Diagram of the ToF "umbrella" and "cube" scintillator paddles (light blue). In this figure, part of the umbrella is hidden to show the cube. Part of the gondola, as well as carbon fiber straps on the cube, are also shown. The outer TOF is highlighted in yellow, and the inner TOF is highlighted in red.

Simplified block diagram of the ToF electronics.

The GAPS time-of-flight system (ToF) provides measurements of a particle's velocity, charge, and trajectory. The ToF consists of 160 plastic scintillator paddles, each between 1.5 and 1.8 meters long. The paddles are assembled into two components, an outer section (the "umbrella" and "cortina") and an inner "cube" section which surrounds the Si(Li) tracker. Paddles are wrapped to be light-tight, and are held in place with a carbon fiber structure. Silicon photomultipliers (SiPMs) are coupled to both ends of each paddle, and are connected to custom preamplifier and readout boards. The readout boards incorporate the Paul Scherrer Institute DRS4 chip, allowing for sample rates of several GHz. A computer organizes and compresses the data from the readout boards for both local storage and telemetry.

An incoming (or outgoing) charged particle produces photons that undergo total internal reflection within the scintillator and are observed by the SiPMs at the paddle ends. Photon arrival times at each paddle end can be determined with sub-nanosecond accuracy. The relative timing between paddle ends gives us the position of a particle hit, and hits on different paddles determine the particle's trajectory. Velocity can then be derived using the timing difference between paddle hits. Charge is estimated using the energy deposited in each paddle, the velocity, and the trajectory.

The ToF also triggers readout of the whole instrument, at a rate of a few 100 Hz. The trigger decision algorithm is designed to initiate readout when particles of interest (e.g. antiprotons, antideuterons) enter the instrument, while simultaneously vetoing background particles (e.g. protons, helium nuclei, carbon nuclei). The trigger decision is made by a master trigger board, which combines information from 20 local trigger boards, and incorporates a particle's speed, charge, and the total number of ToF paddles hit. In general, particles of interest will have lower speed, lower charge, and more ToF paddle hits than background particles. The trigger decision must be made within ~300 nanoseconds.