The Maia 384 detector array in a nuclear microprobe: A platform for high definition PIXE elemental imaging

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Abstract

Application of nuclear microprobe event-by-event data acquisition approaches to synchrotron elemental imaging is at the heart of the design of a large energy-dispersive detector array called Maia, under development by CSIRO and BNL for SXRF elemental imaging on the X-ray microprobe. A new project is aimed at harnessing this development to provide high throughput PIXE imaging on the CSIRO Nuclear Microprobe. Maia combines a 1.2 sr solid-angle 384 detector array, integrated scanning and real-time processing including spectral deconvolution of full-spectral data. Results using a Maia prototype demonstrate the potential using SXRF application data with elemental images of up 100 M pixels.

Introduction

Application of nuclear physics style event-by-event data acquisition approaches used in ion beam analysis to synchrotron elemental imaging is at the heart of the design of a large energy-dispersive detector array and real-time processor called Maia, under development by CSIRO and BNL for SXRF1 elemental imaging on the X-ray microprobe [1], [2], [3], [4]. Conventional full-spectral SXRF imaging relies on the collection of one or more X-ray spectra for each pixel in the raster-scan of the sample through the focussed X-ray beam. Delays in the readout of these spectra limit dwell time per pixel to ∼1 s [5], [6] or perhaps ∼0.1 s in some systems. This limits the number of pixels that can be acquired in a reasonable scan time to a few times 104 pixels, or only ∼103 pixels for a preliminary scan, even if the counting statistics would permit higher definition images to be acquired for at least the major and minor elements.

Ion beam analysis using PIXE2 and the nuclear microprobe has seldom suffered from this problem due to the wide use of the nuclear physics style multi-parameter data acquisition approach, which labels each event with XY position. This allows freedom in choosing the number of pixels, and scan rates are limited only by the method of beam scanning employed. For example, the use of electrostatic scanning of MeV protons using the CSIRO Nuclear Microprobe (NMP) enables typical dwell time per pixel of 50–200 μs and the collection of images up to ∼1–2 M pixels [7].

Initially targeting improved SXRF elemental imaging, the Maia detector system has been developed for the X-ray Fluorescence Microprobe (XFM) at the Australian Synchrotron [8]. It combines (i) a large detector array, which provides large solid-angle and count rates, (ii) a multi-parameter approach to data acquisition with real-time processing of each event, which offers fast scanning and short transit times per pixel, and (iii) integrated stage control and monitoring of stage encoders. Fast scanning and short pixel times pave the way for high definition imaging with high pixel counts. Large solid-angle offers improved sensitivity, and high count rate enables the collection of high-quality major and minor-element images and improves the per pixel detection limit for trace elements.

A new project is aimed at harnessing this development to provide high throughput PIXE imaging on the CSIRO NMP. This paper outlines the new project, discusses issues specific to the NMP and ion beam analysis and illustrates the potential of the new system using SXRF imaging examples obtained recently using a Maia prototype on the XFM beamline.

Section snippets

Maia detector concept

The Maia detector combines a 384 detector solid-state array with integrated custom pulse processing ASICs3 and real-time data processing [4]. It comprises a low-leakage silicon pad array in an annular geometry developed at BNL [9], upstream of the sample (Fig. 1). A mask fabricated in Mo glued to the sample side absorbs X-rays that may give rise to charge-sharing between pads [10]. The detector board is mounted on a water cooled copper block with further

Maia in the nuclear microprobe

The SXRF and PIXE imaging approaches are fundamentally similar. Both typically use a mono-energetic beam focussed to a micron-scale beam-spot to excite fluorescence X-rays characteristic of elements present in the sample under the beam spot. However, the ion beam can be scanned using magnetic or electrostatic deflection while the photon beam is fixed in place and the sample is raster-scanned to form the image. In addition, hybrid schemes can be used with ion beams; the CSIRO NMP combines stage

SXRF demonstration and applications

A 96 detector prototype of the Maia system was trialed on the XFM beamline at the Australian Synchrotron using beams of 11.5–20.1 keV X-rays focussed into a spot size of 1–2 μm using a Kirkpatrick–Baez mirror pair [8]. Tests were also performed at the X27A beamline at the NSLS5 [1], [3]. The detector was located 20 mm from the beam spot at 90° to the beam, providing a solid-angle of 204 msr, with the sample rotated to 45°. Maia directly controlled a stage capable

Discussion

Prior developments have enhanced imaging capabilities through the use of detector arrays for PIXE (e.g. 12 element array in the Sandia NMP [20]) and fast scanning approaches using a limited number of dedicated single-channel analysers, which provide approximate SXRF elemental images without full spectra acquisition [21]. The Maia approach combines (i) a large array for high sensitivity and total count rate, (ii) full-spectral data collection with real-time spectral deconvolution for more

Conclusions

The capabilities of the new Maia detector and imaging system have been demonstrated using SXRF to provide high definition elemental images of up to ∼100 M pixels projected from full-spectral data using the DA method either off-line using GeoPIXE or on-line in real-time. A development is underway to apply this technology to PIXE imaging on the CSIRO NMP to provide an increase in solid-angle to 1.2 sr, count-rate capacity to greater than 10 M/s, images to 100 M pixels and with expansion provision to

Acknowledgements

The authors wish to thank Roland Szymanski for ongoing support and assistance in the development of the CSIRO Nuclear Microprobe and the MARC facility at the University of Melbourne for hosting the instrument on their Pelletron accelerator.

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