Real-time antiproton annihilation vertexing with submicrometer resolution

Abstract

Primary goal of the AEḡIS experiment is to precisely measure the free fall of antihydrogen within Earth's gravitational field. To this end, cold (≈50 K) antihydrogen will traverse a two-grid moiré deflectometer before annihilating onto a position-sensitive detector, which shall determine the vertical position of the annihilation vertex relative to the grids with micrometric accuracy. Here, we introduce a vertexing detector based on a modified mobile camera sensor and experimentally demonstrate that it can measure the position of antiproton annihilations within [Formula: see text] μm, a 35-fold improvement over the previous state of the art for real-time antiproton vertexing. These methods are directly applicable to antihydrogen. Moreover, the sensitivity to light of the sensor enables in situ calibration of the moiré deflectometer, substantially reducing systematic errors. This sensor emerges as a breakthrough technology toward the AEḡIS scientific goals and will constitute the basis for the development of a large-area detector for conducting antihydrogen gravity measurements.

Fig. 1.. Curated selection of antiproton annihilation events as imaged by the CMOS sensor.

The observed shapes are similar to those recorded by the Timepix3 detector (25, 26), albeit at a scale approximately 50 times smaller. Pixels marked in magenta have been deemed non-functional by taking background images. Green arrows indicate examples of ellipse-shaped prongs, cyan arrows indicate examples of thick tracks, and orange arrows examples of thin tracks.

Fig. 2.. Distribution of the widths of recorded tracks.

In blue the distribution in thickness of the tracks in recorded antiprotons events. We attribute the higher-thickness peak of the distribution to tracks left in the sensor by protons and the lower peak to tracks left by pions. In red the distribution in thickness of tracks left by 4-MeV protons within the detector, matching the topmost part of the distribution in thickness of proton tracks. We attribute the difference between the distributions mainly to the difference in energy spectrum of the protons.

Fig. 3.. Reference grid used to determine the sensor resolution.

(A) Optical image of the silicon nitride grid acquired using a point light source before installing the setup in vacuum. (B) A similar image obtained by exploiting the light emitted by a vacuum gauge in the apparatus. Arrows indicate the two grid teeth that were bent by the adhesive deforming under vacuum. (C) The reconstructed position of antiproton annihilations recorded with the grid installed onto the sensor. Toward the left side two distinct areas, nearly devoid of annihilations events can be seen. These are the regions where the thermoplastic adhesive keeping the grid in place was applied. (D) A histogram of the distance of the annihilations events from the nearest edge. Overlayed in orange is the best fitting error function found via maximum likelihood.

Fig. 4.. Precision and accuracy of the grid fitting method.

(Left) Relative drift of the endpoints of the 50 grid teeth as determined by the fitting algorithm described in the “Grid fitting algorithms” section over the course of 1 hour. One of the edges has been highlighted in red to better show the typical progression of the algorithm output over time. The fluctuations in the so reconstructed coordinates are below 0.12-μm root mean square (RMS) for all the grid vertices, with median being 0.031-μm RMS. (Right) Relative position of one grid vertex tracked over the course of the 8-day measurement campaign. The linear fit used to compensate for the grid movement in the data analysis is shown in orange.

Fig. 5.. Detail of the beamline of AEḡIS, showing the injection line from ELENA and part of the trap complex installed inside the 5-T trap.

Individual antiproton bunches from ELENA pass through a degrader and are caught by the AEḡIS C trap (red arrows). Electrode voltages in the beamline are then reconfigured to allow extraction toward the 45° offshoot, after which the trap is opened and the antiprotons implanted into the sensor (green arrows).