by Virginia Greco. Published: 11 August 2017

An educational and outreach project conceived by the ALICE ITS team is now moving forward rapidly thanks to Matthew Aquilina, who has joined the collaboration as a summer student, and his supervisors Magnus Mager and Felix Reidt.


Magnus Mager (left) and Matthew Aquilina (right). In the center, a prototype of compact cosmic ray detector based on the ALPIDE chip. [Credits: Virginia Greco]


Would you like to have your own cheap and compact cosmic ray detector, sitting right on your desk? It sounds much like a nerdy fantasy, but indeed such a device can be realized and become a very useful educational and outreach tool.

This was the idea inspiring the ALICE ITS team at CERN, who decided to use the pixel sensor chip (ALPIDE) to build a small and easy-to-operate cosmic ray detector. The project is now taking off thanks to the involvement of Matthew Aquilina - a summer student from Malta who joined the group at the end of June - and his supervisors Magnus Mager and Felix Reidt.

The ALPIDE chip is a CMOS monolithic active pixel sensor being developed for the upgrade of the ITS of the ALICE experiment and characterized by very high detection efficiency.

Some spare ALPIDE chips could be diverted to this pedagogical project, in which they are used to detect muons and electrons from cosmic rays. By making a stack of up to four chips, connected one-to-one, it is possible to reconstruct the trajectory of a particle crossing them. Considering an average rate of one cosmic ray per square centimeter per minute, with its active area of 1.4cm x 3cm, the ALPIDE chip registers a hit every few seconds. Because of the acceptance limitation in terms of solid angle due to the setup, the reconstruction rate is around 1 cosmic ray track per minute.

The ALPIDE chip is very good for this application since it has very low noise,” explains Magnus. “In addition, it has a multiple-event buffer that allow acquiring new data while we are reading out the previous, so essentially it is dead-time free.

In order to target educational and outreach activities, a dedicated, cost-effective, and easy to use readout system was devised. It was decided to interface the chip to an Arduino microprocessor board, which is largely used for being very versatile and easy to program.

The setup of the compact cosmic ray detector, thus, includes an Arduino card and up to four boards hosting each an ALPIDE chip, one on top of the other. “Programming the Arduino microprocessor to communicate with the chips turned out to be fairly easy,” Magnus comments, “but we still needed an interface to allow people having no specific technical expertise to operate the system.

Here is when Matthew came in. His main task, in fact, is to develop a user-friendly interface to control the system, with the aim to make it ‘plug and play’. He is employing the Unity platform, which is free software meant for developing 3D games but can also be used to make interfaces with 3D objects and operation menus. In this specific case, the user will be able to see on the screen the four detector planes, the pixel detectors on them and, when a cosmic ray crosses their active area, the corresponding hit in each plane. The work is still in progress but is moving forward rapidly.

When I started, first of all I had to study the Arduino-ALPIDE communication protocol, which meant going through the 110-page ALPIDE manual,” Matthew explains; “during the second week, I interfaced the microprocessor with Unity and then I started developing the user-friendly interface”. Indeed, he was chosen by Magnus and his colleagues among many candidates for his previous experience with the Unity software, which he had gained by developing a 3D game with it.

A potential future development for the project is to allow data saving in exportable file formats to be read by other programs, so that some data analysis - such as angular distribution of the cosmic rays, day/night dependence and season dependence - could be performed.

Once the user-friendly interface is done, it will be time to ‘advertise’ the project and make the system available to teachers and students. Some channels to take into consideration are the CERN teacher programmes and the CERN S’Cool Lab. “This device can be useful both for computer science and physics classes,” adds Magnus, “because students can learn about cosmic rays and detectors as well as how to program Arduino to communicate with a custom chip.

It can also be used for outreach purposes in some special event, such as the CERN open days.

Matthew, on his side, is already profiting of this project, since he is enhancing his programming skills and is learning about physics and electronics. At the fourth year of his undergraduate engineering course at the University of Malta, Matthew applied to the CERN summer student programme attracted by the perspective of spending some time at CERN and because he was willing to have an experience outside his country.

I think I will continue my studies enrolling in a Master’s and a PhD programme, but I am not sure about the topic yet,” he declares. “Actually, at high-school I studied mainly chemistry and biology, then at the University I switched to engineering. I think I will continue with something that incorporates programming and electronics, such as robotics”.