We report on the design of the CIBER-X source which is a new laser driven table-top ultrashort electron and x-ray source. X-ray pulses are produced by a three-step process which consists of the electron pulse production from a thin metallic photocathode illuminated by picosecond 213 nm laser pulses with 16 ps duration. The electrons are accelerated in the diode by a cw electric field of 11 MV/m, and the photoinjector produces a single 70 - 100 keV electron pulse of approximately 0,5 nC and approximately 20 A peak current at a repetition rate of 10 Hz. The gun is a standard Pierce diode electrode type, the electrons leaving the diode through a hole made in the anode. The electrons are then transported along a path approximately 20 cm long, and are focused by two magnetic fields produced by electromagnetic coils. Finally, the x-rays are produced by the impact of electrons on a massive target of Tm. Simulations of geometrical and energetic characteristics of the complete source were done previously with assistance of the code PIXEL1. Finally, experimental performances of electron and x-ray bursts are discussed.
Characteristics and applications of a kilohertz-range stroboscopic x-ray generator (SX-T99) are described. This generator is primarily developed in order to increase the maximum photon energy of the pulse x-rays. And the setup is composed of the following essential components: a thyratron pulser, a high-voltage transformer having a ferrite core with a maximum output voltage of 300 kV, a sequence controller, a DC power supply for the hot cathode (filament), and an x-ray tube. The main condenser of 27 nF in the pulser is charged up to 15 kV, and the electric charges in the condenser are discharged repetitively to the primary coil of the transformer. Because the high-voltage pulses from the secondary coil are then applied to the x-ray tube, repetitive harder x-rays are produced. The x-ray tube is of a triode having a filament that is primarily driven at the temperature- limited current region. In this triode, since the grid is connected to the cathode, the tube is driven as a diode. The tube voltage roughly increased in proportion to the charging voltage, and the maximum value was about 300 kV. Thus, the maximum photon energy had a value of about 300 keV. The tube current was primarily determined by the filament temperature and was less than 2 A. The x-ray output displayed plural pulses, and the width of the first pulse was about 400 ns. The maximum repetition rate was about 1 kHz, and the dimension of the x-ray source had values of about 3.5 X 3.5 mm. The high-speed radiography was performed using a computed radiography (CR) system.
A plasma-dielectric waveguide is presented as a possible optical element for enhancing the phase matching condition in high harmonic generation processes. The phase velocity of the pump wave, in a partially ionized plasma, is controlled by its interaction with dielectric walls within the waveguide. The phase velocity is adjusted by changing the thickness and spacing of the dielectric slab walls.
A new class of accelerators Undulated Induction Accelerators (UNIAC -- EH-accelerators) is proposed as a basis for intense X-ray flash sources. Apart from X-ray sources these accelerators can be used for cooling (with simultaneous acceleration) of electron and ion beams, for forming high intensity relativistic beams of neutral molecules, neutrons and plasma fluxes, etc. The key idea of EH-accelerators is the utilization of undulated forms of trajectories of the accelerated particles. They may be trajectories of sinewave form, spiral form, and more complex spatial forms. It is clear that characteristic parameters of modern materials (permanent magnets, accelerator ferrites, and superconductors) allow construction of especially compact, and, at the same time, reliable, and relatively inexpensive acceleration systems in the energy range from hundreds keV to a few GeV. The possibility of constructing compact high intensity X-ray flash sources on the basis of EH-accelerators is substantiated for realization. Special schemes for forming pico- and nano-second intense X-ray pulses are proposed and analyzed. The project peculiarities and state of a preliminary experiment are discussed.
High speed X-ray tomography is being developed for on-line measurement of multiphase flow for well management in the oil industry. To reduce motion artifacts to acceptable levels a source is required that can scan about a 100 mm diameter pipe in approximately 20 ms, thus rendering a rotating source an impractical solution. In order to achieve a spatial resolution of 2 mm in the reconstructed image a total of 105 individual projections over a 210 degree arc are required. The large number of point sources means individual X-ray tubes are not practicable. Our solution is to use multiple electron beams where the active focal spot can be rapidly scanned across the target in an arc about the pipe with the use of electronic grids. This paper describes a prototype of such a tube designed, in the first instance, to cover a 30 degree arc and consisting of 13 individual emitters. Having proved the principle of operation a full system is now in the design stage and shall be briefly discussed.
