Maximum Particle Energy Research Articles

Diffusive shock acceleration at interplanetary perpendicular shock waves: Influence of the large scale structure of turbulence on the maximum particle energy

We calculate the maximum energy that a particle can obtain at perpendicular interplanetary shock waves by the mechanism of diffusive shock acceleration. The influence of the energy range spectral index of the two-dimensional modes of the interplanetary turbulence is explored. We show that changes in this parameter lead to energies that differ in at least one order of magnitude. Therefore, the large scale structure of the turbulence is a key input if the maximum particle energy is calculated.

SOLAR ENERGETIC-PARTICLE RELEASE TIMES IN HISTORIC GROUND-LEVEL EVENTS

Ground-level events (GLEs) are large solar energetic-particle events with sufficiently hard spectra for GeV protons to be detected by neutron monitors at ground level. For each of 30 well-observed historic GLEs from four solar cycles, extending back to 1973, I have plotted onset times versus velocity−1 for particles observed on the IMP-7 and 8, ISEE-3, Wind, and GOES spacecraft and by neutron monitors. A linear fit on such a plot for each GLE determines the initial solar particle release (SPR) time, as the intercept, and the magnetic path length traversed, as the slope, of the fitted line. Magnetic path lengths and SPR times are well determined by the fits and cannot be used as adjustable parameters to make particle and photon emission times coincide. SPR times follow the onsets of shock-induced type II radio bursts and the coronal height of the coronal mass ejection (CME)-driven shock at SPR time can be determined for GLEs spanning an interval of solar longitude of ∼140 deg. For a given GLE, all particle species and energies diverge from a single SPR point at a given coronal height and footpoint longitude of the field line to the Earth. These heights tend to increase with longitudinal distance away from the source, a pattern expected for shock acceleration. Acceleration for magnetically well-connected large GLEs begins at ∼2 solar radii, in contrast to non-GLEs that have been found to be strongly associated with shocks above ∼3 solar radii. The higher densities and magnetic field strengths at lower altitudes may be responsible for the acceleration of higher-energy particles in GLEs, while those GLEs that begin above 3RS may compensate by having higher shock speeds. These results support the joint dependence of maximum particle energy on magnetic field strength, injected particle density, and shock speed, all predicted theoretically.

Secondary charged particle equilibrium in 137Cs and 60Co reference radiation fields

During the calibration or irradiation of dosemeters, typical irradiation geometries (collimated beams) and source-to-detector distances (1-5 m) lead to the fact that for photon energies above a few hundred keV, the secondary charged particle equilibrium is usually not ensured. The reason is that the effective beam radius at the detector position is smaller than the range of the secondary electrons produced in air whose maximum particle energy is as large as the maximum photon energy. Therefore, the International Organization for Standardization (ISO) recommends putting a build-up plate (BUP) made of polymethyl methacrylate in front of the dosemeter to be calibrated in ISO 4037-3. In this paper, the effect of the thickness of the BUP and its distance from the dosemeter at different source-to-dosemeter distances were investigated by means of measurement and calculation. It turned out that the geometrical arrangement of the source, dosemeter and BUP recommended by ISO mostly does not ensure secondary charged particle equilibrium. The consequence is to always place the BUP directly in front of the dosemeter to be calibrated or irradiated.

