II G. Ranucci ISAPP 2011 ISAPP 2011 ISAPP 2011 ISAPP 2011 - - PowerPoint PPT Presentation

Methods and problems in low energy neutrino experiments (solar, reactors, geo-)

II

• G. Ranucci

ISAPP 2011 ISAPP 2011 ISAPP 2011 ISAPP 2011 International School on International School on Astroparticle Astroparticle physics physics THE NEUTRINO PHYSICS AND ASTROPHYSICS July 26th - August 5th, 2011 Varenna - Italy

Some examples of scintillator based detectors

Borexino Borexino (low energy solar neutrino detector) described in the (low energy solar neutrino detector) described in the following at length as paradigmatic example of a following at length as paradigmatic example of a scintillator scintillator detector detector Chooz Chooz (reactor neutrino detector) (reactor neutrino detector) KamLAND KamLAND (reactor neutrino detector) (reactor neutrino detector) Planned: SNO+ and LENS Planned: SNO+ and LENS

Borexino Borexino A real time calorimetric scintillation detector for low energy solar neutrinos installed at the Gran Sasso installed at the Gran Sasso underground laboratory, aimed at detecting solar neutrinos through the scattering off the electrons of the scintillator

Designed for

good performance as instrument precision in

• energy measurement
• position measurement

needs of calibration and Monte Carlo tuning low background low background

• choice of construction materials
• assay of materials during the assembly
• special precautions for installation procedures (clean room,

cleanliness of the surfaces)

• accurate strategy for liquid manipulation and purification
• special issue : particular care for the nitrogen purity
• strategy against the cosmic muon: underground, muon veto,

tagging of the residual cosmogenic products

Main components

• Scintillator
• Nylon (inner and
• uter) vessels
• Buffer liquids
• Stainless steel sphere
• Support of PMT’s
• Containment of

the buffer (zero buoyancy for the nylon vessels) nylon vessels)

• PMT’s
• Concentrators
• Muon veto
• Calibration

equipments

• Water Tank
• Electronics and DAQ

Plants

• Storage vessels
• Scintillator

purification systems

• Water extraction
• Distillation
• Nitrogen sparging
• PPO (solute)

distillation

• Normal nitrogen
• High purity nitrogen
• High purity nitrogen

purified in 39Ar and 85Kr

• Fluid handling system
• Water purification
• Clean room
• CTF, the initial

prototype

Water Tank

Stainless steel sphere

PMT’s on the sphere surface

Vessel before inflation (viewed by CCD cameras)

Vessel after inflation (viewed by CCD cameras)

Detail Detail of

• f the

the south south end end-cap cap of

• f the vessel and

the vessel and of

• f the last

the last mounted mounted PMT’s on the 3 m PMT’s on the 3 m door door of

• f the

the sphere sphere

Muon veto: tyvek (diffusive panels) and phototubes on the external sphere surface

Tyvek on the surface of the Water Tank dome

Electronic racks (cables length more than 50 meters)

Radiopurity construction requirements

Detector and plants materials

Low intrinsic radioactivity Low radon emanation Chemical compatibility with PC

Pipes, vessels and pipes

Electropolished Cleaned with filtered detergents (Detergent-8, EDTA) Pickled and passivated with acids Rinsing with ultrapure water (class

Thorrn-EMI photomultipliers

Low radioactivity Shott borosilicate glass (type 8246) 1.1 ns time gitter for good spatial resolution (Al) light cones for uniform light collection in the fiducial volume mu-metal shilding for the earth magnetic field 384 PMTs with no cones for muon identification in the buffer region

Philadelphia - 30 July, 2008 Gioacchino Ranucci - I.N.F.N. Sez. di Milano

Rinsing with ultrapure water (class 20 – 50 MIL STD 1246 )

Leak tightness

Leak rate < 10-8 atm cc /s Nitrogen blanketing

• n

critical elements like pumps, valves, big flanges Double seal metal gaskets

Nylon vessels

Good chemical and mechanical strength (small buoyancy) Low radioactivity (< 1 count/day/100 tons) Contruction in low 222Rn clean room High purity nitrogen storage

Clean rooms

Mounting room in class 100 Inner detector in class 1.000 Outer detector in class 100.000

Nylon vessels

Requirements:

Chemical resistance to PC,PPO, DMP, water Mechanical strength (20MPa – 5° T) Optical transparency (350-450 nm) Low intrinsic radioactivity (U, Th, K) Clean fabrication (<3 mg dust) Low permeability ti Rn Leak tightness

