Fluorescence from the collimator in Si-PIN and Si-Drift detectors: problems and solutions for the XRF analysis of archaeological and historical materials

The investigation of archaeological and historical materials is one of the most successful applications of portable XRF spectrometers; depending on the situation, instruments with detection limits of a hundred ppm or less may be desirable. One of the technical solutions to achieve this goal still remains working with X-ray tubes at a relatively high voltage and current, i.e. 60 kV and at least 1 mA, which allows to take advantage from both an intense primary beam and the possibility of resolving such important elements as Ag, Cd, In, Sn and Sb. An Amptek SiPIN mod. XR100T and a Ketek Si-drift 5 mm2 were tested with respect to the suitability to work in such ''extreme'' conditions. Both detectors show spurious lines due to the fluorescence from the collimators material: in order to account for these spectral contaminations and analyse samples containing the same element as the collimator, a correction method was developed and tested; this also included the evaluation of the detection limit for the collimator's element actually contained in the sample; in our experimental conditions, the detection limit for zirconium is about 130 ppm. Though developed for the analysis of obsidians with Si-drift detectors, the method is quite general and can be applied to any collimator material.