Electron Microprobe Analysis: Bragg's Law, Electron Generation, and X-Ray Characteristics, Exams of Geochemistry

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Theory and Practice of Electron Microprobe Analysis

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)

You have counted on the X-ray peak produced by element "Q" in a sample, and have obtained 35,000 counts.

(a) What is the 1 sigma standard deviation, in counts, for that determination?

(b) What is the 1 sigma standard deviation, in percent, for that determination?

After completing matrix corrections, you determine that the concentration of "Q" is 50 wt%.

(c) Based on counting statistics, what is the 1 sigma deviation, in percent, for this determination?

(d) What is the 1 sigma deviation, in wt.%, for this determination?

Question 8. (10 points) Curved crystals are used for diffraction of X-rays in the spectometers 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 10 (10 points)

An aperture in the electron column called the “beam regulation aperture”, is particularly important for quantitative analysis. In cross-section, the aperture looks like this:

(a) Describe how this aperture works, and why it is important.

(b) For what type of analysis is the beam regulation aperture most important, and why? For what type of analysis is it least important?