Search for Antimatter in Space

PAMELA is a cosmic ray space experiment that will be installed on board of the Russian satellite Resurs-DK1 whose launch is scheduled at the end of 2002. The duration of the mission will be at least three years in a high latitude orbit at an altitude ranging between 350 and 600 Km. \\ The observational objectives of the PAMELA experiment are the measurement of the spectra of antiprotons, positrons, particles and nuclei in a wide range of energies, the search for antinuclei and the study of the cosmic ray fluxes during a portion of a solar cycle. The main scientific objectives can be schematically summarized as follows: \\ \\ a) measurement of the antiproton spectrum in the energy range 80 MeV-190 GeV;\\ b) measurement of the positron spectrum in the energy range 50 MeV-270 GeV;\\ c) search for antinuclei with a sensitivity of the order $10^{-8}$ in the $\overline{He}/He$ ratio;\\ d) measurement of the nuclei spectra (from H to C) in the energy range 100 MeV/n - 200 GeV/n;\\ e) energy spectrum of the electronic component of the cosmic rays in the range 50 MeV-1 TeV. \\ \\ Moreover, the PAMELA experiment will be able to address the following additional issues:\\ f) continuous monitoring of the cosmic rays solar modulation during the 23rd solar cycle;\\ g) study of the time and energy distributions of the energetic particles emitted in solar flares;\\ h) measurement of the anomalous component of cosmic rays;\\ i) study of stationary and disturbed fluxes of high energy particles in the EarthÕs magnetosphere. \\ \\ Positrons, antiprotons and also primary components of cosmic ray spectra, determined over three decades of energy, will allow to test many theoretical predictions and models; a long duration exposure provides the first opportunity for a detailed study of temporal variations of cosmic rays at relativistic energies. A measurement of the cosmic ray spectra with the same instrument over a significant portion of a solar cycle is of major importance. \\ \\ In the figure a schematic view of the telescope is shown. It is composed by: \\ \\ 1. a magnetic spectrometer (SPE) based on a permanent magnet (Nd-Fe-B, field of 0.4 T at the center) and on 6 silicon microstrip detector planes with excellent spatial resolution (3 $\mu$m for a mip in the bending direction) allowing to determine the sign and the absolute value of the electric charge with a very high confidence level, and to measure the momentum of the particle up to the highest energies given by the Maximum Detectable Rigidity (MDR=740 GV/c); \\ 2. an electromagnetic imaging calorimeter (CAL) made of silicon planes interleaved with tungsten absorbers (for a total of 16 rad. lengths) which gives both the particle interaction pattern inside its volume and the measurement of the energy released by the interacting electrons and positrons. This allows to distinguish electromagnetic from hadronic showers with a high level of confidence and efficiency; \\ 3. a threshold velocity measurement system based on a Transition Radiation Detector (TRD). It is made by carbon fiber radiators and straw tube detectors and its main task is to complement the calorimeter in the particle identification; \\ 4. a plastic scintillator hodoscope system (TOF) providing the triggers, the charge determination and a precise time measurement plus an anti-coincidence system to identify particles interacting in the mechanical structure of the telescope (ANTI). \\ \\ The total height of PAMELA is 120 cm, the mass is 410 kg, the power consumption is 345 W and the geometrical factor is 20.5 $cm^{2}$ sr. \\ \\ Several tests have been performed at CERN PS (t9) and SPS (X7) beams during years 1999 and 2000 on prototypes of the detectors. The complete Engineering Model Ð which has to be delivered by end of 2001 Ð will be tested at the same beams as well. Vibrational, thermal and radiation hardness tests have been performed at various facilities in Europe. The final version of the instrument fully qualified for the flight is scheduled to be delivered to Russian Space Agency by the end of June 2002.