摘要:

Bubble coalescence in solids is assumed to result when collisions occur between bubbles as they migrate by surface diffusion through the solid. The migration of an isolated pore is examined in detail and the results are applied in a finite‐difference approach, programmed for a digital computer, to predict the variation of the number of bubbles per unit volume as a function both of bubble size and of post‐irradiation annealing time. Two idealized models of bubble coalescence have been treated. The first model considers coalescence resulting from random migration of the bubbles, and the second considers coalescence resulting from unidirectional, biased migration, such as would result in the presence of a thermal gradient. Both models are based on surface diffusion migration of randomly distributed bubbles in a perfect, infinite solid. The results give the predicted variation with time of the entire distribution of bubble sizes, so that various parameters, including the mean bubble radius and volume change, can be predicted. Comparison of the predicted values to parameters measured in appropriate experiments could provide a means of measuring fundamental properties of materials. The results can also be applied to the problem of swelling of reactor fuel materials because of trapped fission gases. Although the present analysis is limited by a number of simplifying assumptions, it is clear that the presence of a thermal gradient can greatly enhance the swelling rate.

ISSN:0021-8979    

DOI:10.1063/1.1708962

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Spatially resolved electron density measurements are reported for dc‐excited xenon laser discharges over a pressure range of 12.5 to 25.5 mTorr. Due to electrophoretic effects within the closed capillary tube, anode‐directed gradients in electron density are established which affect local population inversions. The influence of lasering upon these discharges is investigated experimentally by using a 3.51‐&mgr; (5d33−6p22) laser whose optical cavityQis periodically spoiled by a mechanical chopping wheel. It is observed that lasering noticeably alters the state of the electron gas and the populations of excited xenon atoms. The discharge current is decreased due to lasering by up to 0.02%, and spatially resolved microwave cavity measurements reveal that lasering may either increase or decrease the electron density by approximately 109cm−3depending upon local discharge conditions. Laser‐induced changes in the microwave cavityQand the electron drift velocity indicate that lasering also affects the mean electron energy. Ionization via electron impact from the upper laser level and the xenon metastables is suggested as the dominant physical process responsible. Confirming spectroscopic observation of laser‐induced changes in the spontaneous radiation are presented. It is demonstrated that excitation from the xenon metastables populated the 6pand 6p′ levels, degrading the laser population inversion and significantly decreasing the 3.51‐&mgr; laser output power. Radiative and collisional mechanisms responsible for the propagation of the laser‐induced perturbation from the 5d33and 6p22levels to other excited levels are also discussed. It is suggested that laser media in general can be analyzed and improved upon by studying the laser‐induced changes in excitation and relaxation processes.

ISSN:0021-8979    

DOI:10.1063/1.1708963

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Experimental hysteresis loops are presented for thin anisotropic films (81% nickel‐19% iron) in which cusplike bumps exist for angles larger than 20° between the applied field and the hard magnetic direction. The Stoner—Wohlfarth model for a uniaxially anisotropic magnetic material indicates these cusps should vanish for angles larger than 13°, and a new model is developed which accounts for the larger angles which are observed. This new model takes into account, in an approximate way, the effects of the crystalline anisotropy in randomly oriented, polycrystalline magnetic materials. In addition to providing an understanding of the observed hysteresis loop behavior, this model leads to the suggestion of a possible mechanism for creep in magnetic thin‐film memory devices. According to this suggestion, creep may be reduced by reducing the magnitude of the crystalline anisotropy of the magnetic material, and a discussion of some possible alloys for accomplishing this goal is given.

ISSN:0021-8979    

DOI:10.1063/1.1708964

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Lattice parameters of the bcc and hcp phases of iron have been determined as a function of pressure up to 300 kbar at 23°±3°C by means of x‐ray diffraction techniques. Thec/aratio for hcp iron has been determined to be 1.603±0.001 at pressures between 80 and 300 kbar and is independent of pressure. Based upon the extrapolation of the high‐pressure data using an exponential form of the Murnaghan equation of state, the volume of hcp iron at zero pressure is 6.72±0.06 cm3/mole. The volume change for the bcc‐hcp transformation at 130 kbar is −0.34±0.01 cm3/mole. This value satisfies the triple point conditions for the bcc‐hcp‐fcc triple point when published values for the other phase transformations are used.

