Increasing Beam Energy Research Articles

Measurement of TVLs in lead for 4, 6 and 10 MV bremsstrahlung x-ray beams at scattering angles between 30 degrees and 135 degrees.

Measurements have been made of the transmission factors through lead for scattered radiation produced by 4, 6, and 10 MV bremsstrahlung x-ray beams incident on a polystyrene phantom; these measurements cover the range of scattering angles between 30 degrees and 135 degrees. The results show that the tenth value layer (TVL) for scattered radiation decreases sharply with increasing scattering angle and increases slightly with increasing beam energy, although at large scattering angles there is little energy dependence. TVLs range from 3.5 cm to 0.3 cm for 4 MV at scattering angles between 30 degrees and 135 degrees, from 3.8 cm to 0.6 cm for 6 MV, and from 4.2 cm to 0.7 cm for 10 MV, respectively, at scattering angles between 30 degrees and 120 degrees. Monte Carlo calculations, performed at 4 MV to simulate the transmission of scattered radiation, are in good agreement with the experimental measurements.

Lambda production in central Pb + Pb collisions at CERN-SPS energies

In this paper we present recent results from the NA49 experiment for Λ and hyperons produced in central Pb + Pb collisions at 40, 80 and 158 A GeV. Transverse mass spectra and rapidity distributions for Λ are shown for all three energies. The shape of the rapidity distribution becomes flatter with increasing beam energy. The multiplicities at mid-rapidity as well as the total yields are studied as a function of collision energy including AGS measurements. The ratio Λ/π at mid-rapidity and in 4π has a maximum around 40 A GeV. In addition, rapidity distributions have been measured at 40 and 80 A GeV, which allows us to study the /Λ ratio.

Fusion and reaction mechanism evolution in24Mg+12Cat intermediate energies

The formation and deexcitation of fusionlike events selected in events with a total charge equal or greater than 16 in ${}^{24}\mathrm{Mg}{+}^{12}\mathrm{C}$ system has been investigated at 25, 35, and 45 MeV/nucleon with a large multidetector array. Central single-source events are selected by use of the statistical discriminant analysis method applied to a set of 26 global variables. The fusion cross section has been extracted for the three bombarding energies and compared to other experimental data and to theoretical predictions. The total multiplicity is found to first increase to a maximum value and then decrease with increasing beam energy. It is shown that this behavior is connected to the opening of multifragmentation channels at 45 MeV/nucleon and the disappearance of channels with only light charged particles.

Study of low-energy-electron forward focusing effects on highly oriented pyrolytic graphite using scanning auger microscopy

The effects of the electron diffraction in highly oriented pyrolytic graphite are studied in an incident electron beam of energy range 500–700 eV using scanning Auger microscopy. The intensity of the exiting electrons monitored by a cylindrical-mirror analyzer can be well interpreted as the joint effect of incident and exit forward focusing along the close-packed 〈21̄1̄2〉-type atomic chains, with the contribution from forward focusing along the secondary close-packed atomic chains increasing rapidly as the beam energy increases. Incident beam effects contribute greatly to the variation in the intensity of the exiting electrons. Increasing the beam energy in this range also increases the anisotropy observed in the angular distribution of the intensity of both elastically scattered and Auger electrons.

Pulsed electron-beam melting of high-speed steel: structural phase transformations and wear resistance

The structural and phase transformations occurring in the near-surface layers of pre-quenched high-speed steel subjected to pulsed electron beam melting have been investigated. Melting was induced by a low-energy (20–30 keV), high-current electron beam with a pulse duration of 2.5 μs and an energy density ranging from 3 to 18 J/cm 2. Using electron microscopy and X-ray diffraction it has been revealed that with increasing beam energy density gradual liquid-phase dissolution of initial globular M 6C carbide particles occurs in the near-surface layer of thickness up to ∼1 μm. This process is accompanied by the formation of martensite crystals (α-phase) and an increase of residual austenite (γ-phase) content. When the carbide particles are completely dissolved, martensitic transformation is suppressed. In this case, a non-misoriented structure is formed consisting predominantly of submicrometer cells of γ-phase separated by nanosized carbide interlayers. Irradiation of cutting tools (drills) in a mode corresponding to an abrupt decrease in the content of M 6C particles due to their liquid-phase dissolution enhances the wear resistance of the drills by a factor of 1.7. This is associated with the fixation of undissolved particles in the matrix, the formation of residual compressive stresses and of dispersed M 3C carbide particles as well as the high (∼50%) content of the metastable γ-phase in the surface layer.

