Antiproton Annihilation Cross Sections Research Articles

Measurement of the antiproton–nucleus annihilation cross-section at low energy

Systematic measurements of the annihilation cross sections of low energy antinucleons were performed at CERN in the 80's and 90's. However the antiproton data on medium-heavy and heavy nuclear targets are scarce. The ASACUSA Collaboration at CERN has measured the antiproton annihilation cross section on carbon at 5.3 MeV: the value is (1.73 ± 0.25) barn. The result is compared with the antineutron experimental data and with the theoretical previsions.

First measurement of the antiproton-nucleus annihilation cross section at 125 keV

The first observation of in-flight antiproton-nucleus annihilation at ∼130 keV obtained with the ASACUSA detector has demonstrated that the measurement of the cross section of the process is feasible at such extremely low energies Aghai-Khozani, H., et al., Eur. Phys. J. Plus 127, 55 (2012). Here we present the results of the data analysis with the evaluations of the antiproton annihilation cross sections on carbon, palladium and platinum targets at ∼125 keV.

Experimental investigation of ≈130 keV kinetic energy antiprotons annihilation on nuclei

The study of the antiproton (\(\bar {p}\)) annihilation cross section on nuclei at low energies (eV-MeV region) has implications for fundamental cosmology as well as for nuclear physics. Concerning the former, different models try to explain the matter/antimatter asymmetry in the universe assuming the existence of the so-called “islands” where antinucleon-nucleon annihilations occur in the border region (Cohen et al. Astrophys. J. 495, 539–549, 1998), while, from the nuclear physics point of view, the annihilation process is a valuable tool to evaluate the neutron/proton ratio in order to probe the external region of the nucleus (Gupta et al. Nucl. Phys. B 70(3), 414–424, 1974). The existing data of antinucleon-nucleon (or -nucleus) annihilation cross-sections are mainly confined to energies above ≈1 MeV, while the cross section measured at LEAR in the 80’s-90’s (mostly with light targets Agnello et al. Phys. Lett. B 256, 349–353, 1991; Bertin et al. Phys. Lett. B 369, 77–85, 1996; Bertin et al. Phys. Lett. B 414, 220–228, 1997; Zenoni et al. Phys. Lett. B 461, 405–412, 1999; Bianconi et al. Phys. Lett. B 481, 194–198, 2000; Bianconi et al. Phys. Lett. B 492, 254–258, 2000) showed an unexpected behaviour for energies below 1 MeV (Bianconi et al. Phys. Lett. B 483, 353–359, 2000; Bianconi et al. Phys. Rev. C 62, 014611-7, 2000; Batty et al. Nucl. Phys. A 689, 721–740, 2001). The results showed a saturation with the atomic mass number against the A2/3 trend which is observed for higher energies (being A the target mass number). The ASACUSA collaboration at CERN recently measured antiproton annihilation cross section on different kinds of nuclei with a \(\bar {p}\) kinetic energy of 5.3 MeV (Bianconi et al. Phys. Lett. B 704, 461–466, 2011; Corradini et al. Nucl. Instr. Methods A 711, 12–20, 2013). Such results proved compatibility with the black-disk model with the Coulomb correction. But till now experimental difficulties prevented the investigation at energies below ≈1 MeV. In 2012, the 100 keV region has been investigated for the first time (Aghai-Khozani et al. Eur. Phys. J. Plus 127, 125–128, 2012). We present here the first preliminary results of this experiment.

Beam Diagnostics for Measurements of Antiproton Annihilation Cross Sections at Ultra-low Energy

The ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antipro- tons) collaboration of CERN is currently attempting to measure the antiproton-nucleus in-flight annihilation cross sections on thin target foils of C, Pd, and Pt at 130 keV of kinetic energy. The low-energy antiprotons were supplied by the Antiproton Decelera- tor (AD) and a radio-frequency quadrupole decelerator. For this measurement, a beam profile monitor based on secondary electron emission was developed. Data from this monitor was used to ensure that antiprotons were precisely tuned to the position of an 80- mm-diameter experimental target, by measuring the spatial profile of 200-ns-long beam pulses containing 10 5 10 6 antiprotons with an active area of 40 mm 40 mm and a spa- tial resolution of 4 mm. By using this monitor, we succeeded in finely tuning antiproton beams on the target, and observed some annihilation events originating from the target.

Experimental results on antiproton–nuclei annihilation cross section at very low energies

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.

Development of Beam Profile Monitor for Antiproton Annihilation Cross Section Measurements by the ASACUSA Collaboration

The ASACUSA (the Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration has developed various kinds of monitors which both destructively and non-destructively measure the spatial profiles of pulsed antiproton beams at the Antiproton Decelerator (AD) of CERN. These include detectors consisting of foil and wire electrodes based on gas ionization or secondary electron emission, which are compatible with ultra-high vacuum conditions. A new beam profile monitor with a grid of electrode pads fabricated on a normal glass-epoxy circuitboard was adapted for the measurement of antiproton annihilation cross sections. This monitor has an active area of 40 mm × 40 mm and a spatial resolution of 4 mm. It can measure the spatial profile of 40-50-ns-long beam pulses containing > 104 antiprotons striking each anode strip.

Measurement of the antiproton–nucleus annihilation cross section at 5.3 MeV

Antiproton annihilation cross sections on medium-heavy and heavy nuclear targets are measured for the first time at 5.3 MeV kinetic energy at the Antiprotons Decelerator facility of CERN. The results agree with the expected behavior from the black-disk model with the Coulomb corrections.

Saturation of low-energy antiproton annihilation on nuclei

Recent measurements of very low-energy ( p L <100 MeV/ c) p̄ annihilation on light nuclei reveal apparent suppression of annihilation upon increasing the atomic charge Z and mass number A. Using p̄-nucleus optical potentials V opt, fitted to p̄-atom energy-shifts and -widths, we resolve this suppression as due to the strong effective repulsion produced by the very absorptive V opt. The low-energy p̄-nucleus wavefunction is kept substantially outside the nuclear surface and the resulting reaction cross section saturates as function of the strength of Im V opt . This feature, for E>0, parallels the recent prediction, for E<0, that the level widths of p̄ atoms saturate and, hence, that p̄ deeply bound atomic states are relatively narrow. Antiproton annihilation cross sections are calculated at p L =57 MeV/ c across the periodic table, and their dependence on Z and A is classified and discussed with respect to the Coulomb focussing effect at very low energies.

Coulomb corrections to low energy antiproton annihilation cross sections on protons and nuclei

We calculate, in a systematic way, the enhancement effect on $\overline{p}p$ and $\overline{p}A$ annihilation cross sections at low energy due to the initial state electrostatic interaction between the projectile and the target nucleus. This calculation is aimed at future comparisons between $\mathrm{n\ifmmode \bar{}\else \={}\fi{}}$ and $\overline{p}$ annihilation rates on different targets for the extraction of pure isospin channels.

Antiproton annihilation on nuclei at intermediate energies resulting from one, two and an infinite number of collisions

Antiproton annihilation cross sections on nuclei from 20Ne to 208Pb are calculated in the framework of a microscopic optical model in the kinetic energy range EL = 50 – 2000 MeV both directly in the initial channel and after single quasi-free collision. The part of annihilation yield after one quasi-free nucleon knock-out from the nucleus is 20-25% of the contribution to the annihilation cross section in the initial channel. The sensitivity of this cross section to the parameters of antinucleon-nucleon scattering amplitude is investigated. Calculated ratios of volume to surface annihilation cross sections are compared with experimental ones.