stockholm, may 2-6, 2006 snow 20061 sub-mev solar neutrinos: experimental techniques and backgrounds...

Stockholm, May 2-6, 2006

SNOW 2006 1

Sub-MeV solar neutrinos: experimental techniques

and backgrounds

Aldo Ianni

Gran Sasso Laboratory, INFN


SNOW 2006 2

Why do we need to measure sub-MeV solar neutrinos? Neutrino physics Astrophysics

How can they be observed? Upcoming:

100-ton scale ultra-pure organic Liquid Scintillator (high photon yield below 1 MeV)

Elastic Scattering Future:

liquid noble gases, metal loaded LS, TPC ES + Inverse Electron Capture


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Why?


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Only 0.01% of solar neutrino spectrum measured in real time


SNOW 2006 5

Measurements vs unknowns


SNOW 2006 6

MSW-LMA to explain observations

Transition of Pee

Obtained with SSM constraints!


SNOW 2006 7

What do we want to measure and why do it?

( )

pep

ppBeeeppBe

ppBe

CC

ppBeppBe

eeppBe

eeppBe

ppBe

ES

f

Pf

fPPf

measure

Mmeasure/SS

1Mmeasure/SS

,,

,

,,,

,

,

⋅=

⋅−⋅+⋅= ρ

02.099.0 4.1L

L

0.0051.010 02.002.1

07.1 91.0

%)5(2.03.0

Be(5%)

05.007.0

Be(5%)24.062.0

±⏐⏐ →⏐=

±⏐⏐ →⏐±=

⏐⏐ →⏐=

+−

+−

+−

Be

pp

Be

f

f

γ

ν energieslow or middleat up showsmight

neutrino sterilelight a and qq , 0

as such physics standard Non

fl +→+≠ ννμν

astrophysics Neutrino physics


SNOW 2006 8

How difficult is it going to be?


SNOW 2006 9

• What detection channel

– ES: not a specific signature, better with Be and pep + asks for a ultra-pure Fiducial Mass

– CC: strong signature via inverse electron capture. Internal background may become less important


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Signatures and requirements for the ES channel

200 400 600 800 1000

75

80

85

90

95

100 SimulatedseasonalSignal+BackgroundinBOREXINO

S0=33counts?dayB=50counts?day

timeHdaysL

stnuoc

?yad

?001

not

0 20 40 60 80 100 120 140

1

2

3

4

5

6

3s level

5yr

3yr

2yr

BackgroundHevents?dayLC.L.

Hs

L10-16g/g

With U,Th at 10-16 g/g and 40K at 10-14 g/g• internal backg. ~ 20 cpd/100 tonsin [0.25,0.8]MeV


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• ES + Ultra-pure liquid scintillators

– First thoughts/tests ~1988 to address radiopurity issues • High photon yield (104 /MeV) allows to perform spectroscopic

measurements• SSM predicts ~ 0.5 cpd/ton for Be with ES => 100t FM

– Borexino a pioneer experiment with a 4-ton prototype showed (1997):

• that 238U and 232Th can be below or on the order of 10-16 g/g (~10-6Bq/ton)• that 14C/12C ~ 10-18 allows to set a thereshold at 250 keV for ES• that self-shielding design works with organic scintillator ( ~ 1 g/cm3) to reduce external background

– KamLAND (2002) with a 500 ton-scale mass has measured 238U and 232Th at the level of 10-17-10-18 g/g => pep meas. opportunity


SNOW 2006 12

Beyond U and Th

Asking for 1cpd/100tons

[0.1 μBq/m3 PC] it implies:• System sealed against 222Rn

~10-5Bq/ton

• 0.4 ppm 39Ar in N2

• 0.2 ppt 85Kr in N2

210Pb and 210Po are often found not in equlibriumdue to a different chemistry

All spectra normalized to 1


SNOW 2006 13

Removing/Reducing 210Bi, 210Po, 85Kr, 39Ar• High level of cleanliness • Purification of scintillator

– Distillation (thought to be the best method on the basis of small set-up tests)

– Water extraction– High level nitrogen sparging

Beyond U and Th

Check radioisotope impurities before filling!


SNOW 2006 14

What about pep neutrinos?

