A parameter of diffraction experiment with low attention: exposure

Jan Stránský1,2, Leona Švecová1,2, Petr Kolenko1,2, Jan Dohnálek1

1Institute of Biotechnology CAS, v. v. i., Laboratory of Structure and Function of Biomolecules, BIOCEV centre, Průmyslová 595, Vestec, 252 42 Jesenice u Prahy, Czech Republic; stransky@ibt.cas.cz

2Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Břehová 7, 115 19 Praha 1, Czech Republic

Speed and sensitivity of new pixel array detectors bring new possibilities to the measurement of X-ray diffraction data. Due to their point spread function and their ability to measure weak reflections, reliable estimation of exposition parameters by human eye is made more difficult. However, a correct exposure helps increase the information gain of the measurement (more precise measurement of anomalous differences or weak reflections for improvement of electron density maps) and at the same time reduce  radiation damage.

As a signal to noise ratio of individual reflections (I/s) has, with a chosen error model [1], an upper limit (ISa), the optimal exposure can be estimated from the relation of I/s of strong reflections to ISa. The relation can be analyzed e.g. in a graph of I/s to intensity [2], which is informative even for initial data containing few frames. Therefore, a script for fast analysis was developed. The script produces graph for optimal exposure estimation before the final data collection.

A preliminary analysis of earlier measured datasets shows that majority of datasets measured with a PILATUS detector is underexposed. In comparison, datasets measured with a CCD detector (marCCD) show correct exposure or overexposure.

This study was supported by BIOCEV CZ.1.05/1.1.00/02.0109 from the ERDF and institutional support of IBT CAS, v.vi.. RVO: 86652036 and by Ministry of Education, Youth and Sports of the Czech Republic (grant No. LG14009) and Czech Science Foundation, project 15-05228S and project 15-15181S.

[1]        Diederichs, K., Acta Cryst. Section D, 2010, vol. 66, no. 6, 733–740.

[2]        Weinert, T. and Olieric, V. and Waltersperger, S. et al., Nature Methods, 2015, vol. 12, no. 2, 131–137.