Time of flight system (TOF) Author list

Transcription

1 MPD NICA Time of flight system (TOF) Author list V.Babkin, V.Golovatyuk, Yu.Fedotov, S.Lobastov, S.Volgin, N.Vladimirova Joint Institute for Nuclear Research, Dubna, Russia. Dubna 2009

2 TOF MPD Introduction The TOF system of MPD is the main detector for particles identification. In order to separate pion/kaon in the momentum range GeV/c and proton/kaon in the range 0-4,5 GeV/c it has to have time resolution better than 100 ps. The TOF system consists of two subdetectors: one is stations of scintillation counters situated near the beam pipe on both side from the interaction region which give the start signal and the barrel of fast multi gap RPC detectors on the radius 1,3 m (fig.1). Barrel TOF is distinguished by green color on the fig 1. TOF EMC ZDC TPC Si IT Fig 1. Layout of Central part of the MPD detector. TOF both barrel and endcap are distinguished by green color. As a fast detector for time measurement we choose the multigap RPC. This type of the detector has proven to be reliable, rather simple in construction and not expensive. It is developed to be used in TOF system of ALICE, CBM, STAR, PHENIX and other experiments.

3 Requirements to the TOF system The list of requirements to the TOF system is as follow: 1. The total time resolution of the system T start T stop must be better than 100 ps 2. The detector segmentation must allow the occupancy below 10-15% 3. The system must be able to work at particle flux rates up to 100 Hz/cm The detector must be able to work in magnetic field up to 0.5 T. 5. The detector must cover the region -1 < η < 1 at radius R=1.3 m. 6. The detector must be cheap, reliable and simple in construction. For particles identification the information from the TPC about particle momentum with accuracy dp/p~1-2% and track length with accuracy of several mm and will be used. Fig. 2a The distribution of total number of particles (both charge and neutral) in central collision. Fig. 2b The distribution of total number of charged particles in central collision. Fig.2c The distribution of total number of charged particles in central collision η <1, p>100 MeV/c

4 On figures 2a-c the distribution of number of particles generated by UrQMD generator is presented. The average total number of charged particles which cross the barrel covered η <1 is 450. The magnetic field B=0.5 T does not allow particles with momentum below 100 MeV/c reach the radius 1.3 m. Counting rate and occupancy estimation For occupancy estimation the anticipated luminosity for Au+Au collisions L=1.0x10 27 cm -2 s -1 was used. The collision rate for minimum bias events was taken as: L x s = 1x10 27 x 6 barn= 1x10 27 x 6x10-24 = 6000 Hz The interaction rate for central collisions is bellow 1 khz. For simulation of primary products of Au-Au interacted beams with total energy 4,5+4,5 GeV/n we used UrQMD program as a generator and GEANT4 for tracing particles in the detector. However on the first step we didn t put any detector matter in between interacting beams and the TOF system in order to estimate the role of multiple scattering and creation secondary particles as a result of interactions. Impact parameter range for minimum bias b=0-15,8 fm, for central collision, b= 0-3 fm. Fig.3 Primary charged particles multiplicity in one interaction (left pseudorapidity, right in solid angle 10mrad x10 mrad as a function of (θ-90) in mrad.) One may estimate the number of particles in the solid angle 10 mrad x 10 mrad as a function of axial angle from fig.3. On the base of TOF 1.3 meter solid angle 10 x 10 mrad gives projection 1.7 cm 2.

5 From fig.3b one may see that in one event there are charged particles cross the surface with size 1.7 cm 2. Number of charge particles per second crossed the 1 cm 2 surface of TOF is N = 6000Hz x 0.005/1,7 = 17.6 Hz/cm 2 So, the TOF system has to demonstrate reliable for at particle flux up to 20 Hz/cm 2. Multi gap RPC can work with high efficiency and with no time resolution degradation up to 1000 Hz/cm 2 /1/. The main parameters of TOF detector estimation Fig.4 Spectrum of pions, kaons, and protons produced in Au+Au interaction with total energy GeV/n As it was mentioned above the main goal of the TOF system is identification and separation of pion, kaon and proton. The spectra on these products presented on the fig 4 show that momentum range of pions and kaons is up to 1.25 GeV/c, the protons range is limited by 2.5 GeV/c.

