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Analysis of Iron Meteorites by Instrumental Neutron Activation with a Slowpoke Reactor

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Analysis of Iron Meteorites by Instrumental Neutron Activation with a Slowpoke Reactor ANALYSIS OF IRON METEORITES BY INSTRUMENTAL NEUTRON ACTIVATION WITH A SLOWPOKE REACTOR J. Holzbecher and D^ E^ Ryan SLOWPOKE Facility, Dalhousie University Halifax, N. S., Canada, B3H 4J1 R. R. Brooks Department of Chemistry and Biochemistry Massey University Palmerston North, New Zealand In order to devise new methods of chemical classification of iron meteorites - as a refinement of Professor John Wasson's original work [1] - a comprehensive analysis programme has been initiated. The programme involves neutron activation analysis at Dalhousie University; inductively coupled plasma-mass spectrometry (ICP-MS) at the Geological Survey of Canada, Ottawa; mineralogical studies at William Rainey Harper College, Palatine, IL, USA; ICP, graphite furnace, and hydride generatioii atomic absorption spectrometry at Massey University. Analysis of 6 specimens of each of the 14 classes recognized by Professor Wasson are contemplated. This abstract summarizes the experiences in the direct neutron activation analysis of nearly 100 iron meteorites for rhodium, iridium, osmium and gold. At the concentration levels encountered in iron meteorites Rh, Os, Ir, Au, in addition to As and Co, can be readily quantified by direct, instrumental neutron activation analysis thus avoiding more commonly used time - consuming radiochemical separations. Two irradiation, decay and counting time schemes were used: 1) t. =5 min, t, = 1 min, t = 5 min , Samples were irradiated in a Cd-shielded site and counted with an LEP detector; the epithermal irradiation decreases the production of mCo (main activity) much more than that of ^tfi. The LEP detector is advantageous because of the low energies of both nuclides, i.e., 51.5 keV for Rh and 58.6 keV for Co respectively. This rhodium 104m nuclide has not been widely used for rhodium determination in the past because of the self-absorption of its low energy gamma-rays in the sample matrix; the extent of self-absorption is dependent on matrix composition and energy [2] and this is difficult to correct. A simple correction method was devised by taking advantage of the close energies of Rh and finCo. The cobalt content of the meteorite sample is determined via Co after a long irradiation (high energy Co gamma-rays do not suffer from significant self-absorption). The cobalt content is also calculated using Co and the ratio of these two results, which represents the attenuation in the sample matrix, is 68 then used to correct the rhodium results assuming the,-self-absorption is essentially the same at the respective Rh and Co energies. 2) t = 7 h, 2 - 3 d, t = 2 h ± c The irradiation in a thermal flux under these conditions enables determination of Os^Ir, Au, As and Co using their following nuclides: Os (129 keV), Ir (317 keV, 468 keV), Au (412 keV), As (559 keV) and &UCo (1173 keV, 1332 keV). A comparison of the results obtained by the analysis of untreated samples with fire assay collection into lead buttons is shown in the table below. The lead beads from 4 different meteorites and beads of a standard (SARM 7 from South Africa) were provided by International Nickel, Ontario Division, Canada. Corrections for variations in the weight of lead buttons were necessary only for rhodium because of self absorption at the low energy-of its nuclide. • - Comparison of lead bead and direct analysis results Meteorite Ir (UR/S) Rh (ug/g) Au (ug/sO bead direct bead direct bead direct Toluca 2.2 2.2 0.9 1.3 1.5 2.0 North Chile 2.5 2.7 2.5 2.8 0.5 0.6 Mundrabilla 0.8 0.8 1.1 1.6 1.5 1.8 Henbury 11.2 11.0 2.3 2.1 0.5 0.5 The agreement in the iridium results is particularly noteworthy. Our value of 11.2 ug/g Ir for Henbury is reasonably close to the 13 ug/g given by Wasson [1]; the 2.3 ug/g reported by Nichiporuk and Brown [3] is clearly erroneous. Data for all meteorites analyzed will be availabLe at the conference. [1] Wasson, J.T., Meteorites, Springer Verlag, New York (1974) 316 pp. [2] Holzbecher, J. and Ryan, D.E., Evaluation of Some X-rays and Low Energy Gamma-rays in Instrumental Neutron Activation Analysis, J. Radioanal. Chem., Articles, 102 (1986) 507-513. [3] Nichiporuk, W. and Brown, H., The Distribution of Platinum and Palladium Metals in Iron Meteorites and in the Metal Phase of Ordinary Chondrites, J. Geochem. Res., 70 (1965) 459-470. 69 .
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