﻿ X-ray spectra

X-rays are classified as hard or soft, depending on their penetrating ability.

Diffraction will only occur if wavelength of the waves is comparable to the width of the slit used to diffract them.

X-ray spectra are obtained by diffraction, which involves crystallography.

The Braggs pioneered x-ray spectroscopy.

Continuous specra for x-rays are the result of Brehmstrahlung.

X-ray line spectra are the result of K-shell knockout.

X-ray line spectra are characteristic of the elent used as target.

The spectrum of visible light is the familiar display of the colours of the rainbow. This spectrum may be thought of as a set of wavelengths (each shade of colour corresponding to a different wavelength). An x-ray spectrum is a set of wavelengths (all of which are shorter than those of visible light). Those wavelengths, near the ultraviolet end of the electromagnetic spectrum, are known as soft X-rays. The very short wavelengths, close to the gamma-ray range, are known as hard X-rays. Measurement of these wavelengths involves use of a spectrometer and the principle of diffraction.

Diffraction will only occur if the wavelength of the waves is comparable to the width of the slit used to diffract them. Because the wavelength of x-rays is so short conventional diffraction gratings are too broad. The fact that the wavelength of x-rays is similar to the distances between planes of atoms in crystals means that crystals can act as (three dimensional) diffraction gratings for x-rays (as Von Laue first recognised in 1912).

In 1913 W.H. Bragg and his son, W.L. Bragg were the first to perform measurements in x-ray spectroscopy. The Braggs directed x-rays at crystals and observed interference patterns. The process may be broken down as follows:
1. x-rays are diffracted by each individual atom’
2. the combined effect of all the atoms in a single plane is to produce a reflected beam at the same angle to the crystal plane as the incident beam,
3. the reflections from similar planes interfere constructively in certain directions,
4. The condition for constructive interference to arise in radiation scattered by crystals is 2d sin ? = n ? (Bragg’s Law).

Bragg’s law has a somewhat familiar appearance from optics. In the formula d is the spacing between adjacent Bragg planes in the crystal and ? is the wavelength of the x-rays. However, ? is not the angle of incidence but rather the glancing angle.

By using crystals with simple structures where d was known, then the wavelength ? of x-rays was accurately determined. Then with known values of x-ray wavelength, diffraction patterns were examined for more complex crystals resulting in the spacing of the Bragg planes being determined and so the crystal structure established. Hence x-rays helped the development of crystallography. Materials scientists use this information to understand the crystals they have produced while molecular biologists use it to understand the molecular structures of complicated biological molecules.

Because x-rays can be slowed down at a wide range of values of deceleration (Brehmstrahlung) the spectrum of x-rays is a continuous one. However it has two distinctive features: 1.the sudden curtailment at the short wavelength end (due to the finite maximum kinetic energy of the bombarding electron and 2. the presence of a number of prominent lines (corresponding to particular wavelengths) that are characteristic of the metal used as target. Each different element used as target has its own characteristic x-ray (line) spectrum. This feature of the x-ray spectrum relates to the production of x-rays by electron transition between shells.

A spectrometer has three components…Source ….Diffractor…Detector. In the case of an optical spectrometer these are a bulb….a diffraction grating … and the human eye. In the case of an x-ray spectrometer; the source is an x-ray tube and a crystal is used as a three-dimensional diffraction grating. The detector may be a photographic plate or an ionisation chamber.