The ITRAX™ Core Scanner

Background

Many chemical parameters are useful proxies for environmental and diagenetic changes and can also provide valuable correlative and process-determinant tools. Traditionally, acquisition of solid phase geochemical data is time-consuming and results in loss of sediment. However, the recent development of non-destructive, split-core X-ray fluorescence analysis (XRF) core scanners such as the CORTEX system developed by NIOZ (NL) and the ITRAX™ core scanner recently developed by Cox Analytical Systems (Sweden) provides the capability of acquiring high-resolution geochemical records from marine sediments. These instruments allow relatively rapid, continuous XRF and micro-radiographic scanning of split sediment cores, providing records of down-core geochemical changes.

Funding the facilities
The Office of Science and Technology (OST) called for infrastructure bids from Research Council institutes in 2000 and NOCS presented a proposal for new X-ray analytical facilities. In total £0.5 million was granted from OST and NERC. Initially, replacements were made for a wavelength-dispersive X-ray fluorescence spectrometer and an X-ray diffractometer. Later, an innovative polarised-excitation X-ray fluorescence spectrometer was also obtained. These initial purchases provide a powerful analytical capability for sediments and rocks but are destructive techniques. Scanners for split-core, non-destructive analysis scanners are now seen as an essential requirement to provide preliminary geochemical analysis of sediment cores and as such would benefit the BOSCORF user community. The BOSCORF Steering Committee recommended in 2002 that the OST-NERC funds held at NOCS should be used to acquire an ITRAX 1-D split-core scanner.

Itrax™ Core Scanner: general description

The ITRAX™ Core Scanner is a flat beam X-ray scanner providing micro-radiographic images and elemental profiles of sediment half cores. The principle of operation is based on the simultaneous acquisition of microdensity (radiography) and microcompositional variations (XRF) using two separate X-ray detection systems. An optical line-scanning camera is also incorporated into the system to provide an optical image.

The Excitation Source
An intense source of X-rays are provided using a 3 kW Mo side-window tube (other anodes may be used for exciting lighter elements more effectively). These X-rays are squeezed through a proprietary Cox™ flat-beam capillary optic that generates a beam having a rectangular cross-section of nominally 22 mm x 100 microns. The transmitted X-rays are recorded with an array of 1024 diodes, each 25 microns wide.

The detection systems
Micro X-ray fluorescence analysis: The fluorescent or characteristic X-rays are measured at 45º to the incident X-ray beam using a Si drift X-ray detector (energy resolution of 140 eV at 5.9 keV). Digital signal processing provides energy-dispersive spectrometry. Count-rates of up to 200,000 cps can be processed.
During the exposure and recording of the transmitted radiation the characteristic radiation can also be simultaneously obtain one spectrum for each step. Due to the high count-rate capability of the XRF detector system spectrum can be acquired in a short time without 'dead-time' losses.
Microradiography: The Itrax™ core Scanner system is equipped with a high precision slit system that can provide an ultimate resolution down to 20 - 10 µm in the radiographic image. A line camera is used for the transmitted X-ray detection which consists of an array of 1024 diodes. Each diode has a width of 25 µm and a height of 2.5 mm. The 25 µm pixel width will correspond approximately to 20 µm image resolution due to beam divergence. The flat beam optics is designed to avoid any off-axis image distortions and to reduce the 'penumbra' effects in the radiographic projection. The flat-beam geometry technology reduces image blurring due to radiation scattering. Images are almost distortion-free, highly-resolved with high contrast and sensitivity, enabling the detection of very small objects and very small density variations. The system will record the radiographic information line by line as the sample moves through the beam. Selection of an appropriate tube voltage and current (50kV and 50mA), exposure time (400 ms) and step-size (200 µm) the image quality and resolution can be controlled. The sample stage is designed to enable precise movement of the sample at a level of 10 µm.
Optical line camera: The line camera system consists of a CCD colour camera operating in line mode synchronized with the stepper motor movement. The camera has 640 pixels/line and is equipped with a Scheinder CM 120 BK COMPACT XENOPLAN lens system giving a view field of 80 mm (perpendicular to the scanning direction) corresponding to 0.125 mm/pixel.

Text: Dr I.C. Croudace