Question

The counts per second (cps) changes when the spot size is changed. Why?

The counts per second (cps) also changes when kV is changed. Why?

In general, which parameter would you change, spot size or kV, to increase the counts per second? Why?

This is when using an Scanning Electron Microscopy (SEM) or Energy Dispersive Spectroscopy (EDS).

Answer 1

• Spot size, also known as beam current or probe current, indicates the number of electrons in the primary beam of the electron microscope. When the spot size is changed it means the number of electrons in the incident beam change, thus the energy being incident which would also change and results in the change in the number of X-ray counts per second.
• The accelerating voltage (kV) controls the energy of the electrons in the incident (primary) beam. To generate Characteristic X-rays, the electrons in the primary beam should possess sufficient energy to overcome the ionization energy, which is also known as critical ionization energy of the inner shell of electrons in the atoms of the sample. Thus, the change in kV would cause the counts per second (cps) to change.
• kV should be changed because spot size is dependent on kV, thus changing kV would result in the change of the spot size and hence in the change of counts per second (cps).