Measurement of the Time-of-Flight (TOF) of the 511 keV gammas brings an important reduction of statistical noise in the PET image, with higher precision time measurements producing clearer images. Scintillating crystals are used to convert the 511 ke
V annihilation photon to an electron of ~511 KeV energy via the photoelectric effect; it is necessary to determine with precision the position and time of this conversion within the scintillating crystal. We propose using an array of crystals cut into a specific geometry discussed below; these crystals are read out by an array of strip SiPMs. This technique allows individual time measurements of the first arriving photo-electrons and to extract the best time resolution using a specific algorithm. The final result is a precise determination of the 3D position (that includes the depth of interaction) of the photoelectric interaction and an improved time measurement.
The Multigap Resistive Plate Chamber (MRPC) is a gaseous detector; the performance depends very much on the gas mixture as well as the design. MRPCs are used as a timing device in several collider experiments and cosmic ray experiments thanks to the
excellent timing performance. The typical gas mixtures of RPC-type detectors at current experiments are based on the gases $rm C_2F_4H_2$ and $rm SF_6$. These gases have very high Global Warming Potential (GWP) values of 1430 and 23900 respectively. The present contribution has been performed as a part of efforts to reduce the amount of greenhouse gases used in high energy experiments. The performance of MRPC has been measured with two different gas mixtures; $rm C_2F_4H_2$ based gas mixtures and the ecological $rm C_3F_4H_2$ (HFO-1234ze). A small MRPC was used for the tests. It has an sensitive area of 20 $times$ 20 $rm cm^2$; it was been built with 6 gaps of 220 $mu$m. In normal operation, the strong space charge created within the gas avalanche limits the avalanches growth. $rm SF_6$ plays an important part in the process due to its high attachment coefficient at low electric fields. It is thus necessary to find another gas that has a similar attachment coefficient. $rm CF_{3}I$ is a possible candidate. Tests were performed with this gas added to $rm C_3F_4H_2$.
