Presentation on theme: "Production of X-rays (1) X-rays are produced when rapidly moving electrons that have been accelerated through a potential difference of order 1 kV to 1."— Presentation transcript:
Production of X-rays (1) X-rays are produced when rapidly moving electrons that have been accelerated through a potential difference of order 1 kV to 1 MV strikes a metal target. Evacuated glass tube Target Filament
Production of X-rays (2) Electrons from a hot element are accelerated onto a target anode. When the electrons are suddenly decelerated on impact, some of the kinetic energy is converted into EM energy, as X-rays. Less than 1 % of the energy supplied is converted into X-radiation during this process. The rest is converted into the internal energy of the target.
Properties of X-rays X-rays travel in straight lines. X-rays cannot be deflected by electric field or magnetic field. X-rays have a high penetrating power. Photographic film is blackened by X-rays. Fluorescent materials glow when X-rays are directed at them. Photoelectric emission can be produced by X-rays. Ionization of a gas results when an X-ray beam is passed through it.
X-ray Spectra (1) Using crystal as a wavelength selector, the intensity of different wavelengths of X-rays can be measured.
X-ray Spectra (2) The graph shows the following features. A continuous background of X-radiation in which the intensity varies smoothly with wavelength. The background intensity reaches a maximum value as the wavelength increases, then the intensity falls at greater wavelengths. Minimum wavelength which depends on the tube voltage. The higher the voltage the smaller the value of the minimum wavelength. Sharp peaks of intensity occur at wavelengths unaffected by change of tube voltage.
Minimum wavelength in the X-ray Spectra When an electron hits the target its entire kinetic energy is converted into a photon. The work done on each electron when it is accelerated onto the anode is eV. Hence hf = eV and the maximum frequency Therefore,
Characteristic X-ray Spectra Different target materials give different wavelengths for the peaks in the X-ray spectra. The peaks are due to electrons knock out inner-shell electrons from target atoms. When these inner-shell vacancies are refilled by free electrons, X-ray photons are emitted. The peaks for any target element define its characteristic X-ray spectrum.
Uses of X-rays In medicine To diagnose illness and for treatment. In industry To locate cracks in metals. X-ray crystallography To explore the structure of materials.