High resolution search for solar gamma-ray lines

A balloon-borne directional spectrometer for gamma rays from 30 kev to 6.3 Mev is described, based on a high-resolution Ge(Li) detector. The sun was observed from 4.7 g cm−2 on May 5, 1968. Seven lines present in the background are attributed to neutron capture reactions in the detector. No evidence of monochromatic solar gamma-ray emission was detected. An upper limit of 6.9×10−3 cm−2 sec−1 was found for 511 kev radiation from the quiet sun. Upper limits 4.5×10−3 cm−2 sec−1 and 3.2×10−2 cm−2 sec−1 were found for the flux of 2.223-Mev gamma rays at the earth for the quiet sun and for a period of subflare activity, respectively.