Reactor Neutrinos I. Karsten M. Heeger. University of Wisconsin Pontecorvo Neutrino Summer School Alushta, Ukraine

Transcription

1 Reactor Neutrinos I Karsten M. Heeger University of Wisconsin 2012 Pontecorvo Neutrino Summer School Alushta, Ukraine 1

2 Outline Lecture 1 observation of the neutrino reactors as an antineutrino source prediction of the antineutrino flux from reactors detection of reactor antineutrinos oscillation searches with reactors observation of reactor antineutrino disappearance at KamLAND precision oscillation physics with reactor antineutrinos 2

3 Outline Lecture 2 precision oscillation physics: theta13 and beyond the reactor anomaly future reactor experiments θ12 mass hierarchy sterile neutrino searches searches for new physics magnetic moments coherent scattering NSI experiments with antineutrino sources applications of reactor antineutrinos: monitoring & communication 3

4 Neutrino Energies Big-Bang neutrinos ~ ev Neutrinos from the Sun "< 20 MeV depending of their origin. Atmospheric neutrinos" ~ GeV Antineutrinos from nuclear reactors < 10.0 MeV Neutrinos from accelerators up to GeV (10 9 ev)

5 NUSS, July 13, 2009

6 ? What produces the largest neutrino flux on Earth? The Sun, the Big Bang, or a nuclear reactor??? at a distance of 1 km NUSS, July 13, 2009

7 What produces the largest neutrino flux on Earth? The Sun, the Big Bang, or a nuclear reactor? at a distance of 1 km NUSS, July 13, 2009

8 Discovery of the Antineutrino NUSS, July 13, 2009

9 History of the Neutrino Pauli, 1930 N N + e - electrons! some nuclei emit Chadwick, 1914 Fermi,

10 First Proposal For Direct Detection of Neutrino 10

11 Nuclear Reactors as a Neutrino Source Reactors are intense and pure sources of νe B. Pontecorvo Natl.Res.Council Canada Rep. (1946) 205 Helv.Phys.Acta.Suppl. 3 (1950) 97 Good for systematic studies of neutrinos. 11

12 Enrico Fermi and the Neutrino Enrico Fermi proposes "neutrino" as the name for Pauli's postulated particle. He formulates a quantitative theory of weak particle interactions in which the neutrino plays an integral part. 12

13 1953: Project Poltergeist Experiment at Hanford 13

14 Hanford Experiment inverse beta decay ν e + p e + + n Reines, Cowan 300 liters of liquid scintillator loaded with cadmium signal: delayed coincidence between positron and neutron capture on cadmium high background (S/N ~ 1/20) made the experiment inconclusive 0.41+/ events/minute 14

15 1956: First Direct Detection of the Antineutrino 15

16 The Savannah River Detector A new design (1959) tanks I, II, and III were filled with liquid scintillator and instrumented with 5 PMTs target tanks (blue) were filled with water+cadmium chloride inverse beta decay ν e + p e + + n inverse beta decay would produce prompt and delayed signal in neighboring tanks 16

17 Observation of the Free Antineutrino 1959 The Savannah River Detector - A new design Second version of Reines experiment worked! positron annihilation inverse beta decay ν e + p e + + n n capture 17

18 The Savannah River Detector A new design (1959) tanks I, II, and III were filled with liquid scintillator and instrumented with 5 PMTs target tanks (blue) were filled with water+cadmium chloride inverse beta decay ν e + p e + + n inverse beta decay would produce prompt and delayed signal in neighboring tanks 18

19 The Savannah River Detector A new design (1959) tanks I, II, and III were filled with liquid scintillator and instrumented with 5 PMTs target tanks (blue) were filled with water+cadmium chloride inverse beta decay ν e + p e + + n shielding and background reduction is important 19

20 The Savannah River Detector A new design (1959) tanks I, II, and III were filled with liquid scintillator and instrumented with 5 PMTs target tanks (blue) were filled with water+cadmium chloride inverse beta decay ν e + p e + + n first reactor νe spectrum Reines, Cowan, Phys Rev 113, (1959)273 20

