CMOS Camera Counts Antiprotons for GBAR Experiment
Researchers have used a commercial CMOS camera to accurately count antiprotons in the GBAR antihydrogen experiment, achieving a 10% error rate by detecting
A commercial CMOS digital camera is now counting antiprotons for an antihydrogen experiment. Researchers from the GBAR collaboration at CERN's AD/ELENA facility have repurposed the device to monitor the number of keV antiprotons incident on their apparatus, a critical measurement for antihydrogen production.
This method tackles a persistent problem. Knowing the exact number of incoming antiprotons is essential but can be difficult. The team mounted the camera around the experimental vacuum chamber. It images the ionising particles created when antiprotons annihilate on the surface of microchannel plate detectors used for beam imaging.
How the CMOS Detector Works
The detector capitalises on the properties of annihilation products. When an antiproton annihilates with a nucleon at rest, it produces a shower of charged particles. The team's analysis shows the multiplicity of these emerging particles matches expectations, after accounting for the surrounding material. Most of these particles are in the minimal ionising regime.
They are detected with nearly 100% efficiency by the CMOS pixel sensor. The device's thin depletion layer makes it insensitive to background gamma radiation. This selectivity is crucial in a noisy experimental environment.
Reconstruction in a Dense Environment
The system's high granularity and small pixel size enable detailed tracking. Millions of antiproton annihilations can be reconstructed even when tracks are densely packed. The method works over a large dynamic range and offers good resolution.
Cluster length studies provide additional diagnostic data. By analysing non-perpendicular tracks, researchers estimated the thickness of the sensor's depletion zone and its effective detection area. The cluster length also allows for continuous monitoring of particle track angles.
Determining Antiproton Numbers
The actual antiproton count is derived from the number of reconstructed clusters in the CMOS sensor. This is done by calculating the covered solid angle relative to calibration measurements performed with well-known beam intensities at the most upstream location of the GBAR apparatus.
Material effects on the emerging annihilation products were estimated using Monte Carlo calculations with Geant4. A key advantage of this approach is that it cancels out annihilation artefacts on the complex surface of the microchannel plate.
Minimising Systematic Uncertainty
The researchers state this technique largely minimises systematic uncertainties. The lead author notes, "This method minimises largely systematic uncertainties." The result is a final error of roughly 10% for the reconstruction of absolute antiproton numbers. This level of precision supports the GBAR experiment's goal of precisely producing and studying antihydrogen atoms.
