Transcript Pb-210
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)
[email protected]
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 1011 gU 238/ g 3 mg U 238
dN(U 238) 0.693
0.693 3103 g 61023
N
3106 / d
12
dt
T1/ 2
1.6410 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 1016 g
1024
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 2101.6 1024 g 31016 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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