21 1956: First Observation Observation of the Antineutrino by April 1956, a reactor-dependent signal had been observed: signal/reactor independent background ~ 3:1 in June 1956, they sent a telegram to Pauli 21

22 A Lesson from History Ref: R.G. Arms, Detecting the Neutrino, Physics in Perspectives, 3, 314 (2001) 22

23 NUSS, July 13, 2009

24 Reactors as Antineutrino Source NUSS, July 13, 2009

25 Energy Release in Fission and Self-Fusion at A=120: 8.5 MeV at A=240: 7.6 MeV Fig: Basdevant et al. - only nuclei with 40 < A < 95 are stable against both fission and selffusion - Qfis calculated for symmetric fission 25

26 Fission and Nuclear Deformation variation of energy as a function of distortion EA= fission barrier 26

27 235 U Fission distribution of fission fragments asymmetric fission into lighter and heavier nuclei together these have 98 p and 136 n while fission fragments (X1+X2) have 92 p and 144 n on average 6n have to beta-decay to 6p to reach stable matter νe 27

28 Reactors as Antineutrino Sources νe β - decay of neutron rich fission fragments energy per fission pure source of νe some energy taken away by neutrinos, neutrons etc ~ 200 MeV/fission and 6 νe/fission 3 GWth reactor produces ~6x10 20 νe/sec 28

29 Fission with thermal and fast neutrons 235 U 238 U thermal n U can lead to fission of 236 U n U has higher energy than lowest fissionable state some nuclei require thermal neutrons for fission, others require fast neutrons thermal n U does not lead to fission, only radiative capture fission of 239 U requires addition of neutron with kinetic energy Tn=6-4.8=1.2 MeV Nuclei which are used most easily as fuel (fission rapidly by thermal neutron capture): 233 U, 235 U, 239 Pu reactors which burn 239 Pu and which contains 238 U can produce more Pu than it needs breeder reactor 29

30 Nuclear Reactors reactors are an extended neutrino source: 3-4m diameter, 4m high 30

31 Fuel Element for a PWR Reactor 31

32 Reactor Antineutrinos Source ν e from β-decays of n-rich fission products pure νe source typical fuel composition 235 U: 238 U: 239 Pu: 241 Pu = 0.570: 0.078: : > 99.9% of νe are produced by fissions in 235 U, 238 U, 239 Pu, 241 Pu ~ 90% of νe are produced by fissions in 235 U, 239 Pu Plutonium breeding over fuel cycle (~250 kg) changes antineutrino rate (by 5-10%) and spectrum 32

33 Build-Up of Fission Products & Burn-Up Corrections Gram atomic weight per ton of fuel Fig: Basdevant et al. isotope uncertainties of 4-6% for most 0.1% for 238 U, correlated ~5% isotope uncertainty yields ~0.5% uncertainty in neutrino flux 33

34 Reactor Refueling and Time Variation νe flux from reactor has time variation refueling at Palo Verde reactors and predicted antineutrino rate 3-6 week shutdown every months 1/4-1/3 of fuel assemblies are replaced, remaining fuel repositioned Text 3 reactor cores 34

35 Thermal Power Fission Antineutrinos 1. Power Measurement most accurate measurement is secondary heat balance method offline, done weekly, uncertainty ~ % 2. Core Simulation fission fraction of fuel isotopes are obtained by core simulation 3. Energy release per fission in MeV 4. Neutrino Spectra 35

