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Development of a national primary radioactivity measurement standard for 68Ge

¹ Ionizing Radiation, NIST, Gaithersburg, Maryland

492

Objectives: To develop a National radioactivity standard of 68Ge for the US, allowing truly traceable calibration standards for this radionuclide to be made for the first time. This will enable the absolute calibration of activity calibrators and PET scanners for positron emitters such as 18F using a long-lived standard.

Methods: A master solution containing nominally 25 MBq/g of 68Ge in 0.5 N HCl was first measured in 5 activity calibrators maintained at NIST, as well as the NIST "4" secondary standard ionization chamber to determine calibration factors (CF) for those instruments. The master was gravimetrically diluted and its radioactivity concentration determined by 3 liquid scintillation- based techniques: 4β- anticoincidence (AC), Triple-to-Double Coincidence Ratio (TDCR) method, and the CIEMAT/NIST method of 3H-standard efficiency tracing. The activity concentration of the master and the CFs were then calculated using the gravimetric dilution factor.

Results: The AC technique is much less dependent on level scheme data and model-dependent parameters and was thus able to provide an activity concentration value for the master solution with a combined standard uncertainty of less than 0.3%. The other two methods gave activity concentration values with respective differences from the reference value of +1.2% and -1.5%, which were still within the experimental uncertainties. Published dial settings for 18F in Capintec calibrators gave results that were within 0.5 % of the true activity, while AtomLab chambers at the 18F setting gave results that were about 11% low.

Conclusions: The ability to produce standardized solutions of 68Ge presents opportunities for the development of a number of NIST-traceable calibration sources with very low uncertainties (<1%, k=1) that can be used in diagnostic imaging.

Research Support: This study was partially funded by RadQual, LLC.

This Article Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Zimmerman, B. Articles by Fitzgerald, R. PubMed Articles by Zimmerman, B. Articles by Fitzgerald, R.