Ultrahigh energy neutrinos from superconducting cosmic strings

Superconducting cosmic strings naturally emit highly boosted charge carriers from cusps. This occurs when a cosmic string or a loop moves through a magnetic field and develops an electric current. The charge carriers and the products of their decay, including protons, photons and neutrinos, are emitted as a narrow jets with opening angle $\theta \sim 1/\gamma_c$, where $\gamma_c$ is the Lorentz factor of the cusp. The excitation of electric currents in strings occurs mostly in clusters of galaxies, which are characterized by magnetic fields $B \sim 10^{-6}$ G and a filling factor $f_B \sim 10^{-3}$. Two string parameters determine the emission of the particles: the symmetry breaking scale $\eta$, which for successful applications should be of order $10^9$--$10^{12}$ GeV, and the dimensionless parameter $i_c$, which determines the maximum induced current as $J_{max} =i_c e \eta$ and the energy of emitted charge carriers as $\epsilon_x \sim i_c \gamma_c \eta$, where $e$ is the electric charge of a particle. For the parameters $\eta $ and $B$ mentioned above, the Lorentz factor reaches $\gamma_c \sim 10^{12}$ and the maximum particle energy can be as high as $\gamma_c\eta \sim 10^{22}$ GeV. The diffuse fluxes of UHE neutrinos are close to the cascade upper limit, and can be detected by future neutrino observatories. The signatures of this model are: very high energies of neutrinos, in excess of $10^{20}$ eV, correlation of neutrinos with clusters of galaxies, simultaneous appearance of several neutrino-produced showers in the field of view of very large detectors, such as JEM-EUSO, and 10 TeV gamma radiation from the Virgo cluster. The flux of UHE protons from cusps may account for a large fraction of the observed events at the highest energies.

On particle acceleration and very high energy γ-ray emission in Crab-like pulsars

The origin of very energetic charged particles and the production of very high-energy (VHE) gamma-ray emission remains still a challenging issue in modern pulsar physics. By applying a toy model, we explore the acceleration of co-rotating charged particles close to the light surface in a plasma-rich pulsar magnetosphere and study their interactions with magnetic and photon fields under conditions appropriate for Crab-type pulsars. Centrifugal acceleration of particles in a monopol-like magnetic field geometry is analyzed and the efficiency constraints, imposed by corotation, inverse Compton interactions and curvature radiation reaction are determined. We derive expressions for the maximum particle energy and provide estimates for the corresponding high-energy curvature and inverse Compton power outputs. It is shown that for Crab-like pulsars, electron Lorentz factor up to $\gamma \sim 10^7$ can be achieved, allowing inverse Compton (Klein-Nishina) up-scattering of thermal photons to TeV energies with a maximum luminosity output of $\sim10^{31}$ erg/s. Curvature radiation, on the other hand, will result in a strong GeV emission output of up to $\sim(10^{34}-10^{35})$ erg/s, quasi-exponentially decreasing towards higher energies for photon energies below $\sim 50$ GeV. Accordingly to the results presented only young pulsars are expected to be sites of detectable VHE $\gamma$-ray emission.

TH‐C‐350‐06: Laser‐To‐Proton Energy Transfer Efficiency in Laser‐Plasma Interactions

Purpose: In recent years there has been an explosion of research work concerning the topic of charged ion acceleration using high‐power lasers. The maximum particle energy and the shape of the distribution function are the two main parameters influencing the potential utilization of the new technology in radiation therapy. Low energy transfer efficiency from the laser pulse to protons (∼ 5%) is one of the detrimental factors limiting the production of particle beams in the therapeutic energy range. In this study we show that the energy transfer efficiency can be significantly increased through a process of splitting the single interaction stage (conventional interaction design) into multiple sub‐stages. Method and Materials: 2D Particle in Cell simulation code and 3D theoretical model were used to simulate interaction of laser sub‐pulses with multiple targets in a sequential manner. The thin layer of protons is initially located only at the back surface of the first target. The remaining targets are devoid of any contaminant hydrogen‐rich materials. Results: It was shown that in a three‐stage setting, there is ∼ 60% increase in the energy efficiency of the laser accelerator as compared to a single interaction scheme. At the same time according to the results of our 3D model, it should be possible to increase the energy efficiency by more than 100% for a six‐stage interaction setting without using more powerful lasers. Conclusion: The splitting of a single interaction site into multiple stages is an effective way of reducing an irreversible component in the energy exchange process between the laser and protons. As a result more laser energy is transformed into proton kinetic energy, thus increasing effectiveness of the “pump”.

Diffusive Shock Acceleration with Magnetic Amplification by Nonresonant Streaming Instability in Supernova Remnants

We investigate the diffusive shock acceleration in the presence of the nonresonant streaming instability introduced by Bell. The numerical MHD simulations of the magnetic field amplification combined with the analytical treatment of cosmic-ray acceleration permit us to calculate the maximum energy of particles accelerated by high-velocity supernova shocks. The estimates for the Cas A, Kepler, SN 1006, and Tycho historical supernova remnants are given. We also found that the amplified magnetic field is preferentially oriented perpendicular to the shock front downstream of the fast shock. This explains the origin of the radial magnetic fields observed in young supernova remnants.

