Ir544 Barium sulphate method for consecutive determination of radium-226 and radium-228 on the same source

Equations

6

Ingrowth of 228Ac from the parent isotope 228Ra. Equation 1 6

11

Principle of chemical recovery determination. Equation 2 11

11

Chemical recovery determination. Equation 3 11

11

Chemical recovery determination for each peak. Equation 4 11

12

133Ba standard correction factor determination. Equation 5 12

12

Back calculation of 228Ra activity from 228Ac activity measured. Equation 6 12

12

Net count rate determination. Equation 7 12

14

Back calculation of 228Ra activity from measured 228Th activity. Equation 8 14

17

Gamma decay line efficiency determination. Equation 9 17

23

Detection limit determination for 228Ra via ingrowth of 228Th. Equation 10 23

Executive summary

Determination of two isotopes of radium – ²²⁶Ra and ²²⁸Ra – has been an important part of the monitoring and research program of the Environmental Radioactivity group of eriss. Difficulties were encountered with a routine electrodeposition method for the determination of both isotopes in 2001, at which point a faster and more reliable method for the determination of ²²⁶Ra by co-precipitation with barium was developed (the barium sulphate method). This new method was not suitable for the determination of ²²⁸Ra and a reliable method for assessing this isotope was needed. The barium sulphate method for ²²⁶Ra determination was thus expanded to allow for determination of the ²²⁸Ra isotope via measurement of the ingrown daughters, either ²²⁸Ac via gamma spectrometry or ²²⁸Th via alpha spectrometry.

This report provides a detailed description of the radiochemical separation and radiation measurement techniques for the determination of ²²⁸Ra via the barium sulphate method. Detection limits, uncertainty estimation and the applicability of the method to various sample matrices are presented and discussed.

Barium sulphate method for consecutive determination of radium-226 and radium-228 on the same source

P Medley

1 Introduction

Radium was identified as a significant environmental pollutant in the 1950s and since then has been primarily studied due to its hazard to human health. Radium pollution has come mainly from uranium, phosphate and gold production and radium is also an important pollutant in fly ash (Williams & Kirchmann 1990).

There are four naturally occurring radium isotopes (Table 1), of these radium-226 (²²⁶Ra) and radium-228 (²²⁸Ra) have been the most widely studied as they are the most radiotoxic. This is for several reasons, for ²²⁶Ra they have been described in detail in IR501 (Medley 2005), for ²²⁸Ra they are:

• 
  its relatively short half-life,
  
• 
  emitted beta particles have a high potential for causing biological damage,
  
• 
  short lived alpha-emitting daughters will build up over approximately 10 years if ²²⁸Ra is trapped in the body.
  

Table 1 Details and dose conversion factors [Sv/Bq] of the four naturally occurring isotopes of radium

	^228Ra	^226Ra	^224Ra	^223Ra
Half life (t1/2)	5.75 years	1600 years	3.66 days	11.44 days
Primary Alpha particle decay energies and probabilities	None*	4.784 (94.4%) 4.602 (5.6%)	5.685 (94.9%) 5.449 (5.1%)	5.747 (9.1%) 5.716 (52.6%) 5.607 (25.7%) 5.540 (9.1%)
Parent of decay chain	232Th	238U	232Th	235U
ICRP dose conversion factor (Adult)*	6.9 x 10^-7	2.8 x 10^-7	6.5 x 10^-8	1.0 x 10^-7

* Dose conversion factors (DCF) convert actual activity of a radionuclide someone has ingested into an effective committed radiation dose (in Sv) that will be received from the given exposure over their lifetime. The DCFs given are for adults, taken from ICRP Publication 72. All half-lives and alpha decay energies are taken from Martin & Hancock (2004b).

The much lower importance given to the other two naturally occurring isotopes of radium (²²³Ra and ²²⁴Ra) is partly due to their very short half lives (approximately 3 and 11 days respectively). ²²³Ra, as the progeny of Uranium-235 (²³⁵U – 0.7% of natural uranium), has a comparatively low abundance compared with the other naturally occurring radium isotopes. ²²⁴Ra is also usually of less importance due to the lower mobility of its parent ²²⁸Th (and ultimately Thorium-232) in the environment. Consequently these two isotopes do not pose a significant threat to human health.

In biological systems radium tends to follow the biochemical pathway of analogue elements barium, strontium and particularly calcium (Iyengar 1990, Jeffree 1990). This has been demonstrated in several animal species, including humans (Jeffree 1990). It is for the above reasons that ²²⁶Ra and ²²⁸Ra both have high dose conversion factors (Table 1) and a combined proscribed limit in drinking water quality guidelines in many parts of the world, including Australia (NHMRC & NRMMC 2004).

²²⁶Ra being in the uranium series decay chain makes it exceptionally important for any monitoring regime looking at impacts of uranium mining, milling and associated activities (Sauerland et al 2005) and thus is more intensively studied than ²²⁸Ra. ²²⁸Ra is the more radiotoxic but the lesser studied of the 2 isotopes, due to the lower mobility of the parent isotope thorium-232 (²³²Th). Thorium principally occurs in refractory heavy minerals and is not easily leached (Jaworowski 1990), and the short half life of ²²⁸Ra will not allow this isotope to be readily removed from these minerals.

1.1 Radium in the environment

Radium isotopes occur in ultra trace levels in the natural environment due to their short half lives, and the relatively long half lives of parent nuclides. As a result of this, radium tends to follow the behaviour of chemically similar elements, and also tends to adsorb easily to particulate matter (Molinari & Snodgrass 1990, Frissel & Koster 1990, Dickson 1990).

