1. page 1 ISS 2011, Vienna Roland Purtschert, Sophie Guillon Lauren Raghoo, Eric Pili Yunwei Sun, Charles Carrigan A collaboration between University of Bern, GEOTOP, LLNN, CEA Production and transport of Ar-37 in the alluvium New insights from experimental and numerical studies

2. page 2 ISS 2011, Vienna Outline: Ar-37 in OSI Fundamental Question Subsurface Background?

3. page 3 ISS 2011, Vienna Roland Purtschert, Sophie Guillon Lauren Raghoo, Eric Pili Yunwei Sun, Charles Carrigan A collaboration between University of Bern, GEOTOP, LLNN, CEA Production and transport of Ar-37 in the alluvium New insights from experimental and numerical studies

4. page 4 ISS 2011, Vienna Well known starting point Atmospheric background Peak in 2-4 meters depth 2 orders of magnitude higher than in the atmosphere Decrease with depth Riedmann& Purtschert 2011 Production Diffusion Depth z

5. page 5 ISS 2011, Vienna Soil gas sampling locations Radon monitoring Precipitation data Study Site The Belp Field Site

6. page 6 ISS 2011, Vienna Dynamics of water, gas and Ar-37 37 Ar Cosmic Rays (n,  ) Production Soil composition Neutron Moderation Emanation Stopping rate in pore space Transport Soil Permeability (Capping effect) Diffusion Barometric pumping Ca Partitioning Hernry Law

7. page 7 ISS 2011, Vienna Johnson et al 2015, MCNP Modeling Production P(0) Normalized by [Ca] (data from Johnson, 2015) Simplified Ansatz Fabrika Martin (1988) Bulk density

8. page 8 ISS 2011, Vienna Emanation Experiments Soil sample 40 Ca(n,p) 37 Ar 36 Ar(n,  ) 37 Ar Protons from Bern Medical Cylotron 18 O(p,n) 18 F thermal neutrons 190 kg of soil 63 g of soil CR

9. page 9 ISS 2011, Vienna Large Scale Emanation

10. page 10 ISS 2011, Vienna The Bern Cyclotron

11. page 11 ISS 2011, Vienna Emanation Results Within uncertainties all results agree within the range 6-14% Some indication that E wet >E dry The slightly larger emanation for the irradiated sample possibly due to finer material More experiments of this kind are needed

12. page 12 ISS 2011, Vienna 20142015 Belp Data  Variability is large (40-80%) and changes with time  Higher activity in greater depth  Higher variability in shallow depth  Temporal variability pattern similar -> external forcing

13. page 13 ISS 2011, Vienna PrecipitationInfiltration Input for NUFT Code Modelling Sophie Guillon (Guillon, JER, subm.) Environmental Conditions

14. page 14 ISS 2011, Vienna Model Results without Infiltration Model input “Belp Conditions” Soil composition  Production  Diffusion  Permeability Measured Emanation Local meteo data  Barometric Pumping Model results 1m depth without considering infiltration Moderate variations around 32 mBq/m 3 air ) 20142015

15. page 15 ISS 2011, Vienna Model input “Belp Conditions” Soil composition  Production  Diffusion Emanation Meteodata  Barometric Pumping Water infiltration Model Results with Infiltration Model results 1m depth Water infiltration causes significant variations of Ar-37 activity concentrations

16. page 16 ISS 2011, Vienna Comparison with data Good agreement of maximal activities values (no tuning) of variability pattern 2014 2015 Small var Large var Not all variability if covered by the model Hypothesis: Some infiltration effects not completely described in the model

17. page 17 ISS 2011, Vienna Model: Variability Sensitivity Mean value Variabilty Depth Environmental conditions Soil properties 5 m 1 m

18. page 18 ISS 2011, Vienna Variability: Data Data 2.4 m Data 1.4 m Model is not complete

19. page 19 ISS 2011, Vienna Threshold definition- Variability reduction Guillon et al, subm.

20. page 20 ISS 2011, Vienna Summary  Production of Ar-37 in soils can be calculated based on the most relevant parameters like the Ca content and site location  Emanation was the first time determined by direct measurements  Experimental field data and numerical modelling reveal large temporal and spatial variations of Ar-37 activity concentrations caused by changing environmental conditions (water!)  Our understanding of the processes for the definition of a global Ar-37 background in the Alluvium has improved but more work needs to be done Thank you This work is supported by PTS/CTBTO