The Braidwood Neutrino Experiment Ed Blucher Chicago Outstanding
The Braidwood Neutrino Experiment Ed Blucher, Chicago • Outstanding questions in neutrino oscillation physics: importance of 13 • Experimental approaches to 13; motivation for a precise reactor experiment • The Braidwood Experiment 17 November 2005 BNL HEP Seminar 1
Neutrino Oscillations 2 • During last few years, oscillations among different flavors of neutrinos have been established; physics beyond the S. M. • Mass eigenstates and flavor eigenstates are not the same: flavor eigenstates MNSP matrix mass eigenstates • Raises many interesting questions including possibility of CP violation in neutrino oscillations. • CP violation in neutrino sector could be responsible for the matter-antimatter asymmetry (leptogenesis) The antilepton excess is converted to a baryon excess through nonperturbative S. M. B+L violating, but B-L conserving processes.
3 2 -Flavor Neutrino Mixing The time evolution of the flavor states is: For a beam that is pure at t=0,
4 What do we know? Oscillations established with two distinct mass differences 1. Atmospheric: m 2~2. 5 e. V 2 Experiments using neutrinos produced by cosmic rays in atmosphere (e. g. , Super. K); verified with long-baseline accelerator experiment (K 2 K). K 2 K Super Kamiokande
5 2. Solar: m 2~5 5 e. V 2 Series of experiments using neutrinos from the Sun (e. g. , Ray Davis 37 Cl experiment, SNO) and KAMLAND experiment using reactors in Japan. Ray Davis SNO KAMLAND
What about LSND? 6 Unconfirmed observation of oscillations with m 2~1 e. V 2 by LSND does not fit into 3 generation model (with 2 independent mass splittings). Mini. Boone should have results early next year.
Neutrino mixing and masses 12 ~ 30° sin 2 2 13 < 0. 15 at 90% CL What is e component of 3 mass eigenstate? normal inverted 7 23 ~ 45°
Key questions in neutrino mixing • What is value of 13? • What is mass hierarchy? • Do neutrino oscillations violate CP symmetry? • Why are quark and neutrino mixing matrices so different? Value of 3 central to these questions; it sets the scale for experiments needed to resolve mass hierarchy and search for CP violation. 8
Methods to measure sin 22 13 9 • Accelerators: Appearance ( e) at m 2 2. 5 10 -3 e. V 2 NO A: <E > = 2. 3 Ge. V, L = 810 km T 2 K: <E > = 0. 7 Ge. V, L = 295 km • Reactors: Disappearance ( e e) at m 2 2. 5 10 -3 e. V 2 Use reactors as a source of e (<E >~3. 5 Me. V) with a detector 1 -2 kms away and look for non-1/r 2 behavior of the e rate Reactor experiments provide the only clean measurement of sin 22 : no matter effects, no CP violation, almost no correlation with other parameters.
10 Recommendation 2 (of 3): We recommend, as a high priority, a comprehensive U. S. program to complete our understanding of neutrino mixing, to determine the character of the neutrino mass spectrum, and to search for CP violation among neutrinos. This program should have the following components: • An expeditiously deployed multi-detector reactor experiment with sensitivity to disappearance down to sin 22 = 0. 01, an order of magnitude below present limits. • A timely accelerator experiment with comparable sin 22 = 0. 01 sensitivity and sensitivity to the mass hierarchy through matter effects. • A proton driver in the megawatt class or above and neutrino superbeam with an appropriate very large detector capable of observing CP violation and measuring the neutrino mass-squared differences and mixing parameters with high precision.
