The transition radiation photons produced by electrons have wavelengths in the x-ray range, with energies typically in the range from 5 to . However, the number of produced photons per interface crossing is very small: for particles with = 2×103, about 0.8 x-ray photons are detected. Usually several layers of alternating materials or composites are used to collect enough transition radiation photons for an adequate measurement—for example, one layer of inert material followed by one layer of detector (e.g. microstrip gas chamber), and so on.