Optimization of the channeling radiation source crystal to produce intense quasimonochromatic x rays

Abstract

Channeling radiation is a source of intense, tunable, quasimonochromatic x rays produced by electrons traveling along a direction of symmetry through a crystal. We analyze the effect of the channeling source crystal, through both its composition and lattice structure, on the number of photons emitted per electron, the linewidths of the radiation, and the maximum sustainable currents to identify the crystal which will yield the most photons in a bandwidth of less than 10% full width at half maximum (FWHM). Although high atomic number (Z) crystals produce a greater number of photons per electron, given their poor thermal properties these crystals cannot sustain as high average currents as lower Z crystals. The linewidths of the channeling radiation emitted from high Z crystals are as large as 50% FWHM, and electrons are dechanneled more quickly in these crystals, limiting their use as a quasimonochromatic x-ray source. Given its exceptional thermal conductivity and narrow linewidths, diamond is the optimal source crystal for the production of intense, quasimonochromatic x rays using channeling radiation.

Exotic sources of X-rays for iodine K-edge angiography

“…For all its virtues, even with state-of-the-art cooling, diamond can sustain only a few miiliamps of 50 MeV electron beam. With this beam, channeling radiation flux of 4.2×1010 photons/see was obtained in a 5% bandwidth [6]. This is four orders of magnitude less than is required for angiography, and angiography requires a 1% bandwidth or less.…”

“…where p is the electron density, ro is the classical electron radius, k is the x-ray wavelength, and f" and f" are the real and imaginary parts of the the atomic scattering factor. The Cherenkov mechanism works in the case of light elements like carbon, because the number of anomalous electrons (f" = -9.7) near the absorption edge is greater than the number of electrons (6) and this causes the index of refraction to be greater than unity. The mechanism will not work in the case of iodine, because it has 53 electrons and f" is only -10.6 at the K-edge.…”

“…The relativistic beam excites electronic transitions in the solid, and the resulting photon energy is Doppler upshifted by 2_'2, just as in an undulator. Channeling has been likened to an atomic scale undulator with magnetic fields of megagauss [6]. Experiments have shown that 30 keV radiation is readily obtained from crystals with electron beams on the order of 20 MeV.…”

R&D toward a compact high-brilliance X-ray source based on channeling radiation

“…The observation of CR was first reported in Ref. [6,7,8] and has since then been the object of intense researches [9,10,11,12]. The frequency of CR scales as ω = 2γ 2 ω 0 /(1 + γ 2 θ 2 ) where ω 0 ∝ γ −1/2 is the oscillation frequency around the crystal's lattice plane, γ the Lorentz factor, and θ the observation angle with respect to the electron direction.…”

Planar channeling radiation by relativistic electrons in different structures of silicon carbide

“…Binary or polyatomic crystals (e.g. LiH [12], LiF [13,14], GaAs, ruby [15]) have scarcely been utilized. Most of the above mentioned crystals are characterized by a cubic lattice.…”