High-Speed PIXE: Fast multi-elemental analysis with lateral resolution using a Colour X-Ray Camera

A new PIXE-beamline equipped for fast, laterally resolved elemental analysis has recently been put into operation at HZDR. The system uses a full-field energy dispersive X-ray camera, the SLcam® [1,2]. It consists of a poly-capillary lens guiding the proton-induced Xray fluorescence radiation towards a pnCCD-chip with 264×264 pixels, each with an energy resolution of 156 eV (@Mn Kα). The CCD pixel size is 48×48 µm². In combination with a 1:1 X-ray lens, this results in a lateral resolution better than 100 µm with a field of view of 12×12 mm². A 6:1 X-ray lens with a field of view of 2×2 mm² and envisaged lateral resolution < 10 µm is also available.
The camera is mounted on a vacuum chamber containing a precision sample manipulator. The chamber is optimised for high throughput of large samples. A beam broadening system is needed to ensure a homogeneous illumination of the entire detection area. The installed optical microscope together with the image processing software allows correlative superimposition of the PIXE-maps with optical images or electron microprobe element distribution maps. The simultaneous measurement of a large pixel array enables a fast overview over a large region of the sample with first tentative results becoming visible almost immediately.
The new setup is mainly developed for the investigation of geological samples for resource technology research, which comprises the analysis of grain composition and intergrowths as well as the determination of trace element distributions, e.g. rare earth elements. However, the setup can be used for the analysis of every kind ofsample respecting dimensions, roughness and vacuum stability.
First results concerning lateral resolution and detection limits are encouraging. Due to the low background in the PIXE-spectra investigation of trace elements with concentrations below hundreds of µg/g is achievable. The main limitation s for the detection limits are pile-up and the small solid angle of the capillaries. Total count-rate is not a limitation due to the different readout compared to conventional detectors; therefore high beam currents of a few µA can be used.
[1] O. Scharf et al., Anal. Chem., Vol. 83, pp. 2532-2538 (2011).
[2] I. Ordavo et al., NIM A, Vol. 654, pp. 250-257 (2011).