Several Tens Of MeV Research Articles

Tunable Excitons in Biased Bilayer Graphene

Recent measurements have shown that a continuously tunable bandgap of up to 250 meV can be generated in biased bilayer graphene [ Zhang , Y. ; et al. Nature 2009, 459 , 820 ], opening up pathway for possible graphene-based nanoelectronic and nanophotonic devices operating at room temperature. Here, we show that the optical response of this system is dominated by bound excitons. The main feature of the optical absorbance spectrum is determined by a single symmetric peak arising from excitons, a profile that is markedly different from that of an interband transition picture. Under laboratory conditions, the binding energy of the excitons may be tuned with the external bias going from zero to several tens of millielectronvolts. These novel strong excitonic behaviors result from a peculiar, effective "one-dimensional" joint density of states and a continuously tunable bandgap in biased bilayer graphene. Moreover, we show that the electronic structure (level degeneracy, optical selection rules, etc.) of the bound excitons in a biased bilayer graphene is markedly different from that of a two-dimensional hydrogen atom because of the pseudospin physics.

Neutron transport simulation (selected topics)

Neutron transport simulation is usually performed for criticality, power distribution, activation, scattering, dosimetry and shielding problems, among others. During the last fifteen years, innovative technological applications have been proposed (Accelerator Driven Systems, Energy Amplifiers, Spallation Neutron Sources, etc.), involving the utilization of intermediate energies (hundreds of MeV) and high-intensity (tens of mA) proton accelerators impinging in targets of high Z elements. Additionally, the use of protons, neutrons and light ions for medical applications (hadrontherapy) impose requirements on neutron dosimetry-related quantities (such as kerma factors) for biologically relevant materials, in the energy range starting at several tens of MeV. Shielding and activation related problems associated to the operation of high-energy proton accelerators, emerging space-related applications and aircrew dosimetry-related topics are also fields of intense activity requiring as accurate as possible medium- and high-energy neutron (and other hadrons) transport simulation. These applications impose specific requirements on cross-section data for structural materials, targets, actinides and biologically relevant materials. Emerging nuclear energy systems and next generation nuclear reactors also impose requirements on accurate neutron transport calculations and on cross-section data needs for structural materials, coolants and nuclear fuel materials, aiming at improved safety and detailed thermal-hydraulics and radiation damage studies. In this review paper, the state-of-the-art in the computational tools and methodologies available to perform neutron transport simulation is presented. Proton- and neutron-induced cross-section data needs and requirements are discussed. Hot topics are pinpointed, prospective views are provided and future trends identified.

AGILE detection of delayed gamma-ray emission from GRB 080514B

GRB 080514B is the first gamma ray burst (GRB), since the time of EGRET, for which individual photons of energy above several tens of MeV have been detected with a pair-conversion tracker telescope. This burst was discovered with the Italian AGILE gamma-ray satellite. The GRB was localized with a cooperation by AGILE and the interplanetary network (IPN). The gamma-ray imager (GRID) estimate of the position, obtained before the SuperAGILE-IPN localization, is found to be consistent with the burst position. The hard X-ray emission observed by SuperAGILE lasted about 7 s, while there is evidence that the emission above 30 MeV extends for a longer duration (at least ~13 s). Similar behavior was seen in the past from a few other GRBs observed with EGRET. However, the latter measurements were affected, during the brightest phases, by instrumental dead time effects, resulting in only lower limits to the burst intensity. Thanks to the small dead time of the AGILE/GRID we could assess that in the case of GRB 080514B the gamma-ray to X-ray flux ratio changes significantly between the prompt and extended emission phase.

