T. S. Noggle

Low‐Temperature Reactor Irradiation Effects in Metals

The effects of structural and chemical defects on the low‐temperature (30–50°K) annealing peak in low‐temperature reactor‐irradiated aluminum and copper were studied. From the fact that the density of reactor‐induced defects did not affect the annealing kinetics, it was possible to conclude that the low‐temperature annealing process was of the first order without a unique activation energy. The fact fact that both oversized and undersized atoms could suppress this annealing peak led to the conclusion that the radiation‐induced defects were more complicated than simple point defects. The suggestion is made that a defect similar to a crowdion must be created by low‐temperature neutron irradiation. This data also supports to some degree the viewpoint that a radiation‐induced defect, possibly a crowdion, has sufficient knock‐on energy to migrate several hundred atomic distances. The experiments also contain evidence which rule out all forms of vacancy‐interstitial annihilation.

Electron Microscope Observations of Fission Fragment Tracks in Thin Films of UO2

Electron microscope studies of the tracks produced by fission fragments in thin films of UO2 have established a 100% detection efficiency for fission events in films 100 A or less in thickness. A background texture decreases the efficiency in thicker films. The tracks register in the films primarily as a result of a redistribution of surface material arising from the disturbance produced by the continuous loss of energy of the fragment by electron excitation and ionization. The minimum rate of energy loss which registers as a track in the film is on the order of 1000 ev/A. Track length distributions, however, suggest that as yet unrecognized free surface effects may also contribute to the track registration.

TECHNIQUES AND EQUIPMENT UTILIZED IN LOW-TEMPERATURE REACTOR IRRADIATIONS

A cryostat used in hole No. 12 of the graphite reactor at Oak Ridge National Laboratory is described in detail. The cryostat which is made almost entirely of aluminum is supplied with cold helium gas from an Arthur D. Little helium refrigerator. Bombardments have been made in the cryostat at temperatures as low as 15°K for periods as long as two weeks. A discussion of the equilibrium temperature attained by a specimen in the cryostat is given taking into account the reactor gamma ray heating effects. A method for measuring stored energy and specific heats of materials bombarded in the cryostat utilizing gamma ray heating is described. A method for pulse annealing specimens at low temperatures employing the discharge of the energy stored in a bank of condensers is also described. Relatively sharp and quite reproducible thermal pulses can be obtained with this method (which is also suitable for use in the laboratory). The hazard of cryogenic devices in high radiation fields is discussed. It is believed that...

Fission Fragment Tracks in Metal Films

Fission fragment tracks observed in electron microscope studies of platinum and palladium films are interpreted in terms of thermal processes involving melting and vaporization. The results require that the energy lost by the fragment be converted into thermal energy over the entire volume of the track in a time short enough that little of it is conducted away. A model in which free electrons disperse the energy over this volume is used to account for the width and structures of the tracks.

ELECTRON MICROSCOPE STUDIES ON THE ETCHING OF IRRADIATED GERMANIUM

Electron microscope and electron diffraction studies on the surfaces of CP‐4 etched germanium specimens have shown that the changes in etching behavior induced by fast neutron irradiation occur as a result of a gross change in chemical behavior and cannot be due to local variations in the etching rate in the vicinity of the structural defects introduced by the irradiation. Observations on network structures which are produced under certain conditions of etching indicate that these arise as a result of mechanical preparation of the surfaces or from changes in the etching process and are apparently unrelated to any prior substructure in the material.

Victawet and Sodium Metaphosphate as Parting Agents for Electron Microscope Replicas

It is shown that the active material obtained in the use of Victawet as a parting layer for electron microscope replicas is the inorganic decomposition product sodium metaphosphate (S.M.P.). The results of an investigation of the parting layer characteristics of S.M.P. suggest its use in applications which have previously employed Victawet. Measurements with radioactive tracers indicate that on smooth surfaces reproducible stripping can be realized with parting layers of S.M.P. a few molecular dimensions in thickness. The use of a ribbon filament source with this material is recommended to minimize shadowing effects and thus reduce the thickness of S.M.P. required for successful stripping.

Evidence for Dislocation Reactions in Copper

Observations of slip lines and etch pits at dislocations on a copper crystal hardened by irradiation and deformed by bending are interpreted in terms of dislocation reactions.

Direct Replica Technique for Copper

A direct replica technique for the electron-microscope examination of metal surfaces was developed for preshadowed carbon replicas of copper surfaces. This nondestructive method involves the dissolution of the thin oxide layer normally present on copper surfaces in a dilute aqueous solution of ethylenediamine. The stripping action was speeded up by exposing the replicated surface to the fumes of concentrated nitric acid prior to immersion in ethylenediamine. This technique was employed in a study of the effect of reactor irradiation on deformations in copper.