The role of Po-210 and Pb-210 in Low Radioactivity Experiments and Ultrapure Water

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Marco G. GiammarchiIstituto Nazionale di Fisica NucleareVia Celoria 16 – 20133 Milano (Italy)

marco.giammarchi@mi.infn.ithttp://pcgiammarchi.mi.infn.it/giammarchi/

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• Po and Pb Radioisotopes

• Low Radioactivity Experiments

• Role of Po and Pb Isotopes

• Ultrapure Water

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Po and Pb RadioIsotopes

They are located at the end of U,Th radioactive chains

Uranium

Thorium

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The Actinium and Neptunium series(a less important role in experiments)

Uranium 235

Neptunium

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POLONIUM Natural abundance

Half-life Decay Mode and Energy (MeV)

Po-209 trace 105 y α (4.976) and γ

Po-210 trace 138.4 d α(5.407) and γ

Po-211 trace 0.52 s α(7.594) and γ

Po-216 trace 0.15 s α(6.906)

Po-218 trace 3.11 m α(6.114)

LEAD Natural abundance

Half-life Decay Mode and Energy (MeV)

Pb-204 1.4% stable

Pb-205 trace 1.51 107

yEC(0.052) and γ

Pb-206 24.1% stable

Pb-208 52.4% stable

Pb-210 trace 22.3 y β- (0.063) 81%β-(0.061) 19% and γ

Pb-214 trace 26.8 h β-(1.032) 48%β-(0.73) 42% and γ

A sort of summary of abundant isotopes (incomplete)

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Low Radioactivity Experiments

Particle & NuclearPhysics

Astrophysics&

Cosmology

AstroparticlePhysics

Research topics:

Solar Neutrinos Double Beta Decay Proton Stability Geoneutrinos Supernovae detection ….

Physics experiments in which the signal searched for in swamped in a very high (dominant) background

Impact on fundamental physics (Weak Interactions, Neutrino Oscillation, Standard Model)

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1700 m of rock to shield against cosmic rays

3 Experimental Halls, each the size of a football field

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Borexino Detector

Water Tank: and n shield water Č detector208 PMTs in water2100 m3

20 steel legsCarbon steel plates

Scintillator:270 t PC+PPO (1.5 g/l)

in a 150 m thick inner nylon vessel (R = 4.25 m)

Stainless Steel Sphere:

R = 6.75 m2212 PMTs 1350 m3

Outer nylon vessel:R = 5.50 m(222Rn barrier)

Buffer region:PC+DMP quencher (5 g/l)

4.25 m < R < 6.75 m

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The Borexino Detector for Solar Neutrino Physics at Gran Sasso

eLieBe 77

),,( exee xx

Eν = 862 keV (monoenergetic)ΦSSM = 4.8 · 109 ν s-1 cm2

Cross Section 10-44 cm2 (@ 1 MeV)

Electron recoil spectrum

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A tribute to the main author of the Borexino Proposal

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What is the matter with a Low Background Experiment?

SIGNAL

• 50 events/day in 300 tonnes of liquid scintillator in Borexino (Be-7 signal)

BACKGROUNDS

• Cosmic Rays (even undeground!) will need to be reduced by a factor ≈ 103

• Radioactivity of Materials. For U,Th, K normal concentrations, say as an example, 10-11 g/g of 238U

dg

g

dN

Tdt

UdN/103

238

106103

1064.1

693.0693.0)238( 6233

122/1

2383/23810300 11 UmgggUt

Signal to Noise ratio can be as low aso 50 / 3x106 !!

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Role of Po and Pb Isotopes

In the U-238 chain

Po-218: 6.0 MeV, T(1/2)=3.10 min, α decay. The best Rn-222 tracer (after Rn-222 itself)

Po-214: 7.7 MeV α decay. With a half-life of 1.64x10-4 s, it is the basis of the famous Bi-Po tagging (delayed coincidence of 214 isotopes)

Po-210: 5.3 MeV α decay out of equilibrium with the rest of the chain.Half-life of 138.4 days (can move in materials)

Pb-214: A Rn daughter with a 1.024 MeV, 26.8 min beta+gamma to Bi-214.

Pb-210: 0.22 MeV β+γ 22.3 yrs lifetime. Build-up isotope!

Pb-206: stable. Will end the U-238 chain.

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In the Th-232 chain

Po-216: 6.8 MeV, T(1/2)=0.15 s, α decay. The best Rn-220 (Thoron) tracer

Po-212: 1.12 MeV α decay. With a half-life of 3x10-7 s, it is the basis of the famous Bi-Po tagging (delayed coincidence of 212 isotopes)

Pb-212: A Rn daughter with a T(1/2)=10.6 hr min beta+gamma decay to Bi-212.

Pb-208: stable. Will end the Th-232 chain.

What does it mean in a large scale Low Background Experiment?

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Po-210 alpha decay

Borexino single photoelectron spectrum to search for Solar Neutrinos

Not in equilibrium with Rn-222

Concentration of Po-210 in our scintillatorbefore the analysis cuts

62/1 10000,50

693.0

4.138

693.0

dd

dt

dNTN

ggPoP 16246 103106.121010)210( 248

16

10103

103

g

g

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Bi-210 decays

Concentration of Pb-210 in our scintillator atoms

dyr

dt

dNTN 500

23

693.0

3.22

693.02/1

E cannot measure by analytic methods. We can measure by counting.But in doing this we face all radioactivity components

Pb-210 and Bi-210 background

KamLAND SNO+

1000tRunning since 2002.

300tRunning since 2007.

1000tStarting in 2012

700tRunning since 2004.

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Single volume liquid Single volume liquid scintillatorscintillator detectorsdetectors

Min

iBO

ON

EB

orex

ino

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Ultrapure Water

Widely used in Low Radioactivity Experiments:

•Cheap

•Widely available

•Flexible shielding

•Can be purified

•Can be used for cleaning

Purification processes to reach the lowest level of contamination possible

Borexino Main Water Purification Plant

F141

DegassifierLiqui-Cel

PreF140

RO1

RO2 EDI StrippingWhite Tank

POU

F1560.02 μm

Filtering

Ion removal

Reverse Osmosis

Degassing

Ultra-Q

3000 l/h 1500 l/h

1000 l/h

G154

F1530.1 μm

G159

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Water in Water out

U-238 ~ 10-3 3 x 10-7

Ra-226 ~ 0.3 < 1 x 10-6

Rn-222 ~ 10 < 3.4 x 10-6

Th-232 ~ 10-3 3 x 10-7

K-40 ~ 10-3 < 1.6 x 10-6

Summary of the Water Purification System performance (Bq/kg)

Pb-208 measured as a tracer with hi-res mass spectrometry

Unfortunately, no real clue on Pb or Po isotopes

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Conclusions

• Po, Pb (Bi) radiosotopes are important as a limiting background to Low Radioactivity Experiments

• We do not understand Po,Pb radioisotopes in (e.g.) scintillators

• Limited information on Po,Pb radioisotopes in (e.g.) scintillators

• We know nothing about Po,Pb leftover in Ultrapure Water

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Thank you for your attention

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Discrimination between alphas and beta decays:

Discrimination between alpha and beta decays in scintillators (PSD)

A technique to discard alpha decays in the scintillator and extract the signal