The Time-of-Flight


Motivation

ToF-M.jpgThe Time-of-Flight (ToF) system is the stop-watch of AMS-02. In addition to being able to measure a particle's transit time through the detector to a high degree (150 ps), it also serves to notify the other subdetectors, that a particle has passed through the experiment.

The ToF system consists of two planes. When a particle passes through one plane AMS-02's internal timer - the Time-to-Digital Converter (TDC) - starts running. When the particle reaches the other side of the experiment, the timer stops.

If a particle passes both ToF planes, it is said to be "inside the acceptance". In this case, the ToF system alerts all other subdetectors and the data from each of these systems (Tracker, ECAL, TRD, RICH, ACC and the ToF itself) is collected, processed and stored. A signal which can start the data aquisition is called a trigger. The trigger for AMS-02 is composed not only of the ToF, but of a combination of the ToF (for charged particles), the ACC (as a veto for high inclination particles) and the ECAL (for neutral particles).

In addition, the ToF system is also important for antimatter detection. Both particles and antiparticles are bent by a magnetic field, albeit in opposite directions. This fact also means, that the trajectory of a particle travelling downwards is the same as that of it's antiparticle travelling upwards. The ToF seperates both cases by determining the flight direction of each particle.

Physics Background

TOF_paddleM.jpgAs a time measurement is the goal of the ToF system, the signal created by the system must coincide with the actual passage through the detector to a high degree. To achieve this, the process of szintillation is used. When a particle passes through szintillating material it causes molecular excitations in the material. The de-excitation process consists of a rapid (~ 10 ns) emision of flourescence light. This light can be collected by light guides and routed to one or more photomultipliers (PMT).

A PMT is a device capable of efficiently converting light into electrons - and therefore a signal - by means of the photoelectric effect. A series of electrodes - the dynodes - multiply the original electrons before the signal is read out. The multiplication factor - or gain - is usually up to 10^8.

A particular class of PMT - the fine-mesh PMT - has a very compact dynode structure. This minimizes the effects of the stray magnetic field on the multiplication process. These fine-mesh PMTs are used for the AMS-02 ToF.

Construction

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).