INSTRUMENTAL NEUTRON-ACTIVATION ANALYSIS OF GEOCHEMICAL AND COSMOCHEMICAL SAMPLES - A FAST AND RELIABLE METHOD FOR SMALL SAMPLE ANALYSIS

We have developed a fast and reliable procedure to routinely measure the abundances of up to about 35 elements even in small (< 1 mg) samples. Depending on the type of samples, they are either irradiated for about 8 hours at a flux of about 2 . 10(12) n . cm-2 . s-1, or up to 100 hours at a flux of about 6 . 10(13) n . . cm-2 . s-1. As standards. high-purity synthetic multielement standards and well-characterized geological reference materials are used. Synthetic standards are used as primary standards because they have several advantages over secondary (geological) standards. Three to four counts are done one each sample, starting 1-3 days after the end of the irradiation. We use high-purity germanium (HpGe) detectors with high efficiencies and verv good energy resolution (1.6-1.8 keV at 1332 keV). To allow high throughput rates we use fast preamplifiers and gated integrator spectroscopy amplifiers with fast fixed conversion time ADCs. The signals are fed into an acquisition interface module (AIM)and via Ethernet into a MicroVAX. To allow better peak deconvolution, 8k spectra are taken where possible. A specially designed annular NaI(Tl) guard detector allows Compton suppression spectrometry. The system uses standard software and was tested with sets of geological standards and has given reliable results fort wide variety of samples, e.g., cosmic spherules in the 30-200 mug weight range.