2.5.1.6 - Simultaneous Determination of ²²⁶Ra/²²⁸Ra and ²¹⁰Pb Using Radium RAD Disk and TDCR

Introduction

Naturally occurring ²¹⁰Pb²⁺ can be found in altered water samples with originally high Radon concentrations. The maximum concentration in drinking water is 70 mBq/L in order not to exceed 0.1 mSv/a for small children assuming regular water consumption.

A simple tool to estimate the ²¹⁰Pb concentration in water is LS measurement after enrichment and separation on Radium RAD Disk filters. The method makes use of the fact that Lead is quantitatively extracted together with Radium into crown-ether 21 and is thus retained on the filter. Lead may be quantitatively eluted with small amounts of DHC, while Radium isotopes still remain on the filter to more than 90 %. The method in detail is described in chapter 2.2.1.5. After filtration and washing, ²¹⁰Pb is eluted dropwise with 5 mL 0.2 M DHC (fig. 21). The measurement is done after homogenization with 16 mL OptiPhase HiSafe III in a low energetic β-channel. The relatively high detection limit value of 50 mBq/L for the single PM Triathler device is due to the high luminescence background in the low energetic β-area but could be improved with a multi PM low level counter, especially when using TDCR.

²¹⁰Pb can also be determined through the measurement of the high energetic β-emitting daughter ²¹⁰Bi by Cerenkov counting, when equilibrium can be assumed.

Materials and Equipment

• 0.5 M HNO₃

• 0.2 M DHC

• 0.25 M EDTA alkaline

• RAD Disk filter

• OptiPhase HiSafe III

• Gelating cocktail

Procedure

(modified method with Radium RAD Disk in presence of Pb-isotopes, see fiure 21)

• 3 L of the 2 M HNO₃ acidified water sample are sucked through a preconditioned Radium RAD Disk filter.

• After washing the filter with 10 mL 0.5 M HNO₃ and 10 mL H₂O, ²¹⁰Pb is eluted dropwise with 5 mL 0.2 M DHC.

• The filter is washed with further 10 mL H₂ ²²⁶Ra-isotopes are then eluted from the filter twice with 5 mL 0.25 M EDTA.

• Each fraction is mixed with 16 ml OptiPhase HiSafe III and measured with TDCR after 3 hours storage.

Quench correction using β-standards for ²¹⁰Pb may be neglected if the TDCR technique is applied.

Evaluation

The activity concentration of ²¹⁰Pb is calculated according to

whereas

R_N = Net rate (cps)

ε = Measuring efficiency (95 % for ²²⁶Ra and 60 % for ²¹⁰Pb)

h = Elution yield (95 %)

V = Sample volume (3 L)

Detection Limit (MDA): ²²⁶Ra 5 mBq/L, ²¹⁰Pb 70 mBq/L (Triathler) and 10 mBq/L (LL LS counter)

Total analysis uncertainty: for ²²⁶Ra 8%, and for ²¹⁰Pb 15% (due to the calibration uncertainty)

As alternative to the direct measurement, ²²⁸Ra may be quantified by ingrowth of ²²⁸Ac after a storage time of 24 hours (T_1/2 (²²⁸Ra) = 6.13 hours), or after 15 days when ²²⁴Ra with its complex daughter nuclides has been fully decayed (fig. 21 left side). This modification is more time consuming and unsuitable for in-situ analysis, however, allows a considerably lower limit of detection and a higher counting efficiency compared to direct ²²⁸Ra measurement because of the high β-energy of ²²⁸Ac.

(a) With 0.25 M EDTA alkaline

(b) With 0.2 M DHC