Question

1. The elemental composition of a specimen is to be carried out using a Si-based EDS detector. How many electron-hole (charge) pairs will be generated by an oxygen Kα

X-ray with an energy of 525 eV? How many charge pairs will be generated if, instead, a Fe Kα X-ray with an energy of 6400 eV arrive at the detector.

2. You are analyzing a specimen in a Scanning Electron Microscope. As you decrease the working distance, name three things you should be thinking about.

3. When you turn the "focus" knobs on a Scanning Electron Microscope, what are you controlling in the microscope? When do you achieve optimum focus?

Answer 1

1.

where E is the incident X-ray energy and is the enrgy required to create electron-hole pairs.

For Si-based EDS, is 3.8 eV/pair

For oxygen,

n=138 pairs

For Fe X-ray,

n=1684 pairs

2. Things to keep in mind while changing working distance,

Large working distance gives larger spot size but lesser resolution

Convergance angle decreases so we observe an increased depth of focus

Larger working distance increases scan length but reduces magnification

Abberation is also weakened

3. Focus is the position where the cone of electron beam has the smallest diameter. Hence it refers to adjust the working distance. Optimum focusing can be achieved by focusing at higher magnifcation and then adjust to the required magnification