²²⁶Ra is most notably associated with U mining & milling activities, but is also of environmental concern in many other activities including gold mining, coal production and phosphate fertiliser production (eg Leopold et al 2007, Othman & Al-Masri 2007, Williams & Kirchmann 1990). ²²⁸Ra is associated in industry with thorium mining (Campos et al 1986), oil production (Vegueria et al 2002) and can be much more significant to radiological dose than ²²⁶Ra in natural environments (Malanca et al 1995, Iyengar 1990).

Table 2 A range of reported concentrations of ²²⁶Ra and ²²⁸Ra in different environmental sample types, highlighting the considerable range of activities found in the natural environment. Data are summarised from Iyengar (1990).

Matrix	Activity concentration of ^226Ra	Activity concentration of ^228Ra
Rocks	0.037–2220 Bq/kg
Soil	3.7–126 Bq/kg
Continental waters: Rivers Lakes Groundwater	0.074–314 mBq/L 0.37–145 mBq/L 0.74–55500 mBq/L	0.53–133.2 mBq/L 14.8–5610 mBq/L
Deep oceans	2.22–54.4 mBq/L
Land crops	0.01–21.5 Bq/kg (wet weight)	0.074–34.4 Bq/kg (wet weight)
Freshwater biota	0.05–8930 Bq/kg (wet weight)
Marine biota	0.015–66.6 Bq/kg (wet weight)	0.12–33.3 Bq/kg (wet weight)
Terrestrial animals	0.0004–64.2 Bq/kg (wet weight)

The study of radium activity ratios has a wide variety of potential applications. It has been used to trace movement and behaviour of the uranium and thorium parent ions for assessing their movement and sources in groundwater (eg Martin & Akber 1999) or to assess movement of oceanic waters (Jaworowski 1990). Activity ratios have also been used as seepage indicators of tailings water from uranium mining (Martin & Akber 1994) and to gauge erosion and sedimentation rates (Joshi et al 1983). Finally, of importance for the development of the method described in this report, it has been applied to characterising the uptake pathway of radium isotopes in water lilies, a native bush food (Johnston et al 1985).

Radium is known to follow the chemistry of calcium in biological systems, and will accumulate in bones and through the food chain (Iyengar 1990, Vandecasteele 2004). It is for this reason that radium has been studied (primarily ²²⁶Ra) in many different natural systems (Maul & O’Hara 1989, IAEA 1994). These studies have often been used for building radiological dose models to estimate contribution of radium to radiological dose in humans (IAEA 1994, Sam & Eriksson 1995, Lasheen et al 2007, Martin et al 1998).

A range of reported values for ²²⁶Ra and ²²⁸Ra are shown in Table 2.

1.1.1 Radium in drinking water

Radium is a relatively mobile ion, being readily soluble in water and is found in a wide range of environments (Kirby 1964, Iyengar 1990, Maul & O’Hara 1989).

Radium is routinely analysed in drinking water. ²²⁶Ra and ²²⁸Ra are the most likely isotopes to be found in drinking water and they are more commonly found in drinking water supplies derived from groundwater where chloride, carbonate and sulphate anions (among others) tend to increase the mobility of radium (NHMRC & NRMMC 2004, Dickson 1990).

The Australian drinking water quality guidelines (NHMRC & NRMMC 2004) state that analysis for ²²⁶Ra and ²²⁸Ra isotopes is required if gross alpha/beta activity exceeds 0.5 Bq/L. Radium concentrations in Australian drinking water are generally below 0.02 Bq/L, though it is not uncommon for small groundwater sources to exceed these limits (NHMRC & NRMMC 2004, Qureshi & Martin 1996).

1.2 Analysis of radium

High resolution gamma spectrometry, alpha spectrometry techniques, Liquid Scintillation counting and Emanometry are the most common techniques used for radium activity concentration measurements. ²²⁶Ra is usually measured via its direct alpha decay, though it is not uncommon for ²²²Rn and daughters to be used to indirectly measure ²²⁶Ra (Marten 1992). ²²⁸Ra, a beta emitter, is often measured via the alpha decay of its daughter ²²⁸Th (Martin & Hancock 2004a), though ²²⁸Ra determinations can also be performed via gamma spectrometry, using the decay lines of the immediate daughter ²²⁸Ac, or by measuring the beta radiation directly (eg Santos et al 2002).

At the Environmental Research Institute of the Supervising Scientist (eriss) both high resolution gamma spectrometry and alpha spectrometry techniques are used for radium activity concentration measurements. Two separate methods of radium determination via alpha-spectrometry have been used at eriss, the first of these methods (Martin & Hancock 2004b) could be used for determination of all 4 naturally occurring radium isotopes (Table 1), though was a long and costly method. A new method for alpha spectrometric analysis of the isotope ²²⁶Ra only – the BaSO₄ method (Medley et al 2005), based on a method by Sill (1987) – has been used since implementation in 2001, when difficulties with the previous method emerged.

At eriss gamma spectrometry is also routinely used for radium determination, however, with the lower detection limit and the high voltage applied to the high purity germanium (HPGe) gamma detectors (and thus the potential for serious damage if water samples were to leak), this method of analysis is not used for water samples. Thus, ²²⁸Ra determinations on water samples via alpha or gamma spectrometry have not been performed at eriss since early 2001, when difficulties with the ion exchange method were first experienced. With the much higher limits of detection compared with alpha spectrometry, gamma spectrometry is still not suitable for many types of environmental samples other than soil, sediments and water.

This project has extended the Sill (1987) method for evaluation of ²²⁶Ra, to enable analysis on the same sample for ²²⁸Ra. This method for ²²⁸Ra determination is particularly suited for projects requiring very low detection limits, or where significant differences in ²²⁶Ra and ²²⁸Ra activities may be expected, such as bioaccumulation studies on both aquatic and terrestrial biota, ground and surface water monitoring and research, and on sites impacted by uranium mining and milling activities.