36 Fission Products, β-spectra, ν Measurements - β - -spectra resulting from fission of 235 U, 238 U, 239 Pu, 241 Pu have been experimentally measured - use thin layer of fissile material in beam of thermal neutrons, e.g. Schreckenbach et al., Hahn et al. - can be converted to νe spectra Schreckenbach et al. PL160B 325 (1985) reference spectra from ILL over last 25 years -1.MeV -1 fission Pu 239Pu 235U β-spectra emitted ν-spectra detected ν-spectra β spectra ) -1.MeV -1 ( fission ν nu241pu nu239pu nu235u ) 43 x10-1 MeV -1 ( fission Detected ν Pu 239Pu 235U β kinetic energy (MeV) ν kinetic energy (MeV) ν kinetic energy (MeV) Calculations 238 U beta spectra not available, fast neutrons required for fission determined from theory (+/-10%), contributes 7-10% of fissions in a PWR 0 36

37 Neutrino Flux Predictions S k (E)! = Σ!all!fission!products! 37

38 Sk(E) Ref: Lhuillier 38

39 Neutrino Flux Predictions Ref: Lewis 39

40 Goesgen Experiment (1986) Comparison of Predicted Spectra to Observations two curves are from fits to data and from predictions based on Schreckenbach et al. 3 baselines with one detector flux and energy spectrum agree to ~ 1-2% reactors are a well-calibrated source of νe 40

41 Bugey Experiment (1996) Check ν Spectrum Against Data Measured νe spectrum shape and normalization agreed with calculated predictions to ~10% and with converted electron spectra even better Calculation only Klapdor and Metzinger, 1982 Beta calibrated Schreckenbach, 1985 Hahn, 1989 spectra derived from β-spectra: +/-1.4% agreement 41

42 Detection and Studies of Reactor Antineutrinos NUSS, July 13, 2009

43 Reactor Antineutrinos Detection inverse β-decay ν e + p e + + n observable rate and energy spectrum only disappearance experiments possible neutrinos with E < 1.8 MeV are not detected only ~1.5ν e /fission can be detected Arbitrary calculated reactor spectrum Flux From Bemporad, Gratta and Vogel observed Observable spectrum! Spectrum mean energy ~ 3.6 MeV crosssection Cross Section neutrino energy (MeV) cross-section accurate to +/-0.2% ν e scattering Events (/kg/day/kev) ν e e(sm) ν e N(SM) 1 c/kg/kev/d ν e e(mm) 1 recoil energy (kev 43

44 Antineutrino Detection inverse beta decay ν e + p e + + n n+ p D + γ (2.2 MeV) " (delayed) coincidence signature between prompt e + and delayed neutron capture on H, (or Cd, Gd) kev Eν e Ee + + En + (Mn-Mp) + me MeV other detection mechanisms: νe + d e + + n + n νe + e - νe + e - including E from e + annihilation, Eprompt=E ν MeV 44

45 Physics with Reactor νe Discoveries and Precision Measurements of Neutrino Properties Antineutrino Discovery Reactor νe Spectra νe Oscillations Searches for New Physics neutrino magnetic moment and coherent scattering searches Reactor Monitoring and Application fuel burnup and isotopic composition 45

46 Neutrino Oscillation Searches with Reactor Antineutrinos NUSS, July 13, 2009

47 Neutrino Oscillation neutrino flavor change occurs if neutrinos have mass and leptons mix mixing matrix Fig: Kayser mass eigenstates Experiments study flavor conversion as a function of energy, distance and determine mixing angle and mass splitting 2-neutrino case, vacuum L P i i = sin 2 2θ sin Δm 2 E 47

48 ν e /MeV/fisson Reactor Neutrino Oscillation Experiments νe νe,x νe,x 1.1 far N osc /N no_osc Δm 2 θ detector Energy (MeV) Baseline (km) Δm 2 P(v e v e ) 1 sin 2 2θ 13 sin 2 32 L 4E Neutrino Energy (MeV) 48

49 Oscillation Experiments with Reactors for 3 active ν, two different oscillation length scales: Δm 2 12, Δm 2 23 Measure (non)-1/r 2 behavior of νe interaction rate Δm 2 P ee 1 sin 2 2θ 13 sin 2 31 L Δm cos 4 θ 13 sin 2 2θ 12 sin E ν 4E ν L/E Δm 2 amplitude of oscillation θ 2 L Δm 2 12 ~7.6 x 10-5 ev 2 Δm 2 23 ~2.4 x 10-3 ev 2 Δm 2 23 Δm

