The AMS - 02 Transition Radiation Detector

1. Particles Signatures in AMS-02 The spectroscopy of cosmic particles with AMS allows the direct search of Antimatter (antihelium nuclei) and the indirect search of Dark Matter. TRD (Transition Radiation Detector) Particle ID and 3D-tracking The observation of positrons in the cosmic rays spectrum is a signature of the annihilation of dark matter candidates TOF (s1, s2) (Time-of-Flight detector) Trigger - st = 125 ps ACC Anticoincidence (veto) counter Silicon stripTRACKER 2 105 ch with internal laser alignment 6 m2 in (3 double + 2 single) xy layers Charge separation = 1 s up to 1 TeV The cosmic rays spectrum is dominated by protons: Superconducting MAGNET B = 0.9 T V = 0.6 m2 To detect positrons with a 90% efficiency, a proton rejection factor of 106 is needed. In AMS-02 this is achieved by the joint use of EMC and TRD (Transition Radiation Detector) n(p+)/n(e+) = O(104) TOF (s3, s4) (Time-of-Flight detector) Distance from s2 = 1.3 m Separation p+/e+ > 2 s below 2 GeV RICH (Ring Image Čerenkov Counter) For A ≤ 27, Z ≤28, separation > 3s in 1-12 GeV EMC (Electromagnetic Calorimeter) 3D sampling – lead/scintillating fibres p+ rejection > 104 in 10-300 GeV with p-E matching and shower-shape The TRD principle The AMS-02 TRD consists of layers of straw modules interleaved with fibre fleece material, arranged in a conical octagone structure A charged particle crossing the interface between two media with different dielectric constants emits a transition radiation that depends from its mass and momentum This can be used for particle discrimiation at very high energies where e.g. Čerenkov detectors are no longer useful The Transition Radiation photons are detected in the straw tubes filled with a gas mixture with high atomic number Z Different particles leave different energy deposits in the detector after passing through the fleece radiator Fleece radiator TR-yield (test beam results on a 60 cm heigth prototype) Gas Gain Xe & CO2 tanks Circulation pump TRD To obtain the required proton rejection power, a stringent control over gas parameters is necessary. Example: a 3°C temperature change causes a 1% gas density variation, which implies a gas gain variation of about 5%. Manifolds • Polycarbonate endpieces • AW 134 glue for potting • Copper-Tellurium crimp connectors to the electronic board Double O-ring gas connectors UGBS UGBC UGFV USCM 20 GeV Electrons 160 GeV Protons Electronic control Log Likelihood Log Likelihood 0.6 0.6 Gas-gain correction: M’ =M·[1 + (r /<r> - 1) · 5.5] The TRD gas system The electronic control of the system includes a Main DAQ Computer (JMDC) that communicates via CAN bus with a control board (USCM) and then with the dedicated boards that control the electomechanical devices. The control system also monitors pressure and temperature in the gas system and in the TRD modules, and the composition of the gas mixture. In case of overpressure, or power or communication failure, actions are taken that drive the system into a safe status. During the operation the gas is circulated in the TRD through a manifold system from a circulation box (Box-C) and refilled by a supply box (Box-S). Gas flow: 1 l/h per gas circuit (41 l/h) • The Box-S contains the gas tanks: • Xe: 49.5 kg (8420 l @ 1bar) • CO2: 4.5 kg (2530 l @ 1bar) Francesca Spada University of Rome “La Sapienza” and INFN Rome I francesca.spada@cern.ch for RWTH Aachen, MIT Boston, IEKP Karlsruhe, CHEP Knu Daegu, INFN Rome The AMS-02Transition Radiation Detector AMS-02 on the ISS AMS-02 will fly during 3 years at a mean altitude of 400 km on the ISS (International Space Station). The detector has an acceptance of 0.5 m2 sr and will be used to study the flux of particles coming from the space. • The fleece radiator • Radiator material: LRP 375 BK (Freudenberg) • Fleece: 10 µm thick Propylene fibre • Density: ρ = 0.06 g/cm3 • E(g) is O(KeV) The straw tubes • Æ6 mm tubes filled with Xe/CO2[80:20] at a pressure of 1250 mbar • Tubewall: 72 µm Kapton-Aluminium sandwich • Wires: 30 µm W/Au tensioned with 100 g Multilayer tubing Kapton wall Every module contains 16 straws. The structure is stabilized by lengthwise and crosswise stiffeners. Gas Tightness Proton rejection Front end Electronics and DAQ The gas tightness of the straw modules over 3 years is a key point for the operation of the TRD in space. To determine whether a TRD track belongs to a proton or to a positron, a likelihood method is used. • 82 front-end units (UFE), 2 crates (+ power supply) • 5248 channels, double redundancy up to the front-end • 28 V DC connection for each crate • Power to front end: 20 W for 5248 channels The energy deposit distribution is normalized, and for each track hit the probability density functions of the hit to belong to a proton or a positron track are calculated. The combined probabilities of the event are built: The likelihood function is L = We/(We+Wp) Forseen gas storage: 8420 l for Xe at 1 bar (49.5 Kg) 2530 l for CO2 at 1 bar (4.5 Kg) Measured CO2 leak rate (diffusion through the straw walls): 0.23·10-6 l·mbar/s/m Total TRD CO2 leak rate (tubes + polycarbonate endpieces): 1.5·10−2 l·mbar/s This corresponds to a 287 l loss of CO2 over 3 years Safety factor ~ 8 Rel. Gas Gain Fe55 gas-gain corrected Assuming that events with L < 0.6 are from light particles, a proton rejection factor >102 is reached up to 250 GeV with 90% electron efficiency (MC). Rel. Gas Density ADC The TRD Support Structure The support consists of a conical octagon structure of aluminum honeycomb with carbon fibre walls, that matches the stability and lightness requirements. Dimensions: height 62.3 cm,  201.8 cm Weight: 207 kg (including external support) The thermal stability is assured through multi-layer insulation (MLI) Radiator + straws + gas 168 Kg Octagon + support + shielding 207 Kg Gas system 54 Kg Electronics 53 Kg TRD total 482 Kg