Transcript HALO - Helium And Lead Observatory

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. 5 183, 1996.
Philosophy - to produce a
– Very low cost
– Low maintenance
– Low impact in terms of
lab resources (space)
– Long-term, high livetime
dedicated supernova
detector
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue
Motivation / Physics
• Galactic supernova are rare / little known
• Unique opportunity for particle physics, astronomers,
SN dynamics
• SNEWS
• Lead; high  x-sect.,
low n cap. x-sect.
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
SNEWS – Supernova Early Warning System
• 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+
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue
HALO - Phase 1 (80 tonne detector)
Initially use materials on hand
– 80 tonnes of Pb from decommissioned Deep River Cosmicray 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 / block
865 blocks
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue
HALO - SN neutrino signal – Phase 1
• 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)
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
Energy spectrum from
one NCD string with an
AmBe neutron source.
764-keV
peak
191-keV shoulder from
proton going into the wall
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
Monte Carlo Studies – Phase 1
• 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
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue
Monte Carlo Studies – Phase 1
Optimize for capture efficiency as
function of moderator thickness
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, 22-23 August 2007
C.J. Virtue
Monte Carlo Studies – Phase 1
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
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue
Monte Carlo Studies – Phase 1
# 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 m
2.7 kg/cm
41%
36
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
10BF counters
3
– Increased volume of
lead
– Allow for
modification of
block geometry
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue
HALO - Monte Carlo Studies
Tonnes of Pb
Neutron
Capture
Efficiency
(fid. volume)
Length of
NCDs used (m)
Detected
Neutrons
(SN @ 10kpc)
14 kg/cm
--1000
--55%
--700
--372
8 kg/cm
80
560
31%
60%
95
700
22
246
4 kg/cm
80
280
48%
79%
190
700
31
167
2.7 kg/cm
80
189
57%
83%
285
700
36
127
Pb / 3He ratio
F. Fleurot
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue
Monte Carlo Studies
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 …
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue
Further Work
• 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
SNOLAB Workshop VI, Sudbury, 22-23 August 2007
C.J. Virtue