We calculate dilaton and axion radiation generated in the collision of two straight initially unexcited strings and give a rough cosmological estimate of dilaton and axion densities produced via this mechanism in the early universe.
An algorithm to calculate the energy deposition distribution produced by monoenergetic fast electrons normally incident on the semi-infinite absorber is given. While the algorithm is based on an elementary relation that is also a basis of... more
An algorithm to calculate the energy deposition distribution produced by monoenergetic fast electrons normally incident on the semi-infinite absorber is given. While the algorithm is based on an elementary relation that is also a basis of similar work by Kobetich and Katz, higher accuracy has been attained and the region of validity has been extended by using better approximations and new expressions for its evaluation. Empirical equations recently developed for the extrapolated range and the backscattering of electrons have been utilized, and the effect of bremsstrahlung production has been taken into account by the use of a modified KochMotz equation. Expressions for three adjustable parameters introduced into the algorithm have been determined by least-squares fit to published experimental and Monte Carlo results of the energy deposition distribution. The algorithm obtained is valid for incident energies from ~0.1 to 20 MeV and for atomic numbers of the absorber from ~5.3 (the effective atomic number for a light compound) to 82. (Note: The publisher does not allow to put a postprint. So, please ask the author for the copy of the paper.)
The standard model of the post-shock accretion flow in mCVs is discussed. We present some results of the current study of two-temperature flows in mCVs. New observations supporting the standard model are presented. Recent developments in... more
The standard model of the post-shock accretion flow in mCVs is discussed. We present some results of the current study of two-temperature flows in mCVs. New observations supporting the standard model are presented. Recent developments in the studies of the global properties of the accretion stream are briefly discussed.
Sophisticated electronic devices comprising sensitive microelectronic components have been installed in the close proximity of the 720 MeV superconducting electron linear accelerator (linac) driving the FLASH (Free Electron Laser in... more
Sophisticated electronic devices comprising sensitive microelectronic components have been installed in the close proximity of the 720 MeV superconducting electron linear accelerator (linac) driving the FLASH (Free Electron Laser in Hamburg), presently in operation at DESY in Hamburg. Microelectronic chips are inherently vulnerable to ionizing radiation, usually generated during routine operation of high-energy particle accelerator facilities like the FLASH. Hence, in order to assess the radiation effect on microelectronic chips and to develop suitable mitigation strategy, it becomes imperative to characterize the radiation field in the FLASH environment. We have evaluated the neutron and gamma energy (spectra) and dose distributions at critical locations in the FLASH tunnel using superheated emulsion (bubble) detectors, GaAs light emitting diodes (LED), LiF-thermoluminescence dosimeters (TLD) and radiochromic (Gafchromic EBT) films. This paper highlights the application of passive dosimeters for an accurate analysis of the radiation field produced by high-energy electron linear accelerators.
We investigate the hydrodynamics of accretion channelled by a dipolar magnetic field (funnel flows). We consider situations in which the electrons and ions in the flow cannot maintain thermal equilibrium [two-temperature (2T) effects] due... more
We investigate the hydrodynamics of accretion channelled by a dipolar magnetic field (funnel flows). We consider situations in which the electrons and ions in the flow cannot maintain thermal equilibrium [two-temperature (2T) effects] due to strong radiative loss, and determine the effects on the keV X-ray properties of the systems. We apply this model to investigate the accretion shocks of white dwarfs in magnetic cataclysmic variables (mCVs). We have found that the incorporation of 2T effects could harden the keV X-rays. Also, the dipolar model yields harder X-ray spectra than the standard planar model if white dwarf is sufficiently massive (>~1Msolar). When fitting observed keV X-ray spectra of mCVs, the inclusion of 2T hydrodynamics and a dipolar accretion geometry lowers estimates for white dwarf masses when compared with masses inferred from models excluding these effects. We find mass reductions <~9 per cent in the most massive cases.