Philadelphia - 30 July, 2008 Gioacchino Ranucci - I.N.F.N. Sez. di Milano

Solutions and results:

Sniamid Nylon-6 film 125 m thick film Index of refract. = 1.53 with >90% trasmittance U, Th less than 2 ppt Umidification to decrese the Tg glass transition temperature (brittle state)

Scintillator

Solvent: Pseudocumene Solute: PPO (1.5 g/l) Light yield: 11000 ph/MeV Attenuation length (@ 420 nm): 30 m Scattering length (@420 nm): 7 m Scattering length (@420 nm): 7 m Decay time (fast component): 3.5 ns Good α/β properties

Photomultipliers

8” Electron Tubes Limited (ETL) 9351 type P/V : 2.5 (measure of the single electron resolution) Transit Time Spread: 1ns (σ) Dark Count Rate: 1kHz (typical rate at 20 °C) Afterpulsing < 5% (for single electron pulses) Low radioactive glass and internal parts (main contributors to the external background)

Light concentrators

Truncated string cone design Truncated string cone design Optimized to collect the light from the inner vessel and 20 cm beyond it Material: anodized aluminum selected for low radioactivity

Electronics

ADC and TDC circuits Good single electron resolution Time resolution better than 0.5 ns

LAKN – Low Argon and Krypton Nitrogen

Detector fully filled on May 15th, 2007: DAQ starts

May 2007

End October 2006 Ultra-pure water Liquid scintillator Ultra-pure water March 2007

Photos taken with one of 7 CCD cameras placed inside the detector

Ultra-pure water

Neutrino Detection in Borexino Neutrino Detection in Borexino

Detection through the scattering reaction (as in Superkamiokande and in SNO-third method)

e e + → + ν ν

• ff the electrons of the scintillator

The high luminosity (50 times more than the Cerenkov technique) and high radiopuri(huge challenge: fight the natural radioactivity and high radiopuri(huge challenge: fight the natural radioactivity below 3 MeV) ty of the scintillator lead to a low detection threshold: analysis threshold about 200 keV, acquisition threshold about 60 keV It is possible therefore to detect the recoil electrons produced by the monoenergetic (0.862 MeV) 7Be neutrinos - maximum recoil energy: 0.66 MeV Other components of the solar spectrum are detectable, as well - flexibility of the detector

Other capabilities

8B solar neutrinos in the unique energy window 2 - 5 MeV Antineutrino science Geophysical from the Earth from type IIa Supernovae

e

v

e

v

Long baseline from European reactors Investigation of from the Sun Other components of the solar spectrum : pep, CNO, pp

e

v

e

v

Measured quantities

The electronics measures and provides for each triggered events:

• The photomultipliers pulse height

energy measurement

• The photoelectrons arrival times (better than 0.5 ns precision)

position identification The absolute time of the event Expected detector perfomances Effective coverage 30% Photoelectron yield 500 pe/MeV Energy resolution @ 1 MeV 5% Position resolution @ 1 MeV 10 cm

Light Yield

The Light Yield has been evaluated fitting the 14C spectrum,

(Borex. Coll. NIM A440, 2000)

and the 11C spectrum

14C spectrum (β− decay(156 keV, end

point)

11C spectrum(β+ decay(960 keV)

The light yield has been evaluated also by taking it as free parameter in a global fit on the total spectrum (14C,210Po, σ 210Po ,7Be ν Compton edge)

NO-VE April 15-18, 2008

The 11C sample is selected through the triple coincidence with muon and neutron. We limited the sample to the first 30 min of 11C time profile, which reduces the random coincidence to a factor 1/14.

C spectrum(β decay(960 keV)

Light Yield = 500 +( 12 p.e./MeV

The energy equivalent to the sum of the two quenched 511 keV gammas: E2γ(511) = 0.83 +( 0.03 MeV.

Energy resolution: 10% at 200 keV 8% at 400 keV 5% at 1 MeV

Position reconstruction

• Position reconstruction algorithms

– Base on time of flight fit to hit time distribution – developed with MC, tested and validated in CTF – cross checked and tuned in Borexino on selected events (14C, 214Bi-214Po, 11C)

The time and the total charge are measured, and the position is reconstructed for each event . Absolute time is also provided (GPS)

14C

NO-VE April 15-18, 2008

• σ

σ σ σ

• C

Radius (m)

Spatial resolution: 16 cm at 500 keV (scaling as )

N p.e.

