Solar Neutrinos with Borexino

1

Solar Neutrinos with Borexino

Frank CalapriceDepartment of PhysicsPrinceton University

4/10/2014 Stanford Univsersity Seminar May 7 2014

Stanford Univsersity Seminar May 7 2014 2

Solar Neutrino Experiments• Chlorine experiment: (Pontecorvo, Alvarez, Davis)

– First solar neutrino detector was the chlorine radiochemical experiment.– Technique avoided the intense source of radiological backgrounds by detecting

37Ar from the reaction 37Cl(ν,e)37Ar on a target of chlorine atoms.

• Gallium radiochemical experiments: (Gallex, SAGE)– Used simliar radiochemical technique to measure pp neutrinos: 37Ga(ν,e)37Ge

• Water Cerenkov: (Kamiokande, Super-K, and SNO) – Detected high enegy 8B neutrinos (> 5 MeV ) to avoid radiogenic backgrounds

• Borexino Liquid Scintillator– First experiment to directly detect low energy neutrinos in the midst of intense

radiogenic background below 3 MeV.– A breakthrough made possible by “brute force” low-background technology.

4/10/2014


Borexino Solar Neutrinos• Borexino is a large liquid scintillator detector operating in the

Gran Sasso laboratory in Italy.• It the only operating experiment capable of direct detection of low

energy solar neutrinos. • It’s signature feature is unmatched ultra-low background

achieved at start-up of Phase 1 in 2007, and improving in Phase 2 and 3.

• Phase 1 2007-2010 yielded data on 7Be, pep, and 8B solar neutrinos, geo-neutrinos, and other results.

• Phase 2 2010-2014 achieved lower background by scintillator re-purification and aims to measure pp neutrinos.

• Phase 3 2015-201? will exploit improved scintillator re-purification methods for additional reduction of background toward measuring CNO neutrinos and addressing solar metallicity problem.

4/10/2014


Solar Nuclear Fusion Cycles

The pp cycle The CNO cycle

4/10/2014

<1.2 MeV <1.7 MeV

<1.7 MeV

<0.42 MeV 1.44 MeV

0.86 MeV<15 MeV


Neutrino DetectionNeutrino-electron elastic scattering

• Contributions from charged and neutral currents.• Measure energy of recoil electron by number of

detected scintillation photons. • With 500 pe/MeV, energy resolution is about 5% at 1 MeV.

• Position of event is measured by photon time-of-flight.• Position resolution is 10-15 cm.

• Threshold energy is about 60 keV (due to 14C background).

• Calorimetric measurements- no directional sensitivity

4/10/2014


Solar Neutrino SpectraNeutrino Energy

Spectrum Neutrino-Electron Elastic ScatteringEnergy Spectrum

4/10/2014


Solar Neutrino SpectraNeutrino Energy

Spectrum Neutrino-Electron Elastic ScatteringEnergy Spectrum

4/10/2014

X2 for keV


Low Background Strategy• Place unavoidable radioactivity (PMT’s, etc.) far

away and totally shielded from detector.– Use active liquid shields (water, liquid scint.).– Define fiducial mass far from surfaces: use event

position.

• Use a detector that can be purified in situ to remove internal radioactivity.– Liquids and gasses are good. Solids more challenging.

• Develop effective purification method to reduce radioactivity in detector and in shielding.– Distillation, water extraction on liq. scintillator work

well.

4/10/2014


Low Background Features of Borexino

• Water and Scintillator Shielding– Purified for ultra-low internal radioactivity.

• Scintillator Containment Vessel–Nylon balloon with small mass and low

radioactivity.

• Scintillator Purification System – Pseudocumene (PC) & 1.5 g/l PPO– Distillation, water extraction, and N2 gas

stripping.4/10/2014


Overview of the Borexino Detector(Mostly Active Shielding)

• Shielding Against Ext. Backgnd.– Water: 2.25m – Buffer zones: 2.5 m – Outer scintillator zone: 1.25 m

• Main backgrounds: in Liq. Scint.– 14C/12C

• 10-18 g/g. cf. 10-11 g/g in air CO2

– U, Th impurities– Cosmogenic 11C (t1/2 = 20 min)

– 222Rn daught (210Pb, 210Bi, 210Po)– 85Kr (air leak)

• Light yield (2200 PMT’s) – Detected: 500 pe/MeV (~5%)

• Pulse shape discrimination.– Alpha-beta separation

4/10/2014

11

Liquid Scintillator Composition

• Simple two-component liquid scintillator– Pseudocumene + 1.5 g/liter PPO

• Chose two-component without secondary wavelength shifter to simplify purification and re-purification.

