Cosmogenic 3He and 21Ne measured in quartz targets after one year of exposure in the Swiss Alps

Abstract

All currently used scaling models for Terrestrial Cosmogenic Nuclide (TCN) production rates are based on neutron monitor surveys. Therefore, an assumption underlying all TCN studies is that production rates are directly proportional to secondary cosmic ray intensities for all cosmogenic nuclides. To test this crucial assumption, we measured cosmogenic 3He and 21Ne in artificial quartz targets after one year of exposure at mountain altitudes in the Swiss Alps. The targets were inconel steel tubes containing 1 kg of artificial quartz sand (250–500 µm), degassed for one week at 700 °C in vacuum prior to exposure. From August 2006 until August 2007, ten of these targets were exposed at five locations in Switzerland and Italy: Zürich (556 m), Davos (1560 m), Säntis (2502 m), Jungfraujoch (3571 m), and Monte Rosa (4554 m). Additionally, a sixth set of two blank targets was kept in storage and effectively shielded from cosmic ray exposure. Cosmogenic noble gases were measured at room temperature and at 700 °C. Up to 9% of the cosmogenic 3He was measured in the cold step, indicating that 3He diffuses out of quartz at room temperature on short time scales. The remaining 3He and all 21Ne were released at 700 °C, as shown by a repeat measurement at 800 °C for the Monte Rosa target, which yielded no additional cosmogenic helium and neon. As expected, the Monte Rosa target contained the highest cosmogenic nuclide content, with 1.56 ± 0.07 × 10 6 atoms of excess 3He and 4.5 ± 1.2 × 10 5 atoms of excess 21Ne (all errors are 2 σ). The raw measurements were corrected for non-atmospheric blanks, shielding (roof + container wall), tritiogenic helium and solar modulation (normalised to the average neutron flux over the past five solar cycles). The 3He/ 21Ne production rate ratio of 6.8 ± 0.9 indicates that cosmogenic 3He production by the container walls is negligible. The main goal of the artificial target experiment was to determine the production rate attenuation length. Because all our targets had an identical design and were exposed under identical conditions, all systematic errors cancel out in the calculation of an attenuation length. Our best estimates for the 3He and 21Ne attenuation lengths are 134.8 ± 5.9 g/cm 2 and 135 ± 25 g/cm 2, respectively, agreeing very well with currently used scaling models. We conclude that TCN production rates are indeed proportional to neutron monitor count rates, and that 3He and 21Ne production rates follow the same altitudinal scaling relationships as the cosmogenic radionuclides. Finally, the measurements were scaled to sea level and high latitude using the empirical attenuation length, yielding weighted mean production rates of 107.6 ± 6.6 at/g/yr for 3He and 15.4 ± 2.1 at/g/yr for 21Ne. Despite the significant uncertainties associated with the corrections for shielding, solar modulation and especially the 3He/ 3H branching ratio, these estimates are in good agreement with production rates derived from long-term exposure experiments at natural calibration sites and physics-based simulations.