Other
Scientific paper
May 1965
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1965gecoa..29..551s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 29, Issue 5, pp.551-561
Other
1
Scientific paper
For the determination of radiogenic argon by means of thermal neutron activation analysis, the argon extracted from the sample is purified with metallic titanium heated at 800°C then slowly cooled to 200°C, after the sample has melted in vacuo . The argon is then collected in a quartz vial, the gas being adsorbed on 0.240 g of charcoal which is cooled at liquid nitrogen temperature. The vial is provided with a bulb in which the charcoal is placed. After the vial is sealed, the charcoal-containing bulb is heated up to 700°C to ensure the desorption of argon from the charcoal, whereupon the bulb is removed from the vial. This is necessary in order to avoid the irradiation of the calcium present in the charcoal with fast neutrons accompanying the thermal neutron fluxes, since the fast neutrons cause the reaction Ca 40 ( n , ) A 37 . In this manner, A 37 results exclusively from the reaction A 36 ( n , ) A 37 and can therefore be used for the accurate estimation of the radiogenic argon contamination with atmospheric argon. Similarly, a known quantity of argon of normal isotopic composition is introduced into another quartz vial, identical with the first. Both vials are irradiated for ten hours in a 10 13 thermal neutrons/cm 2 sec flux. The radioactive argon from the two vials is introduced into two identical Geiger-Müller counters. The radiogenic argon content in the sample is determined by comparing the counting rates of A 41 and A 37 in the two counters. Since the volume of the vials in which the gas is collected is small, three sample vials and a standard vial can be simultaneously irradiated. In order to check the method, blank runs were carried out which did not show the presence of A 41 and A 37 isotopes. In other vials, known quantities of A 40 and A 36 were irradiated. Assuming these quantities as unknown and using for their determination the same process as for the determination of the argon in the samples, the same quantities of argon were found within the usual experimental error limits of 2-3 per cent. The age determinations obtained by this method on a muscovite and on a biotite sample were compared with the age values obtained on the same samples by classical methods. The results of these verifications prove the accuracy of the present method.
Cerei M.
Danis A.
Ntescu C. M.
Oncescu M.
Soroiu M.
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