Transcript Slide

Real-time Solar neutrino detection
with Borexino
Oleg Smirnov
(JINR, Dubna)
on behalf of Borexino collaboration
5-th International Workshop on Low energy neutrino physics
19 - 21 October 2009, Reims, France
- Borexino goal, 5%
Standard Solar
Model predictions.
measuring neutrino
fluxes one can
discriminate
between different
models.
50 events/d/100t expected (νe and vμ elastic scattering on e-)
Low energy->no Cherenkov light->No directionality,
no other tags-> extremely pure scintillator is needed
BOREXINO Collaboration
Genova
Milano
Princeton University
APC Paris
Perugia
Virginia Tech. University
University of
Massachusetts
Dubna JINR
(Russia)
Kurchatov
Institute
(Russia)
Jagiellonian U.
Cracow
(Poland)
Heidelberg
(Germany)
Munich
(Germany)
Reducing external background with “graded shielding"
Cosmic muons
(LNGS underground labs:
rocks, 3200 m.w.e.)
Increasing radiopurity of materials
Neutrons and external gammas
(ultrapure water layer, 2.15 m, 2400 tones)
γ-s from construction materials
(PC buffer, 700 tones, 2.5 m)
γ-s from construction materials
(outer layer of scintillator, 1.25 m or 200 tones)
Software-defined active volume of scintillator
(fiducial volume, 3m, 100 tones)
Position
reconstruction
needed
BOREXINO
•278 t of liquid organic
scintillator PC + PPO
(1.5 g/l)
• (ν,e)-scattering with
200 keV threshold
•Outer muon detector
13.7m
18m
LS radiopurity in Borexino:
results of 15 yrs work
Background
Typical abundance
(source)
Borexino
goals
Borexino
measured
14C
10-12 (cosmogenic)
10-18
 2·10-18
238U
[g/g]
(by 214Bi-214Po)
2·10-5 (dust)
10-16
(1 μBq / t)
(1.6±0.1)·10-17
232Th
[g/g]
(by 212Bi-212Po)
2·10-5 (dust)
10-16
(5±1)· 10-18
222Rn (238U)
[g/g]
(by 214Bi-214Po)
100 atoms/cm3 (air)
(emanation from materials)
10-16
10-17
( 1 cpd/100 ton)
40K
2·10-6 (dust)
10-18
<3·10-18 (90%)
(surface contamination)
10-2
70 (initial, T1/2=134 d;
/ 12C [g/g]
[g/g]
210Po[cpd
/ t]
not in equilibrium with parent
210Bi);
<5 after 2 yr
85Kr
[cpd / 100 t]
1 Bq/m3 (air)
1
28±7 cpd/100t
39Ar
[cpd / 100 t]
17 mBq/m3(air)
1
<<85Kr
Borexino technical data
1.Light yield: >500 p.e./MeV/2000 PMTs (31% of 4π);
2.Mass: full 278 t; FV (R<3 m && |Z|<1.67 m) mass 78.5 tones (used in 7Be analysis);
3.Energy resolution (1σ) within the FV: ~5% @ 1 MeV;
4.Practical threshold on the electrons recoil is 180 keV (corresponds to 380 keV
neutrino);
5.Muons registering efficiency close to 100%;
6.Triggers rate: 11 cps (mainly
14C,
2.7 ± 0.6 x 10-18 g/g
7.Spatial resolution 14 cm @ 1 MeV
14C/12C
)
Active shielding effectively suppress external
gamma background
210Po
14C
(not in equilibrium with 210Pb)
Kr+Be

