The Agathe experiment is a 1 m2 spark chamber sensitive in the 5–50 MeV energy range designed for use on a stratospheric balloon. The different methods used to determine the efficiency, angular resolution and energy are described. Efficiency calibrations are made in gamma ray beams at different energies. The results are verified by a Monte Carlo calculation then extended to higher energies. The photon energy is determined by calibrations in electron beams and in gamma ray beams; two methods are compared and discussed.

The balloon experiment carried out by the C.E.A. of Saclay and the C.E.S.R. of Toulouse is designed for the study of gamma radiation in the energy range 10–100 MeV. This paper describes the construction of the experimental system, the results obtained during a test flight and the method used for the calibration. This experiment made two successful flights in Brazil in November 1976 and February 1977. The count rates obtained in Brazil and the results of the test flight are compared.