50 Search for Neutrino Oscillations at Reactors early experiments tried to probe atmospheric neutrino anomaly early oscillation experiments didn t know the length scales involved At Δm 2 31 = 2.5x10-3 ev 2, sin 2 2θ 13 < 0.15

51 Neutrino Oscillation Search with Reactor Antineutrinos Oscillation Searches at Chooz + Palo Verde: " ν e ν x ν e νe ν e Distance: 1km Absolute measurement with 1 detector detector size: several tons 51

52 Backgrounds for Reactor Experiments from M. Shaevitz 52

53 Chooz: Positron Spectrum Reactor On/Off - Positron Yields for Reactors I+II - Fit to Spectrum - Comparison to Expected Yield for No Oscillation 53

54 Chooz: Results ~3600 events in 335 days ~2.2 events/day/ton with bkgd events/day/ton 2.7% uncertainty 54

55 Chooz: Degradation of Scintillator 55

56 Reactor νe Flux Measurements at Different Distances Early Reactor ν Experiments flux measurements at distances up to ~1km consistent with expectations 56

57 Reactor Antineutrinos in Japan Japanese Reactors Reactor Antineutrinos Kashiwazaki Takahama Ohi 235 U: 238 U: 239 Pu: 241 Pu = 0.570: 0.078: : reactors ~ 200 MeV per fission ~ 6 ν e per fission Japan Kamioka ~ 2 x ν e /GW th -sec reactor ν flux ~ 6 x 10 6 /cm 2 /sec 57

58 KamLAND Antineutrino Detector ν e + p e + + n n light from the e gives a measu E p + E n MeV, E νe through inverse β-decay liquid scintillator target: - proton rich > protons - good light yield NUSS, July 13, 2009

59 Antineutrino Candidate Event 59

60 First Evidence for Reactor ν e Disappaerance KamLAND 2003 Reactor Neutrino Physics Japan Thermal Power Flux (µw/cm 2 ) mean, flux-weighted reactor distance ~ 180km Many reactors, far away Distance (km) Survival Evis >2.6 MeV Observed ν e 54 events No-Oscillation 86.8 ± 5.6 events Background 1 ± 1 events Livetime: ton-yr PRL 90: (2003) 60

61 Evidence of Spectral Distortion KamLAND 2004 analysis threshold Observed ν e No-Oscillation Background Livetime: 258 events ± 23.7 (syst.) 17.8 ± 7.3 events ton-yr best fit χ 2 =24/17 fiducial volume syst.: 4.7% total systematics = 6.5% 138d, α 210Pb " 210Bi " 210Po " 206Pb 13C(α,n) 16 O (~10-7 ) 222 Rn decay chain introduced in the LS during assembly Spectral Distortions:" A unique signature of neutrino oscillation! Simple, rescaled reactor spectrum is excluded at 99.6% CL(χ 2 =37.3/18) 61

62 Measuring Neutrino Oscillation Parameters solar neutrino problem Ga oscillation searches atmospheric/beam neutrinos θ23, Δm 2 23 Cl SK solar/reactor neutrinos θ12, Δm Present 62

63 Measuring Neutrino Oscillation Parameters Solar Neutrinos Solar Neutrinos + KamLAND 2003 (ν e rate) Solar Neutrinos + KamLAND 2004 (ν e rate+spectrum) Agreement between oscillation parameters for ν and ν Beginning of precision neutrino physics 63