• High light yield (11,000 ph/MeV) with fast timing and excellent alpha-beta pulse shape discrimination.

• Aggressive to acrylic and many plastics, but very safe with nylon.

• Not safest and most environmentally friendly, but not bad.

4/10/2014 Stanford Univsersity Seminar May 7 2014


Nylon Scintillator Containment VesselFabricated in special Princeton Low-Radon Cleanroom

John Bahcall

First hermetically sealedcleanroom with low-radonair was developed to avoidsurface radioactivity dueto 222Rn daughters: 210Pb (22 yr).

Fabrication time: > 1 yrLow-radon cleanroomsare becoming commonlow-background research.

4/10/2014


The 238U and 232Th Decay Chains

Radon in air deposits 210Pb (22 yr) on nylon foil, which later contaminates scintillator with 210Bi (1 MeV b) and 210Po (5 MeV a). 4/10/2014


Pseudocumene Purification During Filling Operations.

4/10/2014


Distillation, Stripping, Water Extraction Columns

“Precision cleaned”, then assembled in low-Rn cleanroom

4/10/2014

Assembly of distillation & stripping columnsIn Princeton Low-Radon Cleanroom.

Low radon clean room

Stripping column

Distillationcolumn

Radio-pure (LAK) nitrogen (Heidelberg)Simgen, Heusser, Zusel, Appl. Rad. 61, 213 (2004)


Purification of Wavelength Shifter (PPO) by

Water Extraction and Distillation • PPO was found to have a high

level of K that could be reduced by water extraction.

• A concentrated solution of PPO in PC was purified by water extraction then distillation.

• Recent studies show that LNGS de-ionized water has significant 210Pb and 210Po.

• The studies also indicate that simple distillation done is ineffective for removing 210Po.

4/10/2014

Afer water extraction, the PPO purification plant used the above simple evaporator to distill the PPO-PC “master solution”. Water extraction may have introducedf 210Po that was not removed by distillation.


Borexino Energy Spectra

PRL 107 141302 (2011)

Data are based on 740.7 live days between May 16, 2007 and May 8, 2010.

Prominent backgrounds are: 210Po 210Bi 85Kr, 11C & 14C (not shown)

CNO obscured mainly by 210Bi due to similar shape.

The 210Po alpha rate was high, but rejected by alpha/beta pulse shape discrimination.

The pep was measured by applying cuts to reduce the 11C. (muon track, neutron, other)4/10/2014

pep77Be

CNO

pp


Phase 1 Borexino Results 2007-2012

Solar Neutrinos✓7Be 46.0 cpd/100t ± 5%. PRL 2011✓ 8B (> 3 MeV) 0.22 cpd/100t ± 19% PRD 2010✓Pep 3.1 cpd/100t ± 22% PRL 2012✓CNO limit < 7.9 cpd/100t PRL 2012 ✓7Be day/night asy. A = 0.001 ± 0.014 PLB 2012 ✓7Be annual modukation PLB 2012

Geo-neutrinos Geo-neutrinos 14.3 ± 3.4 eV/(613 t-yr) PLB

2013

Rare Processes Test of Pauli Exclusion Principle in Nuclei PRC

2010 Solar axion upper limit PRD 2012

4/10/2014


Energy spectrum with backgrounds

4/10/2014

11C

210Po

210Bi

85Kr

CNO


7Be: fit of the energy spectrum

tcpdR syststat 100/5.146 )(5.16.1)(

tcpdR noscillationo 100/2.574

5 s evidence of oscillation

ne flux reduction 0.62 +- 0.05electron neutrino survival probability 0.51 +- 0.07

•Search for a day night effect:•not expected for 7Be in the LMA-MSW model•Large effect expected in the “LOW” solution (excluded by solar exp+Kamland)

)(007.0)(012.0001.02/)(

sysstatDN

DNADN

G. Bellini et al., Borexino Collaboration, Phys. Lett. B707 (2012) 22.4/10/2014


Motivation for pep NeutrinosMSW Theory & Non-standard Interctions

MSW theory of neutrino oscillations in the Sun predictsa transition in survival probabilityof ne from vacuum oscillationsbelow 2 MeV to matter effectoscillation at higher energy.

