Spectrum Of Accelerated Protons 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.

Gamma-Ray Emission from the Impulsive Phase of the 2017 September 6 X9.3 Flare

Abstract We report hard X-ray and gamma-ray observations of the impulsive phase of the SOL2017-09-06T11:55 X9.3 solar flare. We focus on a high-energy part of the spectrum, >100 keV, and perform time resolved spectral analysis for a portion of the impulsive phase, recorded by the Konus-Wind experiment, that displayed prominent gamma-ray emission. Given a variety of possible emission components contributing to the gamma-ray emission, we employ a Bayesian inference to build the most probable fitting model. The analysis confidently revealed contributions from nuclear deexcitation lines, electron–positron annihilation line at 511 keV, and a neutron capture line at 2.223 MeV along with two components of the bremsstrahlung continuum. The revealed time evolution of the spectral components is particularly interesting. The low-energy bremsstrahlung continuum shows a soft–hard–soft pattern typical for impulsive flares, while the high-energy one shows a persistent hardening at the course of the flare. The neutron capture line emission shows an unusually short time delay relative to the nuclear deexcitation line component, which implies that the production of neutrons was significantly reduced soon after the event onset. This in turn may imply a prominent softening of the accelerated proton spectrum at the course of the flare, similar to the observed softening of the low-energy component of the accelerated electrons responsible for the low-energy bremsstrahlung continuum. We discuss possible physical scenarios, which might result in the obtained relationships between these gamma-ray components.

X-Ray and Gamma-Ray Emission of the Shell-Type Supernova Remnant RX J1713.7−3946

To study possible origins of high-energy gamma-ray emission from shell-type supernova remnants, RX J1713.7$-$3946, we model their nonthermal photon spectra from X-ray to very high energy (VHE) gamma-ray bands in the frame of non-linear diffusive shock acceleration, respectively. In our modeling, the accelerated proton spectrum is calculated based on a semi-analytical solution of the problem of particle acceleration at non-linear shock waves with a free escape boundary at some location upstream, and the accelerated electron spectrum is similar to the proton's spectrum when the electron's momentum is much less than its maximum momentum, which is determined by equating the synchrotron loss time with the acceleration time, and the cutoff shapes are assumed to be an super-exponential. In the one-zone model, we calculated the non-thermal photon spectra of the supernova remnants (SNR) for both leptonic and hadronic cases, and found out that the spectra calculated for both cases can reproduce the observed data by Suzaku, Fermi LAT, and HESS under reasonable model parameters. However, high-energy gamma-ray emission would have a hadronic origin if the observed thickness of the X-ray filament of the SNR is taken into account.

Isochoric heating with laser-accelerated proton beams

Numerical simulations are used to study acceleration of protons by intense laser pulses and the induced heating of a secondary target using this proton beam. Laser-plasma interaction and ion acceleration are modeled with a multidimensional, relativistic particle-in-cell code. Proton slowing-down and secondary target heating are computed with a two-dimensional Monte Carlo and a one-dimensional Lagrangian hydrodynamics codes. Strategies for optimization of heating uniformity include tailoring the accelerated proton spectrum with specific target geometries or laser pulse parameters. A trade-off must then be found between heating uniformity and efficiency. The parameters (temperature, depth and radius of heated zone, characteristic heating and hydrodynamics times) that could be obtained on current short pulse laser facilities, using a typical 50 fs Gaussian pulse with an intensity of several 1019 W/cm2, are explored.

High-energy gamma-ray emission from solar flares: Constraining the accelerated proton spectrum

Using a multi-component model to describe the γ-ray emission, we investigate the flares of December 16, 1988 and March 6, 1989 which exhibited unambiguous evidence of neutral pion decay. The observations are then combined with theoretical calculations of pion production to constrain the accelerated proton spectra.

Energetic solar particles and their relation to optical flares

It has been recently suggested by several investigators that the accelerated charged particles provide the energy of the optical flare by the ionization loss process. We have examined this mechanism assuming different forms of the spectrum of the accelerated protons at lower chromosphere. The flux and the energy spectrum of protons of energy 0.1–100 MeV have been calculated at successive heights, from 103 to 40 × 103 km from the solar surface taking into account the ionization loss, pitch angle distribution and density distribution of the neutral and ionized hydrogen in the chromosphere and lower corona. Hence the energy spectrum of the protons escaping from the Sun and the amount of energy dissipated in the solar chromosphere are computed. Comparing the calculated results with the observational data on the solar event of September 28, 1961 it is found that the ionization loss of the accelerated protons and heavier nuclei in the solar atmosphere may supply a significant part of the energy of the optical flare assuming that the fraction, f, of magnetic tubes of force extending out of the solar atmosphere is about 1 %. The accelerated proton spectrum in the form of power law in kinetic energy seems to be the most appropriate form. In the event of September 28, 1961 best estimates are made on this basis of the total number and the energy spectrum of protons at injection, the flux and energy spectrum of escaping protons and the energy dissipated in the solar atmosphere by the accelerated ions. It is found that the possible range of variation of the height of injection level hardly affects the total energy dissipated. The high variability of the intensity of protons released by the Sun is interpreted in terms of the variations of the parameter, f, determined by the configurations of the magnetic field lines.

The gamma‐ray spectrum of the Sun

The γ‐ray spectrum (hν > 10 kev) emitted by the sun is investigated and approximate fluxes at the earth are predicted on the basis of a simple solar‐flare model. The object of this paper is to determine what new information about the sun and, in particular, solar flares we can learn from the detection of this radiation. We also investigate which part of the spectrum is most feasibly detectable and which part yields the most direct information on solar‐flare structure. Our calculations indicate the following sources of γ radiation from the sun. The γ radiation emitted by the quiet sun is negligible compared with emission during a solar flare. The most important emission mechanism in the 10‐kev to 1‐Mev energy region is bremsstrahlung by flare‐accelerated electrons. The photon spectrum is continuous and decreases monotonically with energy. Flare‐accelerated protons will interact with carbon, nitrogen, and oxygen nuclei by inelastic scattering and spallation reactions, producing γ radiation as a result of nuclear de‐excitation. Little effort has been made to detect this line emission. Neutrons resulting from spallation reactions will be partially captured by protons to produce deuterium and 2.23‐Mev γ radiation. The intensity of this line emission is proportional to the neutron density in a flare. Line emission at 0.51 Mev results from positron‐electron annihilation. Positrons are produced by β decay of radioactive nuclei generated by spallation reactions and by the decay of π+ mesons produced in proton‐proton reactions. The primary source of photons with energy greater than 50 Mev is the decay of π0 mesons, which are also produced in proton‐proton reactions. The intensity of these photons is very sensitive to the accelerated proton spectrum in the flare. Flux estimates indicate that the detection of γ radiation resulting from a solar flare is feasible and would yield information on nuclear reactions as well as on the intensity and spectrum of high‐energy protons and electrons.