Transcript Low background radioactivity measurements

Low background radioactivity measurements
Pia Loaiza
Laboratoire Souterrain de Modane CNRS/CEA,
France
 Why do we need ultra-low radioactivity measurements?
 Low background gamma spectrometry:
 How to achieve high sensitivity ?
 Material selection in astroparticle experiments
 Geochemistry applications
 Environmental control
 Other applications
Why do we need measurements of ultra-low
radioactivity levels ?
 Dark matter and 0 experiments : natural radioactivity induces background in
rare event searches experiments  need to reduce drastically the radioactive
background by material selection
n
Source

Shielding
• External gamma radiation,
neutrons
close mat.
Det.
e-
Rn
U,Th,K
(n,,,)
Reduction
Shielding
Injection of
• Rn and its progenies
Radon-reduced
air
• Radioimpurities in shielding
materials
• Radioimpurities in materials
close to detectors
material
screening
• Contaminants in detector itself
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How low is ‘low’ ?
 ROCK in the Laboratoire
Souterrain de Modane:
238U : (10.4  2.5 ) Bq/kg
232Th : (9.96  0.82) Bq/kg
 CABLES in Edelweiss II:
226Ra : (10  7) mBq/kg
228Th < 6 mBq/kg
 COPPER in Edelweiss II:
226Ra : < 40 Bq/kg
228Th : (24 +/- 12) Bq/kg
‘normal’ levels ~10 Bq/kg
1,4 kg of cables ~10 mBq/kg will cause
0.7 neutron/ kg Ge/ year [20-200] keV,
(gamma background shielded by Pb)
 on the limit of acceptable levels
~10-100 Bq/kg is OK
Edelweiss II, NEMO3 materials screened with a sensitivity about 1 mBq/kg
(1 Bq= 1 disintegration/s)
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Present and future
Gamma background in EDWII:
200 evts/kg Ge/day [20-3000] keV
80 evts/kg Ge/day [20-200] keV
EURECA will need Cu with 226Ra, 228Th ~ 20 Bq/kg to reach dark matter
sensitivity goal
SuperNEMO needs 0 sources : 208Tl < 2 Bq/kg & 214Bi < 10 Bq/kg
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Mass spectrometry, Neutron Activation Analysis, Alpha-spectrometry
ICP-MS ~ 0.01 ppb U/Th (about 0.1 mBq/kg)
238U
decay chain
HOW TO MEASURE?
• Mass spectrometry
• Neutron Activation Analysis
• Alpha-spectrometry
• Gamma spectrometry
Sub-chains
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Mass spectrometry, Neutron Activation Analysis, Alpha spectrometry
Gamma emitters
232Th
decay chain
Sub-chains
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Low background Ge detectors for gamma-ray spectrometry
Detection Limit 
Backg. R
 . M. I. t
R = resolution
 = efficiency
I = intensity of the line
M = sample mass
t = time of measurement
To improve sensitivity  BACKGROUND REDUCTION
 Cosmic rays reduction: go underground
 Environmental gamma reduction: shielding
 Intrinsic background reduction: material selection
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Background sources in Ge gamma-ray spectrometry
• Muons on surface
GO UNDERGROUND!
Background of HPGe spectrometer:
2 106 muons/m2 day on surface
15 m.w.e
15 m.w.e + shielding
3300 m.w.e + shielding
26 muons/m2 day at 3300 m.w.e
(Applied Rad and Isotopes 53 (2000) 191)
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Background sources in Ge gamma spectrometry
deep underground
SOURCE
REDUCTION
 External gamma radiation (up to 2.6 MeV 208Tl)
Shielding
 Radioimpurities in cryostat
Material selection
neutron
 Rn and its progenies
Rn reduced air
 Radioimpurities in the shielding materials
Material selection

