X-Rays

''X-Rays is the twelfth lecture in the Properties of Matter section of PH1011. It covers the properties of Xrays, the spectra they produce, and Mosely's realtionship.''

Previous: The Electron II

Next: Atomic Structure

Properties and evidence for
Xrays can be produced from cathode rays, and often affect materials (eg causing phospor to glow) even when inhibited by a shield. They are unaffected by magnetic fields but do cast a shadow, and pass through objects depending on density. This led to use in medical imaging.

Xrays can also be used to study crystal structure; constructive interference occurs when 2dsinθ = nλ (the Bragg equation). This can be used to find the distance between atoms in a lattice structure, eg in finding the structure of DNA.

Xray Spectra
Firing electrons at a target causes EM rays (in this case, Xrays) to be produced, as acceleration or decelleration of a charge produces radiation and vise versa. The incident electron is deflected against the atom and loses energy, which is given out as an xray photon with energy hf. The frequency of the xray is the speed of light x its wavelength. When this is carried out multiple times it gives an xray spectrum, plotting intensity against wavelength. The spectrum has a cutoff wavelength, a broad continous curve and several sharp peaks which are dependent on the anode or target metal due to electron transitions within the atoms. This cause materials to have a unique signature, and thus be identifiable from xray spectra.


 * Example: What is the cutoff wavelength for electron acceleration over 30kV?
 * hf = hc/λo = eV
 * => λo = 6.63x10-34Js x 3x108ms-1/(1.6x10-19C x 30x103)
 * = 4x10-11

Mosely's Relationship
Mosely conducted an experiment in which he used different targets for the x rays, and found that the square root of the frequencies on the xray spectral lines corresponded to the element's atomic numbers. √f α (Z-1) => √f = a(Z-1). If given more than one element here, the formula can be given as √f1/√f2 = (Z2-1)/(Z1-1).

Xrays depend on the charge on the nucleus, and therefore vary by atom. Nuclear charge determines the placement of electrons on the atom, which in turn determines chemical properties.

Summary
Xrays are used in medicine as they can pass through less dense materials such as flesh, and also in crystal structure, as their refraction angles allow the distance between atoms to be calculated. They can be produced by firing electrons at an anode target (as the deceleration of a charge produces EM radiation) and the resulting spectra can be used to identify the metal target. Mosely found that the square roots of xray frequencies varied with atomic number-1, which allows identification of compounds via xray spectra.