FTOF System Overview

The Forward Time-of-Flight System (FTOF) will be a major component of the CLAS12 forward detector used to measure the time-of-flight of charged particles emerging from the target. The average path length from the target to the FTOF counters will be roughly 600 cm. The requirements for the FTOF system include excellent timing resolution for particle identification and good segmentation for flexible triggering and prescaling options. The design parameters were chosen to allow for separation of pions and kaons up to roughly 3 GeV. The most energetic particles are produced at small angles. The system specifications call for a time resolution of σ=80 ps at the more forward angles of CLAS12 and 150 ps at angles larger than 36 deg. The system must also be capable of operating in a high-rate environment. The maximum counting rate occurs in the forward direction where, at an operating luminosity of 1035cm-2s-1, the average rate per scintillator is approximately 250 kHz. The discriminated scintillator signals will be used in certain situations in the CLAS12 Level 1 trigger. Therefore, the system must provide signals representing a uniform response and adequate granularity to select particles reaching the detectors. The timing in the trigger hardware will be limited by the flight time variations between fast particles (e.g. electrons) and slow particles (e.g. protons), which can be as large as 50 ns. Therefore, precise timing information will only be achieved in off-line software analysis using the momentum and position measured with the drift chamber system. The FTOF system will also be used for energy-loss measurements in specific instances. Pulse height information, being directly proportional to energy deposited, provides an independent means for the identification of slow particles. In this regard, the flight time can provide for a more accurate measurement of particle energy than magnetic analysis for slow particles, where the effects of multiple scattering are the largest. In order to meet the requirements for the tight timing resolution, the major considerations in the design of the FTOF system are:

Scintillator Size: The width of each scintillator determines the granularity of the scattering angle definition in the trigger. Also, the overall size of the system will demand careful consideration of light collection in order to optimize the time resolution of the system.

Geometry: The projected space behind the coils of the main CLAS12 torus is inactive and therefore useful for locating the light guides, photomultiplier tubes (PMTs), voltage dividers, and cables. The remaining area in the forward direction is the fiducial region of the detector and must be covered with scintillator counters.

Magnetic Field: The PMTs will have to be properly shielded from the stray magnetic fields of the CLAS12 torus.

Crossing Tracks: Particle trajectories from the target can intersect adjacent TOF counters. Therefore light from both counters will have to be summed to optimize particle identification in the data analysis.

Last modified: May 1, 2013
Daniel S. Carman