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The aim of the GBAR experiment is to measure the gravitational acceleration of antihydrogen by observing the free fall of ultracold anti-atoms. The experiment is installed at CERN’s Antiproton Decelerator/ELENA facility. Positrons are produced by a low energy (9 MeV) linear electron accelerator and captured in a modified Surko (buffer gas) trap. We have recently implemented a silicon carbide-based trapping scheme that replaces the routinely used nitrogen gas with a high quality silicon carbide single crystal in the first phase of the trap. The new setup has been providing stable and efficient positron trapping for more than a year. After a short accumulation in the buffer gas trap, the particles are transported to a high-field (5 T) Penning-Malmberg trap, where a high number of pulses can be collected in a deep potential well. We discuss the performance of the improved positron line and the present status of the experiment.

GBAR is a project aiming at measuring the free-fall acceleration of gravity for antimatter, namely antihydrogen atoms ( H ¯ ). The precision of this timing experiment depends crucially on the dispersion of initial vertical velocities of the atoms as well as on the reliable control of their distribution. We propose to use a new method for shaping the distribution of the vertical velocities of H ¯ , which improves these factors simultaneously. The method is based on quantum reflection of elastically and specularly bouncing H ¯ with small initial vertical velocity on a bottom mirror disk, and absorption of atoms with large initial vertical velocities on a top rough disk. We estimate statistical and systematic uncertainties, and we show that the accuracy for measuring the free fall acceleration g ¯ of H ¯ could be pushed below 10 - 3 under realistic experimental conditions.

Different experiments are ongoing to measure the effect of gravity on cold neutral antimatter atoms such as positronium, muonium, and antihydrogen. Among those, the project GBAR at CERN aims to measure precisely the gravitational fall of ultracold antihydrogen atoms. In the ultracold regime, the interaction of antihydrogen atoms with a surface is governed by the phenomenon of quantum reflection which results in bouncing of antihydrogen atoms on matter surfaces. This allows the application of a filtering scheme to increase the precision of the free fall measurement. In the ultimate limit of smallest vertical velocities, antihydrogen atoms are settled in gravitational quantum states in close analogy to ultracold neutrons (UCNs). Positronium is another neutral system involving antimatter for which free fall under gravity is currently being investigated at UCL. Building on the experimental techniques under development for the free fall measurement, gravitational quantum states could also be observed in positronium. In this contribution, we report on the status of the ongoing experiments and discuss the prospects of observing gravitational quantum states of antimatter and their implications.

We have studied the properties of a commercially available 4H-SiC epitaxial layer and evaluated its potential application as an efficient positron remoderator. A remoderation efficiency of more than 65% has been measured for incident positrons with 1 keV energy. We have determined the work function and the energy distribution of the emitted slow positrons, a property which is essential for practical applications. Comparison of the positron moderation properties of the epitaxial layer with results from a n-type 4H-SiC single crystal, indicate that the epitaxially grown layer is a superior secondary moderator than its substrate counterpart.

A new slow positron beamline featuring a large acceptance positronium lifetime spectrometer has been constructed and tested at the linac-based slow positron source at IRFU CEA Saclay, France. The new instrument will be used in the development of a dense positronium target cloud for the GBAR experiment. The GBAR project aims at precise measurement of the gravitational acceleration of antihydrogen in the gravitational field of the Earth. Beyond application in fundamental science, the positron spectrometer will be used in materials research, for testing thin porous films and layers by means of positronium annihilation. The slow positron beamline is being used as a test bench to develop further instrumentation for positron annihilation spectroscopy (Ps time-of-flight, pulsed positron beam). The positron source is built on a low energy linear electron accelerator (linac). The 4.3 MeV electron energy used is well below the photoneutron threshold, making the source a genuine on-off device, without remaining radioactivity. The spectrometer features large BGO (Bismuth Germanate) scintillator detectors, with sufficiently large acceptance to detect all ortho-positronium annihilation lifetime components (annihilation in vacuum and in nanopores).

The aim of the recently approved GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment is to measure the acceleration of neutral antihydrogen atoms in the gravitational field of the Earth. The experimental scheme requires a high density positronium cloud as a target for antiprotons, provided by the Antiproton Decelerator (AD) – Extra Low Energy Antiproton Ring (ELENA) facility at CERN. We introduce briefly the experimental scheme and present the ongoing efforts at IRFU CEA Saclay to develop the positron source and the positron-positronium converter, which are key parts of the experiment. We have constructed a slow positron source in Saclay, based on a low energy (4.3 MeV) linear electron accelerator (linac). By using an electron target made of tungsten and a stack of thin W meshes as positron moderator, we reached a slow positron intensity that is comparable with that of 22Na-based sources using a solid neon moderator. The source feeds positrons into a high field (5 T) Penning-Malmberg trap. Intense positron pulses from the trap will be converted to slow ortho-positronium (o-Ps) by a converter structure. Mesoporous silica films appear to date to be the best candidates as converter material. We discuss our studies to find the optimal pore configuration for the positron-positronium converter.

Low energy positron beams are of major interest for fundamental science and materials science. IRFU has developed and built a slow positron source based on a compact, low energy (4.3 MeV) electron linac. The linac-based source will provide positrons for a magnetic storage trap and represents the first step of the GBAR experiment (Gravitational Behavior of Antimatter in Rest) recently approved by CERN for an installation in the Antiproton Decelerator hall. The installation built in Saclay will be described with its main characteristics. The ultimate target of the GBAR experiment will be briefly presented as well as the foreseen development of an industrial positron source dedicated for materials science laboratories.

The Gravitational Behaviour of Antihydrogen at Rest experiment - GBAR - is designed to perform a direct measurement of the weak equivalence principle on antimatter by measuring the acceleration ( ) of antihydrogen atoms in free fall. Its originality is to produce + ions and use sympathetic cooling to minimize the initial velocity. These ions are produced using charge exchange reactions with a dense positronium cloud, created by an intense pulse of electron-linac-produced positrons that are accumulated in a Penning-Malmberg trap.

We have installed in Saclay a facility for an intense positron source in November 2008. It is based on a compact 5.5 MeV electron linac connected to a reaction chamber with a tungsten target inside to produce positrons via pair production. The expected production rate for fast positrons is 5·1011 per second. The study of moderation of fast positrons and the construction of a slow positron trap are underway. In parallel, we have investigated an efficient positron-positronium convertor using porous silica materials. These studies are parts of a project to produce positively charged antihydrogen ions aiming to demonstrate the feasibility of a free fall antigravity measurement of neutral antihydrogen.

We report on the first production of an antihydrogen beam by charge exchange of 6.1 keV antiprotons with a cloud of positronium in the GBAR experiment at CERN. The 100 keV antiproton beam delivered by the AD/ELENA facility was further decelerated with a pulsed drift tube. A 9 MeV electron beam from a linear accelerator produced a low energy positron beam. The positrons were accumulated in a set of two Penning–Malmberg traps. The positronium target cloud resulted from the conversion of the positrons extracted from the traps. The antiproton beam was steered onto this positronium cloud to produce the antiatoms. We observe an excess over background indicating antihydrogen production with a significance of 3–4 standard deviations.