New methodologies and techniques for 234 Th analysis Future Applications of 234 Th in Aquatic Ecosystems Woods Hole, August 2004

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

followed by ion exchange separation Bhat et al., 1969Anderson and Fleer, 1982 alfa 230 Th beta 234 Th filtration Ion exchange plating Fe(OH) 3 precipitation U/Th separation 230 Th spike digestion Fe(OH) 3 precipitation Diss Part

Moore and Reid, 1973 Isotope ratios Bacon and Anderson, Th yield tracer MnO 2 impregnated fibre acrylic polypropylene

In-situ pumps Two cartridges Mann et al., 1984 in series Livingston and Cochran, 1987all Th,Ra,Ac, transuranics Ashing leaching Efficiency = 1 – MnB ____ MnA

Soxhlet leaching Hanfland et al.

Non-destructive techniques Buesseler et al., 1992 MnO 2 impregnated acrylic fibre Gamma detector efficiency (63 keV) Melt/ crush 5.3 % Gamma count < 1 % ash 16.1 % Gamma branching ratio: 63 keV93 keV 3.8 %5.4 %

techniqueFe(OH) 3 precipitation ion exchange MnO 2 cartridges ion exchange MnO 2 cartridges non-destructive gamma counting referencesBhat et al., 1969 Anderson and Fleer, 1982 Moore and Reid, 1973 Mann et al., 1984 Livingston and Cochran 1987 Buesseler et al., 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 234 Th techniques by 1992

Beta counting 234 Th filtration MnO 2 precipitation U/Th separation no spike no digestion second filtrationDiss Part Mn 7+ + Mn 2+  MnO 2 MnO 2 precipitation direct beta counting 2MnO Mn H 2 O  5MnO 2 + 4H + RvdL & Moore, 1999

SD 3.2 % SD 2.4 % Precision Deep-water calibration of 20-L method

accuracy On-board Duplicate 20-L samples, acidified and stored 6 months before analysis Surface water samples

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

processSample size (kg) Counting techniqueSD (%) 1-  counting error N NELHA-600m Mn ppt20direct beta Mn ppt2direct beta Mn cart550gamma Fe ppt20direct beta Fe ppt20Chemistry-beta HOT-3500m Mn ppt2direct beta Southern Ocean >250m Mn ppt20direct beta Mn ppt5direct beta Comparison of techniques for total 234 Th Buesseler et al., 2001

10-L Fe(OH) 3 ppt 229 Th Th Coppola et al., L MnO 2 ppt 230 Th Th Pike et al Savoye et al Return of the yield tracer Pike et al uncorrected yield-corrected

Small-volume techniques Still to be worked out: RvdL & Moore L Buesseler et al L Benitez- Nelson et al L Pike et al., L MnO 2 concentration (mg/L) reaction time (hour)

Problems specific to direct beta measurements High self-absorption of thick sources Interference by other beta emitters

Beta detector source with self-absorption I(L) = I(0). (1-e -  L )/  L 0 L Z (cm) absorber 30 mg/cm 2 I(L) = I(0). e -  L absorber I = I(0) Correction required for variable  L But:  ( 234 Th) >>  ( 234m Pa) __ > eliminate  234 Th betas correct only for  ( 234m Pa) 238 U 234 Th 234m Pa 234 U 0.27 MeV 2.19 MeV   Thick beta sources

Beta detector source with self-absorption absorber 30 mg/cm 2 Decayed source with self-absorption absorber 30 mg/cm 2 Step 1: measure filter Step 2: repeat after decay Step 3: measure spike Decayed source with self-absorption Beta detector Step 4: measure  L by sample always use absorber to stop weak betas Determination of self-absorption coefficient  Thick filter measuring procedure

SPIKES Th-232U-238U MeV 2.1 MeV 2.3 MeV 1.0 MeV 3.3 MeV 1.1 MeV 1.4 MeV 6.8 MeV 1.4 MeV alfa beta Beta energies > 1.0 MeV Low Medium High Decay modeParticle reactivity 2.3 MeV 0.27 MeV Interference by other betas

Interference of 226 Ra daughters Benitez-Nelson et al., 2001 recount after 234 Th decay background in open ocean samples approx. 4% 222 Rn vented from counter

Low 234 Th High particle loads – thick filters No equilibrium at depth Other interferences ( 232 Th series) Humics, Colloids MnO 2 ppt /direct beta method of limited value Coastal and fresh waters

200-L method for fresh water Waples et al., 2003 Interference of other Th isotopes Total Th beta count Th-234 Th Th-232 Th-228 Th-229

Cerenkov radiation Cerenkov threshold for betas : MeV 225 photons are produced for every cm When v > c_nc_n

Cerenkov photons produced E max (MeV) Activity (Bq m -3 ) Max # of photons per electron 234 Th m Pa K Bi3.27up to Probability distribution of # of photons produced per decay of 40 K

Separation of 40 K and 234 Th by photon number Coincidence counting 0 # of photons Th 40 K 40 K x 3 countrate (cpm) Distribution of # of photons per event  Too low optical yield

Moore, 1990 Ideas for automation

2004 automation AWI and ISITEC

Claudia asked: What is the best way to convert these measurements into integrated 234 Th deficiencies? Can we assess and reduce the uncertainty in making this conversion? Integrated deficiency Discussion initiated by Moran et al., 2003

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

Current best methods DeficiencyParticulate Open Ocean Small-volume ev. Spike automation Large-volume-filtration + gamma 20-L + beta (note interferences) size-fractionation + Fresh water200-L, Waples et al., 2003 coastalDepends on salinity, particle loads, humics

summary Direct beta counting simplifies 234 Th 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

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/ 234 Th ratio in exported material

IDtypeprocessSample size (kg) Counting technique SD (%) 1-  counting error N NELHA-600m 20-LfiltrateMn ppt20direct beta-up LfiltrateMn ppt20direct beta-down LfiltrateMn ppt2direct beta Fe pptfiltrateFe ppt20direct beta LtotalMn ppt20direct beta LtotalMn ppt2direct beta Mn carttotalMn cart550gamma Fe ppttotalFe ppt20direct beta Fe ppttotalFe ppt20Chemistry-beta Particles> 1 umNuclepore20direct beta Particles> 1 umNuclepore20Chemistry-beta HOT-3500m 2-LTotalMn ppt2direct beta Southern Ocean >250m 20-LTotalMn ppt20direct beta LTotalMn ppt5direct beta

Trapezoidal integration

Other beta emitters E max (MeV) 234 Th m Pa Pb Bi Pb Bi U series 235 U series 232 Th series E max (MeV) 231 Th Ac Pb Bi6.75 E max (MeV) 228 Ra Ac Pb Bi2.25

 = v/c cos  = 1/  n Cerenkov radiation

Coincidence counting set-up