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Detectors

 

Detector Overview

The kinds of detectors commonly used can be categorized as:

• Gas-filled Detectors
• Scintillation Detectors
• Semiconductor Detectors

The choice of a particular detector type for an application depends upon the X-ray or gamma energy range of interest and the application’s resolution and efficiency requirements. Additional considerations include count rate performance, the suitability of the detector for timing experiments, and of course, price.

Detector Efficiency

The efficiency of a detector is a measure of how many pulses occur for a given number of gamma rays. Various kinds of efficiency definitions are in common use for gamma ray detectors:

Absolute Efficiency
The ratio of the number of counts produced by the detector to the number of gamma rays emitted by the source (in all directions).

Intrinsic Efficiency
The ratio of the number of pulses produced by the detector to the number of gamma rays striking the detector.

Relative Efficiency
Efficiency of one detector relative to another; commonly that of a germanium detector relative to a 3 in. diameter by 3 in. long NaI crystal, each at 25 cm from a point source, and specified at 1.33 MeV only.

	Full-Energy Peak (or Photopeak) Efficiency The efficiency for producing full-energy peak pulses only, rather than a pulse of any size for the gamma ray. Clearly, to be useful, the detector must be capable of absorbing a large fraction of the gamma ray energy. This is accomplished by using a detector of suitable size, or by choosing a detector material of suitable high Z. An example of a full-energy peak efficiency curve for a germanium detector is shown in Figure 1.1 below.
Figure 1.1 Efficiency Calibration

Detector Resolution

Resolution is a measure of the width (full width half max) of a single energy peak at a specific energy, either expressed in absolute keV (as with Germanium Detectors), or as a percentage of the energy at that point (Sodium Iodide Detectors). Better (lower) resolution enables the system to more clearly separate the peaks within a spectrum. Figure 1.2 shows two spectra collected from the same source, one using a sodium iodide detector and one using germanium.

 

Even though this is a rather simple spectrum, the peaks presented by the sodium iodide detector are overlapping to some degree, while those from the germanium detector are clearly separated. In a complex spectrum, with peaks numbering in the hundreds, the use of a Germanium detector becomes mandatory for analysis