Borexino Source Experiment

Borexino Source Experiment G. Ranucci On behalf of the Borexino Collaboration SNAC11,Blacksburg, VA 27 September 2011

The idea to use a neutrino source in Borexino and in other underground dexperiments dt dates back to at least 20 years N.G.Basov,V.B.Rozanov, JETP 42 (1985) Borexino proposal, 1991 (Sr90) Bx JNBahcallPI J.N.Bahcall,P.I.Krastev,E.Lisi, KrastevELisi Phys.Lett.B348:121 123,1995 123 1995 N.Ferrari,G.Fiorentini,B.Ricci, Phys. Lett B 387, 1996 (Cr51) Bx I.R.Barabanov et al., Astrop. Phys. 8 (1997) Gll 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

The physics case with a source experiment Neutrino magnetic moment Neutrino electron non standard interactions Probe ν e e weak couplings at 1 MeV scale Probe sterile neutrinos at 1eV scale Probe neutrino vs anti neutrino oscillations on 10m scale

Anomalies/hints for Δm 2 1eV 2 a) The LSND/Miniboone (anti ν and ν) anomalies b) The reactor anomaly arxiv:1101.2755 sin 2 2θ ee =0.1 Δm 2 =0.4 ev 2 Rt Rate only analysis sin 2 2θ ee =0.1 Δm 2 =1.5 ev 2 Rate+shape Workshop on Beyond Three Family Neutrino Oscillations LNGS 3 4 May 2011 c) The Gallium anomaly R=0.86 from Gallex and Sage source tests arxiv1006.3244 Giunti and Laveder sin 2 2θ ee =0.5 Δm 2 =2.24 ev 2 d) Indications from cosmology and BBN of more than 3 neutrinos

Borexino at Gran Sasso: low energy neutrino real time detection Scintillator: 270 t PC+PPO in a 150 μm thick nylon vessel Nylon vessels: Inner: 4.25 m Outer: 5.50 m Neutrino electron scattering ν e > ν e Carbon steel plates Gioacchino Ranucci - SNAC11 Stainless Steel Sphere: 2212 photomultipliers 1350 m 3 Design based on the principle of graded shielding Water Tank: γ and n shield μ water Č detector 208 PMTs in water 2100 m 3 20 legs

Gioacchino Ranucci - SNAC11 UMass

Borexino achievements High precision (better than 5%) 7 Be flux 7 Be day night (absence) Unambiguous geo neutrino detection 8 Be with a 3 MeV threshold First indication of pep solar neutrinos and tight upper limit on CNO neutrinos (Borexino talks at the recent TAUP) All above thanks to the accurate energy position calibration of the detector volume The perfect knowledge of the detector performances witnessed by this impressive list ofachievements is thesolid grounduponwhichupon the proposal for the source test in Borexino is built

7 Be Solar neutrinos in Borexino hep ex/1104.1816v11816v1 7 Be = 46.0 ± 1.5 +1.6 cpd/100 tons 1.5 pp 85 Kr Now removed! 210 Bi pep & CNO

Solar neutrino survival probability Ultimate validation of the MSW LMA survival probability curve

Position and energy calibration On and off axis calibrations sources Rn, AmBe 57Co, 139Ce, 208Hg, 85Sr, 54Mn, Mn 65Zn, Zn 40K, K 60Co The knowledge of the detector and of its performance makes it the ideal environment for a series of source test to shed light on the hints of a new neutrino oscillation mass scale involving steriles Initially external location, later in the center

Source location in Borexino A: underneath thwt D=825 cm No changetopresent configuration B: inside WT D= 700 cm Need to remove shielding water C: center Major change Remove inner vessels To be done at the end of solar Neutrino physics yscs C A B

Source position A

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

Proposed at the workshop Beyond 3 ν at Gran Sasso in May and in arxiv:1107.2335

51Cr Originally proposed by Raju Raghavan ~36 kg of 38% enriched 50 Cr 190 W/MCi from 320 kev γ s 7μSv/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 ~5.4E13 cm 2 2s 1 1 ~1.8 MCi

