Production rates of 7Be and 10Be in the atmosphere

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Abstract

We have estimated the production rates of cosmogenic 7Be and 10Be in the stratosphere and troposphere using the neutron and proton cross-sections for 14N and 16O. The global average production rates (atoms cm−2 s−1) are 0.041 (7Be) and 0.018 (10Be) in the stratosphere and 0.027 (7Be) and 0.018 (10Be) in the troposphere for the solar minimum. To convert to long-term average production rates, these values are multiplied by 0.8–0.9. The correlation between these production rates and the observed 10Be/7Be ratio in the atmosphere and its seasonal variation is shown using a simple box model calculation with adequate parameters.

Introduction

The radionuclides 7Be (53.35 d half-life) and 10Be (1.5 × 106 yr) are produced by cosmic rays in the atmosphere. Although the ratio of the production rates for these two nuclides in the atmosphere is assumed to be constant, the atomic ratio changes with time for the relatively short half-life of 7Be. We have already reported that the 10Be/7Be ratio in the atmosphere changes from 1.3 in the northern hemisphere (0–40°N) to 1.9 in the southern hemisphere (0–60°S) over the Western Pacific Ocean and adjacent areas during December to February [1]. To interpret this result, two factors are required. One is a transport mechanism which is related to time; the other is the initial ratio, i.e., the ratio of the production rates of 10Be to 7Be. The observation of the 7Be and 10Be concentrations in the atmosphere gives important information about the production and behavior of these nuclides in the atmosphere, as pointed out by Raisbeck et al. [2] and Dibb et al. [3].

We present calculations of the production rates, which involve a modification of the nuclide production model presented by Lal and Peters [4], using the neutron and proton cross-sections for 7Be and 10Be production from N and O, and the neutron and proton fluxes in the atmosphere. In addition, we discuss the correlation between the calculated production rates and the observed 10Be/7Be ratio at sea level using a box model.

Section snippets

Cross-sections

The excitation functions used to calculate the production rates for 7Be and 10Be are shown in Fig. 1. These curves were drawn mostly based on the experimental data for the 7Be and 10Be productions from: (i) 14N, 16O + n by Imamura et al. [5]; (ii) 16O + p by Sisterson et al. [6]; (iii) 14N + p compiled by Read and Viola Jr. [7]. In the energy region over 100 MeV, the neutron excitation functions were extrapolated to the corresponding proton excitation functions. The uncertainties in the excitation

Distribution of 10Be/7Be in the atmosphere

The average 10Be/7Be ratios observed in the atmosphere near sea level were 2.00±0.46and1.89±0.42 for the northern and southern hemisphere during the “summer”, respectively, and 1.24±0.27 for the northern hemisphere during the “winter”. The summer corresponds to December to February for the southern hemisphere and July to August for the northern hemisphere and the winter corresponds to December to February for the northern hemisphere in Table 2. In addition, the summer and the winter indicate

Summary

The production rates of 7Be and 10Be in the stratosphere and troposphere, which were calculated using the neutron and proton cross-sections for N and O, are not so far from the estimations given by the previous studies [9], [10]. The uncertainty in our estimation will be decreased when more accurate information about the distribution of the neutron flux and the spectral shape in the atmosphere are obtained.

The 7Be and 10Be concentrations and 10Be/7Be ratios in the atmosphere near sea level give

Acknowledgements

This work was partly supported by a grant-in-aid for Scientific Research (No. 11440168, H. Nagai, Principal Investigator) from the Ministry of Education, Science, Sports and Culture, Japan.

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