Maximum energy of cosmic rays accelerated in supernova remnants

Amplification of the magnetic field in young supernova remnants leads to the corresponding increase in the maximum energy of particles approximately to the knee energy 3 × 1015 eV at the end of the free expansion stage. The knee in the spectrum of cosmic rays is due to the transition of the supernova remnant from the free expansion stage to the Sedov stage. The knee energy is calculated for different types of supernovae. The maximum energy of accelerated particles teaches Z × 1017 eV for the observed remnant expansion velocities.

Selective internal radiation therapy (SIRT) for liver metastases secondary to colorectal adenocarcinoma

Selective internal radiation therapy (SIRT) is a relatively new commercially available microbrachytherapy technique for treatment of malignant hepatic lesions using (90)Y embedded in resin microspheres, which are infused directly into the hepatic arterial circulation. It is FDA approved for liver metastases secondary to colorectal carcinoma and is under investigation for treatment of other liver malignancies, such as hepatocellular carcinoma and neuroendocrine malignancies. A modest number of clinical trials, preclinical animal studies, and dosimetric studies have been reported. Here we review several of the more important results. High doses of beta radiation can be selectively delivered to tumors, resulting in impressive local control and survival rates. Ex vivo analyses have shown that microspheres preferentially cluster around the periphery of tumor nodules with a high tumor:normal tissue ratio of up to 200:1. Toxicity is usually mild, featuring fatigue, anorexia, nausea, abdominal discomfort, and slight elevations of liver function tests. Selective internal radiation therapy represents an effective means of controlling liver metastases from colorectal adenocarcinoma. Clinical trials have demonstrated improved local control of disease and survival with relatively low toxicity. Investigations of SIRT for other hepatic malignancies and in combination with newer chemotherapy agents and targeted biologic therapies are under way or in planning. A well-integrated team involving interventional radiology, nuclear medicine, medical oncology, surgical oncology, medical physics, and radiation oncology is essential for a successful program. Careful selection of patients through the combined expertise of the team can maximize therapeutic efficacy and reduce the potential for adverse effects.

Galactic acceleration phenomena

AbstractI review the current status of our observational knowledge of prominent classes of particle accelerators in the Galaxy, namely shell-type supernova remnants (SNRs) and pulsar wind nebulae. I highlight in particular the contribution of the recent improvement in sensitivity of very-high-energy (VHE) γ-ray observations, which are currently the most direct probe of particle acceleration in the Galaxy up to energies of several hundreds of TeV.Shell-type SNRs have long been proposed as sources of the Galactic cosmic rays. In recent years, X-ray observations have revealed very thin, non-thermal rims in many young SNRs, and I discuss the implications of these observations for magnetic field amplification and the maximum particle energy attainable by acceleration at the blast wave. I then review the current status of the evidence for accelerated nuclei in these objects, and summarise current uncertainties.The most numerous class of identified Galactic VHE gamma-ray sources is currently that of pulsar wind nebulae (PWNe). The emission from these objects is generally assumed to be predominantly leptonic, and I outline the new information provided by VHE gamma-ray observations beyond what could be inferred from observations of synchrotron emission.

TH‐D‐ValA‐02: Prepulse Effect and Maximum Energy of Protons Accelerated by High‐Power Lasers

