SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne1 Calibration of the OMNIS-LPC Supernova Neutrino Detector Outline –OMNIS Experiment and Detectors Lead Slab & Plastic Scintillator Lead Perchlorate –SNS2 OMNIS Calibration Experiment Goals LPC Module Event identification Event rate estimates Background rate estimates –Conclusion

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne2 OMNIS Overview Observation of Neutrinos from Galactic Supernovae –Expected Number of Supernovae: ~3 per century OMNIS: Observatory for Multiflavor Neutrinos from Supernovae  Detection of ν μ ν τ ν e + antineutrinos  Identification of ν e  Sensitive to different type of neutrino than Super-K OMNIS: ν e Super-K: ν e – bar –Planned Lifetime of the Experiment: ~ 50 years –Locations: WIPP and possibly DUSEL –Number of neutrino events from one Supernova ~2,200 from Galactic Center (8kpc); ~400 from far side of the Milky Way

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne3 Two Types of Detectors for OMNIS  2 kT: Lead Slabs & (Scintillators + Gd Sheets) Four ½ kT Modules Detect neutrons produced from ν – Pb cc & nc interactions Number of neutrino events from 8kpc Supernova: ~1,500  1 kT: Lead Perchlorate Dissolved in Water Twenty 50 -Ton modules Detect neutrons produced from ν – Pb cc & nc interactions Detect electrons produced from ν e – Pb cc interactions Measure the Energy spectrum of ν e events Number of neutrino events from 8kpc Supernova: ~ 700

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne4 OMNIS Lead+Plastic Scintillator Detector  Detection Method : ν + Pb  X + (1 or 2 neutrons) “prompt signal” ~1 MeV neutron excites scintillator “delayed signal” ~30 μs later, thermalized n captures on Gd Obtain a rough energy spectrum of neutrinos from rate of single neutron to double neutron events Note: cc and nc signals are indistinguishable Why Lead? Large neutrino cross section and low threshold (7.4 MeV for one-neutron nc events 9.8 MeV for single neutron cc events) High neutron production efficiency Low neutron absorption

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne5 OMNIS Lead Perchlorate Detector A Transparent Lead Perchlorate-Water Solution* (Pb[ClO 4 ] 2, soluble up to 80% by weight  density = 2.7) CC Detection Method: ν e + Pb  e - + Bi + (1 or 2 neutrons) –“prompt signal” e - Čerenkov ring A 15 MeV e - travels only ~ 3cm and emits ~550 photons –“delayed signal” ~ up to 50μs later, n + Cl  γ’s (8.6 MeV) (~25 – 50 us thermalization time constant) Measurement of ν e energy spectrum (E νe = E promt + ηE n threshold ) ηE n threshold accounts for the Q value of the interaction Note: the detected ν e represent the energy spectra of ν μ & ν τ before MSW transitions in the stellar medium * S.R. Elliott Phys. Rev. C (2000)

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne6 SNS 2 OMNIS Calibration Experiment Goals –Demonstrate LPC to be a viable ν detector Identify prompt e - signal from ν e cc interactions –Čerenkov ring, timing Identify delayed n signals from Cl capture –Diffuse spray of photons from multiple gammas Verify the ν e energy spectrum from (E prompt + ηE n threshold ) –Determine cc and nc event rates in LPC Cross sections have been calculated for 208 Pb Expect 206 Pb and 207 Pb to be similar –Extract cross sections: ν e, ν μ and ν μ

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne7 OMNIS LPC Module: Glass-lined Stainless Steel Cylindrical Tank 2 m ~ 30 Tons

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne8 OMNIS LPC Module & CC Event Detect prompt electron & delayed neutron(s) Phototubes 1.5 m ^ v Phototubes are in water-tight enclosures Prompt e - & Č ring Delayed: n capture  several γ V = 10.6 m^ Tons LPC weighs

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne9 CC Events: Prompt electron & delayed neutron ν e Pb  e - + ( 207 Bi + 1n) or ( 206 Bi + 2n) n + Cl  X + γ’s (8.6 MeV) Detection Method –Identify prompt e - path-length is only a few cm characteristic Čerenkov “ring” timing (< ~6 us after start of beam spill ) –Identify neutron(s) via delayed delayed capture on Cl window up to ~ 50 us after spill Consequence: 50 us gate increases probability for cosmic ray background events

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne10 NC Events: Detect the Neutrons ν Pb  ( 207 Pb + 1n) or ( 206 Pb + 2n) n + Cl  X + γ’s (8.6 MeV) But there is no prompt neutron signal –recoil protons don’t produce Č radiation –Neutron capture signal occurs up to ~ 50 us after beam spill Detection Method: Statistical –Look for neutron capture in window up to ~ 50 us after spill n capture exhibits exponential time structure –Subtract signal rate not associated with beam –Understand the beam-associated neutron background Optimize shielding between beam stop & detector Surround LPC with boron-loaded paraffin

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne11 Event Rate Estimate Rates are based on MC simulations of 4m LPC detector Rates for 3m LPC SNS 2 Calibration Detector: Neutrino Flux: 1.7 x 10 7 /s-cm^2 for each of three neutrino species cc events: ~ 230/day (100% ε )  ~60/day with 25% ε nc events: ~ 30/day (100% ε )  ~8/day with 25% ε These are conservative estimates “25% ε” includes geometrical and other efficiencies

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne12 Background Sources Cosmic rays –Muons: 2.5 x 10 8 per day  2,900 Hz “Prompt” gate duration: 6 us after start of spill (~90% u decays) Singles rate  1 Hz “Delayed” gate duration: 50us after start of spill (misidentified neutron) Singles rate  8.7 Hz –Neutrons: 1.4 x10 6 per day  16 Hz “Delayed” gate duration: 50 us after start of spill Singles rate  0.05Hz Question: What is the rate of neutrons that get captured by Cl? This is the number that really matters. Beam-associated neutron backgrounds – 1 accidental event/day implies that 1 beam-associated neutron thermalizes and stops in detector per day. –Question: What is the expected rate of beam-associated neutrons that are captured by Cl?

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne13 Background Rate Estimate Accidental Rates: CC events –Promt = muon; Delayed = muon; 13,000 events/day –Promt = muon; Delayed = neutron: 73 events/day –Reject prompt muons with active Veto System –99% efficient veto: muon-muon rate  1.3 events/day muon-neutron rate  0.7 events/day –Muon Č “ring” signature reduces misidentification by factor > 20 Characteristic signature: bright (~20k-30k photons) and thick “ring” Conclude that CC backgrounds are negligible Accidental Rates: NC events –Delayed = muon : 752,000/day –99.9% veto  752/day –Characteristic Č “ring” signature  ~ 40/day –Delayed = neutron: (assume all n captured)4,300/day Planned neutron shielding will reduce rate by x 100  43/day This can be measured & subtracted very accurately Conclude that NC backgrounds are manageable with a 99.9% veto

SNS2 Workshop August 28-29, 2003 Richard Talaga, Argonne14 Conclusions SNS2 is an excellent facility to calibrate OMNIS LPC Expect ~80 cc and ~10 nc events per day Cosmic ray backgrounds –Negligible for CC –Manageable for NC Beam-associated neutron-capture rate must be low OMNIS requires a specialized detector tank –Can’t use a multi-purpose tank for LPC –Plan to use a 3m x 2m glass-lined stainless steel tank –Might require space for external neutron absorber –A 99.9% efficient veto would be helpful in removing fake muon signals that contribute to nc measurement