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




Yüklə 279.71 Kb.
səhifə3/5
tarix10.04.2016
ölçüsü279.71 Kb.
1   2   3   4   5

2 Methodology


The BaSO4 precipitation method utilises micro-filtration, rather than electro-deposition to deposit radium as a thin source for alpha counting. Poorer resolution as compared to electro-deposition techniques, and unquantifiable radon retention means that only 226Ra can be effectively resolved from the spectrum on these sources (Sill 1987, Medley et al 2005).

A method has been developed for counting the sources prepared for 226Ra determination by the BaSO4 precipitation method, with a High Purity Germanium (HPGe) detector for 228Ra determination. The gamma decay lines of the fast ingrowing 228Ac daughter of 228Ra are utilised for this. The low efficiency, high background counts and low peak decay probabilities of the high energy lines of 228Ac, gives relatively high detection limits for this method. However, the detection limits are lower than the standard gamma method using a pressed geometry due to the thin, flat geometry of the source which improves the efficiency for counting.

To achieve very low-level detection limits for 228Ra determination a complementary method for digestion of the filter paper after a sufficient time for ingrowth of the 228Th daughter and measurement of 228Th via alpha spectrometry has also been developed.

2.1 Gamma spectrometry method development


This section describes the preparation of samples and calibration of the spectrometry systems for determination of 228Ra by the BaSO4 method, utilising high resolution gamma spectrometry. Instrument Detection Limits (IDLs), Method Detection Limits (MDLs), Efficiency, Energy, Recovery and Region of Interest (ROI) calibrations are included.

2.1.1 Source preparation


Source discs for alpha and gamma spectrometry are prepared by methods described in Medley et al (2005). The system set-up and configuration for the alpha and gamma spectrometry systems are described in Martin & Hancock (2004b) and Marten (1992) respectively. Prepared sources are initially counted via alpha spectrometry for 226Ra determination, then for 228Ra via gamma spectrometry.

To accurately analyse prepared sources from the BaSO4 method via gamma spectrometry, the sources must be mounted in a standard geometry. To enable this, sample holders were custom made from PVDF. This material was deemed the most suitable material for the following reasons:



  • Easy to work to necessary dimensions with a high degree of accuracy

  • High chemical resistance, including to H2SO4 and caustic agents – this helped in preparation of standard sources (see method for sealed standards)

  • Resistance to temperatures up to at least 80C

Necessary for preparation of sealed standards

Essential for cleaning of holders which uses an alkaline (pH 10.6) 0.2 M DTPA wash at 80C for removing Ba/RaSO4



  • Relatively low cost; PTFE has a higher chemical resistance and melting point than PVDF but was ruled out due to significantly higher cost

The inner diameter of the PVDF holders is 24 mm to ensure the source discs are not bent while in the holder. The outer diameter is not a factor that will influence the gamma counting efficiency. The thickness of the PVDF holder must be consistent for standards and source discs. Source discs are placed in the base of the holder then the top is placed on to hold the disc centrally and pressed flat to maintain the same geometry.

Figure 1 Illustration of mounting radium source discs in PVDF holders for gamma spectrometry. Polypropylene filters have a 17.5 mm active diameter, and a total diameter of 22 mm.



228Ra is a beta emitter, and cannot be directly measured via alpha or gamma spectrometry. 228Ac, the direct daughter of 228Ra, is very short lived with a half-life of only 6.15 hours (Martin & Hancock 2004b). After a short ingrowth period 228Ac will have sufficiently ingrown to have reached secular equilibrium with 228Ra (Figure 2).

This is calculated using the radioactive decay and ingrowth equation (Equation 1) adapted from Friedlander (1981).

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

Where –

AAc – Activity of 228Ac



ARa – Activity of 228Ra

λRa – Decay constant of 228Ra

λAc – Decay constant of 228Ac

As λRa << λAc equation 1 can be written as:





Figure 2 Time taken for 228Ac to reach equilibrium with 226Ra parent after radium separation.


