Liquid Scintillation Spectroscopy of 227 Ac and Daughters

In order to find a fast and reliable method for measurements of organic and aqueous liquid phases containing αand β-emitters in mixtures we developed a method based on Liquid Scintillation Counting (LSC) using Quantulus Low Level Spectrometer. The use of low level LSC instead of γ-spectroscopy allows reduced sample activity, shorter count rates and use of sample changer. This is of great advantage when many samples have to be measured during a short time period. The main nuclides of interest were Ac and Ra. Ac does not have any appropriate γ-radiation, therefore measurements of the nuclei of interest must be based on αand β-spectroscopy. In this study, it is shown that analyses of the radionuclidic purity of the liquid phases could be determined by αand β-spectroscopy with Quantulus.


Introduction
In radiopharmacy the purity of the species involved is extreemly high both concerning the radioactive and the chemical compounds.Thus, in therapeutic injection fluids there are almost no radioactive or chemical contaminants.As a part of their work on developing new anticancer therapeutic agents, Algeta ASA is extracting radium from actinium.Radium grows in as a daughter nuclide from actinium (through thorium) [1].It is well known that such separation can be achieved by both ion-exchange [2,3] and solvent extraction [4].References 1 and 2 represent prior art of separation of actinides and of radium from their decay products and also from most other elements.This work is a study of di-phosphonate extractant Dipex® supplied by Eichrom as a separation agent for radium and actinium.The main nuclides of interest were 227 Ac, 227 Th and 223 Ra.The series of daughter nuclides is shown in Figure 1 [5], where also the 1.38%-branch [4] of α-radiation creating 223 Fr is shown separately. 227Ac does not have any appropriate γ-radiation, therefore direct measurements must be based on its β-particles whereas the daughter nuclide 227 Th could be measured by its α-particles. 227Ac can be discriminated from the β-emitting daughter nuclides by the energy.Both 211 Pb and 207 Tl have β-particles with maximum energy >1 MeV [5].The time of measurements is important to make proper corrections for the decay of the chain of daughters.In order to find a fast and reliable method for measurements of the liquid phases containing αand β-emitters in mixtures it was decided to develop a method based on Liquid Scintillation Counting (LSC) using Quantulus Low Level Spectrometer.The use of low level LSC instead of γ-spectroscopy allows reduced sample activity, shorter count rates and use of sample changer.This is of great advantage when many samples have to be measured during a short time period.

Experimental
In all samples the liquid scintillator cocktail UltimaGold AB made by Packard was used.Features of Quantulus were utilised, comprising Peak Shape Analysis (PSA) for separating αand β-events, divisions of spectrum in low and high energy windows, and Spectral Quench Parameter (SQP) for control of quenching.γ-spectroscopy was used for qualitative calibration.
Working with α-activity it is an advantage to keep the volumes as small as possible.Thus, most of the extraction tests were performed with small volumes, i.e. 0.5 -5 mL, of both aqueous and organic phases.To ensure that all tests had equal and comparable mixing a rod stirrer in a small cylindrical vial, e.g.counting vial, was used since extraction funnels and manual shaking showed to be inaccurate.The two phases were stirred for three minutes before the sample was set to rest a few minutes until the two phases were separated.As chemical equilibrium was shown to be quickly obtained, three minutes was chosen as mixing time in all extraction and back-extraction (stripping) tests.

