We present an analysis of experimental data on the geographic distributions of the intensity of secondary cosmic rays and the production rates of 7Be and 10Be in water. Two portable, identical, IGY/NM64-design neutron monitors (NMs) were employed in 1998-2001 to map the latitudinal and atmospheric depth dependencies of the sum-intensity of those particles that are understood to produce terrestrial cosmogenic nuclides (TCNs). Carefully prepared containers of ca 100 litre capacity were used to expose pure water for a maximum duration of three years. The data were obtained from wide enough ranges of geomagnetic latitude (20-79°S) and altitude (0-2084 masl) to be useful for calibrating the surface exposure dating (SED) technique. Water targets were exposed at seven different sites (27-79°S) and the monitors were deployed at these plus an additional forty locations.
The required long target-exposure times and the time differences with and amongst the monitor measurements necessitated a correction for solar-cycle phase differences. At the expense of losing some independence, we use data from the global fixed-base NM network to parameterise the amplitude of the solar-cycle variations as a function of magnetic cut-off rigidity and atmospheric depth. Previously reported effective attenuation lengths were used for atmospheric depth corrections when comparing hourly NM count rates with the long-term water target exposures.
The direct comparison between our NM and water target data provides a thorough test of whether, in general, neutron monitor information can be used as a proxy for TCN production across the range of locations relevant to SED. We also tested the geographic scaling models of Lal, Stone, Dunai, and Desilets & Zreda with our observations. We confirm a constant 10Be/7Be production rate ratio of close to 0.50 over the full range of latitude and altitude, allowing us to proceed with the more precise 7Be data set. NM count rates are proportional to 7Be production rates in water between 27 and 50°S geomagnetic latitude and up to 1950 masl. The 7Be/NM proportionality factor is enhanced for the high-latitude sites in the Antarctic, at 201 and 2084 masl, by ca 20%. Comparing the 7Be data with the abovementioned scaling models shows a similar deviation in proportionality. Since these models are largely based on NM survey data, our findings suggest that such efforts can only partially capture the TCN production mechanism, particularly with regards to the dependence on latitude.
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