Electron Microprobe Analysis: Theory and Practice Exam, Exams of Geochemistry

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Theory and Practice of Electron Microprobe Analysis Exam

Answer 10 of the 11 possible questions

Question 2. (10 points) Bragg's Law of Diffraction is represented as follows:

në = 2d sinè

Define:

n

ë

d

è

Indicate ë, d, and è on the following sketch:

Question 4 (10 points)

Define the following:

1. Accelerating voltage. Give a typical accelerating voltage for geological analysis
2. Filament current
3. Beam current
4. Probe current. Give a typical probe current for geological analysis

Question 5. (10 points)

(a) Discuss some characteristics of secondary electrons (SE) generated from a sample surface, and indicate the type of images that can be obtained using SE.

(b) Discuss some characteristics of backscattered electron (BSE) generated from a sample surface, and indicate the type of images that can be obtained using BSE.

Question 7 (10 points)

(a). The electromagnetic lenses in the SX-100 have 3 main functions. Name 2 of these three functions.

(b) What are the lenses made of, and, briefly, how do they work?

(c). The following sketches show schematics of the electron beam passing through a non- energized lense (no current) and a weakly energized lense (low current). Sketch in the path of electrons passing through a strongly energized lense (high current). Which configuration, low current or high current will result in the highest number of electrons reaching the sample?

Question 8. (10 points)

Explain the difference between scanning (raster) mode and fixed beam mode in the electron microprobe.

Which mode would you use for the following types of analyses: Backscattered electron imaging:

Secondary electron imaging:

Qualitative scan:

Line scan:

Quantitative analysis:

X-ray mapping:

Question 9. (10 points)

Question 10. (10 points) Curved crystals are used for diffraction of X-rays in the spectrometers of an electron microprobe.

a) List the 3 types of crystals that are present in the SX-100 at New Mexico Tech

b) What type of elements are each most suitable for? Either list a range of elements, or discuss the general element characteristics.

c) What feature of the crystal makes it appropriate for given elements?

Question 11. (10 points)

You have a sample that contains an unknown mineral phase that you would like to identify.

Describe two electron microprobe techniques that would allow you to identify the unknown phase. Compare the positive and negative features of the two techniques.