Accurate measurement of reactor neutrinos close to surface with STEREO Photo:ILL Vladimir Savu IRFU,CEA, Paris-Saclay University on behalf of the STEREO collaboration Vladimir Savu (CEA) 06.12.2019 { AAP 2019 1 / 23 06.12.2019 { AAP 2019 Motivation Sterile neutrino search and reactorν ¯e measurements Motivation of STEREO I Oscillation test L/E ∼ 10 m/3 MeV ! ∼ 1eV sterile neutrino 2 2 Two new parameters: sin (2θnew ) and∆ mnew Physical Review D 83, 073006 (2011), G. Mention et al. 9 ∆ χ 2 4 1 4 9 5.5 Daya Bay Huber model w/ 68% C.L. I absolute flux 5.0 / fission] 4.5 normalization studies 2 cm 43 4.0 − C.L spectral shape studies [10 I 3.5 68% 239 95% σ −43 σ238 =(10.1±1.0)×10 99.7% σ =(6.04±0.60)×10−43 3.0 241 5.2 5.6 6.0 6.4 6.8 7.2 −43 2 σ235 [10 cm / fission] Phys. Rev. Lett. 118, 251801 (2017) Physics Letters B 773 (2017) STEREO will test precisely these 3 questions with a pure 235U spectrum Vladimir Savu (CEA) 06.12.2019 { AAP 2019 1 / 23 The STEREO experiment Experimental site ILL research facility, Grenoble, France Research reactor core 58 MWth ! 1019 ν¯ s−1 e Water channel 15 m.w.e overburden 235 X Highly U enriched X Compact core (40cm ?) X Short baseline measurement: 9.4m < Lcore < 11.2m 93 tons moved on air cushions AutumnAutumn 2016 2016 I Surface-level experiment I γ and neutron background from neighboring experiments Vladimir Savu (CEA) 06.12.2019 { AAP 2019 2 / 23 The STEREO experiment Data taking Data taking I Phase-I: 66 days reactor ON { 22 days reactor OFF I Phase-II: 119 days reactor ON { 211 days reactor OFF I Data taking efficiency: 98.5%, 14% dead-time+veto-time after off-line cuts 50 days reactor ON cycles with long periods of reactor OFF allowing for a good characterization of the background Vladimir Savu (CEA) 06.12.2019 { AAP 2019 3 / 23 The STEREO experiment The Stereo detector The STEREO detector Acrylic buffers PMT Mineral oil I designed for a relative measurement X six identical Target cells filled with Gd doped LS Gamma-catcher ------ External crown scintillator liquid (no Gd) Target 6 cells scintillator liquid (Gd loaded) Counts L Without oscillation Center cell with oscillation Border cell with oscillation 1 2 3 4 5 6 7 Evisible [MeV] E I reflectivity of VM2000 foils is above 98% in the 400-950 nm range in air Vladimir Savu (CEA) 06.12.2019 { AAP 2019 4 / 23 The STEREO experiment Detector response Light cross-talks I The evolution of the light cross-talks between cells is monitored by looking at muon events I Phase I light cross-talks are evolving due to the liquid scintillator infiltrating into the acrylic walls. X Repaired during summer 2017 X Energy reconstruction formalism to account for the evolving light cross-talks I Phase II light cross-talks are reduced and stabilized 20 20 Cell2 → Cell3 Cell2 → Cell3 [%] [%] ij Cell6 → GC Back ij Cell6 → GC Back L Cell3 → GC D19 L → 15 15 Cell3 GC D19 → Cell5 Cell6 Cell5 → Cell6 10 10 5 5 0 0 10/31/16 12/31/16 03/02/17 10/31/17 12/31/17 03/02/18 Date [MM/DD/YY] Date [MM/DD/YY] Phase I light cross-talks Phase II light cross-talks Vladimir Savu (CEA) 06.12.2019 { AAP 2019 5 / 23 The STEREO experiment Detector response Calibration I Circulation of radioactive sources (68Ge;137 Cs;54 Mn;65 Zn;60 Co;42 K;24 Na; AmBe) along 3 different calibration systems: I internal calibration tubes - 5 z positions I LED system, used to study the single I along the perimeter of the detector photoelectron and the PMT-DAQ I below the central long axis of the detector linearity in the detector Target 3 2 1 Non-linearity [%] 0 −1 −2 −3 0 200 400 600 800 1000 1200 1400 Charge [PE] Top view of STEREO detector No sign of non-linearity up to 10 MeV for all PMTs Vladimir Savu (CEA) 06.12.2019 { AAP 2019 6 / 23 The STEREO experiment Detector response Simulation fine tuning - raw charges I Very good agreement between data and 20 simulation at the raw charge level: most 24Na Data points within ±1% 24Na MC I The energy reconstruction will correct to first order the mean offset between the TG cells Preliminary 24Na Data 24Na MC Dilatation Factor Preliminary Cell 1 Cell 2 Cell 4 Cell 5 Cell 6 10 45 80 30 60 10 45 80 30 60 10 45 80 Height [cm] Measured (black) and simulated (red) collected Summary of all the dilatation factors fitted from charges in cell 1 (top) and in cell 2 (bottom) when the 25 measured and simulated positions of the the 24Na source is located at the middle of cell 1. 