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Virtual Visitor Center at SLAC

High Energy Cosmic Rays, Detector Parts

Schematic diagram of what happens when a cosmic ray passes through the scintillator panel - click on the image to see photographs of a real scintillator panelScintillator Panel

A scintillator is a material which "scintillates": it will give off small flashes of light (photons) when a charged particle passes through it. There are a lot of materials that act as scintillators. In this case, the scintillator panels (see photo) are made mostly of plastic with a small fraction of an organic scintillator. Some of the photons reflect off the highly polished surfaces, bouncing around in the panel as shown. A fraction of the photons eventually emerge from the end of the panel and go into to the light guide. Each passing muon creates about 20,000 photons in a 1 cm thick panel.

Schematic of what happens inside the light guide - click on the image for a photo of a real light guideLight Guide

A light guide (see photo) is a shaped piece of plastic with (almost) the same refractive index as the scintillator to which it is attached. It is usually designed so one end matches the scintillator and the other end matches the light-sensitive area of the photomultiplier tube. Photons entering the light guide from the panel will generally hit its other surfaces at such an angle that they are internally reflected and are eventually directed into the photomultiplier tube.

Photo of a photo-multiplier tube, without it's protective cover.Photomultiplier Tube

A photomultiplier tube (see more photos) is a device that uses the photoelectric effect to convert light into an electrical current. The photoelectric effect is the absorption of a photon in the surface of a material, followed by the emission of an electron: if the photon has enough energy, it can kick out an electron. Usually a visible-light photon does not have enough energy to expel an electron. It has to be helped along by carefully choosing a material for the electron-emitting surface (the photo-cathode) and by applying a high negative voltage to the cathode. Each photon absorbed gives off a single electron. This electron then hits another surface (called the first dynode) at a higher (less negative) voltage, and has enough energy to free several more electrons. By repeating this process, the photomultiplier tube amplifies the signal at each successive dynode, so that the small pulse of electrons produced at the cathode becomes a measurable current pulse when it gets to the last dynode (the anode).

Photo of one of the three circuit boards and digital readoutsCircuit Board

Each circuit board is the electronic control center for a pair of scintillator panels. The current pulses from the two photo-multiplier tubes are converted into counts. Each pulse must be converted from an analog signal (the pulse itself) to a digital one. Then the signals from the two panels must be matched and a new signal generated when both panels have a signal at almost the same time. The circuit board also has electronics to accumulate the counts in a digital readout, and to allow operations such as resetting the counter.

Hidden in the details of the circuit board are many elements of the design of the experiment. How big an analog pulse integrated over what time interval constitutes a single digital pulse? How close to the same time must the two pulses from a pair of panels occur to count as a muon passing through?

Digital Readout

The digital readout is the decimal counter that accumulates the counts from the circuit board.
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