Boris W. Batterman

Sagittal focusing of synchrotron x radiation with curved crystals

Abstract We describe the sagittal focusing of X-rays with singly bent crystals that accept the meridian plane divergence from a similar but flat crystal to form a pair in a nondispersive two-crystal Bragg monochromator. Curved crystals can intercept from 5 to 20 times the sagittal divergence of curved mirrors at X-ray energies above 10 keV. Anticlastic (transverse) bending of the crystal is made negligible in the meridian plane with reinforcing ribs cut parallel to the plane of scattering. Results show that at energies of 10, 20, and 30 keV the bent crystal performs efficiently and images the source size at the Cornell High Energy Synchrotron Source.

CHESS: the new synchrotron radiation facility at Cornell

A new synchrotron radiation laboratory, CHESS, will soon be completed at Cornell University. The facility will operate in a mode parasitic to high energy physics experiments on the new 8-billion-electron-volt electron-positron storage ring (CESR) at Cornell. Electron and positron beams have already been stored and the first photons have been extracted. When completed, the laboratory will be available to the scientific community nationally and will provide the most intense tunable source of high energy x-rays in the country.

A fixed exit sagittal focusing monochromator utilizing bent single crystals

Abstract We have constructed and tested a tunable two crystal, horizontally focusing monochromator for use with synchrotron radiation. Horizontal focusing is achieved by using a segmented rectangular crystal especially cut to have the bending properties of a triangular plate. The bending device was designed to maintain axial symmetry around the central ray independent of bend radius. The entire crystal and bending device is so arranged that, coupled with the first crystal, a fixed exit beam height is maintained for all energies. Other innovative features such as an automatic translation seek circuit will be discussed.

Sagittal focusing of synchrotron radiation

Abstract We describe a simple device that has successfully been used to sagittally focus about 2.5 mrad of synchrotron radiation with an energy resolution equivalent to that of a double crystal channel cut monochromator. Deep narrow slots are cut into a 6 mm thick silicon crystal wafer cut into a triangular shape. The slots leave a connecting link of 0.25 mm thickness. The triangular shape allows a simple means of providing a cylindrical surface approximated polygonally by the thick crystal segments between the narrow slots. The device was tested with 10 and 15 keV synchrotron radiation on one of the CHESS white beam lines at a magnification of 1 3 as suggested by Sparks et al. At both energies the diffracted beam power in the bent crystal was equivalent to that in the unbent state to within a few percent.

The performance of a separated crystal X-ray monochromator during wiggler heating — Preliminary results☆

Abstract We report on the performance of a double crystal monochromator exposed to large heat loads from a 6 pole wiggler magnet running at 15.6 kG peak field and a 5.3 GeV electron beam. This source of synchrotron radiation can produce between 1.8 and 4.5 kW of X-ray power depending on beam conditions. Various single crystals of silicon have been tested in a nondispersive double crystal arrangement over a wide energy range along with several different cooling schemes. We report preliminary measurements of the performance of silicon crystals cooled by ambient helium gas or water cooled through conductive greases or through a 10 μm thick layer of liquid indium and gallium. A side cooled, thick crystal geometry shows good stability for photon energies from 20 to 45 keV. A thin, bottom cooled geometry shows promise at 7.4 keV. We have observed monochromator performance (rocking curve width, shift in edge, X-ray flux output vs electron beam current, harmonic rejection capabilities, etc.) under a variety of heating conditions. We summarize our suggestions for making effective use of monochromators under severe operating conditions.

Stability of some soft-x-ray monochromator crystals in synchrotron radiation

Abstract The stability in synchrotron radiation of β-alumina and beryl has been investigated by exposing the crystals in a white beam at CHESS operating at 5.2 GeV electron energy over a period of ∼ 100 h at an average radiant power of 28 W/cm 2 . Rocking curves at the CuK α energy were used to monitor the diffraction properties before and after exposure to synchrotron radiation. A Si(220) crystal was used as the first crystal. It was found that both materials exhibit no catastrophic material failure. β-alumina is clearly proven stable with no degradation of its rocking characteristics. There was in fact indication of reflectivity improvement after exposure. Results on beryl are not conclusive because the starting material (a mineral specimen) had a lot of inherent microstructural and crystallographic defects. Rocking curves in the soft X-ray region at 10.5 A were also determined using a (10 1 0) RAP crystal as first crystal. A 19-layer 50 A d-spacing Nb-C sputtered film was also characterized for comparison.

Thermal Design of Synchrotron Radiation Exit Ports at CESR

CESR, running at the maximum design parameters (8 GeV, 100 ma), produces 177 watts/mrad of synchrotron radiation at the exit ports for CHESS (Cornell High Energy Synchrotron Source). Due to the low angle of incidence, this corresponds to a linear heat loading of 55 watts/cm at the normal vacuum chamber wall. At the exit line crotch, radiation striking at normal incidence results in an average linear load of 885 watts/cm. For a beam height of 0.12 mm this translates to a power density of 740 watts/mm2. We present a design for a crotch which can effectively dissipate this high power density and will be compatible with the ultra-high vacuum system of CESR. The structure is a composite of a beryllium heat diffuser and an axially cooled copper cylinder. At 8 GeV and 100 ma we anticipate no component temperatures higher than 330°C.

A channel-cut crystal monochromator for x-ray standing wave measurements

Abstract In many X-ray diffraction experiments (e.g., small angle scattering and perfect crystal experiments) it is necessary to use a monochromator with high angular resolution, often much better than the intrinsic Darwin width of a perfect crystal Bragg reflection. A monolithic silicon crystal monochromator with a variable angular resolution function has been designed and fabricated. It is tunable over a wide range of X-ray energies, and is particularly well suited for X-ray standing wave experiments using synchrotron sources. Although independently conceived, this monochromator is similar is design to one developed by Bonse et al. [1] for a different purpose. It is functionally equivalent to an asymmetric crystal monochromator in providing a narrow angular transmission, yet has several advantages over the latter for standing wave experiments, especially at a synchrotron source. Rocking curves and standing wave measurements are presented.

PROPOSED DEDICATED HIGH PRESSURE BEAM LINES AT CHESS

An instrumentation proposal for dedicated high pressure beam lines at CHESS is described. It is the purpose of this proposed program to provide researchers in high pressure science with beam lines for X-ray diffraction studies in the megabar regime. This will involve radiation from a bending magnet as well as from a wiggler. Examples of the high pressure results up to 2.16 Mbar are shown. Diffraction patterns from bending magnet and wiggler beams are shown and compared. The need for this facility by the high pressure community is discussed.

Temperature dependence of intensities from silicon with glancing incidence HEED

Peak reflectivity for the 2nd to 7th order Bragg reflections of the hhh systematic set have been obtained at different temperatures. The results have been interpreted using an n-beam dynamical theory adapted to the Bragg case at glancing incidence. On the basis of this analysis it is concluded that the component of the mean-square vibrational amplitude of the surface atoms normal to the surface is about 40% greater than that of the bulk.