Improved estimate of solar neutron flux measured in an earlier experiment

Abstract

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...

A search for neutrons of solar origin using balloon borne detectors 1967?69

“…would have produced decay protons detectable by OGO-A, yet none was observed. Recently Daniel et al (1971) have revised their previous result downwards by a factor of six, although they still give no errors or confidence limits (result (d) on Figure 10), so that it is now, they claim, consistent with the OGO-A data. But it seems that this agreement still only holds under the improbable assumptions of a flat spectrum and no diffusion.…”

“…Hess and Kaifer (1967) Apparao et al (1966). {c) Bame and Asbridge (1956) (d) Daniel et al (1967Daniel et al ( ,1971) (e! Hess and Kaifer (1967).…”

Estimation of an upper limit for the solar neutron emission during large flares

“…Since the first discussion of a possible solar neutron flux produced by nuclear interactions of solar protons with the photosphere by Biermann et al (1951), model calculations and flux estimations for solar neutrons and solar 7-ray lines, have been made by Hess (1962), Chupp and Razdan (1963), Chupp (1964), Dolan and Fazio (1965), Lingenfelter et al (1965), Lingenfelter and Ramaty (1967), Lingenfelter (1969), Ito et al (1968), Chupp (1970), Svestka (1971), and De Feiter (11971). Detectable fluxes of solar neutrons and solar v-ray lines should appear in the vicinity of the Earth during the acceleration and the slowing down phase of solar protons.…”

“…Apparao et aL (1966), Bame and Asbridge (1966), Hess and Kaiffer (1967), Webber and Ormes (1967), Kim (1968), Daniel et al (1969), Forrest and Chupp (1969), Zych and Frye (1969), Cortellessa et aL (1970), Heidbreder et al (1970, Kondo et aL (1970), Cortellessa et al (1971), and Daniel et al (1971.…”

“…Bame and Asbridge (1966) have performed neutron measurements aboard the Vela satellites and found the flux limit: < 1.5 • 103 N cm-2~ 0.4 N cm-2 s -~, E= 1-100 MeV for flares of importance 3. Positive experimental evidence for a solar neutron flux during a pre-flare phase is reported by Daniel et al (1971): 1.5 • 10 -2 Ncm -2 s -1, E= 50-500 MeV. It is the purpose of this paper to estimate upper flux limits for the solar neutron emission especially during large flares, by superposition of neutron monitor data from low latitude mountain stations.…”