Tentative study on characteristic x-ray enhancement by fluorescent emission of radiation by plasma x-ray source is described. The enhancement was performed by the plasma flash x-ray generator having a cold-cathode triode. And the generator employs a high-voltage power supply, a low-impedance coaxial transmission line with a gap switch, a high-voltage condenser with a capacity of 200 nF, a turbo-molecular pump, a thyristor pulser as a trigger device, and a flash x-ray tube. The high-voltage main condenser is charged up to 60 kV by the power supply, and the electric charges in the condenser are discharged to the tube after triggering the cathode electrode. The flash x-rays are then produced. The x-ray tube is of a demountable triode that is connected to the turbo molecular pump with a pressure of approximately 1 mPa. As the electron flows from the cathode electrode are roughly converged to the target by the electric field in the tube, the plasma x-ray source, which consists of metal ions and electrons, forms by the target evaporating. Both the tube voltage and current displayed damped oscillations, and their peak values increased according to increases in the charging voltage. In the present work, the peak tube voltage was almost equivalent to the initial charging voltage of the main condenser, and the peak current was less than 30 kA. The characteristic x-ray intensity substantially increased according to the growth in the plasma x-ray source. When the linear plasma x-ray source formed, the bremsstrahlung x-rays were absorbed without using a monochromatic filter, and high- intensity characteristic x-rays were produced.
Modern Monte Carlo radiation transport codes can be applied to model most applications of radiation, from optical to TeV photons, from thermal neutrons to heavy ions. Simulations can include any desired level of detail in three-dimensional geometries using the right level of detail in the reaction physics. The technology areas to which we have applied these codes include medical applications, defense, safety and security programs, nuclear safeguards and industrial and research system design and control. The main reason such applications are interesting is that by using these tools substantial savings of time and effort (i.e. money) can be realized. In addition it is possible to separate out and investigate computationally effects which can not be isolated and studied in experiments. In model calculations, just as in real life, one must take care in order to get the correct answer to the right question. Advancing computing technology allows extensions of Monte Carlo applications in two directions. First, as computers become more powerful more problems can be accurately modeled. Second, as computing power becomes cheaper Monte Carlo methods become accessible more widely. An overview of the set of Monte Carlo radiation transport tools in use a LLNL will be presented along with a few examples of applications and future directions.
Unlike x-ray generators, neutron source have inherently low brightness, and care must be exerted in the design of neutron scattering instruments and their coupling to the source to ensure optimal use of the beam. We present a general, versatile Monte Carlo tool for the computer simulation of neutron optics and neutron scattering instruments that allows a user to produce computer models of an instrument and study its performance quantitatively. The Neutron Instrument Simulation Package (NISP) implements a wide range of neutron optics models to describe neutron transport (including gravity) and scattering in the elements making up the instrument. The program is freely available on the world-wide web at http://strider.lansce.lanl.gov/NISP/Welcome.html
Monte Carlo calculations have been widely employed to model the interactions of electrons and photons as they travel through and collide with matter. This approach has been applied with some success to the problem of simulating the response of gas-filled proportional counters, mapping out electron transport through the electric field on an interaction-by-interaction basis. These studies focus on the multiplication of electrons as they drift into the high electric field region of the detector and subsequently avalanche. We are using this technique in our new simulation code to depict avalanching in microgap gas-filled proportional counters, in order to investigate the variation of two principle detector properties with the anode pitch used in the detector. Spatial resolution information can be obtained by measuring the lateral diffusion distance of an electron from the point where it is liberated to the point in the detector where it initiates an avalanche. By also modeling the motion of the positive ions that are left behind from the initial avalanche, we are able to gauge the effect of space charge distortion on subsequent avalanches. This effect is particularly important at the high X-ray count rates that we are interested in for our ultimate aim, which is to use the detectors as part of a high-speed tomography system for imaging multiphase oil/water/gas flows.
