Neutrinos from nuclear reactors

Transcription

1 Neutrinos from nuclear reactors Nuclear reactors are a very intense source of ν e from β decays of the fission fragments. Every fission reaction emits about 00 MeV of energy and 6 ν e. Flux 10 0 ν e s 1 GWatt 1, isotropic, E(ν e ) 0.5 MeV. Latest oscillation experiments look for ν e disappearance at different baselines: L = O(km) Palo Verde L = O(150km) atmospheric regime: Chooz, solar regime: Kamland Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

2 Neutrino flux Detect absolute number of neutrino interaction and distortions of their spectrum prompt positron signal, energy range. ν e p e + n n + p d + γ(. MeV ) τ 186 µs delayed correlated photon. To determine neutrino flux:..1 Measure of the reactor thermal power.. Determination of the neutrino spectrum.. Definition of the experimental observable: positron momentum spectrum. (see annotations) (a) a) ν _ e interactions in detector [1/(day MeV)] b) ν _ e flux at detector [108 /(s MeV cm )] c) σ(e ν ) [10 - cm ] (b) E ν (MeV) (c) Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio 009 / 1

3 Thermal power of the reactor The leading reaction is 5 U fission: 5 U + n X 1 + X + n The lightest fragment have on average A 9, the heavier: A 10. Stable nuclei with A = 9, 10 are 0 Zr 9 e 58 Ce U has 98 protons and 1 neutrons to reach the stability, on average it needs 6 neutron β decays 6 ν e. U 5 fission yield (%) Mass number A The interaction process ν e +p n+e + has a threshold of 1.8 MeV only 5% of neutrinos can be detected. All the neutrinos from low Q-value processes, as nuclear fuel stored in the reactors and radioactivity induced in the nuclear plant structures, don t produce detectable neutrinos. The fuel composition of the reactor core changes with the time, it s under monitor (reactor power depends from its composition). Fissions/Sec U 8 U 9 Pu 0 Pu 1 Pu Pu Days Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio 009 / 1

4 From fission rate to the ν e spectrum The ν e spectrum of three of the four principal fission nuclei: ( 5 U, 9 Pu, 1 Pu), has been derived by measuring the electron spectrum. The fourth: 8 U, has been computed from nuclear models, as well all the processes in the decay chain. Systematic error: 1%. From ν e to positrons ν e + p n + e + cross section: σ (0) tot = σ 0 (f + g ) E e (0) p e (0) = ( ) E e (0) p e (0) 1 MeV 10 cm(1), E e (0) = E ν (M n M p ): positron energy (neglecting neutron recoil, marginal effect) p e (0) momentum, f = 1, g = 1.6 vector and axial coupling constants cosθ e σ tot [10 cm ] E ν [MeV] Solid lines: O(1). predictions ato(1/m n), dashed σ 0 = G F cos θ C (1 + R inner π ), () radiative corrections: R inner 0.0. Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio 009 / 1

5 Data/prediction agreement Experiment Bugey (years 80, now considered a non oscillation experiment): expected and measured ν e spectrum. Curve b) is the most updated prediction a) b) Positron energy (MeV) 7 Systematic errors summary (from hep-ph/010777) Origin and magnitude of systematic errors in Palo Verde and Chooz. Note that the two experiments offer different breakdowns of their systematics. For simplicity we do not show the systematics for the Palo Verde ON-OFF analysis. The Palo Verde results are from the analysis of the full data set (Boehm et al. 001). Systematic Chooz (%) P.V. (%) σ(ν e + p n + e + ) Number of p in target W th Energy abs. per fission Total rate prediction..1 e + trigger eff. -.0 n trigger eff. -.1 ν e selection cuts -.1 (1 ϵ 1 )B pn estimate -. Total ν e efficiency Total.7 5. Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

6 µ (capture) µ (n spallation) Experimental backgrounds Veto Water Buffer Target 511 kev γ e + n 511 kev γ p 8 MeV γ (n capture) p n p n n n ν e Two main categories: Accidental backgrounds from the random superposition of a positron-like" and neutron-like" signals. Directly estimated from the measured rates of the two processes. Backgrounds from neutrons induced by cosmic rays. They can be measured only if the reactor is off (impossible to pay to have a reactor shutdown). Chooz counting rate as function of the reactors power. Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

7 CHOOZ experiment (France-Italy-Russia-USA) Took data in Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

8 CHOOZ detector 5 ton liquid scintillator detector doped with gadolinium. Active liquid scintillator veto. ν e detection: ν e +p e + +n E(ν e ) = E(e + )+1.80 MeV Two signals in delayed coincidence:..1 Prompt: e + followed by e + e γγ.. Delayed: neutron capture in gadolinium, after thermalization, releasing 8 MeV. Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

9 Events e + energy ν signal MC CHOOZ data Positron spectrum ( Measured Expected ) MeV MeV Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

10 How tobuild thesignal/exclusionplot Every sin θ, m cell defines P ν µ ν µ P(ν µ ν µ ) m 10 - ev Grid in the sin θ, m plane E ν (GeV) That modulates thenon-oscillated predicted spectrum a.u E ν (GeV) Thepredictioniscompared tothedata sin (θ) a.u Fill thegridwiththe χ E ν (GeV) Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

11 How to build the signal/exclusion plot (II) The minimum of the χ distribution is the best fit The region at a given confidence level (CL) is defined by the contour at a given χ from the minimum. The CL is computed from the probability distribution of a χ at two degrees of freedom (sin θ, m ) Question: Why χ and not χ? Hint: Why two degrees of freedom? A more formal approach in G.Feldman and R.Cousins, Phys.Rev.D57:87-889, δm Χ map sin (θ) 0 Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

