C. Conner

A solenoidal spectrometer for positron-electron pairs produced in heavy-ion collisions

Abstract A new solenoidal spectrometer, designed to study the production mechanism of electrons and positrons in heavy-ion collisions, has been constructed at Argonne National Laboratory. The spectrometer uses a 300 G magnetic field to transport the leptons to two highly segmented silicon arrays that are centered on the solenoid axis, 1.2 m from the target. Positrons are identified by detecting their annihilation radiation with two arrays of position sensitive NaI(Tl) crystals that surround the silicon arrays. A novel design feature of the spectrometer is the ability to measure the angles of emission of the leptons relative to the solenoid axis. The measured response of the apparatus to electrons and positrons is in very good agreement with the calculated response obtained from Monte Carlo simulations.

Apex and the e+/e− Puzzle: Recent Results

Narrow structures first reported in positron singles energy spectra [1, 2, 3, 4, 5, 6, 7] and later in electron-positron energy distributions [5, 8, 9, 10, 11, 12] associated with collisions of very heavy ions near the Coulomb barrier have been an outstanding puzzle in nuclear physics for nearly fifteen years. In a series of positron and positron/electron experiments by three different groups (EPOS, ORANGE and TORI) ([5] and citations therein) at the GSI UNILAC at Darmstadt, Germany narrow structures (‘lines’) were observed by EPOS and ORANGE for several heavy-ion collision systems, with some features such as the line energies and widths having apparently similar values for the different collision systems.