Anti-ion – ANTION

An intense positron (anti-electron) source is required for positronium production. We will make use of a novel positron source, developed at IRFU CEA/Saclay, based on a compact electron accelerator (linac). The source is compact enough to be implemented at the experimental hall of the Antiproton Decelerator facility at CERN. Positrons will be slowed down and an unprecedented number of them will be accumulated in a high field Penning-Malmberg trap. Positronium will be generated when the accumulated positrons are implanted in a short pulse into a special conversion target. The conversion target, a thin mesoporous silica film, has been developed and optimized for efficiency, positronium energy and radiation hardness by the collaboration. Excitation of the positronium to 3D state will be made in a Dopper-free two-photon reaction using a laser system developed by a group of LKB (Laboratoire Kastler Brossel). The 2P state will be reached by a single-photon excitation using a frequency comb. A new decelerator ring at AD (ELENA) will provide antiprotons for the experiment. A pulsed decelerator, developed by the partner group at CSNSM (Centre de Sciences Nucléaires et de Sciences de la Matière, Orsay), slows further antiprotons to a few kiloelectronvolt kinetic energy. For the first phase of the experiment, a protons source will be provided by CSNSM. For the detection of neutral atoms at kiloelectronvolt energy a multichannel plate detector will be used with optical readout. The theory group (IPCMS Strasbourg) works on the calculation of cross section and on the prediction of the optimal energy and configuration of the reaction zone. Experimental results will be compared with theoretical predictions.

The linac-based positron source at IRFU is operational, we are working on optimization of the positron trapping, slowing down and accumulation process. At CERN, a new, more powerful linac is being contructed for the second part of the project.

The antihydrogen produced by the technology developed and refined by the ANTION project will serve the GBAR experiment. The charged anti-ion will be cooled down to about 10 microKelvin temperature using the most sophisticated laser coolig technology. After removal of one of the two positrons from the ion by photodetachment, gravitational acceleration of the extremely cold neutral atom will be measured. Beyond GBAR (Gravitational Behaviour of Antihydrogen at Rest), technology to create and handle trapped anti-ions opens the way to further precision measurements. Measurement of the reaction cross sections gives the opportunity to compare theoretical models of the relevant three-body and four-body reactions with experimental results.

We reported on the advancement of the project in several invited and regular oral presentations in international conferences. ANTION is a major contribution to the GBAR project.

We propose the production and study, for the first time, of an antimatter ion, the antihydrogen ion, under controlled circumstances in the laboratory. It will be produced in two steps. The first one is the charge exchange reaction pbar + Ps*->Hbar + e- (1), where pbar stands for antiproton, Ps* for positronium (bound positron-electron pair) either in ground state or in 2p or 3d excited state, Hbar for antihydrogen and e- for electron. The second reaction is Hbar + Ps* -> Hbar+ + e- (2), in which the antihydrogen produced in the first reaction interacts with another positronium to create an antihydrogen ion (Hbar+), composed of one antiproton and two positrons.
100 keV antiprotons from the new ELENA facility at the Antiproton Decelerator at CERN will be slowed down to 1-10 keV by a switched decelerator device, that has been developed and is being tested by a partner in the collaboration (CSNSM). The antiprotons will be focused on a dense positronium cloud, where both reactions (1) and (2) will take place. To produce the target cloud, we will use an intense source of positronium that we are developing for the CERN GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment, which is in the final test phase. A low energy linac-based slow positron generator loads a high-field positron trap. An intense positron pulse is then ejected from the trap and implanted on a converter target made of porous silica. The converter has been developed by a member of the collaboration (IRFU). In order to increase the effective density of the target, positronium will be produced in a cavity with reflective walls, at low kinetic energy. The positronium will be excited by a pulsed laser beam to its 3d and 2p levels to enhance antihydrogen (ion) production. 3d excitation will be done with an existing laser setup (developed by LKB), using a Doppler-free two-photon reaction induced by two 410 nm laser pulses. For the 2p excitation a 243 nm pulsed laser will be built with the support of the present project.
In a first step, before working with antiprotons, we will measure the production cross section of normal hydrogen via the charge exchange reaction between protons and excited positronium: p+ + Ps* -> H* + e+ (3), and the subsequent four-body reaction H + Ps* -> H- + e+ (4). In this experiment a proton source, provided by CSNSM, will be used. Reaction (3) will be studied at the existing slow positron source at IRFU. Reactions (4), (1) and (2) will be investigated at CERN with higher positron intensity and the antiproton beamtime made available by CERN for the GBAR collaboration. The cross sections determined by the experiment will be compared with the theoretical predictions carried out in the framework of the ANTION consortium (IPCMS).
Of the four reactions, only the cross section of (3) has been studied experimentally, using exclusively ground state positronium. The results of the experiment will be an important step towards the gravitational measurement of antihydrogen atoms (GBAR project). In GBAR the anti-ions will be cooled down to the ten microKelvin temperature range, to provide, after stripping the extra positron, sufficiently slow atoms for a free fall measurement with antihydrogen. The apparatus developed in the framework of the present project is an essential part of the GBAR instrumentation. Availability of anti-ions will also open the way to further precision measurements on antimatter, as charged particles can be trapped and handled much more easily than neutral atoms.