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Trace element analysis of K, U and Th in high purity materials by Neutron Activation Analysis

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 02139 pila@mit.edu LRT2004, Sudbury, December 12-13, 2004.

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Trace element analysis of K, U and Th in high purity materials by Neutron Activation Analysis

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  1. 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 02139 pila@mit.edu 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 99.9999% 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) • 232Th (n,g) 233Th --> 233Pa half-life 27 d g-ray 312 keV • 238U (n,g) 239U --> 239Np half-life 2.35 d g-rays 106, 222, 228 keV • 41K (n,g) 42K half life12.36h g-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, ScThese 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 239Np 106, 222, 278 59Fe 142, 192, 1099, 1291 233Pa 312 51Cr 321 42K 1524 46Sc 889, 1120 24Na 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 .

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