Weekly Progress

Project Goal

The goal of this project was to investiage the linearity of a curved position sensitive x-ray detector.

Introduction

X-ray powder diffraction is a technique that is used to gain information about the atomic structure of crystalline materials. X-rays illuminating a sample are scattered by atoms in the crystals and interfere constructively and destructively. The interference pattern created by these x-rays is detected and a specturm of x-ray counts vs. scattering angle (2 theta) is obtained. The postion and intensities of the peaks in this spectrum can reveal information about which atoms are present and in which phase.

Often a point-to-point detector is used to record the x-ray spectra. This detector counts the number of x-rays present at one position. The detector is then swept over the 2 theta range to record the whole spectrum. For certain situations, however, it is advantageous to be able to record the entire spectrum at once. One might want to observe the spectrum as a function of time as a phase transition or some other reaction took place. The detector I investigated was of this type. It was an extended detector, curved in order to span a 2 theta range of about 45 degrees.

Experimental Setup

The following diagram displays the experiemental setup.

The source of x-rays was the synchrotron light source. The sample used to test the detector was Alumina (Al₂O₃). The double-crystal monochromator selects the wavelength of the x-rays and the ion chamber is used to determine the intensity of the x-ray beam.

Results

The following is an example of one of the spectra.

The following is an example of a spectrum for alumina obtained by a point-to-point detector.

Analysis

Each peak of the twenty spectra was fit with a Gaussian and the peak positions were obtained.

The deviation of each point from the linear fit was calculated and plotted to get a sense of the linearity of the detector.

It can be seen from this graph that there is some nonlinearity within the detector.