1. Measuring Snow Pack Thickness Using Cosmic Rays Juliana Araujo March 11, 2004

2. Outline Introduction Some definitions Previous attempts to measure snow water equivalent (SWE) Thermal & Epithermal Neutrons Conclusion

3. Introduction Cosmic rays neutrons have been a topic of studies for many years. They are useful in quantifying production of isotopes, such as 36 Cl. Safe alternative to  rays from highly radioactive 60 Co, commonly used in snow gauge

4. Introduction Measuring snow pack thickness and SWE is of great importance for river forecasting and water resources planning. Automation for remote areas: snow pillows radio nuclear devices, Attenuation of  rays in snow monitoring of attenuation of natural isotopes in snow profiling snow density through back scattering X-rays

5. Topics of Discussion Previous Attempts Bissell 1974 Kodama 1975 Kodama 1980 Experiment Basic Theory Equipment Similarities Unanswered questions Thermal & Epithermal Neutrons Purpose Goals Some Preliminary results New Technique

6. Some Definitions Thermal, Epithermal, Fast, and High-energy neutrons Thermal Neutrons: Practically, the Cd-cutoff range Neutrons with an energy <0.6 eV Epithermal Neutrons: Those between the thermal range and 1eV Fast Neutrons: Those that are produced in the atmosphere, due to secondary cascade, through ‘evaporation-like’ process from nuclear interactions of nuclear active particles with higher energies. Some say, 1eV-100Kev, while others define as <10MeV. The energy spectrum peaks at 1 MeV for fast neutrons. High Energy neutrons: Those produced from primary cosmic rays, E > 10MeV

7. More Definitions “evaporation” & “ground albedo” neutrons These are slow neutrons produced in the soil, and escape back into the atmosphere, where it is absorbed by the 14 N(n,p) 14 C reaction (Hendrick & Edge, 1966) ‘Neutrons that are created into the soil and are backscattered from soil to air’ (Kodama 1980) Function of soil moisture content due to diffusion and absorption of “albedo” neutrons.

8. Bissell, 1974 Deep Snow Measurements Highly penetrating cosmic radiation Counts are produced by NaI(TI) scintillator,  rays are >3MeV High-energy, to ensure that what they detect is entirely produced by comic radiation The detectors function primarily by photons generated by cosmic interaction with nuclei in air, water, soil, and in the system.

9. Bissell, 1974 Lake Mead, Nevada test dampening effect in the flux due to water at various depths buried one detector in soil and other, suspended above snow Counts/min >3 MeV, in 10cmX10cm scintillator as function of water depth

10. Bissell, 1974

11. Underground detector counts >3MeV  flux as attenuated by snow cover <3MeV   -flux from radioisotopes in soil, and soil moisture near detector Suspended detector >3MeV  fluxes “unattenuated” by snow <3MeV  natural terrestrial  -radiation attenuated by snow serves as a control from barometric pressures, seasonal and solar variations

12. Kodama, 1975 Preliminary Test to investigate absorption effects of neutrons in water. Type A and WS detector with counting rates ~170- 300n/hr in Tokyo T can be accurate to a % of the depth, with one measurement per day Water absorptions of neutrons in Tokyo, compared with 60 Co measurements of  -rays. Kodama,1975

13. Mt. Norikura (2,770m) One detector was placed inside a snow- free building The other placed on the ground The difference in counts from the two detectors with use of empirical curve  water equivalent of snow pile Experimental error based on counting rates to measuring time and snow depth

14. Kodama, 1975 Time profile of neutron counting rates. a) barometric pressure b) indoor c) outdoor d) water depth 1974NovemberDecember Date water equiv. (depth, cm) neutrons counts/hr Snow fall

15. Kodama,1980 Winter season of 1977-1978 Estimated to be effective for deep snow, >1m Only have statistical errors due to n-counting, and change in moisture content in soil Goal: how cosmic-gauge is useful on continuous observations of SWE.