The effect has been observed

Reducing the uncertainty in pepneutrino rate will improve the confirmation of this MSW feature, and tighten constraints on non-standard interactions.

pep77Bepp

8B

Uncertainty too large.

4/10/2014

Figure from Haxton, et alAnn. Rev. Astron. Astrophys. 2013


Phase 2 Solar Neutrino Goals: 2010-2014

• Technical goals:– Reduce scintillator backgrounds with

loop purification by water extraction. ☐ • 210Bi (210Pb) ☐• 85Kr by nitrogen stripping ☐

• Measurement goals:– pp neutrino measurement ☐– Improve pep, 7Be, 8B measurement ☐– CNO neutrinos detection or lower limit

(?) ☐4/10/2014


Re-Purification of the Liquid Scintillator for Lower Background

• Lower backgrounds essential for Phase 2 solar program.– 210Bi obscures CNO and pep neutrinos.– 85Kr interferes with 7Be neutrinos

• Purification of the scintillator by “water extraction” and “nitrogen stripping” was carried out 2010-2011.– Backgrounds were reduced significantly.

• Lower background is still necessary.– Refinements in water extraction are being

developed

4/10/2014


Background Reduction with Loop Purification of Liquid Scintillator

• “Loop” purification is achieved by draining fluid from bottom of vessel, passing it through purification system, and returning to the top.

• Processes available are:– Water extraction or

distillation – Nitrogen stripping (85Kr)

4/10/2014


Water Extraction and Nitrogen Stripping Performed 2010-2011

• Contacting high purity water with scintillator can remove radioactive impurities from the scintillator.– Works best if impurities have higher

affinity for water, e.g., polar species, but can also be effective if not.

• For water extraction, we use LNGS ground water purified by the following:– Reverse osmosis and Ion-exchange (de-

ionization water plant)– Single stage evaporator distillation.

• Ground water has high levels of radioactivity.– ICPMS studies show that 238U, 232Th, 40K

are removed effectively by de-ionization. – 222Rn is high (10,000 Bq/ton), but can

removed by de-gasification with N2.

– Radon daughters 210Pb, 210Bi, and 210Po studied recently (see below).

The water extraction system. N2 stripping column used in series

4/10/2014

Results of 6 cycles of Re-purification

• 85Kr: 30 cpd/100t → < 5 cpd/100t

• 238U (226Ra): [(530 ± 50) → < 8 x 10-20 gU/g 214Bi-214Po Reduction factor > 77 (< 0.8 count/100t/yr).

• 232Th: [(3.8 ± 0.8 → < 1.0] x 10-18 gTh/g 212Bi-212Po Reduction factor > 3. ( < 0.8 count./100t/yr) • 210Bi: 70 cpd/100t → 20 ± 5 cpd/100t

• 210Po: Essentially not reduced!! WHY???

Rates before purifcation are based on 153.6 ton-yr exposure taken in 740.7 d between May 16, 2007 and May 8, 2010. See Borexino Coll. arXiv 1308.0443v1.

Stanford Univsersity Seminar May 7 2014

264/10/2014


Phase 2 Solar Neutrino Results: 2011-2014

• Technical goals:– Reduce scintillator backgrounds with loop

purification ≈ ✓• 210Bi (210Pb) ≈ (incomplete)✓• 85Kr by nitrogen stripping

• Measurement goals– pp neutrino measurement (to be published) – Improve pep, 7Be, 8B measurement (ongoing)

☐– CNO neutrinos detection or new limit (on-

going) ☐4/10/2014


Spectacular Results for U and Th.

• The low levels of U and Th (< 1 c/y/100ton) are quite likely the lowest levels ever achieved in a counting experiment.

• Results show promise for accurate pep neutrino measurement with future deeper detectors, free of cosmogenic 11C.

• Also promising is use of liquid scintillators for neutrino-less double beta decay: Kamland-Zen & SNO+. – Background from 2448 keV 214Bi and 2614 keV 208Tl

gamma rays could be absent for multi-ton detectors.