Calculated using Equation 2

2.1.2 Gamma counting


The 911 keV and 969 keV gamma lines of 228Ac are used as they are sufficiently separated from interfering lines from the 133Ba tracer, 226Ra and daughters, and have a high decay probability (Table 3). Figure 3 shows a typical spectrum with major lines of 133Ba, 226Ra and daughters and shows that there are no lines interfering in the region of interest of the 911 keV and 969 keV peaks.

Figure 3 Spectrum of sample with 133Ba and 226Ra, counted on an HPGe spectrometer. Major peaks of these 2 isotopes can be seen to be distinctly separate from 228Ac decay lines at 911 and 969 keV. Spectrum produced at eriss.

Table 3 Gamma decay energies and emission probabilities for relevant nuclides and decay lines in samples and standards prepared for 226Ra and 228Ra analysis via the BaSO4 method. All gamma energies are taken from Canet & Jacquemin (1990), except for 228Ac which are taken from Marten (1992).


Isotope

Main gamma emissions (keV)

Probability (%)

Isotope

Main gamma emissions (keV)

Probability (%)

232Th

59

1.9

226Ra

186

3.28

228Ac

338

911


969 & 964
(doublet peak)

12.3

26.6


20.4

214Pb

295

352


18.7

35.8


228Th

84

1.2

224Ra

241

3.9

214Bi*

609

934


964

1120


1238

46.1

3.16


0.38

15.0


5.9

133Ba**

53

81

223



303

356


2.2

33.8


0.5

18.4


62.1


2.1.3 Detection system calibration


Calibration of the gamma spectroscopy system is necessary before any results can be calculated. The parameters that need to be determined are:

  • Channel/energy calibration

  • Detector efficiency

  • Background

  • Detection limits.
2.1.3.1 Energy calibration

Energy calibrations are performed with the sealed standards used for efficiency determination and the 133Ba standard discs. Quality control checks with a standard of known activity, with a broad energy range of gamma emitting nuclides, are counted after each sample to ensure energy calibration is maintained.
2.1.3.2 Efficiency

The efficiency of an HPGe gamma spectrometer varies with source geometry, sample matrix and the energy of the gamma decay line, thus a gamma spectrometry system requires accurate efficiency determination for the geometries used and for each peak of interest to be calculated. To do so a source of known activity with the same geometry and sample matrix (as close as possible) is used. Typical efficiency curves for a series of spectrometers calibrated with a source containing isotopes with a range of gamma photon energies are shown in Figure 4.

Figure 4 Typical efficiency curves for a series (A, D, F, G & K) of gamma spectrometer configurations. Reproduced from Marten (1992).

For this method the chemical recoveries of the sources are determined by comparison with a standard of known activity so determination of the efficiencies of the decay lines for the 133Ba peaks is not necessary. However, the system needs calibration for the 228Ac.

The preparation of standards is often the most critical step in the use of radiation detection systems. For gamma spectrometric measurement of 228Ra via the gamma decay of 228Ac, a pure source of 228Ra is ideally used to calibrate the system. Due to the short half of 228Ra, it is not easily available and quite expensive to obtain as a pure source. Pure thorium salts however are easy to obtain and relatively inexpensive. An isotopically pure 232Th (t½ = 1.41 x 1010 years, Joshi et al 1983) salt will reach secular equilibrium with the daughter 228Ra (t½ = 6.7 years, Joshi et al 1983) after approximately 5 half lives, and thus can be used as an essentially pure 228Ra standard. It is important that 230Th contamination of the thorium salt is negligible, as the daughter 226Ra is unwanted in calibration standards. A review of the main gamma emissions from 232Th and daughters other than 228Ra showed no significant interferences with the high energy 228Ac gamma emissions, though the lower energy emissions could not be used (338 keV, Table 3).


2.1.3.3 Background

There are essentially 2 types of background that need to be considered in gamma spectrometry systems. The background introduced by the source falls in two main categories, either Compton scattering or Bremsstrahlung. Compton scattering is caused by gamma photons imparting energy to weakly bound electrons which in turn release photons. Bremsstrahlung creates a broad spectrum and is caused by beta particles (electrons) interacting with the semiconductor detector and the absorbing material of the sample and holder (Canet & Jacquemin 1990). The type and magnitude of background interference will vary with each source (Marten 1992).

