Using neutrinos to study the Earth

1. Using neutrinos to study the Earth Nikolai Tolich Lawrence Berkeley National Lab 11/28/06 University of Washington Colloquium

2. Outline • Where are these neutrinos coming from? • Short introduction to neutrino oscillations • Recent geoneutrino result • GNuLAND: The future • Other neutrino sources for GNuLAND UW colloquium

3. Structure of the Earth • Seismic data splits Earth into 5 basic regions: inner core, outer core, mantle, oceanic crust, and continental crust. • All these regions are solid except the outer core. UW colloquium

4. Convection in the Earth • The mantle convects even though it is solid. • It is responsible for the plate tectonics and earthquakes. • Oceanic crust is being renewed at mid-ocean ridges and recycled at trenches. Image: http://www.dstu.univ-montp2.fr/PERSO/bokelmann/convection.gif UW colloquium

5. Total heat flow from the Earth Bore-hole measurements • Conductive heat flow measured from bore-hole temperature gradient and conductivity • Deepest bore-hole (12km) is only ~1/500 of the Earth’s radius. • Total heat flow 44.21.0TW (87mW/m2), or 311TW (61mW/m2) according to more recent evaluation of same data despite the small quoted errors. Heat flow Image: Pollack et. al UW colloquium

6. Radiogenic heat • U, Th, and K concentrations in the Earth are based on measurement of chondritic meteorites • Chondritic meteorites consist of elements similar to those in the solar photosphere • U, Th, and K concentrations in Bulk Silicate Earth (BSE) are 20ppb, 80ppb, and 240ppm, respectively • This results in U, Th, and K heat production of 8TW, 8TW, and 3TW, respectively. UW colloquium

7. Discrepancy? • The measured total heat flow is 44 or 31 TW. • The estimated radiogenic heat produced is 19 TW. • Models of mantle convection suggest that the radiogenic heat production rate should be a large fraction of the measured heat flow. • Problem with • Mantle convection model? • Total heat flow measured? • Estimated amount of radiogenic heat production rate? • Geoneutrinos can serve as a cross-check of the radiogenic heat production. UW colloquium

8. 234U 238U 234Pa 230Th 234Th 228Th 232Th 40Ca 228Ac 40K 226Ra 224Ra 228Ra 40Ar 222Rn 220Rn 218At 210Po 214Po 218Po 212Po 216Po 210Bi 214Bi 212Bi 206Pb 210Pb 214Pb 208Pb 212Pb 210Tl 208Tl • Beta decays produce electron antineutrinos Radiogenic isotopes UW colloquium

9. Geoneutrino signal Neutron inverse beta decay threshold UW colloquium

10. U and Th in the Earth • U and Th are thought to be absent from the core and present in the mantle and crust. • The core is mainly Fe-Ni alloy. • U and Th are lithophile (rock-loving), and not siderophile (metal-loving) elements. • U and Th concentrations are highest in the continental crust. • Mantle crystallized outward from the core-mantle boundary. • U and Th prefer to enter a melt phase. • Th/U ratio of 3.9 is known better than the absolute concentrations. UW colloquium

11. Georeactor • It has been hypothesized that a blob of uranium is located at the center of the Earth. • This could then form a natural nuclear reactor produce up to 6TW of heat powering the Earths dynamo. • This could explain the observed anomaly in the 3He/4He ratio. • The only way to test this hypothesis directly is to observe the neutrinos produced by this reactor. UW colloquium

12. The expected geoneutrino flux • The geoneutrino flux per unit energy is given by • A is the decay rate per unit mass • is the number of antineutrinos per decay chain per unit energy • a(L) is the mass concentration as a function of position in the Earth • r(L) is the density as a function of position in the Earth • is the survival probability due to neutrino oscillations UW colloquium

13. Introduction to neutrino oscillations

14. Solar neutrinos Gallium Chlorine SNO + SK UW colloquium

15. Reactor neutrinos

16. Neutrino oscillations • Best fit is tan2q = 0.40 Dm2 = 8.210-5eV • For the geoneutrino energy range, due to the distributed geoneutrino generation points UW colloquium

17. e 2H+n e- d p 6.25 MeV p x 3H x d 35Cl+n p 8.6 MeV n x x 36Cl e- e- SNO CC ne + d  p + p + e- Phase I Phase II NC nx + d  p + n + nx nx + e- nx + e- ES

18. SNO phase III • Added array of 40 3He proportional counters neutral-current detectors (NCDs). • NC signal observed in NCD array via n+3He3H+p. • Improve q by breaking the CC and NC correlation (r = -0.53 in Phase II) UW colloquium

19. Recent geoneutrino result

20. Have geoneutrinos been seen? Fred Reines’ neutrino detector (circa 1953) By Gamow in 1953 UW colloquium

21. Were they geoneutrinos? ~30TW

22. Recent results from KamLAND • From March, 2002 to October, 2004. • 749.1±0.5day of total live-time. • (3.46 ± 0.17)  1031 target protons, 5m radius fiducial volume. • 0.687±0.007 of the total efficiency for geoneutrino detection. • Expect 14.8± 0.7 238U geoneutrinos and 3.9 ± 0.2232Th geoneutrinos. • 152 candidate events • 127±13 background events. Nature 436, 499-503 (28 July 2005) UW colloquium

23. Candidate energy distribution Expected total Candidate Data Expected total background Expected reactor (,n) Expected U Random Expected Th UW colloquium

24. How many geoneutrinos? Expected ratio from chondritic meteorites Best fit 3 U geoneutrinos 18 Th geoneutrinos Expected result from reference Earth model Central value 28

