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| | | Electron Tubes offers integral electromagnetic shields for end window photomultipliers up to 52 mm in diameter. figure 2 shows the construction of an integral shield; the shield is wrapped around the glass envelope, and electrically protected by an insulating sleeve. | | |
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| | | An external magnetic field causes photoelectrons and secondary electrons to deviate from their normal trajectories. The effect is critically dependent on the electron optical design and multiplier structure, the focused structures being most susceptible. Both photoelectron collection efficiency, and electron multiplier gain, g, change giving a combined effect on the overall gain G, as illustrated in figure 1. | | |
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| | | figure 2 Cut-away section illustrating the construction of the integral electromagnetic shield. The co-ordinate axes adopted for the photomultiplier are also shown. | | |
| | | | | | | | | | X-axis | across dynodes | | | | Y-axis | along dynodes | | | | Z-axis | | | | | | | | | | |
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| | | Earth's field ▼ compact focused 9781 | | |
| | | 1.5 1.0 0.5 0.0 1.5 1.0 0.5 0.0 1.5 1.0 0.5 0.0 1.5 1.0 0.5 0.0 | | |
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| | | The relative output of the photomultiplier is shown in figure 3 as a function of external magnetic field with an integral shield. Further details on this technique and the benefits it offers are contained in Integral Mu-metal* Shields', RP 084, available from our website at www.electrontubes.com. | | |
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| | | box & grid 9924 | | |
| | | Earth's field | | |
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| | | 100 80 60 40 20 | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | ......"■■>.. x-axis *•« •.. | | | | | | | | | | | | | | | | | | | | | | | | | dotted line - shielded photomultiplier solid line - unshielded photomultiplier | | | | | | | | | | | | | >s--- | | | | | | | | | | | | | | | | | | |
| | | 3 Q. •M 3 O <U > o> | | |
| | | •M 3 . •M 3 O ai > J2 ai | | |
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| | | -24 | | -16 -8 0 8 applied field (Gauss) | | 16 | | 24 | | |
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| | | | | | | | | | Venetian blind 9956 | | | | | | | | SII | | | | | —[•**—//1 | | | | | -J/ | \__.................. | | | | | | | | | |
| | | figure 3 | | |
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| | | This figure demonstrates how a wrapped electromagnetic shield reduces the sensitivity of a 9106B photomultiplier to external magnetic fields. Field aligned across the first dynode, x-axis. | | |
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| | | -20 | | -10 | | 0 | | 10 | | 20 | | |
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| | | figure 1 applied field (Tesla) x10" Relative output as a function of external magnetic field (for photomultipliers operated at nominal gain). | | |
| | | ' mumetal is a registered trademark of Magnetic Shield Corporation | | |
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| | | The use of an electromagnetic shield is recommended to minimise the effect and, in the most effective designs, extends from the rear of the photomultiplier to a distance half of the diameter beyond the photocathode. | | |
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