Cathodoluminescence microscope

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Thin section of a quartz crystal: above cathodoluminescence, below in transmitted light

A cathodoluminescence microscope is a combination of an optical microscope and a cathode ray tube . It is used to examine the luminescence properties ( cathodoluminescence ) of polished thin sections , thick sections or mounts of solid samples , excited by an electron beam . To prevent the sample from being charged, the surface must be coated with a thin conductive layer of gold or carbon. This is usually achieved by plasma coating in a sputtering device or vapor deposition with carbon.

With the aid of a cathodoluminescence microscope, structures within the crystals or the structure can be made visible that cannot be seen under normal light microscopic conditions. So z. B. important information about the growth of crystals can be obtained. Cathodoluminescence microscopes are used in geology, mineralogy and materials science (examination of rocks, minerals, volcanic ashes, glasses, ceramics , cements , fly ash, etc.). The color and intensity of the cathodoluminescence are essentially dependent on the properties of the sample, but are also strongly determined by the working conditions of the electron source (acceleration voltage and beam current of the electron beam).

KL microscope with “hot” cathode

Two types of cathodoluminescence microscopes are in use today. One works with a “cold cathode”, whereby the electron beam is generated by a gas discharge tube . The other produces an electron beam by means of a hot cathode (engl. "Hot cathode"), whereby the electrons from an incandescent tungsten filament ( hairpin cathode ) is made to be accelerated. The advantage of a hot cathode is the high and precisely controllable intensity of the electron beam, whereby even weakly luminescent materials (such as quartz, see illustration) can be excited to glow.