Truman P. Kohman

Search for extinct natural radioactivity of 205Pb via thallium-isotope anomalies in chondrites and lunar soil

Abstract Thallium and 204Pb contents were determined by stable-isotope-dilution analysis in 16 chondrites, one achondrite, and Apollo 11 and 12 lunar fines. Meteoritic thallium contents vary over a large range, 0.02–100 ppb, corresponding to the fact that thallium is a highly fractionated volatile element. Lunar thallium contents are less than 5 ppb. The 205Tl/203Tl ratio was determined in most of the samples, with precision ranging from 0.03% to several percent depending mainly on the amount of thallium present. No variations from the terrestrial ratio were observed. The chondritic isochron slope for 205Pb (13.8-my half-life) is ≤0.00009 (99% confidence level), corresponding to an interval of at least 60 my and possibly exceeding 120 my between the termination of s-process nucleosynthesis and the lead-thallium fractionations.

Isotopic composition of meteoritic thallium

Thallium contents have been determined as 2.5 ± 0.5 × 10−6 in Ivigtut, Greenland, galena; 7 ± 2 × 10−10 in Plainview chondrite; 1.5 ± 0.3 × 10−9 in Canyon Diablo troilite; and 2.5 ± 0.5 × 10−10 in Canyon Diablo metal. The205Tl/203Tl ratio is identical within about 0.6% of that of terrestrial thallium reagent, thus yielding no indication of extinct natural radioactivity of205Pb (half-life ∼24 m.y.). Using published lead contents of the meteoritic materials, of plot of205Tl/203Tl versus204Pb/203Tl has been made of our results and those of Anders and Stevens [1]. Cogenetic phases initially incorporating205Pb should yield a straight-line isochron. Only limiting slopes can be indicated for various groups of data. The uncertainties of these data and the indicated variability of the Tl/Pb ratio in meteoritic materials suggest that more precise and extensive measurements may reveal radiogenic205Tl.

Lead and Thallium Isotopes in Mare Tranquillitatis Surface Material

Lead from Apollo 11 fines is more radiogenic than any meteoritic lead reported and older than any terrestrial radiogenic lead: 201Pb/206Pb/207Pb/208Pb = 1/99.6/69.0117.1. Comparison with primordial lead from meteoritic troilite yields a 207Pb/ 206Pb age of 4.7 + 0.1 x 109 years. The 238U/204Pb ratio is > 90 and the 232Th/238U ratio is 3.9 �0.1. The lead conitent is > 1.7 x 10-6. Evidently Pb was strongly depleted and Th and U strongly enriched in the formation of this material. Thallium was too low (< 5 x 10-9) to yield mass spectra, but indications are favorable for eventual observation of extinct natural radioactivity of 205Pb.

Coded‐aperture x‐ or γ‐ray telescope with least‐squares image reconstruction. I. Design considerations

Reconstruction of multiplex images produced by coded‐aperture telescopes for extended sources of radiation which cannot be reflected or refracted (moderate‐ to high‐energy x rays and low‐energy γ rays) can be accomplished by least‐squares solution of an overdetermined set of simultaneous equations linking the object, the aperture, and the image. A necessary condition is that the object field (an area of the sky) have a smaller number of pixels than the multiplex image recorded by the position‐sensitive detector, which requires that the projection of the object field onto the detector plane be smaller than the detector. This can be achieved by the use of an auxiliary completely open aperture of carefully chosen size and position (‘‘limiter’’) ahead of the coded aperature. The latter consists of regularly spaced elements which are made open or closed either randomly or according to some pattern, subject to solubility of the resulting set of equations. It may be larger or smaller than the detector, the under...

Coded‐aperture x‐ or γ‐ray telescope with least‐squares image reconstruction. II. Computer simulation

A novel design of coded‐aperture x‐ or γ‐ray telescope is described in a companion article [Kohman, Rev. Sci. Instrum. 60, 3396 (1989)] A fortran computer program xgrtel developed for simulation of its operation is described briefly and illustrated with a representative simulation of the imaging of a synthetic object field with a small system; details are presented in supporting material. The simulation is displayed graphically, and significant results from numerous other simulations are presented. These explore the effects of various instrument parameters, object characteristics, etc., and indicate the overall imaging performance. Under appropriate circumstances excellent imaging, limited only by pixel‐size resolution and statistics of counting, is achieved. The point‐spread function contains no side‐lobes, ghost images, or excess noise when the least‐squares reconstruction method is used with random‐coded apertures, which yield just as good results as uniformly redundant‐array apertures. Disadvantages o...

Coded-aperture x- or γ-ray telescope with least-squares image reconstruction. III. Data acquisition and analysis enhancements

The previously described design of a cosmic x- or γ-ray telescope with least-squares image reconstruction has been enhanced to provide for mask-antimask operation. This cancels and eliminates uncertainties in the detector background. The simulations of its operation have been made more realistic by incorporating instrumental blurring of sources. A second-stage least-squares procedure determines the precise positions and total fluxes of point sources responsible for clusters of above-background pixels in the field resulting from the first-stage reconstruction. Another program converts source positions in the image plane to celestial coordinates and vice versa, the image being a gnomic projection of a region of the sky. A complete sky-to-sky simulation of the imaging of a source field is presented. It is demonstrated that the point-spread function of the overall imaging process is essentially perfect.