Sulak Festschrift Oct 21, 2005 1
Birth of the Large Scale Imaging Water Cherenkov Detector
Bruce CortezSulak FestschriftBoston University
Oct 22, 2005
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Agenda and Sulak Timeline
Jan ‘78 Jan ‘79 Jan ‘80 Jan ‘81 Jan ’82 Jan ‘83
GradStudents
J. StraitW. KozaneckM.Levi
B. CortezG.W. Foster
Location: Harvard Michigan
IMB Collab. Proposal Construction Data
Focus of this talk
S. SeidelD. Casper
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The Beginning September 1978
Larry’s mission A. Salaam’s statement that proton decay in the most important
experiment in physics Grand Unified Theories were now predicting lifetimes of < 1031 years.
Key characteristics Large (lifetimes up to 1033 years) Underground for background rejection Sensitive to large numbers of decay modes
Early October Internal memo on proposed Proton Decay detector
Scale up liquid scintillator detector to 100 T Visit to NY mine
Quickly abandoned effort due to limited lifetime improvement
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October 1978 – The Concept
Visit to U. Chicago / FNAL Bruce Brown water Cherenkov calorimeter prototype detector
DUMAND idea to use water Cherenkov detector technique in massive undersea volume array
Larry realized we can use this concept and scale to massive detector with track detection and particle identification
2 month activity to determine Detector characteristics Signal Background rejection
Presentation by Larry at Madison Seminar on Proton Stability December 8, 1978 – the blueprint for proton decay detector
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December 8, 1978 Paper
Totally active, underground water Cherenkov detector Charged particles detected by
Cherenkov light
Surface array of photomultiplier tubes (PMT)
1033 year limit achievable
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Detector Overview
Cubic – 20 m on each side Fiducial volume of 14x14x14
m3
1.5 x 1033 nucleons (2.5KT) Surface array of 5” diameter
hemisperical photomultiplier tubes (PMT)
Spacing – 0.7m between PMT Total 2400 PMT Energy threshold 30 Mev Muon decay detection eff. 50%
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Cherenkov Geometry
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Dec ‘78Track Geometry
• Initial simulation showing p → e+π0 event with positron and two photons from π0 decay
• (Most showering effects are suppressed)
• Vertex reconstruction and track angle reconstruction requires PMT timing resolution of a few ns.
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How much light?
Requires transparency ( λ > 30m) at the 300-500 nm wavelengths
High efficiency photocathode material (>50%) Single photoelectron detection critical 1 Gev signal (e.g. p → e+π0) requires minimum
200 photoelectrons, for sufficient energy resolution, background rejection, as well as ability to detect decay modes with less light Phototube coverage of surface ~2%.
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Dec ‘78: Background Rejection
Main background is atmospheric neutrinos Estimate background rejection of factor of 2000
for p → e+π0 Requires reconstruction of vertex Requires separation of energy into two
hemispheres for each particle Requires determining angle between two tracks Requires ~10% energy resolution on each particle
Neutrinos could be used for neutrino oscillations study down to 10-3 ev
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Formation of IMB Collaboration
January 1979 letter of intent to William Wallenmeyer, DOE to present proposal Irvine, Michigan, Brookhaven
Co-spokesman Fred Reines (Irvine)Jack Vandervelde (Michigan)
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IMB Collaboration ( April 1980)
Note: Many members missing from picture
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IMB 1987
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IMB Collaboration - Today
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Proposal Presented to DOE: 6/79
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Feasibility of the original design was demonstrated by the IMB collaboration in 1H79 Site selection : Morton Salt Mine outside Cleveland Realistic plans for construction of underground laboratory and
excavation of large cavity Demonstration of water purification (reverse osmosis system)
Supports > 30 m transparency Can be scaled to the necessary size
PMT studies – photcathode efficiency, pulse size, timing resolution, dark noise, etc – on specific EMI 5” and 8” PMT
Low cost electronics proof of concept Waterproof PMT housings Inclusion of more physical effects (nuclear effects, electromagnetic
