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TRANSCRIPT
BELLINI
BOREXINO
Breakthrough in Low Energy Neutrinos
R. S. Raghavan
Virginia Tech Milano, March 23 2009
SSM Prediction
Directly measured so far
Directly
measuredThe New Frontier
SSM Prediction
Directly measured so far
Directly
measuredThe New Frontier
SSM: 4 p 4He
I: p+pd+ e+ + νe (0-0.42 MeV)
II. 7Be 7Li + νe (o.862 MeV)
III 8B 2α + e- + νe (0-15 MeV)
Direct Measurements--only 8B
Kamiokande, SK, SNO
1988—BOREX –NUEX in 11B
1991-Proposal for 7Be
BOREXINO
1976 Proposal for pp
LENS
Borexino LENS
SNO
SK
Radioactivity WAll
BEFORE BOREXINO: After Ga and Kam I B8 nus found: but no Be if SSM holds~~Paradox —What is the matter with Be in Sun?
Radiochemical NOT enough—
Need LO Energy Spectroscopy!
SN in the 1990’s
More intrinsic Motivation for 7Be
•Backbone of the pp chain
•Text book case for nu oscillations
But Be is well below the Radioactivity Wall:
Urgent problem to breakthrough the Wall
Basic Reaction: Nu-electron scattering
νx + e νx + e
Flavor mixed Electron in
Scintillator Recoil Electron
SIGNAL !
BUT: Spectral Signature: Distinct Compton Edge
for monochromatic neutrinos from 7 Be and pep
( see arrows in Fig). No edge for continua.
Background at low energies due to all
sources must be suppressed well below the
solar signal ! Major assests for Be
, Signal Rate, Monoenergetic Nuspectral
Signature—Seasonal Variation from Orbital
eccentricity
Ultrapurity and ultra-low contamination
- U,Th < 10-16 g/g; 14C< 10-18 g/g
Laboratory scale purity data not enough
CTF –5 ton scle
Signal carries NO TAG !!
Detection Method
1988 --Start Borex Project
Start Chemistry of Ultrapurity in Bell Labs
May 1988 First Borex collaboration meeting at Bell Lab-with Blessing of
Arno Penzias
1989—Discovery of ultra-low solubility (~10-17 g/g) of U, Th
in organic liquids ----Antonio di Bari & RSR
Ultrapurification of 100 L –10-15 g/g—Volterra Expt –Radiocarbon—106 depletion needed—only possibility petrochemicals
-Demonstrate that 14C in Natural Gas petrochemical 14C/12C < 10-18
First EvidenceAccc. Mass Spec measurement—Bell Labs+ Univ. Toronto
3 MeV radioacivity WALL broken through for LE neutrinos--
Go for 7Be instead of 8B ( ―test‖ phase of BOREX Borexino! )
Need only a ―small‖ detector~100 tons BOREXINO—little Borex
-1992-- Sufficient experimental data for proposal for CTF (Counting Test Facility
as pre-test for BOREXINO
BOREXBOREXINO
Change of Focus: 8B 7Be
CTF Major Goals:
Dress Rehearsal of OBSERVABLE ultra-purity & low energy
spectroscopyin ton-scale scintillation detector —never done before
Intrinsic sources — Contamination of migrating sources
Bgd from external sources, PMT’s, detector materials, rock, neutrons-----
Spectroscopy at low and v. low energies (--20 keV )
Stability of operations—energy, spatial resolution
Removal of Alphas from actinides even at high purity
via pulse shape spectroscopy
alphas
electrons
Polonium, Krypton—
The “last” obstacles exposed by the CTF
not foreseen In earlier design considerations
Po: Reduced by strict avoidance of
Radon—extreme airtightness
meticulous Cleaning of Rn daughters
—Still not completely solved but
Only minimized enough for seeing Be because
of superior energy resolution and alpha-beta
discrimination
Kr: airtightness <10-8 bar cm3 s-1 Use Very Low Argon Krypton Nitrogen --LAKN—
for all purification, liquid handling etc.
Ar: 0.01 ppm in Nitrogen
Kr: 0.02 ppt in Nitrogen
High Purity Nitrogen:
222Rn < 0.3 μBq/m3
Taken From:
Solar Neutrinos—
From puzzle to
Paradox
Review by
RSR Science ’95
Discussing
―where have all Be
in the sun GONE?
