TRICS PSD Peak Search

For almost any measurement at TRICS a UB matrix has to be determined beforehand. In order to do this a couple of peak must be located by some means. This section describes how the computer can help in finding an initial set of peaks.

The algorithm is quite simple: It consists of a big loop over ranges of the four circle angles two theta, omega, chi and phi. At each position a counting operation is performed. Then peaks are located on all three detectors through a local maximum search. For this, the local maximum search module is used. If a candidate peak is found, it is refined in omega and written to a file. The tricky bit is the adjustement of the local maximum search parameters in order to minimize false maxima caused by a spicky background or powder lines.

The peak search facility need a lot of parameters in order to operate. This includes angle ranges, count parameters and the maximum search parameters. Commands are provided for adjusting these parameters. The general operation of these commands follow a pattern: typing the command alone prints the current values of the parameters. In order to set new values the command name must be typed plus new values for all the parameters listed by this command. An Example:

ps.sttrange

ps.sttrange startval endval step

ps.sttrange

adjustment of the two theta range for the peak search.

ps.omrange

adjustment of the omega range for the peak search.

ps.chirange

adjustment of the chi range for the peak search.

ps.phirange

adjustment of the phi range for the peak search.

ps.countpar

adjustment of the counting parameters for the peak search.

ps.scanpar

adjustment of the parameters used by ps.scanlist for scanning located peaks. See below.

ps.maxpar

Adjusts the maximum finding parameters for the peak search. These parameters need some explanation:

window

window is the size of the quadratic area which will be searched around each point in order to determine if it is a local maximum.

threshold

This is a minimum intensity a candidate local maximum must have before it is accepted as a peak. The value given is multiplied with the average counst on the data frame before use. This threshold is the strongest selection parameter.

steepness

A candidate peak should drop of towards the sides. This is tested for by checking if the pixels on the borders of the local maximum detection window are below maximum value - steepness.

cogwindow

In order to refine the peaks position a center of gravity calculation is perfomed. For this calculation pixels within the cogwindow around the candidate peak position are considered.

cogcontour

In order not to base the COG calculation on background pixels, only pixels above cogcontour * maxvalue are used for the calculation. With the spicky background at TRICS .5 seems a good value.

ps.list

lists all parameters for the peak search.

ps.listpeaks

lists all the peaks already found.

ps.run filename

starts the peak search and stores peaks identified in file filename.

ps.continue

continues a peak search which was interrupted for one reason or another.

ps.scanlist

performs an omega scan for each reflection found in the current peak list.