We examined the application of proton imaging  on thin samples at the proton and light-ion Tandetron accelerator  of the NPI-CAS in Rez near Prague. We make use of high-sensitivity hybrid semiconductor pixel detectors Medipix/Timepix equipped with integrated per-pixel signal processing electronics. We use the Timepix3 ASIC chip  equipped with a 500 µm Si sensor operated with the fast data rate AdvaPix readout electronics interface.
Measurements were performed in air with a 2.9 MeV proton microbeam on thin samples (< 100 µm thick). Mylar and aluminium foils were stacked into closely packed assemblies of varying well-defined thickness. The samples were placed in front of the detector in orthogonal geometry (beam axis perpendicular to the sensor plane – see Fig. 1). Radiographies were collected with standard (few mm size) and a focused microbeam (few µm size).
The imaging principle is based on high-resolution spectrometry of single transmitted particles. Contrast is obtained by registration of small changes in the deposited energy of the proton after passing through the sample . These changes can be measured in wide-range by detailed spectral-tracking analysis of the pixelated clusters in the detector . The track parameters we utilize for imaging contrast are deposited energy, cluster area (number of pixels) and cluster height (maximum energy value of the pixels in the cluster) . The position of interaction in the detector is registered in sub-pixel resolution  down to few µm scale for the particles and geometry used . Radiographies are reconstructed based on these individual parameters imaged in image bins of adjustable size (few µm up to few tens of µm). The radiography of a sample assembled from several aluminium foils stacked in stairs geometry is shown in Fig. 2. No post-processing was applied in the image. The technique developed with different cluster parameters will be presented together with evaluation of image contrast sensitivity on various types of samples and beam energies.
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The research has been carried out at the CANAM (Centre of Accelerators and Nuclear Analytical Methods) infrastructure LM 2015056. This publication has been supported by the project Strategy AV 21 of the Czech Academy of Sciences.