CdTe and CdZnTe detectors

Features

Semiconductor detectors are nuclear measurement detectors which, combined with the appropriate electronics, allow spectrometric, counting or integration measurements. They have the following general characteristics

  • Compactness,
  • Very good energy resolution,
  • Good spatial resolution.

However, these detectors are often :

  • Expensive,
  • Fragile,
  • Susceptible to radiation damage.

Mode of operation

Semiconductor detectors allow the conversion of X-rays or gamma rays into electrons. When a photon interacts with the crystal, an internal charge is created and the application of an external magnetic field allows the collection of these charges. The three modes of interaction of the photon with the crystal are the following:

  • Photoelectric absorption: the absorbed photon transfers all its energy to the electron of the atom in the medium.
  • Compton scattering: this is an inelastic scattering where the photon transfers a fraction of its energy to a weakly bound electron of the atom, which is ejected and the photon is scattered.
  • Pair production: a photon of energy greater than 1.02 MeV interacts with the coulombic field of a nucleus to produce an electron/positron pair.

Innovation

Compared to the more traditional semiconductor detectors, such as silicon or Germanium, semiconductors composed of two or more elements have been particularly studied in recent decades, showing interesting spectroscopic properties.

CdZnTe (or CZT) meaning Cadmium Zinc Telluride and CdTe (Cadmium Telluride) show :

  • High detection efficiency,
  • good spectroscopic performance at room temperature,
  • ideal characteristics for the creation of reliable and compact detection systems.

Data acquisition

Typical CdZnTe are made by depositing a thin layer of metal on the surface of the detector forming the electrodes. These electrodes polarise the detector and create an electrical potential across the crystal. 

Thus, any ionising particle interacting with the polarised crystal (CZT or CdTe) will result in the creation of a number of electron-hole pairs proportional to the energy of the incident particle. The negatively charged electrons and positively charged holes migrate to their respective electrodes and are collected. 

The resulting charge pulse is fed into the preamplifier to produce a voltage pulse whose amplitude is proportional to the energy of the incident particle.

Use of CdZnTe/CdTe

Nowadays, very small CdZnTe detectors are available, operating at room temperature with energy resolutions around 2-3% at 662 keV, allowing good spectroscopic quality while accepting relatively high count rates. 

This is why these detectors are the spectrometers of choice for use on our robot sensors or as a connected sensor.

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