64 Precision Oscillation Physics with Reactor Neutrinos NUSS, July 13, 2009

65 Events / MeV Efficiency (%) Evidence of Spectral Distortion KamLAND Prompt event energy spectrum for νe Selection efficiency KamLAND data no oscillation best-fit osci. accidental C(α,n) O best-fit Geo ν e best-fit osci. + BG + best-fit Geo ν e number of events expected (no-oscillation): 2179 ± 89 (syst) observed: 1609 bkgd: 276 ± 23.5 significance of disappearance (with 2.6 MeV threshold): 8.5σ no-osc χ 2 /ndf=63.9/17 significance of distortion: > 5σ best-fit χ 2 /ndf=21/16 (18% C.L.) previous analysis threshold E p (MeV) - unbinned likelihood fit (rate+shape+time) - 2-flavor oscillation analysis with w/earth matter effects - geo-neutrino U,Th amplitude is a free parameter 65

66 Systematic Uncertainties and Backgrounds Systematic Uncertainties (and mainly affecting θ ) is 4.1%. Detector-related (%) Reactor-related (%) m 2 21 Energy scale 1.9 ν e -spectra [7] 0.6 Event rate Fiducial volume 1.8 ν e -spectra 2.4 Energy threshold 1.5 Reactor power 2.1 Efficiency 0.6 Fuel composition 1.0 Cross section 0.2 Long-lived nuclei 0.3 Estimated Backgrounds TABLE II: Estimated backgrounds after selection efficiencies. fiducial volume systematics reduced from 4.7% 1.8% total systematics: 4.1% Background Contribution Accidentals 80.5 ± Li/ 8 He 13.6 ± 1.0 Fast neutron & Atmospheric ν < C(α,n) 16 O G.S ± C(α,n) 16 O 12 C(n,nγ) 12 C (4.4 MeV γ) 6.1 ± C(α,n) 16 O 1 st exc. state (6.05 MeV e + e ) 15.2 ± 3.5 significantly reduced 13 C(α,n) 16 O 2 nd exc. state (6.13 MeV γ) 3.5 ± 0.2 Total ± 23.5 (number of events) 66

67 4π Full-Volume Calibration Design Concept glovebox with motion spools control cables calibration source calibration pole Z [m] Calibration Data Z [m] Co sources along pole 60Co/ 68 Ge source at end X [m] X [m] the level of detected activity. The source activity traces the outline of the calibration of the system location. The outer dotted line represents the balloon boundary. The inner lots like these were used during the deployment to confirm the location of the system Vertex distribution of 60 Co/ 68 Ge composite source in 4π calibration runs. The progression from left to right illustrates the sequence in which the pole was swept he detector. ing the stability of this temperature gradient is critical to of the Wisconsin success of the low-background Pontecorvo phase School, pu- September 10, 2012 Karsten Heeger, Univ. rification e ort. 67

68 6σ 5σ 4 σ 1 2σ 3 σ σ Oscillation Parameters 2 Δχ Rate-Shape-Time Analysis σ 3σ 2σ 1σ KamLAND only tan 2 Θ= Δm 2 =7.58 x10-5 ev 2 ) 2 (ev 2 21 Δm KamLAND 95% C.L. 99% C.L % C.L. best fit Solar 95% C.L. 99% C.L % C.L. best fit KamLAND+solar (combined under assumption of CPT invariance) tan 2 Θ= θ tan Δχ Δm 2 =7.59 x10-5 ev 2 68

69 KamLAND L/E Dependence 1 Data - BG - Geo ν e Expectation based on osci. parameters determined by KamLAND L0=180km L 0 /E νe (km/mev) Solar neutrino problem solved! first identified by Ray Davis (missing solar νe) SNO observes neutrino flavor change, finds evidence for neutrino mass KamLAND demonstrates ν oscillation, precision measurement of θ, Δm 2 69

70 Pathway Towards Discovery baseline: 1 km size: 5 ton 180 km 1000 ton Survival Probability Data - BG - Geo ν e Expectation based on osci. parameters determined by KamLAND Take big steps - Don t always trust theoretical guidance - A little bit of luck L 0 /E νe (km/mev) 70

