SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Philosophy - to produce a –Very low cost –Low maintenance –Low impact in terms of lab resources (space) –Long-term, high livetime dedicated supernova detector HALO - a Helium and Lead Observatory A “SN detector of opportunity” / An evolution of LAND – the Lead Astronomical Neutrino Dectector, C.K. Hargrove et al., Astropart. Phys , 1996.

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Galactic supernova are rare / little known Unique opportunity for particle physics, astronomers, SN dynamics SNEWS Lead; high x-sect., low n cap. x-sect. Motivation / Physics

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Neutrinos from supernovae Neutrinos leaving star are expected to be in a Fermi-Dirac distribution according to escape depth: Oscillations redistribute neutrino temperatures SK, Kamland are primarily sensitive to ν e HALO’s sensitivity to ν e and NC valuable

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Physics Objectives Contribute to data recorded from next galactic supernova Confirmation of presence of ν e signal Handle on ν temperatures and cooling rate through 1n / 2n ratio Maximize scientific opportunity through participation in SNEWS

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Inter- experiment collaboration to disseminate the news of a galactic SN Coincidence between detectors required in 10 second window SNEWS is “live” – a “GOLD” coincidence would be sent to subscribers; “Individual” non-coincident alerts also possible now > 250 subscribers to distribution list > 2000 amateur subscribers through Sky & Telescope GCN (Gamma-ray burst Coordinates Network) HALO could bridge a gap between SNO and SNO+ SNEWS – Supernova Early Warning System

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Initially use materials on hand –80 tonnes of Pb from decommissioned Deep River Cosmic- ray station (high x-sect., low n-capture x-sect.) –~285 m 3 He proportional counter neutron detectors (NCDs) plus DAQ from SNO –Use lead in its current geometry 88 kg / block 865 blocks HALO - Phase 1 (80 tonne detector)

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue In 80 tonnes of lead for a 10kpc †, –Assuming FD distribution around T=8 MeV for ν μ ’s, ν τ ’s. –68 neutrons through ν e charged current channels 30 single neutrons 19 double neutrons (38 total) –21 neutrons through ν x neutral current channels 9 single neutrons 6 double neutrons (12 total) ~ 89 neutrons liberated; ie. 1.1 n/tonne †- Engel, McLaughlin, Volpe, Phys. Rev. D 67, (2003) HALO - SN neutrino signal – Phase 1

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue HALO - using SNO’s NCD 3 He counters NCD removal from SNO occurred in early Close to 700 m of low background 3 He counters are stored underground awaiting HALO deployment. Space in SNOLAB available early 2008.

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue NCD Energy Spectrum 191-keV shoulder from proton going into the wall 764-keV peak Energy spectrum from one NCD string with an AmBe neutron source.

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Other Backgrounds Internal alphas in n-region –3.5x10 -4 Hz*Length/200m Cosmic ray induced neutrons –1.3x10 -5 (ε) Hz –Multi-neutron bursts thermalize in ~200μs Gamma Backgrounds –< 1x10 -5 Hz ie. small for burst detection, but still a need for more detailed simulation of backgrounds with emphasis on external neutrons

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Choice of moderator – D 2 O versus polypropylene? Twice the volume required; O($700K) in Phase 1 No significant gain in neutron capture efficiency – dominated by neutron leakage not competition for neutron capture Stick with plastics! –Paint / epoxy coating of lead blocks as moderator? Distribution of moderator – various options simulated – best efficiency and least material for moderator immediately surrounding 3 He counters –1-2 mm of coating on lead blocks doesn’t hurt capture efficiency but doesn’t replace need for moderator Monte Carlo Studies – Phase 1

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Optimize for capture efficiency as function of moderator thickness Monte Carlo Studies – Phase 1 42% capture efficiency for 6mm polypropylene moderator Done in a fiducial volume to avoid confusion from edge-effects and to understand maximum efficiency. For single NCD per column. Peaks at 57% for 3 or 4 NCDs Per column.

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue However, with only 80 T the volume-averaged efficiency falls to 17.5% (60% loss relative to “fiducial volume” one)  Add reflector 20 cm water adequate recover to 25% capture efficiency (volume averaged); 40% loss reduces external neutron background from 0.1Hz from thermal flux to 0.002Hz from ~ Hz to 0.04 Hz for fast flux coverage on 5 sides versus 6 has a < 1% effect on efficiency Monte Carlo Studies – Phase 1

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue # NCDs per column Total NCD length Pb / 3 He ratio (80 Tonnes Pb - Phase 1) Neutron Capture Efficiency (vol. aver.) Detected Neutrons 10kpc) 195 m8 kg/cm25% m4 kg/cm35% m2.7 kg/cm41%36 Monte Carlo Studies – Phase 1

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Recovering some of the leaked neutrons… For the phase 1 detector nearly half of the neutrons escape detection by exiting the detector New thoughts (Stan Yen / TRIUMF) are to boost the detected SN signal by replacing the water reflector with Gd-loaded scintillator –Several options being actively investigated –Needs to be incorporated into MC studies –Early days…

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue HALO – Phase 2 (full NCD array) Optimize for –Full ~700m of 3 He counters and possibly 130 m of 10 BF 3 counters –Increased volume of lead –Allow for modification of block geometry

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Pb / 3 He ratio Tonnes of Pb Neutron Capture Efficiency (fid. volume) Length of NCDs used (m) Detected Neutrons 10kpc) 14 kg/cm % kg/cm % 60% kg/cm % 79% kg/cm % 83% HALO - Monte Carlo Studies F. Fleurot

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Phase 2 Interpretation - More is better; but what is optimum? # of 2n events detected varies mass * capture efficiency 2 Optimizing on m* ε 2 with fiducial volume efficiency suggests optimum near 1.5kT, but - insufficient points done - needs further MC work to define Good news – 1 kT of Pb occupies a cube only 4.5 m on a side; O($1M material) Detailed costing and design for Phase 2 still to come … Monte Carlo Studies

SNOLAB Workshop VI, Sudbury, August 2007 C.J. Virtue Continue with refinement of MC work – SN modeling; sensitivity of Phase 2 to additional physics – update Pb cross-sections, neutron energy distributions – Modeling of backgrounds –Addition of Gd-loaded scintillator blanket – finalize design of Phase 2 detector Collaboration building Engineering work for Phase 1 installation Proceed with Full Proposal, Technical Review, and funding application for Phase 1 and R&D for Phase II Further Work