2.5.1.5 - Radium by Derived Radium RAD Disk Method

Introduction

This method is suitable for all Radium isotopes (²²⁶Ra, ²²⁸Ra and ²²⁴Ra).

The decay and ingrowth properties of ²²⁶Ra and its occurrence is described in 2.2.1.4. in detail.

²²⁸Ra is a low energetic β^--emitter with 39 keV maximum energy (56 %), but also possesses a 15.5 keV component of lower intensity (35 %) [Magill 1999]. It is normally present in much lower concentrations compared to ²²⁶Ra. However, in water reservoirs of Thorium containing geological formations such as e.g. Wismut area, Saxonia, Kerala/South India, Sri Lanka, South Thailand, Bahia/North Brazil or in the south of Madagascar, it has been found in much higher concentrations compared to ²²⁶Ra.

The equilibrium conditions with its progenies are substantially more complex. According to

²²⁸Ra forms a variety of α-emitting daughter nuclides.

Its determination is more challenging because of its low energetic β-radiation and the influences of the short-lived α- and high energy β-emitting daughter nuclides.

²²⁸Ra with 3x10^-5 Sv/Bq [MAGILL 1999] (ICRP-68 recommends 6.7x10^-7 Sv/Bq) is estimated as radiobiological more hazardous due to the various α-emitting daughter nuclides in partial equilibrium. In order to limit the effective dose to 0.1 mSv/a, a maximum value of 20 mBq/L in drinking and mineral water for small children should not be exceeded.

The method described here is applicable to all Radium isotopes. It makes use of selective extraction disks and the complexing properties of Radium with EDTA.

Solid Phase Extraction Disks are commercially available for Ra, Sr, Cs and Tc from 3M EMPORE Company (St. Paul, USA) (fig. 19). Radium RAD Disk filters are made of thin membranes which selectively extract Radium and Lead because of their ionic size. 21-crown-7-ether as extractive agent (see fig. 26b) is bound onto a stable inert material of poly-tetrafluoro-ethylene (PTFE). Water samples are extracted through the filter disk and eluted with EDTA [Smith et al. 1997]. According to the recommendations by 3M, the solution should be stored air-tight for equilibration of ²²⁶Ra with ²²²Rn. After 20 to 30 days ²²²Rn can be flushed into a ZnS cell and is determined through its α-scintillations.

For ²²⁸Ra determination, the loaded filter is stored for 1 to 20 hours. The ingrowing ²²⁸Ac is then eluted from the filter by diluted HNO₃, evaporated on a plate and measured in a proportional counter [EPA 1980].

The main advantage of the Radium selective filter is the enrichment of Radium from water samples of up to 3 to 5 L volume. The procedure prescribed by the manufacturer [3M EMPORE 1998] is unsuitable for fast results and in-situ analysis because of the long storage time and the laboratory intensive solid scintillation measurement.

Following our investigations [Möbius et al. 2002], we recommend a simplified and rapid modification. After filtration Radium is eluted dropwise with a small amount of alkaline EDTA. After addition of a gelating cocktail (OptiPhase HiSafe III), the eluate is measured directly in an α/β-LS spectrometer. ²²⁶Ra can be quantified in the a-channel and ²²⁸Ra simultaneously in the β-channel (fig. 21).

We have used the method as well for in-situ water analysis with a filter cartridge (fig. 19 b). The sample is filtered into a syringe, eluted immediately and measured with the mobile HIDEX Triathler instrument.

Materials and Equipment

• Radium RAD Disk filter (3M Empore)

• HNO₃ (conc., 2 M, 0.5 M)

• 0.25 M EDTA solution alkaline

• OptiPhase HiSafe III

• Filtering apparatus (for 48mm filter diameter) with recipient

• Sucking finger (50 mL)

Procedure

[Möbius et al. 2002]

1. 3 L water sample are acidified with concentrated HNO₃ to 2 M (130 mL 12 M HNO₃ per Liter of water).

2. After preconditioning of the Radium RAD Disk with 20 mL 2 M HNO₃ the water sample is extracted by filtration (< 50 mL/min).

3. The filter is washed with 10 mL 0.5 M HNO₃ , with further 10 mL distilled water and then sucked sharply.

4. The Radium isotopes are eluted from the filter by drop wise addition of 5 ml 0.25 M alkaline EDTA (twice for quality control!) and collected in a small recipient.

5. The 5 mL sample is mixed with 16 mL of OptiPhase HiSafe III cocktail in a glass vial (clear gel!) and is stored for 3 hours (decay of ²¹⁴Pb) before measurement.

6. ²²⁶Ra is quantified from the a-PSD-channel and ²²⁸Ra from the low energetic β-channel.

Remark: Do not run the filter dry during extraction!

Modified Procedure for better sensitivity

1. The Radium isotopes are eluted dropwise with 12 mL 0.25 M EDTA alkaline and then covered with 9 mL organic cocktail (BetaPlate Scint or Toluene Scint).

