Abstract
Investigating the antiproton cross section on nuclei at low energies (1 eV - 1 MeV) is of great interest for fundamental cosmology and nuclear physics as well. The process is of great relevance for the models which try to explain the matter/antimatter asymmetry in the universe assuming the existence of the so-called island where antinucleon-nucleon annihilations occur in the border region (1). For the nuclear physics point of view, the annihilation process is considered a useful tool to evaluate the neu- tron/proton ratio probing the external region of the nucleus. Moreover, the cross section measured at LEAR in the 80s-90s showed an unexpected behaviour for energies below 1 MeV. The results showed a saturation with the atomic mass number against the A 2=3 trend which is known for higher energies. The ASACUSA collaboration at CERN measured 5.3 MeV antiproton annihilation cross section on di erent nuclei whose results demonstrated to be consistent with the black- disk model with the Coulomb correction (2). So far, experimental limits prevented the data acquisition for energies below 1 MeV. In 2012 the 100 keV region has been investi- gated for the first time (3). We present here the results of the experiment.
Highlights
At the present days the only existing source of low energy antiprotons is the AD (Antiproton Decelerator), the successor of LEAR at CERN, which is working since 1999
For each target the annihilation cross section is obtained by counting the annihilations in the target and the ones on the lateral wall due to the antiprotons scattered from the target foil
The annihilation cross section of antiprotons on different nuclei has been measured at low energies
Summary
At the present days the only existing source of low energy antiprotons is the AD (Antiproton Decelerator), the successor of LEAR at CERN, which is working since 1999 (see a review article in [4]). A pulsed beam is delivered to the experimental lines with a momentum 100 MeV/c and the main extraction cycle is about 1 spill/minute, being a single spill a ∼200 ns bunch of about 3×107 ps Antiprotons delivered by the AD at 5.3 MeV are addressed to a thin target foil in a region surrounded by layers of scintillating fibers which track the charged pions emitted when an antiproton annihilates This permits the spatial position and time of each annihilation to be measured [5,6,7]. The pbar annihilation cross section on medium and heavy nuclei (Mylar, Ni, Sn and Pt) has been measured for the first time in this energy range [2] and the results (see Fig. 1) are in agreement with the black disk model with the contribution of the Coulomb interaction between the antiproton and the nucleus at low energy [8]
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