ATHENA Experiment Research Articles

The ATHENA experiment for the study of antihydrogen

In 2002, the ATHENA experiment was the first to produce large amounts of antihydrogen atoms at the CERN Antiproton Decelerator (AD). In this review article, we collect and discuss all the relevant results of the experiment: antiproton and positron cooling and their recombination dynamics in the nested Penning trap, the methods used to unambiguously identify the antiatoms as well as the protonium background, the dependence of the antihydrogen formation on mixing time and temperature. An attempt to interpret the results in terms of the two-body and three-body formation reactions, taking into account the complicated nested-trap dynamics, is also made. The relevance of the ATHENA results on future experiments is discussed, together with a short overview of the current antimatter physics at the AD.

Simulation of the formation of antihydrogen in a nested Penning trap: effect of positron density

Detailed simulations of antihydrogen formation have been performed under the conditions of the ATHENA experiment, using several densities of the positron plasma in the range ne = 5 × 1013 m−3 to 1015 m−3. The simulations include only collisional effects, typically resulting in the formation of weakly bound antihydrogen via the three-body process, . (Radiative processes, which are much slower than collisional effects, are neglected.) The properties of these weakly bound anti-atoms are affected not only by further collisions in the plasma but also by the inherent electric fields. The role of field ionization in influencing the distribution of binding energies of the antihydrogen is clarified and the mechanism for this process in the strong B-field nested Penning trap used in the experiment is elucidated. The fate of antihydrogen is explained and the properties of the population detected after having reached the wall of the Penning trap electrodes, as well as those field ionized, are recorded. We find that the yield of detected antihydrogen varies with positron density roughly as n1.7e, rather than the n2e expected from the underlying formation process. As ne is increased, antihydrogen formation is sufficiently rapid that epithermal effects begin to play an important role. In general, the simulated timescales for antihydrogen formation are much shorter than those found from the experiment.

Protonium production in ATHENA

The ATHENA experiment at CERN, after producing cold antihydrogen atoms for the first time in 2002, has synthesised protonium atoms in vacuum at very low energies. Protonium, i.e. the antiproton–proton bound system, is of interest for testing fundamental physical theories. In the nested penning trap of the ATHENA apparatus protonium has been produced as result of a chemical reaction between an antiproton and the simplest matter molecule, H2+. The formed protonium atoms have kinetic energies in the range 40–700meV and are metastable with mean lifetimes of the order of 1μs. Our result shows that it will be possible to start measurements on protonium at low energy antiproton facilities, such as the AD at CERN or FLAIR at GSI.

Production of slow protonium in vacuum

We descrbe how protonium, the quasi-stable antiproton-proton bound system, has been synthesized following the interaction of antiprotons with the molecular ion \( {\rm H}_2^+\) in a nested Penning trap environment. From a careful analysis of the spatial distributions of antiproton annihilation events in the ATHENA experiment, evidence is presented for protonium production with sub-eV kinetic energies in states around n = 70, with low angular momenta. This work provides a new two-body system for studies using laser spectroscopic techniques.

Progress with cold antihydrogen

The creation of cold antihydrogen by the ATHENA and ATRAP collaborations, working at CERN’s unique Antiproton Decelerator (AD) facility, has ushered in a new era in atomic physics. This contribution will briefly review recent results from the ATHENA experiment. These include discussions of antiproton slowing down in a cold positron gas during antihydrogen formation, information derived on the dependence of the antihydrogen formation rate upon the temperature of the stored positron plasma and, finally, upon the spatial distribution of the emitted anti-atoms. We will discuss the implications of these studies for the major outstanding goal of trapping samples of antihydrogen for precise spectroscopic comparisons with hydrogen. The physics motivations for undertaking these challenging experiments will be briefly recalled.

Isobaric production cross sections from 0.6 GeV proton irradiation of neptunium and thorium using mass spectrometry

A high-fluence proton irradiation of neptunium was the last experiment in PSIs programme ATHENA related to accelerator-based transmutation. The principal aim of the programme has been to provide experimental data for the validation of theoretical models in nucleon–meson transport codes, with emphasis on the mass yield distribution of fission and spallation products. An improved mass spectrometry method has allowed the direct derivation of isobaric production cross sections with only minor corrections and an estimate of the fission cross section by integration in the fission hump. In a second sample position of the irradiation head, a repetition of the previous ATHENA experiment with thorium was possible, profiting from the improved mass spectrometry technique. The experimental results are better predicted by the code FUSSPOT than by HETC/RAL, both used at PSI.