Both reactor and accelerator experiments have sensitivity to sin 22 , but accelerator measurements have ambiguities Example: T 2 K. P( e)=0. 0045 sin 22 13=0. 028 dcp — normal — inverted (5 yr ) +/- 0. 028 m 2=2. 5 10 -3 e. V 2 11
12 Reactor and accelerator sensitivities to sin 22 90% CL exluded regions with no osc. signal 90% CL allowed regions with osc. signal Braidwood δCP=0, Δm 2 = 2. 5× 10 -3 e. V 2 (3 yr reactor, 5 yr Nova) sin 22θ 13 = 0. 05, δCP=0, Δm 2 = 2. 5× 10 -3 e. V 2 (3 yr reactor, 5 yr T 2 K)
Resolving the 23 Degeneracy disappearance experiments measure sin 22 23, while P( e) sin 2 23 sin 22 13. • If 23 45 , disappearance experiments, leave a 2 -fold degeneracy in 23 – it can be resolved by combination of a reactor and e appearance experiment. 13 Green: Nova Only Blue: Braidwood Reactor plus Nova Red: Double-Chooz plus offaxis Example: sin 22 23 = 0. 95 0. 01 Δm 2 = 2. 5× 10 -3 e. V 2 sin 22 13 = 0. 05 90% CL Δm 2 = 2. 5× 10 -3 e. V 2 sin 22 13 = 0. 05 Braidwood (3 yrs) + Nova only (3 yr + 3 yr) Double Chooz (3 yrs) + Nova
14 CP Violation and the Mass Hierarchy P( e) T 2 K Nova sin 22 13=0. 1 CP
15 Example: Reactor + T 2 K running T 2 K - 5 years P( e) sin 22 = 0. 01 from reactor sin 22 13=0. 1 Neutrino, normal hierarchy Neutrino, inverted hierarchy CP
Nova and T 2 K Sensitivity to CP and Mass Hierarchy If Braidwood does not see an oscillation signal, it will be difficult for long-baseline “superbeam” experiments to investigate mass hierarchy and CP violation. 16
Reactor Measurements of Neutrino Oscillations per second. ec os ss Cr Flux tio n Reactors are copious sources of Detection of antineutrino by (~100 events /GW/ yr / ton at L= 1500 m) 17
18 Reactor Measurements of Past measurements: 13: Search for small oscillations at Pee 1 -2 km distance (corresponding to Our sensitivity goal: sin 22 ~0. 01. Level at which long-baseline accelerator experiments can be used to measure mass hierarchy, CP violation. Distance to reactor (m)
Chooz: Current Best Experiment P=8. 4 GWth 19 CHOOZ Systematic errors Reactor flux 2% Detect. Acceptance 1. 5% Total 2. 7% L=1. 05 km D=300 mwe m = 5 tons, Gd-loaded liquid scintillator sin 22 < 0. 15 for m 2=2. 5 10 3 e. V 2
20 How to improve on previous reactor experiments? Add an identical near detector Eliminate dependence on reactor flux; only relative acceptance of detectors needed Optimize baseline (1500 m) Larger detectors (5 ton 100 tons) Reduce backgrounds (Go deeper 100 m 150 to 300 m; active veto systems) ~200 m ~1300 m
21 Many sites have been discussed: • Kraznoyarsk (Russia) • Chooz (France) • Kashiwazaki (Japan) • Diablo Canyon (California) • Braidwood, Byron (Illinois) • Wolf Creek (Kansas) • Brazil • Taiwan • Daya Bay (China)
22 Kr 2 Det: Reactor 13 Experiment at Krasnoyarsk Features - underground reactor - existing infrastructure Detector locations constrained by existing infrastructure ~1. 5 x 106 ev/year ~20000 ev/year Reactor Ref: Marteyamov et al, hep-ex/0211070
23 The Chooz site, Ardennes, France
Daya Bay, China 24
25 U. S. Nuclear Power Plants
Braidwood Neutrino Experiment 26 Braidwood Features of Braidwood Site: • 2 3. 6 GW reactors – 7. 17 GW maximum power • Flat: flexibility, equal overburden at near and far sites, surface transportation of detectors • Favorable geology (dolomitic limestone): good for excavation, low radioactivity (order of magnitude lower U, Th than granite)
The Braidwood Collaboration 27 14 Institutions 70 Collaborators
Braidwood Baseline Design 28 Goals: Flexibility, redundancy, cross checks • 4 identical 65 ton fiducial mass detectors; 2 at near site (L=270 m), 2 at far site (L=1510 m) • “Two zone detectors”: inner zone with Gd-loaded LS and r=2. 6 m; outer zone with mineral oil and r=3. 5 m. • Movable detectors with surface transport for crosscalibration; vertical shaft access to detector halls • Oscillation measurements using both rate and energy spectrum • Full detector construction above ground; detectors filled simultaneously with common scintillator. • Near and far detectors at same depth of 183 (464 mwe) gives equal spallation rates that can be exploited for detector and background checks
