Le Naturaliste

Fredlab a écrit : Bref : il y a peut-être meilleur compte à louer une journée un MEB dans un labo.
(réunir une somme d'échantillon pour le jour J : du pollen, des diatomées, des petits arthropodes...)

Pourrions échanger en mp. ?

Plus il y a de technologie et de formation et plus l'humour semble s'émousser. Bigre !

Before the advent of CCD sensors, most TV cameras were fitted with Vidicon or Plumbicon tubes. Both types of electron tubes differ only in the composition of their respective light sensitive layers. In those tubes, similar to CRT display tubes, a fine electron beam is generated with which the photo electric charges on the light sensitive layer are scanned and converted into an electric signal. In opposition to display tubes, the scanned surface is very small, hence the electron beam must be very fine (well focused) to achive the required resolution of several hundred lines.

The fine electron beam inspired Rapp-Instruments to misuse a Vidicon as a scanning electron microscope (SEM). A fine electron beam is used in a SEM to scan the sample, whereby secondary electrons are generated which are used to create the picture. To make it happen, the Vidikon needs to be opened and fitted into a vaccum chamber. This works needs to be done quickly as longer term exposure to air will damage the delicate cathode in the tube. The best thing to do is to fill the tube with baloon gas (helium) immediately after cutting, and to then hold it with the opening facing downwards only. When changing samples later, the changer is never flooded with air but always with Helium. This way, the cathode will survive many cycles of probe changing. For the vacuum chamber, an off-the-shelf T-piece is used to which the opened tube is glued in. For operation, a pressure of less than 10^-4 bar is required, which is reached by using a diffusion pump in this setup.

For the following measurements, a 2/3″ Vidicon type XQ 1272 was used. At 6V heater voltage, the emission regulator is used to set a current of a few µA. The secondary electron current generated at the sample will then be in the range of several nA.

The raster currents for X and Y deflection are generated via D/A converters by a PC. The image signal is read by the PC through an A/D converter.

Normally, secondary electron currents generated at the sample would be accellerated towards a scintillator, whose light would then be measured wiht a photo multiplier. This way, high amplification with and low noise can be achived. For this simple setup, another way has been chosen. The secondary electrons are sucked off by an electrode on high potential. This electron current can be measured at the sample holder and it is used to generate the raster image. This way, the current can be measured at ground potential which simplifies the setup significantly.

These images of fine wireframe at different magnification levels (20-, 40-, and 80 times) demonstrate the usability of this experiment as a proof of concept. The resolution is approx. 20µm, a value that is determined by the electron optics of the Vidicon and is withing the range of what could be expecte.

d'électrons qui ne demande qu'à diverger (les électrons, de même charge, ne demandent qu'à se repousser