Purpose: In recent years there has been an explosion of research work concerning the topic of charged ion acceleration using high‐power lasers. The maximum particle energy and the shape of the distribution function are the two main parameters influencing the potential utilization of the new technology in radiation therapy. The energy spectrum depends on the quality of the laser beam as well as the geometric shape of the interaction target. Recent experiments have shown that it is possible to generate quasi‐monoenergetic protons using a double target system. However, influence of the laser prepulse has not yet been fully quantified. The purpose of this study is to find how the laser prepulse changes the property of the target and under which conditions can one expect the highest proton energy. Method and Materials: A radiation‐magnetodynamics code with inline atomic kinetic modeling was used to simulate interaction of the laser prepulse with initially cold double‐layer target. The simulated target system consists of an aluminum substrate on which a thin layer of hydrogen is located. Different initial substrate thicknesses and laser contrast ratios were simulated to find an optimal interaction conditions. Results of hydrodynamic simulations were used as initial conditions for the simulation of the main pulse interaction with the altered target. Results: It is shown that interaction of the prepulse with the target leads to its partial ionization with subsequent formation of a shock wave. Propagating shock wave destroys initial configuration of matter in a substrate, greatly expanding it in laser's propagation direction. Results of simulations suggest that maximal proton energy is achieved using lasers with smallest contrasts, incident on targets of thicknesses 1–2 μm. Conclusions: Presence of laser prepulse impedes acceleration efficiency of protons. Minimizing the contrast ratio or shortening the duration of the prepulse should result in higher maximal proton energies.

Nonlinear Shock Acceleration beyond the Bohm Limit

We suggest a physical mechanism whereby the acceleration time of cosmic rays (CRs) by shock waves can be significantly reduced. This creates the possibility of particle acceleration beyond the knee energy at ~1015 eV. The acceleration results from a nonlinear modification of the flow ahead of the shock supported by particles already accelerated to the knee momentum at p ~ p*. The particles gain energy by bouncing off converging magnetic irregularities frozen into the flow in the shock precursor and not so much by recrossing the shock itself. The acceleration rate is thus determined by the gradient of the flow velocity and turns out to be formally independent of the particle mean free path (mfp). The velocity gradient is, in turn, set by the knee particles at p ~ p* as having the dominant contribution to the CR pressure. Since it is independent of the mfp, the acceleration rate of particles above the knee does not decrease with energy, unlike in the linear acceleration regime. The reason for the knee formation at p ~ p* is that particles with p > p* are effectively confined to the shock precursor only while they are within limited domains in the momentum space, while other particles fall into loss islands, similar to the loss cone of magnetic traps. This structure of the momentum space is due to the character of the scattering magnetic irregularities. They are formed by a train of shock waves that naturally emerge either from unstably growing and steepening magnetosonic waves or as a result of acoustic instability of the CR precursor (CRP). These losses steepen the spectrum above the knee, which also prevents the shock width from increasing with the maximum particle energy.

Particle Acceleration in Gamma-Ray Burst Jets

Gradual shear acceleration of energetic particles in gamma-ray burst (GRB) jets is considered. Special emphasis is given to the analysis of universal structured jets, and characteristic acceleration timescales are determined for a power-law and a Gaussian evolution of the bulk flow Lorentz factor $\gamma_b$ with angle $\phi$ from the jet axis. The results suggest that local power-law particle distributions may be generated and that higher energy particles are generally concentrated closer to the jet axis. Taking several constraints into account we show that efficient electron acceleration in gradual shear flows, with maximum particle energy successively decreasing with time, may be possible on scales larger than $r \sim 10^{15}$ cm, provided the jet magnetic field becomes sufficiently weak and/or decreases rapidly enough with distance, while efficient acceleration of protons to ultra-high energies $> 10^{20}$ eV may be possible under a wide range of conditions.

The radio–ultraviolet spectral energy distribution of the jet in 3C 273

We present deep VLA and HST observations of the large-scale jet in 3C 273 matched to 0.3" resolution. The observed spectra show a significant flattening in the infrared-ultraviolet wavelength range. The jet's emission cannot therefore be assumed to arise from a single electron population and requires the presence of an additional emission component. The observed smooth variations of the spectral indices along the jet imply that the physical conditions vary correspondingly smoothly. We determine the maximum particle energy for the optical jet using synchrotron spectral fits. The slow decline of the maximum energy along the jet implies particle reacceleration acting along the entire jet. In addition to the already established global anti-correlation between maximum particle energy and surface brightness, we find a weak positive correlation between small-scale variations in maximum particle energy and surface brightness. The origin of these conflicting global and local correlations is unclear, but they provide tight constraints for reacceleration models.

