DEVELOPMENT OF A NEW FACILITY FOR DATING OLD GROUNDWATERS BY USING … · 2013-10-02 · 6 - Kr...
Transcript of DEVELOPMENT OF A NEW FACILITY FOR DATING OLD GROUNDWATERS BY USING … · 2013-10-02 · 6 - Kr...
6 - Kr analysis
,
DEVELOPMENT OF A NEW FACILITY FOR DATING OLD GROUNDWATERS BY USING 81Kr
Bernard Lavielle1 ([email protected]), Eric Gilabert1, Bertrand Thomas1, and Véronique Lavastre2, Romain Rebeix3
1Laboratoire de Chimie Nucléaire Analytique et Bioenvironnementale - University Bordeaux I - CNRS 19 Chemin du Solarium - BP 120 - 33 175 Gradignan, FRANCE
2Laboratoire Transferts Lithospériques - University Jean Monnet 42 023 St Etienne, FRANCE 3Laboratoire de Géochimie ISotopique environnementale (GIS CEREGE) UMR 6635, 150 rue Georges Besse,
30035 Nîmes Cedex 1, France
4 - FAKIR : Schematic diagram of the ion source and sample concentrator*
5 - FAKIR : instrument operation
Références : [1] Thonnard N. et al (1987) NIM B29, 398-406. [2] Lehmann B.E. et al (1991) Appl. Geochem. 6, 419-423. [3] Collon P. et al (2000) EPSL 182, 103-113. [4] Sturchio N.C. et al, (2004) Geophys. Res. Lett. 31, L05503 (1-4). [5] Gilmour J.D. et al (1991) Meas. Sci. Technol. 2, 589-595. [6] Gilmour J.D. (1994) Rev. Sci. Instrum. 65(3), 617-625. [7] Collon P. et al, (2004) Ann. Rev. Nucl. Part. Sci, 54, 39-67.
Ion Source
Cold Trap
2
1 3 4
5
6
7
8
10
TOF
9
11
12 12
11
Fundamental
216.60 nm
216.60 nm
216.60 nm 5p[5/2]2
Dating groundwaters and ice : principle and analytical procedure Radioactive Kr isotopes
Key:
1 - thin SS foil (cold spot)
2 - Ir cap
3 - Cu bar
4 - Insulators
5 - re-entrant tube
6 - copper heat wick
7 - spring
8 - thick Ir foil
9 - cold heat
10 - source plates
11 - windows for UV laser beam
12 - windows for IR laser beam
1 - the cold trap secondary vacuum chamber is pumped to a pressure less than 10-7 mbar.
2 - after 3 hours the cold spot reaches a temperature of 20°K
3 - the gas sample is introduced in the FAKIR mass spectrometer
4 - the heating IR laser beam is fired to release a cloud of Kr atoms trapped on the cold spot
5 - 1.5 microsecond later the UV beam crosses the Kr cloud. Formed Kr ions are accelerated into the flight tube and detected with an electron multiplier
10 Hz pulsed Nd:Yag and Dye lasers (from EXEL Technology) supply 8-10 mj of UV photons @ 216.6 nm for resonant Kr ionization
FAKIR instrument : laser IR (1), cryogenic generator (2), cold trap (3), source chamber (4), flight tube (5), EM detector (6), Nd:YAG:(7) and Dye laser (8).
Acknowledgements The CNAB project was financially supported by ANDRA, by the Région Aquitaine (France), by CNRS (Chemistry and SDU-INSU Institutes, GNR FORPRO, ), by the University of Bordeaux 1 and by ANR. We thank particularly Norbert Thonnard (Institute of Rare Isotope Measurement, Knoxville, University of Tennessee) and Jamie Gilmour (SEAES, University of Manchester) for sharing with us their great expertise in the field of RIS-TOF technique.
