Journal of X-Ray Science and Technology - Volume 4, issue 4
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Journal of X-Ray Science and Technology is an international journal designed for the diverse community (biomedical, industrial and academic) of users and developers of novel x-ray imaging techniques. The purpose of the journal is to provide clear and full coverage of new developments and applications in the field.
Areas such as x-ray microlithography, x-ray astronomy and medical x-ray imaging as well as new technologies arising from fields traditionally considered unrelated to x rays (semiconductor processing, accelerator technology, ionizing and non-ionizing medical diagnostic and therapeutic modalities, etc.) present opportunities for research that can meet new challenges as they arise.
Abstract: Coherent x rays can be produced by scattering of laser light by a beam of relativistic electrons, provided that the electron beam is density modulated or the scattering is done at, or near, 90°. Since the coherent scattering is proportional to N2 , where N is the number of electrons, and the incoherent scattering is proportional to N, also a modest degree of coherence can substantially increase the x-ray yield. The theory of laser-electron-beam scattering is reviewed and compared with the emission of radiation by an electron beam in an undulator. Examples of the practical implementation of an intense source…of coherent x rays are discussed.
Abstract: In this work we combine elements of chirped pulse amplification (CPA) techniques, now familiar in solid-state lasers, with an amplifier based upon a seeded free-electron laser (FEL). The resulting device would produce amplified pulses of unprecedented brevity at wavelengths shorter than can be currently obtained by any tunable laser system. We use a subharmonically seeded FEL to illustrate the concept. Radiation from a Ti:sapphire laser is frequency-tripled and stretched optically to provide a coherent seed pulse for the FEL. When coupled to an electron beam inside a magnetic wiggler, the seed radiation introduces an additional energy modulation on the electron…bunch, which has been prepared with an energy chirp to match the chirp in the optical pulse. The energy modulated electrons are then spatially bunched in a dispersion magnet and introduced to a wiggler configured to be resonant to a harmonic of the seed laser, providing additional frequency multiplication. The coherent radiation produced by these electrons is amplified as it traverses the wiggler and recompressed optically. The preservation of phase coherence provided by this scheme results in a device which can yield 4-fs pulses with 0.3 mJ at a central wavelength of ca. 88 nm, easily the shortest duration amplified pulses produced by any laser. In this paper, we discuss various aspects of the concept, including the generation of short pulses, temporal stretching and compression, and potential applications of the device. The phase distortion during the wide bandwidth FEL amplification is discussed in detail, and is shown to be within the bounds required to produce a 4-fs pulse upon compression.
Abstract: We discuss the production of x rays from relativistic electron beams using synchrotron radiation, free electron lasers, and Compton backscattering of intense laser beams. We review the characteristics, and compare the flux and brightness, of x rays produced from these three processes. We show that free electron lasers can produce coherent x ray pulses in the keV region with a brightness much larger than that of synchrotron radiation facilities, and that in the 10-keV region Compton backscattering systems can be competitive with synchrotron radiation. We describe some of the existing or planned free electron lasers and Compton backscattering facilities.
Abstract: Synchrotron x-ray microtomography has matured into a practical tool for the metrology of small specimens, featuring a spatial resolution of a few micrometers. In this paper, the author examines the practice of monochromatic x-ray tomography, emphasizing the characteristics of synchrotron radiation sources that have enabled the evolution of this new imaging technology. These principles are illustrated using reconstructions of millimeter scale engineered structures fabricated from low atomic weight materials. These structures include spherical shells, considered as model inertial confinement fusion targets, and examples of laser-welded aluminum alloys.
Abstract: The Vanderbilt University Free-Electron Laser Program is developing the capability to create near-monochromatic x rays for medical imaging and other purposes. For this experiment we feed back the normal infrared FEL light to collide with the electron beam. This causes Compton backscattering of the incident photons which creates x rays. This paper is particularly focused on the x-ray beam transport to be used with this experiment. This transport must redirect the x-ray beam to match a beam chase located in the accelerator vault ceiling at a 40° angle to the x-ray creation axis. It has been determined that the most…efficient way to form this transport is by using multiple reflections from mosaic graphite crystals. Samples of these crystals have been obtained and reflection characteristics are being measured. Flat crystals have been investigated at this point. Curved crystals have also been obtained and these will be measured soon. All of these results will dictate the final form of the beam transport.
Abstract: The production of an efficient user friendly ultrafast x-ray source requires an understanding of the role of the various factors which affect the x-ray emission. Here we examine several issues which control the source brightness and the pulse duration. Picosecond time-resolved, high spectral resolution spectroscopy is used to study plasmas produced by a subpicosecond laser pulse with intensity between 1016 W/cm2 and 5 × 1018 W/cm2 .
Abstract: A figure of merit (FOM) has been developed which embodies parameters related to image quality in the numerator and radiation integral dose to the patient in the denominator. In this manner, maximizing image quality and minimizing radiation dose amounts to maximizing the FOM. Furthermore, the FOM is designed to be independent of x-ray exposure (number of photons used), and this eliminates one important parameter in an optimization scenario. Monoenergetic x-ray beams (0% bandwidth) are compared with conventional Bremsstrahlung x-ray sources from a tungsten target, for angiographic imaging systems using 144 mg/cm2 Csl image intensifiers as the detector. Thus the…results are applicable to both digital subtraction angiography (DSA) and digital fluoroscopic procedures involving iodine-based contrast (e.g., roadmapping). The results demonstrate improvement factors (the ratio of the best FOM of the monoenergetic beam over the best FOM of the polyenergetic beam) ranging from 2.3 to 1.4. The improvement factors averaged over four iodine contrast thicknesses (50, 100, 500, and 1000 mg/cm2 ) were 1.61 (σ = 0.159) for the 10 cm thick patient, 1.68 (σ= 0.172) for the 20 cm thick patient, and 1.82 (σ= 0.186) for the 30 cm thick patient. The conclusions are that monoenergetic x-ray beams are capable of delivering the same image quality at about half the radiation dose to the patient compared to conventional X-ray tubes.
Abstract: The Vanderbilt University medical FEL (free electron laser) Compton x-ray program is close to being operational. The FEL modifications necessary for this new capability are near completion. The transport and detection systems for electron and IR beams have been designed, delivered, and tested. We initially expect to produce 108 x-ray photons per second in the 15- to 20-keV region.
Abstract: This paper investigates the feasibility of using large semiconductor image plane arrays (incorporating detector and readout functions) for radiology. It is anticipated that this approach will offer higher sensitivity and spatial resolution than many alternatives. The disadvantage is a smaller object field than can be imaged with film or by using optics after conversion of x rays to visible radiation. Semiconductor image plane arrays (detector plus multiplexing functions) should have utility because of the potential for higher sensitivity and spatial resolution compared with either computed radiography (photostimulable phosphors) or film cassettes. The major limitation is limited image plane size because…of the maximum size of the silicon wafers.