11C
No s
R<3.0 m
(100 t)
214Bi-214Po
8
Spectral components in the Borexino
spectrum (model)
210Po
& 210Bi
Energy scale
•
Calibrated using “internal uniformly distributed sources” taking into account the CTF
calibration experience: 14C (β-,E0=156 keV), 11C (β+ decay), 210Po (α, Eα=5.3 MeV)
•
Monoenergetic line of 210Po has been used to fit the detector’s response width and shape
(non-gaussian shape is used)
•
Careful modeling of the Birks’ ionization quenching at low energies (worked out with the
CTF data); kB~0.017 cm/MeV
•
Two quasi-independent energy variables are used: the total number of registered p.e. (Q)
and the number of triggered PMTs (Npm)
A first calibration campaign with on axis
and off axis radioactive sources has been
performed (Oct 08 on axis, Jan-Feb 09 off
axis).
115 points inside the sphere: ,γ,α,n sources.
The model used is in a good agreement with
measurements.
Also the position reconstruction has been tuned
(source is localized within 2 cm precision
through red laser light and CCD camera).
E, keV
RR(Q)
%
RR(Npm)
%
250
11.1
9.8
400 (210Po) 8.8
7.8
660 (7Be)
7.0
6.2
1000
5.8
5.2
Calibration campaigns 2008-2009
A first calibration campaign with on axis and
off axis sources has been performed (Oct 08
on axis, Jan-Feb09 off axis)
 accurate position reconstruction
 precise energy calibration
 detector response vs scintillation position
Laser ball: check of PMT allignment
100 Bq 14C+222Rn source diluted in PC:
115 points inside the sphere:
 : 14C, 222Rn
: 222Rn
g : 8 sources from 122 keV to 1.4
MeV (54Mn, 85Sr, 222Rn in air)
AmBe source (protons recoil study) :
Source localization within 2 cm
through red laser light and CCD camera;
Accurate handling and manipulation
of the source and of the materials
inserted in the scintillator;
Model used to fit the experimental data
(7Be analysis)
Normalization of main backround components are free:
14C
(with fixed form-factor α);
85Kr
free; in principle can be bounded (correlated with 7Be);
210Po;
(in another approach is removed using α/β statistical subtraction)
210Bi; 11C;
214Pb
fixed at the number of registered events of
222Rn
(anyway negligible).
Other background sources (40K; isotopes from decay chains of
to give negligible contributions.
238U
and 232Th in secular equilibrium) are found
Electrons recoil spectra for solar neutrino are calculated assuming MSW(LMA) scenario:
7Be;
CNO fixed @ SSM+MSW(LMA) (strongly correlated with free
210Bi
component);
pp and other solar neutrino fluxes are fixed @ SSM+MSW(LMA);
Energy scale parameters:
Light yield + 1 energy resolution parameter vT+ 210Po peak position;
Two other parameters pt=0.13 and gc=0.105 (found using MC simulation) for Npm variable are fixed;
For Q variable calibration parameter c is free; parameter feq is fixed (calculated) for both variables;
Birks’ parameter kB fixed at the value found with CTF
“Direct Measurement of the 7Be Solar Neutrino Flux with
192 Days of Borexino Data” PRL 101, 091302 (2008).
49±3stat±4syst cpd/100 t
Main source of systematic uncertainty in this
measurent is error in FV definition
(significantly reduced after position
reconstruction code tuning using calibration
data).
Fit to the spectrum with -subtraction
gives consistent results
210Po
and α/β - discrimination
Optimal Gatti filter
E. Gatti, F. De Martini, A new linear method
of discrimination between elementary
particles in scintillation counters, in: Nuclear
Electronics, vol. 2, IAEA, Wien, 1962, pp.
265–276.
H.O. Back et al. / NIM A 584 (2008) 98–113
Pulse-shape discrimination with the Counting Test Facility
 i  i
Pi 
 i  i
G   Pi S i
Works also for p(n)/ discrimination. Fine tuning in progress
Comparison with theory, 7Be
• Borexino exp. result:
49 ± 3(stat) ± 4 (syst) cpd/ 100t
• High metallicity Solar model MSW/LMA:
48 ± 4 cpd / 100t
• Low metallicity Solar model , MSW/LMA
44 ± 4 cpd / 100t
• High metallicity Solar model,
nonoscillating neutrino (inconsistent with
measurement at the 4 σ C.L.)
74 ± 4 cpd / 100t
The survival probability of the 0.862 MeV 7Be
neutrinos (assuming the BS07(GS98) SSM) is
0.56±0.10.
Constraints on pp and CNO neutrino fluxes
with 192 days of Borexino data
7Be
vs CNO
[Ga+Cl+8B]
f pp  1.0400..13
19SSM (1 )
f pp  1.0400..13
19SSM (1 )
f CNO  6.3 SSM (90%)  lum(CNO)  5.4%
pp vs CNO
f pp 1.00500..008
020 (1 )
f CNO  3.80 (90% C.L.)
with luminosity constraint
f pp 1.00500..008
020 (1 )
f CNO  3.80 (90% C.L.) =>Lum(CNO)<3.3%
Neutrino magnetic moment
From the theoretical point of view, there is no magnetic moment for Dirac massless
neutrino, as well as for Majorana neutrino, massive or massless. Massive Dirac
neutrino should have small m.m.:
  3.2  10
19
 m 

 В
 1эВ 
m.m. can be searched for by studying the deviations
from the weak shape
“flat”
2G m
 d 

