Description du sujet de thèse

Title : Evaluation and validation of the performance of the detectors for the NectarCAM cameras for the CTAO gamma-ray observatory

The Cherenkov Telescope Array Observatory (CTAO), currently under construction, will study cosmic sources of very-high-energy (VHE) gamma rays, in the range from 0.01 to 100 TeV. Astrophysical objects such as black holes, neutron stars, supernova remnants and pulsar-wind nebulae, can accelerate charged particles to ultra-relativistic energies, producing gamma rays through a variety of physical processes (inverse-Compton interactions, pion decay, etc.). The goal of the CTAO observatory are to understand the mechanisms of particle acceleration at play in these sources and to study the interaction of these particles with the interstellar and intergalactic media [1]. To do this CTAO will employ an array of Cherenkov telescopes [2] which detect VHE gamma rays indirectly through their interaction in the Earth's atmosphere. The telescopes employ large mirrors and sensitive cameras, operating at nanosecond timescales, to image the Cherenkov light produced in these interactions. NectarCAM is a project to build nine such cameras for CTAO [3]; each NectarCAM camera is equipped with 1,855 PMT-based photodetectors grouped into 265 modules of 7 detectors each, dubbed the "detector unit" (DU). Each DU contains light concentrators, PMTs along with the electronics to operate the detectors and amplify the signals received, IRAP is leading the development, production and testing of the detector units for NectarCAM [4]. During the camera production phase (2025-2030), the detectors will be produced in industry, validated at IRAP and installed and tested in NectarCAM at IRFU (Saclay). After validation of the full camera in the dark room the cameras will be delivered to CTAO Northern site at La Palma (Canary Islands, Spain) and installed on the telescopes.

The objective of this thesis is to study and monitor the performance of the detector units from their validation at IRAP to their deployment and use at La Palma, and in particular, during the calibration and testing of the completed cameras at IRFU. The PhD student will evaluate the performance of the detectors in the context of the strict specifications for angular, temporal and spectral resolution the camera must attain to be accepted by CTAO. During the beginning of the science operations phase, when observations of astrophysical objects are starting to be made, this detailed knowledge of the performance of the individual detectors as a function of their operating parameters (voltage, limiting current, ...) will be essential to properly configure the camera and to be able to correctly transform the data being taken into scientific results.

The PhD student will first familiarize themselves with the functioning of the detectors, and NectarCAM and CTAO in general. They will analyze the measurements of the individual detectors taken at the test benches in IRAP, and the data taken with the full camera when all the detectors have been installed, with the aim of characterizing the performance of each detector (gain. quantum efficiency, charge resolution, time response, electronic noise, night-sky noise, etc.). The student will produce a model of the response of the detector to photons produced in VHE gamma-ray interactions in the atmosphere, accounting for the background of ambient photons from the night sky and the electronics noise. The model will be compared with the data taken during the first observations of cosmic sources once the camera is installed on the telescope. The student will participate in the installation and commissioning of the camera at La Palma, and in the analysis of the first observations of known VHE gamma-ray emitters, such as the Crab Nebula or the bright active galactic nuclei. They will extract the images of the sources, measure their energy spectra and temporal evolution. The student will participate in the interpretation and publication of the astrophysical phenomena observed,
The PhD thesis will tale place at IRAP (Toulouse) during phases of testing of the individual detector units during (first year), and during the commissioning of the camera at La Palma (final year), and at LLR (Palaiseau, in the Paris region) during the integration and calibration phase at IRFU (second year). Travel for working meetings and scientific conferences will be financed by the project.

The results of the thesis work on the calibration will be published in a high-impact journal specializing on instrumentation (e.g. Nuclear Instrumentation & Method A) and the initial observations of cosmic VHE gamma-ray sources with NectarCAM will be published in an astrophysical journal (e.g. Astroparticle Physics). Software developed will be integrated in the NectarCAM data analysis pipeline (open source) and contributed to the CTAO project. It will be used as part of a data validation system under the responsibility of LLR.
Références :
- [1] Acharya et al., 2019, book : Science with the Cherenkov Telescope Array, arXiv:1709.07997v1
- [2] Acharya et al., 2013, Astroparticle Physics, 43, 3
- [3] Glicenstein et al., 2017, AIPC, 1792
- [4] Tsiahina et al., 2021, NIM A, vol. 1007, 165413.

Contexte de travail

The PhD will be jointly hosted by two labs of the French national research agency (CNRS). The student will spend approximately two-thirds (2/3) of their time at the Institut de Recherche en Astrophysique et Planétologie (IRAP, Toulouse), and at the remaining third (1/3) at Laboratoire Leprince Ringuet (LLR, Palaiseau, in the Paris region). The PhD supervisors are Pierre JEAN (professor at IRAP), and Stephen FEGAN (CNRS researcher at LLR). The PhD contract will last for 3 years, and is financed by CNRS and managed by IRAP.
The doctoral school will be the Toulouse ED SDU2E (Sciences de l'Univers, de l'Espace et de l'Environnement).
The thesis supervisors are all members of the NectarCAM/CTAO collaboration, and have been working together on this project for over 10 years. The IRAP team consists of three researchers and an engineer. It is responsible for supplying the NectarCAM detectors. The LLR team (two researchers and a post-doc) is responsible for Data Quality Monitoring of the cameras during the calibration and observation phases.
The NectarCAM cameras are made up of subsystems designed, developed and produced mainly by French laboratories at CNRS/INSU, CNRS/IN2P3 and CEA. These cameras are France's major contribution to the CTAO observatory.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

Travel is required for: a one-year stay at the LLR (Palaiseau) to monitor camera calibrations, work meetings, NectarCAM and CTAO collaboration meetings, NectarCAM commissioning in La Palma (Spain).
IRAP is a Restricted Zone (ZRR:Zone à Régime Restrictif).

Informations complémentaires

The candidate should hold a Masters 2 in Physics and/or Astrophysics (or equivalent) and have completed a work placement in these fields. He/she should have a sound knowledge of physics, data analysis, programming and good oral and written communication skills in French and English to present his/her results (publications, conferences, meetings).

Application documents:
(a) a CV, (b) a covering letter, (c) Master 2 grades (or equivalent), (d) at least two letters of recommendation: one from the Master 2 course leader (or equivalent) and one from the Master 2 course supervisor (or the person with whom the applicant worked).