Transport container

Cr51 Gallex source

The case of ν 51 e Cr source in Borexino Bismuth210 CNO Source events Be7 Window 0.250 0.700 0 KV KeV Background : solar neutrinos + Bismuth210

37 Ar ν e source 37 Ar(τ=50.55 days) 37 Cl 813 kev (9.8%) 811 kev (90.2%) From irradiation of CaO using fast neutrons 40 Ca(n,α) 37 Ar ~16 W/MCi from 2.6 kev X rays Used in SAGE with ~0.4 MCi SAGE coll., PRC 73 (2006) 045805 Gaseous source Mentioned here for historical reasons

90Sr 90Y ν e source τ Sr = 28.79 years τ Y = 3.8 days 90 Sr Inverse beta decay <E>=2±0 2±0.2MeV2MeV <σ>=7.2 10 45 cm 2 90 Y 7.25 kg/mci Product of nuclear fission ~6700 W/MCi Used in thermoelectric generators including Bremsstrahlung Known technology for 0.2 MCi sources

3

Spatial profile of detected events for a monoenergetic 120 source (Cr51) in the tunnel 100 80 Δm 2 sin 2 2θ 8 0.07 007 1 0.1 60 no oscillation 40 Ideal case no spatial resolution no background 20 0 400 500 600 700 800 900 1000 1100 1200 1300 1400 Cmfrom the source

How to exploit the rate and waves information Oscillometry measurements Standard way to convey the predicted sensitivity of an oscillation experiment Exclusion plotsexploiting orate only (usual approach) orate plus waves (specificity of the present case) oobtained through the likelihood ratio method, testing against the oscillation hypothesis simulated data which insteadareproduced are unaffected by oscillations o Practically derived by comparing the corresponding χ 2 (average data with no oscillation model with oscillation) with the desired quantile (90%, 99%, 99.73%) of the χ 2 distribution with two degrees of freedom

Complementary, less used approach: Discovery plots to determine the existence of the effect, exploiting again only the rate or the rate+waves combination oobtained i d through h the likelihood lih ratio method, but testing against the no oscillation hypothesis simulateddata data which instead are produced as affected by oscillations o Practically derived by comparing corresponding χ 2 (average data with oscillation model with no oscillation) with the desired quantile (90%, 99%, 99.73%) of the χ 2 distribution ib ti with two degrees of freedom othe determination of the oscillationparameters is also carried out oexclusion and discovery contours are practically similar At high Δm 2 effectively rate only analysis (in both cases)

Cr51 Exclusion plots Sr90 Ce144 In the tunnel (A) and (in some cases) in the center (C). B is very similar to A Detector characteristics affecting the sensitivity of the measure Spatial lresolution about t15 cm Systematic error on spatial reconstruction (Fiducial Volume systematic error) 1% Background Solar l neutrinos and Bismuth for ν source about 40 events per day Geo and reactor anti ν (a dozen of events per year) for anti ν source The precision ofthe knowledge ofthe source intensity is another systematic error assumed 1%

At high Δm 2 the fast oscillations are smeared by the detector spatial resolution effectively rate only analysis Δm 2 10 1 0.1 90% C.L. Cr51 10 Mci (2 expositions) Tunnel l(d=825 cm) 200 days 1% err. source intensity 1% err. FV Reactor anomaly JointReactor+Ga anomaly Rate + waves 90 % C.L.. e xcluded Solar+KamLAN Dconstraints accounting for the θ 13 0 from the T2K result A. Palazzo arxiv:1105.170 5 and talk at TAUP Here and in the following Δm 2 in unit ev 2 0.01 Rate only 001 0.001 0.01 0.1 1 sin 2 2θ

10 99% C.L. Δm 2 1 0.1 Cr51 10 Mci (2 expositions) Tunnel l(d=825 cm) 200 days 1% err. source intensity 1% err. FV 90% C.L.. excluded Reactor anomaly 0.01 Rate + waves Rate only 0.001 0.001 0.01 0.1 1 sin 2 2θ

Cr51 in the center Enhanced sensitivity due both to the pattern and the increased number of events 1.4 1.2 1 0.8 0.6 Oscillation waves 0.4 02 0.2 Resolution effect non gaussianity at center 0 0 100 200 300 400 500 600 Distance from the center

