The Upper and Lower ToF in a assembly test before the integration in AMS.

The Time-of-Flight system (ToF) is the AMS stopwatch. It is able to measure with a high level of precision (1.5×10^-10 s) the particle transit time into AMS. The main ToF goal is to warn the other sub-detectors of the incoming of an incident cosmic-ray.

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How does the ToF work?

The ToF system behaves just like a stopwatch. When a particle passes through a ToF plane the timer – the Time-to-Digital Converter, or TDC – starts running. When the particle exits from the other side of the magnet barrel passing through the opposite ToF planes the timer stops.

The AMS ToF system can reach a timing precision of 150 ps. Since the distance between Upper and Lower ToF is approximately 1.2 m, the ToF is able to measure particles velocity up to 98% of the speed of light!

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Why do we need the ToF?

A particle able to traverse the Upper and Lower ToF is said to be inside to the AMS acceptance. In the case of such a particle the ToF alerts all the AMS sub-detectors and data from Tracker, TRD, RICH, ECAL, ACC and ToF itself are collected, processed and stored. A signal capable of starting the data acquisition is called the trigger of the experiment. The main trigger of the experiment, the Level 1 trigger, is constructed using information from ToF (for charged particles), ACC (as veto for high inclination particles) and ECAL (for neutral particles).

The ToF is also important for the antimatter detection. Indeed in a magnetic field an upward going electron trajectory is equivalent to the downward going positron one. If you mistake the direction of a particle, you can have the wrong charge sign! Don’t be worried, our ToF is able to distinguish 1 up-going particle for every 10⁹ down-going particles.

In addition the ToF system gives a reliable evaluation of the absolute charge of a particle – to distinguish among nuclei such as Helium, Carbon, Silicon, … – contributing to the astonishing chemical distinction capability of AMS.

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How is the ToF built?

The AMS-02 ToF system is composed by 4 planes of scintillation counters 2 above and 2 below the magnet. The four planes contain, beginning from top, 8, 8, 10 and 8 scintillator paddles. Each ToF plane consists of paddles aligned along the x and y coordinates, respectively.

A ToF paddle consists of 1 cm thick polyvinyltoluene scintillator of trapezoidal or rectangular shape (of dimension approximately 1×12×120 cm³). The scintillators are coupled at both ends via plexiglass light guides to 4/6 Photo-Multipliers (PMTs).

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Design of a ToF counter.

In Depth: How does a Scintillation Paddle work?

A charged particle passing through a scintillating medium causes molecular excitations in the material. The de-excitation process is characterized by the fast emission (τ ∼ 10⁻⁸ s) of fluorescence light. This light can be collected by the mean of light guides on the PMT.

The PMT is a device able to convert efficiently the fluorescence light into electrons, through the photoelectric effect. The produced electrons are focused on a multiplication chain. It consists of a series of electrodes, the dynodes, that are able to accelerate the electrons producing additional low energy electrons on the impact of the accelerated electrons on the dynode structure. At the end of the dynode cascade the initial signal is amplified by a factor up to 10⁸.

A particular class of PMTs, the fine-mesh PMT, have a compact dynodes structure in order to reduce the dependence of the photo-induced multiplication by the stray magnetic field, as in the case of the AMS ToF. Tilted and bent light guides have been used to optimize the angle between PMT aligned along the y-axis and the magnetic field.

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If you want to know more:

» AMS INFN Bologna Group

» Stopwatch (Wikipedia)

» Photoelectric effect (Wikipedia)