Calibration of the OMNIS-LPC Supernova Neutrino Detector

1. 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 Richard Talaga, Argonne

2. OMNIS Overview Observation of Neutrinos from Galactic Supernovae • Expected Number of Supernovae: ~3 per century OMNIS: Observatory for Multiflavor Neutrinos from Supernovae • Detection of νμντ νe + antineutrinos • Identification of νe • Sensitive to different type of neutrino than Super-K • OMNIS: νe • Super-K: νe–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 Richard Talaga, Argonne

3. Two Types of Detectors forOMNIS • 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 νe – Pb cc interactions • Measure the Energy spectrum ofνe events • Number of neutrino events from 8kpc Supernova: ~ 700 Richard Talaga, Argonne

4. OMNISLead+Plastic Scintillator Detector • Detection Method: ν + Pb  X + (1 or 2 neutrons) • “prompt signal” ~1 MeV neutron excites scintillator • “delayed signal”~30μs 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 Richard Talaga, Argonne

5. OMNISLead Perchlorate Detector A Transparent LeadPerchlorate-Water Solution* (Pb[ClO4]2, soluble up to 80% by weight  density = 2.7) • CC Detection Method: νe + 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 ofνeenergy spectrum(E νe = Epromt + ηEnthreshold) • ηEnthreshold accounts for the Q value of the interaction • Note: the detected νerepresent theenergy spectra ofνμ & ντbefore MSW transitions in the stellar medium *S.R. Elliott Phys. Rev. C62 065802 (2000) Richard Talaga, Argonne

6. SNS2OMNIS Calibration Experiment • Goals • Demonstrate LPC to be a viable ν detector • Identify prompt e- signal from νe cc interactions • Čerenkov ring, timing • Identify delayed nsignals from Cl capture • Diffuse spray of photons from multiple gammas • Verify the νe energy spectrum from (Eprompt + ηEn threshold ) • Determine cc and nc event rates in LPC • Cross sections have been calculated for 208Pb • Expect 206Pb and 207Pb to be similar • Extract cross sections: νe ,νμand νμ Richard Talaga, Argonne

7. OMNIS LPC Module: Glass-lined Stainless Steel Cylindrical Tank 2 m ~ 30 Tons <------------------- 3 m ------------------> Richard Talaga, Argonne

8. OMNIS LPC Module & CC EventDetect prompt electron & delayed neutron(s) V = 10.6 m^3 Phototubes LPC weighs Prompt e- & Č ring ^ 28.6 Tons 1.5 m Delayed:n capture severalγ v Phototubes Phototubes are in water-tight enclosures Richard Talaga, Argonne

9. CC Events: Prompt electron & delayed neutron • νe + 208Pb  e- + (207Bi+ 1n) or (206Bi+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 beamspill) • 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 Richard Talaga, Argonne

10. NC Events: Detect the Neutrons • ν + 208Pb  (207Pb+ 1n)or(206Pb+2n) n + Cl  X + γ’s (8.6 MeV) But there is no promptneutron 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 Richard Talaga, Argonne

11. Event Rate Estimate Rates are based on MC simulations of 4m LPC detector Rates for 3m LPC SNS2 Calibration Detector: Neutrino Flux: 1.7 x 107/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 Richard Talaga, Argonne

12. Background Sources • Cosmic rays • Muons: 2.5 x 108 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 x106 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? Richard Talaga, Argonne

13. 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 Richard Talaga, Argonne

14. Conclusions • SNS2 is an excellent facility to calibrate OMNIS LPC • Expect ~80 cc and ~10 nc events per day • Cosmic raybackgrounds • 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 Richard Talaga, Argonne