There may also be systematic background as a result of contamination of the counter or counting well and natural system (ie – electronic interference in old counting systems, cosmic and other natural radiation). Lead and other types of shielding are routinely used to minimise natural background. Interferences in the production of a final spectrum will vary with each detector system (Marten 1992).


2.1.3.4 Detection limits

Detection limits represent the point at which a detection system can no longer be considered to have detected a peak.

There are several types of detection limits described for radiation detection systems – instrument detection limits, IDLs, method detection limits, MDLs, and overall detection limits, DDLs. There are also a number of ways to determine the detection limits.



Detection limits are essentially the measure of the relative effectiveness of:

  • IDL – the detector system configuration only

  • MDL – IDL and the method components – any processing that affects each sample

  • DDL – MDL and the preparation and counting conditions that are unique for each prepared source.
Instrument detection limit

The IDL for 228Ra for this method can be determined by measuring:

  • the efficiency of the detector for the 228Ac peaks to be used

  • the background spectrum as a result of the detector and sample holder configuration only.

Detector efficiencies for the method were determined from a series of sealed standards prepared at varying activities from an isotopically pure, calibrated thorium nitrate (Th(NO3)2) solution in secular equilibrium with 228Ra. The solution was pipetted onto a small disc of absorbent paper (17.5 mm diameter – the same size as polypropylene filter papers) in the PVDF sample holders for gamma measurement. A small volume of 60% ethanol was added. Th(NO3)2 is highly hygroscopic so it is essential the standards are dried when weighed to ensure accurate calibration (this was performed on a hotplate at 70°C), once dried, it is also essential that the standards are sealed. The sealed standards were counted under identical conditions to those as for samples, and the efficiency of each 228Ac line was determined (assuming 100% recovery). The IDL can be calculated after measuring a number of instrument background spectra, using methods derived by Currie (1968).
Method detection limit

The method detection limit, uses the same factors as for the IDL, but also takes into account the processes samples undergo for sample preparation. For this method these are:

  • chemical separation and associated typical recovery of the analyte of interest

  • additional background counts resulting from trace contaminants in chemical solutions and laboratory equipment

  • the accuracy of other preparation steps – eg weighing of the sample (only has a small influence on uncertainty in radiation measurement techniques, De Regge & Fajgelj 1999)

  • Appropriate sample size and type may also be influencing factors – for example there are limits on the amount of organic material that can be tested before the chemical separation and deposition of the analyte is adversely affected.

Unsealed standards were prepared to determine the method detection limit and to assess whether 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 made from the prepared and calibrated Th(NO3)2 tracer solutions in secular equilibrium, and an isotopically pure 133Ba tracer solution.

Unsealed standards were also prepared with 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 given a period of time (approximately 6 ½ months) to allow ingrowth of the 228Th daughter from 228Ra, so they could be used for calibration of the ingrowth part of this method.

Overall detection limit

Overall detection limits, are affected by all of the above considerations, but vary with each sample, this is due to several factors:

  • Counting time – the most significant source of uncertainty in radiological measurements are counting statistics (De Regge & Fajgelj 1999), with longer count times uncertainty is reduced, and therefore the detection limits are reduced

  • Individual sample composition

Some samples contain much higher levels of trace contaminants which may affect the chemistry (such as barium) than others

Samples with higher levels of 226Ra can affect the overall Compton background due to the 934 keV gamma line of 214Bi (Table 3), and in extreme cases (very high 226Ra: 228Ra activity ratios, eg >1000) another very low probability gamma emission of 214Bi at 964 keV can also affect the spectrum due to its proximity to the 228Ac peak at 969 keV



  • Chemical recovery of the sample.

2.1.4 Chemical recovery determination


A barium standard disc is prepared, and the chemical recovery of the disc determined using a NaI detector according to methods described by Medley et al (2005).

This disc is then counted in an HPGe detector and the count rate compared with that of the sample is used for chemical recovery determination. Actual tracer activities are not needed as the relative recovery of each sample is determined by comparison with the count rates obtained for the standard disc, using the same amount of tracer.