4/10/2014


Backgrounds before & after Water Extraction + N2 Stripping

Region sensitive to CNO & 210Bi

Before re-purification 2008-2010Rates in parentheses are in cpd/100t.Without 11C cuts. See arXiv1308.0443v1.

After re-purification 2012-2013 (with 11C cuts)

4/10/2014


• Why did we have so much 210Po at start of Borexino in 2007 compared to 210Bi (210Pb)? They should be in secular equilibrium, but they are far from it.– 210Bi initial rate: ~15 cpd/100t (increased later to ~40

)– 210Po initial rate: ~8000 cpd/100t (decaying with t1/2 =

138d)

• Why did water extraction work so well for U and Th, but not as well for Pb, and not at all for Po?”– We have reduced 210Po with water extraction in 1996 CTF

studies, but with 210Po at higher concentrations.

• Is the water used for water extraction really free of radioactivity, especially radon daughters?

4/10/2014

The Mystery of 210Bi and 210Po


The 238U and 232Th Decay Chains

A lower 210Bi rate compared to 210Po implies a chemical separation process that removes Pb more efficiently than Po.4/10/2014


Water Research: 2011-2014

• The puzzling results on 210Po and 210Bi (210Pb) motivated a series of off-line studies at LNGS and Princeton for the past 3 years, eventually producing surprising results for radioactivity in “purified water”.

• Due to limited time, we won’t discuss details today, but summarize results and discuss impact on further purification.

• Designs for an upgrade of the water extraction system will then be described.

4/10/2014


About Borexino Water

• The water is obtained from natural drainage from the rock into the laboratory.

• The water has 222Rn at a concentration of 10,000 Bq/m3, typical for many sites. – Typical concentrations of 210Pb and 210Po are ~ 10 Bq/m3

• Water purification system based on reverse osmosis and ion exchange is used to remove radioactive impurities.– Nitrogen stripping removes 222Rn. – [U], [Th] are very low: < 10-15 g/g by ICPMS.

• The water in then distilled before being used for water extraction.

4/10/2014


Effectiveness of Water ExtractionAnd Radio-purity of Water

• Water extraction of scintillator was very effective in removing the radioactive impurites in the U and Th chains.– This indicates that the water itself had low levels

of these elements. – 238U: Measured by 214Bi-214Po delayed coincidences.

• The results indicate low concentration in water of elements above 214Bi in the U decay chain, in particular 226Ra, which feeds the A=214 isotopes.

– As an alkaline earth, 226Ra should be removed well by ion exchange processes. As example, Mg reduction is ~106.

4/10/2014


Water Discoveries

1. Water purification systems based on reverse osmosis and ion exchange do a poor job of removing 210Pb and 210Po.– Measured reductions of ~1000 (Pb) and ~10 (Po).

• Methods: ICPMS on 208Pb and alpha counting on 210Po.

– Results imply activities > mBq/m3 before distillation.

2. Polonium in ground water is processed biologically by micro-organism into a volatile dimethyl polonium compound.– Recent publications were found by student Brooke Russell. – With an estimated boiling point of 138 C, 210Po is difficult to

remove from water by single stage evaporator distillation.

4/10/2014


Test of Small Fractional Distillation System to Remove 210Po from Water.

• Small-scale fractional distillation system designed and tested at Princeton to remove dimethyl polonium from well water.– 6-foot tall column with structured

packing and high reflux designed for x1000 reduction.

• Princeton well water has x5 more 210Po than LNGS.

• Results:– Operated as simple evaporator,

high 210Po in product.– Operated with high-reflux column,

no 210Po in product.– Reduction factor > 300.

Prototype distillation system with two 6-foot columns: 2 l/hr capacity.Students : B.Russell, C. Aurup, W. Taylor

4/10/2014


Credit goes to Brooke and Team

4/10/2014

Brooke Russell ‘11found paperson volatile Po& directed team of rising seniorsand technicalspecialist Allan to develop andtest a newdistillation systemthat workedbeautifully to remove 210Po.

For a Professorit doesn’t get better than that!

Will Taylor

Brooke RussellAllan Nelson

Christian Aurup


Implications of Water Discoveries

• Due to limited effectiveness of reverse osmosis, ion exchange, and distillation, the water used for water extraction had relatively high levels of 210Pb and 210Po.

