Calibration of the gamma spectrometric method for 228Ra measurement via the 228Ac daughter, and development of a digestion procedure for radium sources prepared by BaSO4 co-precipitation to enable alpha spectrometric measurement of the 228Ra daughter, 228Th, was undertaken. A detection limit of 70 mBq was determined for the indirect measurement of 228Ra via gamma spectrometry. A detection limit of 5 mBq, after a 12 month waiting period, was determined for indirect measurement of 228Ra through ingrowth of 228Th.
The developed technique showed suitability for relatively fast, cheap and accurate analysis of drinking water for assessment of compliance with the Australian Drinking Water Quality Guidelines which sets a guideline limit of 0.5 Bq/L for combined 226Ra and 228Ra if gross alpha/beta activity exceeds 0.5 Bq/L (excluding radon and 40K; NHMRC & NRMMC 2004). Analysis of 226Ra and 228Ra activity to ADWG guidelines via this method is more rapid and has lower detection limits than common methods currently employed (Medley 2007, Table 8, p 39).
The developed technique showed suitability for analysis of potentially contaminated water form uranium mining and milling activities and the low detection limits provided with the 228Th ingrowth method for 228Ra measurement also provides an excellent tool for analysis of low-level environmental samples and 226Ra/228Ra activity ratios for a wide range of applications.
The high backgrounds associated with the gamma spectrometry techniques and peak tailing interferences from higher energy alpha particles with the alpha spectrometry techniques limit the range of 226Ra /228Ra activities that can be measured on the same prepared source. With the gamma spectrometry method for 228Ra determination developed in this project it is estimated that 226Ra can be at least 4 orders of magnitude higher that 228Ra activity before the interference from the 964 keV 214Bi in the 969 keV 228Ac peak becomes significant (Table 3). Even so, the 911 keV peak could still be used alone (with reduced MDL due to lower overall counts) for 228Ra determination. With the complementary 228Th ingrowth method for 228Ra determination there is no realistic difference in activity of 226Ra and 228Ra isotopes that could not be measured.
With 226Ra and 228Ra concentrations spanning ranges shown in Table 2 in natural environments, these techniques are useful for low-level studies of radium transport in the environment, especially if time to allow for ingrowth of 228Th is possible. Examples of this include movement of radium from past and present coal, uranium and radium mining activities (Fernandes et al 2006, Carvalho et al 2007, Leopold et al 2007) and assessment of radium uptake in natural bush foods (Martin & Ryan 2004). If combined with additional pre-concentration steps such as manganese dioxide precipitation this method could prove useful for studies in environmental transport of radium in saline coastal, estuarine and ocean waters (Okubo 1990).
Combined with pre-concentration techniques such as dry-ashing to remove excess organics for large volumes, this technique may be applied to more in-depth studies of radium migration within terrestrial and marine biota (Justyn & Havlik 1990, Simon & Ibrahim 1990, Ryan et al 2005). Difficulties of such techniques with radium due to plating of radium on walls of nickel containers or fusion with ceramics can make expensive platinum crucible or microwave digestions (pers comm Elizabeth Manickam) the only alternative.
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Samples are prepared for 228Ra determination using the same methods for source preparation for 226Ra determination as described in IR501 (Medley 2005). The system set-up and configuration for the alpha and gamma spectrometry systems are described in SSR180 (Martin & Hancock 2004) and IR76 (Marten 1992) respectively.
Additional apparatus required to the above referenced publications include only a calibrated 232Th tracer solution, and the PVDF holders described in the main text (Figure 1).
Appendix 2 Calibration of gamma and alpha spectrometers
Calibration of the spectroscopy system is necessary before any results can be calculated. The gamma spectrometry system requires accurate efficiency determination for each peak of interest to be calculated. Counting of a high activity energy calibration source for 10 minutes after each sample count provides a means of ensuring energy calibration of each spectrum can be adjusted for minor variations after counting. Although there are several types of detection limits described for these systems – IDLs, MDLs, overall DDLs, the detection limits used in this report are based on a 30% relative standard deviation cut-off limit.
