Marian Smoluchowski
Institute of Physics

Defence year: 2024

Positron emission tomograph (PET) is a device widely used in medical diagnostics. It is used both to detect the location of tumors in the patient's body and to monitor the treatment process. It is used, for example, in combination with proton therapy. The principle of operation of this device is based on the detection of radiation from the annihilation of positrons created in the process of ?+ decay, which occurs in the patient's body after the injection of a radiopharmaceutical into the body. All PET scanners currently produced are made using crystal scintillators, which are significant in mass and have high production costs. The J-PET group led by prof. dr hab. Paweł Moskal set the goal of constructing a PET scanner using plastic scintillators, which are much lighter than crystal scintillators. A scanner made in this way will be much more mobile and low-cost compared to classic tomographs of this type. In addition to the aforementioned scintillators, a very important element of the PET scanner are also photomultipliers, which together with scintillators are used to convert the aforementioned radiation into a measurable electrical signal. Due to the desire to create a mobile tomograph, photomultipliers should also be as small as possible and at the same time as accurate as possible. For this reason, it was decided to use sillicon photomultipliers (SiPM), which are the main focus of the work. Many parameters characterizing SiPM significantly depend on temperature. The following work focuses on examining one of such dependencies, in order to exclude potential errors related to temperature changes in the PET scanner being created at the Jagiellonian University. The work presents the results of measurements carried out in a narrow temperature range (17?C ? 23?C) for two Hamamatsu S14160-6050HS and two Onsemi MICROFJ-60035-TSV SiPM photomultipliers. An analysis of the signals collected during the measurements was carried out. Using the author's program, histograms of quantities calculated based on the above-mentioned signals (amplitude and total charge) were created, which are a estimate of the energy of the ? quantum on the scintillator. Then the Compton edges appearing in the above-mentioned histograms were determined. The final result of the work is the obtained temperature dependencies of the determined Compton edges. They show a significant decrease in the Compton edge position values with increasing temperature. For Onsemi photomultipliers the temperature changes were ?1.61mV/K and ?1.22mV/K for the amplitude case and ?0.96pC/K and ?0.481pC/K for the charge case, respectively. For the Hamamatsu photomultipliers, these values were ?0.845mV/K and -0.56mV/K for the amplitude case and ?0.340pC/K and ?0.406pC/K for the charge case, respectively. These mean a jump in the Compton edge value by a few percent with a temperature drop of 1?C.