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Borexino Solar Neutrino Program Toward CNO Neutrinos Better 7 Be and pep Data

Borexino Solar Neutrino Program Toward CNO Neutrinos Better 7 Be and pep Data. Frank Calaprice Princeton University. Borexino. It is the only operating detector capable of direct detection of low energy solar neutrinos. It’s signature feature is unmatched ultra-low background.

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Borexino Solar Neutrino Program Toward CNO Neutrinos Better 7 Be and pep Data

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  1. Borexino Solar Neutrino ProgramToward CNO NeutrinosBetter 7Be and pep Data Frank Calaprice Princeton University Borexino General Meeting Milan 12/16-17/14

  2. Borexino • It is the only operating detector capable of direct detection of low energy solar neutrinos. • It’s signature feature is unmatched ultra-low background. • Lower background by scintillator re-purification was successfully demonstratedin 2010-2011. • Future solar neutrino program depends on reducing background and systematics, energy calibration, etc. Borexino General Meeting Milan 12/16-17/14

  3. Motivation for CNO Neutrinos • Detection of CNO neutrinos would be first confirmation of fusion process that powers most stars. • The CNO neutrino rate is proportional to abundance of elements (e.g. C) heavier than 4He in Sun. • Can help resolve the solar “metallicity problem”. • Measurement, or even detection, of CNO neutrinos would be the crowning achievement of Borexino. • Voted #1 Science Priority by BorexinoCollaboration (LNGS 2011). Borexino General Meeting Milan 12/16-17/14

  4. Standard Solar Model Fluxes Accurate measurements of CNO (+/-10%) and 7Be(+/- 3%) coupled with improved nuclear reaction cross sections (LUNA) can determine if the Sun has high or low metallicity. Low metallicity has serious problems with helioseismology. High metallicity implies standard solar model is wrong and needs modification, such as allowing for different formation scenarios with accretion and planetary formation. Borexino General Meeting Milan 12/16-17/14

  5. Motivation for pep NeutrinosMSW Theory & Non-standard Interctions MSW theory of neutrino oscillations in the Sun predicts a transition in survival probability of ne from vacuum oscillations below 2 MeV to matter effect oscillation at higher energy. The effect has been observed Reducing the uncertainty in pep neutrino rate will improve the confirmation of this MSW feature, and tighten constraints on non-standard interction. pep pp Uncertainty too large. 77Be 8B Borexino General Meeting Milan 12/16-17/14

  6. Requirements for Future Program • Lower 210Bi background • CNO high metallicity rate is ~ 5 cpd/100 t • 210Bi is too high at ~ 20 cpd/100 t • Factor of 10 reduction to 2 ± 1 cpd/100t is needed. • Reduction of 210Pb, or subtraction with 210Po, or both. • Better energy calibration ( ± 3%  ± 1 %) • Better understanding of energy dependence of quenching of scintillation light for β’s and γ’s. • Separate measurement of energy spectrum of 210Bi. • Control of detector temperature. • Slow drifts over few months may be ok. • Changes must be small and slow, < 0.1 C/month Borexino General Meeting Milan 12/16-17/14

  7. Background Reduction by Scintillator Re-purification • The main backgrounds are 210Bi (parent: 210Pb) and 210Po in the liquid scintillator. • We can remove these backgrounds from the scintillator by a “loop-mode” re-purification: • Continuously remove scintillator from the bottom of the vessel, passs it through a “purification system”, and return it to the top of the vessel. • We have 3 powerful re-purification methods: • Nitrogen stripping to remove volatile compounds. • Water extraction to remove many impurities, especially highly polar compounds, but also 210Pb, and 210Po. • Takes advantage that scintillato is immiscible in water and mixing intimately in a column in cross-current mode will transfer impurities from scintillator to water. • Water must be free of the radioactive impurities. • Distillation to remove non-volatile compounds. PPO will be removed with impurities and needs to be separately purified and mix in with the PC again to reconstitute the scintillator. Borexino General Meeting Milan 12/16-17/14

  8. Overview of the Borexino Detector(Mostly Active Shielding) • Shielding Against Ext. Backgnd. • Water: 2.25m • Buffer zones: 2.5 m • Outer scintillator zone: 1.25 m • Main backgrounds: in Liq. Scint. • 14C/12C • 10-18 g/g. cf. 10-11 g/g in air CO2 • U, Th impurities • 222Rn daught (210Pb, 210Bi, 210Po) • 85Kr • Light yield (2200 PMT’s) • Detected: 500 pe/MeV (~4%) • Pulse shape discrimination. • Alpha-beta separation Borexino General Meeting Milan 12/16-17/14

  9. Background Reduction with Loop Re-purification of Liquid Scintillator • “Loop” purification is achieved by draining fluid from bottom of vessel, passing it through purification system, and returning it to the top. • Processes available are: • Water extraction • Distillation • Nitrogen stripping (85Kr) Borexino General Meeting Milan 12/16-17/14

