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Scintillation Counters and Photomultiplier Tubes

                             The dynode chain

High voltage applied to dynodes creates electric fields

  which guide electrons between from stage to stage

Process of secondary emission yields more electrons

  at each stage

This is the “multiplication” in “photomultiplier”

Process is similar to photoelectric effect, with incident

         photon replaced by incident electron

Composition of dynodes

Ag - Mg

Cu - Be         Deposited in thin layer on

Cs - Sb          conducting support

Sensitivity to earth’s magnetic field

Earth’s magnetic field is typically 0.5 - 1.0 Gauss

Trajectories of charged particles moving in a magnetic

         field will curve, depending on field orientation

Can cause photoelectrons and secondary-emitted

         electrons not to reach next stage

First few stages, when there are few electrons,

         most vulnerable

Use of magnetic shields

Should extend shield beyond front of tube

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