Fluorescent screens

Certain materials (CaWO₄, CsI etc.) emit visible light upon irradiation by x-rays, the intensity of which is related to the magnitude of x-radiation they are exposed to. This light, the wavelength of which is in the optical range and which is due to fluorescence, can be utilized for radiological imaging. The characteristics and speed of the process make it possible to produce prompt optical images, which is especially advantageous for live diagnistic examinations. Due to the absorption of the body examined by x-rays it is not always possible to obtain images directly visible for the human eye. Therefore it is necessary to enhance the intensity of the image. That is done in the image enhancer devices, which operate on the principles of photoelectric effect and fluorescence. The beam of x-rays first fall onto a fluorescent screen, in the immediate vicinity of which there is a photocathode. Due to the light emitted by the screen electrons emit from the photocathode. The electrons are accelerated and focused by an electric field and then fall onto a second fluorescent screen. This screen has a significantly smaller area than the first screen had, so the intensity of light per unit area will be much higher here. The accelerated electrons have much higher energy so they are capable of creating higher intensity fluorescent light than the primary radiation could. Obviously the entire device must be put into a vacuum enclosure.

 

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