78Kr 80Kr 82Kr 83Kr 84Kr 86Kr 0.35% 17.3% 57.0% 11.5% 11.6% 2.25%
77Kr 74.4 min
85Kr 10.76 a
81Kr 2.29 105 a
79Kr 35.04 h
81Rb37
81Kr36
81Br35
32 min
2.29 105 a 13 s
β++CE 4.58 h
CE 73%
CE
85Br35
85Kr36
85Rb37
4.48 h 10.76 a
β− 79%
β− 21%
β−
• 97,5% in atmosphere with a constant spatial distribution • Nuclear reaction induced in the atmosphere by cosmic ray protons and neutrons with stable isotopes of Kr : 80Kr(n,γ)81Kr… • other production sources (nuclear industry, natural radioactivity) negligible <10-3
• nuclear industry (U and Pu fission product)… • other production sources are negligible (natural radioactivity , cosmic ray…)
85Kr in atmosphere
t = 1λ81
⋅ln81Kr /Kr
air
81Kr /Kr
water
81KrKr
water
=81KrKr
air
exp(−λ81⋅t)
81Krwater=81Krair ⋅exp(−λ81⋅t)
81Kr (229 000 yrs) • dating old groundwaters • dating old polar ice Age range (105 to 106 yrs )
85Kr (10,76 yrs) • modern groundwaters and ice (<50 yrs) • monitoring air contamination
1 2 4 3 5 6
7 8
50 cm
Old Woman Iron meteorite
80Kr 82Kr
• mass resolution about 400 • detection limit <1000 atoms
Old Woman Kr spectrum • sample size 40 mg • 100 sec integration time
He Ne Ar Kr
Xe
Nd:YAG 1.6 J , 10 Hz
8 mj @ 216,6 nm
2+1 photons ionization
Water Infiltration and gas dissolution:
T°, P, salinity
Conservation of relative abundances of
noble gases
Decay of radionuclides 39Ar, 81Kr, 85Kr(14C, 36Cl…)
Absolute dating of groundwater
residence time : 81Kr
In atmosphere 81Kr/Kr = (5.2 ± 0.4) 10-13
(See Collon et al, [7 ])
Concentration measurements of 81Kr,85Kr require 4 steps : 1) Water sampling and storage 2) Gas extraction from water & Kr Purification and Separation 3) 81Kr and 85Kr isotopic enrichment 4) Analyses using FAKIR mass spectrometer
2 major problems - sentivity of the instrument must be extremely high less than 1200 atoms of 81Kr in modern groundwaters - Isotopic abundance Kr/ 81Kr is about 1013
FAKIR : Facility for Analysis of Krypton Isotopic Ratios
1 - Sampling and storage
Principle of gas extraction: • the 20 L cells is emptied into the 50 L tank • the gas is extracted by nebulization and pushed by He flux and collected on a cold trap • extraction yield 60-85%
Principle of gas purification: • gas chromatography facility designed for 20L of
water • separation of CO2, Ar, Ne, He, O2, N2, CH4
1000 cc gas 0.002 cc Kr
Magnetic sector
Faraday cup
Slits
ion deflector Quadrupoles 2 doublets
sapphires
VG PT2 (modified design)
A Turbo-molecular pump is used for recycling the Kr gas expended in the MS
3 - 81Kr and 85Kr isotopic enrichment 2 - Gas extraction, Kr separation & purification Ion energy : 6 keV
Selection slits
Source FEBIAD designed for high efficiency allowing a gas injection using a turbo-molecular pump
Sapphire with Al layer 78
80
81
82
83
84
86
85
3.7 mm 0.4 mm
Turbo Molecular
pump
Source FEBIAD
81Kr and 85Kr ions are implanted into a thin Al layer on sapphire. Gas is released by vaporizing Al using a laser beam before introducing in FAKIR.
81Kr measurements were first demonstrated by Thonnard et al [1], and Lehmann et al [2] in groundwaters. Dating were performed by Collon et al [3] using AMS and Sturchio et al [4] by using the ATTA technique with large water samples.
* Design after Gilmour et al [5,6]
Mass Resolution : 400
source
Magnetic sector
ESA