Search

Simple defect production and annealing processes during pulsed irradiation were investigated. Exponential decay to fixed sinks and bimolecular recombination were handled analytically but computer calculations were needed when both modes of annealing were operative simultaneously. The defect concentrations reach steady state after a large number of pulse cycles and these steady state values were compared with the steady state values produced during continuous irradiation using the time average production rate. The maximum defect concentrations, attained at the end of the irradiation pulse, are always higher than those produced by the comparable continuous irradiation. The opposite is the case for the minimum concentrations reached at the end of the annealing pulse. The defect concentrations averaged over the pulse cycle are generally lower, while the averaged squares of the concentrations are higher than those produced by the comparable continuous irradiation.

We have studied solute clustering arising from fission-fragment (FF) irradiation of a Au-Fe alloy (17 at.%) by measuring the magnetic ordering temperature through Mössbauer spectroscopy. Up to about 3 × 10-3 displacements per atom (dpa) and a dose rate of about 3 × 10-7 dpa/s, the FF-irradiation results are very similar to those obtained by neutron irradiation. At high-dose, high-dose-rate FF irradiations, a large and rather abrupt increase in the magnetic ordering temperature was observed, suggesting a fundamental change in the microstructure of the alloy. This change is apparently submicroscopic, as preliminary electron transmission microscopy showed some coarsening of the grains, but no evidence of segregation. Although requiring further development, fission-fragment damage experiments are very promising as a technique for achieving very high dpa levels in conveniently short times.

Enhancement of substitutional diffusion is observed in a brass (30 wt% Zn) by following the decrease in electrical resistivity with neutron irradiation of a thermally equilibrated alloy; the decrease arises from the increase in short- range order. It was determined by previous research that this diffusion enhancement is largely caused by the annealing of radiation-produced vacancies in excess of the thermal equilibrium concentration. Therefore, the results reported here are based upon a well-established technique.The rate of resistivity change per neutron of different energies will give the average relative number of free vacancies produced per neutron.This experiment compares the effect of 14.8-MeV neutrons with neutrons from a fission reactor. The results indicate that 14.8-MeV neutrons produce 10 + 2 times as many free vacancies as reactor neutrons when the latter are expressed in terms of those neutrons with energies greater than 0.1 MeV.

The energies characterizing helium migration and trapping in α-Al2O3 have been calculated within the framework of the polarizable point-ion shell model developed for α-Al2O3. The helium self-energy, in the octahedral position, is 2.9 eV and the helium migration energy is 2.6 eV. If defects are present in α-Al2O3 (arising, for example, from radiation damage) strong trapping of the helium occurs at oxygen vacancies (binding energy 1.7 eV) and at aluminum vacancies (binding energy 1.3 eV). The strong interaction with defects indicates that helium migration as well as the annealing of the defects are retarded by these trapping effects.

α-brass was subjected to low amplitude cyclic straining at 20 cycles per second at a series of temperatures. The electrical resistivity decreased with increasing number of cycles up to about 2 × 104, the higher temperature runs showing the greater decrease. The interpretation is based on an excess number of vacancies enhancing the diffusion rate and hence the degree of short-range order. The kinetics indicate a bimolecular loss of defects which is interpreted as the recombination of equal numbers of interstitials and vacancies. Equilibrium resistivity is not achieved at the measured temperatures and a model is proposed in which the number of potential defects depends solely on the initial preparation of the material and in which diffusion enhancement stops when these are exhausted by recombination.

Recent contributions in Phys. Rev. Letters indicate that the interpretation of fluorescence decay experiments in anthracene is by no means clear-cut.1,2 The basic dilemma is how to reconcile the fluorescence experiments of Bergman et al.1 with the photogeneration of free carriers experiments of Silver et al.3 Bimolecular rate constants are derived from both of these experiments which differ by a factor of about 104. Thus, if these experiments are measuring the same basic processes a serious discrepancy exists.

An approximate kinetic description is developed for interstitial clustering in the presence of impurities, or other nucleating faults, under the assumption that equilibrium among the clusters prevails and that the vacancies resulting from neutron damage are primarily in clusters rather than randomly distributed.

Strips of α-brass (30% Zn) were subjected to cyclic straining within the elastic limit at 95°c at frequencies from 2 to 20 c.p.s. The resistivity decreased noticeably for about 103 sec, the higher the frequency the greater being the decrease. The results are analysed on the basis of a model proposed earlier in that jogs on dislocations evaporate equal numbers of vacancies and interstitials which either return to the dislocations or annihilate each other. This model predicts several characteristics, such as the change of the resistivity decrease with time, the saturation resistivity versus frequency, the behaviour of the rate constant with frequency and the effect of initial treatment of the sample. All of these predictions are confirmed by the experiments.