THE NEUTRINO PHYSICS AND ASTROPHYSICS

Methods and problems in low energy neutrino

experiments (solar, reactors, geo-)

II

G. Ranucci

ISAPP 2011ISAPP 2011ISAPP 2011ISAPP 2011

International School on International School on AstroparticleAstroparticle physicsphysics

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 scintillatorscintillator detectordetector

ChoozChooz (reactor neutrino detector)(reactor neutrino detector)

KamLANDKamLAND (reactor neutrino detector)(reactor neutrino detector)

Planned: SNO+ and LENSPlanned: SNO+ and LENS

BorexinoBorexino

A real time calorimetric

scintillation detector for low

energy solar neutrinos

installed at the Gran Sassoinstalled 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 backgroundlow 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

outer) 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)

DetailDetail ofof the the southsouth endend--capcap ofof the vessel and the vessel and ofof the last the last mountedmountedPMT’s on the 3 m PMT’s on the 3 m doordoor ofof the the spheresphere

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 requirementsDetector and plants materialsLow intrinsic radioactivityLow radon emanation Chemical compatibility with PC

Pipes, vessels and pipesElectropolishedCleaned with filtered detergents

(Detergent-8, EDTA)Pickled and passivated with acidsRinsing with ultrapure water (class

Thorrn-EMI photomultipliersLow 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 volumemu-metal shilding for the earth

magnetic field384 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 (class20 – 50 MIL STD 1246 )

Leak tightnessLeak rate < 10-8 atm cc /sNitrogen blanketing on critical

elements like pumps, valves, bigflanges

Double seal metal gasketsNylon vessels

Good chemical and mechanicalstrength (small buoyancy)

Low radioactivity (< 1 count/day/100tons)

Contruction in low 222Rn cleanroom

High purity nitrogen storageClean rooms

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

Nylon vessels

Requirements:Chemical resistance to PC,PPO, DMP,

waterMechanical strength (20MPa – 5°∆T)Optical transparency (350-450 nm)Low intrinsic radioactivity (U, Th, K)Clean fabrication (<3 mg dust)Low permeability ti RnLeak tightness

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

Solutions and results:Sniamid Nylon-6 film125µm thick filmIndex of refract. = 1.53 with >90%

trasmittanceU, Th less than 2 pptUmidification 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 mScattering 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 designTruncated 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 15 th, 2007: DAQ starts

May 2007End 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 BorexinoNeutrino Detection in Borexino

Detection through the scattering reaction (as in

Superkamiokande and in SNO-third method)

ee +→+ ννoff 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

ev

ev

Long baseline from European reactors

Investigation of from the Sun

�Other components of the solar spectrum : pep, CNO, pp

ev

ev

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 keV8% at 400 keV5% 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

The fit is compatible with the expectedr2-like shape with R=4.245m. The σσσσ of the position reconstruction algorithm is found to be 35 cm.

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 2R x y z= + + 2 2cR x y= +

FV

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

γ from PMTs that penetrate the buffer

α/β discrimination

αααα particles

Small deformation due to average

SSS light reflectivity

ββββ 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/βα/βα/βα/β Gatti parameter α/βα/βα/βα/β Gatti parameter

Any instrument must be calibrated: Calibration campaign with sources

Am-Be source

LNGS 13/4/2011Gianpaolo 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


Milano

@ Spatial reconstruction

± 10-12 cm from 200 keV to few MeV

Calibration is one of

the ingredient for a

good measure, the

others are a low and

under control

background and a

suitable model of the

detector behavior

(Monte Carlo)


Milano31

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, 232Th is measured with the delayed concidence:

212Bi 212Po 208Pbβ β β β αααα

ττττ = 432.8 ns

2.25 MeV ~800 KeV eq.

Specs: 232Th: 1. 10-16 g/g

0.035 cpd/ton

212Bi-212Po

Time (ns)

ττττ=423±42 ns

Events are mainly in the south vessel surface (probably particulate)