Results and Discussion
To enable αand β-separation the PSA-parameter must be set manually.In Figure 2, the relative αand β-activities as functions of the PSA-value is shown.It was a definite minimum of α's in β's and vice versa for PSA = 60.This value was therefore used throughout the tests.Also, the SQP versus counting efficiency was determined.However, all samples had SQP-values > 850 which was a region where the efficiency was constant.There was therefore no need for to compare the results.It must be noted, however, that the SQP is a relative measure of the maximum energy of an induced Compton electron spectrum from an external γ-source, and is thus only a measure of the quenching of the β-spectra, not the α-spectra.
In Figure 3 γ-spectra of the aqueous and organic phases after extraction are shown.The organic phase is containing 227 Th, whereas the aqueous phase is containing 223 Ra and its daughter nuclides.As is seen, 227 Ac has no appropriate γ-energies and cannot be determined.Included in the figure are two β-spectra showing 227 Ac in the low energy region (organic phase) and 223 Ra and its daughter nuclides in the high energy region (aqueous phase).
Figure 4 shows LSC-spectra after extraction with three different concentrations of Dipex in hexane, 0.008%, 0.2%, and 2.15% by volume, respectively.The aqueous phase was 3 M HCl in all cases.These spectra  This cannot be any of the elements in the main chain of daughter nuclides of Ac as they are strongly extracted by Dipex at high pH-values.The only plausible element that would seek a neutral aqueous phase is an alkaline element, i.e.Fr.This may not be in accordance with Mitsugashira et al. [4] who used a buffer solution at pH 2.2 to recover 223 Fr from 1M HDEHP-solution in benzene.We have not compared the extraction ability of 1 M (33%) HDEHP with 2% Dipex, but presumably it is much easier to recover Fr from the latter.It is noteworthy that the measured β-spectrum (Figure 5) is not in accordance with the decay characteristics reported by Abdul-Hadi et al. [6], but there seems not to be published β-spectra of 223 Fr.The closest we have found is a spectrum calculated from data given by Table of Isotopes [7].Except for 223 Fr we have no suggestion for which nuclide the measured β-spectrum belong to.Unfortunately, the half-life was not determined.

Conclusion
We have developed a method of using low level LSC for measurements of α-and β-emitting nuclides in mixtures.Features of modern liquid scintillation spectrometers like αand β-separation (peak shape analysis), quench corrections, etc must be utilised.For the samples of interest in this project there was, however, no need for quenching corrections.The method applies e.g.very well to solvent extraction samples requiring to be measured within a relatively short period of time after separation and is a relevant alternative to gamma spectroscopy for nuclides with low gamma abundance.

Figure 1 .
Figure 1. 227Ac and the series of daughter nuclides.Also shown is the low abundant (1.38%) 227 Ac -transformation to 223 Fr.

Figure 2 .
Figure 2. The relative and -activities as functions of the PSA-value.

Figure 3 .
Figure 3. γ-spectra of the aqueous and organic phases after extraction.The organic phase contains 227 Th, whereas the aqueous phase contains 223 Ra and its daughter nuclides.Included are two -spectra measured by LSC showing 227 Ac in the low energy region (organic phase) and 223 Ra and its daughter nuclides in the high energy region (aqueous phase).confirm that separation of Ac and Ra may be obtained with solvent extraction.The lower portion of the figure (2.15% Dipex) shows an organic phase containing solely 227 Ac, while the high energy β-emitting daughter nuclei of 223 Ra, i.e. 211 Pb and 207 Tl, are present in the 3 M HCl aqueous phase.It also shows that the small peak in the low energy part of the β-spectrum in Figure 3 is contribution from 227 Ac in the aqueous phase.A strip with pure water of an organic solution containing 227 Ac gave the spectrum in Figure 5.This cannot be any of the elements in the main chain of daughter nuclides of Ac as they are strongly extracted by Dipex at high pH-values.The only plausible element that would seek a neutral aqueous phase is an alkaline element, i.e.Fr.This may not be in accordance with Mitsugashira et al.[4] who used a buffer solution at pH 2.2 to recover223 Fr from 1M HDEHP-solution in benzene.We have not compared the extraction ability of 1 M (33%) HDEHP with 2% Dipex, but presumably it is much easier to recover Fr from the latter.It

Figure 4 .
Figure 4. LSC-spectra after extraction with three different concentrations of dipex in hexane, 0.008%, 0.2%, and 2.15% respectively.The aqueous phase was 3 M HCl in all cases.

Figure 5 .
Figure 5. Spectrum obtained from a strip of an organic solution containing 227 Ac with pure water.The most probable nuclide is 223 Fr based on the chemistry.Its spectrum is however, not in accordance with literature.