Beam current normalization in an X-ray microanalysis instrument

In an electron beam column instrument for quantitative energy dispersive x-ray microanalysis including an optical system for establishing a focused electron beam flux of a preset probe current level directed onto an analysis specimen, means for analyzing the spectrum of x-rays emitted from the specimen to accumulate a count of x-ray intensities, an oscillator producing clock pulses, an elapsed time counter receiving clock pulses for timing out an analysis time period, the improvement comprising:

• means for monitoring electron beam current stability;
• means for inhibiting the counting of clock pulses by the elapsed time counter, the count inhibiting means being responsive to an actuating signal pulse to inhibit counting for an interval of time that corresponds to the duration of the actuating signal pulse;
• a pulse generator for generating a signal pulse of a preselected duration in response to a trigger pulse input thereto;
• means for producing an actuating signal pulse between pulse generator signal pulses;
• and
• a current-to-frequency converter for generating pulses to trigger the pulse generator, the converter generating trigger pulses at a frequency that varies in direct proportion to beam current variations.

Apparatus for providing beam current normalization in an EDS analyzer having an elapsed analysis time counter, which comprises:

• means for producing a current proportional to beam current;
• a digital current integrator producing pulses, each representing the integration of a defined amount of charge, the time interval between pulses being inversely proportional to the current from the current producing means;
• a one-shot connected to the digital current integrator for producing a pulse of prescribed duration in response to each pulse from the digital current integrator;
• and
• means connected to the one-shot for producing a delay time signal between the prescribed duration pulses to inhibit the elapsed time counter of the EDS analyzer.

Apparatus for providing beam current normalization in an EDS analyzer having a dead time control section producing a dead time signal and a timer responsive to the dead time signal for establishing the length of an analysis time period, which comprises:

• an electron beam current sensor for providing a current signal proportional to electron beam current;
• a digital current integrator connected to the beam current sensor for producing an output pulse waveform signal;
• and
• means coupled to the digital current integrator for modulating the analyzer dead time signal in response to the integrator output signal to correct for variations in electron beam current during EDS analyzer data acquisition.

In an electron column instrument for quantitative energy dispersive x-ray microanalysis which includes an optical system for establishing a focused electron beam of a preset beam current level directed onto an analysis specimen, a solid state detector for detecting x-rays emitted from the analysis specimen, signal processing circuitry coupled to the solid state detector for producing a voltage pulse representative of detected x-rays, a pulse height analyzer for analyzing the spectrum of x-rays detected and accumulating a count of x-ray intensities, an elapsed time counter for timing out an analysis time period, an oscillator generating clock pulses to drive the counter, a clock gate coupling the oscillator to the counter for inhibiting or allowing the passage of clock pulses to the counter, and a dead time control circuit for generating a signal indicative of the time period the signal processing circuitry and the analyzer require to process and analyze a detected x-ray, the improvement comprising:

• a beam current sensor for providing a current proportional to electron beam current;
• a digital current integrator coupled to the beam current sensor for producing an output signal comprising pulses representative of the integration of a fixed amount of charge, the average spacing in time between integrator pulses being inversely proportional to beam current;
• and
• logic circuitry operably connected to the digital current integrator and to the dead time control circuit for combining the dead time signal and the integrator output signal and producing a clock gate control signal.