Removed from a Gamma ray Spectrometer. The flat end of the tube would have been in contact with a gamma ray detector eg a block of Thallium-activated Sodium Iodide. The gamma ray produces a flash of light in the crystal, the intensity being proportional to the gamma ray energy. The light impinges on a photo cell in the tube, which converts the photons into an electrical charge. These primary electrons enter a series chain of dynodes, each with a PD of around 1kV wrt the previous dynode. There are 10 dynodes in the chain with a final voltage of 10kV to 12kV in the chain. The impact of the accelerated primary electrons against a dynode creates secondary electrons, thus multiplying the number of available electrons. After the last dynode in the chain, the electron current generates a voltage in a resistor. The voltage is amplified by standard amplification techniques. The voltage is proportional to the gamma ray energy.