showers) in simulations Event reconstruction software shown to be better than smearing due
to above physical effects
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What Changed from December
Actually – very little – proposed experiment design very similar to original paper
Small difference: More detailed light collection estimates plus
budgetary constraints increased PMT spacing to 1.2m (with 8” PMT) or 1.0m with 5” PMT
Closer to 1% photocathode coverage of surface
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Competing Proposal - HPW
Harvard Purdue Wisconsin Water Cherenkov detector with PMT distributed
throughout volume with mirrors at edges to increase light collection
We had rejected this idea Mirrors will confuse the track/particle detection Even if the later reflected light can be eliminated, the
prompt light has fewer PMTs listed by ~ factor of 2 making track reconstruction difficult
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Surface array has twice as many lit PMT as volume array (ignoring mirrors
More PMTs in surface array means better track reconstruction and better background rejection
Reflected light in volume array increases the total amount of light collected, but only confuses the track reconstruction ability
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DOE Decision
DOE picked IMB as the primary detector IMB given sufficient funding to go ahead with
construction programHPW given some funding to continue
“Underground physics” (non-accelerator) given boost by DOE
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Kamioka Early Feb 79 Proposal
Initial concept for water Cherenkov detector Slab design – thin veto on top,
followed by iron slab followed by larger detector
Much higher photocathode coverage proposed (> 10%)
Eventual cylindrical design, based on 20” hemispherical PMT. Timing electronics not in original
detector
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Kamioka Feb ‘79
Ref to Sulak paperFewer PMTs as
proposed by Sulak makes Kamioka proposal more practical
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1979-1982: IMB Detector
Detector excavation constraints - slightly non-cubical detector 23m x 17m x 18m
5” PMT chosen: 2048 total 1 meter spacing
Fall 1981 : Initial fill Aborted due to leaks due to stretching beyond elastic limit in
corners Summer 1982: Final fill
Lightweight concrete poured into corners behind liner as fill occurred to reduce/eliminate stretching
First good data Aug 1982
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First IMB Results – 6.5x1031 year limit on p → e+π0
Additional data / analysis extended this limit by about a factor of 5, and also set limits between 1031 and 1032 for many decay modes
The Dec ‘78 assertion by Larry that the detector would detect proton decay events, and reject neutrino background (for e+π0 ) to a factor of 2000 was nearly borne out (including IMB III upgrade)
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Mock Up in U. Mich (“Disco Room”)
Larry with approx 100 5” PMT
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Fully Assembled and filled
2048 PMT with supports
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Early (Aug ‘82) 2-track event - Classified as neutrino event with ~130° opening angle
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Epilogue I (1986-1988) Limitations of first generation water Cherenkov
detectors became clear Kamioka II upgrade (1986) (with U.Penn)
included timing electronics and led to solar neutrino measurements
IMB III upgrade increased light collection by factor of ~4 with 8” PMT and waveshifter plates
Both experiments detected the neutrinos from SN1987a - Neutrino Astronomy
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Epilogue 2 (1995-present)
Based on success of IMB/Kamioka, consensus established to push the water cherenkov technology to the limit to get best physics results on proton decay, solar neutrinos, neutrino oscillations, etc Joint US / Japanese funding required
SuperK experiment had size (30KT), photocathode coverage (40%), fiducial volume, timing resolution, and depth sufficient for physics objectives Joint US-Japanese effort that included members from both first
generation experiments Positive neutrino oscillation signal reported for atmospheric neutrinos
SNO experiment used water Cherenkov techniques as well, but with D2O to allow detection of neutral current interactions for more solar model independent measurement of neutrino oscillation from solar neutrino
Nobel prize 2002 awarded to M. Koshiba of Kamioka experiment for “pioneering … detection of cosmic neutrinos”
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