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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.
The above plots show relative output as a function of external magnetic field (for photomultipliers operated at nominal gain).
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.
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.
figure 3 demonstrates how a wrapped electromagnetic shield reduces the sensitivity of a 9106 photomultiplier to external magnetic fields.
Further details on this technique and the benefits it offers are contained in 'integral Mu-metal* shields', RP 084, which can be requested via this form.
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* Mumetal is a registered trade mark of Magnetic Shield Corporation.