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Sterile Neutrinos at BorexinoSOX

G. Ranucci – INFN MilanoOn behalf of the Borexino Collaboration

European Strategy for Neutrino Oscillation Physics - II CERN

15 May 2012

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Borexino at Gran Sasso: real time detection of low energy neutrinos

Water Tank:g and n shieldm water Č detector208 PMTs in water2100 m3

20 legsCarbon steel plates

Scintillator:270 t PC+PPO in a 150 mm thick nylon vessel

Stainless Steel Sphere:2212 photomultipliers 1350 m3

Nylon vessels:Inner: 4.25 mOuter: 5.50 m

Design based on the principle of graded shielding

Neutrino electron scattering

n e -> n e

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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 (Sr90) Bx– J.N.Bahcall,P.I.Krastev,E.Lisi, Phys.Lett.B348:121-123,1995– N.Ferrari,G.Fiorentini,B.Ricci, Phys. Lett B 387, 1996 (Cr51) Bx – I.R.Barabanov et al., Astrop. Phys. 8 (1997)– Gallex coll. PL B 420 (1998) 114 Done (Cr51)– A.Ianni,D.Montanino, Astrop. Phys. 10, 1999 (Cr51 and Sr90) Bx– A.Ianni,D.Montanino,G.Scioscia, Eur. Phys. J C8, 1999 (Cr51 and Sr90) Bx– SAGE coll. PRC 59 (1999) 2246 Done (Cr51 and Ar37)– SAGE coll. PRC 73 (2006) 045805– C.Grieb,J.Link,R.S.Raghavan, Phys.Rev.D75:093006,2007– V.N.Gravrin et al., arXiv: nucl-ex:1006.2103– C.Giunti,M.Laveder, Phys.Rev.D82:113009,2010– C.Giunti,M.Laveder, arXiv:1012.4356

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Source Experiment: Physics Case

• Probing Short Baseline Flavor Oscillations in disappearance

• Search for Neutrino Magnetic moment• Probe neutrino-electron scattering at 1 MeV

scale– Weinberg’s angle– gV and gA coupling (NSI)

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Source location in Borexino

• A: underneath WT– D=825 cm– No change to presentconfiguration

• B: inside WT– D = 700 cm– Need to removeshielding water

• C: center– Major change– Remove inner vessels– To be done at the end of solarNeutrino physics

A

BC

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Source position A

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Sources Activity: several 1000 n evts within 1 year E >250 keV (14C background) Half-life ≥1 month Compact Limited heat Efficient shielding Low impurities level

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A similar option, but less viable, is 106 Ru– 106 Rh

Neutrino source

Anti-Neutrino sources

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51Cr

~36 kg of Cr 38% enriched in 50Cr190 W/MCi from 320 keV g’s7mSv/h (must be < 200)

SAGE coll., PRC 59 (1999) 2246Gallex coll., PL B 420 (1998)

Done two times for Gallex at 35 MW reactorwith effective thermal neutrons flux of ~5.4E13 cm-2s-1

~1.8 MCi

Originally proposed by Raju Raghavan

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51Cr source in Gallex

shielding size dictated by g-emitting impurities

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The case of ne 51Cr source in Borexino

Window 0.250-0.700 MeVBackground perfectly known : solar neutrinos + Bismuth210

Bismuth210

Source events

CNO

Be7

Detection as 7Be solar neutrinos

The uncorrelated nature of the measure forces the external deployement of the source: too much backg. from the shield for internal deployment

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90Sr-90Y

tSr= 28.79 yearstY= 3.8 days

90Sr

90Y

Inverse beta decay

Product of nuclear fissionUsed in thermoelectric generatorsKnown technology for 0.2 MCi sources

7.25 kg/MCi~6700 W/MCiincluding Bremsstrahlung

<E>=2±0.2MeV<s>=7.2×10-45cm2

e source

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106Ru-106Rh

tRu= 539 daystRh=29.8 s

106Ru

106RhInverse beta decay

Product of nuclear fission

<E>=2.5±0.2 MeV<s>=89.2×10-45cm2

e source

Similar option: 144Ce– 144Pr Advantage w.r.t. 90Sr: lower activity affordable

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Anti-nu AdvantagesBackground free measure (delayed coincidence)

-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-anti n 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

Th= 1.8 MeV

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51Cr externalCannot 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 internalInternal 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

Staged two –phase approach

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51Cr Source under the detector

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400 500 600 700 800 900 1000 1100 1200 1300 14000

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40

60

80

100

120Spatial profile of detected events for a monoenergetic source (Cr51) in the tunnel

Cm from the source

Dm2 sin22q

8 0.07

1 0.1

no oscillation

Ideal case no spatial resolutionno background

At high Dm2 the fast wiggles are washed out when the resolution is included

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Example of a simulation of the 51Cr source externally positioned

The fit allows also to determine precisely the oscillation parameters

Oscillometry analysis: total rate + waveshape of the profile of the detected events

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Reach of the sterile neutrino search with the 51Cr source

2 analysis of the 51Cr source outside BX

• activity=10MCi;

• Error on activity=1%;

• Error on FV=1%;

Reactor anomaly

Exclusion contoursSensitivity to the rate only

FV error better than 1% already achieved in BX (calibration)

Error of 1% on the source intensity is agressive – important effort to achieve it

Sensitivity to the rate + waveshape

Green region 90% CL excluded from Solar+KamLAND constraints accounting for the 13 0 value A. Palazzo - Phys. Rev. D 85, 077301 (2012)

Rate + shape + additional handle: time decay of the source event rate to better discriminate against the background

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2 analysis of the 51Cr source outside BX

• activity=10MCi;

• 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

Reach of the sterile neutrino search with the 51Cr source

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Weinberg’s Angle @ 1 MeV

10 MCi source 5 MCi source d(sin2qW) = 2.6%

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Neutrino Magnetic Moment

Reactor anti-neutrinos:~6×10-11 mB (90% CL)

From Borexino (solar):~5×10-11 mB (90% CL)

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144Ce Source at the center of the detector

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0 100 200 300 400 500 6000

0.2

0.4

0.6

0.8

1

1.2

1.4

Distance from the center

Resolution effect non gaussianity at center

Waves from a source in the center

Enhanced sensitivity due both to the pattern and the increased number of events

Oscillation waves

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Other simulations – 90Sr at the center

Good agreement with the analytical oscillation curves

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0.001 0.01 0.1 10.01

0.1

1

10

100

sin22q14

214

Dm

Ce144 50 kCiCenter365 days1% err. source intensity1% err. FV

90% C.L. excluded

Reach of the sterile neutrino search with the 144Ce source

Error of 1% on the source intensity is agressive – but the FV error could be omitted – included as safety margin

Adequate coverage of the region of interest of the oscillation parameter plane

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EW couplings

• Standard Model– gV = -1/2+2sin2qW = -0.038

– gA = -0.5

• Use three-level cross-section• Use 51Cr and 144Ce source

51Cr144Pr

CHARM II with nme ES

90 % C.L.

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Status of the investigation51Cr

Enriched Cr used for Gallex still available at CEA Saclay Research reactor

• A) High thermal neutron flux throughout the entire target ideally 1E15 n/cm2/sec

• B) Enough space to accommodate the material• C) Flexible enough to allow the reconfiguration of the core

The Siloe’ reactor at Grenoble met this requirements, but it is no longer available, no other suitable reactors available in France

AlternativesPetten 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 could be suited to do the irradiation

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More investigations required for the anti- n sources: 90Sr 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

Status of the investigationAnti-n

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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 51Cr 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