The new HADES ToF Forward Detector

Abstract

The High-Acceptance DiElectron Spectrometer (HADES) at GSI Darmstadt consists of a 6-coil toroidal magnet centered on the beam axis and six identical detection sections located between the coils and covering polar angles between $18^\circ$ and $85^\circ$. The physics aims include the study of the properties of hot and dense hadronic matter as well as elementary and pion-induced reactions. To increase the acceptance of HADES at very low polar angles in the forward region, between $0.5^\circ$ and $7^\circ$, a new detector, the Forward Detector (FD), has been built. The FD is composed of a tracking and a Time Of Flight (TOF) detector based on Resistive Plate Chamber (RPC) technology. The TOF detector, covering an area of around $2$ m$^2$, is composed by $128$ strip-like shielded RPC cells, with two different widths $22$ mm and $44$ mm and $750$ mm length distributed in four modules symmetrically placed around the beam axis. Each cell is composed by four gas gaps, $0.270$ mm, delimited by three ($2$ mm) aluminum and two ($1$ mm) glass electrodes. In order to cope with an expected maximum particle load of around $400$ Hz/cm$^{2}$, close to the beam axis, the detector is operated above room temperature in order to decrease the resistivity of the glass and increase the count rate capability. Details of the system construction and results concerning timing precision are described in this communication. The detector was operated at $31.5^\circ$C with a maximum particle load of around $600$ Hz/cm$^2$ during a production beam time for six weeks in early $2022$ showing an average time precision of around $80$ ps.