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Geology from Geo-neutrino Flux Measurements

Geology from Geo-neutrino Flux Measurements. Eugene Guillian / Queen’s University DOANOW March 24, 2007. Content of This Presentation. KamLAND: The Pioneering Geo-neutrino Detector Proved that geo-neutrinos can be detected, but under very unfavorable conditions

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Geology from Geo-neutrino Flux Measurements

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  1. Geology from Geo-neutrino Flux Measurements Eugene Guillian / Queen’s University DOANOW March 24, 2007

  2. Content of This Presentation • KamLAND: The Pioneering Geo-neutrino Detector • Proved that geo-neutrinos can be detected, but under very unfavorable conditions • How to proceed in the next generation • 10  KamLAND (size  time) • Low background • Simple neighboring geology • Multiple sites • How to extract geological information from flux measurements • 1 site • 2 sites • Statistical sensitivity • Effect of nuclear reactor background • Possible applications

  3. Geo-neutrinos • Produced in the radioactive decay of unstable isotopes • The flux of geo-neutrinos depends on: • Total mass of these isotopes in the earth • The distribution of the isotopes in the earth

  4. Total Isotopic Mass in the Earth • An educated guess: • CI carbonaceous chondrite meteorite • Representative of the raw material from which the earth was formed • Based on the isotopic abundances in this type of meteorite, estimate the initial elemental abundance • Evolution of the early earth • Core separation • Bulk Silicate Earth (BSE) • Crust extraction from BSE

  5. Crust Extraction from the Mantle • Uranium, thorium, and potassium are all lithophile elements • They have a strong tendency to leave the mantle and stay in the crust • A good starting guess about isotope concentrations:

  6. More Detailed Earth Models • Examples: • Mantovani et al., Phys. Rev. D 69, 013001 (2004) • S. Enomoto, Ph. D. Thesis, Tohoku University (2005) • Turcotte et al., J. Geophys. Res. 106, 4265-4276 (2001)

  7. The Mantovani et al. Reference Model • Note: These are just educated guesses • There is considerable spread in what could “reasonably” be assigned to these values

  8. S. Enomoto Reference Model

  9. Turcotte et al. Models • These models argue for significant level of selective erosion of crustal uranium, and subsequent recycling into the upper mantle. • Mantle convection boundary is deeper than the 660 km seismic discontinuity (1200 ± 200) km

  10. The Overall Picture of Geo-neutrinos • The models differ in: • The number of geological subdivision • The assignment of isotopic concentrations in each subdivision • But, at the very basic level, similar (i.e. within a factor of several) geo-neutrino fluxes are predicted • The flux at the surface of the earth is ~106 cm-2 s-1 • The flux varies by a factor of several depending on the location

  11. A Neutrino Flux Map: Example • The color scale is Y = yield (number of detected events per unit of exposure)

  12. A Note on Units: The Scale of Things • Geo-neutrino flux units • Several  106 cm-2 s-1 • Geo-neutrino detection rate (yield) • 1032 p-yr • The number of geo-neutrino events that can be detected with 1032 free protons exposed for 1 year • For a typical target, 1032 free protons is about 1000 tonne • The volume is about the size of a large room Several million geo-neutrinos stream through a penny every second

  13. e+ kinetic energy M(e+) = 0.5 MeV En - 1.8 MeV 1.8 MeV Mn - Mp = 1.3 MeV Detecting Geo-neutrinos with a Liquid Scintillator Detector • Inverse Beta Decay Anti-neutrino Free Proton Neutron Positron Prompt energy deposition En - 0.8 MeV • 1.8 MeV energy threshold Energy Input Deposited Energy e+ kinetic energy En - 1.8 MeV M(e+) = 0.5 MeV 1.0 MeV from e+-e- annhilation M(e-) = 0.5 MeV

