Programs[ edit ] Transport stream has a concept of programs. For instance, a transport stream used in digital television might contain three programs, to represent three television channels. Suppose each channel consists of one video stream, one or two audio streams, and any necessary metadata. A receiver wishing to decode a particular "channel" merely has to decode the payloads of each PID associated with its program.
Schematic of a photomultiplier tube coupled to a scintillator.
This arrangement is for detection of gamma rays. Typical photomultiplier voltage divider circuit using negative high voltage. Photomultipliers are typically constructed with an evacuated glass housing using an extremely tight and durable glass-to-metal seal like other vacuum tubescontaining a photocathodeseveral dynodesand an anode.
Incident Pmt papers strike the photocathode material, which is usually a thin vapor-deposited conducting layer on the inside of the entry window of the device.
Electrons are ejected from the surface as a consequence of the photoelectric effect. These electrons are directed by the focusing electrode toward the electron multiplierwhere electrons are multiplied by the process of secondary emission.
The electron multiplier consists of a number of electrodes called dynodes. A small group of primary electrons is created by the arrival of a group of initial photons.
The primary electrons move toward the first dynode because they are accelerated by the electric field. Upon striking the first dynode, more low energy electrons are emitted, and these electrons are in turn accelerated toward the second dynode.
The geometry of the dynode chain is such that a cascade occurs with an exponentially-increasing number of electrons being produced at each stage. This last stage is called the anode. The necessary distribution of voltage along the series of dynodes is created by a voltage divider chain, as illustrated in Fig.
In the example, the photocathode is held at a negative high voltage of order V, while the anode is very close to ground potential. The capacitors across the final few dynodes act as local reservoirs of charge to help maintain the voltage on the dynodes while electron avalanches propagate through the tube.
Many variations of design are used in practice; the design shown is merely illustrative. There are two common photomultiplier orientations, the head-on or end-on transmission mode design, as shown above, where light enters the flat, circular top of the tube and passes the photocathode, and the side-on design reflection modewhere light enters at a particular spot on the side of the tube, and impacts on an opaque photocathode.
The side-on design is used, for instance, in the typethe first mass-produced PMT. Besides the different photocathode materials, performance is also affected by the transmission of the window material that the light passes through, and by the arrangement of the dynodes.
Many photomultiplier models are available having various combinations of these, and other, design variables. The manufacturers manuals provide the information needed to choose an appropriate design for a particular application.
History[ edit ] The invention of the photomultiplier is predicated upon two prior achievements, the separate discoveries of the photoelectric effect and of secondary emission. Photoelectric effect[ edit ] The first demonstration of the photoelectric effect was carried out in by Heinrich Hertz using ultraviolet light.
Historically, the photoelectric effect is associated with Albert Einsteinwho relied upon the phenomenon to establish the fundamental principle of quantum mechanics in an accomplishment for which Einstein received the Nobel Prize.
It is worthwhile to note that Heinrich Hertz, working 18 years earlier, had not recognized that the kinetic energy of the emitted electrons is proportional to the frequency but independent of the optical intensity.
This fact implied a discrete nature of light, i. Secondary emission[ edit ] The phenomenon of secondary emission the ability of electrons in a vacuum tube to cause the emission of additional electrons by striking an electrode was, at first, limited to purely electronic phenomena and devices which lacked photosensitivity.
In the effect was first reported by Villard. The primary goal for many, if not most, workers was the need for a practical television camera technology. Television had been pursued with primitive prototypes for decades prior to the introduction of the first practical camera the iconoscope.
Early prototype television cameras lacked sensitivity. Photomultiplier technology was pursued to enable television camera tubes, such as the iconoscope and later the orthiconto be sensitive enough to be practical.
So the stage was set to combine the dual phenomena of photoemission i. First photomultiplier, single-stage early [ edit ] The first documented photomultiplier demonstration dates to the early accomplishments of an RCA group based in Harrison, NJ.
Harley Iams and Bernard Salzberg were the first to integrate a photoelectric-effect cathode and single secondary emission amplification stage in a single vacuum envelope and the first to characterize its performance as a photomultiplier with electron amplification gain.
These accomplishments were finalized prior to June as detailed in the manuscript submitted to Proceedings of the Institute of Radio Engineers Proc.
Magnetic photomultipliers mid — [ edit ] Higher gains were sought than those available from the early single-stage photomultipliers.1 The NASA Program Management Tool: A New Vision in Business Intelligence David G.
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