Analyses of Local Open Circuit Voltages in Polycrystalline Cu(In,Ga)Se2 Thin Film Solar Cell Absorbers on the Micrometer Scale by Confocal Luminescence

Polycrystalline chalcopyrite absorbers in thin film solar cells exhibit distinct spatial inhomogeneities which are obviously introduced during growth of the grainy structure. Amongst several structural inhomogeneities in scale lengths of grains of one ?m or below, spatial variations of optoelectronic properties in much larger scales of some ?m can be observed, such as fluctuations in the splitting of quasi-Fermi levels in the range of several tens of meV. These have been quantitatively deduced via Planck's generalized law from spectrally resolved confocal room temperature luminescence scans. Since the necessary absorption length in thin film semiconductors is in the neighborhood of the wavelength of photons, for the evaluation of luminescence spectra wave optics with multireflection effects have been applied.

PAMELA observational capabilities of Jovian electrons

PAMELA is a satellite-borne experiment that has been launched on June 15th, 2006. It is designed to make long duration measurements of cosmic radiation over an extended energy range. Specifically, PAMELA is able to measure the cosmic ray antiproton and positron spectra over the largest energy range ever achieved and will search for antinuclei with unprecedented sensitivity. Furthermore, it will measure the light nuclear component of cosmic rays and investigate phenomena connected with solar and earth physics. The apparatus consists of: a time of flight system, a magnetic spectrometer, an electromagnetic imaging calorimeter, a shower tail catcher scintillator, a neutron detector and an anticoincidence system. In this work a study of the PAMELA capabilities to detect electrons is presented. The Jovian magnetosphere is a powerful accelerator of electrons up to several tens of MeV as observed at first by Pioneer 10 spacecraft (1973). The propagation of Jovian electrons to Earth is affected by modulation due to Corotating Interaction Regions (CIR). Their flux at Earth is, moreover, modulated because every ∼13 months Earth and Jupiter are aligned along the average direction of the Parker spiral of the Interplanetary Magnetic Field. PAMELA will be able to measure the high energy tail of the Jovian electrons in the energy range from 50 up to 130 MeV. Moreover, it will be possible to extract the Jovian component reaccelerated at the solar wind termination shock (above 130 MeV up to 2 GeV) from the galactic flux.

Local Tunneling Spectroscopy across a Metamagnetic Critical Point in the Bilayer RuthenateSr3Ru2O7

The local spectroscopic signatures of metamagnetic criticality in Sr(3)Ru(2)O(7) were explored using scanning tunneling microscopy (STM). Singular features in the tunneling spectrum were found close to the Fermi level, as would be expected in a Stoner picture of itinerant electron metamagnetism. These features showed a pronounced magnetic field dependence across the metamagnetic critical point, which cannot be understood in terms of a naive Stoner theory. In addition, a pseudogap structure was observed over several tens of meV, accompanied by a c(2 x 2) superstructure in STM images. This result represents a new electronic ordering at the surface in the absence of any measurable surface reconstruction.

Orbital contribution to the magnetic properties of iron as a function of dimensionality

The orbital contribution to the magnetic properties of Fe in systems of decreasing dimensionality bulk, surfaces, wire, and free clusters is investigated using a tight-binding Hamiltonian in an s, p, and d atomic orbital basis set including spin-orbit coupling and intra-atomic electronic interactions in the full Hartree-Fock HF scheme, i.e., involving all the matrix elements of the Coulomb interaction with their exact orbital dependence. Spin and orbital magnetic moments and the magnetocrystalline anisotropy energy MAE are calculated for several orientations of the magnetization. The results are systematically compared with those of simplified Hamiltonians which give results close to those obtained from the local spin density approximation. The full HF decoupling leads to much larger orbital moments and MAE which can reach values as large as 1B and several tens of meV, respectively, in the monatomic wire at the equilibrium distance. The reliability of the results obtained by adding the so-called orbital polarization ansatz OPA to the simplified Hamiltonians is also discussed. It is found that when the spin magnetization is saturated, the OPA results for the orbital moment are in qualitative agreement with those of the full HF model. However, there are large discrepancies for the MAE, especially in clusters. Thus, the full HF scheme must be used to investigate the orbital magnetism and MAE of low dimensional systems.