54Mn source. Vladimir Savu (CEA) 06.12.2019 { AAP 2019 7 / 23 The STEREO experiment Detector response Simulation fine tuning - Quenching dE I Quenching for high dx leads to a non-linear Preliminary response at low energy deposits dL dE ≈ dX dX dE 1 + KB dx Norm. Calib. Coefficient I The effective Birks coefficient KB is adjusted in the MC in order to match the experimental quenching curve I Agreement between experimental and simulated curves reaches sub-% accuracy Vladimir Savu (CEA) 06.12.2019 { AAP 2019 8 / 23 The STEREO experiment Detector response Energy reconstruction −! Q are the collected charges −1 M matrix constructed from regular monitoring: mij = Ci · Lji −−! −1−! Erec = M Q 54 I Ci calibration coefficients( Mn radioactive source) I Lji cross-talks between cells (cosmics) Preliminary Cell 5 Data/MC agreement of the reconstructed Vertex distribtution for the 54Mn source deployed inside the energy distribution for a 54Mn calibration internal calibration tube Vladimir Savu (CEA) 06.12.2019 { AAP 2019 9 / 23 The STEREO experiment Detector response Energy reconstruction for all the calibration sources I Residuals from all the radioactive sources I Energy resolution (δE=E) is well reproduced are contained in a ±1% band in the MC for all the radioactive sources I Energy resolution saturates at 5% due to volume effects Cell 1 Cell 2 Cell 3 Cell 4 Preliminary Preliminary Cell 5 Cell 6 Vladimir Savu (CEA) 06.12.2019 { AAP 2019 10 / 23 The STEREO experiment Detector response Detector response - Y-axis I Same quality and pattern of residuals as for I Neutrons from AmBe source are able to the X,Z axes tested with the calibration traverse the GC cells and reach the Target tubes cells I No further correction needs to be applied I We look at the n-H capture peak Target & Gamma-Catcher AmBe n-H (mid-height) Preliminary Cell ID Data/MC Erec of neutron captures by hydrogen, for the average of 3 horizontal deployment Y-axis scan with AmBe source positions of an AmBe source at mid-height along the Y-axis of the detector Vladimir Savu (CEA) 06.12.2019 { AAP 2019 11 / 23 The STEREO experiment Detector response 12B β-decay spectrum I Complementary data to the calibration sources I extended energy range Preliminary I continuous spectrum I extended vertex distribution I β-decay spectrum of 12B ! selection of cosmic muon captures on 12C: − 12 12 µ + C ! B + νµ I Good agreement between the measured and simulated 12B spectra Use the 12B spectrum and the calibration sources to perform a global fit of the energy scale Vladimir Savu (CEA) 06.12.2019 { AAP 2019 12 / 23 The STEREO experiment Detector response 12B β-decay spectrum - global fit Preliminary I Global fit of sources + 12B spectrum I Supplementary constraints on the energy Cell 4 scale I Polynomial fit function: Global Fit Data MC Erec = Pol(Erec ) I Distortions of the measured neutrino spectrum induced by the fitted deviation of Data MC Erec to Erec are contained in an envelope obtained by varying the calibration coefficients with ±1% Vladimir Savu (CEA) 06.12.2019 { AAP 2019 13 / 23 The STEREO experiment Detector response Time stability Fit of the n-H capture peak in Cell 2 Relative deviation to the mean n-H Preliminary n-H capture of neutrons from cosmic rays I Preliminary I Distribution of vertices similar to νe I Stable detector response at 0.3% level in each cell Vladimir Savu (CEA) 06.12.2019 { AAP 2019 14 / 23 The STEREO experiment Background and shielding Signal and background Inverse beta decay (IBD) interaction: n thermalisation νe diffusion p γ + e+ νe + p e + n energy deposit γ Gd(n,γ)γ)) ! annihilation ~ 8MeV γ cascade e- e+ γ Delayed event ! neutron capture Prompt event ! e+ P Gd: i Eγ;i ∼ 8MeV E + ≈ E − 0:8MeV vis;e νe H: Eγ ∼ 2:2MeV Accidental background Correlated background from cosmic rays I natural radioactivity I stopping muons I reactor induced neutrons and gammas I spallation neutrons Vladimir Savu (CEA) 06.12.2019 { AAP 2019 15 / 23 The STEREO experiment Background and shielding Background and shielding I Passive shielding for γ, neutrons and magnetic fields I Water channel and active veto for cosmic muons Accidental background Correlated background from cosmic rays I natural radioactivity I stopping muons I reactor induced neutrons and gammas I spallation neutrons Vladimir Savu (CEA) 06.12.2019 { AAP 2019 16 / 23 The STEREO experiment ν¯e selection and efficiency studies ν¯e signal selection and efficiency studies n thermalisation νe τ ~ 20μs γ diffusion p + I Mean cut efficiency: e γ Gd(n,γ) energy deposit ~ 8MeV γ cascade annihilation 61.4±0.9% e- e+ γ I Dominated by neutron efficiency (delayed Clean after Clean before correlated pair t signal) 1.6 < Eprompt < 7.1 MeVE delayed > 4.5 MeV AmBe source at z=45cm 3000 Data 2500 Simulation 2000 events 1500 1000 500 10 20 30 40 50 60 70 correlation time [µs] 1.5 1.0 0.5 data/MC 10 20 30 40 50 60 70 correlation time [µs] Good agreement with Monte-Carlo Good agreement with Monte-Carlo for the neutron in correlation time detection efficiency at the % level Vladimir Savu (CEA) 06.12.2019 { AAP 2019 17 / 23 The STEREO experiment ν¯e selection and efficiency studies Improved Gd Gamma cascade simulation I Delayed signal: gamma cascade