Radioactivity observed in scintillation counters during the HEAO-1 mission

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Abstract
The UCSD/MIT Hard X‐ray and Low‐Energy Gamma‐Ray Experiment (A4) was carried on the HEAO‐1 satellite to perform a survey of the sky at energies between 10 keV and 10 MeV. Observation of radiation from cosmic sources in this energy range has been difficult because of high internal background counting rates from radioactivity induced in the material of the detectors. The activation is caused by ambient charged particles and neutrons, and leads to complex, time‐variable energy loss spectra. We report here on the results of an extensive analysis of the radioactivity induced in the NaI Medium Energy Detectors of this experiment during the 500‐day HEAO‐1 mission. These 7.5 cm dia by 2.5 cm thick crystals operated from 40 keV to 2 MeV. Various radioactive nuclides were identified through their observed spectra and decay times. The incident charged particle flux was monitored with three NE 102 plastic scintillators. Model activities at a variety of expected half‐lives were calculated from this charged particle input. The amplitude of each of these activation terms was treated as a free parameter in standard multiple regression fits at each energy. Since the dominant behavior of each term is exponential with time, the problem is ill‐conditioned. Judicious selection of data from the entire mission nevertheless permitted a reliable separation of activities. The resulting energy spectra and nuclide identification are discussed.

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