Time dependent worldwide distribution of atmospheric neutrons and of their products: 2. Calculation

Abstract

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.

Absolute Production of Radiocarbon and the Long-Term Trend of Atmospheric Radiocarbon

“…Nevertheless, if we assume that the specific 14 C activity in the preindustrial atmosphere (∆ 14 C=0‰) was 14.1dpm/gC, the PANDORA model gives preindustrial steady-state 14 C decay rate of 1.66 dps/cm 2 . This value is distinctly lower than the absolute production rate, which for the period 1964-1976 averaged at 2.2-2.35 at/cm 2 s (compiled from Light et al 1973;Korff and Mendel 1980;Castagnoli and Lal 1980;Lal 1988Lal , 1992. The 14 C production and decay may be balanced when we allow for an escape of 1.85GtC/yr "out" of the system.…”

Surface exposure geochronology using cosmogenic nuclides : applications in Antarctic glacial geology

“…Calibration of 3 He production rates has concentrated so far on 3 Hec in olivine and clinopyroxene from dated basalt flows in Hawaii and Utah . Because the cosmic ray flux at the earth's surface is a function of altitude, latitude, and the solar cycle (e.g., Merker et al, 1973), calibrated production rates at other locations must be scaled for these effects. The altitude dependence is normally considered an approximately exponential function 0033-5894/93 $5.00 Copyright C 1993 by the University of Washington All nghts of reproduction in any form reserved.…”

“…3 He CHRONOLOGY crepancy was also noted by . This may be at least partially because the latitude scaling is based on data for high altitude (-10 km: Light et al, 1973;Merker et al, 1973). Given that the pole-equator difference in cosmic ray intensity increases with altitude , these data cannot be used as general scaling factors for production rates at all altitudes.…”

A Dynamic Model for Estimating Radiation Dose to the World Population From Releases of 14C to the Atmosphere

“…The interaction of secondary neutrons from cosmic rays with I4N atoms in the upper atmosphere produces some 27,OOo-38,OOO Ci of 14C per year (Fa70; Li73). The lower estimate and the radioactive decay constant of 1.21 x lo4 yr-' imply a steady-state inventory of about 2 x 10s Ci, which sustains a specific activity of -6 X Ci (gC)-' in the atmosphere and man's food chain, from which each individual receives 1.3 mremlyr radiation dose rate to total body (Ki78a), or approx.…”