Differential sampling allows Monte Carlo to calculate the derivatives of a response with respect to some problem parameter, simultaneous with the calculation of the response itself. Such derivatives can be then used to construct new responses due to changes in the parameter. This capability has been added to our Monte Carlo code which simulates digital images from mammography systems, using realistic pixel sizes. The differential sampling method and its application to the image simulation code are described. Using the derivatives of an image, an example problem dealing with the visibility of a tumor is used to demonstrate the power of differential sampling.
Dedicated X-ray tubes and thermoelectrically cooled detectors have revolutionized in the last years the field of energy dispersive X-ray fluorescence (EDXRF) analysis. The small size of both X-ray tubes and detectors allowed the construction of portable systems, with characteristics similar to those of laboratory systems. The portability of the new EDXRF systems opened also all the new analytical fields where the 'sample' cannot be transported to the Laboratory (analysis of works of art in museum, analysis of minerals, of soil, and etc.).
The image information transfer properties of a number of x-ray fluorescent screens have been calculated for x-ray energies from 10 to 160 keV. The detective quantum efficiency of the screens at each x-ray energy has been determined by separate calculations of the x-ray absorption efficiency and the statistical factor associated with the emission of light photons upon absorption of an incident x ray. The point spread function of the screens at each x-ray energy has also been calculated. These basic physical quantities will be useful for the prediction of the information transfer properties of X-ray intensifying screens.
COG is a major multiparticle simulation code in the LLNL Monte Carlo radiation transport toolkit. It was designed to solve deep-penetration radiation shielding problems in arbitrarily complex 3D geometries, involving coupled transport of photons, neutrons, and electrons. COG was written to provide as much accuracy as the underlying cross-sections will allow, and has a number of variance-reduction features to speed computations. Recently COG has been applied to the simulation of high- resolution radiographs of complex objects and the evaluation of contraband detection schemes. In this paper we will give a brief description of the capabilities of the COG transport code and show several examples of neutron and gamma-ray imaging simulations.
The modified formula Bethe for stopping power is applied to Monte Carlo calculations of secondary electron transport in x- ray irradiated targets. The method used allows to increase considerably the calculation performance. The modification concept related to exchange effect is discussed. Calculated values of stopping power for different materials are compared with those obtained by other methods. Secondary electron yields and spectra for Al, Ge, Ag and Au are presented.
To facilitate the design and characterization of scintillation detectors we are implementing a new Monte Carlo code. The code handles the transport of optical photons through plastic scintillators, and is being written to ensure accurate interaction of photons at surfaces and to allow an analysis of photon arrival times at the detector entrance window. The code is being written in C++ with the long term view of forming an extension to the EGS4/51 code system. This paper will discuss the implementation of the code, outlining the means by which geometry is specified and how photon interactions have been modelled.
A low energy electron expansion of the EGS4 code system (briefly called EGS4/GOS) has been used in the simulation of X-ray tubes operating at diagnostic energies. In this new code, electron-atom inelastic collisions take into account atomic binding effects, and the emission of K-fluorescence radiation is simulated fully. Improved models for the generation of bremsstrahlung radiation in condensed media have also been developed. Benchmarks against experimental data show that the new code simulates realistically the production of bremsstrahlung and fluorescence radiation in X-ray tubes, with an accuracy of the order of 10% for the K(alpha ) line. The EGS4/GOS code has been used in studies aimed at understanding the physics that constrains transmission target X-ray tubes. In a first set of studies, the tube's operational characteristics (namely the efficiency and the ratio of fluorescence to bremsstrahlung radiation in the measured spectrum) have been evaluated. It is shown that tubes with thinner targets exhibit higher fluorescent-to-bremsstrahlung ratios (F/B), but this gain is paid for in terms of a loss in efficiency. Further results indicate that increased tube voltages benefit both the tube efficiency and F/B. The choice of substrate materials for thin target applications is also discussed. It is shown that low Z materials, where the probability for generating bremsstrahlung radiation is low, may represent feasible alternatives. However, these materials also turn out to be more transparent to low energy radiation, and this may reduce the observed fluorescence-to- bremsstrahlung ratios. The effectiveness of different filter materials to be used in the development of 'quasi- monochromatic' X-ray tubes is also discussed.