We have calculated the global distribution of atmospheric neutrons and their products by a Monte Carlo simulation of nucleon transport, in the internuclear cascade followed by neutron transport below 19 Mev. First, we present the results generated by monoenergetic primary protons and alpha particles entering the top of the atmosphere. Second, the kernels derived from the monoenergetic cases are used to determine the spatial and energy distributions of neutrons and their products from the protons and alpha particles in the cosmic radiation; solar modulation effects are included. The calculation is compared, in the 1‐ to 10‐Mev region, with the results of our fast neutron experiment; the agreement is within the uncertainties of the primary spectrum and of the experimental results over most of the atmosphere. The calculation is then normalized to the experiment in the fast neutron region. The results of the normalized calculation include the steady state neutron spectrum, the neutron production rates, the radiocarbon production rates, the neutron leakage rates from the top of the atmosphere, and the production rates of other nuclides. The normalized calculation reproduces experimentally observed slow neutron densities and the observed neutron flux and spectrum above 1 kev, and it predicts features of the atmospheric neutron morphology not yet observed. The points of agreement and divergence with earlier calculations are discussed, including the radiocarbon production rates and the neutron leakage rates during solar cycle 20, which is near the mean of the last 10 solar cycles.
The fast-neutron flux in the atmosphere has been measured during solar minionurn. Data from ground level to about 4 g/cm 2 were obtained in a series of seven' high-altitude balloon flights. The flights were conducted between September 1964 and August 1965 at four locations between X _--8øN and X _--69øN, conventional geomagnetic latitude. The detector, a phoswichtype scintillator, was sensitive to neutrons in the range 1-10 Mev. A seven-channel pulseheight analyzer permitted the evaluation of the ,energy spectrum in this range. The neutron flux at the transition maximum increased from 0.17 ___ 0.02 cm -2 sec -• at X _--8øN to 1.9 ___ 0.1 cm -• see -• at X ----69øN. Neutron leakage from the top of the atmosphere was estimated by extrapolation. The magnitudes of the leakage fluxes, as well as the pole-to-equator ratio of 16, were in substantial agreement with the results of the diffusion calculation of Lingenfelter. The best fit to an inverse power law of the differential neutron energy spectrum was found to be independent of latitude within the limits o.f our experimental precision. Averaged over all flights, the best fit was for a special index of n ----1.05 ___ .15. This spectrum 'is harder than that calculated by Lingenfelter. Differences in the shapes of the neutron profiles of this experiment and of the calculation are consistent with the difference in spectrum.
We have investigated solar phenomena associated with unusual changes in the production rates of 14C in the atmosphere. 14C is produced in interactions of cosmic ray neutrons with nitrogen in the atmosphere. Intensity of the neutrons varies globally and fluctuates with time as a result of interactions of galactic cosmic rays which generate neutrons with plasma and magnetic fields of the solar wind. We estimate the total mean production rate of 14C for solar cycle 20, specifically 1965 to 1975, to be 2.25 ± 0.1 nuclei-cm−2sec−1 from galactic cosmic rays alone, with negligible integrated contribution from solar particle events. Annual averages of Rz, the Zurich sunspot number, and the production rate of 14C, n(14C), were related by n(14C) = 2.60–5.53 × 10–3 Rz ± 3 percent. The contribution of solar flare particles and the zero sunspot limit are discussed with relation to major fluctuations that appear in the radiocarbon versus dendrochronology over short (∼100 years) integration times.
From an analysis of the time variations during 1968–1971 of the fast neutron flux in the upper atmosphere (mean energy of response to primaries, 1–2 GeV per nucleon) versus those of ground‐based neutron monitors we have identified two classes of transient intensity decrease on the basis of differences in their spectral responses, time histories, and flare associations. Type I events are found to be classic Forbush decreases, sharp declines accompanying a geomagnetic storm sudden commencement, following by 1–3 days a large optical flare with radio noise and energetic particle production, whereas type II events are more symmetric in their time histories and are therefore not associated with a particular flare. There are also differences in the spectral responses of the two types. During a type I decrease the flux change of the lower‐rigidity cosmic ray particles lags the flux change of the high‐rigidity particles both in the decline and in the recovery, tracing out a hysteresis loop. During a type II event, if there is any hysteresis at all, the lower‐rigidity primaries tend to ‘overrecover’ in comparison to the higher‐rigidity primaries. Intercomparison of neutron monitor data for median response rigidities from 10 to 30 GV reveals that the spectral response in type II events is softer on the average for low‐rigidity (< 10 GV) primaries and harder for high‐rigidity (> 10 GV) primaries than that in type I events. Comparison of intermixed sequences of type I and II events with recurrences of active regions reveals an identifiable but complex evolutionary relation between decrease occurrence and active region development. Long‐lived (of the order of days) low‐energy (<1 MeV) proton events occur during all but one of the type II events identified, supporting an association with solar active region transits. We interpret type II events as either a subsequent evolution of type I (Forbush decrease) events or a quasi‐stationary ‘corotating’ spatial structure loosely associated with an active region. Therefore both type I and type II decreases occur in intermixed recurrence series at intervals of 20–30 days.
A fast‐neutron detector was carried by balloon up to 27.4 km on November 8, 1962, from Sioux Falls, South Dakota. The detector consisted of an inner liquid scintillator, employing pulse‐shape discrimination, and a thin 4π phosphor shield for rejection of charged particles. Both scintillators were viewed by a single photomultiplier. The counting rates produced by neutrons in two energy intervals between 1 and 10 Mev were recorded. The best fit to a power law spectrum was N(E) dE = 2.6E−1.16±0.2 exp(−0.0069x;) dE neutrons/cm2 sec Mev in the equilibrium region. The counting rate rose to a broad maximum at about 75 g/cm2, where the total flux of neutrons between 1 and 10 Mev was 1.9 neutrons/cm2 sec. The flux at floating altitude was 1.4 neutrons/cm2 sec. The results are compared with those of Newkirk and of Hess.
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