The mass spectrum of xenon from spontaneous fission in a laboratory sample of plutonium-244 is precisely what meteoriticists predicted it would be; this discovery completes a web of proof that this nuclide is a bona fide extinct radioactivity of galactic origin, that r-process nucleosynthesis was ongoing in the galaxy at the time of the birth of the sun, and that the early meteoritic abundances of plutonium-244, heretofore tentative, can be utilized with confidence in models for the chronology of galactic nucleosynthesis. The search for an explanation for anomalous fission-like xenon in carbonaceous chondrites can now be narrowed.

Plutonium-244: confirmation as an extinct radioactivity.

This study examined the development of individual axon terminal fields in n. laminaris (NL) of the chicken brainstem. In their mature form axons from the nucleus magnocellularis (NM), second-order auditory neurons in the chicken brainstem, project bilaterally onto the NL. Axons from the ipsilateral and contralateral NM neurons form spatially segregated, elongated arbors in the dorsal and ventral neuropil of NL, respectively. The long axes of these arbors correspond to physiologically defined isofrequency bands. To assess the development of this stereotyped arborization pattern, 6-17-day embryonic chicken brain stems were maintained in vitro while injecting horseradish peroxidase into small groups of axons. Three-dimensional reconstructions were made from serial sections and projected onto a Cartesian plane for quantitative analyses. At embryonic day 6 (E6), the ventral axons already course beneath the recently migrated NL neurons. The arrival of the dorsal NM axon branches is delayed and their paths are indirect. They first loop dorsally into the the ventricular layer, where they seem to make specific connections with migrating NL neurons and use these as guides to their appropriate positions ~~ in the NL. During the period from E9 to El7 the dorsal and ventral terminal fields become similar,kach adopting properties of the other’s initial pattern. The dorsal terminal fields extend to form bands similar to the early ventral terminal fields, while the ventral terminal fields narrow and appear to shift position in order to achieve the tonotopic specificity characteristic of the early dorsal terminal fields. The results show that a complex, mature pattern of neuronal connections can be formed during development by the combination and reorganization of two simple patterns-each shaped, in turn, by its respective axonal trajectory.

http://depts.washington.edu/rubelab/personnel/young_rubel.pdf

Embryogenesis of arborization pattern and topography of individual axons in N. laminaris of the chicken brain stem.

Behavior of tungsten under irradiation and plasma interaction

Chemical separations for super-heavy element searches in irradiated uranium targets

REVIEW ARTICLE: Superheavy nuclei

Evidence is presented for the possible existence of a superheavy element produced by secondary reactions in a tungsten target bombarded by 24 GeV protons.

Evidence for the possible existence of a superheavy element with atomic number 112.

Excitation functions and isomer ratios in the reactions 93Nb(α, xn) (x = 1−4)

PREVIOUSLY1, we have presented evidence for the possible existence of a superheavy element with atomic number 112. The principal evidence for the possible existence of this element was based on the observation of spontaneous fission in a mercury source separated from a tungsten target which had been irradiated by 24 GeV protons. Spontaneous fission activity was observed, using polycarbonate foils, in mercury sources separated from two tungsten targets. These latter are identified by the symbols W2 and W3. In the case of the W3 source the proton dose was probably less than for W2 and also only about 30% of the mercury added as carrier was recovered in the final source (as measured by colorimetric techniques). The observed fission activity was smaller than that observed with the W2 source. In the latter case (W2) the measured activity was originally about three fissions/day and all experimental work reported here has been carried out with this sample.

V.J. Robinson

Papers

Evidence for the possible existence of a superheavy element with atomic number 112.

Excitation functions and isomer ratios in the reactions 93Nb(α, xn) (x = 1−4)

Spontaneous Fission Previously Observed in a Mercury Source

103Rh(α, xn) 107−xAg reactions

Rare earth yields from bombardment of tungsten targets with 24 GeV protons