With great sadness, we share the news of the passing of our dear friend and colleague, Berrie Giebels. Berrie left us at the age of 52 on the 20th of November 2023.

Berrie undertook his PhD from 1995 to 1998 in Bordeaux on the Cherenkov experiment CELESTE. After a year at Collège de France and three years at SLAC, he joined the gamma-ray astronomy group at Laboratoire Leprince Ringuet (LLR) to work on H.E.S.S. and Fermi-LAT. At LLR, Berrie led the development of the mechanical structure of the LAT calorimeter, and worked on the early beam tests of the calorimeter at SLAC and CERN. His astrophysical interests converged around Active Galactic Nucleus (AGN) physics, and he became an expert on AGN variability. He was an enthusiastic advocate of “GeV-TeV” astronomy, and pushed hard to improve the connection between the Fermi satellite and the TeV ground-based instruments. Berrie was instrumental in the development of the NectarCAM at LLR, the Medium-Sized Telescopes’ camera for the CTAO-North, assembling the team and securing the financing needed to develop the mechanics of the first camera at LLR. In later years, Berrie became the scientific director for astroparticle physics and cosmology at IN2P3/CNRS, where he was an unwavering supporter of the CTAO. He played a crucial role in getting national funding for the project in France. He served as CNRS Delegate in the CTAO Council, and was Deputy Director of IN2P3 from 2021 until his death.

Berrie was always eager to share his enthusiasm for and knowledge of astronomy and physics, seeking out opportunities to work with undergraduate and master’s students at École Polytechnique. He cared and took the time to organize enriching educational experience: on one occasion, he led a project to develop a cloud chamber at LLR, pushing the undergraduate students to think seriously about the project, treating it as if it were to be launched into space. He accomplished this by having them defend their design to a product assurance engineer and by engaging in extensive GEANT4 simulations. Berrie was the PhD advisor of a number of current CTAC members, such as David Sanchez and Jonathan Biteau. He was a mentor and friend to many junior physicists at LLR, including ourselves.

Berrie was highly regarded and respected by his colleagues for his extensive expertise. He was generous in the sharing of his knowledge and skills. In addition to the qualities that made him an outstanding scientist, Berrie was also a great friend to many. His kind and gentle nature, combined with his wonderful sense of humour and his sincerity, made people feel at ease around him. This contributed to his many friendships and his success in leading and managing teams of scientists. We will always remember Berrie for his kindness, his sense of humour, and his enthusiasm, aptitude and passion for science. We will remember him for the smaller things too, such as his fondness of the Dutch Goose bar in Menlo Park, and his love of orange clothing, as a way of celebrating his native Netherlands.

You will be missed, Berrie.

In memoriam written by Stephen Fegan and Deirdre Horan.