NIST contacts are: Ken Inn 301 975-5541 kenneth.inn@nist.gov He prepared the current NIST 'standard'. This is a series of natural standards that were simply characterized as to content so as not to change their chemical character. and J. Hutchinson 301 975-5532 j.hutchinson@nist.gov He is head of the Radioactivity Group A little risk adds spice to life. slaback@MICF.NIST.gov