• Distillation removes most of the 210Pb, but leaves significant 210Po in the water.

4/10/2014


210Pb and 210Po Mysteries Solved!

1. The initial high level of 210Po with low level of 210Pb(210Bi) is due to water extraction of PPO.– Water extraction and distillation were used to

purify the PPO master solution• Water extraction introduced 210Po and 210Pb.• Distillation removed 210Pb, but not 210Po due to volatile

dimethyl polonium compound.• Predicted 210Po rate agrees roughly with what we

observed.

2. Failure of water extraction to reduce 210Po in scintillator.– This is due to 210Po in water at roughly same level

as lowest concentration in scintillator achieved.

4/10/2014


Moving Forward to Phase 3…

• Reduce 210Pb and 210Po radioactivity in water used for water extraction.– Fractional distillation system was

designed and tested for removing 210Po from well water.

• Upgrades to existing system have been designed and changes are modest.– Two new columns will be added to

existing evaporators.4/10/2014

Borexino Water Extraction SystemsCurrent & Proposed Upgrade with 2 Fractional Distillation Columns


Make-up water system not shown.

ProposedSystem

PresentSystem

Make-up water system not shown.

Proposed SystemCurrent

System

4/10/2014


Phase 3 Neutrino Goals2015-20??

• The Solar Metallicity:– Measurement of CNO neutrinos (±10%??) – Accurate measurement of 7Be neutrinos (±3%?) – The 750 keV mono-energetic 51Cr neutrino source for SOX

will provide good energy calibration for measurement of the 862 keV 7Be neutrino, reducing a major systematic error.

• Sterile Neutrino Searches: The “SOX” Experiment.– Intense neutrino sources will be placed in tunnel under

Borexino to search for short-baseline neutrino oscillations.– 10 MCi 51Cr 750 keV mono-energetic neutrinos and 100 kCi

144Ce anti-neutrino source (Emax = 3 MeV) to be used.

4/10/2014


Standard Solar Model Fluxes

Accurate measurements of CNO (+/-10%) and 7Be(+/- 3%) neutrinos, coupled with improved nuclear reaction cross sections, especially 3He( ,a g)7Be (LUNA), will probe metallicity in Sun’s core.

4/10/2014

Note 10% difference in 7Be rates

Note ~30% difference but 15% errors.

Table from Haxton, et alAnn. Rev. Astron. Astrophys. 2013


The Solar Metallicity Problem

• In 1998 the metallicity (abundance of elements heavier than 4He) determined from line spectra in Sun’s atmosphere agreed well with other data.– Standard solar model based on uniform composition.– Helioseismology data– Solar neutrino data (8B by SNO)

• Improvements were made in the analysis of solar atmospheric spectra over next 10 years (3D model, etc.)– A 2009 assessment of data resulted in a lower metallicity.

• Z /X = metal/hydrogen ratio = 0.024 (GS98) 0.018 (AGSS09).

– The new results are in conflict with helio-seismic data that probe the Sun’s composition at greater depths.

• This is a serious problem for stellar models because it implies that the chemical composition is not uniform.– New models involving accretion and planetary formation by

Haxton et. al.4/10/2014


Short distance ne Oscillations with Borexino (SOX)

4/10/2014


Sterile Neutrino Search Sources: 51Cr and 144Ce-144Pr

4/10/2014


Conclusions• Continuing reduction of backgrounds toward ~1

event/100 tons/yr opens new opportunities for future research.– New solar neutrino measurements will improve oscillation data

in the vacuum to matter-effect transition.– pep neutrinos will be improved, but would be easier to measure

with high accuracy at SNOlab.– Precision 7Be (3%) and CNO (10%) may resolve the metallicity

problem. Deeper site is better for CNO.– Low backgrounds demonstrate possibilities in other rare-event

experiments, such as searches for dark matter or neutrino-less double beta decay.

• Ultra-low background is possible, but requires persistence, investment in big detectors, and time to acquire data. – Demonstrating 1 c/yr/100t requires a 100-ton detector and >1

yr

4/10/2014

Stanford Univsersity Seminar May 7 2014 484/10/2014

The End

Documents

Solar Neutrinos with Borexino