Log Spiral (Bent Crystal Laue Analyzer)

Currently still in the testing stages, the Log spiral is used in x-ray fluorescence and XAFS experiments.  It is a device that is placed between the detector and the sample and is used to filter out unwated fluorescence in samples with low concentrations of the element of interest.  The silicon crystal is bent in the shape of a logarithmic spiral which causes incident light rays from a point source to diffract at the same energy. The log spiral is especially beneficial in samples that contain elements close to the atomic number of the element of interest (particularily z-1).  The high energy resolution that it provides can allow the user to see seperate peaks corresponding to the fine structure of the atom.  There is a different log spiral for each characteristic energy in the range of 5.3- 22 kev.  Currently we are equipped with only a few but hope to acquire more so that the full range can be covered.  Typically the detector used with the log spiral set up is the cyberstar fast scintillation detector but the 32 element Ge solid state detector can also be used depending on the specifics of the experiment.  So far, inadequate shielding has been provided for the log spiral and detector set up and more testing needs to be done to optimize positioning.

Above is a picture of the current log spiral set up.  The log spiral and detectors are enclosed with lead tape to filter out unwanted background noise.