The additional solid angle of the 50 mm² detector yields improved sensitivity and faster measuring times in materials analysis where the 30 mm² detector would be signal starved, but not in cases where the 30 mm² detector is saturated. Saturation refers to when the detector's rate of signal collection exceeds the throughput of the analyzer electronics. Hence, sensitivity and measurement speed are limited by throughput. In this analytical situation, the X-ray tube is generally running below maximum power limiting the input XRF signal to the analyzer throughput. Typical materials analysis problems which fall into this category include measurement of major elements in steels, heavier transition metal alloys and measuring thin residues, corrosion, and coatings on substrates comprised of heavier transition metal alloys. Having equal throughputs, both the 30 mm² and 50 mm² SDDs provide similar speed and sensitivity for samples which can saturate the detectors; however, the 50 mm² SDD allows for a greater reduction in X-ray tube power, which in turn improves tube life.

The data in Table II does show that the 80 mm² Si{Li} has better sensitivity for Cd measured at the 23.1 keV Ka line. The Si{Li} detecting crystal is about 10 times thicker than the SDD detecting sensor allowing > three times the absorption efficiency for the Cd(Ka) X-ray signal. Hence, the better Si{Li} detection limit for Cd is the result of superior solid angle and absorption at high X-ray energies.

Conclusion

Two detector operating regimes have been identified, that is,starvation and saturation. These regimes are dictated by the types of samples analyzed. When the detector is signal starved, the Apollo XRF ML-50 SDD can provide nominally 20 to 30% better sensitivity or 40% reduction in measurement time in comparison to the standard 30 mm² SDD. In saturation, using the ML-50 SDD, tube current can be reduced to a greater extent, extending X-ray tube life.

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