CATCHING A PEAK BY THE TAIL: INTENSITY CORRECTION FOR THE FINITE SIZE OF THE "SEED-SKEWNESS" PEAK MASK Robert Bolotovsky and Philip Coppens, Chemistry Department, State University of New York at Buffalo, Buffalo, NY 14260
In [1] a novel technique for integration of peaks on imaging plates has been described. The seed-skewness method finds an optimal mask for every peak based on the statistics of the pixel intensities. Being finite in size, the optimal mask inevitably cuts the weak "tails" of the peaks, because at a certain intensity level the peak contribution to the pixel intensity vanishes and yields to the background noise. This effect causes the optimal masks for the weak peaks to be smaller than those for the strong peaks. A simple Gaussian-peak model gives an estimate of the intensity bias due to this effect. The correction can be as high as 10-20% for the very weakest observed peaks. The missing tail effect may also cause the optimal mask to split into two parts when the peak is bimodal, as happens when the a1 and a2 positions are sufficiently separated. A correction for this effect is based on the a1/a2 intensity ratio, determined either in a separate counter experiment or directly from the imaging plate data. The algorithms have been implemented in the program HIPPO [1], and applied to both synchrotron and rotating anode data. Applications to several low-temperature data sets will be presented.
Support of this work by the National Science Foundation (CHE9317770) is gratefully acknowledged. The SUNY X3 beamline at NSLS is supported by the Division of Basic Energy Sciences of the U.S. Department of Energy (DE-FG02-86ER45231). Research carried out in part at the NSLS at BNL which is supported by the U.S. DOE, Division of Materials Sciences and Division of Chemical Sciences.
1. R. Bolotovsky, M. A. White, A. Darovsky and P. Coppens, J. Appl. Cryst. 28, 86 (1995).