On the basis of solar flare forecasts, balloon flights were made from Hyderabad, India (vertical geomagnetic threshold rigidity of 16.9 GV), to detect the possible emission of high energy neutrons during solar flares. The detector comprised of a central plastic scintillator, completely surrounded by an anticoincidence plastic scintillator shield. The instrument responds to neutrons of about 15-150 MeV and gamma rays of about 5-30 MeV with about the same efficiency. The detector was flown to an atmospheric depth of 25 g cm -2 on February 26, 1969; while the balloon was at ceiling a flare of importance 2B and one of 1N occurred. No perceptible flare associated increase in the counting rate was observed. Using the observed counting rates, an upper limit of 1.2 • 10 -2 neutrons cm -2 sec -1 is obtained for the first time for a flare of importance 2B for neutrons of energy 15-150 MeV. The corresponding upper limit for gamma rays of energy 5-30 MeV is found to be ,-~ 10 -2 photons em -~ sec -1. The neutron flux limits are compared with the recent calculations of Lingenfelter.

The isotopic composition and the energy spectrum of low‐energy helium nuclei have been investigated, using a nuclear emulsion stack flown from Fort Churchill, Canada, on August 4, 1962. The proportion of He3 nuclei at the top of the atmosphere was determined to be 18±5% within the kinetic energy interval of 160–370 Mev/nucleon and 24±8% within the rigidity interval 1.1–1.4 bv/c. The differential energy spectrum in the above energy interval was found to have a maximum around 275–300 Mev/nucleon. Assuming that there is no He3 at the cosmic‐ray sources, and that the observed abundance reflects the interactions of the primary radiation in its passage through interstellar matter, we conclude that the low‐energy primary radiation has traveled a total of 6±2 g/cm2 in this medium. This result is found to be consistent with the values obtained from the low‐energy lithium, beryllium, and boron experiment. The implications of these results are examined on the basis of different models used for the propagation of cosmic radiation through interstellar matter.

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In an earlier paper from this laboratory [Daniel et al., 1967], we presented evidence for the emission of solar neutrons of energy between 50 and 500 Mev, from a spark chamberscintillation counter detector assembly flown in a balloon from Hyderabad, India, on April 15, 1966. A careful and detailed analysis of the data, a comparison of the observed counting rate with that of a bare Geiger counter flown in the same balloon, and the association of the enhanced detector counting rate with a brightening in the optical region in Ha and an increased emission level of solar radio flux at 100 MHz, supported the conclusion that the enhanced counting rate, which persisted for about 90 rain, was due to the emission of energetic neutrons from an active region of the sun. The next step in reliably estimating the absolute flux of solar neutrons was difficult because of the lack of calibration measurements using accelerator neutrons or reliable flux determinations of atmospheric neutrons relevant to our experiment. Nevertheless, a rough estimate was made by using a selected class of spark chamber events and by comparing them with neutroninduced interactions in nuclear emulsions [Apparao et al., 1966]; the flux value thus deduced was 10 -• neutrons cm -2 sec -• in the energy interval 50 to 500 Mev. Subsequent to the emulsion investigation by Apparao et al. [1966], we realized that the application of the knock-on proton method in emulsions for deducing the absolute flux of neutrons of energy •>50 Mev becomes progressively uncertain because of the increasing contribution of single-pronged stars induced by energetic neutrons. Prior to our spark chamber experiment, Roelof [1966] considered the problem of diffu-Copyright ¸ 1971 by the American Geophysical Union. sion in interplanetary space of protons arising from neutron decay and showed that neutrondecay protons can be used as an insensitive indicator of the presence of neutrons in solar flares. By using proton observations on the earth satellite Ogo A at the time of our experiment, Holt [1967] applied the calculations of Roelof to our experiment and concluded that, under the least favorable conditions (according to Holt [1967] ), the neutron flux at I AU could not have been more than % of our reported flux of 10 -• neutrons cm -2 see -•. This paper reports a re-evaluation of our earlier data, possible because of new measurements, to arrive at a very much more reliable flux of solar neutrons, which is about 6 times smaller than our previ. ous value.In a recent experiment we reliably determined for the first time [Daniel et al., 1970] the flux of energetic atmospheric neutrons over the same launch site from which the spark chamber experiment was performed. This was determined by employing the technique of pulse shape discrimination to select events induced by energetic neutrons in a CsI(T1) crystal and the known cross sections for the production of different kinds of neutron events in the crystal; also a power law type of energy spectrum has been assumed for the energetic atmospheric...