−1/ 2

Fiducial volume

Radial distribution z vs Rc scatter plot

the nominal Inner Vessel radius: 4.25m (278 tons of scintillator) the effective I.V. radius has been reconstructed using: # 14C events # Thoron on the I.V. surface (emitted by the nylon(τ=80s) # External background gamma # Teflon diffusers on the IV surface maximum uncertainty : ???%

z < 1.8 m, was done to remove gammas from IV endcups NO-VE April 15-18, 2008

R2 gauss

2 2 2

R x y z = + +

2 2 c

R x y = +

FV FM: by rescaling background components known to be uniformly distributed within the LS and using the known LS mass (278.3 t)

γ from PMTs that penetrate the buffer

α/β discrimination

α α α α particles

Small deformation due to average SSS light reflectivity

β β β β particles Full separation at high energy

ns

NO-VE April 15-18, 2008

250-260 pe; near the 210Po peak 200-210 pe; low energy side of the 210Po peak

2 gaussians fit 2 gaussians fit

ns α/β α/β α/β α/β Gatti parameter α/β α/β α/β α/β Gatti parameter

Any instrument must be calibrated: Calibration campaign with sources Am-Be source

LNGS 13/4/2011 Gianpaolo Bellini Universita' e INFN- Milano

222 Rn loaded scintillator 214(Bi-Po) α/β discrim.

Low energy (0.14-2 MeV)

R(m)

Resolution

@ Energy scale ± 1.2% from 200 keV to 2 MeV

Over 2 MeV: A little worse due to the less accuracy in the calibration

@ Spatial reconstruction

LNGS 13/4/2011 Gianpaolo Bellini Universita' e INFN- Milano

@ Spatial reconstruction ± 10-12 cm from 200 keV to few MeV

Calibration is one of the ingredient for a good measure, the

• thers are a low and

under control background and a suitable model of the detector behavior (Monte Carlo)

LNGS 13/4/2011 Gianpaolo Bellini Universita' e INFN- Milano 31

Ultra low background requirements are the ultimate challenge for a detector aiming at neutrino spectroscopy in the sub-MeV range

Same problems for double beta decay and dark mater search

210Pb and associated 210Bi and 210Po

Requirement for Th and U about 10-16 g/g Limits the lower threshold 14C/12C found in BX at 2x10-18

Background: 232Th content

Assuming secular equilibrium, is measured with the delayed concidence:

212Bi 212Po 208Pb

β β β β α α α α τ τ τ τ = 432.8 ns

2.25 MeV ~800 KeV eq.

Specs: 232Th: 1. 10-16 g/g 0.035 cpd/ton

! "

• τ

τ τ τ #$%$&%'

NO-VE April 15-18, 2008 From 212Bi(212Po correlated events : 232Th: =(6.8±1.5)x 10-17 g/g

2 2 2

R x y z = + +

2 2 c

R x y = +

Only few bulk candidates

#$%$&%'

Background: 238U content

Assuming secular equilibrium, ()is measured with the delayed concidence:

214Bi 214Po 210Pb

β β β β α α α α τ τ τ τ = 236 µ µ µ µs

3.2 MeV ~700 KeV eq.

214Bi-214Po

τ(exp)=240±8µs

µ

NO-VE April 15-18, 2008

µ

Background: 238U content

!"$$*

• 214Bi-214Po

214Bi-214Po

z (m) z (m)

NO-VE April 15-18, 2008

• NOTES

– With these figures, bulk 238U and 232Th contamination is negligible – The 210Po background is NOT related neither to 238U contamination NOR to 210Pb contamination

2 2 c

R x y = +

2 2 c

R x y = +

• < 2 cpd/100 tons

238U: = (1.6±0.1) x10-17 g/g

Specs: 238U: 1. 10-16 g/g

Background: 210Po

+,#-

• The bulk 238U and 232Th

contamination is negligible

• The 210Po background is NOT

related neither to 238U contamination NOR to 210Pb contamination

210Po decay time:

60 cpd/1ton

• Not in equilibrium with 210Pb !
• 210Po decays as expected

NO-VE April 15-18, 2008

• .!

no direct evidence((((> free parameter in the total fit cannot be disentangled, in the 7Be energy range, from the CNO

Background: 85Kr

85Kr is studied through : 85Kr β decay :

(β decay has an energy spectrum similar to the

7Be recoil electron )