The first cold antihydrogen

Antihydrogen, the atomic bound state of an antiproton and a positron, was produced at low energy for the first time by the ATHENA experiment, marking an important first step for precision studies of atomic antimatter. This paper describes the first production and some subsequent developments.

Real-time detector for plasma diagnostic in antimatter experiment

In the ATHENA experiment, which has recently produced and detected cold antihydrogen, the antiatoms formation is performed by mixing two cold (meV) charged clouds of positrons and antiprotons. The antihydrogen production is strictly dependent on positron plasma parameters. For this purpose we developed a new system to investigate such properties in a non-destructive way. The method is based upon the measurement of the plasma response under a frequency sweep RF excitation and its subsequent analysis. Plasmas trapped in Penning trap exhibit typical resonant collective modes characterized by frequencies, amplitudes and widths dependent on the particle number, density, spatial extent and temperature. With this system it is so possible to have a real-time monitor of the plasma during antihydrogen production.

Production and detection of cold antihydrogen atoms

The production and observation of cold antihydrogen atoms have been recently reported by the ATHENA experiment at the CERN Antiproton Decelerator. The antiatoms were produced by mixing low-energy antiprotons and positrons in an electromagnetic trap. The antihydrogen detection is based on the observation of a characteristic signature in the annihilation of the neutral antiatoms on the trap walls by means of an imaging particle detector. An overview of the apparatus is given and the results obtained are discussed.

First production and detection of cold antihydrogen atoms

The ATHENA experiment recently produced the first atoms of cold antihydrogen. This paper gives a brief review of how this was achieved.

Temperature dependence of anti-hydrogen production in the ATHENA experiment

The ATHENA experiment recently produced the first sample of cold anti-hydrogen atoms by mixing cold plasmas of anti-protons and positrons. The temperature of the positron plasma was increased by controlled RF heating and the anti-hydrogen production rate was measured. Preliminary results are presented.

Detection of antihydrogen annihilations with a cryogenic pure-CsI crystal detector

In the framework of the ATHENA experiment a combined tracking and photon detector, operating close to liquid nitrogen temperatures, was developed to determine simultaneously the vertex of an antiproton annihilation and energy and direction of photons stemming from a positron annihilation. The photon detector consists of 192 pure-CsI scintillation crystals (≈4cm3) coupled to 5×5mm2 avalanche photo-diodes, which are read by electronic chips realised in VLSI CMOS technology. These read out chips are also operational at liquid nitrogen temperature and feature a self-triggering capability.

A cryogenic silicon micro-strip and pure-CsI detector for detection of antihydrogen annihilations

Abstract A new kind of detector for the end products of antihydrogen annihilations, operating at liquid nitrogen temperatures, was designed, developed and built for the ATHENA experiment at CERN. Requirements to be met were low-temperature operation (80 K ) in a high magnetic field (4 T ) , compact size, low-power consumption (5 W ) and high granularity. The detector consists of two layers of each 16 double-sided silicon micro-strip modules ( 16 cm long), surrounded by 192 pure-CsI scintillation crystals (≈4 cm 3 ), read by avalanche photo diodes. The frontend electronic also working at 80 K is realised in VLSI CMOS technology and features a self-triggering capability.

Performance of ultra-thin silicon detectors in a [formula omitted] antiproton beam

The performance of ultra-thin silicon detectors (6, 55 and 67 μm ) was tested in order to provide beam position information, intensity monitoring and a fast trigger signal for the ATHENA experiment. The aim of this experiment is to produce cold antihydrogen using a low energetic ( 5 MeV ) bunched ( ≈500 ns ) antiproton beam from the CERN Antiproton Decelerator (AD). The detector has to be operated in vacuum at ≈10 K and inside a 3 T superconducting magnet. To avoid absorption in the low energetic beam it is imperative to use a thin detector, produced by special lapping and etching techniques. Several detectors were tested in one of the extraction lines of the AD. Additional tests were performed using a laser setup at 660 and 1060 nm to prove the functionality of the complete system. One detector was finally installed in the ATHENA experiment for the August 2000 run where currents of up to 1 A were observed in the detector with simple 50 Ω resistive readout. Results are presented from this detector showing beam profiles, intensities and time structure.

Producing Slow Antihydrogen for a Test of CPT Symmetry with ATHENA

The ATHENA experiment at the Antiproton Decelerator facility at CERN aims at testing CPT symmetry with antihydrogen. An overview of the experiment, together with preliminary results of development towards the production of slow antihydrogen are reported.