eTraining Introduction

Specimen Preparation

Leica Ultracut UCT

Histology

JEOL JSM-6400 SEM

Hitachi S-4700 FE-SEM

Hitachi FB-2000A FIB

Microanalysis

Veeco Dim 3000 AFM

Fluorescence Microscopes

Support

Basic Science

Energy Dispersive Spectrometer

The Energy Dispersive Spectrometer (EDS Detector) is a semi-conductor based detector. It measures X-rays signals and can detect characteristic X-rays of all elements above atomic number 4, given typical beam currents used for secondary electron imaging. It provides a means of rapidly evaluating the elemental constituents of a specimen and gives accurate quantitative analysis. It is less expensive than WDS and can sense an entire X-ray signal at once. It is less sensitive to element concentrations than WDS, however, and will not separate peak overlap. Usually EDS is used on major elements, while trace/minor elements are analyzed with WDS. EDS is a cooled solid state detector mounted on a electron beam instrument. The system and associated counting electronics comprise an EDS system. ED detectors were first introduced in the late 1960's and have been improved over the years. Today, ED systems are the most common X-ray analysis tool found on electron beam instruments.

EDS Spectrometer ArmThe ED spectrometer is comprised of a liquid nitrogen (LN2) dewar, Si crystal, and preamplifier. The special Si crystal is mounted at the end of the detector located closest to the specimen. When X-rays strike the Si crystal very small signals are produced. These signals are amplified in the detector and sent on to the analysis electronics in the computer for further processing. The crystal is cooled by a cold finger, an extension of the LN2 dewar. Liquid nitrogen cools and therefore stabilizes the electronic properties of the crystal, improving the measurements of minute X-ray signals.

X-ray photons generated in the specimen pass through a thin window isolating the detector crystal from the specimen chamber. The magnitude of the pulse is proportional to the energy released into the crystal by the X-ray photon. The signals, also called pulses, are compiled into a histogram, or spectrum, expressing X-ray counts versus energy. The analyst then identifies the resulting peaks to determine the elements present in the excited volume on the sample—a process known as qualitative analysis.

The number of X-ray signals are quantified in terms of count rate or counts per second. The count rate is influenced by the position of the detector relative to the sample and the electron beam current. The detector can be mechanically moved in and out relative to the specimen. The analyst can increase the X-ray signal by varying the condenser lens, the first lens that the electron beam encounters on its way through the column.

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