Different potential systematic uncertainties involved in 210Pb dating method

Begy R.-Cs.1,2 , Reizer E. 1, Forray F.-L. 3, Simon H. 1, Gabor A.-I. 1,2,

This work is financially supported by PN-II-RU-TE-2012-3-0351 project

1Babeş-Bolyai University, Faculty of Environmental Science and Engineering, 30 Fântânele Street, 400294, Cluj-Napoca, Romania; 2INCDO-INOE 2000, Research Institute for Analytical Instrumentation, ICIA Cluj-Napoca, Romania3Babeş-Bolyai University, Faculty of Geology, M. Koganiceanu Street nr.1 , Cluj-Napoca, Romania;

About us• Nuclear spectrometry laboratory from “Babes-

Bolyai” University (Faculty of Environmental Science and Engineering)

• Team: Begy R.-Cs.(Phd. head), Simon H. (Phd stud.), Reizer E.(Msc. Stud.), Vasilache D. (Phd stud.), Kelemen Sz. (Res. asist)

• Facilities: two HpGe gamma spectrometer (GEM and GMX type) 8 alpha chamber with 900 mm2 PIPS detector, scintillations cell for Rn and Ra measurements etc.

• Task: Pb-210 dating(Starts in 2005 for first time in Romania (in our laboratory)) and Monitoring of Environmental radioactivity (NORM, TENORM)

Aims of this studyFor Pb-210 dating

Determination of Pb-210• Alpha spectrometry (chemical preparation)

• Gamma spectrometry

To analyze the effect of the undisolved (insoluble) materials especially for silicates found in the sediments Estimate the uncertainty caused by the silicatesImproved the chemical separation process

To analyze the variation of the mass attenuation in sediment layers (compared with IAEA reference material)Establish a correction for the self attenuation effect

Study AreaName Location Type

1.Buhăescu 47°35’18,52″N 24°38’ 35,56″E GLACIAL

2.Știol 47°35’ 08,97″N 24°48’ 59,50″E GLACIAL

3.Muced 47°34’26,63″N 24°32’41,64″E GLACIAL

4.Saint Anna 46°07’66″É 25°53’14,52″K VOLCANIC

5.Iacob 45° 07' 49,30'' N 29° 23' 38.80'' DELTAIC

Alpha spectrometry• The total 210Pb can be determine by measuring its daughter

isotope 210Po ( one of the most precise method)is generally based on various acids leachings procedures.

EPA -David N.Edgington, 1975

Leaching with mineral acids, partial digestion

without HF

Wojciech Tylmann, 2013Rolf Aalto, 2012Begy et al. 2015

digestion totality of sediment samples using HF

Negative effect •their difficult decontaminations

• their costly prices• Most time consuming

• (1 week)

• quick contamination of teflon disches

Negative effect

Positive effect

Positive effect

Lose of Po-210

Less time consuming (max 1

day)

Getting the total amount of Po from

sample

1.

2.

Just HCl and H2 O2

Materials and Methods 1 (EPA -David N.Edgington, 1975)

• Application of the first method, and the residual was digested in Teflon dishes in presents of HF, HNO3 until total dissolution.

• Total activity represent the activity measured in first and second step together

• Chemical processes was yielded by adding Po-209 as tracer

• Measurements with PIPS detector alpha-spectrometric system

• Total time consumption is 1 day

Control of measurements

1.Results of Leaching with mineral acids, partial digestion without HF

Știol Buhăescu Muced Iacob St Anne Total Activity of

sed. (Bq/kg) 57260 2727400 33029 20030 16120

Activity of silicate content

(Bq/kg)915 (16%)

22618 (9%) - 609

(30%) 787(45%)

Undissolved silicate content

(mass) 25% 36% - 48% 22%

The uncertainty of the value are represented in 2σ confidence interval

Activity of 226Ra 55460%

76533% - 202

33%50464%

2. Total digestion of sediment samples using HF (Wojciech Tylmann, 2013 Rolf Aalto, 2012

Begy et al. 2015)Activity in residuals (Bq/kg)

