CdTe and CdZnTe detectors

Characteristics

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

  • compactness,
  • very good energy resolution,
  • good spatial resolution,

On the other hand they are detectors which are often :

  • expensive,
  • fragile,
  • that may be degraded by radiation,

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 as follows:

  • Photoelectric absorption: the absorbed photon transfers all its energy to the electron of the middle atom.
  • Compton scattering: it is an inelastic scattering where the photon transfers a fraction of its energy to a weakly bound electron of the atom, this electron 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 classically known semiconductor detectors, such as Silicon or Germanium, semiconductors composed of two or more elements have been particularly studied in recent decades, presenting interesting spectroscopic properties.

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

  • high detection efficiency,
  • good spectroscopic performance at room temperature,
  • ideal features for the creation of reliable and compact detection systems,
Kromek monolithic CZT detectors (source : https://www.kromek.com/cadmium-zinc-telluride-czt/)

Indeed, for spectroscopic purposes, the phenomenon we wish to promote is photoelectric absorption whose effective cross section varies in Zn where Z is the effective atomic number and n is between 4 and 5. CdZnTe and CdTe have an effective Z of 50.

Data acquisition

Typical CdZnTe are manufactured by depositing a thin layer of metal on the surface of the detector constituting the electrodes.
These electrodes are used to polarize the detector and create an electrical potential across the crystal.
Thus, any ionizing particle interacting with the polarized 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 sent into a preamplifier to produce a voltage pulse whose amplitude is proportional to the energy of the incident particle.

Uranium oxide spectrum obtained with a CdTe Amptek XR-100 (source: http://amptek.com/uranium-and-plutonium-spectra)

Using CdZnTe/CdTe

Nowadays, very small CdZnTe detectors are available, operating at room temperature with energy resolutions around 2-3% at 662 keV allowing a good spectroscopic quality while accepting relatively high counting rates. This is why these detectors are the spectrometers of choice to be mounted on our robotic sensors or as a connected sensor.