Pulsed Tunable Monochromatic X-Ray Beams from a Compact Source: New Opportunities

Abstract

1197he science of X-ray production and application is now a little more than a century old [1] but is still an active field of research and development [2].Historically, X rays for imaging and crystallography have generally been produced through the use of bremsstrahlung and line X rays from electrons impinging on a metallic anode. Such sources are inexpensive, simple, and robust but provide little control over the X rays produced. More recently, synchrotron sources have been used for both applications, with good results. Unfortunately, synchrotrons are large, expensive facilities with less than ideal beam geometry and are therefore not entirely practical for routine imaging applications.The excellent results of experiments with monochromatic sources [3][4][5] show the desirability of improving on the current broadband X-ray imaging practice. No alternative has existed for experiments that need to operate at various X-ray energies. The availability of such a source may fundamentally change the practice of X-ray imaging and provide much wider availability of tuned X rays to crystallographers.A compact source of pulsed tunable monochromatic X rays has been designed, built, and tested. This device can deliver "hard" X rays from 10-to 50-keV at narrow bandwidths (1-10%), with a flux of 10 10 photons in each 8-psec pulse. These are produced in a cone-beam area geometry useful for human imaging, small animal imaging, protein crystallography, and nondestructive testing in industry. The machine integrates a laser with a linear accelerator (LINAC) and can be used in an unshielded environment.The source described here is a tabletop-terawatt (T 3 ) laser-based Compton backscattering system, which uses few-joule pulses from a 1,052-nm laser to collide with a 20-to 50-MeV electron beam to produce an intense pulse of narrowband X rays. The entire system footprint is 4 m wide by 10 m long, and it requires no shielding vault. It produces X rays in a smallangle cone-beam geometry in the 10-to 50-keV range, with up to 10 10 photons in an 8-psec pulse, which is sufficient flux for medical and industrial imaging to be performed in a single shot. This source is certainly not the first Compton backscattering or laser-synchrotron X-ray source built. Experiments have been carried out at a number of the large accelerator facilities [6, 7] that have produced modest fluxes of photons over a wide range of interesting energies. Also, sources similar in concept to this one have been proposed [8,9] and operated on a small scale. However, none of these sources has been designed and built in a small practical form and with a high enough flux to be deployed as a common laboratory-scale or clinical resource. Further, most of the current generation of sources produce high levels of background radiation from the linear accelerator and require the source to be embedded in a shielding vault.The source at Vanderbilt University has its roots in a project that was built as an add-on to the free-electron laser at Vanderbilt that was proposed in 1987. It p...

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