Time of flight

The HELIX time-of-flight and charge (TOF) system consists of three layers of 1cm-thick plastic scintillator paddles. One layer is located at the top of the spectrometer and another at the bottom, with a total separation of 2.3 m. A third layer (the bore paddle) is located just below the magnet, above the RICH radiator. The scintillators are read out with Silicon Photomultiplier (SiPM) made by Hamamatsu. SiPM are used because they are immune to magnetic field and they can be operated with relatively low voltage (~40 V).

The TOF system has a timing resolution of better than than 50 ps for nuclei with charge greater than 3. This allows for accurate velocity measurement for energies up to 1 GeV/n. The charge of the incoming particle can also be identified by the TOF as it will produce scintillation light with intensity proportional to Z².