25. Reality check… • Could all “geoneutrinos” come from an undiscovered uranium deposit? • Not likely. The antineutrino flux from a 100kton uranium deposit (the world’s largest) located 1km away from KamLAND would be only 3% of expected geoneutrino flux. UW colloquium

26. The future

27. KamLAND and geoneutrinos • KamLAND was designed to measure reactor antineutrinos, these are the most significant background and are irreducible. • Reactor antineutrino signals are identical to geoneutrinos except for the prompt energy spectrum. • Working on purifying the liquid scintillator, which will reduce the (,n) background events. KamLAND UW colloquium

28. Homestake: reactor background ~7% of KamLANDs Future locations • No nearby nuclear reactors • On oceanic crust to probe mantle • On continental crust to probe continental crust • Needs to be shielded from cosmic muons UW colloquium

29. 111 93 75 56 37 21 3 Geoneutrino flux Homestake: geoneutrino flux 50% larger than at KamLAND 54 per 1032 target protons per year • www.fe.infn.it/~fiorenti UW colloquium

30. Local heat flow Homestake • The local heat flow is consistent with the continental crust average. • A measurement within DUSEL would allow a constraint on the local contribution to the geoneutrino flux. UW colloquium

31. Nearby uranium deposits • Assuming 3600kton of uranium (the Earth’s total reasonably assured reserves) were located 100km away this would contribute less than 0.03% to the expected signal. Homestake UW colloquium

32. Expected signal U geoneutrinos Th geoneutrinos 6TW georeactor Commercial reactors UW colloquium

33. Local contribution Mantovani F. et. al Phys Rev D 69 (2004) 013001 • On continental crust approximately 50% of the flux comes from the surrounding 500km. UW colloquium

34. Crust age UW colloquium

35. Crust type UW colloquium

36. GNuLAND requirements • GNuLAND: Geo-Neutrino Liquid scintillator Anti-Neutrino Detector • Expected geoneutrino flux has 9% error due to neutrino oscillations. • To make a ~10% measurement of the geoneutrino flux requires exposure of 2.31032 proton years. • ~2-3 year measurement with KamLAND sized detector. • To observe a 6TW reactor at the Earths core at 3 would require exposure of 0.81032 proton years. UW colloquium

37. Palo Verde style detector • Palo Verde ~11tons liquid scintillator. • Needs to be ~100 times larger (~4.5 times each dim.). • Could be rearranged to meet cavity shape. • Modular: • Maybe easier to install • Could be moved? • Scalable UW colloquium

38. KamLAND style detector • 1kton liquid scintillator. • ~20m diameter sphere. • Monolithic: • Lower radioactive backgrounds • Fully contained events UW colloquium

39. Future U prospecting Conclusion Nature Journal says that “Future observations at KamLAND, and at the Borexino detector under the Gran Sasso mountain in central Italy, which begins operation in 2006, will generate more data and provide greater sensitivity in testing the nature and sources of geoneutrinos.” "Before the revolution really comes to fruition, I think it'll take some time," Gratta told Live Science, "I would imagine one or two decades, before we have more of those detectors and maybe larger ones built in the appropriate place for geophysics.'' Clearly this is still some time off commercial application in the mining sector, and these scientists probably aren’t too concerned with how this could improve exploration success like 3-D seismic has done for oil and gas. They are looking at things like the heat of the Earth’s core. But when the BHP’s of this world get their hands on this, they could well put it to good use, as long as it doesn’t have the unintended consequence of scaring up too many new deposits and depressing commodity prices. Either way, it will make for an interesting future. Science buffs can access the tremendously complicated results of the KamLAND study here.

40. Other neutrino sources

41. Supernova 1987A • Kamiokande II detected 11 neutrinos • IMB detected 8 neutrinos • Baksan detected 5 neutrinos • In a burst lasting less than 13 seconds • Told us a lot about supernovae and neutrino properties UW colloquium

42. nm, nt, nm, nt(T = 8 MeV) ne (T = 5 MeV) Supernovae detection • Observe ~200 neutrinos from a supernova via proton scattering. • This process would be the only model independent method capable of determining the total energy and nx temperature. Total energy 21053 ergs at 10 kpc. 1kton. ne(T = 3.5 MeV) Beacom J. et. al Phys. Rev. D 66, 033001 (2002) UW colloquium

43. Homestake 4850’ Homestake DUSEL Solar neutrinos Galbiati C. et. al Phys. Rev. C 71, 055805 (2005) 11C, Gran Sasso UW colloquium

44. pep neutrinos • Tests transition from vacuum to matter oscillations • The pep neutrinos give essentially the same information as the p-p neutrinos • Measures total solar neutrino flux independent of luminosity constraint Vaccum Matter pep SK, SNO Ga Cl Bahcall J.N. et. al JHEP 11, 004 (2003) UW colloquium

45. GNuLAND status • Have money at LBNL to pursue detector Monte Carlo for next 3 years. • Requested funding from NSF (&DOE) to pursue scintillator research. • Plan on having a proposal ready within 3 years. UW colloquium

46. Conclusions • KamLAND observed 4.5 to 54.2 geoneutrinos with 90% C.L. This is consistent with the current geological models. • Current errors in our knowledge of the neutrino oscillation angle mean we should not plan on measuring the geoneutrino flux to better than ~10%. • A ~1 kton detector is the ideal size. However, due to reactor backgrounds KamLAND will never reach this sensitivity. • GNuLAND would allow us to strongly constrain the total amount of Uranium in the crust, which is vital for models of Earth formation. • GNuLAND could also observe a natural reactor at the Earth’s core. UW colloquium

48. Other geoneutrino detectors UW colloquium

49. Oceanic crust heat flow UW colloquium Stein and Stein, 1992

50. Relic supernovae neutrinos UW colloquium