Experiment X1 - Forschungs-Highlights
1. Transition Radiation in the X-ray Region (XTR)
Experiments with TR show that a hard x-ray beam with photon energies up to about 35 keV can be produced from a stack of 30 beryllium foils. The spectral brilliance should reach, at 855 MeV and 0.1 mA, that of the most powerful synchrotron radiation sources from bending magnets. A highly oriented pyrolytic graphite crystal has been used to prepare a quasi-monochromatic 33 keV photon beam. At the K-absorption edge of titanium at 4.96 keV narrow band transition radiation is emitted from a stack of four foils originating from effects of the anomalous dispersion. A novel type of interferometer has been developed with which the real part of the refrective index decrement of thin self-supporting foils can be measured in the hard X-ray region. It consists of two foils at which the low emittance 855 MeV electron beam of the Mainz Microtron MAMI produces transition radiation, a single crystal spectrometer with a flat crystal in Bragg geometry, and a pn CCD X-ray detector. Distinct interference oscillations have been observed as function of both, the photon emission angle and the distance between the foils. The dw of a 2 µm thick nickel sample foil has been measured at X-ray energies around the K-absorption edge at 8333 eV and at 9930 eV with an accuracy of better than 1.5%.
2. Parametric X-ray Radiation
The angular distribution and energy width of PXR from a silicon single crystal has been studied at a beam energy of 855 MeV. The measurements of the angular distributions and photon fluxes are in accord with the Feranchuk-Ivashin theory of PXR production. An intrinsic energy width of less than 3 eV has been measured for the (111) reflex with a critical absorption technique at the K-absorption edge of titanium at 4.96 keV .
The energy width is limited by geometrical line broadening effects which can be optimized to reach the ultimate limit given by the finite length of the wave train. Experimental and Theoretical studies of the line shape have been performed in backward direction in which geometrical line broading effects could be kept to a minimum. A silicon single crystal served as monochromator. Line shapes have been measured for the (111) up to the (555) reflection showing that the natural linewidth th the order of a few meV is significantly broaded due to small angle scattering of the electrons in crystal.
3. Undulator Radiation
A low energy photon beam in the energy range between 15 eV and about 1 keV could be produced at MAMI with a short period undulator. The feasibility has been demonstrated with a 10 period prototype hybrid undulator (period length 12 mm, gap size 3 mm, magnetic field on axis 1 Tesla). Radiation up to the 7th harmonic has been observed. For a 100 period undulator the spectral brilliance of the first harmonic at 350 eV would amount to about 2*10^15 / (s mm^2 mrad^2 0.1% BW) at a beam current of 0.1 mA.
A novel interferometer , consisting of two undulators and a grating spectrometer, has been developed. It allows the measurement of the complex index of refraction of thin self supporting foils in the VUV
and soft X-ray region
4. Smith-Purcell Radiation
Smith-Purcell radiation, generated when a beam of charged particles passes close to the surface of a diffraction grating, has been studied in the visible spectral range at wavelengths of 360 nm and 546 nm with the low emittance 855 MeV electron beam of the Mainz Microtron MAMI. The beam focused to a spot size of 4 µm (FWHM) passed over optical diffraction gratings of echelle profiles with blaze angles 0.8°, 17.27° and grating periods of 0.833 µm and 9.09 µm. Taking advantage of the specific emission characteristics of Smith-Purcell radiation a clear separation from background components, such as diffracted synchrotron radiation from upstream beam optical elements and transition radiation, was possible. The intensity scales with a modified Bessel function of the first kind as function of the distance between electron beam and grating surface. Experimental radiation factors have been determined and compared with theoretical predictions.