The design study of the new generation in-beam PET scanner to be developed and installed at the dedicated ion beam tumour therapy facility of Heidelberg, Germany, demands a deep understanding of physical processes affecting in-beam PET acquisition. The current experience in the monitoring of carbon ion therapy at GSI Darmstadt shows that techniques well established in radiotracer imaging may fail in the non-conventional case of in-beam PET therapy monitoring. In fact the data acquired by the in-beam positron camera during particle extraction (spill) from the GSI synchrotron are corrupted by a high noise level. They are therefore discarded for tomographic reconstruction. Previous investigations have suggested the reason of high noise to be random concidences not properly corrected for during beam extraction. The standard random correction technique implemented by the manufacturer of our PET data acquisition system (CTI PET Systems, Knoxville, TN, USA) is based on subtraction of delayed (by 128 ns) from prompt coincidences. The failure of the correction during particle extraction is thought to be due to a non-stationary (in the sub-µs scale) γ-background originating from nuclear reactions induced by the beam and hence following the time microstructure of the carbon ions . The time dependence may result in a reduction of delayed coincidences affecting the proper correction for the random coincidences detected in the prompt window. This conjecture has been supported by previous in-spill measurements of time correlation between the γ-rays produced in 12C irradiation of organic matter and the synchrotron radiofrequency (RF) using a GSO scintillating crystal (2 cm radius and 2.5 cm thickness) coupled to a fast PMT . In the presented experiment an improved electronics was set-up in order to add photon energy discrimination to the time information. A further record of the spill number was introduced in order to perform dy
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