On the spectrum of high-energy cosmic rays produced by supernova remnants in the presence of strong cosmic-ray streaming instability and wave dissipation

The cosmic-ray streaming instability creates strong magnetohydrodynamic turbulence in the precursor of a SN shock. The level of turbulence determines the maximum energy of cosmic-ray particles accelerated by the diffusive shock accelera- tion mechanism. In this paper we present the continuation of previous work (Ptuskin & Zirakashvili 2003). We assume that Kolmogorov type nonlinear wave interactions together with ion-neutral collisions restrict the amplitude of the random mag- netic field. As a result, the maximum energy of the accelerated particles strongly depends on the age of a SNR. The average spectrum of cosmic rays injected in the interstellar medium in the course of the adiabatic SNR evolution (the Sedov stage) is approximately Q(p)p 2 ∝ p −2 at energies larger than 10−30 GeV/nucleon and with a maximum particle energy that is close to the position of the knee in the cosmic-ray spectrum observed at ∼4 × 10 15 eV. At an earlier stage of SNR evolution - the ejecta-dominated stage described by the Chevalier-Nadyozhin solution, the particles are accelerated to higher energies and have a rather steep power-law distribution. These results suggest that the knee may mark the transition from the ejecta-dominated to the adiabatic evolution of SNR shocks which accelerate cosmic rays.

Non-thermal X-ray emission from young supernova remnants

The Galactic (nucleonic) cosmic-ray spectrum up to the knee (E 10 15 eV) is attributed to acceleration processes that take place near the external shocks around supernova remnants (SNRs). Theoretical predictions based on the theory of diffusive shock acceleration give a similar estimate for the maximum particle energy that can be reached at these shocks: E ∼ 10 14 -10 15 eV. Electrons with energies E ∼ 10 14 eV radiate X-ray photons in the ∼10-100 μG magnetic fields present in many young SNRs. These electrons near the knee give rise to a non-thermal X-ray component in the spectrum of young supernova remnants. Recent observations of SN1006 and G347.3-0.5 confirm this prediction. We have combined hydrodynamical calculations of the evolution of a young remnant with an algorithm that simultaneously calculates the acceleration of electrons, their radiation losses and the synchrotron spectrum of a young supernova remnant. The electrons are treated using a test-particle approximation. We give a semi-analytical estimate of the maximum electron energy and typical synchrotron frequencies for young remnants at the end of the free-expansion stage of their evolution. We present spectra of the energy distribution of the electrons in a young supernova remnant, and construct a synchrotron map in the X-ray domain, assuming Bohm diffusion within the remnant and a shock-compressed magnetic field.

Particle Orbit Analysis under the Ion Cyclotron Range of Frequency Heating in the Large Helical Device

Particles under the influence of the ion cyclotron range of frequency (ICRF) electromagnetic field were analyzed in the large helical device (LHD) by numerically solving the equation of motion instead of the guiding-center equation. Behaviors of the ICRF-heated particles in three cases of slowing-down by thermal electrons were compared. We also compared the characteristics of the ICRF-heated particles in the standard magnetic configuration (Rax = 3.75 m) with those in the inwardly shifted magnetic configuration (Rax = 3.6 m). It was found that the maximum energies of particles starting from the core plasma region exceed 300 keV and that such particles are confined within the vacuum vessel wall for 10-4 s. It was confirmed that the ICRF-heated particles with energies of around 400 keV are lost through the divertor field lines. The maximum energy of the ICRF-heated particles starting from the core region rises with increasing electron temperature. As a result, the energy level relevant to the proton (p)–boron (11B) fusion reaction (≃ 650 keV) was obtained when Te > 30 keV. Through acceleration in both the parallel and the perpendicular direction to the magnetic field, the high-energy chaotic orbit particles were produced by ICRF heating. It was also found that the energy at which the transition to the high-energy chaotic orbit particle occurs determines the upper energy limit of the ICRF-heated particle in LHD. A high confinement performance was found for the high-energy chaotic orbit particles produced by the ICRF field. The particle orbits in the inwardly shifted magnetic configuration were more widespread within the core region. Thus, it was concluded that the ICRF heating efficiency of the core plasma region in the inwardly shifted magnetic configuration exceeds that in the standard magnetic configuration.

