Even changing the orientation of a photomultiplier in the Earth's field can show a detectable effect. We offer a selection of different types of magnetic shields to cover the entire range of ET Enterprises' photomultipliers.
There is more than one type of shield available for some tube types and the final choice of shield will depend on the magnitude of shielding required. Magnetic fields affect the collection efficiency and the gain of a photomultiplier, to a degree dependent on the type of dynode structure. Performance curves for the four multiplier structures are given in the technical document: understanding photomultipliers. Information is also given in the individual data sheets.
integral magnetic shields
These are offered for all end-window photomultipliers up to 52 mm diameter. The photograph to the right shows a photomultiplier fitted with an integral magnetic shield. A 0.125 mm thick mumetal* sheet is wrapped around the envelope and protected by an insulating sleeve. There is direct electrical contact from the shield to the cathode pin. These are the most cost effective, but other options with a higher degree of shielding may be necessary in place of, or in addition to, an integral shield.
Split cylinder magnetic shields
These are available for end window photomultipliers up to 52 mm in diameter. The shield is in the form of a split cylinder of thickness 0.5 mm. An internal grip pad that adheres to the photomultiplier is included. The range is listed in the table below.
Contoured magnetic shields
These 0.5 mm thick magnetic shields are available for side-window, and end window photomultipliers with diameters in the range 52 mm to 200 mm.
The range is listed in the table below. To view an outline drawing click on the shield type number.
|MSSW||30 (side window)||B11A no lugs|
It is important to connect the magnetic shield to the photocathode potential; this will prevent noise generation from potential gradients across the glass envelope of the photomultiplier.
* Mumetal is a registered trade mark of Magnetic Shield Corportion.