16. Kodama, 1980 Experiment: Takada, 13m a.s.l. Hirosaki (302m) Oritate, 1330m Ohtawa, 1440m Instrumentation: moderated BF 3 counter, 2 cm polyethylene constant response in 1ev-1MeV range two sensors, WS, and HP After corrections for barometric pressures they used the following to convert the counts to water equivalents: (1) N w =N o exp(1-0.753(1-exp(-0.77w))); w<30cm (2) N w = N 30 exp(1-0.00578(w-30)); w>30cm (3) w 1 =13ln(0.753/(0.753-ln(N o / N w ))), cm (4) w 2 =173ln(N 30 /N w ) if w1>30cm

17. Kodama, 1980

18. Correlation between barometric pressure and cosmic ray neutron flux, under snow

19. Kodama, 1980 Good correlation between cosmic-ray gauge, and snow sampler, except near the snow cover maximum due to discordance or field discrepancies Atmospheric pressure effects, varies with barometric pressure The greater the snow cover depth, the harder the energy spectrum Primary Cosmic ray modulation daily variations affects the apparent swe of snow pack Statistical Fluctuations

20. Conclusions from the past All three experiments are used for high energy neutrons. Whether they use  -ray or neutrons, they measure these effects under snow shielding Related to the attenuation of neutrons in the snow, and moisture in soil. They do not look at lower energies. As in the case of Kodama 1980, the method was successful for long term measurements, and can be used for deep snow packs

21. The new technique Uses thermal and epithermal neutrons, as means to quantify moisture in the soil, and possibly applicable to snow pack thickness Unique, because there has been no previous work of this nature, with thermal neutrons My definition: Thermal neutrons: 0-0.6eV Epithermal neutrons: 0.6-100KeV

22. The new technique Comparison of depth profiles for measured and calculated thermal neutron fluxes. Desilets—Personal Communication Depth in concrete (g/cm 2 )

23. The new Technique Scalable— volume could be corrected by adjusting the height of the instrument * F. M Phillips et al., 2000. Fig.1: Comparison of epithermal and thermal neutron fluxes in a concrete block at Los Alamos National Laboratory

24. The new technique *Edge, R.D. 1958 Fig. 8. Approximate neutron density near a water surface Speculation: Snow pack depresses neutron flux Along with ponding, it could significantly skew the count rates of thermal neutrons.

25. Fig. 3a. Epithermal neutron flux as a function of water content (%) * F. M Phillips et al., 2000. Fig. 3b. Thermal neutron flux as a function of water content. The thermal flux data of Hendrick and Edge 1966 are shown for comparison.

26. What This Means Made-up Basalt varying water content, 3%, 20%, 40% varying amount of water on top of saturated soil, 5cm, 10cm, and 20cm of water. Results are a good indication that we are on the right path Although, some of it still is ambiguous

27. Real Life Simulation, MCnp For a typical Montana soil, 20% water content Thermal & Epithermal above and below ground

28. Real Life Simulation, MCnp Thermal & Epithermal above and below ground 20 cm of water

29. Fake Basalt, variable WC Thermal Epithermal total Thermal Epithermal total Thermal Epithermal total

30. Air/soil boundary effect Fig. 4 Slow cosmic- ray neutron density below water. *Edge, R.D. 1958

31. Variable Water Depth

32. Next Steps Analysis of current results, what do they mean in terms of estimating snow pack thickness? Correlation to snow water equivalent? Boundary affects between air/water/soil. Function of thickness of water in between?

33. References: Avdyushin, S.I., V.V. Abelentsev, E.V. Kolomeets, V.V. Oskomov, R.G.-E. Pfeffer, K.O. Syundikova, and S.D. Fridman, 1988. Estimating snow moisture reserve and soil humidity from cosmic rays. Izvestiya Akademii Nauk SSSR, Seriya Fizicheskaya 52, 2454-2456. Bissell, V.C., and Z.G. Burson, 1974. Deep snow measurements suggested using cosmic radiation. Water Resources Research 10, 1243-1244. Kodama, M., S. Kawasaki, and M. Wada, 1975. A cosmic-ray snow gauge. International Journal of Applied Radiation and Isotopes 26, 774-775. Kodama, M.,1980. Continuous monitoring of snow water equivalent using cosmic ray neutrons. Cold Regions Science and Technology, 3: 295-303.

34. Above ground 3% 20% 40%

35. Thermal & Epithermal 3% 20% 40% 3% 20% 40%