Search

Characteristics of CR-39 plastic containing chlorinated compounds such as HCB and DCD have been studied using relativistic heavy ions from the LBL Bevalac. These results are compared with those of pure CR-39, focusing in particular on etching properties and on whether a penetrating etch hole is produced along each particle path by a long duration etch. The relation between the reduced track etch rate and the etchant concentration for CR-39 containing HCB is quite different from that of pure CR-39. Dilute etchants gives much higher track sensitivities than do more concentrated solutions in a wide range of REL. Since type HCB/DCD CR-39 loss its sensitivity in the interior of the bulk material for low REL produced by 2.1 GeV/ n Ne ions, penetrating etch holes are not produced along the particle paths after a heavy etching.

We have conducted an underground search for supermassive relic particles by an 80 m 2 array consisting of three layers of track detector CR-39. The non-observation of penetrating tracks places the velocity-dependent flux upper limits down to 2 × 10 −14 cm −2 s −1 sr −1 for various electrically or magnetically charged particles under assumptions on their energy losses in the low- velocity region. Assuming that the nuclear recoils are effective to the track formation of CR-39, this experiment sets the best flux limits for such relic particles in the velocity region 4 × 10 −5 β −4 .

A search for massive magnetic monopoles, predicted by the grand unified theories, has been performed using plastic nuclear track detectors (cellulose nitrate) with a total area of about 100 m 2 , which were exposed at ground level for 3.3 yr. The effective solid angle for detection of monopoles is estimated over the velocity range 0.03 c v c . An upper limit of 5.2 × 10 −15 cm −2 s −1 sr −1 at 90% CL is obtained for the flux of unit magnetic monopoles with velocities higher than 0.04 c .

We are setting up a large scale (1000 m2) monopole search experiment using a highly sensitive plastic track detector CR-39. By applying a heavy etching method, this detector will perform a background-free search for GUT monopoles down to a flux of 10-16 cm-2 sec-1 sr-1 in the velocity range 10-3 < β < 1. To realize such an experiment, we have made some effort for improving the sensitivity of CR-39 plastic and the uniformity of thickness, and for reducing the number of background holes. These results will be reported as well as our experimental plan.

The cosmic-ray spectrum has an intensity enhancement in the energy range 10 to the 14th to 10 to the 16th eV per nucleus. Recent observations of heavy cosmic rays in this energy range indicate that the Ca/Fe ratio may be as large as 10 times the solar value. It is suggested that pulsars in type-II supernova remnants are the origin of this component of the cosmic-ray spectrum.

The measurement of the registration-temperature effect on track response of CR-39 plastic has been made in vacuum over a wide temperature range. The result is almost the same as that of Thompson and co-workers (1982), which was obtained in dry-air environment. Also, it is found that the fractional change of track response for alpha-particles per unit registration temperature is smaller than that for higher LET particles.

The microstructure and the magnetic properties of co-sputtered cobalt-carbon thin films were analyzed. Cobalt-carbon thin films consisting of nano-crystalline cobalt grains separated by graphite-like carbon were obtained after subsequent annealing. The phase of the cobalt grains depended on the carbon concentration; a hexagonal close-packed phase and a heavily faulted close-packed phase were observed. The grain size ranged from 4 to 20 nm. The in-plane magnetic hysteresis loops measured at room temperature showed that the coercivity and the squareness were drastically changed with the carbon concentration. These changes can be related to a change in the magnetocrystalline anisotropy of the cobalt grains due to a difference in the phase and/or size effect that can be explained by the presence of superparamagnetic particles. The highest coercivity (660 oersteds) was measured in a film with a large grain size as well as a hexagonal close-packed cobalt phase. The annealing temperature was found to have an optimum value; the magnetic properties were improved upon annealing due to carbide decomposition, but at high annealing temperatures, coalescence of the grains resulted in sample degradation.

Cobalt–carbon thin films were deposited with a carbon concentration ranging from 27 to 57 at.% at different substrate temperatures. The morphology and phases of as-deposited films were investigated by transmission electron microscopy and x-ray diffraction. The effect of annealing on the microstructure is reported. Under particular conditions of substrate temperature, carbon concentration, and subsequent annealing, a granular morphology consisting of nanocrystalline cobalt grains embedded in graphitelike carbon was obtained. The cobalt grains were uniform in size. The particle size could be controlled in the range from 4 to 7 nm by varying the carbon concentration. The cobalt phase was found to depend on the carbon concentration and substrate temperature. The hexagonal close-packed cobalt phase was observed only via the formation of the metastable carbide δ′-Co2C and its subsequent decomposition upon annealing. Otherwise the cobalt phase has a heavily faulted close-packed structure or a random stacking str...

Cross-sectional transmission electron microscopy observation of CoPtC thin films showed that 10 nm sized ultrafine particles of CoPt typically were elongated along the substrate normal. Analysis of the superposition of 40 micro-electron diffraction patterns showed that there was no preferred crystal orientation of CoPt particles. This superpositioning technique can be applied to thin films, whose X-ray diffraction analysis is difficult due to the small size of the crystals.