12 δm (ev ) 1 ν e ν x CHOOZ final results analysis A analysis B analysis C 90% CL Kamiokande (multi-gev) 90% CL Kamiokande (sub+multi-gev) sin (θ) Analysis A ν e spectrum after background subtraction. Both the absolute rate and the spectrum are used. Analysis B Uses the different baseline ( L = m) of the two reactors. Many systematic errors cancel, but statistical errors are bigger and the m sensitivity is reduced by the shorter baseline. Analysis C Only spectrum information is used. Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

13 Top Cited Sperimentali in QSPIRES (al 6/11/05) 850 WMA P First Year Wilkinson Microwaveobservations: determination 00 of cosmologicalparameters. 9 S. Perlmutter et al., Measurements of Omega and Lambda from high redshift 1999 supernovae. 6 SuperKamiokande Evidencefor oscillation of atmospheric neutrinos Adam G. Riess et al., Observational evidencefrom supernovaefor anacceler- ating universeand acosmologicalconstant David J. Schlegel, Douglas P. Finkbeiner, Marc Davis, Maps of dust IR emission for usein estimation of reddening and CMBR foregrounds. 15 WMA P First year Wilkinson Microwave observations: preliminary 00 mapsand basicresults. 195 S. Ting et al., Experimental observation of a heavy particle J B. Richter et al., Discovery ofanarrow resonancein e+ e- annihilation J. Ashman et al., Ameasurement of the spin asymmetry in deepinelastic 1998 muon -proton scattering. 111 COBE Structure inthe COBE dmr Þrst year maps CDF Observation of TOP quark production in anti- ppcolli sions. 118 D0 Observation of the TOP quark SNO Measurement ofthe rate of νe + D p+ p+ e interactionsproducedby 001 B-8solar neutrinosat the Sudbury Neutrino Observatory V.L. Fitch, J.W. Cronin et Evidencefor the Pi decay ofthe K()0 meson 196 al. 105 SNO Direct evidencefor neutrino a ν flavor transform. from 00 neutral current interactions inthe Sudbury Neutrino Observatory. 105 CHOOZ Limits on neutrino oscillations from the Chooz experiment S.W. Herb et al., Observation of adimuon resonance at9.5-gev in GEVproton -nucleuscollisions. 10 ARGUS Observation of B0 -anti-b0 mixing Homestake (R. Davis et Measurement of the solar flux with the Homestake 1998 νe al.) chlorine detector Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

14 Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

15 Kamland and the far east reactors Site Japan Kashiwazaki Ohi Takahama Hamaoka Tsuruga Shiga Mihama Fukushima 1 Fukushima Tokai II Shimane Onagawa Ikata Genkai Sendai Tomari Korea Ulchin Wolsong Yonggwang Kori Distance # of P(ther.) flux Signal _ (km) cores (GW) (ν cm s 1 _ ) ( ν/yr) ~750 ~690 ~90 ~ x X x10 1.0x x x10 1.0x105 5.x10.9x10 1.7x10 9.9x10 9.8x10 8.x10 5.x10.5x10.x10 8.8x10 7.5x10 8.x10 8.0x Total 69 1.x Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

16 Kamland and systematic errors Detector-related (%) Reactor-related (%) m1 Energy scale 1.9 ν e -spectra 0.6 Fiducial volume 1.8 ν e -spectra. Event rate Energy threshold 1.5 Reactor power.1 Efficiency 0.6 Fuel composition 1.0 Cross section 0. Long-lived nuclei 0. Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

17 Kamland Results (I) # of Observed and Expected Events KL1 KL KL Exposure (ton yr) Observed ev (E prompt : MeV) (>.6) (>.6) (>0.9) Expected ev ± ± ± 89 Background ev. 0.95± ± ±.5 accidental ± ± 0.0 ± Li/ 8 He (β, n) 0.9 ± ± ± 1.0 fast neutron 0 ± 0.5 < 0.89 < C(α, n) 16 O gs, 1st, nd 10. ± ± (±stat ±syst) ±0.085±0.01 ±0.0±0.07 ±0.00± % CL % CL 8.5 σ Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

18 6σ 5σ σ 1 σ σ σ Kamland Results (II) χ σ σ σ 1σ - 10 KamLAND 95% C.L. 99% C.L. 99.7% C.L. best fit 1 (ev m ) Solar 95% C.L. 99% C.L. 99.7% C.L. best fit tan θ χ Fit to scaled no-oscillation spectrum : exclude at 5.1 σ m = x 10-5 ev tan θ = Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

19 Kamland Results (III) Survival Probability Data - BG - Geo ν e Expectation based on osci. parameters determined by KamLAND L 0 /E νe (km/mev) Mauro Mezzetto, INFN Padova L/<E> () Neutrini da Reattori Corso di Dottorato, Maggio / 1

20 Kamland Geoneutrinos Geo-Reactor Natural nuclear reactor in the center of the Earth was proposed in 001 as the energy source of geo-magnetic field. Not a mainstream theory, but not ruled out by any evidence. Explains mechanism for flips of the geomagnetic field. 8 Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1

21 Signature from Geo-Reactor Kamland Geoneutrinos: results big earthquake Kashiwazaki power station :. GW Y-intercept : Geo-Reactor + BG theoretical prediction : TW Mauro Mezzetto, INFN Padova () Neutrini da Reattori Corso di Dottorato, Maggio / 1