The Specimen Chamber and Detectors

The Si(Li) detector usually views the specimen at either 90° or 135° to the beam direction, either through a thin window in the chamber wall, or for detecting the K X-rays of the light elements below atomic number 20 the detector may be placed within the chamber itself, thus dispensing with the need for a window. 

The detector itself may be shielded from background y-rays by means of an annular shield of W or Pb, and absorbers in the form of appropriate metal foils are placed between the detector and the specimen. These reduce the intensity of the continuum of Bremsstrahlung radiation and also prevent back-scattered protons from entering the detector which would degrade the observed spectrum. 

The chamber may also be equipped at 180° to the beam with a (silicon surface barrier) detector for analysis of scattered protons, which provides the option of performing quantitative light element analysis by RBS (q.v.). In certain applications RBS can determine most of the matrix composition and PIXE the trace element contribution. 

In the case of thin samples described so far, the absorption of x-rays in the sample is negligible, and it is not necessary to apply any corrections for the slowing down of the particles or the absorption of X-rays. 

If thick samples are placed in the specimen chamber for analysis, the particles are slowed down and eventually stopped in the sample, so the calculation of the X-ray yield and their absorption is more complicated. Some objects may be too large to be placed in the specimen chamber, in which case the external beam technique is employed. The particle beam passes through a window at the end of the beam-line into the air where an object of any size (e.g. an archaeological artefact) may be analyzed. 

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