Stark shift of interband transitions in AlN∕GaN superlattices

The e1h1, e1h2, and e1h3 transitions of AlN∕GaN superlattices with different well widths were detected by electroreflectance measurements in dependence on the externally applied voltage. The quantum confined Stark effect of several tens of meV is observed, whose energy shift increases for larger well widths. The experimental results agree with quantum mechanical calculations at the Brillouin zone center. For well widths of 2.3 and 1.4nm an intrinsic electric field strength in the wells of 5.04 and 6.07MV∕cm is calculated.

Energetic particles in the paleomagnetosphere: Reduced dipole configurations and quadrupolar contributions

The Earth’s magnetosphere acts as a shield against highly energetic particles of cosmic and solar origin. On geological timescales, variations of the internal geomagnetic field can drastically alter the magnetospheric structure and dynamics. This paper deals with energetic particles in the paleomagnetosphere. Scaling relations for cutoff energies and differential particle fluxes during periods of reduced dipole moment are derived. Particular attention is paid to high rigidity galactic cosmic ray particles in the GV range and to lower rigidity solar energetic particles. We find that in the paleomagnetosphere of a strongly reduced dipole moment, solar protons of several tens of MeV may access the Earth’s atmosphere even at midlatitudes. Higher‐order core‐field components can widen the polar caps and open new particle‐entry regions around the equator. Potential field modeling of the paleomagnetosphere and magnetohydrodynamic (MHD) simulations are utilized to study the impact of solar particles on the Earth’s atmosphere for more complex field configurations that are likely to occur during geomagnetic polarity transitions.

Creation of multiple nanodots by single ions

In the search to develop tools that are able to modify surfaces on the nanometre scale, the use of heavy ions with energies of several tens of MeV is becoming more attractive. Low-energy ions are mostly stopped by nuclei, which causes the energy to be dissipated over a large volume. In the high-energy regime, however, the ions are stopped by electronic excitations, and the extremely local (approximately 10 nm3) nature of the energy deposition leads to the creation of nanosized 'hillocks' or nanodots under normal incidence. Usually, each nanodot results from the impact of a single ion, and the dots are randomly distributed. Here we demonstrate that multiple, equally spaced dots, each separated by a few tens of nanometres, can be created if a single high-energy xenon ion strikes the surface at a grazing angle. By varying this angle, the number of dots, as well as their spacing, can be controlled.

Laser-accelerated high-energy ions: state of-the-art and applications

The acceleration of high-energy ion beams (up to several tens of MeV per nucleon) following the interaction of short (t < 1ps) and intense (Iλ2> 1018 W cm−2 μm−2) laser pulses with solid targets has been one of the most important results of recent laser-plasma research. The acceleration is driven by relativistic electrons, which acquire energy directly from the laser pulse and set up extremely large (∼TV/m) space charge fields at the target interfaces. In view of a number of advantageous properties, laser-driven ion beams can be employed in a number of innovative applications in the scientific, technological and medical areas. Among these, their possible use in hadrontherapy, with potential reduction of facility costs, has been proposed recently. This paper will briefly review the current state-of-the-art in laser-driven proton/ion source development, and will discuss the progress needed in order to implement some of the above applications. Recent results relating to the optimization of beam energy, spectrum and collimation will be presented.

LASER-ACCELERATED PROTONS: PERSPECTIVES FOR CONTROL/OPTIMIZATION OF BEAM PROPERTIES

The acceleration of high-energy ion beams (up to several tens of MeV per nucleon) following the interaction of short ( t &lt; 1ps ) and intense (Iλ2 &gt; 1018W cm-2μm2) laser pulses with solid targets has been one of the most active areas of research in the last few years. The exceptional properties of these beams (high brightness and high spectral cutoff, high directionality and laminarity, short burst duration) distinguish them from those of the lower energy ions accelerated in earlier experiments at moderate laser intensities. In view of these properties, laser-driven ion beams can be employed in a number of groundbreaking applications in the scientific, technological and medical areas. This paper reviews the current state-of-the-art, highlights recent developments and indicate future directions of this research area.

