1. 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 EGU

2. 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 EGU

3. No

4. Questions?

5. 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.

6. 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?

7. 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 m ( 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 m ( g/cm2)

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

9. Locations of neutron measurements

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

11. Water targets for production of 10Be

12. Summary of water target sites on Hawai`i

13. 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, ; NIMP B 172, ;

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

15. 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

16. 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 ( ):
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

17. 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.

18. 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
This work NM granite
LSD* LSD n (0-4 km)
LSD* 10Be (0-4 km)
This work 10Be in water
* Lifton et al., 2014, EPSL 386,

19. 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
Kelly et al
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,
Kelly et al., 2015, Quat. Geochronol. 26,

20. 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

21. 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