Activity standardization of ³²Si at PTB

Within the scope of the SINCRON project, several ³²Si solutions were measured by means of liquid scintillation (LS) counting techniques at PTB to determine the activity concentration. Initial results revealed limited long-term stability of the samples, and a discrepancy between the TDCR method and the CIEMAT/NIST efficiency tracing method was found. In some cases, the sample instability could not be completely avoided but there is evidence that the results of the first measurements which are carried out within a few days after sample preparation can be used for an activity determination, though with increased uncertainty. Various sample compositions were tested, and a systematic study of long-term measurements and further experiments indicates that the sample instability is due to an adsorption-like effect. The discrepancies between the two LS methods were significantly lower when measuring other ³²Si solutions. The initially observed discrepancies are likely due to low-energetic radioactive impurities that can be present in some of the ³²Si solutions. A spectral analysis supports the thesis that tritium is present in the first solution and even allows a rough quantification of the activity ratio A(³H)/A(³²Si/³²P). This value allows impurity corrections to be applied, which leads to a noticeable improvement in the agreement between TDCR and CIEMAT/NIST efficiency tracing. Finally, a new LS sample composition with 15 mL Ultima Gold and 1 mL of HCl (0.5 mol/L) was found to yield stable LS samples. The activity determinations presented in this paper represent a fundamental step towards a new ³²Si half-life determination in the framework of the SINCHRON project.