Silicon monochromator crystal

Comparison between the core-level X-ray absorption spectroscopy (XAS), emission (XES), and X-ray photoemission spectroscopies (XPS) usually shows that the spectral edges rarely coincide with each other and with the Fermi level. It is common practice, however, to place F at the emission threshold which corresponds to a fully relaxed ion core (16). For defining the structure of the edge, an energy resolution of at least 1-2 eV is required in the range of 5-20-keV X-ray photons. This can be achieved with Bonse-Hart channel-cut silicon monochromator crystals. 

The XAS spectrometer is similar to a UV-visible system in that it consists of a source, a monochromator, and a detector. The most favorable XAS source, synchrotron radiation, is tunable to different wavelengths of desirable high intensity. A laboratory instrument for analysis of solids and concentrated solutions may use a rotating anode source (further described in Section 3.3). The monochromator for X-ray radiation usually consists of silicon single crystals. The crystals can be rotated so that the wavelength ( i) of the X-rays produced depends of the angle of incidence (0) with a Bragg lattice plane of... 

Figure 3 shows one of our photoacoustic cell for X-ray spectroscopy of solid samples The cylindrical cell has a sample chamber at the center with volume of 0.16 cm which has two windows of beryllium (18 mm x 0.5 mm thickness). A microphone cartridge is commercially available electret type (10 mm ) and the electronics of preamplifier for this microphone is detailed elsewhere Figure 4 shows the typical experimental setup for spectroscopic study X-ray was monochromated by channel cut silicon double crystal (111) and ion chamber was set to monitor the beam intensity. Photoacoustic signal intensity was always divided by the ion chamber current for the normalization against the photon flux. X-ray was modulated by a rotating lead plate (1 mm thick) chopper with two blades. 

The original synchrotron Laue diffraction patterns from protein crystals recorded at Daresbury using a broad bandpass were conducted on this instrument with the monochromator removed (see Helliwell (1984)). Some preliminary multiwavelength experiments with a silicon double crystal monochromator (Si (111) triangle removed) were conducted. The growth of the Laue and MAD experiments has led to two further stations at Daresbury (SRS-3 and SRS-4). 

Section A3.4 gives the cell parameters for silicon and germanium monochromator crystals as well as the respective spectral bandpasses of specific reflected beams. These monochromator crystals are the ones most extensively used at SR sources for macromolecular crystallography. 

A3.4 CELL PARAMETERS OF SILICON AND GERMANIUM MONOCHROMATOR CRYSTALS... 

The situation is different if a semiconductor detector (lithium-doped silicon crystal) and the proper electronics are available. This setup can separate K a and K radiation, so that the beam does not have to be passed through a monochromator. The counter tube itself has nowadays become easier to operate, for the semiconductor crystal no longer has to be maintained at liquid nitrogen temperature—the problem has been solved electronically. Efforts continue to find broader applications for this type of detector, since replacement of the monochromator by electronics means that there is no longer a 50 % loss of primary intensity through diffraction by the monochromator crystal or absorption by the /( filter. 

In some cases, when attempting to improve the monochromaticity of the diffracted beams, a plane back monochromator is inclnded on which the beams diffracted by the film are diffracted once more [FEW 91]. These analyzer crystals are usually made out of silicon or germanium. A back monochromator like this is comprised of one, two or three crystals. For a specific position of the sample,... 

The electrons emitted by the photocathode are subsequently accelerated to 50 kV and focused on to a toroid-shaped anode. The anode is made of oxygen-free, high conductivity copper and is maintained at a high positive potential. The electron pulses interact with the copper anode forcing the emission of Cu-Ka x-ray photon pulses, which exit the vacuum chamber through a thin beryllium-foil window. A bend germanium crystal monochromator disperses and focuses the x-rays onto the sample. The duration of the x-ray pulses is measured by a Kentech x-ray streak camera fitted with a low density Csl photocathode. The pulse width of the x-rays at 50 kV anode-cathode potential difference is about 50 ps. This value is an upper limit for the width of the x-ray pulses because the transit time-spread of the streak camera has to be taken into consideration. A gold photocathode (100 A Au on 1000 A peiylene) is used to record the 266-nm excitation laser pulses. The intensity of the x-rays is 6.2 x 10 photons an r (per pulse), and is measured by means of a silicon diode array x-ray detector which has a known quantum efficiency of 0.79 for 8 kV photons. 

---