The IRIS group of IFIC (Valencia) has recently developed and characterized a new version of a three-layer Compton camera (CC) for hadron therapy monitoring, known as MACACO (Medical Applications CompAct COmpton camera). With the main purpose of improving the timing response of 25 ns of the current MACACO II, a new system, MACACOp, has been assembled employing the novel TOFPET2 ASIC (PETsys, Portugal) to read and process the photosensor signals. This prototype is composed by two detector planes, each one consists of a monolithic Lanthanum (III) Bromide (LaBr3) crystal from Epic-Crystal coupled to a silicon photomultiplier (SiPM) array with 64 elements from KETEK. The ASIC employed provides time and energy digitization of signals from 64 channels, each containing ADCs and TDCs, allowing to read out each one individually. By using a low threshold for timing and a high one for accepting events, the ASIC can discard dark counts without introducing dead time and discard pulses with low amplitude. Characterization and coincidence tests have been performed in laboratory, at a controlled temperature of 20 ◦C, using the MACACOp two-layer prototype. Promising results have been obtained, with a fully working system. Namely an energy resolution of 6.2% Full-Width at Half Maximum (FWHM) at 511 keV and a preliminary time resolution of 2.2 ns FWHM. Moreover, images of a 22Na have been successfully reconstructed. In order to test the device in a more realistic scenario, at high photon energies, in-beam tests with proton beams have been carried out at the National Accelerator Centre (CNA, Seville). A graphite target was irradiated by an 18 MeV proton beam, to produce 4.4 MeV photons. Data were taken at three different beam intensities (250 pA, 2.5 nA and 3.6 nA) with the first detector at 135 mm from the target. The distance between planes was set to 60 mm. Different measurements were taken by moving the graphite target in 1 mm steps with a linear motorised stage with micrometric precision. Preliminary data analysis has been carried out. Energy spectra were obtained in both singles and coincidences data acquisition modes. The measurements allowed to reconstruct the photon emission distribution at target positions separated by 1 mm. The results obtained demonstrate the potential of the MACACOp as a monitoring device for hadron therapy.

Trabajo presentado a las Jornadas de Física Médica, organizadas por la Real Sociedad Española de Física y el IFIC a través de la instalación de Física Médica IFIMED, celebradas virtualmente del 14 al 15 de diciembre de 2020.

Peer reviewed