ISSN:0021-8979    

DOI:10.1063/1.1708965

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Experimental results indicate that, under certain conditions, ion beams undergo severe deflection preceding the region where neutralization of the beam occurs. It is believed that this effect is related to the existence of a conducting plasma which forms in the neutral region of the beam. This effect is analyzed in terms of an axially symmetric model. The plasma is simulated by assigning appropriate boundary conditions at the plasma surface. The method consists of specifying the electric field on a trial boundary, then examining the resulting potential distribution for a unipotential surface on this boundary. A sequence of numerical solutions is found in which the trial boundary is perturbed for each iterate until the Cauchy conditions are satisfied. After the location and shape of this boundary are determined, the effect of variations in electrode potentials is examined and compared with observation. It is shown that under some conditions the plasma can significantly influence the structure of the ion beam.

ISSN:0021-8979    

DOI:10.1063/1.1708966

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The absorption spectra and decay lifetimes of theMcenter triplet state (3M) andRcenter quartet state (4R) measured in irradiated KCl are similar to previous results in additively colored crystals. The wavelength dependence of the light‐induced decay indicates that the3Mcenter is deactivated by illumination in its absorption bands at 505, 590, and 685 m&mgr; (85°K), and pumped by illumination in theM1band and also in theFband andKband regions. The initial yield of3Mcenters induced by pulse illumination is proportional to theMcenter density, both for crystals of different initialFcenter density bleached to maximumMband and for a crystal that was bleached through the growth and decline of theMband. The temperature dependence of the dark decay follows a configuration coordinate model in which a transition of 45‐sec half‐time competes with a thermally‐activated step of 0.064 eV activation energy. The 14‐sec dark decay half‐time of the4Rcenter absorption at 710 m&mgr; was found to be independent of temperature from 85° to 200°K. Possible mechanisms of metastable center formation and decay are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1708967

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Kikuchi maps have been obtained for bcc and hcp crystals, and their applications to the determination of orientations and Burgers vectors are described. The hcp map is particularly useful and time‐saving, and enables orientations to be rapidly and uniquely determined. These cannot be readily obtained from hcp spot patterns in many cases. It is shown that the basal‐plane orientation, which can be tilted by about 20 deg to any of the six ⟨11¯03⟩ poles, is sufficient to provide the unique solution for determining any of the possible 21 Burgers vectors in hcp structures, and that the forbidden ⟨1¯21¯1⟩ reflections formed by double diffraction can be utilized to simplify such determinations. Kikuchi maps become increasingly valuable for investigating materials of crystal structures which have lower symmetry than that of the cubic systems.

ISSN:0021-8979    

DOI:10.1063/1.1708968

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The properties of CdTe have been evaluated by investigating the pulse‐height response of surface‐barrier devices to monoenergetic alpha particles incident normal to the surface. The energy &egr; required to form a hole‐electron pair was found to be 4.65±0.2 eV. The count rate efficiency and energy resolution that have been obtained indicate that crystals can be grown without major fluctuations in the densities of trapping or recombination centers. The effective lifetime values within the high field regions range from less than 10−9sec to 4×10−8sec for holes and electrons. These values represent a marked increase over the previously established value of 10−10sec.

ISSN:0021-8979    

DOI:10.1063/1.1708970

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Transmission electron microscopy was used to study the effects of fission fragment irradiation on single crystalline gold films as a function of the fission‐fragment energy. It is found that the density of black spots increases as the average energy of the fission fragment decreases. The observations indicate that black spots are produced in displacement cascades initiated by energetic primaries. The results are consistent with a model based on the assumption that only those events are observed in which the primary energy is greater than 2.5×104eV.

ISSN:0021-8979    

DOI:10.1063/1.1708971

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Topographic x‐ray diffraction images of ultrasonic waves in a thick silicon crystal are presented. The stationary nodal pattern of an acoustic field in the crystal is displayed, including that due to a set of plane standing waves. The images were achieved by making use of the anomalous transmission of x rays, and are of preliminary nature.

ISSN:0021-8979    

DOI:10.1063/1.1708972

出版商:AIP    

年代:1967

数据来源: AIP