Nitrogen Gas-Cluster Ion Beam – A New Nitrogen Source for GaN Growth

ABSTRACTIn this paper we report sapphire nitridation and GaN film growth by using ionized nitrogen clusters as a nitrogen source. The clusters, typically 2000–3000 molecules/charge, are generated by a prototype GCIB source. The clusters are accelerated to 10–25 kV and disintegrate upon impact with the substrate surface where they react with Ga atoms to form GaN. The efficiency of this novel nitrogen source was tested by studying the nitridation of (0001) sapphire substrates at relatively low temperatures of 200 - 400°C. The effect of exposure of the substrate to the nitrogen cluster-ion beam was examined by XPS, RHEED and AFM. It was found that the amount of retained surface nitrogen increases nonlinearly with increasing beam energy. There exists a threshold energy, ∼ 20 kV, above which nitrogen retention is significantly enhanced.GaN films were grown with such nitrogen clusters heteroepitaxially on sapphire/AlN-buffer (MBE grown) and homoepitaxially on thick GaN on sapphire (HVPE grown).TEM cross-section images indicate that the heteroepitaxial GaN films had defect density similar to that of MBE and MOCVD grown films. The homoepitaxially-grown GaN films were found to replicate the GaN templates and show strong cathodoluminescence (CL) emission at 363 nm with FWHM of 9 nm. Furthermore, the spectra show no evidence of yellow band emission.

Antiproton production inp+Acollisions at 12.3 and 17.5GeV/c

Inclusive and semi-inclusive measurements are presented for antiproton ($\bar{p}$) production in proton-nucleus collisions at the AGS. The inclusive yields per event increase strongly with increasing beam energy and decrease slightly with increasing target mass. The $\bar{p}$ yield in 17.5 GeV/c p+Au collisions decreases with grey track multiplicity, $N_g$, for $N_g>0$, consistent with annihilation within the target nucleus. The relationship between $N_g$ and the number of scatterings of the proton in the nucleus is used to estimate the $\bar{p}$ annihilation cross section in the nuclear medium. The resulting cross section is at least a factor of five smaller than the free $\bar{p}-p$ annihilation cross section when assuming a small or negligible formation time. Only with a long formation time can the data be described with the free $\bar{p}-p$ annihilation cross section.

Electron beam induced degradation of a pulsed laser deposited ZnS:Cu,Au,Al thin film on a Si(1 0 0) substrate

ZnS:Cu,Au,Al thin film phosphors grown on Si(1 0 0) were subjected to an electron beam irradiation with beam energies ranging from 1 to 5 keV and current densities between 16 and 89 mA at O 2 and CO 2 partial pressure of 5×10 −7 Torr . Auger electron spectroscopy (AES) and the cathodoluminescence (CL), both excited by the same primary electron beam were utilised to monitor the changes in the surface composition and the luminous efficiency of the films during electron bombardment, respectively. The degradation mechanism for these reactive gases in ambient was manifested by the depletion of S and the formation of a non-luminescent ZnO layer on the surface of the phosphor. The degradation was more severe under O 2 ambient compared to the same partial pressure of CO 2 during electron beam bombardment, which is attributed to the free energy of formation of ZnO from ZnS when these respective gases are used. The degradation rate also depends on the energy of the electron beam, decreasing with increasing beam energy. This was interpreted according to the ionisation energy cross-section profile. The CL brightness increased exponentially with the increasing beam energy as more free carriers that will subsequently recombine yielding CL, are excited at higher beam energies.

Impact of simple tissue inhomogeneity correction algorithms on conformal radiotherapy of lung tumours

