Limit on the Continuous Solar Flux of the 2.22-MeV Neutron-Proton Capture Gamma Ray

An upper limit on the continuous emission of 2.22-MeV photons (neutron-proton capture $\gamma$ rays) from the sun has been determined from day and night balloon flights in May and June of 1967, from Palestine, Texas at an atmospheric depth of ∼3.8 g/${\mathrm{cm}}^2$. The experimental apparatus consisted of a 3 in.×3 in. CsI(Tl) $\gamma$-ray spectrometer and airborne telemetry to transmit the pulse-height distribution to the receiving station, where it was recorded on magnetic tape for later analysis by a laboratory multichannel analyzer. On-board calibration sources giving lines below 2.22 MeV were always present in the spectrum and allowed a continual check on the stability of the apparatus. Comparison of the day and night average counting rates as well as the daytime absence of any line leads to a null result for any statistically significant solar flux of 2.22-MeV photons. Our resulting upper-limit flux is comparable with the value of 5×${10}^{-3\mathrm{}}$ photon ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ determined from the Ranger-3 experiment in January 1962 at a considerably different phase of the solar cycle.