The NA61/SHINE experiment at the CERN SPS ,  was designed to study the properties of ion-ion collisions, in particular, the onset of deconfinement  as well as to search for the critical point of strongly interacting matter. These goals are achievable by investigating p+p, p+A and A+A collisions at different beam energies from 13A to 158A GeV for ions and up to 400 GeV/c for protons. A schematic view of the NA61/SHINE experiment is shown in Fig.1 (left). An overview of the collision systems and the energies of the physics program is presented in Fig.1 (right).
2. Hadron calorimeter (Projectile Spectator Detector - PSD)
The Projectile Spectator Detector (PSD) ,  in NA61/SHINE is a segmented forward hadron calorimeter. It is used to determine the collision centrality as well as orientation of the event plane. The PSD allows to select the collision centrality of events at the trigger level. The modular structure of calorimeter is suitable also for studies of collective flow.
The PSD calorimeter consists of 16 central small modules with size 10x10cm and 28 large modules with size of 20x20cm . Each module contains 60 layers of 16 mm lead plates and 4 mm scintillator tiles (sampling ratio 4:1). Wavelength shifting fibers (WLS) are used to collect light from scintillator tiles which is read out by silicon photomultipliers. In total, the PSD has 5.6 hadronic interaction lengths. One short module of 10x10cm is used in front of the PSD to minimize shower leakage for forward going high momentum particles. A photo of the PSD calorimeter and schematic view of the PSD front face are shown in Fig.2.
3. PSD performance.
Calibration has to be performed in order to reconstruct the energy of particles in the PSD. In the first step each module of the calorimeter was exposed to through-going minimum ionizing particles by using a muon beam. The spectrum of one PSD section for muons is shown in Fig.3 (left). Then the response to proton showers was studied at several beam energies to determine the PSD calibration parameters (see Fig.3 (right)). Figure 4 shows results of linearity and energy resolution obtained for proton beams after applying the calibration.
In 2016 the PSD was used to study Pb+Pb collisions at maximum SPS ion energy of 150 AGeV as well as at intermediate energy 30 AGeV. Using the trigger from the PSD the 25% most central events were selected. Figure 5 shows a plot of track multiplicities registered in the TPCs versus energy in the PSD, normalized to the beam energy per nucleon. A linear fit extrapolates to a value close to the atomic number of Pb for very peripheral collisions.
4. PSD upgrade for the future continuation of the NA61/SHINE experiment.
A new physics program is under consideration for NA61/SHINE experiments beyond 2020, which includes open charm measurements. The beam rate will be increased by an order of magnitude (up to 5x10 ions/sec) for these experiments. To survive in such high radiation conditions the PSD must be upgraded. It is proposed to use two calorimeters instead of the current single one (see Fig. 6). PSD1 is the present calorimeter with the 16 small central modules replaced by 4 new large ones with transverse size of 20x20cm2 and truncated edges to leave a 60mm diameter hole for the beam in the center. The second calorimeter (PSD2) consists of 9 modules of the same large transverse size (20x20cm ). It will be placed downstream of PSD1. All PSD2 modules have 10 longitudinal readout sections (5.6 interaction lengths, the same as for PSD1) excepting the central module which will have 14 longitudinal readout sections corresponding to 7.8 interaction lengths.
GEANT4 simulation shows that this new configuration of calorimeters shower leakage decreases from 11% in present PSD to 4% in new PSD1+PSD2 configuration for Pb+Pb reaction at 150 AGeV. This will improve the purity of centrality selection for the most central collisions, see Table 1.
|Event centrality class [%]||0-5||5-10||10-15||15-20||20-25||25-30|
|PSD1 + PSD2||96||92||88||88||80||63|
The proposed new configuration will help to solve the radiation hardness problems of the forward hadron calorimeter for NA61 experiments beyond 2020. The beam hole in the PSD1 will help to avoid radiation damage of scintillator tiles due to the radiation dose from ionizing particles (see Fig. 7 (left)). The expected neutron flux at the location of silicon photomultipliers in PSD1 and PSD2 will also be at acceptable level for the envisaged beam rates (see Fig. 7 (right)).
PSD2 will measure practically only the energy of heavy fragments and can be operated with reduced voltages on silicon photomultipliers. Only the central PSD2 module will suffer radiation damage in the scintillator tiles which will lead to a decrease of the light yield. However, this effect can be compensated by increasing the bias voltages.
Results of performance studies of the existing segmented hadron calorimeter PSD in the NA61/SHINE experiment were reported. Proposed ideas to upgrade the PSD calorimeter system for future heavy ion experiments at the CERN SPS beyond 2020 were presented.