Background and purpose: Conformal radiotherapy requires accurate dose calculation at the dose specification point, at other points in the planning target volume (PTV) and in organs at risk. To assess the limitations of treatment planning of lung tumours, errors in dose values, calculated by some simple tissue inhomogeneity correction algorithms available in a number of currently applied treatment planning systems, have been quantified. Materials and methods: Single multileaf collimator-shaped photon beams of 6, 8, 15 and 18 MV nominal energy were used to irradiate a 50 mm diameter spherical solid tumour, simulated by polystyrene, which was located centrally inside lung tissue, simulated by cork. The planned dose distribution was made conformal to the PTV, which was a 15 mm three-dimensional expansion of the tumour. Values of both the absolute dose at the International Commission on Radiation Units and Measurement (ICRU) reference point and relative dose distributions inside the PTV and in the lung were calculated using three inhomogeneity correction algorithms. The algorithms investigated in this study are the pencil beam algorithm with one-dimensional corrections, the modified Batho algorithm and the equivalent path length algorithm. The calculated data were compared with measurements for a simple beam set-up using radiographic film and ionization chambers. Results: For this specific configuration, deviations of up to 3.5% between calculated and measured values of the dose at the ICRU reference point were found. Discrepancies between measured and calculated beam fringe values (distance between the 50 and 90% isodose lines) of up to 14 mm have been observed. The differences in beam fringe and penumbra width (20–80%) increase with increasing beam energy. Our results demonstrate that an underdosage of the PTV up to 20% may occur if calculated dose values are used for treatment planning. The three algorithms predict a considerably higher dose in the lung, both along the central beam axis and in the lateral direction, compared with the actual delivered dose values. Conclusions: The dose at the ICRU reference point of such a tumour in lung geometry is calculated with acceptable accuracy. Differences between calculated and measured dose distributions are primarily due to changes in electron transport in the lung, which are not adequately taken into account by the simple tissue inhomogeneity correction algorithms investigated in this study. Particularly for high photon beam energies, clinically unacceptable errors will be introduced in the choice of field sizes employed for conformal treatments, leading to underdosage of the PTV. In addition, the dose to the lung will be wrongly predicted which may influence the choice of the prescribed dose level in dose-escalation studies.

Ion-beam-assisted deposition of CN x films

Carbon nitride thin films were prepared by the electron-gun evaporation of high purity carbon under concurrent bombardment with nitrogen ion-beam. The base vacuum reached 10 −4 Pa and the operating vacuum pressure was approximately 10 −2 Pa . The purity of the nitrogen gas was 99.99% and the ion-beam energy was 500–1000 eV. It was found that the deposition rate decreased with increase of the ion beam energy. The films deposited at different conditions were studied by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-rays (EDX). The XPS data showed that CN sp 2 and CN sp 3 bonds existed in the films. The result with the maximum N/C atomic ratio of 0.43 was obtained from EDX measurements. The films tested by nano indentation had the maximum knoop hardness value of 23 GPa. The experiments of atomic oxygen action and cold welding in an ultra high vacuum environment proved that the films had good anti-atomic oxygen and anti-cold welding properties, and would have broad application prospects in space environments.

Transverse flow of nuclear matter in collisions of heavy nuclei at intermediate energies

The Quantum Molecular Dynamics Model (IQMD) is used to investigate the origin of the collective transverse velocity observed in heavy ion experiments. We find that there are three contributions to this effect: initial–final state correlations, potential interactions and collisions. For a given nuclear equation of state (eos) the increase of the transverse velocity with increasing beam energy is caused by the potential part. For a given beam energy the collective transverse velocity is independent of the nuclear eos but the relative contributions of potential and collisions differ. In view of the importance of the potential interactions between the nucleons it is not evident that the similarity of the radial velocities measured for fragments at beam energies below 1 A GeV and that for mesons at beam energies above 2 A GeV is more than accidental.

Fe3+/∑Fe vs. FeLα peak energy for minerals and glasses: Recent advances with the electron microprobe

This paper describes a preliminary study that attempts to determine the oxidation state of Fe (Fe3+/ΣFe) with the electron microprobe (EMP) by measuring the self-absorption induced shift of the FeLα peak emitted from minerals and glasses. In transition metals of the first row, the L-spectra exhibit common distortions, namely peak position shifts, peak shape alterations, and changes in the Lβ/Lα ratios, caused by the large difference in the self-absorption coefficients (μ/ρ) on either sides of the L3 absorption edges that are in close proximity to the Lα peak maxima. Measurements performed on α-Fe2O3 and FexO oxides have shown that self-absorption effects are stronger for the later oxide, leading to enhanced Fe2+Lα peak shift toward longer wavelengths as the beam energy increases. First measurements performed on silicates have confirmed that enhanced self-absorption of FeLα occurs on Fe2+ sites. The measurements consisted of plotting the FeLα peak position at a fixed beam energy (15 keV) against the total Fe concentration for two series of Fe2+- and Fe3+- bearing silicates. In a first step, these data have shown that both Fe2+Lα and Fe3+Lα peaks shift continuously toward longer wavelengths as the Fe concentration increases, with enhanced shifts for Fe2+Lα. For silicates containing only Fe2+ or Fe3+, no effects of the site geometry were detected on the variations of the FeLα peak position. A second set of plots has shown the variations of the peak position relative to the previous Fe2+-Fe3+ curves of step 1, as a function of the nominal Fe3+/ΣFe, for a series of reference minerals (hydrated and non-hydrated) and basaltic glasses. Data from chain and sheet silicates (e.g., pyroxenes, amphiboles, micas) exhibited strong deviations compared to other phases (e.g., garnets, Al-rich spinels, glasses), due to reduced self-absorption of FeLα. Intervalence-charge transfer (IVCT) mechanisms between Fe2+ and Fe3+ sites may be the origin of these deviations. These crystal-structure effects limit the accuracy of the method for mixed Fe2+-Fe3+ valence silicates. Precisions achieved for further Fe3+/ΣFe measurements strongly depend on the total Fe concentration. For basaltic glasses containing an average of 8 wt% Fe and 10% Fe3+/ΣFe, the precision is about ±2% (absolute). For low Fe concentrations (below 3.5 wt%), the uncertainty in the peak position measured by the EMP spectrometers leads to error bars that are similar to with the separation of the curves fitted to the Fe2+ and Fe3+ plots, which is propagated as prohibitive lack of precision for Fe3+/ΣFe (>70% relative). A major limitation of microbeam methods in general deals with beam damage. This aspect has been carefully studied for basaltic glasses, and optimal beam conditions have been established (in general, electron doses higher than those corresponding to 130 nA and 30 μm beam diameter should be avoided to prevent large beam induced oxidation phenomena). Additional work, in progress, concerns: (1) other beam-sensitive phases such as hydrated glasses; and (2) minerals in which FeLα is affected by large matrix effect corrections (e.g., Cr- and Ti-rich oxides where FeLα is strongly absorbed), for which the self-absorptioninduced shift of FeLα is different from that of common silicates and glasses.