Built-in electric field effects on donor bound excitons in wurtzite InGaN strained coupled quantum dots

Considering the three-dimensional confinement of the electrons and holes and the strong built-in electric field induced by the spontaneous and piezoelectric polarizations of the wurtzite In x Ga 1− x N/GaN strained coupled quantum dots (QDs), the positively charged donor bound exciton binding energy is calculated within the framework of the effective-mass approximation and variational method. Our results clearly indicate that the donor bound exciton binding energy sensitively depends on the donor position in the system, the structural parameters of the coupled QDs and the strong built-in electric field. The variation of this energy versus the donor position is in several tens of meV.

Anomalous blueshift in emission spectra of ZnO nanorods with sizes beyond quantum confinement regime

Cathodoluminescence (CL) spectroscopy has been employed to study the electronic and optical properties of well-aligned ZnO nanorods with diameters ranging from 50to180nm. Single-nanorod CL studies reveal that the emission peak moves toward higher energy as the diameter of the ZnO nanorod decreases, despite that their sizes are far beyond the quantum confinement regime. Blueshift of several tens of meV in the CL peak of these nanorods has been observed. Moreover, this anomalous energy shift shows a linear relation with the inverse of the rod diameter. Possible existence of a surface resonance band is suggested and an empirical formula for this surface effect is proposed to explain the size dependence of the CL data.

Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders

Streamers are thin filamentary plasmas that can initiate spark discharges in relatively short (several centimeters) gaps at near ground pressures and are also known to act as the building blocks of streamer zones of lightning leaders. These streamers at ground pressure, after 1/N scaling with atmospheric air density N, appear to be fully analogous to those documented using telescopic imagers in transient luminous events (TLEs) termed sprites, which occur in the altitude range 40–90 km in the Earth's atmosphere above thunderstorms. It is also believed that the filamentary plasma structures observed in some other types of TLEs, which emanate from the tops of thunderclouds and are termed blue jets and gigantic jets, are directly linked to the processes in streamer zones of lightning leaders. Acceleration, expansion, and branching of streamers are commonly observed for a wide range of applied electric fields. Recent analysis of photoionization effects on the propagation of streamers indicates that very high electric field magnitudes ∼10 Ek, where Ek is the conventional breakdown threshold field defined by the equality of the ionization and dissociative attachment coefficients in air, are generated around the tips of streamers at the stage immediately preceding their branching. This paper describes the formulation of a Monte Carlo model, which is capable of describing electron dynamics in air, including the thermal runaway phenomena, under the influence of an external electric field of an arbitrary strength. Monte Carlo modeling results indicate that the ∼10 Ek fields are able to accelerate a fraction of low‐energy (several eV) streamer tip electrons to energies of ∼2–8 keV. With total potential differences on the order of tens of MV available in streamer zones of lightning leaders, it is proposed that during a highly transient negative corona flash stage of the development of negative stepped leader, electrons with energies 2–8 keV ejected from streamer tips near the leader head can be further accelerated to energies of hundreds of keV and possibly to several tens of MeV, depending on the particular magnitude of the leader head potential. It is proposed that these energetic electrons may be responsible (through the “bremsstrahlung” process) for the generation of hard X rays observed from ground and satellites preceding lightning discharges or with no association with lightning discharges in cases when the leader process does not culminate in a return stroke. For a lightning leader carrying a current of 100 A, an initial flux of ∼2–8 keV thermal runaway electrons integrated over the cross‐sectional area of the leader is estimated to be ∼1018 s−1, with the number of electrons accelerated to relativistic energies depending on the particular field magnitude and configuration in the leader streamer zone during the negative corona flash stage of the leader development. These thermal runaway electrons could provide an alternate source of relativistic seed electrons which were previously thought to require galactic cosmic rays. The duration of the negative corona flash and associated energetic radiation is estimated to be in the range from ∼1 μs to ∼1 ms depending mostly on the pressure‐dependent size of the leader streamer zone.

