CALIBRATION DEVICE FOR ACCURATE CURRENT MEASUREMENT ON A CAMECA SX100 EPMA

摘要

The beam current I_b in a CAMECA SX100 electron probe microanalyser (EPMA) is measured with a pico Ampere meter that makes use of an operational amplifier that is optimised for current to voltage conversion. The feedback element of the operational amplifier is a resistor R and the following equation is valid: V_(out) = I_b * R (1) Currently a problem exists with resistor R: "Unless the resistive element is hermetically sealed in a glass or metal container with glass-to-metal seals, moisture can permeate a resistor and change its resistance'' [1]. This means that the factory calibration of the pico Ampere meter cannot be guaranteed for the whole lifetime of the instrument. In practice, a deviation of 5 % and more can be expected after several years of operation, while for accurate quantitative EPMA analysis this deviation should be less than 0.5 %.