Radiation Detection

Since the discovery of x-rays by Wilhelm Röntgen in 1895 radiation detectors have experienced a constant evolution. The phosphorescent screen where Roentgen observed x-ray was the first real-time detector and the precursor to the scintillation crystal detectors still in use today. The gas filled radiation detector was discovered by Hans Geiger while working with Ernest Rutherford in 1908. The design of this device was later refined by Hans Geiger and Wilhelm Mueller, in the 1920s. The design of this device was later refined by Hans Geiger and W. Mueller, in the 1920s. It is sometimes called simply a Geiger counter or a G-M counter and is the most commonly used portable radiation instrument. The main drawback of the G-M counter is its inability to provide information on the energy of the radiation it detects.

Most modern spectrometers depend on scintilation crystals or semiconductor radiation detectors. Scintillation crystals respond to radiation by emitting a flash of light proportional to the energy of the photon that is stopped in the crystal. CsI, and NaI are the most common compounds used in this application. Scintillation crystals can be very efficient due to the size of crystals that can be grown, yet their resolution is relatively poor. Photomultiplier tubes are used to convert the light emitted by these detectors into electrical pulses which can then be processed. Temperature drift, size, and power requirements are the main obstacles to overcome in designing systems that use these detectors.

The most recent class of detector developed is the solid state detector. These detectors convert the incident photons directly into electrical pulses. Solid state detectors are fabricated from a variety of materials including: germanium, silicon, cadmium telluride, mercuric iodide, and cadmium zinc telluride. The best detector for a given application depends on several factors. For instance, germanium detectors have the best resolution, but require liquid nitrogen cooling which makes them impractical for portable applications. Silicon, on the other hand, needs no cooling, but is inefficient in detecting photons with energies greater than a few tens of keV (kilo electron volts). In the last few years detectors fabricated from high Z semiconductor materials have gained acceptance due to their ability to operate at room temperature and their inherent high efficiency. Detectors made from cadmium telluride, mercuric iodide, and cadmium zinc telluride are routinely used.

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