[ Identification | Description | Input parameters | Output parameters | Links ]
Single_crystal
ComponentSingle crystal with mosaic. Delta-D/D option for finite-size effects. Rectangular geometry. Multiple scattering and secondary extinction included. The mosaic may EITHER be specified isotropic by setting the mosaic input parameter, OR anisotropic by setting the mosaic_h, mosaic_v, and mosaic_n parameters. Crystal structure is specified with an ascii data file. Each line contains 4 or more numbers, separated by white space. The first three numbers are the (h,k,l) indices of the reciprocal lattice point, and the last number is the value of the structure factor |F|**2, in barns. (The rest of the numbers are not used; the file is in the format output by the Crystallographica program). Example: Single_crystal(xwidth=0.01, yheight=0.01, zthick=0.01, delta_d_d=1e-4, mosaic = 5, ax = 3.8186, ay = 0, az = 0, bx = 0, by = 3.8843, bz = 0, cx = 0, cy = 0, cz = 11.6777, reflections="YBaCuO.lau") Overview of algorithm: (1). The neutron intersects the crystal at (x,y,z) with given incoming wavevector ki=(kix,kiy,kiz). (2). Every reciprocal lattice point tau of magnitude less than 2*ki is considered for scattering. The scattering probability is the area of the intersection of the Ewald sphere (approximated by the tangential plane) with the 3-D Gaussian mosaic of the point tau. (3). The total coherent scattering cross section is computed as the sum over all tau. Together with the absorption and incoherent scattering cross section and known potential flight-length l_full through the sample, we can compute the probability of the four events absorption, coherent scattering, incoherent scattering, and transmission. (4). Absorption is never simulated explicitly, just incorporated in the neutron weight. (5). Transmission in the first event is selected with the Monte Carlo probability p_transmit, which defaults to the actual transmission probability. After the first event, transmission is selected with the correct Monte Carlo probability. (6). Incoherent scattering is done simply by selecting a random direction for the outgoing wave vector kf. (7). For coherent scattering, a reciprocal lattice point is selected using the relative probabilities computed in (2), and the weight is adjusted with the contribution from the structure factors (this way all reflections will get equally good statistics in the detector). (8). The outgoing wave vector direction is picked at random using the intersecting 2-D Gauss computed in (2). The vector is normalized to the length of ki (elastic scattering) to account for the error caused by the planar approximation of the Ewald sphere. (9). The process is repeated from (2) with kf as new initial wave vector ki.
Name | Unit | Description | Default |
---|---|---|---|
reflections | string | File name containing structure factors of reflections | |
xwidth | m | Width of crystal | |
yheight | m | Height of crystal | |
zthick | m | Thichness of crystal (no extinction simulated) | |
delta_d_d | 1 | Lattice spacing variance, gaussian RMS | 1e-4 |
mosaic | -1 | ||
mosaic_h | -1 | ||
mosaic_v | -1 | ||
mosaic_n | -1 | ||
ax | 3.8186 | ||
ay | 0 | ||
az | 0 | ||
bx | 0 | ||
by | 3.8843 | ||
bz | 0 | ||
cx | 0 | ||
cy | 0 | ||
cz | 11.6777 | ||
p_transmit | -1 | ||
absorbtion | 0 | ||
incoherent | 0 | ||
aa | 0 | ||
bb | 0 | ||
cc | 0 |
Name | Unit | Description | Default |
---|---|---|---|
hkl_info |
Single_crystal.comp
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[ Identification | Description | Input parameters | Output parameters | Links ]
Generated automatically by McDoc, Peter Willendrup <peter.willendrup@risoe.dk> / Feb 28 2006