A light proof, hermetically sealed, dehumidified cylindrical housing houses the photomultiplier with electronics for high voltage power and for the high-speed amplifier for photon counting. High voltage DC power to operate the photomultiplier is usually set at a voltage between 1,200 and 1,400 volts, depending on tube characteristics. A photon arriving at the light sensitive cathode initiates a burst of electrons that cascades and is amplified as is passes through the various stages of the tube. The burst of electrons arriving at the anode is then amplified to yield a pulse that is counted in the counting register. A discriminator level is set at about 30 milli-volts to filter out low voltage pulses, which are more likely electronic noise and not light signal.
An order-blocking filter is placed between the exit slits and the cathode of the photomultiplier to remove radiation from grating orders that are not used. For the Mark II instruments, the second order radiation (~280 nm - 350 nm) is used and the first order (~560 nm - 700 nm) is blocked by a nickel sulfate/UG-11 filter combination. In the Mark IV, the first order (~840 nm -1,050 nm) is not used and a filter wheel inserts an order-blocking filter. Second order visible radiation (~420 nm - 525 nm) for measurements of NO2 is selected with a BG-12 filter, and third order (~280 nm - 350 nm) for measurements of ozone, SO2, and spectral UV radiation is selected by a nickel sulfate/UG-11 filter combination. An order-blocking filter is not used in the Mark EI instruments that are equipped with gratings operating in the first order.
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