New methodologies and techniques for 234Th analysis

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New methodologies and techniques for 234Th
analysis

• Future Applications of 234Th in Aquatic
  Ecosystems
• Woods Hole, August 2004

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Talk outline

• Classical techniques
• New developments
• Marine vs Fresh water
• Alternative approaches, Automation
• Calculation of deficiency
• Outlook

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Fe(OH)3 precipitation
followed by ion exchange separation Bhat et al.,
1969 Anderson and Fleer, 1982
U/Th separation
230Th spike
Fe(OH)3 precipitation
Diss
plating
digestion
Part
Ion exchange
filtration
beta 234Th
alfa 230Th
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MnO2 impregnated fibre
Moore and Reid, 1973
acrylic
polypropylene
Isotope ratios Bacon and Anderson, 1982 234Th
yield tracer
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In-situ pumps
Two cartridges Mann et al., 1984 in series
Livingston and Cochran, 1987 all Th,Ra,Ac,
transuranics

• Ashing
• leaching

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Soxhlet leaching
Hanfland et al.
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MnO2 impregnated acrylic fibre
Non-destructive techniques Buesseler et al., 1992
Gamma branching ratio 63 keV 93 keV 3.8 5.4

Gamma detector efficiency (63 keV)
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234Th techniques by 1992
technique Fe(OH)3 precipitation ion exchange MnO2 cartridges ion exchange MnO2 cartridges non-destructive gamma counting
references Bhat et al., 1969 Anderson and Fleer, 1982 Moore and Reid, 1973 Mann et al., 1984 Livingston and Cochran 1987 Buesseler et al., 1992
reliable well-defined no interferences Spin-off of multi-tracer studies No chemical treatments
- - - lengthy procedure, digestion and ion- exchange on board (1988 Polarstern) Large volumes Ship time ashing /leaching Needs same cartridge efficiency Large volumes Ship time Needs same cartridge efficiency

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MnO2 precipitationdirect beta counting
Mn7 Mn2 ? MnO2
2MnO4- 3Mn2 2H2O ? 5MnO2 4H
U/Th separation
no spike
MnO2 precipitation
second filtration
Diss
Beta counting 234Th
no digestion
Part
filtration
RvdL Moore, 1999
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Precision
Deep-water calibration of 20-L method
SD 2.4
SD 3.2
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accuracy
Surface water samples
On-board
Duplicate 20-L samples, acidified and stored 6
months before analysis
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Small-volume techniques
20-L is too much for a Rosette cast Counting
statistics are not limiting Reduction to 5, 4
and 2 liter versions
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Comparison of techniques for total 234Th
process Sample size (kg) Counting technique SD () 1-s counting error N
NELHA-600m NELHA-600m NELHA-600m NELHA-600m NELHA-600m NELHA-600m
Mn ppt 20 direct beta 9.8 2.6 18
Mn ppt 2 direct beta 5.6 3.4 4
Mn cart 550 gamma 10.9 3.5 7
Fe ppt 20 direct beta 15.5 3.9 6
Fe ppt 20 Chemistry-beta 5.1 4.1 2
HOT-3500m HOT-3500m HOT-3500m HOT-3500m HOT-3500m HOT-3500m
Mn ppt 2 direct beta 7.4 3.0 26
Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m
Mn ppt 20 direct beta 3.2 1.1 10
Mn ppt 5 direct beta 3.2 1.9 18
Buesseler et al., 2001
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Return of the yield tracer
10-L Fe(OH)3 ppt 229Th 232Th Coppola et al.,
2002 4-L MnO2 ppt 230Th 229Th Pike et al.
2004 Savoye et al. 2004
uncorrected
yield-corrected
Pike et al. 2004
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Small-volume techniques
Still to be worked out
RvdL Moore 1999 20-L Buesseler et al. 2001 5-L Benitez- Nelson et al. 2001 2-L Pike et al., 2004 4-L
MnO2 concentration (mg/L) 1 0.2 0.1 0.25
reaction time (hour) 8 1 8-12 12
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Problems specific to direct beta measurements
High self-absorption of thick sources Interfere
nce by other beta emitters
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Thick beta sources
Beta detector
I(L) I(0) .  (1-e-aL)/aL
I I(0)
I(L) I(0) .  e-aL
__gteliminate 234Th betas correct only for a
(234mPa)
absorber 30 mg/cm2
source with self-absorption
absorber
Correction required for variable aL But a
(234Th) gtgt a (234mPa)
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Thick filter measuring procedure
Beta detector
always use absorber to stop weak betas
Step 1 measure filter
Step 2 repeat after decay
absorber 30 mg/cm2
source with self-absorption
Decayed source with self-absorption
Determination of self-absorption coefficient a
Beta detector
Step 3 measure spike
absorber 30 mg/cm2
Step 4 measure aL by sample
Decayed source with self-absorption
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Interference by other betas
SPIKES
Th-232
U-238
U-235
2.3 MeV
Decay mode
Particle reactivity
alfa
Beta energies gt 1.0 MeV
Low Medium High
beta
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Interference of 226Ra daughters

