We have observed the spatial distribution of coherent or resonance transition radiation (RTR) in the soft-x-ray region of the spectrum (1 -3 keV). Resonance transition radiators were constructed and tested at two accelerators using electron-beam energies ranging from 50 to 228 MeV. These radiators emitted soft x rays in a circularly symmetrical annulus with a half-angle divergence of 2.5-9.0 mrad. The angle of peak emission was found to increase with electron-beam energy, in contrast to the incoherent case, for which the angle of emission varied inversely with electron-beam energy. By careful selection of foil thickness and spacing, one may design radiators whose angle of emission varies over a range of chargedparticle energies. A particular RTR mode (r =m =1) was found to give a sharp annular ring that becomes more accentuated as the number of foils is increased. The RTR effect has application in particle detection, beam diagnostics, x-ray source brightness enhancement, and x-ray free-electron-laser emission.

By designing transition radiators to emit x rays at the foil material's K-, I.-, or M-shell photoabsorption edge, the x-ray spectrum is narrowed. The source is quasimonochromatic, directional, and intense and uses an electron beam whose energy is considerably lower than that needed for synchrotron sources. Depending upon the selection of foil material, the radiation can be produced wherever there is a photoabsorption edge. In this paper we report the results of the measurement of the x-ray spectrum from a transition radiator composed of 10 foils of 2-pm titanium and exposed to low-current, 90.2-MeV electrons. The measured band of emission was from 3.2 to 5 keV. In addition, a measurment was performed of the total power from a transition radiator composed of 18 foils of 2.0-pm copper exposed to a high-average-current electron beam of 40 pA and at energies of 135, 172, and 200 MeV. The maximum measured power was 4.0 mW. The calculated band of emission was from 4 to 9 keV.

We have measured the total soft-x-ray power from a transition radiator composed of a stack of 25 beryllium foils each 1.0 μm thick which were penetrated by a relativistic electron beam whose maximum power was approximately 7 kW. The maximum total soft-x-ray power was measured to be 15.2 mW for a 245 MeV, 37 μA electron beam. The bandwith of the radiation at the full width half maximum points was calculated to be between 0.6 and 1.6 keV. In addition, we have exposed photoresist-coated silicon wafers at a distance of 3 m from the radiator. Exposure times of the bare resist were as short as 120 s for 5 cm2 of wafer are (resist sensitivity is 55.6 mJ/cm2). The shortest time for mask/wafer exposure was 180 s for 5 cm2.

Channeling radiation could provide a viable source for digital energy subtraction angiography (DESA). A signal to noise ratio (SNR) of 6.2 for a resolution of 0.5 mm x 0.5 mm could be achieved using a 6-mA 100-ms 20-MeV electron-beam pulse and a diamond channeling crystal as the x-ray source. This article investigates the choice of a DESA contrast agent and the parameters of a channeling-radiation x-ray source to develop a channeling-radiation DESA imaging system. The production of dual-energy peaks, the maximum available x-ray flux, the advantages of an area exposure, the necessity of a mosaic Bragg-crystal filter to reduce patient dose, the optimal energy separation of the peaks for a quasi-monochromatic x-ray source, and the reduction of the signal from bone are discussed, leading to estimated SNRs and image resolution for a channeling-radiation imaging system. The computer analysis developed to calculate the image quality is also discussed.

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