85Kr

β

85Rb

687 keV

τ = 10.76 y ( BR: 99.56%

85Rb 85Kr 85mRb

τ= 1.46 µs ( BR: 0.43% 514 keV β 173 keV γ

NO-VE April 15-18, 2008

Inferred 85Kr contamination 30.4±5.3(stat)±1.3(syst) counts/day/100 tons

τ = 10.76 y ( BR: 99.56%

τ= 1.46 µs ( BR: 0.43%

Cosmic µ

µ are identified by the OD and by the ID

• OD eff: ~ 99%
• ID analysis based on pulse shape variables

– Pulse mean time, peak position in time

• Estimated overall rejection factor:

– > 104 (still preliminary) A muon

NO-VE April 15-18, 2008

ID efficiency A muon in OD Muon flux:(1.21±0.05)h(1m(2 Muon angular distributions After cuts, µ are not a relevant background for 7Be analysis

– Residual background: < 1 c/d/100 t

With a calibrated instrument a tuned MC and a low, well known background it is possible to predict the detected spectrum!

Did it work? The answer is yes

The spectrum after cuts is very similar to the MC prediction Main purposes of cuts

• Remove external

gammas (fiducial volume)

11C 7Be 14C

The PSD of the properties of the scintillator described before are extremely useful to tackle this alpha peak due to 210Po gammas (fiducial volume)

• Remove muons and

cosmogenics

MC- fit range: 250-1600 keV Soft α subtraction # pp, pep, CNO fixed, according MSW-LMA high metallicity # free parameters: 7Be,85Kr,

210Bi ( βemitter) ,11C, 210Po (α emitter), 14 C, 214 Pb (β emitter)

Eps-Hep2011 Grenoble 22/7/2011 Gioacchino Ranucci INFN- Milano

Analytical- fit range 300- 1250 keV statistical α subtraction

214 Pb (β emitter)

The 7Be flux is extracted via a multi- component fit

First selective measurements of the 7Be neutrinos from Sun

Summary of solar neutrino results

Direct result from each experiment flux of one (or more) components of the solar neutrino spectrum-direct comparison with the SSM expectation (two versions High metalliciy, low metallicity of the solar surface) Day night asymmetry of the measured flux(es) – indication of matter effects in the Earth Combined analysis of all experiments Determination of the allowed region of the oscillation parameters ∆m12 and θ12(either sin or tan) Combination with KamLAND reactor experiment to sharpen the ∆m12 determination

Gallex/GNO

1 SNU equals 1 interaction per second per 1036 target atoms

Output (measured neutrino flux) of the Gallex/GNO and Sage experiments compared to the model prediction

8B SNO Flux Result

Φ Φ Φ ΦNC = 5.140 +4.0 -3.8 %

(x106cm-2s-1)

8B SNO Flux Result

(x106cm-2s-1)

8B Flux Result

Φ Φ Φ ΦNC = 5.140 +4.0 -3.8 %

(x106cm-2s-1) (x106cm-2s-1)

• J. N. Bahcall, A. M. Serenelli, and S. Basu, AstroPhys. J. 621, L85 (2005)

8B Elastic Scattering Result

ACC= (0.056 ± 0.074 (stat.) ± 0.051 (syst.) ANC= 0.042 ± 0.086(stat.) ± 0.067 (syst.) AES= 0.146 ± 0.198(stat.) ± 0.032 (syst.) (CC, ES spectrum shapes unconstrained in this analysis)

      + − ≡ D N D N A ) ( 2

SNO Day(Night Asymmetries (I) ACC and ANC are correlated (ρ = (0.532) In standard neutrino

• scillations, ANC should be

zero…

SK-III solar neutrino results

• Total live time : 548 days, Etotal ≥ 6.5 MeV

289 days, Etotal < 6.5 MeV

• Energy region: Etotal=5.0-20.0MeV
• 8B Flux: 2.32+/-0.04(stat.)+/-0.05(syst.) (x106/cm2/s)

– SK-I: 2.38+/-0.02(stat.)+/-0.08(syst.) – SK-II: 2.41+/-0.05(stat.)+0.16/-0.15(syst.)

