Characterizing the NBSR for Evaluating k0-NAA at NIST

 

Rachel S. Popelka-Filcoff and Richard M. Lindstrom

 

Since the 1950’s, several laboratories including NIST, have used neutron activation analysis (NAA) at their reactors to perform multi-elemental analysis of artifacts from around the world.  NAA is a multi-elemental analytical technique that is matrix independent, has a large dynamic range of analysis, and is independent of chemical states.  In addition, it fits international analytical guidelines on precision, comparability, accuracy, traceability and long-term stability.  Historically, comparator NAA has been the method of choice for determining concentrations of elements for many applications.

 

However, comparator NAA relies on well-characterized standards for each element studied.  In contrast, k0-NAA is a single comparator method, based on ratios to compound nuclear constants that have been determined with high accuracy.

This poster describes the methods used to evaluate the k0-NAA method at NIST.  Three pneumatic irradiation tubes (RT1, RT2 and RT4) at the NBSR are characterized, including the flux ratio (f), deviation of epithermal flux from ideality (α).  These initial experiments were performed by irradiation of k0 monitors Au/Al wire (IRMM 530), and metal foils of Zn, Zr, Cu and Mo.  Samples of each foil and wire were packaged “bare” in standard irradiation containers, and a second set were packaged between 1 mm thickness cadmium cover, and then both sets were separately irradiated under the same conditions.  Once the reactor and pneumatic tubes were characterized, k0 measurements of NIST SRMs (SRM 679-Brick Clay, SRM 278-Obsidian Rock, SRM 1633b-Coal Fly Ash, SRM 688-Basalt Rock, SRM 1577a-Bovine Liver and SRM 1572-Citrus Leaves).

 

Calculations of f and α and results from the irradiation of the foil and wire monitors were performed using spreadsheet calculations, and established software packages.  Established k0 calculations such as the bare triple probe (BTP) (Au-198, Zr-95 and Zr-97) and the cadmium ratio and cadmium-covered multi-monitor methods were used for characterization of the pneumatic irradiation tubes.  Since the NBSR is highly thermalized, cadmium-covered multi-monitor methods may be more reliable than measurements based entirely on BTP measurements.  Using the same spreadsheets software packages, values were calculated for SRM analysis.  k0-NAA results are compared with certified values for the SRMs examined.

 

Author:                  Rachel S. Popelka-Filcoff

Mentor:                  Richard M. Lindstrom

Division:                Analytical Chemistry

Laboratory:           CSTL

Building:                235

Room #:                 B169

Mail Stop:              8395       

Phone:                    301-975-4611

Fax:                         301-208-9279

Email:                      rachel.popelka-filcoff@nist.gov

Sigma Xi:               Member of Sigma Xi

Category:               Chemistry