Compact Linear Accelerator Sources for Gamma-ray Generation

Content Provider	Semantic Scholar
Author	Anderson, Scott G. Barty, Christopher P. J. Beer, G. K. Cross, Robert R. Ebbers, Christopher A. Gibson, David Hartemann, F. Houck, Timothy L. Marsh, R. A. Adolphsen, Chris Jongewaard, E. N. Li, Zhiqiang Limborg-Deprey, Cecile Raubenheimer, T. O. Tantawi, Sami G. Vlieks, Arnold Wang, W. J.
Copyright Year	2011
Abstract	High precision accelerator technology is required for the development of tunable, monochromatic gamma-ray sources capable of generating MeV photons with very high brightness. High gradient accelerator technology is capable of producing high brightness electron beams in relatively short lengths. X-band technology has been demonstrated extensively in the development of the Next Linear Collider program at SLAC, and has been adapted for use in a Compton scattering gammaray source at LLNL. Critical components and technologies will be discussed including the use of high repetition rate solid state modulators, XL4 klystrons, SLED-II pulse compression, RF distribution geometries, a novel X-band RF photoinjector, high gradient traveling wave accelerator structures, and linear accelerator design and layout. Selection of beam properties relevant to gamma-ray production will be discussed, especially with respect to future improvements in gammaray brightness and dose specific to nuclear management applications. INTRODUCTION Nuclear Resonance Fluorescence (NRF) [1] is an isotope specific process in which a nucleus, excited by gamma-rays, radiates high energy, narrow bandwidth photons. Because NRF energy levels depend on the exact nuclear structure, the NRF spectrum is isotope-dependent. NRF lines are found in the MeV energy range, where photons are most penetrating, near the absorption minimum between photo-ionization and pair production. Although NRF is a process that has been well known for several decades, with the advent of MonoEnergetic Gamma-ray (MEGa-ray) sources from Compton scattering, it has now potential high impact applications in homeland security, nuclear waste assay, medical imaging and stockpile surveillance, among other areas of interest. Although several successful experiments have demonstrated NRF detection with broadband bremsstrahlung gamma-ray sources [2], NRF lines are more efficiently detected when excited by narrowband gamma-ray sources. Currently, Compton scattering is the only physical process capable of producing tunable narrow bandwidth radiation (below 1%) at gamma-ray energies, with state-of-the art accelerator and laser technologies. In Compton scattering sources, a short laser pulse and a relativistic electron beam collide to yield tunable, monochromatic, polarized gamma-ray photons. Several projects have recently utilized Compton scattering to conduct NRF experiments: Duke university [3], Japan [4], and Lawrence Livermore National Laboratory (LLNL) [5-7]. In particular, LLNL’s Thomson-Radiated Extreme X-rays (TREX) project demonstrated isotope specific detection of low-density materials behind heavier elements [5]. The proposed NRF applications need high average photon flux at a specified energy (i.e., to maximize Nγ / eV / sec at the NRF resonance lines) while concurrently minimizing background noise from off-resonance radiation. For the Compton source, these requirements motivate the use of small laser and electron beam sizes, σ x , at the interaction point (IP) to increase flux, yet maintain a small normalized beam divergence, γσ ′ x , to decrease the bandwidth of the γ-rays. In fact, it can be shown [8] that the accelerator design should seek to maximize the quantity: Nγ / eV / sec ∝ I εn 2 , where is the average current of the accelerator, and εn is the normalized beam emittance. This paper describes the VELOCIRAPTOR linac (Very Energetic Light for the Observation and Characterization of Isotopic Resonances and the Assay and Precision Tomography of Objects with Radiation), designed to drive a precision, compact, γ-ray source by optimizing I εn 2 . Rf photoinjector and traveling wave linac technology operating at X-band (11.424 GHz) was chosen for this application due to recently demonstrated advancements in accelerating field gradients [9], and the potential for very high brightness beam generation. Below, the photoinjector design, traveling-wave accelerator design, X-band rf compression and distribution systems, and electron beam dynamics simulations will be presented with emphasis given to design requirements imposed by this light source application. RF PHOTOINJECTOR The X-band rf photoinjector was based on an earlier design developed by A. Vlieks at SLAC [10,11]. The Vlieks design was the first X-band photoinjector, and was operated successfully at cathode electric fields of 200 MV/m. The Vlieks photoinjector was a 5.5 cell design, and as such Figure 1. CAD rendering of VELOCIRAPTOR X‐band rf photoinjector. Labels indicate major design improvements over the Vlieks design. Longer
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Language	English
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Resource Type	Article