Nikhef Special Seminar: Polychromatic X-ray imaging (Martin Fransen)

Thursday, 6 July 2017 from to (Europe/Amsterdam)
at Nikhef ( H331 )
- Looking at colours the eye can’t see -
Conventional X-ray images are grey scale images. Those images are often made using X-ray tubes. Common energy ranges of spectra emitted by X-ray tubes lie in a range between 5 keV to 150 keV. Grey scale images only represent total intensity and most of the spectral information is lost. Lacking spectral information can result in an ambiguity between material composition and amount of material in the sample. If the X-ray intensity as a function of the energy can be measured (i.e. a ‘colour’ X-ray image) more information can be obtained from a sample. This translates to less required dose and/or to a better understanding of the sample that is being investigated. 
X-ray detectors based on Medipix/Timepix pixel chips have spectral resolving capabilities and can be used to make polychromatic X-ray images. Medipix and Timepix chips have branched from pixel chips developed for detectors for high energy physics collider experiments. 
I will discuss how Medipix and Timepix based detectors can be used for polychromatic X-ray imaging. Subsequently I will  show, with a few examples, how polychromatic X-ray imaging offers an improvement over existing techniques. 
Spectral resolving capabilities allows identifying absorption lines. Absorption lines are sudden changes in X-ray absorption for changing X-ray energy and are related to the binding energies of electrons. The binding energies of electrons are unique for each element. Identifying absorption lines identify the elements in a sample. In addition, when ‘searching’ for a known element, the sudden change in absorption helps to distinguish it. This works for example in the detection of iodine contrast agent used for mammography. The aim is to minimise the amount of required contrast agent and dose for a diagnostic measurement. Measurements to determine the minimum detectable amount of iodine with a Timepix based detector show promising results when compared to the sensitivity of existing methods. 
I conclude with the results of an experiment in which I show how defects down to one μm thin can be measured in a 0.23 mm thick metal sheet. A practical mechanical trick was required to achieve the required sensitivity while keeping the measurement ‘simple and fast’. 
The discussed experiments show that there are many fields that can benefit from polychromatic X-ray imaging. Polychromatic X-ray imaging is a growing field of interest, partly driven by technological advancements made in high energy physics experiments. Research continues to identify and access new fields outside high energy physics that benefit from this advancing technology. 

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