6 - Kr analysis

,

DEVELOPMENT OF A NEW FACILITY FOR DATING OLD GROUNDWATERS BY USING 81Kr

Bernard Lavielle1 (lavielle@cenbg.in2p3.fr), 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