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

2 2 2R x y z= + + 2 2cR x y= +

Only few

bulk candidates

Events are mainly in the south vessel surface (probably particulate)

z (m

)

Background: 238U contentAssuming secular equilibrium, 238U is measured with the delayed concidence:

214Bi 214Po 210Pbβ β β β αααα

ττττ = 236 µµµµs

3.2 MeV ~700 KeV eq.

214Bi-214Po

τ(exp)=240±8µs

µs

NO-VE April 15-18, 2008

µs

Background: 238U content214BiPo events behavior during time:

June 2007Setp - Oct 2007

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 2cR x y= +

2 2cR x y= +

214BiPo content in the FV

< 2 cpd/100 tons238U: = (1.6±0.1) x10-17 g/g

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

Background: 210PoNOTES• 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

• 210Bino 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 )

85Krβ 85Rb

687 keV

τ = 10.76 y - BR: 99.56%

85Rb85Kr 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 µµ 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)

11C7Be14C

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

Eps-Hep2011 Grenoble

22/7/2011Gioacchino 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

ΦΦΦΦNC = 5.140 +4.0 -3.8 %(x106cm-2s-1)

8B SNO Flux Result

(x106cm-2s-1)

8B Flux Result

ΦΦΦΦNC = 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)

+−≡

DN

DNA

)(2SNO Day-Night Asymmetries (I)

ACC and ANC are correlated (ρ = -0.532)

In standard neutrino oscillations, 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.(013.0)stat.(031.0056.02/)(

)(±±−=

Φ+ΦΦ−Φ

=NightDay

NightDayDNA

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 tons5.16.1

+−

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= 07.051.0 ±No oscillation

hypothesis

excluded at 5 σ

(expected from

SSM 74±5.2 counts)

Error dominated by

theoretical uncertainties

Eps-Hep2011 Grenoble

22/7/2011Gioacchino Ranucci INFN- Milano

SSM 74±5.2 counts)

Tight constraints on

pp

and CNO (<1.7% 95%

C.L. of solar

luminosity) fluxes

003.0010.0013.1 +

−=ppf

Accurate low energy validation of the MSW-LMA oscillation

paradigm

ννννee survival probability

Summary of the 8B results

Global neutrino

oscillation 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 antiReactor anti--

neutrino neutrino

experimentsexperiments

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

kindAn 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

DetectorElectronics 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 ββββ-decay

νep

e+

γγγγ (0.511 (0.511 (0.511 (0.511 MeV))))

Evisible = Te + 2*0.511 MeV =

= Tgeo-νννν – 0.78 MeV

PROMPT SIGNALPROMPT SIGNALγγγγ (0.511 (0.511 (0.511 (0.511 MeV)

Energy threshold of T geo-νννν = 1.8 1.8 1.8 1.8 MeV i.e. Evisible ~ 1 MeV

γγγγ (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 SIGNALDELAYED SIGNAL

mean n-capture time on p

256 µµµµsReactor antinu

but also

Geoneutrinos neutron thermalization

The coincidence technique makes the background requirements

much less challenging !

KamLAND usesthe entire Japanese

nuclear powerindustry as a

long-baseline source

KamLAND

Kashiwazaki

KamLAND

80% of flux frombaselines 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 on 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 detectionKamLAND

Geo-neutrinos: anti-neutrinos from the EarthGeo-neutrinos: anti-neutrinos from the EarthU, Th and 40K in the Earth release heat together with anti-neut rinos,

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 EarthOpen questions about natural radioactivity in the Earth1 - 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 signalneutrino signal

With the help of a MC to disentangle the geo- and

reactor- contributions

Geo-ν

reactorsreactors

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 µ in Water Tank (measured) < 0.01

Fast neutrons from µ in rock (MC) < 0.04

Non-identified muons 0.011 ± 0.001

Accidental coincidences 0.080 ± 0.001

Time correlated background < 0.026Time 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-νννν/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

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