LiF crystals

Gupta [28] presents results on the effect of crystal orientation on shock propagation in LiF crystals. This work supports earlier studies and shows... 

For the case of LiF crystals, both the dislocation concentration and the incremental stress caused by plastic deformation are proportional to the amount of deformation. This indicates that the hardening is caused by impediments created by dislocations and dipoles to the motion of subsequent dislocations. 

J. J. Gilman and W. G. Johnston, Behavior of Individual Dislocations in Strain-hardened LiF Crystals, Jour. Appl. Phys., 31, 687 (1960). 

The author believes that dipoles cause deformation hardening because this is consistent with direct observations of the behavior of dislocations in LiF crystals (Gilman and Johnston, 1960). However, most authors associate deformation hardening with checkerboard arrays of dislocations originally proposed by G. I. Taylor (1934), and which leads the flow stress being proportional to the square root of the dislocation density instead of the linear proportionality expected for the dipole theory and observed for LiF crystals. The experimental discrepancy may well derive from the relative instability of a deformed metal crystal compared with LiF. For example, the structure in Cu is not stable at room temperature. Since the measurements of dislocation densities for copper are not in situ measurements, they may not be representative of the state of a metal during deformation (Livingston, 1962). 

Figure 13.9—Schematic of a sequential, crystal-based spectrometer and the spectrum obtained using the sequential method with an instrument having a goniometer. The Soller slit collimator, made of metallic parallel sheets, collimates the primary X-ray beam emitted by a high power source (SRS 300 instrument, reproduced by permission of Siemens). A typical spectrum of an alloy, obtained by an instrument of this category, having an LiF crystal (200) with 26 angle in degrees as the abscissa and intensity in Cps as the ordinate). Model Philips PW2400 Spectrum, reproduced with permission of VALDI-France.

The magnification attended in the experiment with the photoelectron microscope was M = 10s, and the spatial resolution was around 30 nm, which proved sufficient for the visualization of individual color centers in a LiF crystal with the concentration of such centers less than 10l7cm 3. The results obtained in Ref. 9 may be considered the first successful implementation of laser resonance photoelectron microscopy possessing both subwavelength spatial resolution and chemical selectivity (spectral resolution). It will be necessary to increase the spatial resolution of the technique by approximately an order of magnitude and substantially improve its spectral resolution by effecting resonance multistep photoionization by means of tunable ultrashort laser pulses. 

Ionic radii are discussed thoroughly in Chapters 4 and 7. For the present discussion it is only necessary to point out that the principal difference between ionic and van der Waals radii lies in the difference in the attractive force, not the difference in repulsion. The interionic distance in UF, for example, represents the distance at which the repulsion of a He core (Li+) and a Ne core (F ) counterbalances the strong electrostatic or Madelung force. The attractive energy for Lt F"is considerably over 500 kJ mol"1 anti the London energy of He-Ne is of the order of 4 kJ mol-1. The forces in the LiF crystal are therefore considerably greater and the interioric distance (201 pm) is less than expected for the addition of He and Ne van der Waals radii (340 pm). 

Fig. 6.2.1. Starlike dislocation rosette around pyramid impression on (100) face in LiF crystal, after etching. (After Shaskalska and Dobzhanski, 1962, from Yushkin, 1971)...

Gragert and Meyer (Fig. 6.2.1) and Boyarskaya (Fig. 6.2.2) by observation of surface deformations induced by indentation with a tungsten carbide ball and by scratch. The observations were carried out using secondary electron beam and in cathodoluminescence. They demonstrated on MgO and LiF crystals the occurrence of cracks around the impression of the ball similar to those induced by a Vickers indenter, and also the occurrence of a concentration of screw and edge dislocations in the area of the cracks. 

The resolution of the light into its various frequency components is accomplished by (i) gratings or prisms in dispersive instruments, (ii) interferometers (such as the Michelson124 interferometer see below) in Fourier transform spectrometers (Fellgett s125 advantage), or (iii) for X rays, bent LiF crystal or graphite monochromators. 

When a metal was exposed to X rays, it emitted X rays of a different wavelength. The emitted X rays were diffracted by a LiF crystal (d = 201 pm), and first-order diffraction (n = 1 in the Bragg equation) was detected at an angle of 34.68°. Calculate the wavelength of the X ray emitted by the metal. 

The very first demonstration of molecule interference dates back to the days of Estermann and Stern [Estermann 1930] who demonstrated experimentally diffraction of 11-2 at a LiF crystal surface in 1930. Further experiments with diatomic molecules had to await progress and interest in atom optics. A Ramsey-Borde interferometer was realized for the iodine dimer in 1994 [Borde 1994] and was recently used [Lisdat 2000] with K. Similarly, a Mach-Zehnder interferometer was demonstrated [Chapman 1995 (a)] for Na2. The nearfield analog to the Mach-Zehnder interferometer, a Talbot-Lau interferometer, was recently applied to experiments with L12 [Berman 1997], Diffraction at nanofabricated gratings also turned out to be the most effective way to prove the existence of weakly bound helium dimer [Schollkopf 1996] and to measure its binding energy [Grisenti 2000],... 

Fig. 8-33 Different aspects of the (100) face of a LiF crystal, (a) Optical micrograph of the etched surface, (b), (c), (d) Berg-Barrett x-ray topographs of the same area of the unetched surface, made with three different hkl reflections the arrow on each topograph is the projection of the incident-beam direction on the (100) face. Newkirk [8.18].

In the Berg-Barrett topographs of Fig. 8-33 explain why only screw dislocations are revealed in (b), only edges in (c), and screws and edges in (d). Give the indices of the operating slip systems, i.e., the indices of each slip plane and of the slip direction in that plane. (Slip in a LiF crystal occurs on 110 planes in <110> directions, but the experimental observations in Fig. 8-33 are inconsistent with the assumption that slip occurred on all possible slip systems. Note also that only those dislocations which intersect a crystal face will be distinctly observed by x-ray examination of that face.)... 

Fig. 15-3 Recording of fluorescent spectrum of a stainless steel containing 19.4 Cr, 9.5 Ni, 1.5 Mo, 1.4 W, 1.0 Mn (in weight percent), balance mainly Fe. Flat LiF crystal analyzers. Platinum-target x-ray tube, 50 kV, 40 mA. (Courtesy of Diano Corporation.)...

Verify the statement in Sec. 15-8 regarding spectrometer resolution by wavelength dispersion (WD) and energy dispersion (ED) by calculating the percent resolution AA/A for each type and for wavelengths of 0.5, 1.0, and 1.5 A. For the WD spectrometer, assume a LiF crystal with 2d = 4.03 A (200 reflection) and line width B = 0.5°. For the ED spectrometer, assume a Si(Li)-FET counter and Eq. (7-5). Assume also that the line or pulse-distribution separation must be twice the breadth for adequate resolution. 

Two dosimeters suitable for monitoring single pulses of x-rays with doses in the range 1 to 100 rads at dose rates greater than 103 rads/sec. are described. Both systems were independently referred to Fricke dosimetry as the absolute standard and cross checked under pulse conditions. In one system the transient hydrated electron absorption produced by the pulse is measured by kinetic spectrophotometry as an indication of the dose. In the other, doped LiF crystals of about 50 mg. are irradiated in sealed polyethylene bags under conditions of electronic equilibrium. Readout of the irradiated crystals was done on a standard commercial machine. Both methods were readily capable of 5% precision and with a little care better than 3% is obtainable. 

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