Annihilation in a smartphone: A phone matrix as a particle detector for CERN

A modified CMOS photosensitive matrix, an element of a smartphone camera, works great as a particle detector, e.g. for detecting antimatter annihilation. This is good news - particle detectors can be smaller, more accurate and cheaper - showed a team from CERN with the participation of Poles.

The CERN AEgIS team, which also includes Polish physicists (cern-aegis.pl), was looking for a way to more precisely than before determine the place where annihilation occurred – the collision of matter and antimatter. The researchers noticed that a slightly modified photosensitive CMOS matrix used in smartphones is great for detecting such processes. It is more precise and cheaper than traditional particle detectors, so why not use it?

Scientists used a commercial CMOS chip, which is a photosensitive element in a smartphone camera. This chip has a much better resolution than the silicon detectors typically used in large experiments. It turns out that in order for it to effectively collect information about particles, it only needs a little modification. Then, a single matrix was created from several dozen such elements, creating a large detector. The results were published in "Science Advances".

"The AEgIS research team has built a unique detector that is thousands of times cheaper than silicon detectors manufactured specifically for such research. In addition, our detector has a much better resolution and allows for the analysis of particle collisions live," explained Prof. Mariusz Piwiński from the Institute of Physics of the Nicolaus Copernicus University, a member of the AEgIS team, in an interview with PAP.

In professional silicon detectors, a high-energy particle passing through a layer of a semiconductor - silicon, leaves a trace in its electronic structure, similar to how an airplane leaves a trace in the sky. By analyzing such a trace, it is possible to determine where the particles collided and what kind of particles they were. At CERN, huge databases have been created of what traces in silicon left by different particles look like.

It turns out that silicon-based smartphone matrices are also able to register when a high-energy particle passes through them. And the data about this trace is recorded in real time. In addition, the traces that the particles leave there are the same as in previous detectors. In order to use the capabilities of "smartphone" detectors, there is no need to build a new database of traces.

In the detectors used so far, a single square pixel is about 30 micrometers in size. In the CMOS matrix, such a single pixel is smaller than 1 micrometer. It will therefore be possible to record events with even greater precision.

"To have better and better cameras in smartphones, better and better matrices are needed. There is already a well-known technology for producing photosensitive systems that have tiny pixels, are based on silicon, and are also miniature because they fit in the phone's casing," said Prof. Mariusz Piwiński from the Nicolaus Copernicus University.

He explained that smartphone matrices are not manufactured with particle annihilation in mind, but they can be prepared to do so. He explained that elements that serve as microlenses are sputtered onto the smartphone matrix. Scientists had to remove these elements to expose the photosensitive element.

The matrix size is small: 3.7 mm by 2.8 mm, but if several dozen such elements are placed next to each other, they can be used to build a matrix that will cover a suitably large surface.

Scientists in the AEgIS experiment are investigating how gravity affects antimatter.

Antimatter particles can be treated as "mirror images" of matter particles. For example, an electron is a particle with a negative charge, and its corresponding antimatter particle, a positron, has an identical mass, spin, but a positive charge. When these two objects meet, annihilation occurs: the conversion of their mass into energy in the form of high-energy photons (according to Einstein's famous formula E=mc2).

In the AEgIS project, scientists are investigating whether the acceleration with which antihydrogen atoms fall to Earth - a planet made of ordinary matter - is exactly the same as the acceleration of gravity for hydrogen atoms. Our current state of knowledge and technology already allows us to create and study antihydrogen in laboratory conditions.

Does hydrogen fall to Earth? Yes, but in Earth's air conditions, hydrogen molecules are pushed upward by heavier nitrogen and oxygen molecules.

Although a balloon (or airship) filled with hydrogen rises upwards, it does not escape Earth's orbit. And in a vacuum chamber, when there is no buoyancy effect, hydrogen atoms can be observed to actually fall to Earth. Scientists want to investigate whether antihydrogen atoms behave in exactly the same way. The observed possible differences can help answer many important questions about the structure and occurrence of matter in the universe.

In a vacuum, atoms move along paths that can be precisely described. Thus, an antihydrogen atom released horizontally in a laboratory vacuum chamber - if it behaves like its material equivalent - over a distance of several meters should, due to gravity, fall by several dozen micrometers. Of course, it ultimately depends on its flight time, i.e. the speed at which it will move. For us, this is next to nothing, but for scientists - a lot. They are able to register this using precise detectors. And it is precisely such detectors with even better resolution that are needed to check this.

Prof. Mariusz Piwiński explained that for now, CMOS matrices have been used to detect the site of antiproton annihilation, but he hopes that the idea will be taken up in other experiments to detect the annihilation of other antimatter particles.

This could lead to smaller, cheaper, more accurate particle detectors. Why are the particle detectors used at CERN so huge? In experiments of this type, detectors are needed that collect information about the place where the annihilation occurred. And the resolution of silicon detectors is limited. In order to fit a lot of pixels around the collision site, the matrix must be moved away from the center. If the pixels are smaller (as in CMOS matrices), they can be moved closer to the collision site.

The AEgIS CERN project involves Polish scientists from the Warsaw University of Technology, the Institute of Physics of the Polish Academy of Sciences, Nicolaus Copernicus University in Toruń and the Jagiellonian University.

Ludwik Tomal (PAP)

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