Luminescent screens
When a beam passes through a luminescent screen, part of the deposited energy results in excited electrostatic states in the material from which a light emission at a defined wavelength will follow. The light emission originates in impurity inclusions, the so-called activators, in most of the materials used. Luminescent screens are used in all accelerators during the running-up periods and when problems occur. They allow a direct observation of the beam position and shape on a TV monitor. They are necessarily single-pass monitors.
A typical proton machine monitor can have three different screens inserted at 45 degrees in the beam path. The mechanism has a fourth empty position which is used for the free passage of the beam through the monitor. A TV camera observes the light spot generated by the beam passage.
The screen and the detectors characteristics have to be matched to each other. There are two main type of detectors: TV tubes with various photocathode types (which are more resistant to radiation), and solid-state detectors (CCD sensors, for example).
The simplest use of these monitors is to observe the beam-generated light on a TV monitor. In general, a reference grid is deposited on the screen or superimposed optically on the image to get an accurate spatial and dimensional reference. The TV signals can also be recorded on an analog video recorder or on a 'frame grabber' digitizer. For the solid-state detectors, which have a geometrically well-defined matrix structure, it is possible to digitize each picture element (pixel) separately, thus obtaining the best geometrical resolution. Furthermore, owing to their good dynamic range, of the order of 1:1000 in single-shot operation, is is possible to use them to measure beam profiles. These can be used to display density cross-sections, to make projections on a chosen axis, and to plot density contours or 3-D representation of the light, i.e. beam density.
Some luminescent screens (LS) are provided in LEP, most of them mounted on beam stoppers. Being destructive devices to the beam, they can only be used for exploring the first turn trajectory. They are made of chromium doped Al2O3 plates, 2 mm thick. All screens are placed perpendicular to the beam for better image sharpness and are seen through stainless steel mirrors at 45 degrees and a CCD TV camera.