Spurion/ScanMapper Quick Start

(just enough information to be dangerous)

'Spurion' has been incorporated into DAVE and is now called TAS Scan Mapper.  It can be found under TAS Tools in the Planning section. It can be used to make a plot of the scattering plane, accessible reciprocal space, and aluminum diffraction lines.  The program can trace the wave vectors and energies for various higher-order scattering processes. Extensive help is available in the program itself, or on the web.

The program can produce hardcopy output of reciprocal space (see figure), and an energy plot.  A GUI interface has been developed and a detailed help is available within the program.

Figure of reciprocal space, showing the Bragg peaks, aluminum (sample holder) powder peaks, and a few possible spurious processes.

The easiest way to get started is input a data file that has been collected and contains the information about the instrumental configuration and lattice.

To add aluminum or copper powder (sample-holder) lines to the reciprocal space plot, or higher-order processes on and off.  The trajectory of the second, third, and fourth order processes are shown on the reciprocal space plot, and the corresponding energy ranges are shown on the energy plot.

Incoherent scattering process can also be indicated.  The plots can be zoomed, and output to a color postscript file that can be sent (outside of Scan Mapper) to the printer.

Background

A neutron incident on a sample with wave vector k_i and energy E_i is scattered into a final wave vector k_f and final energy E_f.  The total momentum and energy must be conserved in the scattering process, and thus there must be a corresponding change in the crystal momentum and energy.  The changes in wave vector Q and energy ΔE can be written as

(1)

and

(2)

The monochromator can scatter higher-order wavelength neutrons, and the analyzer can also scatter higher-order wavelength neutrons.  So Eq. (1) can be written more generally as

,        (3)

.        (4)

Then when you perform a scan using Eqs. (1) and (2) with n=m=1 you are simultaneously scanning (inadvertently) using higher-order processes.  We can use filters or monochromators/analyzers (such as silicon or germanium) that have systematic absences to eliminate some of these possibilities, but there are usually some processes that are possible.  Spurion will show you where these processes occur in reciprocal space, and in energy.

Another possible spurious process can occur via incoherent scattering from the monochromator or analyzer crystal (or holder).  If we have an incident wave vector k_i, then a wave vector of the same length can scattering from the analyzer (i.e. k_i = k_f) via incoherent scattering.  Conversely, you can scatter incoherently from the monochromator and then Bragg scattering from the analyzer.  This is elastic scattering, and if the difference is close to a reciprocal lattice vector this process can produce a spurious peak, even though the incoherent cross section is orders-of-magnitude smaller than the Bragg scattering.  The spurion program warns then this happens by color coding this elastic Bragg condition red.