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Magnetic Defocusing Assembly
The Products for Research model PR411 Magnetic Defocusing Assembly is designed to provide reduced dark current as shown in Figures 1 and 2 by virtue of a reduction in effective cathode area. The PR411 can provide dark currents less than 10% of the unfocused dark current at ambient temperature when used with most 2 in. nominal diameter end on photomultiplier tubes with semi-transparent photocathode. In addition, the PR411 is also very effective in damping out large unwanted pulses such as those induced by cosmic rays passing through the front windows of these tubes. Because different applications may require different effective photocathodes, the PR411 consists of three permanent magnets mounted in threaded rings which when used in various combinations provide effective cathodes of 8 mm diameter, 10 mm diameter and 15 mm diameter. Figure 3 shows the relative shapes of the effective photocathodes on a 2 inch diameter tube, scanned by a 1 mm spot.
The PR411 Magnetic Defocusing Assembly is designed to be used with any standard PFR housing, cooled or uncooled without effecting the external configuration of the housing.

The principle of operation of the PR411 is that thermionic or photo-electrons are prevented from getting from the cathode to the first dynode by virtue of the shape of the magnetic field. Again, due to the field shape, the central region of the photocathode is not affected. For these reasons, the PR411 can not be used effectively with tubes that have reflective photocathodes or off axis first dynode aperatures. In addition, tubes with inherently low cathode dark currents may not show the same dark current reduction as the reduction in dark counts when used in the photon counting mode. Finally, the effects of the PR411 will be in addition to any dark current reductions accomplished by cooling, since the methods are independent of each other.

 

Chart recording showing effects of the PR411

on the dark current of a 2 inch (52 mm) diameter S-20 PMT.

Graph (7K)




Figure 3   •   BACK TO TOP
 

Relative shapes of the effective photocathode of a 2 inch (52 mm) diameter PMT,

scanned by a 1 mm spot of light.

Graph (6K)




BACK TO TOP

 

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