Question

Bnefly, but informatively, describe how X-rays are generated for analytical purposes 147 Bnefly, but informatively, describe how X-ray radiation is detected. [4]

Answer 1

1. X-rays can be generated using an X-ray tube. X-ray tube consists of a vacuum tube. X-ray tube uses high voltage of electricity to accelerate electrons produced from a cathode at high temperature. These accelerated high velocity electrons then collide with an anode, which is essentially a metal, creating X-rays. In crystallography, a copper target (anode) is most common, with cobalt often being used when fluorescence from iron content in the sample might otherwise present a problem.

Characteristic X-Ray emission or X-ray electroluminescence is often used in crystallography. If the electron from the cathode, which is accelerated with electric voltage of x-ray tube, has enough energy, it can knock out an inner shell electron from the target anode. The vacancy that has been generated due to this process is then filled by electrons from the higher energy levels of anode atom and consequently X-ray photons are emitted. This process produces an X-rays at a few discrete frequencies, sometimes referred to as spectral lines. Usually these are transitions from the upper shells to the K shell (called K lines), to the L shell (called L lines) and so on. If the transition is from 2P to 1S, it is called K_α, while if it is from 3P to 1S it is K_β. The frequencies of these lines depend on the material of the target and are therefore called characteristic lines. The K_α line usually has greater intensity than the K_β one and is more desirable in diffraction experiments. Thus the K_β line is filtered out by a filter. The filter is usually made of a metal having one proton less than the anode material (e.g., Ni filter for Cu anode or Nb filter for Mo anode).

2. X-rays are detected using sophisticated instruments called detectors.The two most common types of X-ray detector used in the laboratory for powder diffraction (excluding the case of X-ray film) are the scintillation and the gas-filled detectors.

Scintillation Detectors

In scintillation detectors the X-rays are converted to electrical signals with the help of a two stage process. The X-ray photon collides with a phosphor screen, or scintillator, which forms the coating of a thallium-doped sodium iodide crystal. The latter produces photons in the blue region of the visible spectrum. These are subsequently converted to voltage pulses by means of a photomultiplier tube attached directly behind the scintillator. The number of electrons ejected by the photocathode is proportional to the number of visible photons which strike it, which in turn is proportional to the energy of the original X-ray photon.

Limitation: the energy resolution of the detector is poor, and as such it cannot be used to resolve X-ray photons due to Kα and Kβ radiation.

Advantage: It has very high quantum efficiency and a very low dead time making it the ideal detector for the point intensity measurements required for step-scanning diffractometers.

Gas-filled Detectors

This detector works on the principle that X-ray photons can ionize inert gas atoms such as argon or xenon into an electron (e^-) and ion (e.g. Ar⁺) pair. The ionization energy required to eject an outer electron is low (10-20 eV) compared to the energy of the X-ray photon (8 keV) so that one X-ray photon can produce several hundred ion pairs. A wire placed inside the detector is set to a potential of about 1,000 V. This accelerates the electrons of the ion pair towards the wire causing further ionization and an enhanced signal by gas amplification. The burst of electrons on the wire is converted into a voltage pulse which is then shaped and counted by the electronics. In order to minimise the dead time of the system, a quenching gas such as methane (CH₄) is mixed with the inert gas (e.g. 90% Ar : 10% CH₄).

Disadvantage: loss of linearity at high count rates

Advantage: They have a better energy resolution than scintillation detectors. The simple gas-filled detector is much less-commonly used than a more sophisticated form known as a position sensitive detector or PSD.