Sterile Neutrinos at Borexino SOX G Ranucci INFN
Sterile Neutrinos at Borexino SOX G. Ranucci – INFN Milano On behalf of the Borexino Collaboration European Strategy for Neutrino Oscillation Physics - II CERN 15 May 2012 1
Borexino at Gran Sasso: real time detection of low energy neutrinos Scintillator: 270 t PC+PPO in a 150 mm thick nylon vessel Nylon vessels: Inner: 4. 25 m Outer: 5. 50 m Neutrino electron scattering n e -> n e Stainless Steel Sphere: 2212 photomultipliers 1350 m 3 Design based on the principle of graded shielding Water Tank: g and n shield m water Č detector 208 PMTs in water 2100 m 3 20 legs Carbon steel plates 2
The idea to use a neutrino source in Borexino and in other underground experiments dates back to at least 20 years – – – – N. G. Basov, V. B. Rozanov, JETP 42 (1985) Borexino proposal, 1991 (Sr 90) Bx J. N. Bahcall, P. I. Krastev, E. Lisi, Phys. Lett. B 348: 121 -123, 1995 N. Ferrari, G. Fiorentini, B. Ricci, Phys. Lett B 387, 1996 (Cr 51) Bx I. R. Barabanov et al. , Astrop. Phys. 8 (1997) Gallex coll. PL B 420 (1998) 114 Done (Cr 51) A. Ianni, D. Montanino, Astrop. Phys. 10, 1999 (Cr 51 and Sr 90) Bx A. Ianni, D. Montanino, G. Scioscia, Eur. Phys. J C 8, 1999 (Cr 51 and Sr 90) Bx SAGE coll. PRC 59 (1999) 2246 Done (Cr 51 and Ar 37) SAGE coll. PRC 73 (2006) 045805 C. Grieb, J. Link, R. S. Raghavan, Phys. Rev. D 75: 093006, 2007 V. N. Gravrin et al. , ar. Xiv: nucl-ex: 1006. 2103 C. Giunti, M. Laveder, Phys. Rev. D 82: 113009, 2010 C. Giunti, M. Laveder, ar. Xiv: 1012. 4356 3
Source Experiment: Physics Case • Probing Short Baseline Flavor Oscillations in disappearance • Search for Neutrino Magnetic moment • Probe neutrino-electron scattering at 1 Me. V scale – Weinberg’s angle – g. V and g. A coupling (NSI) 4
Source location in Borexino • A: underneath WT – D=825 cm – No change to present configuration • B: inside WT – D = 700 cm – Need to remove shielding water C B • C: center – Major change – Remove inner vessels – To be done at the end of solar Neutrino physics A 5
Source position A 6
Sources § Activity: several 1000 n evts within 1 year § E >250 ke. V (14 C background) § Half-life ≥ 1 month § Compact § Limited heat § Efficient shielding § Low impurities level 7
Neutrino source Anti-Neutrino sources A similar option, but less viable, is 106 Ru– 106 Rh 8
51 Cr Originally proposed by Raju Raghavan ~36 kg of Cr 38% enriched in 50 Cr 190 W/MCi from 320 ke. V g’s 7 m. Sv/h (must be < 200) SAGE coll. , PRC 59 (1999) 2246 Gallex coll. , PL B 420 (1998) Done two times for Gallex at 35 MW reactor with effective thermal neutrons flux of ~5. 4 E 13 cm-2 s-1 ~1. 8 MCi 9
51 Cr source in Gallex shielding size dictated by g-emitting impurities 10
The case of ne 51 Cr source in Borexino Bismuth 210 CNO Source events Window 0. 250 -0. 700 Me. V Background perfectly known : solar neutrinos + Bismuth 210 Be 7 The uncorrelated nature of the measure forces the external deployement of the source: too much backg. from the shield for internal deployment Detection as 7 Be solar neutrinos 11
90 Sr-90 Y source t. Sr= 28. 79 years t. Y= 3. 8 days 90 Sr Inverse beta decay <E>=2± 0. 2 Me. V <s>=7. 2× 10 -45 cm 2 90 Y 7. 25 kg/MCi ~6700 W/MCi including Bremsstrahlung Product of nuclear fission Used in thermoelectric generators Known technology for 0. 2 MCi sources 12
106 Ru-106 Rh source 106 Ru t. Ru= 539 days t. Rh=29. 8 s Inverse beta decay 106 Rh <E>=2. 5± 0. 2 Me. V <s>=89. 2× 10 -45 cm 2 Product of nuclear fission Similar option: 144 Ce– 144 Pr Advantage w. r. t. 90 Sr: lower activity affordable 13
Anti-nu Advantages Background free measure (delayed coincidence) Th= 1. 8 Me. V -Higher counting rate due to the possibility to exploit the full volume, in this case the FV error can be ignored – the coincidence technique enables to fight efficiently the extra background added from the shield and makes it suited to be located in the center -> more events and less intensity required - Higher energy -> more events because of the quadratic dependence of the cross section from the energy - Same as geo-antin measure in Borexino – bckg. totally negligible - Future scalability: in a post solar phase of the experiment the entire sphere can be filled with scintillator - Issues to be considered : heat dissipation, high energy gammas and bremmstralung background – shielding and “shadowing” around the center 14
Staged two –phase approach 51 Cr external Cannot be deployed internally because of background consideration – the test has zero impact on the apparatus and on the «solar» data taking - feasible within a couple of years Anti-nu source internal Internal deployment possible thanks to the coincidence measurement – but huge (and very pure) shield Require a major refurbishment of the detector for the support of the source Nylon vessel removed and the whole sphere converted into active volume Done by 2017 15
51 Cr Source under the detector 16
