The Transition Radiation Detector for the PAMELA Experiment M. Ambriola for the PAMELA Collaboration University of Bari and INFN email: ambriola@ba.infn.it 1

PAMELA expected results The PAMELA experiment 2 Primary objectives: measurement of the p spectrum in the energy range 80 MeV ÷ 190 GeV; measurement of the positron spectrum in the energy range 50 MeV ÷ 270 GeV; search for antinuclei with a sensitivity of 10¯7 in the He/He ratio; measurement of the nuclei spectra (from H to C) in the energy range 100 MeV/n ÷ 200 GeV/n; measurement of the electron spectrum in the energy range 50 MeV ÷ 270 GeV. Secondary objectives: continuous monitoring of the cosmic solar modulation after the 23rd maximum of the solar activity; study of the time and energy distributions of the energetic particles emitted in association with solar flares and in CMEs; research on Earth’s magnetosphere and magnetic field; study of stationary and disturbed fluxes of high energy particles in the Earth’s magnetosphere. PAMELA expected results

The PAMELA telescope 3 It consists of: a magnet spectrometer with a permanent MAGNET and a microstrip silicon TRACKER capable of measuring charged particles up to a maximum detectable rigidity of ~ 740 GV/c ; a plastic scintillator system, including a time of flight counter (TOF) and an anti-coincidence system (ANTI); an electromagnetic imaging CALORIMETER, made of silicon sensor planes interleaved with tungsten absorber layers. Its total depth corresponds to 0.9 interaction lengths and 16.3 radiation lengths; a transition radiation detector (TRD) composed of 9 active layers of proportional straw tubes, interleaved with carbon fibers radiators. ~ 1 m Launch date: end of 2002 Spacecraft: russian satellite Resurs DK Orbit characteristic: sun-synchronous at 350÷600 km of altitude and 70.4 º of inclination Foreseen mission life: three years 3

The PAMELA TRD and the……. The goal of the Transition Radiation Detector is to complement the calorimeter in the identification of p and e+. A hadron rejection factor of 5% at an electron efficiency of about 90% is required. The TRD has a modular structure The basic component is a straw proportional counter: 4 mm in diameter 28 cm in length walls made of 30 mm thin kapton foil inside a tungsten anode wire, 25 mm in diameter, is stretched to a tension of 50÷55 g the operating HV ~ 1400 V, with a gas mixture of: Xe(80%)-CO2(20%) 28 cm straw tube 32 straws are grouped to form a module. 4

…..TRD structure the modules are placed sideways, on a special frame, to form a plane. a special gas manifold brass plate keeps the straw tubes in site. the full TRD will be made of 9 of these planes for a total of 32 modules and 1024 straws. The radiators are made of carbon fibers packed in bags at 60 g/l density. The radiators are placed in the space left between consecutive planes. 5

PAMELA TRD beam-tests 6 calorimeter A PAMELA prototype has been tested during beam-test campaigns at the CERN PS and SPS facilities. Selected particles: p -, m - and e- . Momentum ranges: 2÷5 GeV/c at PS and 10 ÷100 GeV/c at SPS. The TRD prototype consisted of 10 radiators and 9 straw modules . calorimeter TRD TRD Tracker PAMELA prototype at CERN-PS

TRD data analysis 7 The data analysis requires: pedestal subtraction and noise suppression; selection of single track events in the TRD; particle tagging by means of the calorimeter; calibration of the TRD data by means of a dedicated simulation code. The TRD performances have been estimated by means of a likelihood estimator which takes into account the energy released by the particles as sampled on each TRD planes. SPS data: 40 GeV/c ionization from p ionization from e ¯ ionization + TR from e ¯ Example of energy ditribution for p ¯ (non- radiating particle) and e ¯ (radiating particle) in a single plane of the detector. The Transition Radiation (TR) contribution is clearly visible in addition to ionization.

Simulation and …….. 8 calorimeter A full simulation of the PAMELA apparatus has been developed based on the Geant 3.21 package. In order to reproduce a reliable ionization in the TRD, the simulation has been interfaced to the Garfield package. TOF TRD TRACKER ANTI calorimeter Simulated positron of 2 GeV/c in the PAMELA apparatus.

……data calibration The response of the TRD to ionization has been studied. Comparison between simulated and calibrated data is shown in figure. 9 Most probable ionization energy loss in Xe-CO2 versus momentum for pions, showing the relativistic increase of the ionization energy loss.

Likelihood method 10 log Le = log (Pni=1 Pie/ Pni=1 Pip) The TRD performances have been estimated from a likelihood distribution calculated from the energy released on each TRD plane. First the probability distributions of released energy have been estimated from electron (radiating) and pion (non-radiating) reference samples. Then using these reference distributions the following likelihood indicator has been calculated for each event: log Le = log (Pni=1 Pie/ Pni=1 Pip) where Pie(p) is the probability of an electron (pion) to produce the detected signal in plane i. The amount of pion contamination in the electron sample has been estimated in correspondence to a cut on this indicator.

TRD performances 11 The values of pion contamination versus electron efficiency have been estimated in the momentum ranges of 2 ÷ 4 GeV/c and of 40 ÷ 80 GeV/c. As shown in figure 1, an efficiency of the order of 90% can be obtained at a level of 5% contamination from 2 GeV/c pions. As expected, the electron efficiency (at a given level of pion rejection) decreases with increasing momentum due to the increasing energy loss from relativistic pions. At the higher momenta of the SPS, particles begin to radiate transition radiation as shown in figure 2. 5 % Figure 2: energy loss distributions of pions of 40 GeV/c (g ~ 290) and muons of 80 GeV/c (g ~ 750) in one plane. The contribution of transition radiation to the signal is clearly visible for muons. 40 Gev/c p 80 Gev/c m Figure 1: Pion contamination versus the electron efficiency. At the level of 5% contamination from 2 GeV/c pions an efficiency of the order of 90% can be obtained.

Conclusions 12 A full length prototype of the PAMELA TRD has been tested at the CERN PS and the SPS facilities. The TRD performances estimated from this analysis comply with the design specifications: a rejection factor of the order of 5% for non-radiating particles (pions of few GeV/c) is obtained at an electron efficiency of about 90 %. Further analysis and software simulation of the TRD are in progress.