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. 5

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

Calculated using Equation 2 6

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. 7

Figure 4 Typical efficiency curves for a series (A, D, F, G & K) of gamma spectrometer configurations. Reproduced from Marten (1992). 8

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. 14

Figure 6 Flow diagram of standard method for thorium isotope analysis (Martin & Hancock 2004b). Steps highlighted have been omitted for this project. 15

Figure 7 Flow diagram of digestion procedure for 228Th ingrowth method 15

Figure 8 Combined net count rates of 228Ac decay lines at 911 and 969 keV of sealed standards vs. activity of sealed standards 18

Figure 9 HPGe gamma spectrum of the most active of the sealed standards. No additional lines close to the 911 and 969 keV lines of 228Ac can be seen, even in the logarithmic scale. 18

Figure 10 Recovery corrected combined net count rates of 228Ac decay lines at 911 and 969 keV of unsealed standards vs. activity of unsealed standards 19

Figure 11 Combined normalised net counts of 228Ac 911 and 969 keV peaks vs. %RSD for 6 unsealed standards 21

Figure 12 Measured vs actual 228Ra activity in unsealed standards as determined by ingrowth of 228Ra and measurement via alpha spectrometry 23