The upper limit on the quiet time solar neutron flux from 1-20 MeV has been measured to be less than 2 • 10 -2 n cm -~ s -1 at the 95 ~ confidence level. This result is deduced from the OGO-6 neutron detector measurements of the 'day-night' effect near the equator at low altitudes for the period from June 7, 1969, to December 23, 1969. The OGO-6 detector had very low ( < 4 ~) counting rate contributions from locally produced neutrons in the detecting system and the spacecraft and from charged-particle interactions in the neutron sensor.
1. Introduction
Biermann et al. (1951) first suggested that solar flare protons with energies of about 100 MeV could interact with solar atmosphere to produce a secondary flux of neutrons. Later theoretical calculations by Hess (1962), Lingenfelter et aL (1965a, b), and Lingenfelter and Ramaty (1967) gave further support to the possible existence of a solar neutron flux and showed that the only significant source of solar neutrons is from protons accelerated and slowed down during flares. For almost two decades efforts have been made to detect such a neutron flux. The measurement of a solar neutron flux is important because this is a means to evaluate some parameters of the accelerating mechanism of solar cosmic rays such as the time scale for acceleration, the nuclear reactions which may occur, and the region in the solar envelope in which acceleration takes place. Attempts to measure this flux have been made both during small solar flares and during relatively quiet periods. The existence of a detectable solar neutron flux has not been confirmed in the searches by Hess and Kaifer (1967), Bame and Ashbridge (1966), Webber and Ormes (1967), Forrest and Chupp (1969), Zych and Frye (1969), Kim (1970), Heidbreder et aL (1970), Cortellessa et aI. (1970, 1971), Daniel et al. (1971), andEyles et al. (1971). No attempt will be made here to discuss all these results. For the details, reference should be made to the review of solar flare neutron production by Chupp (1971) and the survey of the experimental efforts to detect solar neutrons compiled by Lockwood (1973).
We report here the results of the OGO-6 neutron experiment which set a new lower limit to the 1-20 MeV neutrons produced at the Sun during a relatively quiet period. These new measurements will be compared with others in the energy region 1-100 MeV.