Thus the chemical recovery is determined according to the following basic principle:

Principle of chemical recovery determination. Equation 2

Where –


Rsample – Sample recovery

Nsample – Net count rate for the sample

Nstandard – Net count rate for the standard disc

The 302 keV and 356 keV gamma decay lines of 133Ba are used, other lines with a high decay probability are interfered with by lines from daughters of 226Ra, and so cannot be used for accurate recovery determination. To account for the varying peak decay probabilities of the two gamma decay lines, and the varying count times and tracer masses of the sample and standard, the following set of Equations 3–5 are used for chemical recovery determination:

Chemical recovery determination. Equation 3

Where –

R302 – Sample Recovery calculated using the 302 keV gamma line

R356 – Recovery calculated using the 356 keV gamma line

Recovery at each 133Ba decay line is calculated using Equation 4.

Chemical recovery determination for each peak. Equation 4

Where –

RPeak – The sample recovery at a given peak

CPeak – The gross counts in the peak

BL – The background counts in a region to the left of the gamma line spanning half the width of the main region of interest

BR – The background counts in a region to the right of the gamma line spanning half the width of the main region of interest

t – The count time in kiloseconds (ks)

PDP – The Peak Decay Probability of the peak. For the 302 keV peak this is 0.1833, and the 356 keV peak is 0.6205

Msample – The mass of the tracer used in the sample

CFStd – The 133Ba standard correction factor

Equation 5 is used to calculate the 133Ba standard correction factor.

133Ba standard correction factor determination. Equation 5

Where –

CFStd – Gross counts of a given peak of the standard disc

RStd – The recovery of the standard disc, this is calculated from methods described in Medley et al (2005)

MStd – The mass of the tracer used in the sample.

All other parameters are the same as described above, though for the standard disc.


2.1.5 228Ra activity determination


Based on the above calibrations of the spectroscopy system 228Ra activity is determined using Equation 6.

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

Where –


Net count rate determination. Equation 7

and –

Nc – Combined net counts of the 911 and 969 keV regions of interest

C911 – Counts in the 911 keV region of interest

C969 – Counts in the 969 keV region of interest

BL911, BL969, BR911, BR969 – The background counts in a region to the left and right of the 911 keV and 969 keV gamma lines spanning half the width of the main region of interest

PDPc – The Combined Peak Decay Probability of the 911 and 969 keV which is 0.4705

ε – The calculated combined efficiency of the 911 and 969 keV peaks which is 2.70%.

2.1.6 Quality control and quality assurance


Quality assurance and quality control (QA/QC) is an important part of implementing a developed method to ensure calibrated parameters remain within acceptable limits.

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.

Regular background checks are conducted for a standard pressed geometry, though this does provide relevant information regarding the background spectrum of the detector system, these shall be implemented for unused and chemical blank polypropylene discs at bi-monthly intervals. It will take some time to fill control charts with sufficient data points to provide meaningful QC data (>20 data points, Currie 1968).

Due to the discs being used for determination of 226Ra by alpha spectrometry as well as 228Ra by gamma spectrometry, all samples are counted with a NaI detector for chemical recovery determination prior to alpha counting. Alpha counting should always precede gamma counting to prevent losses on the PVDF containers used for gamma counting from affecting final results.

Comparison of the chemical recovery obtained through NaI and HPGe spectrometers provides an additional QA process. The calibration of high resolution gamma spectrometers allows for accurate determination of chemical recovery in samples for 226Ra analysis where 226Ra activity levels are very high. The gamma emissions of radon daughter 214Pb, and increased Compton background from higher energy gamma emissions of 210Bi and others (Table 3) can increase the number of counts in the broad region of interest used for chemical recovery determination through the NaI spectrometer, increasing the measured recovery.

2.2 Alpha spectrometry method development


This section describes the preparation of samples and calibration of the spectrometry systems for determination of 228Ra by the BaSO4 method and subsequent separation and measurement of the 228Th daughter utilising high resolution alpha spectrometry. Instrument Detection Limits (IDLs), Method Detection Limits (MDLs) and Recovery calibrations are included. Efficiency, Energy and Region of Interest (ROI) calibrations are detailed in (Martin & Hancock 2004b).