DET.
-
210Bi (210Pb)
e511 keV
Rn
Pb
e+
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Ge detector types
COAXIAL
WELL
high sample mass
high efficiency
PLANAR
high resolution
at low energies
The choice depends on what we want to measure
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Gamma-ray spectrometry at LSM
13 HPGe detectors for:
• Material screening for
SuperNEMO, Edelweiss and ultra
low background instrumentation
(coaxial, planar)
• Environmental studies
(well type, planar)
• Environmental monitoring
(well type)
Developpement of low background Ge for -spectromety:
 Planar, P. Loaiza et al, NIM A 634 (2011) 64–70
 Coaxial (arrived 2011)
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Where do we stand in terms of sensibility?
Selected results of radioactivity measurements for material selection:
Material
Detector
Mass
(g)
Time
(h)
210Pb
234Th(238U
226Ra
228Ra
228Th
(mBq/kg)
)
(mBq/kg)
(mBq/kg)
(mBq/kg)
(mBq/kg)
Aluminium
Mafalda
(Planar)
1025
132
<9
<3
<0.9
<1
1.00.3
Epoxy
Mafalda
(Planar)
47
384
357
143
92
<6
103
Glue
Iris
(Coaxial)
2500
768
< 0.135
< 0.274
< 0.174
Copper
GeMPI2
(coaxial)
42500
564
<0.04
<0.06
0.020.01
Low energies:
46 keV, 63 keV, 92 keV
Higher energies:
200 keV < E < 3000 keV
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Geodynamic studies
Activité (Bq/kg)
Studies in lake sediments use radionucleide profiles to date and obtain the
sedimentation rates. The requirement on sensitivity is less stringent than those
for material selection, but still need low background detectors.
137Cs
1986 Site B,core 2 Cs1963
Lac du Bourget
Nuclear weapons 241Am
Tchernobyl
8
250
tests
200
6
Dating using artificial radionucleides
150
137Cs and 241Am
4
100
2
50
0
0
0
5
10
15
20
25
30
35
40
Prof (cm)
y = 186e-0,075x
R2 = 0,95
210Pb P2
1000
210Pb
excess is used to determine
the sedimentation rate
( in this case 3.9 mm/year)
210Pbexc
V= 3,9 mm/an
100
10
1
0
5
10
15
20
25
30
35
40
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Geodynamic studies in the southern hemisphere
Kastner et al, Global and Planetary
Change 72 (2010) 201-214
Lago del Desierto
• Lakes Puyehue and Icalma :
F. Arnaud et al, Science of the Total
Environment 366 (2006) 837-850
• Chile and Peru:
Muñoz et al, Deep-Sea Research II 51
(2004) 2523-2541
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Environmental monitoring
Monitoring of radioactive contamination in the atmosphere
Measurements of artificial radionuclides in certain samples require
very low backgrounds.
7Be
and 137Cs concentration in the atmosphere
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Other applications
Material selection for
integrated circuits
Bordeaux wine dating
The concentration of 137Cs provides a
simple method to estimate the wine age.
Philippe Hubert,
Europhysics News (2005) Vol. 36 No. 1
Atmospheric neutrons
and on-chip radioactive
impurities ( -particle emitters),
induce soft-errors
in the semiconductors
Material selection using
high sensitive gammaspectrometres is being
explored
G. Warot, P. Loaiza
REE 3, Mars 2010, 51
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SUMMARY
 Ultra-low radioactivity measurements are needed for material selection in rare
event searches, like 0 experiments and dark matter
Required levels today ~ 1 - 0.1 mBq/kg
Required levels in future ~ 10 Bq/kg
 Method of measurement depends on the radioelement: Mass spectrometry
(ICP-MS) for long-lived isotopes, gamma-ray spectrometry for gamma-ray emitters
 To improve sensitivity in Ge for -ray spectrometry background must be reduced:
- Cosmic rays reduction: go underground
- Environmental gamma reduction: shielding
- Intrinsic background reduction: material selection
 Low-background gamma-ray spectrometry used in several fields: astroparticle
physics, geodynamic studies, environmental monitoring, …
 Low background Ge for gamma-spectrometry is an ideal tool to be placed in a
young underground laboratory: needed for rare-event searches but also may be used
for several applications. Low cost.
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Costs
 Detector with dedicated low-background developpement:
between 100 kEuro and 200 kEuro, depending on crystal mass,
cooling system,…
 Shielding: Archeological lead: about 200-250 Euros/kg,
Low activity lead: about 2 Euros/kg
Lead casting: around 20 kEuros
 Commercial acquisition system (hardware + software) : about 10 kEuro
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Which sensitivities for the future experiments?
EURECA: • Present  rejection factor ~ 105
• According to simulations: ~105 evts/year in 10 keV<E<50 keV in 1000 kg
of Ge from Cu 226Ra, 228Th : 20 Bq/kg
SuperNEMO
40 mBq/kg in 214Bi
3 mBq/kg in 228Th
needed for PMTS
The necessary sensitivity levels are reached,
but time-consuming measurements needed
need more detectors
further reduce background?
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Performances Planar
Resolution: 850 eV at 122 keV
Integral count rate 20 keV <E < 1500 keV :
150 cpd
All peak-rates < 1 c/day, except 210Pb
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Bruit de fond intégral de quelques détecteurs Ge pour la spectrométrie gamma, divisé par la
masse du cristal, en fonction de la profondeur des différents sites souterrains. Le détecteur
‘LSCE’ est de type puits et installé au Laboratoire Souterrain de Modane, ‘JRC-IRMM’
correspond à un détecteur de type coaxial installé au Laboratoire Hades, en Belgique, ‘LNGS’
correspond au taux d’un détecteur coaxial appartenant au groupe du Max Planck Institute de
Heidelberg, installé au Gran Sasso.
La composante cosmique ne contribue pas au bruit de fond au IRMM : le taux intégral
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des détecteurs IRMM est comparable à ceux de sites plus profonds
Radionuclides in the U and Th
decay series are useful
chronometers for the
determination of many processes
in the environment. The low
natural radioactivity encountered
necessitate instrumentation
capable of measuring very low
radionuclide concentrations.
Some applications :
Quantitative evaluation of both
horizontal and vertical mixing rates
in the open ocean.
Determination of the rate of
particle deposition on the marine
sediment layer (originated by both
biological and physical processes).
The decay of 210Pb provides a
dating method which has been
applied to lake sediments.
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