The additional solid angle of the 50 mm2 detector yields improvements in speed and sensitivity in analytical problems where the 30 mm2 detector would be signal starved. When a detector is signal starved, the detector's processing electronics still have available
capacity to process more signal. In these cases, having a detector with a greater solid angle allows for more X-ray signal
to be passed to the processing electronics. As long as the larger SDD's spectral resolution and throughput are similar to
that of the smaller SDD, which is the case here, improvements in speed and sensitivity can be achieved with the larger detector.
Typical materials analysis problems which fall into this category include measurements of light element matrices such as glasses,
plastics, and aluminum alloys; measurements of trace elements where heavier filters are needed for best sensitivity such as
trace Pb and Cd in solders and plastics and measuring thin residues, corrosion, and coatings on light element matrices. Table
II compares the limits of detection for several elements spanning the XRF spectral range.
Table II: Limits of detection (ppm) for detectors available on Orbis
For the data collected in Table II, the SDD detectors were operating in the signal starved regime where the X-ray tube is
run at full power for maximum excitation. Improvements in sensitivity of the 50 mm2 over the 30 mm2 SDD are approximately consistent with the increase in solid angle of the 50 mm2 over the 30 mm2 SDD.
In applications where improved sensitivity is not needed, the 50 mm2 SDD can be used for increased productivity. The proportionality between sensitivity as described by the limit of detection
(LOD) and measuring time is shown in equation 2.
Hence, the increased sensitivity provided by the 50 mm2 SDD can be traded for faster measuring time. For example, using equation 2 and the improvements in measuring sensitivity
shown in Table II, the 50 mm2 SDD can achieve the same measurement sensitivity obtained with the 30 mm2 SDD with a nominal 40% reduction in measuring time.