71 Neutrino Discoveries - A Success Story 1968 Ray Davis detects 1/3 of expected solar neutrinos. (Nobel prize in 2002) 1998 SuperK reports evidence for oscillation of atmospheric neutrinos. 2001/2002 SNO finds evidence for solar νe flavor change KamLAND discovers disappearance of reactor νe 71

72 55 years of liquid scintillator detectors A story of varying baselines Precision measurement of Δm12 2. Evidence for oscillation First observation of reactor antineutrino disappearance Nobel Prize to Fred Reines at UC Irvine 1980s & 1990s - Reactor neutrino flux measurements in U.S. and Europe First observation of (anti)neutrinos Chooz Chooz KamLAND Past Reactor Experiments Hanford Savannah River ILL, France Bugey, France Rovno, Russia Goesgen, Switzerland Krasnoyark, Russia Palo Verde Chooz, France Savannah River 72

73 Measurement of Fundamental Parameters Mass Splittings KamLAND 2010 MINOS Nu2012 normal inverted KamLAND has measured Δm12 2 to ~2.8% 73

74 Neutrino Oscillation Mixing Angles # U e1 U e2 U e 3 & # U e 3 & % ( U = U µ1 U µ2 U µ 3 % ( = % ( % ( $ U "1 U " 2 U % " 3 ' $ ( ' L P i i = sin 2 2θ sin Δm 2 E U MNSP Matrix Maki, Nakagawa, Sakata, Pontecorvo # & # cos) 13 0 e *i, CP % ( sin) & # 13 cos) 12 sin) 12 0& # & = 0 cos) 23 sin) 23 % ( + % ( % ( % ( + *sin) 12 cos) 12 0 $ 0 *sin) 23 cos) % 23 ' *e i, CP $ sin) 13 0 cos) ( % ( + % 0 e i- / 2 ( 0 % i- / 2+i. ( 13 ' $ 0 0 1' $ 0 0 e ' atmospheric, K2K reactor and accelerator SNO, solar SK, KamLAND 0νββ Schwetz et al arxiv: updated as of

75 Neutrino Oscillation - Before 2011 Mixing Angles # U e1 U e2 U e 3 & # U e 3 & % ( U = U µ1 U µ2 U µ 3 % ( = % ( % ( $ U "1 U " 2 U % " 3 ' $ ( ' U MNSP Matrix Maki, Nakagawa, Sakata, Pontecorvo # & # cos) 13 0 e *i, CP % ( sin) & # 13 cos) 12 sin) 12 0& # & = 0 cos) 23 sin) 23 % ( + % ( % ( % ( + *sin) 12 cos) 12 0 $ 0 *sin) 23 cos) % 23 ' *e i, CP $ sin) 13 0 cos) ( % ( + % 0 e i- / 2 ( 0 % i- / 2+i. ( 13 ' $ 0 0 1' $ 0 0 e ' atmospheric, K2K reactor and accelerator SNO, solar SK, KamLAND 0νββ small? zero? maximal? large, but not maximal! 75

76 Neutrino Oscillation - Before 2011 Mixing Angles # U e1 U e2 U e 3 & # U e 3 & % ( U = U µ1 U µ2 U µ 3 % ( = % ( % ( $ U "1 U " 2 U % " 3 ' $ ( ' U MNSP Matrix Maki, Nakagawa, Sakata, Pontecorvo # & # cos) 13 0 e *i, CP % ( sin) & # 13 cos) 12 sin) 12 0& # & = 0 cos) 23 sin) 23 % ( + % ( % ( % ( + *sin) 12 cos) 12 0 $ 0 *sin) 23 cos) % 23 ' *e i, CP $ sin) 13 0 cos) ( % ( + % 0 e i- / 2 ( 0 % i- / 2+i. ( 13 ' $ 0 0 1' $ 0 0 e ' atmospheric, K2K reactor and accelerator SNO, solar SK, KamLAND 0νββ 76

77 NUSS, July 13, May 2003 S. Glashow

78 NUSS, July 13, 2009