29 Braidwood Site Near Detector Far Detector
Bore Hole Project at the Exelon Site Bore hole project completed in January 2005 – Bore holes drilled to full depth (200 m) at near and far shaft positions on Braidwood site. – Provided detailed information on geology, ground water, radioactivity, etc. – Confirmed feasibility of detectors down to depths of 460 mwe. – Reduces contingency required for underground construction – Demonstrated willingness of Exelon to allow construction on their site. 30
Braidwood Design Sensitivity GOALS: 1. Discovery potential (at 3 ) for sin 22 13 > 0. 01 2. Sensitivity (90% CL) down to the sin 22 13 = 0. 005 level With cross checks and redundancy to establish signal and check systematic errors • See signal in both rate and energy spectrum measurements • Cross calibrate detector pairs at high-rate near site • Cross calibrate near/far detectors using spallation isotopes like 12 B • Multiple near and far detectors give direct cross checks on detector systematics at 0. 05% for the near set and 0. 3% for far • Large detectors allow studies of the radial dependence of the IBD signal and backgrounds. 31
Normalization and spectral information Predicted spectrum 13=0 (from near detector) Observed spectrum (far detector) sin 22 13=0. 04 E (Me. V) 32 Counting analysis: Compare number of events in near and far detector Systematic uncertainties: • relative normalization of near and far detectors • relatively insensitive to energy calibration Energy spectrum analysis: Compare energy distribution in near and far detectors Systematic uncertainties: • energy scale and linearity • insensitive to relative efficiency of detectors
Detectors and analysis strategy designed to minimize relative acceptance differences Central zone with Gd-loaded scintillator surrounded by buffer regions; fiducial mass determined by volume of Gd-loaded scintillator Neutrino detection by n m. Gd → m+1 Gd s (8 Me. V); =20 sec Events selected based on coincidence of e+ signal (Evis>0. 5 Me. V) and s released from n+Gd capture (Evis>6 Me. V). No explicit requirement on reconstructed event position; little sensitivity to E requirements. 33 Shielding e+ e n 6 meters Gd-loaded liquid scintillator To reduce backgrounds: depth + active and passive shielding
Conceptual Mechanical Design – Outer steel buffer oil containment vessel (7 m diameter) • 1000 low activity glass 8” PMTs evenly distributed on inside surface (25% coverage) – Inner acrylic Gd-loaded scinitillator containment vessel (5. 2 m diameter) – Top access port can be used to insert calibration sources 34
Detector With Moveable Veto System and Shielding 35
36 Acceptance Issues Must know: (relative) number of protons in fiducial region (relative) efficiency for detecting IBD events Known volume of stable, identical Gd-loaded liquid scintillator in each detector Well understood efficiency of positron and neutron energy requirements
Monte Carlo Studies 37 Reconstructed e+ and n-capture energy Reconstructed Positron Energy ~E 0. 8 Me. V Studies based on hit-level simulation with parameterizations of many detector effects. Studies using full GEANT 4 simulation are underway. Reconstructed Neutron Capture Energy n Capture on H n Capture on Gd Reconstructed Energy Cuts: • positron: Evis > 0. 5 Me. V • n-Gd capture: Evis > 6 Me. V
38 Energy Scale Calibration from neutron capture peaks 0. 1% uncertainty Use neutron capture peaks from IBD events to measure energy scale. In each far detector, E scale can be measured to 0. 3% every 5 days. (This calibration averages over detector in exactly the same way as signal events. ) Acceptance uncertainty from energy scale should be ~0. 1%.