Directed flow of lambda hyperons in (2-6 )A GeV Au+Au collisions.

Directed flow measurements for Lambda hyperons are presented and compared to those for protons produced in the same Au+Au collisions (2A, 4A, and 6A GeV; b<5-6 fm). The measurements indicate that Lambda hyperons flow consistently in the same direction but with smaller magnitudes. A strong positive flow [for Lambdas] has been predicted in calculations which include the influence of the Lambda-nucleon potential. The experimental flow ratio Lambda/p is in qualitative agreement with expectations (approximately 2/3) from the quark counting rule at 2A GeV but is found to decrease with increasing beam energy.

Baryon rapidity loss in relativistic Au + Au collisions.

An excitation function of proton rapidity distributions for different centralities is reported from AGS Experiment E917 for Au+Au collisions at 6, 8, and 10.8 GeV/nucleon. The rapidity distributions from peripheral collisions have a valley at midrapidity which smoothly change to distributions that display a broad peak at midrapidity for central collisions. The mean rapidity loss increases with increasing beam energy, whereas the fraction of protons consistent with isotropic emission from a stationary source at midrapidity decreases with increasing beam energy. The data suggest that the stopping is substantially less than complete at these energies.

Deposition of carbon nitride films by filtered cathodic vacuum arc combined with radio frequency ion beam source

Atomically smooth carbon nitride films were deposited by an off-plane double bend filtered cathodic vacuum arc (FCVA) technique. A radio frequency nitrogen ion source was used to supply active nitrogen species during the deposition of carbon nitride films. The films were characterized by atomic force microscopy (AFM), XPS and Raman spectroscopy. The internal stress was measured by the substrate bending method. The influence of nitrogen ion energy (0–1000 eV) on the composition, structure and properties of the carbon nitride films was studied. The nitrogen ion source greatly improves the incorporation of nitrogen in the films. The ratio of N/C atoms in the films increases to 0.40 with an increase in the ion beam energy to 100 eV. Further increase in the ion beam energy leads to a slight decrease in the N/C ratio. XPS results show that nitrogen atoms in the films are chemically bonded to carbon atoms as CN, CN, and CN bonds, but most of nitrogen atoms are bonded to sp2 carbon. The increase in nitrogen ion energy leads to a decrease in the content of nitrogen atoms bonded to sp2 carbon, and an increase in the content of nitrogen atoms bonded to sp3 and sp1 carbon. Raman spectra indicate an increase in the sp2 carbon phase in carbon nitride films with an increase in nitrogen ion energy. The increase in sp2 carbon fraction is attributed to the decrease in internal stress with increasing nitrogen ion energy.

Laser-driven electron cyclotron autoresonance accelerator with production of an optically chopped electron beam.

Analysis is presented of the gyroresonant acceleration of electrons in a vacuum using a focused laser. Continuous and equal acceleration is shown for electrons injected at all optical phases over an interaction length of tens of centimeters. Beam stalling is avoided as beam energy increases. Acceleration from 50 to 178 MeV is predicted for a 4 TW, 10.6-microm laser focused to a waist radius of 1.0 mm; these parameters correspond to a planned experiment. A beam stop with an off-axis hole after acceleration is shown to create a train of optically chopped bunches with 3-fs bunch lengths and a 35-fs period.