  14. Detecting Geo-neutrinos • Delayed energy deposition • Neutron thermalization & capture on free proton • ~200 micro-second • 2.2 MeV gamma rays • Prompt-delayed correlation • Reduces background noise to a very low level Delayed Prompt

  15. 1.8 MeV Energy Threshold • Only the highest-energy anti-neutrinos from 238U and 232Th are detectable • 40K is not detectable with this technology Nature 436, 499-503 (28 July 2005)

  16. Inverse Beta Decay Cross Section • Cross section • The effective cross sectional area of a free proton from the point of view of a geo-neutrino • ~10-43 cm2 • Geo-neutrino flux: • ~106 cm-2 s-1 = ~1013 cm-2 yr-1 • Probability that a particular free proton will be hit by a geo-neutrino in one year: Extremely small! ~10-43 cm2 ~1013 cm-2 yr-1 = ~10-30 per year • This determines the necessary target size • A detector with 1032 free protons should see ~1032 10-30 = ~100 events

  17. The Fine Print • Detection efficiency ≈ 70% • Neutrino oscillation • When geo-neutrinos travel more than ~50 km, it becomes a mixture of undetectable types of anti-neutrinos • This effect reduces the detectable flux by about a factor of 0.57

  18. Extracting Geological Information from a Geo-neutrino Flux Measurement • Example: Flux at Sudbury • Assumes the S. Enomoto reference model, which determines: • The total flux at Sudbury • The relative contributions from 238U and 232Th 0.459·NU 0.541·NU NTh • Region 1: • N1 = NTh + 0.459·NU • Region 2: • N2 = 0.541·NU Region 1 Region 2

  19. Extracting Geological Information from a Geo-neutrino Flux Measurement • N1 and N2 are the measured quantities • NU and NTh are quantities that carry geological information It is possible to separately measure the uranium and thorium flux

  20. Upper Limit on the Sensitivity • The statistical error sets the upper limit on the sensitivity to the geo-neutrino flux measurement Mantovani et al. Ref. Model, 1033 p-yr The sensitivity scales with exposure as:

  21. What Does the Flux Tell Us about the Earth?

  22. Analyzing the Flux Formula Earth models (not well known) Constant (accurately known) Relatively well-determined through seismic tomography The goal of neutrino geology is to learn about aX(r) from measurements of FX(r)

  23. What Can We Learn about Isotope Concentrations from a 1-site Measurement? • We can get the isotope concentration averaged over the entire earth • But the information about the isotope distribution in the earth and the total amount of isotopes is poorly determined • One can distribute X between the mantle and crust to produce the same answer • The constraint on models is weak

  24. Neutrino Geology in the Near Future • KamLAND was the pioneering neutrino geology detector

  25. Goals for the Coming Generation

  26. A 2-site Geo-neutrino Measurement: An Example • Two measurements • Can solve for two unknowns • The continental crust and mantle account for most of the observed geo-neutrinos, regardless of the detector location • The two unknowns: • Average isotope concentration in the continental crust • Average isotope concentration in the mantle • The small contribution from other geological subdivisions is approximated as being zero

  27. An Example of a 2-Site Measurement • The mantle contribution is the same at both sites • Assume that the mantle is spherically symmetric • A large contrast in the continental crustal component exists • The contribution from other geological structures is negligible

  28. Flux vs. Concentration Equations

  29. Geologic Integrals Unit: g/cm Continental Crust ( 1016 g/cm) 148 Mantle

  30. Solving for the Concentrations 22 Matrix Equation 1016 Uranium Flux Geologic Integral Matrix Solution of the unknown quantities in terms of the measured ones

  31. Statistical Sensitivity of 2-Site Measurements • Exposure = 1033 p-yr • Model = Mantovani et al. Reference • Oceanic site = Hawaii • Vary the “continental” sites Concentration / 238U / Continental Crust Concentration / 238U / Mantle Upper CC Input Lower Mantle Input Middle CC Input Lower CC Input Upper Mantle Input Statistical Uncertainty ≈ 12% Statistical Uncertainty ≈ 24%

