Detectors

In principle, consist of a diode, operated in the reverse direction and provided with a large-volume barrier layer which is read out with a charge-sensitive preamplifier.

Germanium detectors (HPGe) have the best possible energy resolution but they need to be cooled by liquid Nitrogen and it’s quite effortful to put them in operation and nevertheless they are quite expensive. Germanium detectors are typically available with sensitivities between medium-sized NaI detectors and large CdZnTe detectors.

Cadmium Zinc Telluride detectors (CdZnTe or CZT detectors) also belong to the semiconductor detectors but operate at room temperature. From the resolution, CZT detectors are significantly worse than nitrogen-cooled germanium detectors, but significantly better than the best scintillation detectors.

Typically for CZT is that only relatively small detector sizes are available, therefore the general sensitivity is quite low.

However, this can be an advantage if highly radioactive objects have to be measured or a high spatial resolution is required.

There are CZT detectors in the versions planar and semi-spherical. Planar detectors are good for energies <100 keV, semispherical detectors have well-evaluable spectra up to 2MeV.

This kind of detectors consisting mostly salt crystals which emit light flashes during the absorption of a gamma quant. These light flashes will be collected by a photo multiplier, or in some cases, with a sensitive photodiode.

Scintillation detectors generally have a lower energy resolution than semiconductor detectors. Another characteristic is that the energy calibration is relatively prone to temperature fluctuations. However, they can be produced at acceptable prices with very high sensitivity.

Most widely used are scintillation detectors made of NaI (sodium iodide). They also show the best resolution among the scintillators.

Recent developments include detectors from LaCl3 (lanthanum chloride) or LaBr3 (lanthanum bromide), which have better resolution than NaI detectors up to factor 2 and are even more temperature-stable.

LaCl3 detectors have got a side effect of self-radioactivity due to the element lanthanum which makes them unsuitable for measurements at low radiation levels.

Also widely used are detectors from CsI (caesium iodide) and BGO (bismuth germanate) which have an even higher volume-specific sensitivity than NaI detectors. CsI detectors are also often offered with photodiode and are therefore quite compact.

Plastic detectors are also included in the section of scintillation detectors. Their resolution is poor and hardly useful for spectroscopy, but are the most cost-effective one`s at high sensitivity.