We have performed precise measurements of the energy spectrum of bremsstrahlung from a single crystal. The results are compared with the theory of coherent bremsstrahlung and found to be sensitive to the choice of the functional form of the atomic form factor. The Si data agree with the theoretical curve which is obtained with the form factor based on the Hartree-Fock model within a systematic error of 2%. It is pointed out that measurements sensitive to the atomic form factor to the 0.5% level will become feasible for various materials by improvement of the present method. PACS numbers: 61.80.Mk, 71.20.-bSince the atomic form factor is a Fourier transform of the electron distribution function of an atom, its accurate measurement is of fundamental importance. Precise knowledge of the atomic form factor has also some practical importance as exemplified by the problem associated with the computation of x-ray attenuation coefficients. l It has been pointed out that the coherent bremsstrahlung from a single crystal is sensitive to the shape of the atomic form factor of the crystal. 2 " 4 The theory of bremsstrahlung of high-energy electrons is well established; the electron emits a photon with momentum k when it is deflected by the Coulomb potential of the atomic nucleus. The atomic electrons surrounding the nucleus tend to screen off the positive charge of the nucleus so that the bremsstrahlung cross section is affected by the atomic form factor F(q), where q is the momentum transfer defined by q=po""Pik with po and pi being the incident-and outgoing-electron momentum, respectively. For coherent bremsstrahlung from a single crystal the integration over q is replaced by the summation 5 over the reciprocal-lattice points g: do_ y [l-F(g)] 2 dk "t g 4
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