39 3 -zone versus 2 -zone detectors (Braidwood 2 -zone Design) I. Gd-loaded liquid scintillator II. catcher: liquid scintillator (no Gd) III. Non-scintillating buffer I II III
Acceptance Sensitivity to Energy Scale 40
Gd - Liquid Scintillator (Gd-LS) 41 • Detectors must be filled simultaneously common scintillator; relative volume measurement with <0. 2% uncertainty. • We plan to use 0. 2% Gd + 20% PC + 80% dodecane mixture developed by BNL Nuclear Chemistry group. Stability of Gd-LS (Dick Hahn, Minfeng Yeh, et al. ) (Absorbance of 0. 002 corresponds to – Long-term stability tests in progress – So far, stable with attenuation length > 18 m. attenuation Length of ~20 m). Chooz degradation was 0. 4%/day x - Braidwood scintillator
Movable Detectors 42 • Transport is necessary to move detectors from construction/filling area to below ground halls • Movable detectors allow direct check of relative detector acceptances at near site Period Near Far • Possible scenario: Initial 3 months A B 3 year data run A C B D Final check A D B C • Possible method: Use climbing jack system with cable to lift and put detectors on multi-wheeled trailer (standard method used in industry). Goldhofer Trailer Moving 400 tons
Using Isotope Production to Measure Fiducial Mass • Unique feature of the Braidwood site: Near and far detectors have equal, well-understood, substantial overburden Can use produced 12 B events to measure: • Near/far relative target mass from the total rate • Near/far energy calibrations from the relative energy distribution 12 B Beta decays t 1/2 = 20 ms (can tag to muon) 13. 4 Me. V endpoint • ~50, 000 12 B beta-decay events per year per detector can be tagged and isolated giving a statistical uncertainty of 0. 45% • Systematic uncertainties related to the knowledge of relative near/far overburden; must be known to few percent from: – Geological survey information (Bore hole data: near/far agreement <1%) – Cosmic muon rates in the near and far locations 43
44 Summary of Acceptance Uncertainties
Backgrounds 45 Even though near and far shielding is the same, backgrounds do not cancel: signal/background ratios in the near and far detectors are different. – Uncorrelated backgrounds from random coincidences (not a problem) • Reduced by limiting radioactive materials • Limestone rock at Braidwood site has low radioactivity • Directly measured from rates and random trigger setups – Correlated backgrounds • Neutrons that mimic the coincidence signal • Cosmogenically produced isotopes that decay to a beta and neutron (9 Li and 8 He).
Cosmic Muon Rates at Braidwood Depths • Calculation of muon rate at 464 mwe (600 ft) – Used data from boreholes for density and material – Average muon flux = 0. 213 /m 2/sec – Average muon energy = 110. 1 Ge. V Material Chemical composition Density (g/cm 3) Depth of top of layer (m) Soil Si. O 2 1. 60 0. 0 Mudstone Si. O 2 2. 46 11. 3 Mudstone Si. O 2 2. 52 27. 1 Limestone Ca. CO 3 2. 61 42. 7 Limestone Ca. CO 3 2. 63 61. 0 Mudstone Si. O 2 2. 60 63. 1 Dolomitic Limestone 0. 63* Ca. CO 3 + 0. 37*Mg. CO 3 2. 58 82. 6 Dolomitic Limestone 0. 63* Ca. CO 3 + 0. 37*Mg. CO 3 2. 62 98. 8 Dolomitic Limestone 0. 63* Ca. CO 3 + 0. 37*Mg. CO 3 2. 71 116. 4 Dolomitic Limestone 0. 63* Ca. CO 3 + 0. 37*Mg. CO 3 2. 62 135. 0 Limestone Ca. CO 3 2. 63 142. 3 Limestone Ca. CO 3 2. 71 157. 6 Dolomitic Limestone 0. 63* Ca. CO 3 + 0. 37*Mg. CO 3 2. 66 168. 9 46