10 The effect on the exclusion plot of the enhanced sensitivity in 1 the center location is striking, the reactor anomaly 0.1 region would be fully covered. Δm 2 90% C.L. Cr51 10 Mci (2 expositions) Center 200 days 1% err. source intensity 1% err. FV Reactor anomaly Joint Reactor+Ga anomaly 0.01 Rate + waves Technically very challenging g size and Rate only shielding (320 kev γ s and γ s from impurities) issues more plausible 0.001 0.001 001 001 0.01 01 0.1 to deploy the anti ν 1 source sin 2 2θ 90% C.L.. excluded d

1.005 1 0.995 0.99 0.985 0.98 09 0.975 0.97 0.965 0.96 Sr90 source oscillation probability as function of distance and energy Δm 2 θ 05 0.5 01 0.1 0.955 0 200 400 600 800 1000 1200 1400 1600 Cm from the source location The different curves correspond to energies ranging from 1.8 to 2.28 MeV A twofold energy distance approach is needed For simplicity here we have integrated over the energy

Sr90 source event spatial profile variation over the energy interval 1.8 to 2.28 MeV 4.50E 02 02 4.00E 02 3.50E 02 3.00E 02 2.50E 02 2.00E 02 1.50E 02 1.00E 02 5.00E 03 03 0.00E+00 0 200 400 600 800 1000 1200 1400 1600 1800 Distance from the source (cm)

Sr90 source expected event spatial profile after energy integration over energy Δm 2 =1 sin 2 2θ =0.2 Δm 2 =2 sin 2 2θ =0.1 Amplitu ude (a.u.) Sr90 in the tunnel Amplitu ude (a.u.) Sr90 in the tunnel 0 0 500 1000 1500 2000 Distance from the source (cm) 0 0 500 1000 1500 2000 Distance from the source (cm) Δm 2 =1 sin 2 2θ =0.1 Δm 2 =0.1 sin 2 2θ =0.2 itude (a.u.) Sr90 in the tunnel litude (a.u.) Sr90 in the tunnel Ampli Ampl 0 0 500 1000 1500 2000 0 500 1000 1500 2000 Distance from the source Gioacchino (cm) Ranucci SNAC11 Distance from the source (cm) 0

Sr90 Advantages Background 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 following plots in which the FV error is maintained are therefore conservative) the coincidence technique makes it suited to be located in the center Future scalability: in a post solar phase of the experiment the entire sphere can be fll filled with scintillator Issues to be considered : heat dissipation and bremmstralung background shielding and shadowing around the center

10 90% C.L. Δm 2 Sr90 1 Mci Tunnel (d=825 cm) 1 365 days 1% err. source intensity 1% err. FV 0.1 90% C..L.. exclu ded Reactor anomaly 0.01 Joint Ga+Reactor anomaly Rate + waves Rate only 0.001 0.001 0.01 0.1 1 sin 2 2θ

10 Realistically the 90% C.L. FV error can be ignored due to Sr90 1 Mci the delayed Tunnel (d=825 cm) coincidence 365 days 1 measurement. 1% err. source The 90% intensity reactor anomaly region is almost 0.1 fully covered Reactor anomaly Δm 2 90% C..L.. exclu ded 001 0.01 Joint Ga+ Reactor anomaly Rate + waves Rate only 0.001 0.001 0.01 0.1 1 sin 2 2θ

10 Decent sensitivity even at 99% C.L. Further improvable 1 ignoring the FV error 99% C.L. Sr90 1 Mci Tunnel (d=825 cm) 365 days 1% err. source intensity 1% err. FV 90% C.L L.. exclud Δm 2 0.1 ed Reactor anomaly 0.01 Rate + waves Rate only 0.001 0.001 0.01 0.1 1 sin 2 2θ