  F e

 dT W
2
1/T behaviour


T
 g L2  g R2 1 

 E
2

meT 
  g L g R 2 
E 


2
1 
 d 
2 em  1








2 
me  T E 
 dT  EM
Limit on effective solar neutrino
magnetic moment
•
•
•
•
•
with 192 days of live-time statistics the 90% c.l. limit is:
µeff<5.4·10-11 µB
stronger limits with the same statistics can be obtained bounding some spectral
contributions (i.e. 85Kr);
The limit is model-independent, defined only by the shape of the spectra, also
no systematics is attributed to the uncertainty of the FV.
The best up-to-date existing limit comes from the measurements with high
purity 1.5 kg Ge detector at Kalinin Nuclear Power Plant, GEMMA experiment
(arXiv:0906.1926):
µ<3.2·10-11 µB
For flavour components one can write [D.Montanino et al. PRD 77, 093011 (2008)]:
(eff2 ) MSW  Pee e2  (1  Pee )(cos 2  23 2  sin 2  232 )
where Pee=0.552±0.016 is the survival probability at Earth for
electronic neutrino at E=0.863 MeV, sin2θ23=0.5+0.07-0.06
New limits on μ and τ neutrino
magnetic moments
Applying constraints on μνe of Gemma experiment:
  12 10 B
11
  12.5 10 B
11
•
•
•
•

Present limits on the neutrino magnetic moments are:
μe < 3.2×10-11 μB by GEMMA (elastic scattering)
μμ < 68×10-11 μB by LSND (elastic scattering)
μτ < 39000×10-11 μB by DONUT (elastic scattering)
8B
neutrino flux meaurement
Energy spectrum after statistical 208Tl subtraction.
Measurement of the solar 8B neutrino flux
with 246 live days of Borexino and
observation of the MSW vacuum-matter
transition
by Borexino coll. arXiv:astro/ph 0808.2868v1
[see also Nucl.Phys.Proc.Suppl. 188:127-129,
2009]
0.26±0.04stat±0.02 syst cpd/100 t
The 8B mean electron neutrino survival
probability, assuming the BS07(GS98)
SSM, is 0.35±0.10 at the effective
energy of 8.6 MeV in agreement with
water Cherenkov detectors.
The ratio between the measured
survival probabilities for 7Be and 8B
neutrinos is 1.60±0.33, 1.8σ different
from 1.
Borexino is the first LS experiment
observing 8B neutrinos.
Update of 8B analysis
•
•
•
•
•
•
•
•
Principal sources of systematic error on measured 8B flux: energy threshold, fiducial
volume, detector stability
Statistical error remains the limit: 250 days (stat error 17%) -> 500 days analyzed (12%) ->
600 days collected (11%).
Preliminary analysis of 500 days data has been performed, the results are in agreement
with published ones.
Improved understanding of energy scale: energy calibration with 12 sources with energy
from 120 keV up to 9.3 MeV; PRELIMINARY: uncertainty in energy threshold <1%. Monte
Carlo code tuned to take into account non- linearities of the energy scale (ionization
quenching, electronics);
Improved position reconstruction (calibrated with sources). PRELIMINARY: error on FV
could be as low as 3% (FV: R<3 m @ E>2.8 MeV red).
Currently finalizing impact of stability and overall systematic error.
The study in progress: tagging of 208Tl events in coincedence with 212Bi-208Po (b.r. 36%).
11Be contribution in E>2.8 MeV (Q=11.5 MeV, τ=19.9 s): Hagner et al measurements
N(11Be)<0.02 cpd (90%), scaling the value measured by KamLAND N(11Be)=0.02±0.004 cpd
in Borexino. Preliminary analysis shows no significant presence of 11Be in Borexino (about
10 times lower than scaled KamLAND value), while other important cosmogenic
backgrounds are in agreement with KamLAND data.
Borexino provided measurement of
electron neutrino survival probability
in two different energy ranges
Time variations of 7Be neutrino flux
±3.5% variations due to the seasonal variation of Earth-Sun
distance: need more statistics, feasibility of measurement
depends on stability of backgrounds and strategy chosen for
(possible) repurification. For the moment no statistically
significant measurement is available.
Preliminary “negative” result on day/night assimetry (see
G.Testera’s talk at Neutrino Telescopes in March 2009) with
422 days statistics (213 “nights” + 209 “days”) is in agreement
with MSW/LMA predictions:
ND
ADN 
 0.02  0.04
ND
Solar CNO- neutrino cycle:
a clue to the chemical composition of the Sun
dominates in massive stars
“bottle-neck” N(p,γ) reaction, slower
than expected (LUNA result)
A direct test of the heavily debated solar C, N and
O abundances would come from measuring the
CNO neutrinos.
The feasibility of the CNO neutrino
detection in Borexino is under study
(depends on the possibility of background reduction)
Spectral components in the experimental
spectrum (model)
11C
background suppression
12C>11Cn+
11B+e++
e
n capture
g(2.2 MeV)
Muon track
Spherical cut
around 2.2 gamma
to reject 11C event
Cylindrical cut
Around muon-track
Neutron
production
Borexino collaboration: “CNO and pep neutrino spectroscopy in Borexino: Measurement of the deep-underground
production of cosmogenic 11C in an organic liquid scintillator” PHYSICAL REVIEW C 74, 045805 (2006)
Detecting antineutrino
• Inverse beta-decay [high c.s. ~10-42 cm2]