Veto (Tagging) System 47 Goal: < 1 n background event/day/detector. Strategy: tag muons that pass near the detector. Use shielding to absorb neutrons produced by muons that miss the veto system. Residual n background: Shielding Veto Detectors p 1. Veto inefficiency ─ 99% efficiency → 0. 25/detector/day 2. Fast neutron created outside the shielding ─ 0. 5/detector/day n n 6 meters m With μ rate in the veto system of 21 Hz and the tag window of 100 μs → 0. 2% dead time m Muon identification must allow in situ determination of the residual background rate
48 Background Simulations Detector For a veto system with 2 mwe of shielding, both a GEANT 4 and a MARS calculation give: • 170 n/ton/day produced in the surrounding rock • 4500 n/day emerging from the rock • Background rate of ~0. 75 events/ day after the veto requirements Fraction of Neutrons that reach the vessel wall Untagged neutrons
9 Li and 49 8 He Isotopes like 9 Li and 8 He can be created in μ spallation on 12 C and can decay to β+n. Long lifetimes make veto difficult: 9 Li~178 ms KAMLAND found isotope production correlated with muons that shower in the detector. from thesis of Kevin Mc. Kinny Tagging showering muons and rejecting events in a 0. 5 s window eliminates 72% of 9 Li and results in 7% deadtime. Expect 0. 078 9 Li/ton/day; half decay in β+n modes; 72% are tagged 0. 7/detector/day. More…
50 Background Summary Background Rate (<) Dead Time Random 0. 4/detector/day Fast Neutron (inside) 0. 3/detector/day Fast Neutron (outside) 0. 5/detector/day none 0. 2% none 9 Li 7% ~7% Total 0. 7/detector/day 1. 9/detector/day Compare to 160 signal/detector/day at the far site (S/N~85)
Sensitivity and Discovery Potential 51 Summary of Uncertainties for 3 yr Data • For three years of Braidwood data and m 2 > 2. 5 x 10 -3 e. V 2 90% CL limit at sin 22 13 < 0. 005 3 discovery for sin 22 13 > 0. 013 With two near and two far detectors, the total uncertainty in the near/far ratio is 0. 33%
90% CL Sensitivity vs Years of Data • Information from both counting and shape fits • Combined sensitivity for sin 22 13 reaches the 0. 005 after three years 52
Braidwood Measurement Capability For 3 years of data and a combined counting plus shape analysis m 2 = 2. 5 x 10 -3 e. V 2 and sin 22 13 = 0. 02 53
Other Physics: Neutrino Electroweak Couplings Braidwood experiment can isolate about 10, 000 e – e events that will allow the measurement of the neutrino g. L 2 coupling to ~1% – This is 4 better than past -e experiments and would give an error comparable to g. L 2(Nu. Te. V) = 0. 3001 0. 0014 g. L 2 - g. L 2(SM) Precision measurement possible since: – Measure elastic scattering relative to inverse beta decay – Can pick a visible energy window (3 -5 Me. V) away from background 54
Status of Project 2004 Engineering/R&D proposal 2005 Nu. SAG Review 2006 Full proposal submission 2007 Project approval; construction start 2010 Start datataking Cost Estimate: Civil Costs: $34 M + $8. 5 M (Cont. ) 4 Detectors and Veto Systems: $18 M + $5 M (Cont. ) Exelon enthusiastic supporter of project 55
56 Conclusions • The worldwide program to understand oscillations and determine the mixing parameters, CP violating effects, and mass hierarchy will require a broad range of measurements – a reactor experiment to measure 13 is a key part of this program. • A reactor experiment will provide the most precise measurement of 13 or set the most restrictive limit. • Reactor experiment with sensitivity of sin 22 ~1% will give information needed to understand future roadmap of neutrino program. • Braidwood offers an ideal site to perform an experiment with the required sensitivity (sin 22 13 = 0. 005 at 90% c. l. )
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