Diffraction Efficiency Of Crystals Research Articles

X-ray diffraction efficiency of bent GaAs mosaic crystals for the Laue project

In a Laue lens made of single crystals oriented to diffract parallel x-rays at the lens focus, the energy and angular resolution are limited by the crystal size and by the crystal mosaicity. The use of extended crystals bent according to the lens curvature provides better focusing, with the resolution given essentially by the crystal mosaicity. With this approach, a crystal mosaicity as low as 15–25 arcsec, well below the mosaicity value of copper crystals, was found suitable for the new design of the Laue lens. The reflectivity and transmission profiles and the integrated intensity have been measured in flat and bent GaAs and Si crystals prepared by the method of surface damaging by using sandpaper of different grain size. The surface grinding induces a local lattice strain which produces a self-standing bent crystal. Bent crystals with radius of curvature lower than a critical value given by the extinction length behave as ideal mosaic crystals, maximizing the diffraction efficiency at high x-ray energies. It is found that the surface grinding does not affect the crystal diffraction efficiency, the damage thickness being limited to a few tens microns near the crystal surface.

A new method of anti-jamming ability improvement for Michelson Interferometer

In order to improve anti-jamming capability of Michelson interferometer system, replace the traditional structure of the moving mirror scanning was replaced, an interference system based on electro-optic modulation of crystal refractive index was designed to achieve optical path scanning. The system modulated voltage signal on the variable refractive crystal, to generate cyclical changes, changed the refractive index to control optical path difference in the original optical path system. Using electronic scanning to replace of mechanical scanning, improved the system's noise immunity was improved. In the electro-optic modulation process, computed the maximum optical path difference of the system was computed, and analyzed of the crystal thickness and crystal diffraction efficiency of the modulation process were analyzed. The simulation experiment shows that, with the modulation voltage range increasing, the available range of the optical path is also increased, and the system spectrum resolving power will also increase accordingly. Meanwhile, in the modulation process set the modulation range was set to make the energy of diffraction energy losses less than 10% of the total energy, so as to ensure a better signal to noise ratio. Experimental results show that, as the modulation voltage changes, interference fringes occurred continuously moved. When the voltage is further increased, the nonlinear error appears. After non-linear error correction for the system, spectrum resolution reached to 7. 2 cm-1, slightly lower than the original system. But its anti-jamming capability is greatly enhanced, as in the absence of experimental platform for seismic conditions, conventional interferometer relative error is more than 20%, while the relative error of the system is less than 5%, in line with the design requirements. It was proved that the anti-jamming capability of the system was enhanced greatly, when the static electro-optical modulation was used.

High diffraction efficiency in crystals curved by surface damage

Diffraction profiles of curved Si and GaAs crystals obtained by a controlled damage process on one side of planar crystals have been investigated at X-ray energiesE= 17, 59 and 120 keV. AtE= 17 and 59 keV in the condition of slight curvature, that is when the diffracting plane bending over the extinction length is lower than the Darwin width, the Laue diffraction profiles with lattice planes parallel or inclined with respect to the curvature radiusRshow an enhancement of integrated intensity proportional to 1/R, much larger than in the corresponding perfect bent crystals. AtE= 120 keV, in the condition of strong curvature, the crystals behave as bent perfect crystals with integrated intensity corresponding to that of a mosaic crystal. These crystals are proposed as optical elements for focusing hard X-ray beams.

Study of X-ray detector system characteristics on the 3B3 medium energy beamline in BSRF

On the 3B3 medium energy X-ray beamline, a new monochromatic X-ray source tunable in the 2—6 keV energy range bas been constructed and implemented. The characteristic parameters of the X-ray source having been determined, with the help of the light source, we have calibrated the sensitivity of X-ray detector,thickness of filter,diffraction efficiency of crystals and energy response of the BAS5000-RS, which showed the high performance of the monochromator and detecter system.

Measurement of diffraction efficiencies relevant to crystal lens telescopes

The Toulouse/Argonne collaboration is working on a novel detector for gamma-ray astronomy based on a crystal lens that uses Bragg reflection of Ge crystals to concentrate photons onto a small detector in the image plane. The diffraction efficiency of the crystals is decisive for the energy bandpass, the field of view and the effective area of the crystal lens telescope. We have measured diffraction efficiencies of Ge crystals from 200 to 500 keV at the Advanced Photon Source synchrotron at Argonne National Laboratory in two experiments carried out at the sector 1 bending magnet beamline. The high brilliance and high particle energy of third-generation synchrotrons permit measurements at gamma-ray energies. The low angular divergence of synchrotron radiation is similar to that of an astronomical point source. The efficiencies ranged from 20% to 31% for diffraction of Ge (1 1 1) and (2 2 0) using crystals with a mosaic width of a few arc seconds. Neglecting absorption, the values ranged from 31% to 49%. The continuum flux diffracted in second-order diffraction (4 4 0) was a factor of 2.4–2.6 less than in first-order diffraction (2 2 0) . For third-order diffraction (the (3 3 3) and the (1 1 1) planes), the flux was reduced by a factor of 6.8. The peak efficiency and the rocking curve integral for mosaic widths ranging from a few arc seconds to 0.4° were measured. The results agree with the expectation of the Darwin model for mosaic crystals. This gives us confidence in the calculations of the crystal lens telescope performance, and allows an optimization of a balloon-borne and a satellite-based telescope configuration. Measurements were carried out with a wedged crystal, that was used to increase the diffraction efficiency of the lens crystals for a source at finite distance. The efficiency increase is due to both a bending and a straining of the crystal. We measured rocking curve widths of a crystal before and after etching to determine the effect of surface damage on the diffraction efficiency. The mean decrease in rocking curve width due to the etching was 1.7 arc sec in the center of the crystal and 6.9 arc sec at the crystal edge.

An experimental assessment of X-ray diffraction theory in high purity aluminium crystals

An experimental test of Zachariasen's X-ray diffraction formula was carried out for high purity aluminium crystals with a wide range of mosaic block sizes and mosaic block misorientations. These crystals were produced by cold rolling to different strains three samples (crystals 1, 2 and 3) cut from a large single crystal grown by the Bridgman method. The (100), (200) and (220) X-ray integrated intensities were measured for each crystal, and the block sizes and block misorientations were obtained using transmission electron microscopy techniques (the mean linear intercept method for the block size and Kikuchi line techniques for the block misorientation). With the exception of crystal 2 in the undeformed condition, for a given state the three crystals showed a common diffraction efficiency, the value of which increased with increasing deformation. The different diffraction efficiency of crystal 2 with respect to crystals 1 and 3 in the undeformed condition is rationalized in terms of a segragation of impurities at the bottom of the large single crystal. Comparison between the integrated intensities determined experimentally and the intensities calculated by substituting the measured block size and block misorientation parameters into Zachariasen's formula showed good agreement between the two sets of values. In view of this, it was concluded that Zachariasen's formula is closely consistent with the diffraction efficiency of the crystals under study; the behaviour of the crystals is characteristic of Zachariasen's type II crystals, i.e. the extinction is primarily controlled by the mosaic block size rather than by the mosaic block misorientation.