UDL UDL - UDL UDL

Teflon dish contamination

%20 46 - 35 22

Improvement for avoid the using of Teflon:The sample is place first in a single use plastic glass with 2.5 ml HF and 3 ml of HNO3 for two days. After, 2 ml of 0.8 mol boric acid is added and replaced in Erlenmeyer glasses with 10 ml of HNO3 and placed on a hot plate at 90 ˚C .Evaporation at almost dryness 10ml HNO3 Evaporation 10 ml HCl Evaporation 10 ml HCl EvaporationAdding H2O2 and three times distilled WaterDeposition on a Stainless Steel disc at 0.4-0.9 pH fixed with HCl on 82 ˚C

UDL – Under Detection Limit that in our case is 3 mBq

Mineralogy of the residuals 1• The samples mineral composition were identified using powder X-ray diffraction

(XRD) analysis with a Bruker D8 Advance powder diffractometer using Bragg-Brentano geometry.

• The samples from Muced, Buhăescu and Știol Lakes contain quartz with undulatory extinction typical for metamorphic rocks. Besides quartz, muscovite and feldspar where identified by x-ray diffraction Other minerals identified by polarised optical microscopy, besides those identified by x-ray diffraction, are amphiboles and chlorite.

• Sediments accumulated in St. Anna Lake consist of minerals originated for the alteration of volcanic rocks (dacite). The dominant minerals identified by X-ray diffraction are feldspar and quartz The quartz is of non-undulatory type, typical for magmatic rocks. Small quantities of amphibole, pyroxene, biotite where identified by polarised optical microscopy.

• Jacob lake samples have predominantly quartz with undulatory extinction,

muscovite, and rare feldspars, pyroxene and amphiboles.

Mineralogy of the residuals 2

Power X-ray diffraction patterns for sediments from Buhaiescu Lake, St. Anna Lake. Identified minerals are: MS-muscovite, Pl-plagioclase and Qtz-quartz

Resulted UncertaintiesLakes Știol Buhăescu Muced Iacob St Anne

Effect of silicates to 210Pb dating

method

Difference in

Ages(y)%

2,05 (max 8,6)

-1,03(max 8,3) - -5,9

(max 21)-11

(max 17)

Difference in

Dates%

0,78(max 4,5)

0,54(max 3,4) - 0,2

(max 3,11)0,45

(max 1,3)

uncertainties in 210Pb dating method (%)

7 12 - 5 19

uncertainties in 210Pb dating method with

silicates effect

19% 20% - 26% 36%

Gamma spectrometry• The main problem is regarding to the matrix effect,

the self attenuation in the sample

• Three possibilities:– Computing software for self attenuation corrections

(monte-carlo simulation)– Analytical techniques for density correction (empirical

equations)– Simple determination of the self attenuation coefficient

and recalculate the peak intensity (very important in relative method )Unfortunately it must to be apply for each analyzed sediment layer

Self attenuation coefficient in sediment column

yI apabsabsprobgeom det

rR

xarctggeom 2

1

4

1

xabsprob

me

Empirical approach

xgeom

mexryI 2det

det2rya mb

xbh

geom exaI 0

5

223344

32

242412424

6

1

222

2

111

4

1

b

bhhbhbhbe

rR

b

hbhbe

rRb

hbeI

bh

hbhb

Correction by recalculation the peak intensity trough mass attenuation correction

Before Correction After Correction

Conclusions• Leaching with just mineral acids (HCl and H2O2) can result residue of

22-48% depending on the origin of the sediments

• The remaining activity of 210Pb(210Po) in the undisolved material can vary in the range of 9-45%

• The remaining residual activity found in sediments which have predominantly quartz or are originating from volcanic rocks

• Second digestion procedure was developed, in this case the residue was negligible and the remaining activity was under the detection limit

• The highest uncertainty is produced in case of volcanic rocks, total error can reach 30%

• In gamma spectrometric measurements the matrix effect is very important to correct, the simplest way being with the determination of the mass attenuation coefficient for each sediment layer

Thank you for attention

This work is financially supported by PN-II-RU-TE-2012-3-0351 project, Radionuclides as tracers of the anthropic influence on the Danube Delta sedimentary processes National project, initiated by the Romanian Guverment