What are X-rays?
In this article:
- X-rays are a form of electromagnetic radiation emitted by X-ray tubes, consisting of a heterogeneous spectrum of wavelengths, including both continuous and characteristic lines.
- The energy and spectrum of X-rays depend on the voltage applied to the X-ray tube and the material of the target, with higher voltages producing shorter wavelengths and greater penetration power.
- The Duane-Hunt formula defines the shortest possible wavelength in the X-ray spectrum, which is directly related to the maximum energy of the incident electrons.
- X-ray spectra vary by generator type, such as constant potential (CP) or self-rectified systems, and are influenced by inherent filtration materials like glass, aluminum, or beryllium.
- In industrial radiography, X-rays are described by their peak voltage (e.g., 120 kV), not a single energy value, due to their broad spectral nature and application in non-destructive testing (NDT).
The radiation which is emitted by an X-ray tube is heterogeneous, that is, it contains X-rays of a number of wavelengths, in the form of a continuous spectrum with some superimposed spectrum lines. The shortest wavelength of the spectrum is given by the Duane-Hunt formula:
The average shape of the X-ray spectrum is generally the same however not truely identical for different X-ray sets; it depends chiefly on the energy range of the electrons striking the X-ray tube target and, therefore, on the voltage waveform of the high-voltage generator. A constant potential (CP) X-ray set will not have the same spectrum as a self-rectified set operating at the same nominal kV and current. The spectrum shape also depends on the inherent filtration in the X-ray tube window (glass, aluminium, steel or beryllium). The energy imparted to an electron having a charge e, accelerated by an electrical potential V is (eV) so the energy of the electrons can be quoted in eV, keV, MeV. These same units are used to denote the energy of an X-ray spectrum line. The energy of a single wavelength is:
The heterogeneous X-rays emitted by an X-ray tube do not however have a single wavelength, but a spectrum, so it would be misleading to describe the X-rays as (say) 120 keV X-rays. By convention therefore, the ‘e’- in keV- is omitted and the X-rays described as 120 kV, which is the peak value of the spectrum.
