snolab workshop vi, sudbury, 22-23 august 2007 c.j. virtue philosophy - to produce a –very low...
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SNOLAB Workshop VI, Sudbury, 22-23 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 ObservatoryA “SN detector of opportunity” / An evolution of
LAND – the Lead Astronomical Neutrino Dectector, C.K. Hargrove et al., Astropart. Phys. 5 183, 1996.
SNOLAB Workshop VI, Sudbury, 22-23 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, 22-23 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 νe
• HALO’s sensitivity to νe and NC valuable
SNOLAB Workshop VI, Sudbury, 22-23 August 2007 C.J. Virtue
Physics Objectives
• Contribute to data recorded from next galactic supernova
• Confirmation of presence of νe signal
• Handle on ν temperatures and cooling rate through 1n / 2n ratio
• Maximize scientific opportunity through participation in SNEWS
SNOLAB Workshop VI, Sudbury, 22-23 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 e-mail 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, 22-23 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 3He proportional counter neutron detectors
(NCDs) plus DAQ from SNO– Use lead in its current geometry
88 kg / block865 blocks
HALO - Phase 1 (80 tonne detector)
SNOLAB Workshop VI, Sudbury, 22-23 August 2007 C.J. Virtue
• In 80 tonnes of lead for a SN @ 10kpc†,– Assuming FD distribution around T=8 MeV for
νμ’s, ντ’s.– 68 neutrons through νe charged current channels
• 30 single neutrons• 19 double neutrons (38 total)
– 21 neutrons through νx 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, 013005 (2003)
HALO - SN neutrino signal – Phase 1
SNOLAB Workshop VI, Sudbury, 22-23 August 2007 C.J. Virtue
HALO - using SNO’s NCD 3He counters
NCD removal from SNO occurred in early 2007.
Close to 700 m of low background 3He counters are stored underground awaiting HALO deployment.
Space in SNOLAB available early 2008.
SNOLAB Workshop VI, Sudbury, 22-23 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, 22-23 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, 22-23 August 2007 C.J. Virtue
• Choice of moderator– D2O 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 3He 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, 22-23 August 2007 C.J. Virtue
Optimize for capture efficiency as function of moderator thickness
Monte Carlo Studies – Phase 1
42% capture efficiencyfor 6mm polypropylenemoderator
Done in a fiducial volumeto avoid confusion fromedge-effects and to understand maximumefficiency.
For single NCD per column.Peaks at 57% for 3 or 4 NCDsPer column.
SNOLAB Workshop VI, Sudbury, 22-23 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, 22-23 August 2007 C.J. Virtue
# NCDs per column
Total NCD length
Pb / 3He ratio(80 Tonnes Pb
- Phase 1)
Neutron Capture
Efficiency(vol. aver.)
Detected Neutrons
(SN @ 10kpc)
1 95 m 8 kg/cm 25% 22
2 190 m 4 kg/cm 35% 31
3 285 m2.7
kg/cm41% 36
Monte Carlo Studies – Phase 1
SNOLAB Workshop VI, Sudbury, 22-23 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, 22-23 August 2007 C.J. Virtue
HALO – Phase 2 (full NCD array)
Optimize for– Full ~700m of 3He
counters and possibly 130 m of 10BF3 counters
– Increased volume of lead
– Allow for modification of block geometry
SNOLAB Workshop VI, Sudbury, 22-23 August 2007 C.J. Virtue
Pb / 3He ratioTonnes of Pb
Neutron Capture
Efficiency(fid. volume)
Length ofNCDs used
(m)
Detected Neutrons
(SN @ 10kpc)
14 kg/cm---
1000---
55%---
700---
372
8 kg/cm80
56031%60%
95700
22246
4 kg/cm80
28048%79%
190700
31167
2.7 kg/cm
80189
57%83%
285700
36127
HALO - Monte Carlo Studies
F. Fleurot
SNOLAB Workshop VI, Sudbury, 22-23 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*ε2 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, 22-23 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
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