<sod>The '''A4''', or '''Hard X-ray / Low Energy Gamma-ray Experiment''', used [[sodium iodide]] (NaI) [[scintillation counters]] to cover the energy range from about 20 keV to 10 MeV.<ref>Peterson, Laurence E, ''Instrumental Technique in X-Ray Astronomy''. in ''Annual Reviews of Astronomy and Astrophysics'' '''13''', 423 (1975)</ref><eod>
<sod>The '''A4''', or '''Hard X-ray / Low Energy Gamma-ray Experiment''', used [[sodium iodide]] (NaI) [[scintillation counters]] to cover the energy range from about 20 keV to 10 MeV.<ref>Peterson, Laurence E, ''Instrumental Technique in X-Ray Astronomy''. in ''Annual Review of Astronomy and Astrophysics'' '''13''', 423 (1975)</ref><eod>