New Time-of-Flight Wall TOFD

TOFD is the next-generation time-of-flight detector designed for R³B which will enable to fully exploit the potential of FAIR beams at the R³B setup - heavy beams up to ²³⁸U and high counting rates up to 1 MHz. This detector will be situated about 20 m down-stream of the reaction target, behind the large dipole magnet GLAD, and will measure the time-of-flight and the nuclear charge of the fragments produced in the target. Together with the measured trajectory through a dipole field, the mass of the fragments can be identified. The charge is obtained by precise energy loss measurements of the fragments passing through the detector.

In order to separate the neighbouring fragments in mass and atomic numbers also in the mass region 200 it is mandatory to reach a relative resolution in the energy-loss measurement σ_E/E better than 1% and in time-of-flight (TOF) measurement better than 2E-4. For a beam with 1AGeV kinetic energy and 20 m flight path, TOF has to be measured with a resolution better than σ_TOF = 20ps. Moreover, it will be necessary to maintain these values up to 1MHz beam rate.

The detector concept is based on the use of plastic scintillators. TOFD will have four planes of scintillators and the active part will cover an area of 120x80cm². Each plane consists of 44 scintillator paddles with the size 800x27 mm². The first two planes will have scintillators with a thickness of 3 mm and the last two planes a thickness of 5 mm, respectively. Each scintillator paddle will be directly coupled to photomultipliers on both far ends. The photomultipliers of choice have fully active base enabling that, even for large currents, the voltages at the last dynodes of the PMs could be kept constant and, consequently, the constant gain of the PM. This guarantees the stable peak position in the energy spectrum for different counting rates and, therefore, good energy resolution regardless on the counting rate. Moreover, the chosen electronic read-out allows extracting small currents/charges from the PMs which will furthermore guarantee stability of measurements even at the highest rates. The electronic readout is based on the new multichannel front-end electronic card TAMEX. This card has been designed by the GSI EE group for high-resolution time and charge measurements and is a combination of the existing LAND TACQUILA FEE and a FPGA TDC from the VFTX module.

More details can be found in the dedicated EPJA paper from M. Heil at al  10.1140/epja/s10050-022-00875-8.

In 2014 during the S438 Experiment the prototype of the TOFD detector was tested with secondary beams in the Pb-U region. Preliminary results of the data analysis show that the design goals can be reached. As an example, results obtained with ¹⁹⁴Bi entering the Cave C are shown in figure below: the nuclear-charge resolution of σ_Z=0.33 charge units (or equivalently 0.4%) and ToF resolution between two neighbouring paddles of 22 ps have been obtained. This ToF resolution corresponds to 8 ps of ToF resolution for the whole detector.

In the same experiment, the prototype of the TOFD detector has been tested with 58Ni at 500AMeV. With this light beam, we have measured a nuclear charge with a resolution of 0.8 % (sigma) which is the design goal. Measured time resolution of the full detector was 17 ps (sigma). This value is better than needed for this Kind of beam, see table above.