BRAGG'S LAW

BRAGG'S LAW

In 1913, the father-son team of William H + William L Bragg built an X-ray spectrometer, using NaCl to resolve Pt X-rays. For this they received the Nobel Prize in 1915.

where :                         is the order of the x-ray (1st, 2nd, etc.)

                          is the wavelength in angstroms

                         is the analyzing crystal 2d spacing in angstroms

                          is the angle of diffraction

There is a more precise form of Bragg's Law that allows for refraction of x-rays by the crystal or diffractor; this can be shown as a modification of the 2d such that d' = d[1-(k/n²)] where n is the order of reflection and k is a constant for each crystal.

Plot from Julien Allaz

These constants range from 6x10-5 for LiF to 2.2x10-3 for TAP (Reed, 1993). They can cause higher order lines to "cross over" and appear on the opposite side of a peak, such as the 3rd order P Ka upon the F Ka for LDEs. Probe for EPMA automatically uses this precise calculation.