Heating and non-inductive current drive by negative ion based NBI in JT-60U

The current drive and heating properties of negative ion based NBI have been studied comprehensively in JT-60U. It has been confirmed from shine-through measurements of the injected beam (350 keV) that multistep ionization processes are essential in the ionization processes of high energy particles. The profile of the current density driven by a negative ion based NB (N-NB) has beendetermined experimentally. This is in good agreement with the theoretical prediction, and N-NB driven current reached 0.6 MA with EB = 360 keV and PINJ = 3.7 MW. The current drive efficiency ηCD is increased by increasing electron temperature and improved by increasingbeam energy. The fast ions from N-NBs are well confined in the enhanced confinement core by the weak poloidal magnetic field of reversed shear plasmas. A clear H mode transition was obtained with N-NB dominant heating, where the net absorbed power required for an H mode transition seemed similar to the previous result obtained in JT-60U using a low energy beam (90 keV). With the strong electron heating by N-NBI (80% absorbed by electrons), an H factor ( = τE/τITER-89PLE) of 1.64 with Te(0) = 1.4Ti(0) was obtained in the steady state ELMy phase.

ELECTROMAGNETIC DISSOCIATION OF 60 A GeV16O IN EMULSION

The electromagnetic dissociations (EDs) of 60 A GeV16O in nuclear emu lsion is investigated with high statistics for the first time. It is found that the interaction cross section for EDs increases with increasing beam energy, and the charge distribution of projectile fragments is the same as the results at 2 00 A GeV, but the production probability of projectile fragments with charge 3≤ Z≤5 is smaller than that of the results at 200 A GeV. These results can be expl ained by use of the Weizscker-Williams method for calculating the EDs contributi ons. The percentage abundances of various decay modes for EDs at 60 A GeV is the same as the results at 200 A GeV,but different from the results at 14.6 A GeV. The EDs of 60 A GeV16O is mainly caused by the giant dipole and quadr upole resonance of E1 and E2 and by the quasi deuteron effect, which can be qual itatively explained by the multiplicity distribution of projectile fragmentation proton in EDs. The multiplicity distribution of α fragments in EDs and nuclear events have different functional forms. This difference may be a consequence of the different reaction mechanisms involved in EDs and nuclear events.

Crystallographic and magnetic properties of CoCrPt/Cr films produced by pulsed laser deposition

The crystallographic structure and magnetic properties of CoCrPt(30 nm)/Cr(50 nm) films fabricated by pulsed laser deposition have been studied at various substrate temperatures and ion bombardment energies. Cr(002) texture is developed at 350–500° C where increased coercivity is observed from the Co-alloy layers, while Cr(110) texture is formed at 300° C. The average grain size of the Cr layers gradually increases with substrate temperature from ~10 nm to ~20 nm, which is comparable to that of sputtered films. A Cr(002) X-ray peak is not observed from films grown at 400° C with Ar ion bombardment (> 50 eV). The coercivity and squareness of the magnetic hysteresis loops decrease as the beam energy increases, which may be attributed to the absence of Cr(002) texture.

Effects of oxygen ion beam plasma conditions on the properties of Indium tin oxide thin films

Tin-doped indium oxide (ITO) films were depositied at room temperature by ion beam-assisted evaporation and the effects of oxygen ion beam conditions on the properties of room-temperature ITO thin films were investigated. It is well known that high conductivity of ITO is caused by intrinsic defect(oxygen vacancy) and dopant(tin). One of the techniques to obtain highly conductive ITO film at room temperature is to cause electron degeneracy in the band gap by introducing non-stoichiometry in the ITO film using oxygen ion beam plasma while evaporating ITO films using an electron beam evaporation. In this study, flux and energy of oxygen ion beam plasma were varied to modify depositing ITO thin films by controlling gas flow rates and grid bias potential. The structure of deposited ITO thin film was amorphous for all of the experimental conditions. The increase of oxygen ion beam flux increased the optical transmittance of the deposited ITO film, however, at a given ITO evaporation rate, the resistivity showed a minimum in a certain range of ion beam flux. The increase of oxygen ion beam energy up to 900 V decreased the resistivity and increased the optical transmittance of deposited ITO thin films. Using the oxygen ion beam-assisted method, ITO thin films with the resistivity of 5.2×10 −4 Ω cm and optical transmittance of above 90% could be deposited at room temperature.