The component of energy deposition due to radiative processes (bremsstrahlung component) in absorbers irradiated by electron beams has been computed together with the total energy deposition by using the ITS Monte Carlo system version... more
The component of energy deposition due to radiative processes (bremsstrahlung component) in absorbers irradiated by electron beams has been computed together with the total energy deposition by using the ITS Monte Carlo system version 3.0. Plane-parallel electron beams with energies from 0.1 to 100~MeV have been assumed to be incident normally on the slab absorber, whose thickness is 2.5 times the continuous slowing-down approximation (csda) range of the incident electrons. Absorber materials considered are elemental solids with atomic numbers between 4 and 92 (Be, C, Al, Cu, Ag, Au and U). An analytic formula is given to express the depth profile of the bremsstrahlung component as a function of scaled depth (depth in units of the csda range), incident-electron energy and absorber atomic number. It is also applicable to compounds.
The dependence of the alpha particle counting rate from samples of powdered sherds on the size of grains selected for counting is interpreted with the aid of photographs of polished sections of the sherds showing the sizes of the... more
The dependence of the alpha particle counting rate from samples of powdered sherds on the size of grains selected for counting is interpreted with the aid of photographs of polished sections of the sherds showing the sizes of the inclusions. The measurements are discussed in terms of the errors possible in thermoluminescence dating when determining the internal beta dose-rate due to the thorium and uranium series from alpha particle counting rates.
We use a two-temperature hydrodynamical formulation to determine the temperature and density structures of the post-shock accretion flows in magnetic cataclysmic variables (mCVs) and calculate the corresponding X-ray spectra. The effects... more
We use a two-temperature hydrodynamical formulation to determine the temperature and density structures of the post-shock accretion flows in magnetic cataclysmic variables (mCVs) and calculate the corresponding X-ray spectra. The effects of two-temperature flows are significant for systems with a massive white dwarf and a strong white-dwarf magnetic field. Our calculations show that two-temperature flows predict harder keV spectra than one-temperature flows for the same white-dwarf mass and magnetic field. This result is insensitive to whether the electrons and ions have equal temperature at the shock, but depends on the electron-ion exchange rate, relative to the rate of radiative loss along the flow. White-dwarf masses obtained by fitting the X-ray spectra of mCVs using hydrodynamic models including the two-temperature effects will be lower than those obtained using single-temperature models. The bias is more severe for systems with a massive white dwarf.
A project is in progress for a systematic, rigorous validation of Geant4 electromagnetic physics. This paper presents the first results concerning the validation of Geant4 bremsstrahlung models against experimental data.
Sophisticated electronic devices comprising sensitive microelectronic components have been installed in the close proximity of the 720 MeV superconducting electron linear accelerator (linac) driving the FLASH (Free Electron Laser in... more
Sophisticated electronic devices comprising sensitive microelectronic components have been installed in the close proximity of the 720 MeV superconducting electron linear accelerator (linac) driving the FLASH (Free Electron Laser in Hamburg), presently in operation at DESY in Hamburg. Microelectronic chips are inherently vulnerable to ionizing radiation, usually generated during routine operation of high-energy particle accelerator facilities like the FLASH. Hence, in order to assess the radiation effect on microelectronic chips and to develop suitable mitigation strategy, it becomes imperative to characterize the radiation field in the FLASH environment. We have evaluated the neutron and gamma energy (spectra) and dose distributions at critical locations in the FLASH tunnel using superheated emulsion (bubble) detectors, GaAs light emitting diodes (LED), LiF-thermoluminescence dosimeters (TLD) and radiochromic (Gafchromic EBT) films. This paper highlights the application of passive dosimeters for an accurate analysis of the radiation field produced by high-energy electron linear accelerators.
