1.3.3
Crystals
The sensing element of the Water Cooled Crystal Holder is a quartz crystal. The crystal is a
plano-convex plate which is excited into thickness shear-mode vibrations by signals from the
Oscillator Unit. When new, the crystal oscillates at a frequency of about 6 MHz.
The frequency of oscillation is determined by the mass of deposited layer on the crystal surface
when it is exposed to the deposition source. As the deposited layer builds up, the oscillations
slow down. Edwards use a plano-convex crystal since this shape induces most of the oscillations
to occur in the central exposed region of the crystal. This increases the sensitivity of the crystal
to the mass of the deposited layer and means that the crystal may be firmly clamped around the
rim to improve heat transfer to the Water Cooled Crystal Holder
4
Water Cooled Crystal Holder
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