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SNO Liquid
Scintillator
Project
NOW 2004
17 September 2004
Mark Chen
Queen’s University &
The Canadian Institute for
Advanced Research
Introduction
• Fall 04 to Dec 06: SNO Phase III
– 3He proportional counter array now in place
• dedicated Neutral Current Detectors (NCD’s)
– nominal end date: 31 Dec 2006
• bring total uncertainty on 8B solar n NC signal
below 5%
– physics with heavy water will be complete
what should be done with the detector after?
Fill with Liquid Scintillator
• SNO plus liquid scintillator → physics
program
– pep and CNO solar neutrinos
– geo-neutrinos
– 240 km baseline reactor oscillation
confirmation
– supernova neutrinos
• working name: SNO+
Low Energy Solar Neutrinos
• test solar models: 7Be, pep,
CNO
• precision survival probability
measurement: pep
• observe rise in survival
probability at lower energies:
lower energy 8B, 7Be, pep
from Peña-Garay
Survival Probability Rise
SSM pep flux:
predicted to ±1-2%
allows precision test
Dm2 = 7.9 × 10−5 eV2
tan2q = 0.4
SNO CC/NC
pep n
Event Rates (Oscillated)
7Be
solar neutrinos
3000 pep/year/600 tons >0.8 MeV
using BPB2001
and best-fit LMA
3900 CNO/year/600 tons >0.8 MeV
11C
Cosmogenic Background
these plots from KamLAND proposal
muon rate in
KamLAND: 26,000 d−1
compared with
SNO: 70 d−1
Real KamLAND Backgrounds
external
pep window
pep Solar n Backgrounds
•
11C
cosmogenic production
– t1/2 = 20 min makes this difficult to veto at shallower depths
– positron decay guarantees >1 MeV energy deposited, right in the
pep n-e− recoil window
– but at SNO depths, muon rate is small enough to allow easy
tagging (or even tolerate this background without veto)
• CNO neutrinos are a “background”
– good energy resolution desired to see clear “recoil edge” for
monoenergetic pep n
– clearly interesting, for astrophysics, first observation of CNO n
• radiopurity requirements challenging
–
–
40K, 210Bi
(Rn daughter)
85Kr, 210Po (seen in KamLAND) not a problem since pep signal is
at higher energy than 7Be
– U, Th not a problem if can achieve KamLAND-level purity
More on pep Solar Neutrinos
from J. Bahcall and C. Peña-Garay
“Our global analyses show that a measurement of the
n-e scattering rate by pep solar neutrinos would yield
essentially equivalent information about neutrino
oscillation parameters and solar neutrino fluxes as a
measurement of the n-e scattering rate by pp solar
neutrinos.”
• which is to say that a pep solar neutrino experiment
would be an alternative to a pp solar neutrino
experiment, in some regards…
Antineutrino Geophysics
•
•
can we detect antineutrinos
from b- decay of U and Th in
the Earth’s mantle and crust?
knowing Earth’s total
radioactivity would be very
important for geophysics
–
–
–
•
understanding thermal history
of the Earth
thought to account for ~40%
total heat generation
dominant heat source driving
mantle convection
how much in the mantle and
the crust?
More on Geo-Neutrinos
• detecting geo-neutrinos from natural
radioactivity in the Earth (U, Th) helps to
determine the radiogenic portion of Earth’s
total heat flow
• by doing so, it also tests theories of
Earth’s origin based upon the “Bulk
Silicate Earth”…e.g. see Rothschild, Chen,
Calaprice, Geophys. Res. Lett., 25, 1083 (1998)
• e.g. see NOW 2004 talk by G. Fiorentini…
Geo-Neutrino Signal
terrestrial antineutrino event rates:
• Borexino: 10 events per year (280 tons of C9H12) / 29 events reactor
• KamLAND: 29 events per year (1000 tons CH2)
• Sudbury: 64 events per year (1000 tons CH2) / 87 events reactor
Rothschild, Chen, Calaprice
(1998)
above plot for Borexino…geo/reactor ratio
at Sudbury would be twice as high
KamLAND will soon make
first detection…
SNO+ Geo-Neutrinos
from G. Fiorentini
“SNO is considering move to liquid scintillator after
physics with heavy water is completed. With very low
reactor background, well in the middle of Canadian
shield (an “easy” geological situation) it will have have
excellent opportunities.”