 e  p  e  n
  250s
n  p  d  g (2.2MeV )
• Evisible = E – 0.78MeV [E>1.MeV]
Reactor antineutrino
in Borexino:
~15 ev/yr are expected for 100% reactors
duty cycle.
15 ev/yr
207 Nucl. power plants in 17
countries.
13 Plants give 40% of total
signal.
3 most powerful power plants in
France give 13% of the total
signal.
28 April 2009 Milan
Geoneutrinos study is promising due to the location of
the Borexino far away from the European reactors.
Emax(U) = 3.26 MeV
Emax(Th) = 2.25 MeV
Emax(K) = 1.3 MeV
Energy “window”:
1.81-3.26 MeV
Expected 6 ev/yr in the geoneutrino region.
28 April 2009 Milan
Earth heat flow
Φ≈ 60 mW/m2
Full flux:
HE = (30- 44)ТW
44±1 TW (Pollack 93)
31 ±1 TW (Hofmeister & Criss 04)
Cosmochemistry (meteorites) estimates of
radiogenic heat give from 19 to 31 ТW : only limiting
values are consistent with heat balance, existing
estimates shows the lack of heat up to 25 TW
•Radiogenic heat (HR) is connected with the antineutrino number (Lν):
HR
= 9.5 M(U) + 2.7 M(Th) + 3.6 M(40K)
L
= 7.4 M(U) + 1.6 M(Th) + 27 M(40K)
• H [TW] ;
M [1017kg] ; L[1024 1 /с]
•M(U), M(Th) and M(K)
Expected antineutrino signal
for 1 yr of the data taking
no FV cut (278 t), detection efficiency about 85%
Geo 232Th
Geo 238U
Reactor
Total
Random
1-1.5
1.2
2.1
0.5
3.8
0.3
1.5-2.6
0
2.3
3.3
5.6
0.2
2.6-10
0
0
8.5
8.5
0.0
For reactor neutrino 0.8 duty cycle has been used.
13C(α,n)16O background is negligible.
Other (from random) backround sources are muon-induced -n decaying isotopes
(8He+9Li) and fast neutrons induced by muons missed by MVS are effectively
removed applying 2 seconds cut after each muon crossing the LS, the introduced
dead time is about 11%
Borexino potential on supernovae neutrinos
Detection channel
Expected number of
events in 300 t LS for
standard SN @ 10kpc
ES
(E > 0.25 MeV)
5
Electron antineutrinos
(E > 1.8 MeV)
78
-p ES
(E > 0.25 MeV)
52
12C(,)12C*
18
(Eg = 15.1 MeV)
12C(anti-,e+)12B
3
(Eanti- > 14.3 MeV)
12C(,e-)12N
(E > 17.3 MeV)
9
Borexino has
entered
SNEWS
(Super Nova
Early Warning
System)
Summary/What’s next?

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
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



Borexino operates at purity levels never achieved before, it demonstrated the feasibility of the
neutrino flux measurement in sub-MeV region, under the natural radioactivity threshold (4.2
MeV);
Solar 7Be- flux has been measured with 10% accuracy;
a first measurement of 8B- in LS with threshold below 5 MeV (2.8 MeV);
Borexino results are compatible with MSW/LMA;
strong limit on neutrino effective magnetic moment is obtained;
extremely high sensitivity to electron antineutrino has been experimentally confirmed, waiting for
more statistics.
Further calibration and reduction of the error on the 7Be flux down to 5% (further improvements if
constraining 85Kr, in this case also the limits on the effective magnetic moment will be
improved);
Seasonal variations of the neutrino fluxes (detector stability, more statistics); other time
variations
More precise measurement of the oscillation probability in the transition region (either due to the
higher statistics or due to increase of the FV);
 The CNO and pep-neutrino fluxes measurement (requires cosmogenic

11C
tagging);
The feasibility of the pp-neutrino flux measurement is under study (better understanding of the
detector at low energies and the precise spectral shape of 14C is needed);
 Antineutrino studies: geo, reactor, supernova.