Electronic structure of free-standingGaAs∕AlGaAsnanowire superlattices

Based on an atomistic tight-binding model, extensive calculations for the electronic structure of free-standing $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ nanowire superlattices, oriented along the [100] crystallographic direction, have been performed. Miniband formation and energy gap opening in the conduction and valence bands are studied in detail for different lateral sizes and layer thicknesses of the nanowire superlattice. Generally, the conduction minibands show relatively simple dispersion relations, while in contrast the valence minibands show complicated dispersion relations. For the nanowire superlattice with a GaAs layer thickness $(w)$ and a lateral size $(d)$ of a few nanometers (e.g., $w\ensuremath{\sim}8$ and $d\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$), distinct energy gaps in the order of several tens of meV can appear in the conduction and valence bands. The wave functions of conduction and valence miniband states at the $\ensuremath{\Gamma}$ point have also been studied for the nanowire superlattices. It is shown that at strong lateral confinement situations the wave functions of the conduction miniband states of the nanowire superlattice can be localized in the barrier layers. However, it is generally difficult to observe the same phenomenon from the valence miniband states.

Development of the Monte Carlo model CASCADE-2004 of high-energy nuclear interactions

The Monte Carlo code CASCADE for the calculation of inelastic hadron- and nucleus-nucleus interactions at energies from several tens of MeV up to several tens of GeV and for modelling of nuclear-physical processes accompanying the transport of particles and nuclei in matter is improved by considering a more detailed model of decay of highly excited residual (after-intranuclear-cascade) nuclei. Results of calculations are in good agreement with experiment. However, there are some deviations for light-isotope production, which prompt the necessity of developing more correct models of evaporation and strong asymmetric high-energy fission.

Determination of shock parameters for the very fast interplanetary shock on 29 October 2003

We study a very fast interplanetary shock (IP shock) event observed on 29 October 2003 based on the Geotail particle and field measurements in the solar wind. During this event the intensity of high‐energy solar energetic particles (greater than several to several tens of MeV) was quite high, causing a serious background problem for plasma particle measurements on Geotail as well as on the other spacecraft. The magnetic/electric field measurements and the plasma wave measurement aboard Geotail, on the other hand, were free from such a background problem and provided a reliable estimate for the local plasma parameters including the plasma density. From these measurements, our best estimation for the local shock velocity is ∼2000 km/s in the observer's rest frame or ∼1400 km/s in the upstream plasma rest frame. The corresponding Alfvén Mach number is ∼12. It is found that the timing analysis of the shock arrivals at ACE and Geotail gives a shock velocity significantly lower than the above value. We argue that this difference is due to the shock surface rippling by 15–20 deg. We also comment that this IP shock had a property of “cosmic‐ray‐mediated” shock, namely a shock having a spatial structure affected by pressures exerted by nonthermal particles accelerated by the shock itself.

Analysis of transient ion beam induced current in Si PIN photodiode

Transient ion beam induced current (TIBIC) system combined with microbeam and single-ion-hit system is a useful tool for analyzing single event effects (SEEs) such as single event transient (SET) current. SET current contributes to the bit error rate (BER) degradation of optical data links. BER degradation due to high-energy protons has attracted special interest recently. When high-energy protons traverse the photodiode being more prone to BER degradation than the external electronics, both inelastic and elastic reactions are responsible for producing many high-energy secondary ions with energies up to several tens of MeV. The energy-loss of these ions induces SET current on the terminal of the photodiode. Therefore the analysis of the heavy-ion induced SET current is an important issue for estimating and understanding proton induced BER degradation. In this study, we measured and analyzed TIBIC using angled C, N, O and Si microbeam with energies up to 18MeV from the Tandem accelerator at JAERI, Takasaki.

Neutrino-nucleus interactions: open questions and future projects

We discuss various issues concerning the interactions of nuclei with neutrinos of low impinging energies (i.e. having several tens of MeV to a few hundred MeV) of interest for particle physics, nuclear physics and astrophysics. We focus, in particular, on open questions as well as possible strategies to obtain more experimental information. The option of a low-energy beta-beam facility is extensively discussed. We also mention its potential concerning the neutrino magnetic moment.