Diapositive 1

1
Low background gamma spectrometry at the
Laboratoire Souterrain de Modane
The LSM laboratory HPGe detectors development at
LSM LSM HPGe detectorss performance Costs Futu
re needs in gamma spectrometry
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Laboratoire Souterrain de Modane
Deepest underground laboratory in Europe 4800
m.w.e 2nd deepest underground in the world
Located in the Fréjus tunnel at the
french-italian border
Modane
Operated by CNRS and CEA
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Total Vol 3500 m3
Ge room 70 m2 2 secondaries hall of 18 m2
and 21m2
Main hall 30m x 10 m x 10m
Muon Flux 4.2 ? m-2 d-1
(5.14 0.39) 10-9 cm-2 s-1
NIM A262 (1987) 463
Neutron flux (3.57?0.05stat?0.27sys )x 10-6 n
cm-2 s-1 thermal (1.1 ?0.1stat) x 10-6 n cm-2
s-1 E gt 1 MeV
subm. To J. Phys. G Nucl. Phys.
Radon concentration
Astrop. Phys. 9 (1998) 163
5 to 15 Bq/m3
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Radon reduction facility
Principle
HPGes
Output
Same as Kamioka system
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HPGe Gamma-spectrometers at LSM
13 HPGe detectors for

• Material screening for EDELWEISS, SuperNEMO and
  ultra low background instrumentation.

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Environmental studies

• Measurement of environmental radioactivity
  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.

Monitoring of radioactive
contamination in the atmosphere Measurements of
artificial radionuclides in certain samples
require very low backgrounds. Those measurements
are carried out at the LSM.
7Be and 137Cs concentration in the atmosphere
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HPGes internal background
Detector Type Volume Total and peak background rate (counts/day) Total and peak background rate (counts/day) Total and peak background rate (counts/day) Total and peak background rate (counts/day)
Detector Type Volume 40-2700 keV 352 keV 583 keV 1461 keV
MONDEUSE well 220 cc 770 4.2 2.7 5
ROUSSETE well 430 cc 692 4.1 2.9 7.2
ABYMES well 980 cc 828 5.6 5.6 5.6
XXL well 844 cc 821 6.8 lt1.8 11.6
HERMINE N 197 cc 313 1.2 1.5 2.3
HELLAZ P 204 cc 515 4.5 0.5 1.4
JASMIN P 380 cc 529 2.0 1.41 1.71
GENTIANE N 215 cc 178 lt 0.21 0.38 0.65
IRIS P 400 cc 282 1.02 1.46 3.01
MAFALDA plan 150 cc 150 lt 0.3 lt 0.3 lt 0.4
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HPGe detectors development Planar HPGe
Collaboration with CANBERRA and CENBG (Centre
dEtudes Nucléaires de Bordeaux Gradignan)
Crystal dimensions ? 80
mm h30 mm
Vol 150 cm3 Mass0.8 kg
Suited for E lt 600 keV Good resolution at low
energies
Pulse-tube
SELECTION OF ALL MATERIALS MODIFIED CONFIGURATION
Pulse-tube controller
We use a pulse-tube to cool down instead of
liquid Nitrogen (for the first time in a HPGe in
LSM)
5 cm archeological lead, 210Pb lt 0.1 Bq/kg 15 cm
Pb standard lead , approx 10-20 Bq/kg
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Performances
Resolution 850 eV at 122 keV Integral count rate
20 keV ltE lt 1500 keV 150 cpd
All peak-rates lt 1 c/day, except 210Pb
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Where do we stand in terms of sensibility?
Example of measurements at LSM
Mafalda detector-planar
Sample Mass (g) Time (h) 210Pb (mBq/kg) 234Th (mBq/kg) 226Ra (mBq/kg) 228Ra (mBq/kg) 228Th (mBq/kg)
Aluminium 1025 132 lt 9 lt3 lt0.9 lt1 1.0?0.3
Epoxy 47 384 35?7 14?3 9?2 lt 6 10?3
IRIS detector-coaxial, measurements by Abdel
Nachab for SuperNEMO coll.
Sample Mass (g) Time (h) 40K (mBq/kg) 137Cs (mBq/kg) 226Ra (µBq/kg) 228Ra (µBq/kg) 228Th (µBq/kg)
GLUE 1 2500 768 lt 1.5 lt 0.1 lt 135 lt 274 lt 174
GLUE 2 2373 869 lt 1.5 lt 0.3 lt 170 lt 274 lt 166
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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
  keVltElt50 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
Specific detector
2 ?Bq/kg 208Tl 10 ?Bq/kg 214Bi
double beta sources
Need to gain around factor 3 in sensibility to
reach 20 ?Bq/kg in Cu Time-consuming measurements
2 to 3 months
need more detectors
further reduction of background
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Sharing knowledge
We know its possible to get a sensitivity of 20
?Bq/kg Our colleagues from Gran Sasso have
obtained
Material Weight days 226Ra ?Bq/kg 228Th ?Bq/kg 40K ?Bq/kg 60Co ?Bq/kg
Copper 125 kg 100.7 lt16 lt19 lt88 lt10
Today - sharing knowledege on good materials -
on clean methods for cutting, shaping
pieces
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Summary

• Long tradition at LSM for development of
  low-background HPGe detectors
• Sensitivities at LSM today 10 mBq/kg 210Pb, 3
  mBq/kg 238U (234Th) planar detector
• 
  200 ?Bq/kg for 226Ra, 228Th coaxial detector
• Sensitvities needed for future experiments,
  namely EURECA 10 ?Bq/kg
• Need further reduction of intrinsic background
• Taking into account the Gran Sasso experience,
  the sensitivities needed are
• reacheable

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Detector-specific radioctivity measurements Exampl
e the BiPo detector for SuperNEMO

• Measurement of 208Tl and 214Bi concentration in
  foil-sources and other thin materials
• Goal measurement of 10 m2 of foil-sources (40
  mg/cm2) in 1 month with sensibility
• Volume concentrations 208Tl lt 2 ?Bq/kg
  214Bi lt 10 ?Bq/kg
• Surface concentrations 208Tl lt 0.1 ?Bq/m2
  214Bi lt 0.5 ?Bq/m2
• Detection of the BiPo cascade ? delayed ?

Plastic scintillateurs
Source foil