Birth of the Large Scale Imaging Water Cherenkov Detector

1
Birth of the Large Scale Imaging Water Cherenkov
Detector

• Bruce Cortez
• Sulak Festschrift
• Boston University
• Oct 22, 2005

2
Agenda and Sulak Timeline
Focus of this talk
Michigan
Location
Harvard
Construction
Data
IMB Collab.
Proposal
Grad Students
B. Cortez
G.W. Foster
J. Strait W. Kozaneck M.Levi
S. Seidel
D. Casper
Jan 78
Jan 79
Jan 80
Jan 81
Jan 82
Jan 83
3
The Beginning

• September 1978
• Larrys mission
• A. Salaams statement that proton decay in the
  most important experiment in physics
• Grand Unified Theories were now predicting
  lifetimes of lt 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

4
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

5
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

6
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

7
Cherenkov Geometry
8
Dec 78Track Geometry

• Initial simulation showing p ? ep0 event with
  positron and two photons from p0 decay
• (Most showering effects are suppressed)
• Vertex reconstruction and track angle
  reconstruction requires PMT timing resolution of
  a few ns.

9
How much light?

• Requires transparency ( ? gt 30m) at the 300-500
  nm wavelengths
• High efficiency photocathode material (gt50)
• Single photoelectron detection critical
• 1 Gev signal (e.g. p ? ep0) 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.

10
Dec 78 Background Rejection

• Main background is atmospheric neutrinos
• Estimate background rejection of factor of 2000
  for p ? ep0
• 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

11
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)

12
IMB Collaboration ( April 1980)
Note Many members missing from picture
13
IMB 1987
14
IMB Collaboration - Today
15
Proposal Presented to DOE 6/79
16
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 gt 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

17
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

18
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

19
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

20
DOE Decision

• DOE picked IMB as the primary detector
• IMB given sufficient funding to go ahead with
  construction program
• HPW given some funding to continue
• Underground physics (non-accelerator) given
  boost by DOE

21
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 (gt
  10)
• Eventual cylindrical design, based on 20
  hemispherical PMT.
• Timing electronics not in original detector

22
Kamioka Feb 79

• Ref to Sulak paper
• Fewer PMTs as proposed by Sulak makes Kamioka
  proposal more practical

23
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

24
First IMB Results 6.5x1031 year limit on p ?
ep0

• 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 ep0 ) to a factor of
  2000 was nearly borne out (including IMB III
  upgrade)

25
Mock Up in U. Mich (Disco Room)
Larry with approx 100 5 PMT
26
Fully Assembled and filled
2048 PMT with supports
27
Early (Aug 82) 2-track event - Classified as
neutrino event with 130 opening angle
28
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

29
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