THE NEUTRINO PHYSICS AND ASTROPHYSICS
Transcript of 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
LNGS 13/4/2011Gianpaolo 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
others are a low and
under control
background and a
suitable model of the
detector behavior
(Monte Carlo)
LNGS 13/4/2011Gianpaolo Bellini Universita' e INFN-
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