Browsing by Author "Allen, GL"

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    59Ni production rates in mesosiderites measured with AMS
    (Wiley, 1993-07-01) Fink, D; Tuniz, C; Herzog, G; Albrecht, A; Fifield, LK; Allen, GL; Paul, M
    The cosmogenic radionuclide 59 Ni(t1/2=76ka) has great potential as a monitor of thermal neutrons in metal-rich extraterrestrial materials. In deep samples from larger meteoroids (which can support a big neutron flux) containing >1% or so of nickel, thermal neturon capture on 58Ni (delta th=4.6b) is the dominate production mechanism. Near the surface of millimeter-sized bodies production occurs via primary proton, fast neutron, and a reaction channels on Fe, Co, and Ni. We have applied AMS to the measurement of 59Ni activities (see [1] for details) in four samples from the metal phase to f the mesosiderites Estherville (fall,1879) and Budulan(find). The activities range from 1.5 to 3.5 dmp/g-Ni. Related work is described in [2,3]. To discuss neutron fluxes in Budulan, we must correct the measured 59Ni activities for terrestrial age. By using measured 41Ca activities (13-19 dpm/kg-Fe [4]) and a maximum production rate PFe(41Ca), in stony irons of 21dpm/kg-Fe [5] we deduce a maximum terrestrial age of 35 ka. After correction for this terrestrial age and normalization of L-chondrite composition [6], the production rates of 59Ni,PFe(59Ni), range from 5-13 dpm/g-Ni; these values are 2-3x greater than those reported in [7] for large irons and ~10x those for chondrites. Albrecht et al. [4] and Fink et al. [8] present 41Ca data in the silicate and metal phases from the same Estherville and Budulan samples. If thermal neutron production were solely responsible for PFe(59Ni) and PS2(41Ca) (the latter corrected for spalliation of oxidized iron in pyroxene), the thermal neutron fluxes, o, inferred from each nuclide in a sample should be the same. We deduce ratios of o(59Ni)/o(41Ca) that range from 0.75 to 1.65. Differences in epithermal yields can account for only a minor fraction of this variation as the ratio of the total resonant neutron absorption intergrals for 40Ca and 58Ni is within 10% of the ratio of the thermal neutron cross sections alone. A twofold change in Budulan's terrestrial age alters the flux ratio by 10% at most. Like 41Ca[9,10], PFe(59Ni) can be used to estimate shielding depths and lower limits on the preatmospheric radius. Calculations by [11] give a maximum value for PFe(59Ni) of 22 atoms/min/g-Ni at the center of an L chondrite with a radius of 300 g/cm2. The 10Be and 26A1 activities in Estherville [5] and respective semi-empirical production rate formulas [12] set a maximum meteoroid radius of 300 g/cm2. Our measured value for 59Ni implies a lower radius limit of 150 g/cm2 and shielding depths of 60-150g/cm2. Similarly, we suggest a radius of 200< R < 400 g/cm2 and shielding depths from 40-200 g/cm2 for Budalan. We infer that the above samples originated at relatively large depths (except for perhaps Budulan 2428) in meteoroids with preatmospheric radii >30cm, assuming a mesosiderite density of 5.5 g/cm3. Interestingly, those samples (Budulan2357 and Estherville 3311) having 41Ca production rates that indicate a higher degree of shielding have flux rations equal to or less than 1; the other two samples have 41Ca contents typical of near-surface exposure and have ratios o(59Ni)/o(41Ca) larger than unity. This correlation indicates that P59 from fast neutron reactions on 60,61Ni enhances 59Ni production at near-surface regions.