Trace element analysis of K, U and Th in high purity materials by Neutron Activation Analysis P. ILA Dept. of Earth Atmospheric & Planetary Sciences Massachusetts Institute of Technology Cambridge, MA LRT2004, Sudbury, December 12-13, 2004

2 What are high purity materials? Purity level Common usage 99.9% High Pure (3 nines purity) 99.95% Analytical Grade 99.99% Spectroscopically Pure % High Pure (6 nines purity) Fe wire, Au foils, Silica powder, HPGe, Cu

3 Determination of Th, U and K by Instrumental Neutron Activation Analysis (INAA) 232 Th (n,  ) 233 Th --> 233 Pa half-life 27 d  -ray 312 keV 238 U (n,  ) 239 U --> 239 Np half-life 2.35 d  -rays 106, 222, 228 keV 41 K (n,  ) 42 K half life12.36h  -ray 1524 keV Usually, we do not do K by INAA, but can be done if necessary

4 Gamma activity measurement by Ge detectors – Gamma Spectroscopy  The gamma activities from the irradiated materials are measured by a gamma spectrometer consisting of HPGe detector and the related nuclear instrumentation like the amplifier, analog to digital converter, multi-channel analyzer and a computer with data acquisition and reduction software and other peripherals.

5 Interference-free limits Best limits in clean materials like acrylic U, Th : 1 ppt; K : 50 ppb Typical limits U, Th : 20 ppt, K : 50 ppb The lower limit of detection is set by the matrix activities because of the Compton continuum from higher energy gamma rays than those of interest.

6 Interferences from Cr, Fe, Co, Sc & Na  Depends on the major element matrix, and trace elements like Cr, Fe, Co, Sc These four elements are contaminants arising from handling of high purity materials ( by way of molding, milling, cutting etc.)  Contamination of Na arises from as much as touching or breathing on the sample.

7 Interferences … Gamma ray Energies keV 239 Np  59 Fe 142, 192, 1099, Pa Cr K Sc 889, Na 1368, 2754

8 Extending the limits of detection Limits can be extended below pg/g by a judicious choice of mass of sample, irradiation time, irradiation flux

9 INAA and Low Background Counting If the activities other than from Th and U are negligible, LBC will be useful to go below pg/g

10 Radiochemical Neutron Activation Analysis (RNAA) In extreme cases, post irradiation separation of Pa and Np activities are necessary and have been done as required

11 Conclusion INAA is a very useful direct tool but RNAA coupled with LBC becomes very powerful when the impurities to be determined are below ppt level. The relative ratio of Th or U to total matrix trace impurities is one issue. The other issue is the major matrix itself. We cannot use NAA for determining Th and U in Fe, or Ge, or Co or Cu, or any other element where the major element gets activated.