• Cosmogenic background: 11C• Possible 11C background

reduction by tagging the sequence (in Borexino and KamLAND): muon + neutron capture + 11C decay [see Galbiati et al, PRC, 71, 055805 (2005)]

• Method already tested with Borexino prototype [see hep-ex/0601035 ]

• SNO+ main goal due to SNOlab depth


SNOW 2006 15

Reduction of background for pep neutrinos

Cylindrical cut Around muon-track

Spherical cut around⏐neutron capture vertex to reject 11C event correlated in time and space

Neutron production

vertex

Muon going through

In 95% of cases a neutron is produced together with a 11CSignal/Noise as large as 2 with only 3% of data rejected @ Gran Sasso


SNOW 2006 16

11C tagged with the Borexino prototype

Taken from Borexino coll. hep-ex/0601035

11C decays + with Q~1MeV and min Measured production rate ~0.14 events/day/ton at Gran Sasso depth


SNOW 2006 17

Backgrounds for pep besides U,Th,11C


SNOW 2006 18

Next generation projects

Goals• real-time observation of pp (CC/ES)• real-time observation of Be with a CC channel

ProjectsXMASS : LXe, ESCLEAN : LNe, ES(see D. McKinsey this workshop)

MOON : 100Mo CCLENS : 115In CC


SNOW 2006 19

Liquid Xe [XMASS]• Multi-purpose detector• Channel: ES• No 14C!• Target: 23t [10t FV] of LXe• Design: use of 30cm self-

shielding ( = 3.06 g/cm3)• Backgrounds (main tasks):

– 85Kr: to be reduced to 4x10-15g/g from 10ppm (by distillation)

– 136Xe 0ν: isotope separation (<1/100 of natural)

• 100kg(NOW)->1E3->1E4

2.5 m


SNOW 2006 20

LENS SneIne115115 s]4.5 [delayed, 2 [prompt] +=+→+ − μγν

•115In abundance = 95.7%•Threshold of capture = 0.114 MeV•B(GT) = 0.17 [precise measurement with neutrino source in TF]•LS stability tested : > 2yr•Backgrounds:

decay of 115In + following Bremsstrahlung

Multiple In decay •Desing: high segmentation with 125t on LS [10t of In]

~4% pp meas. in 5yr


SNOW 2006 21

MOON• Multi-purpose detector [0ν, supernova ν’s]• Channel: inverse e- capture (prompt) + delayed b decay•Threshold of capture = 0.168 MeV•(gA/gV)2 B(GT) = 0.52±0.06•Backgrounds:

• U,Th at 10-3 Bq/ton • 214Pb->214Bi->214Po•2ν•Surface contamination

•Design: •3.3ton 100Mo •module 6m x 6m x 5m•Mo foils 0.05 g/cm2

•x, y reading with scintillators•~10-9 spatial resolution required

•Signal: • pp: ~337 events/yr/3.3tons•Be: ~167 events/yr/3.3tons

•Test Facility in operation since April 2005 @ Oto underground lab.


SNOW 2006 22

Conclusions• It is great opportunity to measure low energy

solar neutrinos• First thoughts: 1988!• When: Borexino and KamLAND->2007• 10% Be meas. [5%] gives 10%Be,1%pp [5%Be,0.5%pp]• New goal: pep neutrinos. Precise meas. @

SNO+• Complementary projects under-way >=2010(?)


SNOW 2006 23


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SNOW 2006 25

From Galbiati et al, PRC, 71, 055805 (2005)


SNOW 2006 26



SNOW 2006 27



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SNOW 2006 29

He Ne Ar Kr XeA 2 10 18 36 54

Ion. Potent. (eV)

24.6 21.6 15.7 14 12.1

Boiling point (K)

4.2 27 87 119.8 165

p.e./MeV 4E4 4E4 4.3E4

long lived isotopes

39Ar,42Ar 85Kr

Density (g/cm3)

0.125 1.2 1.4 2.6 3.06

Rad. Length (cm)

756 24 14 2.4

From hep-ph/0008296


SNOW 2006 30

800kg detector

10 ton detector

～ 30cm～ 80cm

～ 2.5m

R&DDark Matter Search

Multipurpose Detector

(DM, Solar Neutrino, )

With light guide

XMASS Program with LXe

100kg Prototype

M. Nakahata


SNOW 2006 31

General Properties

νx+ e- → νx+ e- in 22 tonnes Helium

Ultra-pure (superfluid self-cleaning)

Scintillation + rotons or e-bubbles

Event discrimination

Position & energy reconstruction

ν

R. Lanou

Coded ApertureWafer Array

Progress

Frozen N2 + acrylic to replace graphite

moderator

Successful extraction of electrons from

drifted e-bubbles

… more powerful than rotons

Prospects

Technique & physics potential established

… Requires large scale prototyping

stockholm, may 2-6, 2006 snow 20061 sub-mev solar neutrinos: experimental techniques and backgrounds...

Documents

mev slide

infn slide

important slide

opportunity slide

snolab depth slide

real time slide

es channel

c background reduction