SNOLAB Workshop VII, Sudbury, 4-5 October 2008 S. Yen /C.J. Virtue H elium A nd L ead O bservatory A...

SNOLAB Workshop VII, Sudbury, 4-5 October 2008 S. Yen /C.J. Virtue

H eliumA ndL eadO bservatory

A lead detector forsupernova neutrinosin SNOLAB


no nuclear energyfrom iron core→ core collapse

The end of a massive star's life:


Gravitational energy lowered by nuclear protonsabsorbing orbital electrons and emitting ν

e .The

iron core shrinks to only ~ 1/25,000 of its former diameter, and becomes a dense sphere of mostly neutrons of nuclear density

“football stadium” -------> “cherry”

νe

νe


Gravitational pot energy -> kinetic energy ofinfalling material. In this hot environment bremsstrahlungand e+-e- annihilation _produces e+-e- pairs as well as ν ν pairs of all flavours.

e

e e+ e-

_

e

e

Z0

ν νe

Z0

e-e+

Z0

ν ν

Z0

e+ e-

e+ e-

_


The shockwave takes several hours to reach thesurface and blow away the outer layers of the star.That is when the optical radiation is produced, andeven that constitutes only ~1% of the energyemitted. 99% of energy is emitted as prompt ν’s.

Observing a supernova explosion in opticalwavelengths is like sweeping up the confetti hours after the parade went by. The real action has long passed!

Neutrinos give us a window to view the core collapseas it happens – one of the motivations for a supernova neutrino detector.


In normal matter (e.g. lead), the mean free pathof a 1 MeV neutrino is ~4x1020 cm (400 light years).

But in the collapsed core of a supernova, whichis of nuclear matter density, the mean free pathof a neutrino is only ≤ 1 km

The neutrinos cannot freely escape – they are trapped, thermalize to a Fermi-Dirac energydistribution, and gradually diffuse outthe surface.


Because they interact less strongly with matter, the ν

μ and ν

τ are more penetrating, and come

from regions closer to the core, and therefore have a hotter spectrum than do the ν

e. .

If we can measure the temperature of the differentflavours of neutrinos as a function of time, we could verify this picture of the basic physical processes of the core collapse


Measuring the neutrino temperature, in eachneutrino flavour, as the neutron star coolsdown, would be the astrophysicist's dream!

However

The pristine picture is complicated by flavourchanges.

These are of three types:

1. vacuum neutrino oscillations

2. matter induced (MSW) neutrino oscillations

3. neutrino-neutrino interactions


3. Neutrino induced oscillations

In the collapsed core, the neutrino densityis so large that the neutrinos exert asignificant potential on each other,and a MSW-like flavour transitiondue to ν-ν interactions (rather thanν-e interactions) can occur. This leads tospectacular flavour swapping and spectralsplitting for the ν

e, but only if we have inverted

mass hierarchy and if θ13

≠ 0.

Fogli et al. hep-ph/07071998

NEW!


Initial flux at r~10 km

Final flux at r~200 kmafter collective ν-νinteractions

from E. Lisi, TAUP'07, Sendai 2007

anti-neutrinos undergo flavor swap

high energy neutrinos undergo flavor swap, butlow energy neutrinos don't (spectral split)

most dramatic for νe


Comparison of rate in HALO (νe) with prediction based on measured

anti-νe spectrum in SNO+ or Super-K could be a signature for

flavour-swapping.

Without flavour swapping, νe spectrum is softer than anti-ν

e,

so using measured anti-νe as a proxy for ν

e overpredicts

νe interaction rate in HALO

Very New !


With flavour-swapping, the high energy νe and anti-ν

e spectra are identical.

Using the measured anti-νe spectrum from SNO+ as a proxy for ν

e yields a

good prediction of the rate in HALO.

A robust signature of flavour-swapping independent of supernovasize, temperature, distance.


Water Cerenkov (Kamioka)

electron antineutrino + p

any neutrino + e

any neutrino + O

Water Cerenkov andLiquid Scintillatormostly sensitive to‗ν

e + p → e+ + n

Main Title

electron neutrino + Pb (CC)

any neutrino + Pb (NC)

Pb mostly sensitive to νe

νe + n (nuclear) → e - + p

because the large neutronexcess Pauli blocks theanti-ν

e + p reaction

Pb


Why observe supernova neutrinos?

For the astrophysicist: - early warning of a SN for optical observations - observe signals directly from the core collapse - observe formation of a Black Hole - measure effective temperature of matter at different stages, as the proto-neutron star forms and cools

For the particle physicist: - observe ν's from the only situation in the universe where ν's are trapped and thermalize - observe ν-ν interactions and collective behaviour of a large density of neutrinos—not possible elsewhere! - resolve normal versus inverted ν mass hierarchy—even a crude measurement of ν

e energy can do this if θ

13≠0.

