Performance Evaluation of the TOF-Wall Detector of the FOOT Experiment

Abstract

The correct quantification of the dose released in charged particle therapy treatments requires the knowledge of the double differential fragmentation cross section of particles composing both beam and target. The FOOT experiment aims at measuring these cross sections for ions of interest for charged particle therapy applications. The paper describes the performance of the TOF-Wall detector of the experiment. The detector is composed of two layers of 44 cm x 2 cm x 3 mm plastic scintillator bars (20 for each layer), arranged orthogonally and read out by silicon photomultipliers. The detector is designed to identify the charge of fragments ranging from protons to oxygen ions, with a maximum energy of 700 MeV/u, by measuring the energy released in the scintillators and the time of flight with respect to a start counter. In this study, the detector was scanned with carbon ions of energy between 115 MeV/u and 400 MeV/u and with a 60 MeV proton beam. The measurements show an energy resolution (σE/µE) between 6% and 4% and a contribution of the detector to the TOF system time resolution between 25 ps and 20 ps (standard deviation) for carbon ions and between 100 ps and 80 ps for protons.

Elemental fragmentation cross sections for a 16O beam of 400 MeV/u kinetic energy interacting with a graphite target using the FOOT ΔE-TOF detectors

“…The Tof-Wall detector (TW) is composed of two orthogonal layers of 20 plastic scintillator bars (EJ-200 by Eljen Technology) wrapped with reflective aluminum and darkening black tape [30,33]. Each bar is 0.3 cm thick, 2 cm wide and 44 cm long.…”

“…This extremely low value, combined with SC contribution, provides an overall TOF resolution of ~75 ps [30]. Such resolution on the time measurement allows for the hit position reconstruction along the bar achieving a spatial resolution σ pos of ~8 mm for the heavier ions [33], even better than the granularity obtainable from the bar crossing of 2 cm. To discard signals from the bars in Frontiers in Physics frontiersin.org which the fragments did not interact (zero suppression), a low amplitude threshold to reject noise has to be set for each TW channel directly at DAQ level.…”

“…To improve the data/MC agreement, a calibration of the MC energy response was implemented (using the detector response for known energies to find the calibration function) and a Gaussian smearing was applied to the energy loss and time of flight observables computed with FLUKA. To do that, the resolution curves for energy loss and time of flight, obtained by means of dedicated detector calibration data acquisition campaigns, were used [30,33].…”

“…The algorithm is applied in the same way both to the MC simulation, once the TW experimental ΔE and TOF resolutions are included, and to the data-set collected at GSI. To do so, the ΔE and TOF have to be properly calibrated, as discussed in Section 2.3 and shown in [30,33]. In Figure 6 the BB curves extracted from the MC truth, as discussed above, are superimposed to the (ΔE, TOF) distribution in the case of the reconstructed MC (left) and in the case of data after the calibration procedure (right).…”

“…The simple bar crossing provides in this case 4 possible positions, but only two are related to the real fragment hitting positions, while the other two are the result of the combinatorial algorithm that computes all the possible pairs. In that case, to select the closest hit among orthogonal bars the propagation of the light signal along a given bar and the time difference between the bar edges, as calibrated in [33], is exploited to provide an additional…”