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

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.


• 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


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


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


• 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


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)


• 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


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.


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.


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


• 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


Optimize for capture efficiency as function of moderator thickness


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.


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



# 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



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…


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


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


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


• 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

Documents

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