using a simple model for the plasma. From the time of appearance of the lines, conclusions on the electron temperature in the ``heating zone'' can be drawn as well as on an over-all heating rate.

Investigation of charge exchange in a dense plasma flow from a magnesium target. (USSR): K B Kartashev et al, Zh Eksper Teor Fiz, 59 (3), Sept 1970, 779–784 (in Russian)

We consider the motion of a particle in a plane under the gravitational action of 3 fixed centers (the 3-center planar problem). As it is well known (Vestnik Moskov. Univ. Ser. 1 Matem. Mekh 6 (1984) 65; Prikl. Matem. i Mekhan. 48 (1984) 356; Classical Planar Scattering by Coulombic Potentials, Lecture Notes in Physics, Springer, Berlin, 1992) on the non-negative level sets of the energy E there do not exist non-constant analytic first integrals, and moreover the system has chaotic trajectories. These results were proved by variational methods.Here we investigate the problem in the domain of small negative values of E. Moreover, we assume that one of the centers is far away from the other two. Then we get a two-parameter singular perturbation of an integrable dynamical system: the 2-center problem on the zero-energy level. The main problem we deal with is to prove that the Poincaré–Melnikov theory applies in the limit case E→0.

Inelastic scattering of 100 MeV protons from Mg and Si using a Ge(Li) total absorption proton counter

A cooled Ge(Li) detector was used as the total absorption proton counter for the energy analysis of 100 MeV protons scattered from targets of enriched magnesium (99.7 % 24Mg) and silicon (natural isotopic composition). A total energy resolution of 400 keV (FWHM) was achieved; it is comparable to the best resolution presently attainable with magnetic spectrometers in the 100–200 MeV energy region. The measurements cover the 10°–60° lab angular region; and angular distributions are presented for 23 peaks in the energy spectra. A qualitative discussion of the excited states which correspond to the observed peaks is based mainly on the shapes of the distributions. Comparisons are made with results from similar experiments at 55 MeV and 185 MeV. Shapes of angular distributions for strongly excited 2+, 3− and 0+ transitions are independent of energy, but significant differences exist for 4+ transitions. The excitation of a previously unknown level in 28Si at 11.45±0.12 MeV energy has been observed. The level most probably has Jπ = 5−.