  32. Statistical Sensitivity for Th Concentrations Concentration / 232Th / Continental Crust Concentration / 232Th / Mantle Upper CC Input Lower Mantle Input Middle CC Input Upper Mantle Input Lower CC Input Statistical Uncertainty ≈ 34% Statistical Uncertainty ≈ 77%

  33. S. Enomoto Reference Model ≈ 12% ≈ 22% ≈ 68% ≈ 36%

  34. Turcotte et al. Model I ≈ 22% ≈ 14% ≈ 38% ≈ 49%

  35. Turcotte et al. Model II ≈19% ≈17% ≈ 40% ≈58%

  36. Radiogenic Heat Measurement Heat from continental crust • The radiogenic heat is derived from the concentrations 40K term Heat from mantle

  37. Heat Measurements Mantovani Turcotte I Turcotte II Enomoto Continental Crust ≈25% ≈23% ≈19% ≈18% ≈28% Mantle ≈36% ≈40% ≈24% ≈17% ≈16% Total ≈15% ≈16% Dashed blue line: Estimated 40K contribution

  38. Background Noise • KamLAND from several years ago tells us a lot about background noise Nature 436, 499-503 (28 July 2005) • Internal background • 13C(a,n)16O (radon gas contamination) • External background • Anti-neutrinos from nuclear reactors N = 152 Nuclear Reactor 13C(a,n)16O

  39. Internal Background • A lot of R & D by the KamLAND team and others have taken place since the first geo-neutrino measurement • Make use of the R & D results, and learn from experience: • Make sure the liquid scintillator and other internal detector components have minimal exposure to radon gas • Use newly developed purification techniques to remove 210Pb (radioactive lead) from the liquid scintillator Assume that the internal background can be reduced to a negligible level

  40. Reactor Anti-neutrino Background • The only way to minimize this is to place the detector as far as possible from nuclear reactors Map of heat production by world-wide nuclear reactors ≈ 478 nuclear reactors world-wide ~30 to 40 TWt total heat Total generated heat ≈ 1.1 TWt ~20 to 30 TWt radiogenic heat

  41. Log-scale background rate (arbitrary units) Reactor Anti-neutrino Background Rate • Exposure = 1033 p-yr • Detection Efficciency = 0.70 • Reactor Duty Cycle = 0.80

  42. Subtracting the Reactor Background Region 3 En - 0.8 MeV Region 1 Region 2

  43. Sensitivity with Reactor Background

  44. Sensitivity with and without Reactor Background Example: Mantovani et al. Ref. Model No Reactor With Reactor

  45. Change in Sensitivity: Concentrations

  46. Change in Sensitivity: Heat • Red Points: No Reactor • Blue Points: With Reactor • Vertical Axis: Sensitivity (%) • Horizontal Axis: Location

  47. Testing Geological Models • Examples of what kind of sensitivity the next-generation geo-neutrino measurements might have to geological models • Distinguishing the Turcotte et al. models from the “Reference” models • What can we say about the Th/U ratio? • How well can we constrain radiogenic heat?

  48. The Turcotte et al. Models Oxidized atmosphere (2 Ga) made U soluble in H2O, but not Th. U gets recycled into the upper mantle. Mantle convection occurs only in UM. Mass of UM ≈ 0.5 times total mantle mass. • k = Th/U concentration ratio • BSE: k = 4 • Continental Crust: k = 5-6 • Upper Mantle: k = 2.5 • Lower Mantle: k = 4 • Mass balance of 238U between CC and UM • k for CC and UM combined must be 4 Primitive value

  49. Concentration Measurements for Different Models We can distinguish the mantle concentration of 238U of Turcotte I from those of “Reference” models • 2 sites = Hawaii & Sudbury • 1033 p-yr 238U/CC 238U/Mantle 232Th/CC 232Th/Mantle

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