EGU 2018, Vienna, 8 April 2018; abstract EGU2018-12052 Are neutron-monitor data suitable for scaling production rates of cosmogenic 10Be with altitude? Tim Corley and Marek Zreda University of Arizona, Tucson, Arizona, USA Field assistance by D. Desilets, C. Zweck and A. Sarikaya EGU 2018, Vienna, 8 April 2018; abstract EGU2018-12052

EGU 2018, Vienna, 8 April 2018; abstract EGU2018-12052 Are neutron-monitor data suitable for scaling production rates of cosmogenic 10Be with altitude? Tim Corley and Marek Zreda University of Arizona, Tucson, Arizona, USA Field assistance by D. Desilets, C. Zweck and A. Sarikaya EGU 2018, Vienna, 8 April 2018; abstract EGU2018-12052

No

Questions?

Question Previous work by Nishiizumi et al. (1996) and Brown et al. (2000) suggests that at high latitudes production rates of 10Be in water targets change with altitude similarly to the intensity of high-energy neutrons. Hence, neutron monitor data are a reasonably good proxy for production of 10Be. What about low latitudes? Desilets et al., 2006, EPSL 246.

Problem statement Do production rates of 10Be and the intensity of high-energy neutrons at low latitudes change with altitude at the same rate? In other words, do the two have the same attenuation length? If they do, neutron monitor data can be used to scale production rates of 10Be with altitude. But computational evidence suggests that altitudinal variations of production rates do not follow that of high-energy neutrons. And geological samples from low latitudes and high altitudes are inconsistent with the “global” data set. If so, how different are the two respective attenuation lengths?

Compare neutron measurements with production of 10Be in water targets Methods Compare neutron measurements with production of 10Be in water targets Intensity of high-energy neutrons Two neutron monitors Measurements at 27 locations At latitude 20°N (12.5 GV) At altitudes 0-4200 m (1035-629 g/cm2) Production of 10Be in water 12 water targets at six locations Exposed for 3 years At latitude 20°N (12.5 GV) At altitudes 0-4200 m (1033-629 g/cm2)

Neutron monitor Two neutron monitors: - lead producer - granite producer

Locations of neutron measurements

Neutron intensity, muon-corrected, Hawai`i Data in context: Source L, g/cm2 Method This work 149 NM granite This work 146 NM lead C&B 147 NM LSD 150 model DZP 147 NM C&B: Carmichael & Berkovich, 1967, Can. J. Phys. 46, S1006-S1013. LSD: Lifton et al., 2014, EPSL 386, 149-160. DZP: Desilets & al., 2006, EPSL 246, 265-276.

Water targets for production of 10Be

Summary of water target sites on Hawai`i

Cosmic-ray neutron intensity: temporal variations Time series: Measured with neutron monitors at Jungfraujoch, rigidity 5 GV (grey line). Computed for Hawaii, rigidity 12.5 GV (cyan line). Horizontal bars: Water target exposure for 10Be. N96: Nishiizumi 1996; B00: Brown 2000; Z14: Zreda 2014 (this work). JGR 101, 22225-22232; NIMP B 172, 873-883;

Production rates of 10Be in water targets, Hawai`i Production includes: neutrons muons exposure during transit exposure in the lab

Correction for exposure in lab Method 1 Measure thickness of concrete ceilings Estimate material density Calculate total shielding mass (g/cm2) Calculate ratio of 10Be production rate in lab to that outside Calculate inventory of 10Be produced while samples were in the lab Method 2 Use neutron monitor Measure high-energy neutrons in the lab and outside Calculate ratio of neutron intensity inside to that outside Use this as a proxy for production of 10Be Calculate inventory of 10Be produced while samples were in the lab

Production rates of 10Be in water targets, Hawai`i Production corrected for: exposure during transit exposure in the lab Total production rate: Hawai`i (rigidity ~12 GV), Sea level (1035 g/cm2), Solar minimum (2006-2010): Water: 4.9±0.5 a10 g-1 y-1 Quartz: 2.9±0.3 a10 g-1 y-1 Recomputed to high latitudes (rigidity 0 GV; Lifton et al., 2014): Quartz: 5.2±0.5 a10 g-1 y-1

10Be vs neutrons, Hawai`i Attenuation length L for 10Be is much longer than that for neutrons. The slopes can match if fraction of muogenic production is 0.3 at sea level. This is an order of magnitude higher than the usual estimates.

Results in context Source Description Atmospheric depth Lambda Scaling Relative sea level summit g/cm2 factor factor g/cm2 g/cm2 This work NM lead 1033 629 146 15.76 1.32 This work NM granite 1033 629 149 15.00 1.26 LSD* LSD n (0-4 km) 1033 629 150 14.78 1.24 LSD* 10Be (0-4 km) 1033 629 151 14.44 1.21 This work 10Be in water 1033 629 163 11.92 1 * Lifton et al., 2014, EPSL 386, 149-160.

Implications Recent production rates of 10Be at low latitudes and high altitudes Source Latitude Altitude P10 PSLHL °S m a g-1 y-1 a g-1 y-1 Martin et al. 19 3800 30.8 3.8 Kelly et al. 14 4857 43.3 3.9 PSLHL are considered ~15% lower than “global” estimates (~4.5). Longer attenuation length computed from Hawai`i water target experiment would make them higher, possibly reconciling them with “global” rates. Martin et al., 2015, Quat. Geochronol. 30, 54-68. Kelly et al., 2015, Quat. Geochronol. 26, 70-85.

Conclusions Neutron monitor data appear unsuitable for scaling neutrogenic production rates of 10Be with altitude Production rates of 10Be change more slowly with altitude than do neutron intensities

Assessment Possible reasons for mismatch: (1) Calculated production in transit and in lab underestimated here (2) Muogenic production underestimated by previous work (3) Laboratory mistakes (4) Other (unknown?) factors Advised future work: Repeat this experiment to assess the significance of 1 and 3 above