What is a photoconductor?
A photoconductor is a material that converts absorbed X-ray photons directly into electrical charge. When an X-ray photon is absorbed within the material, it generates mobile charge carriers that can be collected to form an electrical signal.
A photoconductor converts absorbed X-ray photons directly into electrical charge, enabling direct digital radiography without an intermediate light conversion step.
Photoconductors are used in direct digital radiography detectors, where they allow X-ray energy to be converted directly into an electronic signal without an intermediate light conversion step.
Understanding the physics
In a photoconductor, incoming X-ray photons interact with the detector material and deposit energy through interactions such as the photoelectric effect or Compton scattering. This deposited energy excites electrons within the material, creating electron–hole pairs.
An electric field is applied across the photoconductor layer. This field causes the negatively charged electrons and positively charged holes to move in opposite directions toward collection electrodes.
As these charges move through the material, they produce an electrical signal proportional to the amount of X-ray energy absorbed at that location. The signal is then read out by an array of thin-film transistors (TFTs) that form the detector matrix.
The most widely used photoconductor material in medical imaging detectors is amorphous selenium (a-Se). Selenium is particularly suitable because it efficiently generates charge carriers when X-ray photons are absorbed and can be manufactured as a uniform thin layer over large detector areas.
Because the signal is generated directly within the photoconductor, there is minimal lateral spread of signal, which helps preserve the spatial location of the interaction.
Where this matters clinically
Photoconductors are used in direct digital radiography detectors, which can offer excellent spatial resolution because the electrical signal is generated at the location where the X-ray photon is absorbed.
Amorphous selenium detectors are widely used in mammography, where imaging is performed at relatively low photon energies and high spatial resolution is required to detect small calcifications.
At higher photon energies, however, the probability of X-ray absorption in selenium decreases, which can reduce detector efficiency. For this reason, most general radiography systems use indirect detectors with scintillators, which absorb higher-energy X-rays more efficiently.
Understanding how photoconductors convert X-ray energy into electrical charge helps explain how different detector technologies are optimised for different imaging applications.