(SK-I,II were recalculated using the Winter06 B spectrum) (SK-I,II were recalculated using the Winter06 8B spectrum)

– SK-III official: 2.32 ± 0.04(stat.) ± 0.05(syst.) – SK-IV: 2.28 ± 0.04

• Day / Night ratio:

) syst. ( 013 . ) stat. ( 031 . 056 . 2 / ) ( ) ( ± ± − = Φ + Φ Φ − Φ =

Night Day Night Day DN

A

Preliminary

50

From SK-I

SK-III 8B energy spectrum

Preliminary

T~4.0MeV

51

Consistent with no distortion

(Etotal=4.5-5.0MeV data not used in the oscillation analysis)

Borexino Result

7Be(0.862): 46±1.5 (stat.) (syst)cpd/100 tons

5 . 1 6 . 1 + −

Other components in the fit

Corresponding to an un-oscillated νe flux of (2.78±0.13)x109 cm−2s−1 By assuming the MSW-LMA solution the absolute 7Be solar neutrino flux measure is (4.84±0.24)×109 cm−2s−1 The ratio the measurement to the SSM prediction is fBe=0.97±0.09

Eps-Hep2011 Grenoble 22/7/2011 Gioacchino Ranucci INFN- Milano

Other components in the fit

85Kr in very good agreement with the correlated coincidence determination

Unprecedented better than 5% precision in low energy solar neutrino measurements

Adn= 0.007±0.073 sys. error negligible (day-night asymmetry)

Implications of the Borexino result Survival Probability : Pee=

07 . 51 . ±

No oscillation hypothesis excluded at 5 σ (expected from SSM 74±5.2 counts)

Error dominated by theoretical uncertainties

Eps-Hep2011 Grenoble 22/7/2011 Gioacchino Ranucci INFN- Milano

SSM 74±5.2 counts)

Tight constraints on pp and CNO (<1.7% 95% C.L. of solar luminosity) fluxes

003 . 010 .

013 . 1

+ −

=

pp

f

Accurate low energy validation of the MSW-LMA oscillation paradigm ν ν ν νee survival probability

Summary of the 8B results

Global neutrino

• scillation analysis of

all solar experiments

Identification of the so called LMA (large mixing angle) solution The addition of the reactor antineutrino data from KamLAND further sharpens the further sharpens the determination of the mass difference

Reactor anti Reactor anti-

• neutrino

neutrino experiments experiments

Liquid scintillator based detectors, gadolinium loaded to increase the neutron capture rate The technique is therefore the same discussed before Chooz set the most stringent limit, up to the beginning of this year of the the mixing angle θ13 , now T2K discussed before The main difference is the detection reaction: inverse beta decay Chooz and KamLAND are the more recent example of successful experiments of this kind An interesting round of new generation experiments is in preparation : Double Chooz, Daya Bay, Reno (lecture of Lothar Oberauer) Historical remark: the precursor of this class of experiment is the Reines- Cowan’s Savannah River experiment which marked the first ever detection of (anti) neutrinos

KamLAND Detector

Electronics Hut Steel Sphere of 8.5m radius Inner detector 1325 17” PMT’s 1km (2700 m.w.e) Overburden

2/6/2007 57

Water Cherenkov outer detector 225 20” PMT’s 1 kton liquid- scintillator 1325 17” PMT’s 554 20” PMT’s 34% coverage Buffer oil Transparent balloon of 6.5m radius

A picture of the interior before the fill

Detecting anti-ν: ν: ν: ν: inverse β

β β β-decay νe p e+

γ γ γ γ (0.511

(0.511 (0.511 (0.511 MeV) ) ) )

Evisible = Te + 2*0.511 MeV = = Tgeo-ν

ν ν ν – 0.78 MeV

PROMPT SIGNAL PROMPT SIGNAL

γ γ γ γ (0.511

(0.511 (0.511 (0.511 MeV)

Energy threshold of Tgeo-ν

ν ν ν = 1.8

1.8 1.8 1.8 MeV i.e. Evisible ~ 1 MeV

γ γ γ γ (0.511

(0.511 (0.511 (0.511 MeV) ) ) )

n p n γ γ γ γ (2.2

(2.2 (2.2 (2.2 MeV) ) ) ) DELAYED SIGNAL DELAYED SIGNAL

mean n-capture time on p 256 µ µ µ µs

Reactor antinu but also Geoneutrinos neutron thermalization

The coincidence technique makes the background requirements much less challenging !

KamLAND uses the entire Japanese nuclear power industry as a long-baseline source

KamLAND

Kashiwazaki

KamLAND

80% of flux from baselines 140-210 km

Takahama Ohi

Pure anti-ν flux Flux from reactor is well known Low energy anti-ν

Which is the method? Observe the spectral distortion of the energy of the detected prompt events (positron)

Prompt Energy Distribution

2/6/2007 62

• KamLAND saw an antineutrino energy spectral distortion at

99.6% significance neutrino oscillation !

