The role of Po-210 and Pb-210 in Low
Radioactivity Experiments and Ultrapure Water
Marco G. Giammarchi
Istituto Nazionale di Fisica Nucleare
Via Celoria 16 – 20133 Milano (Italy)
marco.giammarchi@mi.infn.it
http://pcgiammarchi.mi.infn.it/giammarchi/
• 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
Thorium
Uranium
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The Actinium and Neptunium series
(a less important role in experiments)
Neptunium
Uranium 235
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A sort of summary of abundant isotopes (incomplete)
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)
Decay Mode and
Energy (MeV)
LEAD
Natural
abundance
Half-life
Pb-204
1.4%
stable
Pb-205
trace
1.51 107 y
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
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EC(0.052) and γ
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Low Radioactivity Experiments
Particle & Nuclear
Physics
Astroparticle
Physics
Astrophysics
&
Cosmology
Research topics:
 Solar Neutrinos
 Double Beta Decay
 Proton Stability
 Geoneutrinos
 Supernovae detection
 ….
Impact on fundamental
physics (Weak Interactions,
Neutrino Oscillation, Standard
Model)
Physics experiments in which the signal searched for in swamped in a very high
(dominant) background
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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
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 m
2212 PMTs
1350 m3
Buffer region:
PC+DMP quencher (5 g/l)
4.25 m < R < 6.75 m
Water Tank:
 and n shield
 water Č detector
208 PMTs in water
2100 m3
Outer nylon vessel:
R = 5.50 m
(222Rn barrier)
Carbon steel plates
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20 steel legs
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The Borexino Detector for Solar Neutrino Physics at Gran Sasso
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Be  e  7Li  e
Eν = 862 keV (monoenergetic)
ΦSSM = 4.8 · 109 ν s-1 cm2
Electron recoil spectrum
 x  e  x  e ( x  e, , )
Cross Section  10-44 cm2 (@ 1 MeV)
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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
300t 1011 gU 238/ g  3 mg U 238
dN(U 238) 0.693
0.693 3103 g 61023

N
 3106 / d
12
dt
T1/ 2
1.6410 d
238g
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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Borexino single photoelectron
spectrum to search for Solar
Neutrinos
Po-210 alpha decay
Not in equilibrium with Rn-222
Concentration of Po-210 in our scintillator
before the analysis cuts
3 1016 g
 1024
8
3 10 g
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N
T1/ 2 dN 138 .4 50,000

d
10 6
0.693 dt 0.693
d
P ( Po210)  106  2101.6 1024 g  31016 g
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Pb-210 and Bi-210 background
Bi-210 decays
Concentration of Pb-210 in our
scintillator
N
T1/ 2 dN 22.3
23

yr
 500 atoms
0.693 dt 0.693
d
E cannot measure by analytic methods.
We can measure by counting.
But in doing this we face all radioactivity components
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Single volume liquid scintillator detectors
SNO+
Borexino
KamLAND
300t
1000t
Running since
2002.
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MiniBOONE
Running since 2007.
1000t
Starting in 2012
700t
Running since 2004.
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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
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Borexino Main Water Purification Plant
Filtering
Reverse Osmosis
Ion removal
Degassing
Ultra-Q
F153
0.1 μm
Pre
F140
1000 l/h
RO2
EDI
Stripping
White
Tank
F141
3000 l/h
F156
0.02
μm
1500 l/h
G159
RO1
Degassifier
Liqui-Cel
G154
P
O
U
Summary of the Water Purification System performance (Bq/kg)
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
Unfortunately, no real clue on Pb or Po isotopes
Pb-208 measured as a tracer with hi-res mass spectrometry
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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 alpha and beta decays in scintillators (PSD)
A technique to discard alpha decays in the scintillator and extract the signal
Discrimination between
alphas and beta decays:
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