We report experiments in which we attempted to grow Ge and Si 0.2 Ge 0.8 films with a tetragonal crystal structure on InP(001). The substrate was chosen because it has approximately the same lattice constant as the tetragonal Si x Ge 1− x a -axis lattice constant. The tetragonal structure has been observed previously in Ge, and has a 1.5-eV bandgap. Theory predicts that the gap is direct, which implies that tetragonal Si x Ge 1− x films are suitable for optoelectronic devices. Growth on the InP(001) (2×3) and (2×4) reconstructed surfaces was investigated. Some evidence for the tetragonal phase was obtained for the growth of Si 0.2 Ge 0.8 at 100 °C on the two reconstructions, but no evidence was obtained for growth of either tetragonal Si 0.2 Ge 0.8 or Ge on the (2×3) reconstruction for growth temperatures at or above 130 °C. We conclude that tetragonal Si x Ge 1− x films on InP(001) does not seem to be a technologically feasible material system.

In nanogranular CoPtC thin films, the CoPt nanocrystals embedded in graphite-like C were found to have an elongated prolate ellipsoid-like shape. The long axes of the ellipsoids were perpendicular to the film plane, that is, in the film-growth direction. The shape magnetic anisotropy resulting from the shape of the CoPt ellipsoids gave rise to a magnetization component out of the film plane. The easy axis of the magnetization remained as that of the film plane.

The structure of Ba2YCu3O7-x(BYCO) has been almost completely analyzed using neutron,x-ray, and electron diffraction techniques. However, most of these analyses have studied bulk polycrystallinites or single crystals of BYCO, not the thin film structure. In this study, cross sections of thin annealed films on SrTiO3 substrate are observed using transmission electron microscopy (TEM). The crystal structure near the film/substrate interface is examined.The samples for TEM observation were cut from the same film which showed an onset temperature of 80 K. Samples for cross-sectional TEM observation were prepared by the ordinary method without using water. Milling was performed using a 4 kV Ar+ beam at an incident angle of about 15°. Observation was performed using a JEOL JEM 4000EX microscope at an accelerating voltage of 400 kV.A low magnification cross-sectional image of the BYCO annealed film is shown in Fig. 1. The film thickness is about 0.7 μm.

We have investigated the crystallographic properties of a silicon substrate with a buried SiO2 layer formed by oxygen-ion implantation (16O+ dose: 1.2 to 2.4×1018/cm2) and an epitaxial silicon layer on the substrate using the step-by-step sample thinning technique. The surface silicon layer on the buried SiO2 layer was found to have a dislocation-free single-crystalline silicon region, but with small precipitates of oxide. The epitaxial silicon layer grown on the surface silicon layer was found to have a high density of dislocations. The dislocation density was 2×109/cm2 and was almost constant for a substrate oxygen dose above 0.6×1018/cm2. The dislocations are considered to be caused by precipitates in the surface silicon layer. However, the epitaxial layer had fairly high mobility and no stacking faults.

Properties of the thermal oxide films of PbIn alloys have been studied by means of XPS. Prior to the oxidation, Ar sputtering and subsequent storage in UHV were carried out in order to relax the sputtering effect. The oxide films were then formed by exposing the alloys to O2 for 30 minutes. Pb oxide and In oxide were determined to be PbO x and In2O3 according to their XPS chemical shifts. By using angle resolved-XPS, PbO x was found to exist in a small oxide layer. On the assumption that an oxide layer consists of an overlying PbO x , layer and an underlying In2O3 layer, the thickness for each layer was also measured by XPS. The total oxide thickness has a maximum at In ∼12 wt.% in alloy. The In2O3 volume fraction in oxide layer increases monotonically with an increase of In concentration in alloys, and PbO x is formed only in the region below In ∼30 wt.%.

Depth distributions of crystallographic and chemical properties in oxygen ion-implanted Si layers have been studied by TEM, AES and XPS. Oxygen ions (16O+) were implanted into (100)-Si wafers at 150 keV to a dose of 1.2×1018/cm2. The dose rate was about 15 µA/cm2. The implanted oxygen forms a buried SiO2 layer after annealing. The Si layer on this buried SiO2 layer is not completely amorphized by implantation, and changes after annealing to a single crystal Si layer at the surface region and a deeper polycrystalline Si layer. A twin layer is formed at the interface region between these two layers. The polycrystalline Si layer consists of a mixture of Si and SiO2.

Ba2YCu3O7-x (BYCO) films epitaxially grown on (100) SrTiO3 substrates were examined by cross-sectional TEM. Near the film/substrate interfaces, the films consist of crystal grains whose boundaries have a regular atomic arrangement over the entire boundary area (coherent boundaries). The c axis of each crystal grain is oriented parallel to one of the three axes of the SrTiO3 substrate. Lattice constants measured with electron diffraction are a=b=c/3=0.390 nm. There are no mixed or amorphous layers at the interfaces.

A stack of CR-39 sheets has been exposed to 96 MeV/n and 2.1 GeV/n Ar beams in order to study uniformity of response and the response curve. Variation in response of the area of about 50 cm × 50 cm for one sheet was within 1.5 %. Roughness of the surface after 27 hr etching and the charge resolution were improved by changing temperature-time curing cycles. Steeper response curve at high REL region was observed and it was confirmed that our CR-39 has higher sensitivity in the recording of nuclear tracks down to Z/β ≃ 6 for vertical incidence.