• recount after 234Th decay
• background in open ocean samples approx. 4
• 222Rn vented from counter

Benitez-Nelson et al., 2001
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Coastal and fresh waters
Low 234Th High particle loads thick filters No
equilibrium at depth Other interferences (232Th
series) Humics, Colloids MnO2 ppt /direct beta
method of limited value
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Interference of other Th isotopes
200-L method for fresh water Waples et al., 2003
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Cerenkov radiation
c _ n
When v gt
Cerenkov threshold for betas 0.256 MeV 225
photons are produced for every cm
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Cerenkov photons produced
Emax (MeV) Activity (Bq m-3) Max of photons per electron
234Th 0.27 41 2
234mPa 2.27 41 238
40K 1.31 12000 130
214Bi 3.27 up to 3.3 368
Probability distribution of of photons
produced per decay of 40K
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Separation of 40K and 234Th by photon number
Distribution of of photons per event
200
40K x 3
countrate (cpm)
234Th
Coincidence counting
40K
0
0 of photons
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? Too low optical yield
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Ideas for automation
Moore, 1990
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automation
2004
AWI and ISITEC
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Integrated deficiency
Claudia asked What is the best way to convert
these measurements into integrated 234Th
deficiencies? Can we assess and reduce the
uncertainty in making this conversion?
Discussion initiated by Moran et al., 2003
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Integrated deficiency

• 1. Depth of integration
• Bottom of mixed layer?
• Depth of equilibrium?
• Depth of sediment trap?
• Depth of export calculation
• 2. Integration method
• Sampling integration (yo-yo)
• Trapezoidal integration

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Current best methods
Deficiency Particulate
Open Ocean Small-volume ev. Spike automation Large-volume-filtration gamma 20-L beta (note interferences) size-fractionation
Fresh water 200-L, Waples et al., 2003 200-L, Waples et al., 2003
coastal Depends on salinity, particle loads, humics Depends on salinity, particle loads, humics
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summary

• Direct beta counting simplifies 234Th analysis in
  seawater
• for better precision yield measurement at home
• Thick sources corrections required for self
  absorption
• Fresh waters and sediment samples large
  contribution from other beta emitters

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Outlook

• Surface waters Continuous transects in surface
  waters on ships of opportunity
• High-resolution profiles should allow to make
  full budget in water column surface sediment
• Parallel collection of size-fractionated
  particulate fraction for determination of X/234Th
  ratio in exported material

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ID type process Sample size (kg) Counting technique SD () 1-s counting error N
NELHA-600m NELHA-600m NELHA-600m NELHA-600m NELHA-600m NELHA-600m NELHA-600m NELHA-600m
20-L filtrate Mn ppt 20 direct beta-up 7.3 2.8 17
20-L filtrate Mn ppt 20 direct beta-down 7.0 2.8 20
2-L filtrate Mn ppt 2 direct beta 5.3 3.9 8
Fe ppt filtrate Fe ppt 20 direct beta 13.8 4.9 3
20-L total Mn ppt 20 direct beta 9.8 2.6 18
2-L total Mn ppt 2 direct beta 5.6 3.4 4
Mn cart total Mn cart 550 gamma 10.9 3.5 7
Fe ppt total Fe ppt 20 direct beta 15.5 3.9 6
Fe ppt total Fe ppt 20 Chemistry-beta 5.1 4.1 2
Particles gt 1 um Nuclepore 20 direct beta 21.4 4.8 18
Particles gt 1 um Nuclepore 20 Chemistry-beta 7.9 4.2 3
HOT-3500m HOT-3500m HOT-3500m HOT-3500m HOT-3500m HOT-3500m HOT-3500m HOT-3500m
2-L Total Mn ppt 2 direct beta 7.4 3.0 26
Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m Southern Ocean gt250m
20-L Total Mn ppt 20 direct beta 3.2 1.1 10
5-L Total Mn ppt 5 direct beta 3.2 1.9 18
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Trapezoidal integration
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Other beta emitters
238U series
235U series
232Th series
Emax (MeV)
234Th 0.27
234mPa 2.27
214Pb 1.02
214Bi 3.27
210Pb 0.06
210Bi 1.16
Emax (MeV)
231Th 0.39
227Ac 0.045
211Pb 1.37
211Bi 6.75
Emax (MeV)
228Ra 0.05
228Ac 2.13
212Pb 0.57
212Bi 2.25
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Cerenkov radiation
b  v/c cos q   1/ b  n
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Coincidence counting set-up