2. The vial is closed and stored with the cover downwards in a refrigerator.

3. After equilibration with ²²²Rn (minimum 20 days) the vial is shaken vigorously (time t₀), stored for another 3 hours and then measured in the α-channel.

Evaluation

Measurement of the EDTA eluate directly:

The activity concentration A_C of the water sample is calculated by

with f_t1 = 1 / exp-(t₁/T_1/2²²²Rn) _* ln2

and f_t2 = 1 / 1 - exp-(t₂/T_1/2²²²Rn) _* ln2

R₀ = Background 0.5 count per hour

ε= Measuring effiviency 300 % (²²²Rn, ²¹⁸Po, ²¹⁴Po)

V = Sample volume (3 L)

f_t1 = Correction factor for Rn decay (time between extraction and mean counting time)

f_t2 = Correction factor in case of unequilibrium

t₁ = Storage time

t₂ = Ingrowth time

Detection Limit (MDA) for modified method: ²²⁶Ra 0.1 mBq/L

²²⁴Ra interferes the measurement because of the formation of several α-emitting daughter products. We therefore recommend a storage of 2 to 3 weeks until ²²⁴Ra has completely been decayed. We have shown that Bi, Po, Th and U do not disturb, however, Pb-isotopes as well as Ba, Sr and K are retained quantitatively (> 90 %) from 2 M HNO₃ solution. The presence of Barium in higher concentration reduces the Ra extraction considerably. 3M EMPORE recommends a maximum of 1 ppm for Pb and Ba and 2 ppm for Sr when using 1 L water sample.

Detailed results for Rn and Ra analysis from the workshop along with the LSC 2001 Conference can be found in “LSC-Handbuch” [Möbius and Möbius 2008], page 71.

Elution Behavior

Our investigations concerning the elution behavior of different radionuclides on Radium RAD Disk filters have shown that:

(1) With 6 mL 0.25 M alkaline EDTA, more than 95 % of Radium could be eluted drop wise. Pb-isotopes (²¹⁰Pb) in contrast to ²¹⁰Po and ²¹⁰Bi are also retained quantitatively on the filter; they are eluted as well in alkaline EDTA quantitatively within the first 6 mL (fig. 20).

(2) Radium shows a higher affinity on RAD Disk filters compared to Lead as can be seen from figure 15. By using 0.2 M alkaline DHC (di-hydrogen citrate) or neutral EDTA as weaker eluent, the elution of ²²⁶Ra is less effective than ²¹⁰Pb. Consequently, interferences from Pb-isotopes may be eliminated by elution with 5 mL of 0.2 M DHC in the first step. Stripping of Pb-isotopes with 6 ml DHC reduces the yield for Radium by approx. 5 % (fig. 20b).

(3) Ingrowing radionuclides from elements which are not retained on the filter may be separated by diluted HNO₃ at any time. This concerns ²²⁸Ac as daughter of ²²⁸Ra, as well as ²²²Rn, ²¹⁸Po, ²¹⁴Bi and ²¹⁴Po as daughter nuclides of ²²⁶Ra, and additionally ²¹²Bi as daughter of ²¹²Pb. Rn-isotopes exhale during filtration and do not interfere in contrast to RADAEX as extractive scintillator. ²¹⁴Pb and its direct daughters ²¹⁴Bi and ²¹⁴Po decay after 3 hours storage. ²²⁸Ra may be selectively determined through the elution of ingrowing ²²⁸Ac after storage (fig. 21).

For the measurement of the EDTA eluate, we recommend a quench resistant gelating cocktail like OptiPhase HiSafe III. With the latter a maximum of 5 to 6 mL EDTA eluate with 16 ml cocktail can be mixed to become a clear gel. Larger volumes of EDTA result in an opaque solution with high quenching and inefficient α/β-PSD separation. In presence of ²¹⁰Pb we recommend the modification described below under 2.2.1.6.

Daughter ingrowth from ²²⁶Ra has to be taken into account if separation and measurement are not done in timely conformity.

EPA 1980: Environmental Protection Agency. “Radium-228 in drinking water – Method 904.0.” Section 8, Prescribed Procedures for Measurement of Radioactivity in Drinking Water, EPA-600 4-80-032

Magill J. 1999: Nuclides 2000, Joint Research Centre, Karlsruhe

Möbius S., Kamolchote K. and Rakotoarisoa T. 2002: Extractive methods for fast Radium analysis; in: S. Möbius et al. “LSC2001 Advances in Liquid Scintillation Spectrometry”,pp 281-290, Radiocarbon 2002, Tucson

Möbius S. and Möbius T. L. 2008: LSC-Handbuch, DGFS e.V. und Forschungszentrum Karlsruhe GmbH, Karlsruhe 2008

Smith L.L., Alvarado J.S., Markun F.J., Hoffmann K.M., Seely D.C. and Shannon R.T. 1997: An evaluation of Radium-specific, solid-phase extraction membranes; Radioactivity and Radiochemistry 8 (1) (1997) 30-37

3M EMPORE 1998: 3M Empore Method Summary: 3M Test Method RA-195, RA-295, RA-395, St. Paul