Cadmium Zinc Telluride (commonly known as CdZnTe or CZT) is a semiconductor that, at room temperature, directly converts x-ray or gamma-ray photons into electrons.
Traditional semiconductors, such as silicon (Si) and germanium (Ge) have been used f o r many years as radiation detectors, but compound semiconductors, comprising of two or more elements,with their range of phy sical properties (band gap, atomic number and density) have been investigated for many years as alternatives. CZT (and CadmiumTelluride) has attracted particular interest as a detector for both X and gamma rays.
These semiconductor detectors create internal charges when exposed to photon interactions. These charges can be collected by the application of an external electric field.
There are three effects which are important:
Photoelectric Absorption: where an absorbed photon transfers all its energy to an atomic electron.
Compton Scattering: where an absorbed photon transfers a fraction of its energy to an atomic electron to produce an ‘hot’ electron and a degraded photon
Pair Production: where a photon, with energy above 1.02MeV, interacts with the Coulomb field of a nucleus to produce an electron and a positron.
Each of the above interaction cross-sections varies with the effective Z (atomic number) of the semiconductor material, the highest being the photoelectric absorption which varies as Zⁿ where n is in the range 4 to 5. As a consequence, radiation detectors required for spectroscopic measurements, favour the the use of high Z materials. CZT has an effective Z of 50.
Typical CZT detectors are fabricated with thin layer of metal deposited on the detector surfaces to act as electrodes. These electrodes then allow the detectors to be electrically biased to creating an electrical potential across the detector. Any ionizing radiation interacting with the biased CZT, will result in the generation of electron hole pairs which are proportion to the energy of the incoming and absorbed radiation. Negatively charged electrons and positively charged holes migrate to the their respective electrodes and are collected.
The resulting charge pulse can then be fed into a suitable preamplifier to produces a voltage pulse, its height being proportional to the incident energy of the absorbed photon. Tpypically, these preamplifier output pulses are fed into shaping amplifier, which converts the pulse into into a Gaussian shape and at the same time providing further amplification. These clean pulses can then typically fed into a suitable counter or a Multichannel Analyzer (MCA) which enables the characteristic spectrum for the incoming photons to be generated.