  10. Results of 6 cycles of Re-purification • 85Kr: 30 cpd/100t → < 5 cpd/100t • 238U (226Ra): [(530 ± 50) → < 8 x 10-20 g/g 214Bi-214Po Reduction factor > 77 (< 0.8 count/100t/yr). • 232Th: [(3.8 ± 0.8 → < 1.0] x 10-18 g/g 212Bi-212Po Reduction factor > 3. ( < 0.8 count./100t /yr) • 210Bi: 70 cpd/100t → 20 ± 5 cpd/100t • 210Po: Essentially not reduced!! Why? Rates before purifcation are based on 153.6 ton-yr exposure taken in 740.7 d between May 16, 2007 and May 8, 2010. See Borexino Coll. arXiv 1308.0443v1. Borexino General Meeting Milan 12/16-17/14

  11. Spectacular Results for U and Th. • The low levels of U and Th are quite likely the lowest levels ever achieved in a counting experiment. • Results deserve a separate publication. • Why did water extraction work so well for these but not so well for Pb and Po? • Answer: De-ionization reduces Ra with large reduction factor, while reduction of Pb and Po is poor. • Mg, an alkaline earth like Ra, is reduced by >106. Borexino General Meeting Milan 12/16-17/14

  12. Backgrounds before & after Water Extraction + N2 Stripping Region sensitive to CNO & 210Bi After re-purification 2012-2013 (with 11C cuts) Before re-purification 2008-2010 Without 11C cuts. See arXiv1308.0443v1. Borexino General Meeting Milan 12/16-17/14

  13. “Purified” Water Surprises • LNGS round water has relatively high levels of 222Rn (10,000 Bq/m3) and its daughters 210Pb-210Bi-210Po (1-10 Bq/m3) • The inability to remove 210Po and some problems with 210Pb led to investigations of the water. • Surprise #1 • Water purification by de-ionization is not very effective in removing 210Pb, and especially not effective for 210Po. • Surprise #2 • 210Po has a volatile compound (B.P. 138 C) that is not removed by simple distillationof water. • Fractional distillation of water is needed. Borexino General Meeting Milan 12/16-17/14

  14. Surprise #1 Nov 2012 • Reduction of 210Pb (210Bi) was inferred from ICPMS measurements of stable 208Pb. • 208Pb reduced by x700 by de-ionization(modest) • Typical reduction is x105-6 for dissolved salts. • Reduction of 210Po was measured by direct counting of alphas, using deposition on silver. • 210Po reduced by X10 by de-ionization (very poor!) • 210Po in de-ionized water is ~0.1 Bq/m3(very high). Borexino General Meeting Milan 12/16-17/14

  15. Recent Measurements • New Water Plant Installed 2012-2013 • New Measurements of 210Po Oct 2013 • [210Po] = 0.1-0.5 Bq/m3, even higher than before. • Output water: 5.3 MeV 210Po alpha peak (~0.5 mBq/liter.) Borexino General Meeting Milan 12/16-17/14

  16. Water Purification Plant (new version) Filtering Reverse Osmosis Ion removal Degassing Ultra-Q 3 F153 0.1 μm Pre F140 1000 l/h 2 RO2 EDI Stripping White Tank F141 1 1500 l/h 3000 l/h F156 0.02 μm 4 G159 RO1 Degassifier Liqui-Cel POU G154 Borexino General Meeting Milan 12/16-17/14

  17. Surprise #2 • Publications claimed that some 210Po in well water is in a volatile compound produced by micro-organism processes. • “Bio-Volatilization of Polonium: Results from Laboratory Analyses”, N. HUSSAIN, T. G. FERDELMAN, T. M. CHURCH, Aquatic Geochemistry 1: 175-188, 1995 • Dimethyl polonium: Boiling point ~138 C (est.) • Poor removal of 210Po by de-ionization and distillation due to volatile polonium compound could explain long-standing problems Borexinohas with this isotope. Borexino General Meeting Milan 12/16-17/14

  18. Test of Small Fractional Distillation System to Remove 210Po from Water. • Small-scale fractional distillation system designed and tested at Princeton to remove dimethyl polonium from well water. • 6-foot tall column with structured packing and high reflux designed for x1000 reduction. • Princeton well water has x5 more 210Po than LNGS. • Results: • Operated as simple evaporator, high 210Po in product. • Operated with high-reflux column, no 210Po in product. • Reduction factor > 300. Prototype distillation system with two 6-foot columns: 2 l/hr capacity. Students : B.Russell, C. Aurup, W. Taylor Borexino General Meeting Milan 12/16-17/14

