Soller Slits Automatic Focusing Method For Multi-element Fluorescence Detector

The X-ray absorption fine structure(XAFS) is one of the most powerful tools for describing local structure. It provides a practical way to determine the chemical state and local atomic structure for a selected atomic species under solid or liquid conditions. For fluorescence measurements in XAFS experiment, the detectors are always combined with a filter and Soller slits to eliminate the influence caused by scattered X-rays. Thus, Soller slits are used together with 32-element Ge solid-state fluorescence detector in the BL14W1 beamline at Shanghai Synchrotron Radiation Facility(SSRF). It is time-consuming to perform high accuracy manual focusing operations. Therefore, an automatic focusing method for Soller slits in multi-element fluorescence detectors is introduced.This paper first discussed the theory of focusing operation. By simulating the filtering process of Soller slits to the fluorescence photon, an approximate model is established. Then, approximates are made to build a geometric model after the character analysis processes of simulated data. The three-dimensional geometric model deduces the functional relation between the position of excitation point and the fluorescence intensity distribution coefficients. Finally, the automatic focusing method, solving the fluorescence intensity distribution coefficients by the least square fitting process to deduce the position of excitation point, is proposed.Second, this paper describes the process to set up the automatic focus system at BL14W1 beamline. The system is divided into four parts: the acquisition system, the master control program, the fitting program and the electromechanical system. The acquisition system is to obtain the fluorescence counts through DXP-XMAP module. The master control program is to control the focusing process and transfer important data. The program is developed by EPICS and LabVIEW on Linux and Windows separately. The fitting program is to calculate the position of excitation point by fitting the fluorescence intensity distribution. This part is developed by MATLAB. The last part is the actuator which is combined with motor and EPICS motor driver. This paper also discussed the environment configuration in consideration of the finiteness of laboratory computer performance.Finally, this paper introduces three experiments at BL14W1 beamline to verify the reliability and accuracy of the method. The first experiment made a contrast between the actual position of excitation point with the calculated one. The results indicate that the fitting results are linear with the actual one and have high accuracy near the focal point. The second experiment measured four fluorescence spectrum of CuSO4 to illustrate the influence of filter and Soller slits in XAFS experiments. The results demonstrates that, with a filter and focused Soller slits, the scattered signals are greatly reduced and target fluorescence intensity meets the highest value. The third experiment compared three XAFS spectrum of MnCl2 to illustrate the influence of automatic focusing method for XAFS experiments. It suggests that XAFS experiments meets the highest signal to noise ratio with focused Soller slits comparing to the unfocused conditions.