The Background in this case is everything mimicking the delayed coincidence signal

• Accidentals: uncorrelated events due to the radioactivity in

the detector mimicking the inverse beta decay signature.

• 13C(α,n): 210Po (introduced as 222Rn) emits an α particle,

which reacts with naturally occurring 13C (~1.1% of C). There is a lot of Polonium in the scintillator

2/6/2007 63

1H(n,n)1H: the neutron collides with protons (prompt) and later captures

• n a proton (delayed).

12C(n,nγ)12C: the neutron excites a 12C producing a 4.4 MeV γ (prompt),

and later captures on a proton (delayed).

13C(α,nγ)16O: the 16O* de-excites with a 6 MeV γ (prompt), and the

neutron later captures on a proton (delayed).

• Neutron can be also cosmogenic or from fissions due to natural

radioactivity

Energy and position measurements, as well as calibration issues, are similar to the Borexino case explained before

Geo-neutrinos

Methods and associated issues for geo-antineutrino detection resemble those described in the reactor study Only two experiments have detected geo-neutrinos so far via the same inverse beta decay reaction shown before for reactor antineutrino detection KamLAND

Geo-neutrinos: anti-neutrinos from the Earth Geo-neutrinos: anti-neutrinos from the Earth

U, Th and 40K in the Earth release heat together with anti-neutrinos, in a well fixed ratio:

• Earth emits (mainly) antineutrinos

whereas Sun shines in neutrinos.

• A fraction of geo-neutrinos from U and Th (not from 40K) are above

threshold for inverse β on protons:

• Different components can be distinguished due to different energy

spectra: e. g. anti-ν with highest energy are from Uranium.

• Signal unit: 1 TNU = one event per 1032 free protons per year

p e n 1.8 MeV

+

ν + → + −

How does Earth’s interior work?

Open questions about natural radioactivity in the Earth Open questions about natural radioactivity in the Earth

1 - What is the radiogenic contribution to terrestrial heat production? 2 - How much U and Th in the crust and in the mantle? 3 – A global check of the standard geochemical model (BSE)?

The top 25 big questions facing science by 2030

4 - What is hidden in the Earth’s core? (geo-reactor, 40K, …)

• They escape freely and instantaneously from Earth’s

interior.

• They bring to Earth’s surface information about the

chemical composition of the whole planet.

Geo-neutrinos: a new probe of Earth's interior

But we focus here with the detection issues!

Select events via the inverse beta decay against

Generic Background mimicking delayed coincidences Specific background represented by the reactor neutrino signal neutrino signal With the help of a MC to disentangle the geo- and reactor- contributions

Geo-ν

reactors

reactors Sum NON oscillation

Theoretical spectra: input to MC

MC output:

includes detector response function Geo-ν

Geo-ν energy window Reactor energy window

USED IN THE UNBINNED MAXIMUM LIKELIHOOD FIT OF THE DATA

68.3 % 99.7% 68.3 % 99.7%

Example from Borexino

Background source events/(100 ton-year)

Cosmogenic 9Li and 8He 0.03 ± 0.02 Fast neutrons from in Water Tank (measured) < 0.01 Fast neutrons from in rock (MC) < 0.04 Non-identified muons 0.011 ± 0.001 Accidental coincidences 0.080 ± 0.001 Time correlated background < 0.026 Time correlated background < 0.026 (γ,n) reactions < 0.003 Spontaneous fission in PMTs 0.003 ± 0.0003 (α,n) reactions in the scintillator [210Po] 0.014 ± 0.001 (α,n) reactions in the buffer [210Po] < 0.061

TOTAL 0.14 ± 0.02

To be compared: 2.5 geo-ν ν ν ν/100 ton-year assuming BSE)

Conclusions

The neutrino detection technology has reached a mature stage where different techniques coexist to cope with the multiple experimental challenges posed by the different neutrino sources to be investigated In particular Cerenkov , Scintillator and Radiochemical methods have proved to be essential in the long quest towards the experimental assessment of neutrino oscillations Surely Scintillator and Cerenkov methodologies will continue to play a fundamental ole in the next research frontiers : from high energy cosmic neutrinos to sub-MeV solar neutrinos In this interesting future the achievement of ultra-low background level will continue to be a key factor, also in other rare process research field like neutrinoless double beta decay and dark matter