Sr90 in the center The averaging effect over the energy range is less important than for the external location 7.00E 03 03 6.00E 03 5.00E 03 4.00E 03 3.00E 03 2.00E 03 03 1.00E 03 Energy lower limit Energy range upper limit Middle energy range No oscillation 7.00E 03 6.00E 03 5.00E 03 4.00E 03 3.00E 03 No oscillation 2.00E 0303 Energy 1.00E 03 averaged 0.00E+00 0 100 200 300 400 500 600 cm from the center 0.00E+00 0 100 200 300 400 500 600 cm from the source Δm 2 =2 sin 2 2θ=0.1

Potential improveme nts w.r.t. this plot: a)no FV error b) Extended dd data taking time (three years) c) Entire SS sphere filled with scintillator Δm 2 10 1 0.1 filled with 0.01 90% C.L. Sr90 1 Mci Center 365 days 1% err. source intensity 1% err. FV Reactor anomaly Joint Reactor+Ga anomaly Rate + waves Rate only 90 % C.L.. e xcluded 0.001 0.001 0.01 0.1 1 sin 2 2θ

Extremely good sensitivity also at the 99% C.L. 10 1 99% C.L. Sr90 1 Mci Center 365 days 1% err. source intensity 1% err. FV 90 0% C.L.. excluded d Δm 2 0.1 Reactor anomaly 0.01 Rt Rate+ waves Rate only 0.001 0.001001 001 0.01 01 0.1 1 sin 2 2θ

106Ru 106Rh ν e source 106 Ru τ Ru = 539 days τ Rh =29.8 s 106 Rh Inverse beta decay <E>=2.5±0.2 2MeV <σ>=89.2 10 45 cm 2 Product of nuclear fission Similar option: Ce144 Pr144 Advantage w.r.t. S90 Sr90: lower activity ii affordable

Following the concepts devised in arxiv:1107.2335 W+Cu shield

With a modest source activity a good sensitivity reach is anyhow ensured Δm 2 100 90% C.L. 10 1 Ce144 50 kci Center 365 days 1% err. source intensity 1% err. FV 90% C..L.. exclu ded Joint Reactor+Ga anomaly 0.1 Reactor anomaly Rate+waves 0.01 0.001 0.01 0.1 1 sin 2 2θ

100 99% C.L... Even at 99% C.L. Δm 2 10 1 Ce144 50 kci Center 365 days 1% err. source intensity 1% err. FV 90% C.L.. exc luded Joint Reactor+Ga anomaly 90% C.L. 0.1 Reactor anomaly Rate + waves 0.01 0.001 0.01 0.1 1 sin 2 2θ

100 90% C.L. A reach capability at some level even if located externally Δm 2 Ce144 50 kci Tunnel (d=825 cm) 365 days 10 1% err. source intensity 1% err. FV 1 Reactor anomaly 90% C.L.. excluded d 0.1 Joint Reactor+Ga anomaly Rate + waves 0.01 0.001 0.01 0.1 1 Sin 2 2θ

test sta atistic t Discovery curves Likelihood ratio test statistics to identify the oscillation effect, if exists L ( no oscillation) ) t = 2ln L( oscillation) Example from a specific simulation case: Δm 2 =1 sin 2 2θ=0.2 Blind search over Δm 2 and sin 2 2θ 160 Cr51 10 MCi Δm 2 =1 sin 2 2θ=0.2 200 Cr51 10 MCi Δm 2 =1 sin 2 2θ=0.22 140 120 100 80 60 40 20 0 0 2 4 6 8 10 Δm 2 150 100 Oscillation effect unambiguously detected True parameters correctly identified 50 0 0 0.2 0.4 0.6 0.8 1 1.2 sin 2 2θ

Why consider explicitly the discovery scenario? It provides a more suitable framework to address the request coming from the community to illustrate the sensitivity of the proposed p experiments to an existing effect in term of actual discovery capability Eligio Lisi talk at TAUP 2011