6 It is has to mentioned that due to the magnetic field B=0.5T the low momentum particles don t reach the TOF detector, with inner radius 1.3 m. As an example on fig.5 modification of pion spectrum after applying magnetic field is shown. Fig.5 Modification of pion spectrum from Au+Au collision after applying magnetic field B = 0.5 T On fig.5 we present the part of the particles (pion, kaon) below of momentum level. These curves are obtained from the corresponding spectra on fig.4. One may conclude that TOF system can resolve pions on the level of 96% and kaon almost 99% up to particles momentum 1.2 GeV/c. Fig.6 Part of the pions and kaons below some momentum levels

7 From fig 7a one may estimate that with TOF base as much as 1.3 m one can separate π/κ in momentum range up to 1.25 GeV/c with 3 standard deviations of the detector time resolution. a b Fig 7 Calculation of the separation in units of standard deviation as function of TOF base for different momentum of particles and TOF time resolution (7a) and for the fixed base and time resolution 100 ps as function of the particle momentum. On figure 8 and 9 mass separation capabilities of the TOF MPD system is presented. The base of TOF is 1.3 m, time resolution is 100 ps. We have made crude estimation of efficiency and contamination level from the plot 9. We put simple boundaries as straight lines to define type of the particles. Counting particles of given type are outside of the boundary as lost of efficiency and a number of cases with particles of different type in the region as a contamination. Results for pions, kaons and protons are presented on fig.10. Barrel Fig.8-9 Mass separation with the TOF (momentum reconstructed in the TPC) as a function of the momentum.

8 The main parameters of TOF Radius from the beam line - 1,3 m Time resolution -100 ps Max momentum of π/k system separated better than 2,5 σ - 1,3GeV/c Detector Design Mechanical construction of barrel The RPC TOF system looks like barrel with the length of 355 cm and radius of 130 cm. Along the beam the TOF detector will cover the region η < 1. The barrel surface is about 30 m 2. The dimension of one multigap RPC counter is 7 cm x 67 cm, it has 150 pads with size 2.3cm x 2 cm. The RPC counters will be placed in the modules. In total there are 12 modules in the barrel. The full barrel is covered by 560 counters. The total number of readout channels is Geometry efficiency in the region η < 1 is 93% Fig.10 Barrel of TOF system

9 Organization of one module The layout of timing RPC positioning in the module is presented on Fig.11 and 12. One detector of the multigap RPC has outer dimension 120 cm x 670 cm. The detectors will be placed in the module perpendicular to the beam axes. In total there are 43 detectors in each module. The detectors are placed in such a way that readout pad is perpendicular to the line coming from the interaction point. The detectors in the module are put with overlap in order to exclude dead zones. In total there will be 43 x 12 = 516 multigap RPCs. Fig.11 Distribution of timing RPCs in the barrel Fig 12 View of the multigap RPCs distribution in the module box.

10 Design of the multigap RPC module The multigap Resistive Plate Chamber consists of a stack of 12 resistive plates of glass spaced from each other by spacers of 220 μm thick creating a 10 equal gas gaps. We choose 10 gaps construction in order to avoid a problem connected with processing of small signal and to diminish multi hits signals. ALICE TOF collaboration /1/ demonstrated that RPC made of commercial sodalime glass with bulk resistivity of ~10 13 Ω*cm can operate at a flux in excess of 1 khz with no degradation in the performance. In case of NICA MPD the flux is expected to be less than 20 Hz/cm 2. Fig.13 Drawing of the basic element of timing RPC The scheme of the detectors basic element is presented on fig. 13. The detector consists of two parts 5 gaps each. The outer glass electrodes have thickness 0.8 mm. The internal glass electrodes have thickness 0.5 mm. The fishing line as a spacer defines the 220 μm gap between all electrodes. The outer part of two external glass electrodes is covered by conductive tape with surface resistivity about 1-5 MΩ /cm 2 to apply high voltage and the ground. All internal electrodes are floating. The pick up pad are made on the PCB board of mm thick. Fig.14 Layout of pads on the readout plane. One have to note that the readout pad geometry and dimension is the subject of further studies. Final decision will be taken after optimization of tracking capabilities of the whole MPD detector. Coordinate from the TOF could be used as a

11 seed coordinate in the track find procedure for track reconstruction using data from TPC and inner detector. Reference 1. Alice addendum to the Technical Design Report of the Time of Flight System. CERN/LHCC Addendum to the ALICE TDR 8, 24 Apri CBM Technical Status Report 2006

12 Particle Spectra Fig.15 Momentum spectra of pions, kaons and protons in different regions of the pseudorapidity of MPD detector. Spectrum of the primary protons is presented for demonstration that the low energy peak on the spectrum of all protons is due to secondary protons created in the matter before TOF in both Barrel and End cap regions.

13 Fig.16 The momentum dependence of efficiency and contamination of the TOF for barrel red and blue for endcap regions of the MPD detector

14 End Cap Fig.11 Mass separation with the TOF (momentum reconstructed in the End Cap Straw Tracker) as a function of the momentum

15 Mechanical construction of the End cap TOF on base of mrpc TOF End Cap plane structure. There are 2 types of mrpc with size of active are 800x120 mm 2 and 100 x 120 mm 2. These detectors placed in the four type of modules. To ovoid dead space the mrpc in the module are placed overlapped. Also the modules positioned overlapped in the End Cap.

16 a b MRPC types for End Cap and four types of the containers used for assembled mrpcs. The End Cap TOF wall consist of four type of the panels (a) which contain mrpc strips of two length 80 cm and 110 cm. (b).