Nonthermal Particles and Radiation Produced by Cluster Merger Shocks

We have developed a numerical model for the temporal evolution of particle and photon spectra resulting from nonthermal processes at the shock fronts formed in merging clusters of galaxies. Fermi acceleration is approximated by injecting power-law distributions of particles during a merger event, subject to constraints on maximum particle energies. We consider synchrotron, bremsstrahlung, Compton, and Coulomb processes for the electrons, nuclear, photomeson, and Coulomb processes for the protons, and knock-on electron production during the merging process. The broadband radio through gamma-ray emission radiated by nonthermal protons and primary and secondary electrons is calculated both during and after the merger event. Using ROSAT observations to establish typical parameters for the matter density profile of clusters of galaxies, we find that typical merger shocks are weak and accelerate particles with relatively soft spectra. We consider the prospects for detecting nonthermal radio and gamma-ray emission from clusters of galaxies and implications for the origin of ultra-high energy cosmic rays and the diffuse gamma-ray background. Our results suggest that only a few of the isotropically-distributed unidentified EGRET sources are due to shocks formed in cluster mergers, and that only a minor contribution to the diffuse extragalactic gamma-ray background can originate from cluster merger shocks. Cluster merger shocks can accelerate protons to $\lesssim 10^{19}$ eV for the standard parameters considered here. We predict that {\it GLAST} will detect several cluster mergers and, depending on the mean magnetic fields in the intracluster medium, the Low Frequency Array could detect anywhere from several to several hundred.

Particle acceleration through multiple conversions from a charged into a neutral state and back

We propose a new means for a quick and efficient acceleration of protons and/or electrons in relativistic bulk flows. The maximum attainable particle energies are limited either by radiative losses or by the condition of confinement in the magnetic field. The new mechanism takes advantage of a conversion of particles from the charged state (protons, electrons or positrons) into a neutral state (neutrons or photons) and back. In most cases, the conversion is photon induced and requires the presence of intense radiation fields, but under special circumstances the converter acceleration mechanism may operate via other charge-changing reactions, for example, inelastic nucleon-nucleon collisions. As in the traditional, ``stochastic'' (or diffusive) acceleration models, the acceleration cycle in the proposed scenario consists of the escape of particles from the relativistic flow followed by their return back after deflection from the ambient magnetic field. The difference is that the charge-changing reactions, which occur during the cycle, allow accelerated particles to increase their energies in each cycle by a factor much larger than 2 and usually roughly equal to the bulk Lorentz factor squared. The emerging spectra of accelerated particles can be very hard and their maximum energy in some cases is larger than in the standard mechanism. This significantly reduces the required energy budget of the sources of the highest-energy particles observed in cosmic rays. The proposed acceleration mechanism has a distinctive feature---it unavoidably creates neutral beams, consisting of photons, neutrinos, or neutrons, whose beam pattern may be much broader than the inverse Lorentz factor of the relativistic flow. Also, the new mechanism may serve as an efficient means of transferring the energy of bulk motion to gamma radiation and, if the accelerated particles are nucleons, inevitably produces high-energy neutrinos at a relative efficiency approaching $\ensuremath{\gtrsim}50%.$

Limits on diffusive shock acceleration in supernova remnants in the presence of cosmic-ray streaming instability and wave dissipation

The instability in the cosmic-ray precursor of a supernova shock moving in interstellar medium is studied. The level of magnetohydrodynamic turbulence in this region determines the maximum energy of particles accelerated by the diffusive shock acceleration mechanism. The high efficiency of cosmic ray acceleration is accepted, and the consideration is not limited by the case of weak turbulence. It is assumed that Kolmogorov type nonlinear wave interactions together with the ion-neutral collisions restrict the amplitude of random magnetic field. As a result, the maximum energy of accelerated particles strongly depends on the age of supernova remnant. The maximum energy can be as high as $\sim10^{17}Z$ eV in young supernova remnants and falls down to about $\sim10^{10}Z$ eV at the end of Sedov stage. Thus the standard estimate of maximum particle energy based on the Bohm limit calculated for the interstellar magnetic field strength is not justified in this case. This finding may explain why the supernova remnants with the age more than a few thousand years are not prominent sources of very high energy gamma-rays.