• which is to say that fundamental models are tested by
experimental values…if those model calculations and
measurements (for Sudbury) have smaller uncertainties
(than for Kamioka), what we learn from the experimental
measurements (at Sudbury) has potentially greater value
Reactor Antineutrinos
• SNO+ can try to confirm reactor neutrino
oscillations
• move KamLAND’s spectral distortion to
higher energies by going to a longer
baseline
• this moves KamLAND spectral distortion
features away from the geo-neutrinos
– improves geo-neutrino detection
– spectral shape confirmation
Top Ten List
• table from
Suekane’s
NOON2003 talk
Location, Location, Location
Bruce
Bruce-SNO+
• 240 km baseline – places 2nd oscillation
maximum in the middle of the reactor neutrino
positron spectrum
• 51 events per year (no oscillation expectation)
from 6 reactors at full power 14 GWth
• there are 2 more reactors at Bruce that may be
restarted
• not a precision test, will not further constrain
oscillation parameters…just a confirmation, with
statistics like K2K (e.g. in 3 years, expectation of
~150 events, observation of ~100 events…)
KamLAND Spectral Distortion
T. Araki et al., hep-ex/0406035 (2004)
SNO+ Spectral Distortion
3 Measurements for Low Cost
• for relatively little cost, there is an
opportunity to use existing equipment (i.e.
most of the SNO detector) to enable new
measurements
• costs are:
– liquid scintillator procurement
– mechanics of new configuration
– fluid handling and safety systems
– scintillator purification
Supernova Neutrinos
• 1 kton organic liquid scintillator would
maintain excellent supernova neutrino
capability
–
–
–
–
–
ne + p
ne + 12C (CC)
ne + 12C (CC)
nx NC excitation of 12C (NC)
nx + p elastic scattering (NC)
[large rate]
[large rate]
see Beacom et al., PRD 66, 033001(2002)
SNOLAB LOI
• letter of interest submitted on 12 April 2004
• SNO+ option “study group”
M. Chen*, A. Hallin, C. Kraus, J.R. Leslie, J. Maneira,
R. MacLellan, A.B. McDonald, A. Wright Queen’s
M. Boulay Los Alamos
D. Hahn, M. Yeh Brookhaven
X. Dai Carleton
B. Cleveland, R. Ford SNOLAB
D. Hallman, C. Virtue Laurentian
R.G.H. Robertson U of Washington
• potential collaborators from outside SNO have indicated
some interest
• fully funded expansion of SNO underground site into an
international facility for underground experiments
–
–
–
–
double beta decay
dark matter
solar neutrinos
supernova neutrinos
• excavation expected to begin late 2004, completed by 2006
• space ready for experiments in 2007
Technical Aspects of R&D
• liquid scintillator cocktail design
– optimize optical properties (attenuation length, light yield, pulseshape discrimination, scattering)
– chemical compatibility with acrylic
– high density preferred (r = 1 g/cm3) to use with existing H2O
buffer outside the acrylic vessel
•
•
•
•
•
mechanical “hold-down” system
cover gas improvements (lower radon)
safety, fluid handling underground
scintillator purification
SNO detector state (surviving PMT’s, acrylic vessel
certification)
• calibrations and operations
Schedule
• SNO+ R&D: one year
–
–
–
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complete technical description
full cost estimates
completed feasibility studies
fully-developed science goals
• if above okay, full proposal(s) to be submitted
11/2005
• call for new collaborators in parallel with above
• when above approved, 2 years to first fill
(04/2008)
Double Beta Decay: SNO++
• SNO plus liquid scintillator plus double beta
isotopes: SNO++
• add bb isotopes to liquid scintillator
– dissolved Xe gas (2%)
– chemical loading (Nd, Se, Te)
– dispersion of nanoparticles (Nd2O3, TeO2)
• enormous quantities (high statistics) and low
backgrounds trade off for poor energy resolution
of liquid scintillator
Candidate Selection
2n bb Background
Test <mn> = 0.150 eV
• 0n: 1057 events per year with 1% Ndloaded liquid scintillator (natural Nd)
• S/B: 0n/2n (upper half peak) = 2.3
• crude illustration below:
statistical test of the shape to extract 0n and 2n components!
Summary
• R&D to develop SNO+ underway
• staged approach envisioned:
– deployment of pure scintillator for antineutrinos
– next stage: go for purification to try for low energy
solar neutrinos
– next stage: deploy double beta (e.g. nanoparticles),
would jump to this stage ASAP
– long-term program provides steady and “early”
science output for SNOLAB
• new collaborators are welcome