2.2.1 Methodology


Prepared sources are screened for 228Ra activity via HPGe gamma spectrometry using the 228Ac daughter. If the activity is below the detection limit for this method, the samples are stored until the 228Th daughter has ingrown sufficiently to be measured via alpha spectrometry (Figure 5). The maximum activity of 228Th is reached after approximately 1650 days, or just over 4 years. Suitable activities for analysis of many environmental can be reached after 12 months ingrowth.

Figure 5 Ingrowth of 228Th from the parent isotope 228Ra, based on Equation 1. Note that this ingrowth curve is valid regardless of the initial activity of 228Ra.



228Th activity was determined by methods described in Martin & Hancock (2004b), and 228Ra activity was back calculated using Equation 8 given below.

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

Where –


ARa - The activity of a 228Ra at time

ATh - The initial activity of 228Th

λRa – The decay constant of 228Ra

λRa – The decay constant of 228Th

t - Time since separation of 228Ra from the sample

2.2.2 Source preparation for 228Th measurement


A standard method for separation of thorium isotopes and analysis via alpha spectrometry used is described in Martin & Hancock (2004b), a flow diagram summary is given in Figure 6. Based on a number of assumptions, this method has been modified for the current application.

Figure 6 Flow diagram of standard method for thorium isotope analysis (Martin & Hancock 2004b). Steps highlighted have been omitted for this project.

The source disc filter paper used in radium determination is 100% polypropylene. To break this down into a suitable chemical form and release for thorium analysis, a digestion process is required (Figure 7). Concentrated sulphuric acid (98% H2SO4), is used first to char the filter, and then concentrated nitric acid/hydrogen peroxide (69% HNO3/35% H2O2) treatment is used to remove excess carbon from the sample matrix. Once the digestion is complete, the sample can be taken up in appropriate acid medium, and undergo tributyl phosphate (TBP) extraction for thorium, eliminating the iron hydroxide precipitation step.

Figure 7 Flow diagram of digestion procedure for 228Th ingrowth method

Due to the very similar chemical properties of thorium and uranium, an anion exchange step is usually employed to ensure complete separation of these two elements (Martin & Hancock 2004b). For the current application it can be assumed that uranium has already been effectively removed from the sample in the initial radium separation, and thus the anion exchange step can be eliminated. To prevent from allowing organic residue to carry through from the TBP extraction step (usually removed in the anion exchange step), after evaporation of the extract, concentrated HNO3 is added, and evaporated at moderate heat (approximately 80°C).

The solution is then passed through the electrodeposition process, whereby thorium is electroplated onto a stainless steel planchet, and counted via alpha spectrometry according to methods described Martin & Hancock (2004b).There are several assumptions to this approach for 228Ra determination, and a number of calibration and performance assessment procedures were undertaken to ensure the validity of this method.

Th(SO4)2 is very soluble in water and acid media (Hyde 1960), and there is an assumption that there is a 100% recovery of 228Th from the digestion procedure. This was confirmed by the analysis of unsealed standards prepared initially for calibration of both the gamma and alpha spectrometric techniques.

2.2.3 Alpha counting


Alpha counting for 228Th calibration of the detection system, and QA/QC for the alpha spectrometry determinations is performed according to procedures detailed in Martin & Hancock (2004b) and others detailed below:

  • Bi-monthly blanks are counted to monitor background levels

  • Efficiency calibration of the spectrometry system is performed by calibration with a known activity alpha-emitting standard

  • Peak tailing calibrations are performed with prepared standards of isotopically pure 228Th, 229Th and 230Th

  • Energy calibrations are performed with a prepared standard of 232Th in secular equilibrium with its decay series

  • Chemical blanks are run with each batch of samples to monitor procedural contamination

  • All equipment used for chemical separation and handling is washed according to procedures described in Appendix 1, section A1.4.1 of IR501 (Medley et al 2005).

2.2.4 228Ra activity determination


Back calculation of 228Ra activity from 228Th determination is performed using Equation 8.

1   2   3   4   5


Verilənlər bazası müəlliflik hüququ ilə müdafiə olunur ©azrefs.org 2016
rəhbərliyinə müraciət

    Ana səhifə