(Became folk song)
Burning question
After Gallex resul:
Imperative to directly
measure Be7
Picture of CTF detector
CTF RESULTS—Major Success
U/Th ~10--16 g/g
C14 <2x10-18 /C12
Energy Resolution Satisfactory and Expected by Monte Carlo
Spectroscopy down to 20 keV—C14 beta (E<156 keV) spectral shape
Vast experience in radon control, contamination pathways …..
CTF taught us every thing and more
Discovered unforeseen sources of contaminants
Po: Reduced by strict avoidance of
Radon—extreme airtightnessmeticulous cleaning of Rn daughters
—Still not completely solved but
Only minimized enough for seeing Be because
of superior energy resolution and alpha-beta discrimination
Kr: airtightness <10-8 bar cm3 s-1
Use Very Low Argon Krypton Nitrogen --LAKN—
for all purification, liquid handling etc.
Then the roof fell in.
Small release of scintillator liquid into outside stream
Environmental emergency
Experiment quarantined for 3 years for installation of
Safety measures by National Agencies and Tunnel
Authorities
Collaboratioin showed extraordinary strength and
Continued together ---
On to Borexino experiment
Science Problem Recognized ultrapurity
Practical Detector Next ultra- Engineering
Magical Mystery Tour of Borexino Engineering
The starting point:
no cut spectrum
14C dominates
below 200 KeV
210Po NOT in
eq. with 210Pb
Mainly external
γs and cosmos
After muon cut
7Be ν Rate:
192 days
Rate consistent with Standard Solar
and MSW-LMA Nu oscillation models
Expect to reduce the systematic
error (Mainly from determination
of Fiducial Mass) via Source
Calibration of Detector (VT led)
49±3stat±4syst counts/(d·100 t).
Solar Neutrinos
Improve statistical precision (and reduce systematics
Aim5% precision of flux 3% hopefully finally
Clean up C-11 Cosmogenic events by neutron tagging
Wait for Po decay (2 years 6 half lives)
New Data on pep neutrinos (variant of pp neutrinos)
CNO in the sun—Discovery experiment on Chemical
composition of light elements in the sun
Solve current mystery
Get Be-7 nu flux to 5%3%?
CNO flux to ~10-15%
pep nu flux to 15%?
Recent analysis shows that the sun
may have 30-50% LESS C, N, O….
Opacities changed, Fluxes changed
significantly
Above all agreement with
helioseismology destroyed.
Crisis===Solution—Measure CNO
flux
Borexino, LENS
The New Solar Neutrino Problem!—
C, N, O in the Sun?
1. Non-standard Weak Interactions
Look at the Recoil Electron Profile
The shape as used so far is the result of averaging over
angular distributions of CC and NC interactions
i.e νe + e (CC) and νμ,τ +e (NC).
The sun emits substantial flux of τau-neutrinos!!
Poser: If the τau-coupling constant is different from standard V-A, then the
angular distribution is different, thus the averaged
recoil electron profile is changed—can probe this in Borexino data
if the Po and Kr problems are cleaned up
Be Be
Analysis
Po Po removed
by α-β disc.
Observed recoil profile
Standard universal V-A profile
Deviation of recoil profile via
Nonstandard tau couplings
Berezhiani, RSR, Rossi
hep-ph/ 0111138
standard Shape can be obtained
from artificial nu source (planned)
which emits ONLY νe
β α
P=0.5 P=0.5 P=0.5
Std Model NSI єeR : NSI єτR
-0.3 0.6 -0.3 0.4
3. Geophysics Antineutrinos from the earth—very low fluxes can be detected
Borexino site has little antineutrino bgd compared to KamLAND
Kamland and Borexino located geophysically different sites
4. Neutrinoless Double beta decay
Noble gases have high solubility in organic liquids
Xe, an excellent candidate for 0nubb studies has 2 wt% in PC
Can put 2tons of Xe in BOREXINO
Ton-scale 0nubb search for the first time
-Detector is ready –needs only to optimize existing puming systems
Energy of onubb signal at 2.5 MeV well outside SN spectrum
Already know background rates at these energies are very low
In any case target in-target out measurements allow no uncertainty of Bgd
Scintillation performance Unaffected—High resolution achieved (500 pe/MeV)
Expected nu mass sensitivity <0.1 meV with enriched Xe
Competitive—FAST –can be down quicker than any other planned expt
MOST EXCITING PROSPECT WITH BOREXINO
Major Background in 0nuBB window
2nuBB bgd
Cosmogenics—10C
Ext Bgd
Int Bgd
dBB signal window
Ultimately we hope to achieve
dBB sensitivity
<mν> ~ 100 meV