The stand-off shock formed in the accretion flow on to a stationary wall, such as the surface of a white dwarf, may be thermally unstable, depending on the cooling processes which dominate the post-shock flow. Some processes lead to... more
The stand-off shock formed in the accretion flow on to a stationary wall, such as the surface of a white dwarf, may be thermally unstable, depending on the cooling processes which dominate the post-shock flow. Some processes lead to instability, while others tend to stabilize the shock. We consider competition between the destabilizing influence of thermal bremsstrahlung cooling, and a stabilizing process which is a power law in density and temperature. Cyclotron cooling and processes which are of order 1, 3/2 and 2 in density are considered. The relative efficiency and power-law indices of the second mechanism are varied, and particular effects on the stability properties and frequencies of oscillation modes are examined.
The temperatures of electrons and ions in the post-shock accretion region of a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass flow rates or for sufficiently weak magnetic fields. At lower mass flow rates or in... more
The temperatures of electrons and ions in the post-shock accretion region of a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass flow rates or for sufficiently weak magnetic fields. At lower mass flow rates or in stronger magnetic fields, efficient cyclotron cooling will cool the electrons faster than the electrons can cool the ions and a two-temperature flow will result. Here we investigate the differences in polarized radiation expected from mCV post-shock accretion columns modeled with one- and two-temperature hydrodynamics. In an mCV model with one accretion region, a magnetic field ≳30 MG and a specific mass flow rate of ∼0.5 g cm−2 s−1, along with a relatively generic geometric orientation of the system, we find that in the ultraviolet either a single linear polarization pulse per binary orbit or two pulses per binary orbit can be expected, depending on the accretion column hydrodynamic structure (one- or two-temperature) modeled. Under conditions where the physical flow is two-temperature, one pulse per orbit is predicted from a single accretion region where a one-temperature model predicts two pulses. The intensity light curves show similar pulse behavior but there is very little difference between the circular polarization predictions of one- and two-temperature models. Such discrepancies indicate that it is important to model some aspect of two-temperature flow in indirect imaging procedures, like Stokes imaging, especially at the edges of extended accretion regions, were the specific mass flow is low, and especially for ultraviolet data.
The construction characteristics and the performances of an x‐ray diode working at 200 Hz are described. The temporal behavior of both the plasma cathode emission and the diode discharge has been studied, including the measurements of... more
The construction characteristics and the performances of an x‐ray diode working at 200 Hz are described. The temporal behavior of both the plasma cathode emission and the diode discharge has been studied, including the measurements of vacuum effects on the repetition rate and lifetime capability, preionization versus electrical features, and ionization density versus dosage. This simple and low‐cost x‐ray diode has been used to uniformly preionize a half‐liter XeCl laser, delivering an output laser energy of 800 mJ/shot at a 100 Hz repetition rate.
We have tested an X-ray diode working in reflection geometry, which uses commercial spark-plugs as plasma cathode. The results of the characterisation work confirm that the spark-plug plasma cathode is a reliable and long-lifetime... more
We have tested an X-ray diode working in reflection geometry, which uses commercial spark-plugs as plasma cathode. The results of the characterisation work confirm that the spark-plug plasma cathode is a reliable and long-lifetime electron source for X-ray diodes. After more than 10^6 shots, the X-ray dose, uniformity, and ionisation rate values guarantee an effective preionisation of excimer laser discharges. Owing to both the long lifetime and the substantial absence of maintenance, this X-ray diode seems suitable to preionise commercial gas lasers, such as excimer and TEA CO2 lasers.
The construction characteristics and the performance of an X-ray generator working at 200 Hz are described. The possibility to scale this system up to 1 kHz for the preionization of high-average power gas lasers is discussed.
The Scienta ESCA300 is a state-of-the-art x-ray photoelectron spectrometer with high intensity, and high energy and spatial resolution. This unique combination of features is made possible by the use of a high-power rotating anode x-ray... more
The Scienta ESCA300 is a state-of-the-art x-ray photoelectron spectrometer with high intensity, and high energy and spatial resolution. This unique combination of features is made possible by the use of a high-power rotating anode x-ray source with monochromatization, high transmission and imaging electron optics and multichannel detector. We report on the performance of this instrument for conducting samples and as a microanalytical device
