The New Maia Detector System: Methods For High Definition Trace Element Imaging Of Natural Material

Abstract

Motivated by the need for megapixel high definition trace element imaging to capture intricate detail in natural material, together with faster acquisition and improved counting statistics in elemental imaging, a large energydispersive detector array called Maia has been developed by CSIRO and BNL for SXRF imaging on the XFM beamline at the Australian Synchrotron. A 96 detector prototype demonstrated the capacity of the system for real-time deconvolution of complex spectral data using an embedded implementation of the Dynamic Analysis method and acquiring highly detailed images up to 77M pixels spanning large areas of complex mineral sample sections.

Maia X-ray fluorescence imaging: Capturing detail in complex natural samples

“…The key capabilities of the Maia detector system that enable the high throughput of the XFM beamline are (i) transit times per pixel (referred to below as "dwell time") down to 50 µs [14], (ii) close detection geometry providing a solid angle of 1.3 sr [5], (iii) high sustained count rates, normally up to 12 M/s, (iv) 100 × 100 mm 2 scan range and image area (600 × 300 mm 2 on the XFM large format stage), (v) image pixel count beyond 10 8 pixels, (vi) real-time spectral deconvolution and image display, and (vii) moderate inelastic scattering signal that provides good collective images of light elements [18], especially useful for biological samples [19][20][21][22]. Typically, pixel times and image areas are not generally a limitation, and these are chosen to suit a particular application.…”

“…The full non-linear fit is performed on the SXRF spectrum at the top beam energy for the XANES scan, in which the shape of the background and the inelastic scattering distributions are determined. Then a series of DA matrices are generated as the beam energy is reduced according to the method of Ryan et al [12,14] with the elastic and inelastic peaks shifted to track the change in beam energy. During processing of the event stream, the lookup table operation first selects the correct DA matrix for the beam energy and then selects the matrix column based on event X-ray energy to increment a 2D result array indexed by XY position repeated over beam energy.…”

“…Using a fundamental parameters approach to SXRF analysis, the element intensities can be related to concentration, which results in a matrix transformation directly from input spectrum to element concentration vector [12]. In the Maia case, the model includes the effects of the Mo mask on individual detector solid-angles and the changing spectral relative intensities across the array, as an extension of the layered sample model in the GeoPIXE software [5,13,14]. The model produces a set of relative sensitivity factors (RSFs) for the individual detectors across the array reflecting changing distance, angle, absorption paths and intrinsic efficiency.…”

“…Some of these contributions are negative, which is how overlap and background subtraction are accomplished [12]. In the Maia detector processor, these contributions are accumulated for all events in the current image pixel in what reduces to a simple lookup-accumulate operation implemented in the HYMOD FPGA [14]. When the stage moves out of the pixel, the accumulator is read-out as a concentration record, which is displayed immediately by the real-time display client.…”

“…Larger areas to 10 cm 2 are typically done at 4-6 µm resolution. Typical image size is 10-100 M pixels acquired over 2-10 hours [14]; the largest image to date is 620 M pixels. Using a collimated beam, larger objects, such as works of art, have been imaged at 50-100 µm resolution [18] over areas up to 600 × 300 mm 2 .…”