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Spatial profile of detected events for a monoenergetic 120 source (Cr 51) in the tunnel Dm 2 sin 22 8 0. 07 100 80 1 0. 1 no oscillation 60 Ideal case no spatial resolution no background 40 20 0 400 600 800 1000 Cm from the source 1200 At high Dm 2 the fast wiggles are washed out when the resolution is included 1400 18
Example of a simulation of the 51 Cr source externally positioned Oscillometry analysis: total rate + waveshape of the profile of the detected events The fit allows also to determine precisely the oscillation parameters 19
Reach of the sterile neutrino search with the 2 analysis of the 51 Cr source outside BX Sensitivity to the rate + waveshape 51 Cr source Rate + shape + additional handle: time decay of the source event rate to better discriminate against the background • activity=10 MCi; • Error on activity=1%; • Error on FV=1%; Reactor anomaly Sensitivity to the rate only Exclusion contours FV error better than 1% already achieved in BX (calibration) Green region 90% CL excluded from Solar+Kam. LAND constraints accounting for the 13 0 value A. Palazzo - Phys. Rev. D 85, 077301 (2012) Error of 1% on the source intensity is agressive – important 20 effort to achieve it
Reach of the sterile neutrino search with the 51 Cr source 2 analysis of the 51 Cr source outside BX • activity=10 MCi; • Error on activity=2%; • Error on FV=1%; Reactor anomaly Exclusion curves Error of 2% on the source intensity as achieved in the framework of the Gallex calibration 21
Weinberg’s Angle @ 1 Me. V 10 MCi source 5 MCi source d(sin 2 W) = 2. 6% 22
Neutrino Magnetic Moment Reactor anti-neutrinos: ~6× 10 -11 m. B (90% CL) From Borexino (solar): ~5× 10 -11 m. B (90% CL) 23
144 Ce Source at the center of the detector 24
Waves from a source in the center Enhanced sensitivity due both to the pattern and the increased number of events 1. 4 1. 2 Oscillation waves 1 0. 8 0. 6 0. 4 Resolution effect non gaussianity at center 0. 2 0 0 100 200 300 400 Distance from the center 500 600 25
Other simulations – 90 Sr at the center Good agreement with the analytical oscillation curves 26
Reach of the sterile neutrino search with the 144 Ce source 100 Dm 214 Adequate coverage of the region of interest of the oscillation parameter plane 90% C. L. excluded 10 Ce 144 50 k. Ci Center 365 days 1% err. source intensity 1% err. FV 1 0. 01 sin 22 q 14 0. 1 1 Error of 1% on the source intensity is agressive – but the FV error could be omitted – included as safety margin 27
EW couplings • Standard Model 90 % C. L. – g. V = -1/2+2 sin 2 W = -0. 038 – g. A = -0. 5 • Use three-level cross-section • Use 51 Cr and 144 Ce source 51 Cr 144 Pr CHARM II with nme ES 28
Status of the investigation 51 Cr ü Enriched Cr used for Gallex still available at CEA Saclay ü Research reactor • A) High thermal neutron flux throughout the entire target ideally 1 E 15 n/cm 2/sec • B) Enough space to accommodate the material • C) Flexible enough to allow the reconfiguration of the core ü The Siloe’ reactor at Grenoble met this requirements, but it is no longer available, no other suitable reactors available in France ü Alternatives Petten reactor (Netherland) - promising, complete feasibility evaluation to be started soon Possibility in USA - the “Advanced Test Reactor” at Idaho National Laboratory, featuring neutron fluxes at the required level Opportunities in Russia are being investigated as well, a couple of reactors 29 could be suited to do the irradiation
Status of the investigation Anti-n More investigations required for the anti-n sources: 90 Sr can be available from the Companies who separate it from the other fissions products- Experience in Russia (heating equipments upto 1993) The same consideration apply to the 144 Ce source Joint (with potential supplier) feasibility study of the source preparation and delivery to be done 30
Conclusions Borexino is well suited for a possible source based short baseline ne disappearance test - performances and background perfectly known In a first step a totally non invasive measurement can be performed by deploying externally a 51 Cr n source in the Tunnel underneath the Water Tank specifically prepared for this purpose during the construction of the detector, affording already an interesting sensitivity limit capable to address a sizable portion of the joint reactor and Gallium In the post solar phase scenario an anti-n source can be deployed in the center and the target volume increased achieving the ultimate sensitivity capable to cover a wide region of the oscillation parameter plane, thus fully addressing the reactor anomaly indication Investigations for the sources preparation and procurement in progress Opportunity for LNGS to maintain and strengthen the leadership role gained in the context of neutrino oscillation through the Gallex—GNO and Borexino results in the solar neutrino sector 31
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