Limiting factor of the discovery capability Since the maximum of the test statistic t over the spanned region of the oscillation parameters is the criterion to define a discovery, the corresponding limiting factor is represented by the distribution of t when actually there is no oscillation (this is equivalent to the noise distribution in a signal over noise search) MC distributions The no oscillation curve follows closely the expected ideal χ 2 (2) distribution The threshold is set in this example for a three sigma detection 0.3 0.25 0.2 0.15 0.1 0.05 0 Δm 2 =5 3σ threshold sin 2 2θ=0.11 sin 2 2θ=0.2 no oscillation sin 2 2θ=0.05 0 5 10 15 20 25 30 35 40 45 t= 2ln(Lnum/Lden) When t is found above the threshold discovery is declared. For a given Δm 2 the boundary between the no discovery and discovery regions of the parameter sin 2 2θ is its specific value at which discovery happens 50% of the times, e.g. 0.11 in this example

Example of individual simulations Cr51 in position B The fit allows also to determine precisely the oscillation parameters

Other simulations Sr90 at the center Good agreement with ihthe analytical loscillation i curves

In practice the discovery plot does not differ from the exclusion plot for the same C.L. m 2 Δm 51 Cr Discovery plot 10 1 0.1 Cr51 10 MCi Tunnel (d=825 cm) 200 days 1% err. source intensity 1% err. FV 90% C.L.. ex xcluded Reactor + Ga 90% C.L. 0.01 3 sigma discovery plot Reactor anomaly 90% Reactor anomaly 99% 0.001 0.001 0.01 0.1 1 Sin 2 2θ

100 144 Ce discovery plot The superior performance of the anti ν source in the center is confirmed 10 Ce144 50 kci Center 365 days 1% err. source intensity 1% err. FV 90% C. L.. exclud ded Δm 2 1 Joint reactor+ga anomaly 90% C.L. 0.1 actor anomaly 99% C.L. reactor anomaly 90% C.L. 3 sigma discovery plot 0.01 0.001 0.01 0.1 1 Sin 2 2θ

Discovery plots only shape analysis vs. rate + shape 10 51 Cr 10MCi IP 200 days 3σ CL 1 1% FV 1% Source 90% exc luded Δm 2 0.1 51 Cr 10 MCi external 3 sigma rate+waves 0.01 0.001 0.001 0.01 0.1 1 Sin 2 2θ

Status of the material for the Cr51 source and investigations in progress The isotopically enriched Cr (40% Cr50) material is stored at Saclay in form of small chips for a total of 35.5 Kg It is perfectly suited to undergo a new irradiation We are currently in contact with Michel Cribier and the Saclay group in order to bring back the material to Italy. It will be stored in a special location while waiting for the new irradiation campaign : Forli company Protex SPA Shipping company : MIT nucleare the same Company can do the transportation after the irradiation (it will be needed a special Transport Container ) We have contacted the LNGS expert for handling of radioactive material Luciano Lembo (involved also in the Gallex calibration) to discuss boththe the issues connected with the transportation of the a) inert and b) activated Cr material: transport regulations and authorizations complex but addressable matter

Search of an irradiation facility 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 We visited the Delft reactor (Netherland) which meets the requirements B and C, but not A cost would have been very limited We visited the Petten reactor (Netherland), promising, complete feasibility evaluation being started Possibility in USA : the Advanced Test Reactor at Idaho National Laboratory, featuring neutron fluxes at the required dlevell

Further considerations Opportunities in Russia are being investigated as well additional transportation difficulties Starting point GNO 2001proposal for irradiation at SM3 reactor of Dimitrovgrad cost estimate was 1.3 M$ of dollars kick off meeting in Moscow at the beginning of July More investigations required for the anti ν sources: Sr90 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 very recent proposal of Ru106 and Ce144 sources Joint (with potential suppliers) feasibility studies of the source preparation and delivery to be started imminently

Conclusions Borexino is in a very favorable position to address the hot topic of possible short baseline ν e disappearance due to oscillation to sterile via a series of neutrino source tests, through which perform powerful oscillometry measurements exclusion and discovery plots In a first step a totally non invasive measurement can be performed by deploying externally a 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 the Gallium anomaly and to start the exploration of the reactor anomaly Alternative for the first phase: source immersed in water in the Water Tank In the post solar phase scenario the source(s) 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 We have started the investigation for the source(s) preparation and procurement. For Cr51 Gallex experience and GNO proposal of 2001 Opportunity for LNGS to maintain and strengthen the leadership role gained through the Gallex GNO and Borexino results in the solar neutrino sector