Sealed standards prepared to determine the Instrument detection limits for the method. These standards were prepared from a relatively isotopically pure, calibrated thorium nitrate (Th(NO3)2) solution in secular equilibrium with 228Ra, a beta emitter, and its direct daughter 228Ac, a gamma emitter.1
A2.2 Preparation of unsealed standards
Unsealed standards were prepared to determine the method detection limit and if it is significantly affected by the 133Ba activity which is used as a tracer to determine the chemical recovery of radium from the procedure. These standards were prepared from a calibrated Th(NO3)2 tracer solution in secular equilibrium, and an isotopically pure 133Ba tracer solution (reference to standard material details here).
Unsealed standards were also prepared from varying amounts of the 133Ba tracer solution only in order to assess the correlation of 133Ba activity to background counts in the 228Ac peaks.
After counting, these standards were held for approximately 8 months to allow ingrowth of the 228Th daughter from 228Ra, then digested according to the procedure given in Appendix 3, section A3.3.
Appendix 3 Methods
A3.1 Preparation of sealed standards
Cut a 25 mm disc from thin absorbent paper and place in the base of a sample holder, use the top of the holder to press it flat (remove the top of the holder after pressing) – provide a photograph here.
Weigh, then add a known volume of calibrated Th(NO3)2 tracer solution in secular equilibrium
Add a small volume of 60% EtOH to prevent the acid in the tracer solution from reacting with the absorbent paper
Heat the sample holder at 60C on a hotplate until completely dry
Once dry, leave the sample holder on the hotplate to avoid the Th(NO3)2 adsorbing moisture from the air (see footnote)). Place the top of the sample holder on firmly, then seal the holder with a high strength plastic glue (brand name given here)
Count in an HPGe gamma spectrometer (for how long? How many counts?)
A3.2 Preparation of unsealed standards
Measure into a 50 mL centrifuge tube approximately the same amount (weighed to 4 decimal places) of 133Ba standard as used per sample
Evaporate the samples in a water batch to near dryness (low recovery was noted for original standards prepared, and this step is a modification of procedures followed for initial calibration of the method).
Add desired volume of Th(NO3)2 tracer solution (see footnote or something)
Add 0.5 mL of Ba carrier solution
Add 10 mL 0.2 M DTPA, 1 drop of thymol blue and 2 drops of methyl red
Simultaneously add 6 mL of 5:1, 20% Na2SO4:Acetic acid mix, and 0.5 mL Ba seeding solution, then leave to stand for at least 30 minutes before proceeding to the next step
Filter as per samples (steps 18–23 from A1.4 of IR501), then remove filter and allow to air dry
After drying place in the base of a PVDF gamma sample holder, use the top of the holder to press it flat, then count using an HPGe gamma spectrometer.
A3.3 Digestion of polypropylene filter, thorium extraction and deposition
Place the filter to be digested in a tall form 50 mL beaker, and heat to ~80°C. The filter paper should go semi-transparent after 5–10 minutes.
Add 5–10 drops of concentrated (98%) H2SO4, to char the remaining filter paper and continue heating (ensure all of the filter paper is charred before proceeding to the following steps) until most of the H2SO4has evaporated
Add 10 mL concentrated (69%) HNO3 and heat the solution to 140C
Add 2 mL of 30% H2O2 in 0.2 mL aliquots, waiting for the foaming to subside between addition of each aliquot
Repeat step 3 every 30 minutes for ~1–2 hours or until the black colour has been completely removed from the sample. Add extra concentrated HNO3 when necessary to prevent the solution from drying out
Evaporate the sample to dryness.
Uptake in 20 mL 8 M HNO3, and separate and deposit 228Th using method ‘8.3 Separation and deposition procedure’ from SSR180 with the following exceptions:
The Anion Exchange step is eliminated.
TBP extraction is followed by gently heating the solution with concentrated HNO3 to remove excess organic material from the extraction step – add 10 mL conc. HNO3 and evaporate to dryness at 60°C
Continue with the electrodeposition step as for a normal sample
Count the sample in an alpha spectrometer (see SSR180 for system set-up) for 228Th determination.