- a Pb detector offers unique sensitivity to the interesting ν

e channel


• Design Overview• Areas of progress

– Funding– Physics motivation (S. Yen)– Active Personnel– Preparation for moving Pb– Mechanical studies– Simulation work– Summary budget and

timetable

HALO - a Helium and Lead Observatory

Progress Report


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 - Design Overview

“Helium” – because of the availability of the 3He neutron detectors from SNO + “Lead” – because of high -Pb cross-sections, low n-capture cross-sections,

sensitivity to e (dominantly) and x complementing water Cerenkov

and liquid scintillator detectors


HALO-I - Design Overview

• Lead Array– 32 three meter long columns

of annular Lead blocks– 76 tonnes total lead mass

(864 blocks)

• Neutron detectors– Four 3 meter 3He detectors per

column– 384 meters total length

• Moderator– 250mm Schedule 40 PP tubing

• Reflector– 20 cm thick graphite blocks


• In 76 tonnes of lead for a SN @ 10kpc†,– Assuming FD distribution around T=8 MeV for

νμ’s, ντ’s.– 65 neutrons through νe charged current channels

• 29 single neutrons• 18 double neutrons (36 total)

– 20 neutrons through νx neutral current channels• 8 single neutrons• 6 double neutrons (12 total)

~ 85 neutrons liberated; ie. 1.1 n/tonne

†- Engel, McLaughlin, Volpe, Phys. Rev. D 67, 013005 (2003)

HALO - SN neutrino signal – Phase 1


Areas of Progress

• Funding– Two years support from NSERC ($50K + $40K)– necessitates a reduced scope budget / timetable at end of

talk

• Active Personnel– C. Virtue (50%)– S. Yen (25%)– T. Shantz (100%) HALO MSc student – HALO design /

construction– S. Korte (10%) SNO+ MSc student – SN simulation– J. Farine (10% **) Backgrounds and Calibration– F. Duncan (10% **) – A. Habig (10% **) Slow Controls

** soon to be harnessed, declared interests


Areas of Progress• Preparation for moving Pb underground

– inspection August 2008


Lead Blocks in Storage

• Presence of lead carbonate (white powder) on many blocks


Paint Selection – Taylor Shantz

• Re-affirms need to paint blocks to halt deteriorization and to immobilize residual lead carbonate

• Paint Selection Criteria: – great adhesion due to the handling requirements – ability to bond on surfaces with abrasion resistant thin

layers of lead carbonate– Short curing time– Minimal paint-paint welding together in stacked geometry– long-lasting tough finish; a paint that physically

deteriorates in the SNOLAB clean environment is to be avoided

– minimal U and Th content of paint; we would assay prior to application; a clear coat without pigments likely avoids these concerns


Paint Selection

• Following discussions with Industrial Hygenists at SLAC and PPPL, and EXO engineers, seven candidate paints were considered and three selected for standardized testing

• ASTM D 3359-08 – “Measuring Adhesion by Tape Test” is currently being used to compare the adhesion of the three candidate paints

• We are also assessing the paint to paint welding issue in the same tests


Short term Paint plan

• Select an appropriate paint• Complete design of lifting jig (Oleg Li) and

fabricate• Finish developing the production painting

plan– Labour intensive / costly (handling ~900 88kg

pieces)– Requires lead handling precautions / procedures

• Will collect statistics on block dimensions during handling for painting– input into lead array structural support


Mechanical Studies

• Options considered Default stacking

Narrow Stacking On-edge stacking Vertical Stacking

Oleg Li


Mechanical Studies• Preliminary Stress analysis (1200 lbs)• Stress, Deformation, Safety Factor

Safety Factor (Min = 4.10)


Mechanical Studies

• Long-term creep tests

1 tonne load


Characterization of Creep in Lead

• program is to measure creep rate as function of load and then leave under maximum load as a long term test

• hope is to use creep data to “calibrate” a structural mechanics simulation package modelling the compression test geometry and then to model long term creep in real HALO geometry

• still considering whether an accelerated test to rupture in a 5 tonne press would give useful information at lower creep rates

Generic Creep-Rupture curve withexaggerated times to properly displaythe three stages.

Secondary Creep

TertiaryCreep

PrimaryCreep


Simulation Work – Stephen Korte

• GEANT code that follows neutrons produced uniformly in volume of lead, with initial energies selected from expected SN -induced n energy distribution– exists / was used to optimize HALO design– has been previously reported at other workshops

• this code is not well-suited to physics sensitivity studies that we would like to perform, so

• Stephen is adding HALO specific cross-sections to SNGEN (J. Heise)

• he will also add the spectral splitting and flavour swapping phenomenology to permit such physics sensitivity studies

• SNGEN will be the physics generator that passes particle lists to the GEANT code to complete the simulation.


Budget2008-09 2009-10 Revised (over 2

years)

Salaries 76K 76K 24K

Materials 16K 16K 16K

Travel 18K 18K 12K

Capital

Mechanical 44K 0K 24K

Electronics 65K 0K 45K

Labour 20K 0K 20K

Contingency 27K 0K 0K

Travel 8K 0K 8K

Totals 274K 110K 149K


Budget• savings achieved by

– reducing / deferring # of electronics channels (daisy-chaining)

– deferring graphite reflector or using water– Stan Yen’s travel supported by TRIUMF outside of grant

• short-fall is of order $60K to complete detector• we will submit a request for SNOLAB

infrastructure support (cranes, lifting jigs, shielding, assembly labour, etc. for discussion)

• a further NSERC grant will likely be required to complete the detector


“First Pass” ScheduleHALO Construction

SNOLAB Workshop VII, Sudbury, 4-5 October 2008 S. Yen /C.J. Virtue H elium A nd L ead O bservatory A...

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