  19. Basic requirement for W.E. to work. • The impurities will partition between the two fluids according to the ratioof their solubility “S” and concentration “C” in the two fluids. • If there is good contact between the fluids, with high surface area, the impurities will move between the two fluids and eventually reach an equilibrium. • [S]water x [C]water= [S]scintx [C]scint • This occurs multiple times (stages) in a tall column with reduction of impurity in scintillator for each stage provided: • [S]water]x [C]water > [S]scint x [C]scint. Borexino General Meeting Milan 12/16-17/14

  20. Observation Radioactivity in water must be lower than in scintillator, for water extraction to work. We have terrible water with high 210Po. • Water extraction works when 210Po in scintillator is high. We have marginal water for 210Pb • Water extraction worked, but we may be near limit. Borexino General Meeting Milan 12/16-17/14

  21. While we’re at it… • Let’s estimate what would happened when we used water extraction to purify the PPO Master Solution, a process that used 4 water extractions. • Say water has 210Po @ 0.1 Bq/m3 ~ 9000 cpd/t. • We use the water to purify PPO Master Solution with ~120 g PPO/liter PC, then add PC to make 1.5 g/l scintillator. Dilution factor of ~100 to make scintillator • Distillation of PPO Master Solution is at high temperature. • Doesn’t help to reduce the 210Po added in water extraction. • This means scintillator made with freshly purified PPO could have ~100 cpd/ton, assuming equal partitioning. Borexino General Meeting Milan 12/16-17/14

  22. 210Po in Borexino Filling & Re-filling #1 DMP Revovals • 210Po Filling: 80cpd/t • Estimate PPO M.S. ~ 100 cpd/t • Three refillings with LVD PPO • First Refill Oct-Nov 09 • Done before DMP removal. • Rate increase: 2 cpd/t • Total in 300 t ~600 cpd • 2.5t LS added: ~240 cpd/t • Expect PPO MS ~100 cpd/t • 210Po is overestimated (mixing) • Get rough agreement! Borexino General Meeting Milan 12/16-17/14

  23. Re-fillings 2 and 3 • June 9 ’09: Refilling 2 • Added 9.5 m3 of PC+PPO • 210Pb went up ~ 20cpd/t in whole IV • Huge increase about 10 times bigger that RF #1! • Incompatible with contamination due to W.E. of M.S. • DMP removal #1 occurred between RF 1 and RF 2 • March 10 -25: Refilling 3 • Added 7.5 m3 PC+PPO • Another big increase in 210Po, 15 cpd/t in whole vessel • Again not compatible with W.E. of Master Solution. • DMP removal # 2 occurred between RF 2 and RF 3. • DMP operations followed by inadequate cleaning of the plant probably caused most of the increases in the 210Po during refillings. • The contamination persisted for first Water Extractions, WE 1, WE2, then decreased with each WE as the system cleaned up. Borexino General Meeting Milan 12/16-17/14

  24. Conclusions on Filling and Refillings • Initial 210Po in Borexino is consistent with use of contaminated water to purify PPO Master Solution. • First re-fill with 2.5 t PC+PPO produced small increase in 210Po, consistent with Master Solution. • DMP operations followed by inadequate cleaning of the plant caused most of the increases in the 210Po during refillings. • The contamination persisted for first Water Extractions(WE 1, WE2) then decreased with each WE as the system cleaned up. Borexino General Meeting Milan 12/16-17/14

  25. What about Water Extractions and 210Po? • Why didn’t it decrease? • Could contaminated water explain this? • This is more difficult to estimate because the water is distilled multiple times during water extraction. • But we can make a comparison with CTF 3 experience. Borexino General Meeting Milan 12/16-17/14

  26. Comparison of CTF and BX Water Extraction Lowest 210Po Levels 22 cpd/t 36 cpd/t 40 30 20 [210Po]minCTF3: 28 cpd/t [210Po]min BX: 30 cpd/t 210Po was reduced in CTF1 and 3 but stopped at ~30 cpd/t, lower than our estimate in water. Similar level is found in BX WE 6. Extra distillations help but not much. Borexino General Meeting Milan 12/16-17/14

  27. Major Milestone Achieved • After more than 20 years we now have evidence for a major source of 210Po that is capable of explainig210Po problems in Borexino. • Estimates show that water extraction of PPO master solution can produce ~ 100 cpd/t in scintillator • This is a likely explanation for our original source of 210Po without 210Pb, and first refilling. • 210Po in water can limit the effectiveness of water extraction. A lower level of 30 cpd/t in scintillator is reasonable. Borexino General Meeting Milan 12/16-17/14

  28. Moving Forward… • We need to reduce radioactivity in water used for water extraction. • Fractional distillation has been designed and successfully tested for removing 210Po from well water. • Designed have been made and changes are modest: • Two new columns will be added to existing equipment. Borexino General Meeting Milan 12/16-17/14

