Hexapod Component of Extrapolation Chamber for Measuring Absorbed Dose from Alpha-emitting Sources

Procurement Summary

The National Institute of Standards and Technology (NIST) Radioactivity and Dosimetry Groups have initiated a collaborative project with University of Wisconsin (UWI) to characterize radionuclides for application in targeted alpha therapy to treat various types of cancer. The Radioactivity Group will develop methods used to fabricate alpha-particle-emitting radioactive sources and measure the absolute activity of these sources. The Dosimetry Group will measure the absorbed dose due to the sources, which will complement the activity measurements, fully characterizing the source and providing valuable information relevant to clinical treatment planning. In order to perform absorbed dose measurements, a specialized instrument called an extrapolation ionization chamber is required. The UWI has built an extrapolation chamber specifically designed to measure the absorbed dose from alpha-emitting sources, which is the first of its kind as commercially available extrapolation chambers only measure beta-emitting sources. It is critical for the instrument at NIST to be a copy of the one at UWI in order to allow for transfer of the NIST measurements to the UWI as a secondary standard and to enable measurement comparisons between the two institutions. To fabricate the same instrument for NIST, specific Hexapod components, available only from one source, are required as an integral part of the extrapolation chamber. Any difference in physical dimensions and mounting platform geometry of different Hexapod components would not allow the radioactive source/shield assembly to be an exact copy of that of the UWI extrapolation chamber. Measurements of absorbed dose from alpha-particle-emitting radionuclide sources are extremely sensitive to the source-detector geometry due to the short range of the alphas. If this geometry is not the same between the NIST and UWI extrapolation chambers, there will be unacceptable differences in the measurement results, therefore making it impossible to transfer the NIST absorbed-dose measurements to the UWI and invalidating any measurement comparisons between the two institutions. Without the ability to transfer NIST standard measurements, the alpha sources cannot be fully characterized for eventual use in a therapy clinic, and the goal of this collaborative project cannot be achieved. Having such an instrument at NIST is essential to the success of the project, and the required Hexapod components are only available from one source.

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