The Saint Mary's College field portable XRF is a energy dispersive instrument.  It has a minature X-ray tube as a source.

The XRF Process

This Figure is from the Innovative Technology Verification Report on the Niton XLt 700 series XRF analyzer. EPA/540/R-06/004, page 4.

It provides an appropriate visual to the text in the left hand column.

The Saint Mary's College XRF is a 700 series instrument.

A good deal of information on the U.S.EPA supported development of XRF instrumentation is available at the CLU-IN website .

Recently an instrument manufacturer has developed a website with more details on XRF methods. It may serve as a useful resource to read ( learnxrf.com ).

This paragraph in quotes below states succinctly how the XRF spectra are generated and it was a part of an US EPA report on this technology as it was developed using radioisotopes sources for these instruments. 

EPA Environmental Technology Verification Report, Field Portable  X-ray Fluorescence Analyzer; Spectrace TN 9000 and TN Pb FP-XRF,  EPA-VS-SCM-03, p. 64-98.

“FPXRF analyzers operate on the principle of energy dispersive XRF Spectrometry.  This is a nondestructive qualitative and quantitative analytical technique that can be used to determine the metals composition in a test sample.  By exposing a sample to an X-ray source having an excitation energy close to, but greater than, the binding energy of the inner shell electrons of the target elements in the sample, electrons are displaced.  The electron vacancies that result are filled by electrons cascading in from the outer shells.  Electrons in these outer shells have higher potential energy states than inner shell electrons, and to fill the vacancies, they give off energy as they cascade into the inner shell vacancies.  This release of energy results in an emission of X-rays that is characteristic of each element.  This emission of X-rays is termed XRF. …A qualitative analysis can be made by identifying the characteristic X-rays produced by the sample.  The intensity of the characteristic X-rays emitted is proportional to the concentration of a given metal and can be used to quantitate each element.”

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