A novel method of hit time and hit position reconstruction in scintillator detectors is described. The method is based on comparison of detector signals with results stored in a library of ...synchronized model signals registered for a set of well-defined positions of scintillation points. The hit position is reconstructed as the one corresponding to the signal from the library which is most similar to the measurement signal. The time of the interaction is determined as a relative time between the measured signal and the most similar one in the library. A degree of similarity of measured and model signals is defined as the distance between points representing the measurement- and model-signal in the multi-dimensional measurement space. Novelty of the method lies also in the proposed way of synchronization of model signals enabling direct determination of the difference between time-of-flights (TOF) of annihilation quanta from the annihilation point to the detectors. The introduced method was validated using experimental data obtained by means of the double strip prototype of the J-PET detector and 22Na sodium isotope as a source of annihilation gamma quanta. The detector was built out from plastic scintillator strips with dimensions of 5mm×19mm×300mm, optically connected at both sides to photomultipliers, from which signals were sampled by means of the Serial Data Analyzer. Using the introduced method, the spatial and TOF resolution of about 1.3cm (σ) and 125ps (σ) were established, respectively.
Recent tests of a single module of the Jagiellonian Positron Emission Tomography system (J-PET) consisting of 30 cm long plastic scintillator strips have proven its applicability for the detection of ...annihilation quanta (0.511 MeV) with a coincidence resolving time (CRT) of 0.266 ns. The achieved resolution is almost by a factor of two better with respect to the current TOF-PET detectors and it can still be improved since, as it is shown in this article, the intrinsic limit of time resolution for the determination of time of the interaction of 0.511 MeV gamma quanta in plastic scintillators is much lower. As the major point of the article, a method allowing to record timestamps of several photons, at two ends of the scintillator strip, by means of matrix of silicon photomultipliers (SiPM) is introduced. As a result of simulations, conducted with the number of SiPM varying from 4 to 42, it is shown that the improvement of timing resolution saturates with the growing number of photomultipliers, and that the Formula: see text configuration at two ends allowing to read twenty timestamps, constitutes an optimal solution. The conducted simulations accounted for the emission time distribution, photon transport and absorption inside the scintillator, as well as quantum efficiency and transit time spread of photosensors, and were checked based on the experimental results. Application of the Formula: see text matrix of SiPM allows for achieving the coincidence resolving time in positron emission tomography of Formula: see text0.170 ns for 15 cm axial field-of-view (AFOV) and Formula: see text0.365 ns for 100 cm AFOV. The results open perspectives for construction of a cost-effective TOF-PET scanner with significantly better TOF resolution and larger AFOV with respect to the current TOF-PET modalities.
In this paper we estimate the time resolution of the J-PET scanner built from plastic scintillators. We incorporate the method of signal processing using the Tikhonov regularization framework and the ...kernel density estimation method. We obtain simple, closed-form analytical formulae for time resolution. The proposed method is validated using signals registered by means of the single detection unit of the J-PET tomograph built from a 30 cm long plastic scintillator strip. It is shown that the experimental and theoretical results obtained for the J-PET scanner equipped with vacuum tube photomultipliers are consistent.
► Description of the annular two phase flows in pipe with respect to the liquid viscosity. ► Behaviour of these flow when submitted to a shearing and pressure gradient. ► Measurement of the velocity ...of the interfacial waves with respect to the flow condition and liquid properties. ► Characterisation of the atomisation of the annular liquid film in the convergent of a venturi metre. ► Relationship between the atomisation rate and the liquid film characteristics (film thickness, wave amplitude).
The aim of this study is to better understand and model atomisation phenomena occurring in a venturi metre under air/oil and air/water annular flows. First, the structure of the liquid film was observed using a high-speed video camera in the pipe section upstream of the venturi, and along the convergent section: the surface waves and regimes visualised were described and compared with the literature, which highlighted the influence of the liquid film physical properties. Some of these surface waves, corresponding to disturbance waves as described in the literature, seem to be at the origin of consequent atomisation. These disturbance waves were then characterised through calculation of their velocities and comparison with the literature: a calculation procedure was defined and showed the influence of parameters such as air superficial velocities, wave height, liquid viscosity, interfacial friction velocity and flow regimes. Another aspect of the atomisation modelling consists in modelling both the location of the beginning of atomisation and the atomisation rate inside the venturi. The video records showed that, contrary to what is often assumed in some studies, a consequent atomisation may start at the middle of the convergent section, depending on the surface waves occurring at that time. Finally, a correlation law for the atomisation rate is proposed, based on the literature and measurements on air/water and air/oil flows.
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