Thursday, April 12, 2012

EDS Detector Window Absorption

This graph shows the transmission through several different EDS detector windows. The solid line is for the Moxtek AP1.3, the dot-dashed line for the Moxtek HT2.2 and the dashed line for the Moxtek AP3.3. Our EDS detector has an AP3.3 window. The graph is borrowed from F. Scholze and M. Procop, X-Ray Spectrom. 2005; 34: 473–476. There are all light element EDS windows-- meaning they are for doing very low energy spectroscopy-- and contain C, N and O in a polymer film that is coated with a very thin layer of Al and often a B compound. The absorption edges of B (0.18 keV), C (0.28 keV), N (0.40 keV), O (0.53 keV) and Al (1.56 keV) are all visible in this transmission data measured at the beamline at Physikalisch-Technische Bundesanstalt in Berlin. These absorption edges must be considered in modeling X-ray transmission in standardless EDS. The complexity of this low energy transmission function itself argues for the use of standards.

This last image shows the transmission through an AP3.3 window that is contaminated with water at a level of 2 µg/cm2. The AP3.3 window is shown by the solid circles, the contaminated window by the solid line. Note the drastic changes in window transmission. While the bare window data can be used to determine the absorption of any window contamination layer, small chemical shifts can change the absorption edge structure and thus the detection efficiency in the near the edges.

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