Born as a collaboration between CSIRO and Brookhaven National Laboratory in the US, Maia is an array detector comprised of 384 detector elements each wire-bonded to a dedicated pulse-processing circuit that passes events via fibre-optic link to the HYMOD real-time image processor. HYMOD contains a field programmable gate array (FPGA) processor to record events at up to 50 MHz along with a processing pipeline to deal with gain trimming, energy calibration mapping and pile-up rejection that can be problematic with high X-ray count rates. In parallel, HYMOD measures beam fluxes on a per-pixel basis, tags each X-ray event with a time and stage position, outputs these as an event stream in list-mode, handles event transmission over Ethernet and performs real-time spectral deconvolution for image reconstruction. The innovative combination of the high X-ray collection efficiency and smart data handling won a 2011 R&D 100 award.
What does this mean? When an X-ray hits an X-ray detector, the detector electronics measure the energy of the X-ray and the time duration of the pulse, known as time over threshold. This usually causes dead-time in the detector while it handles the X-ray. If two X-rays arrive at the same time the detector records a single event with a higher energy, a so-called pile-up. Dead-time and pile-up are two major problems for conventional detectors especially where there are high count-rates induced by an intense X-ray beam. Maia partly gets around these problems by having 384 detectors working simultaneously to collect many more X-rays than a single detector. Online in the HYMOD processor, or in offline analysis using the GeoPIXE software, piled-up X-rays are rejected, and other artifacts are corrected for by using the relationship between the pulse duration and the X-ray energy.
Using event mode enables continuous fly-scanning and short times per pixel. The result is that Maia and GeoPIXE working together can produce real-time megapixel images of samples being mapped and, by measuring standards of known compositions, produce quantified chemistry maps for elements with concentrations below 10 parts per million.