  29. Borexino Water Extraction SystemsCurrent & Proposed Upgrade with 2 Fractional Distillation Columns Proposed System Present System Proposed System Current System Make-up water system not shown. Make-up water system not shown. Borexino General Meeting Milan 12/16-17/14

  30. As Built Water Extraction System Borexino General Meeting Milan 12/16-17/14

  31. Upgraded Water Extraction System Borexino General Meeting Milan 12/16-17/14

  32. Upgrade of PPO Distillation Column to Purify Water from D.I. Plant Present distillation unit with 5-ft packed column. Reflux not shown. Thanks to Andrea Ianni for nice drawings! Upgraded fractional distillation unit with 10-ft packed column Borexino General Meeting Milan 12/16-17/14

  33. Temperature Instabilities As Nicola Rossi pointed out, activities in Hall C can affect the temperature, which can have bad affects on the data. The annual on-off of the OPERA magnet is well known. The Darkside filling (day ~600) also had a noticeable affect. The CNO group noticed that this affects the 210Po rate, due to convective movement of activity from the IV to the FV. The change in 210Po affects the extrapolated “constant rate” due to 210Pb, and spoils the connection of 210Po and 210Bi that we would like to exploit. Stabilizing the temperature is important for CNO and SOX. 210Po increased in bottom of IV Borexino General Meeting Milan 12/16-17/14

  34. Stabilizing the Temperature • It will be difficult to stabilize the air temperature of Hall C, especially during the upcoming dismantling of OPERA and possible installation of the new LUNA accelerator. • Dampening the temperature swings, with a time constant comparable to the 210Po half-life, seems feasible with commercial insulation. • With 20 cm of standard insulation (k = 0.045 W/(m-K) the time constant for changing the temperature of the WT steel plate is ~125 d. • New material is becoming available with 10 times better insulation. Borexino General Meeting Milan 12/16-17/14

  35. Model of 210Pb - 210Bi - 210Po • Scintillator contains 210Pb that produces 210Bi and 210Po. • This can be removed by water extraction. • Nylon vessel has radioactivity on vessel surface and within nylon film. • Short-lived activity on (or in) nylon, 222Rn (3.8 d) and 210Bi (5.0 d), cannot travel far and decays near nylon, except if there are big temperature changes. • Movement is too slow by diffusion, but not by convection driven by temperature swings. • 210Po (138 d) can travel farther due to its longer half-life. • Improving temperature stability may keep it near the vessel. • 210Pb (22 year) can leach off surface and contaminate the scintillator. • We have not seen evidence of this from 210Bi, but it could be a problem. Borexino General Meeting Milan 12/16-17/14

  36. Temperature Stability • With the model outlined above, stabilizing temperature could minimize convection currents and keep 210Po that emanates from the vessel away from the fiducial volume. • With some uncertainty, measuring 210Po in the fiducial volume could determine 210Bi, then CNO. • If temperature stability works well to stabilize 210Po, the best strategy would be to use both methods. • Reduce 210Bi background for a spectral fit directly to get CNO. • Use secular equilibrium to determine 210Bi from 210Po, subtract 210Bi and then do another fit. • With two independent methods, we could end up with a very reliable measurement of CNO. Borexino General Meeting Milan 12/16-17/14

  37. Solar and SOX Compatibility • SOX benefits from lower background and more stable detector. • Solar benefits from energy calibration with Cr source. • Detector must be at low background before SOX if calibration is meaningful. • Both benefit if purification is done as soon as possible. • Both benefit with better temperature stability. Borexino General Meeting Milan 12/16-17/14

  38. Four-part Proposal • Begin construction of new column as soon as possible, aim for readiness in summer 2014. • Clean plants extensively with new water and do test of water extraction in Borexino. • Need with sufficient scintillator to be meaningful. • If test is positive proceed as soon as possible with goal of 210Bi < 2 cpd/ton. • Insulate BX Water Tank to stabilize temperature of detector. Borexino General Meeting Milan 12/16-17/14

  39. The End“The day he was released from prison game me a sense of what people can do when they are guided by their hopes, not their fears” Obama on Mandela Borexino General Meeting Milan 12/16-17/14

  40. Addendum: Kamland Experience • Kamland copied much of Borexino including the purification of the PPO by water extraction. • Kamioka mine has higher radon levels than LNGS, which is relatively low due to non-granite rock. • Their experience supports conclusions reached here. Borexino General Meeting Milan 12/16-17/14

  41. KamLand after 4 Cycles of Distillation14C 232Th 238U Borexino ~ 2 x 10-18 < 0.1 x10-17 < 0.08 x10-18 Borexino General Meeting Milan 12/16-17/14

  42. KamLand after 4 Cycles of Distillation85Kr 210Bi 210Po BX BX BX 55000 cpd/t <0.0005 0.002 0.10 Reduction factor: 25,000 2,400 15. Borexino General Meeting Milan 12/16-17/14

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