Showing papers on "Fluence published in 1972"

Fast neutron tolerance of GaAs JFET's operating in the hot electron range

Abstract: Carrier removal and mobility degradation in epitaxially grown n-type GaAs, due to exposure to fast neutrons, is reported. The variations in carrier concentrations and mobilities are utilized to predict the transconductance degradation of GaAs JFET's as a function of neutron fluence. Experimental results up to a fluence of 8 \times 10^{15} n/cm2are reported and correlated with theory.

Anisotropy of the neutron fluence from a plasma focus.

Abstract: The fluence of neutrons from a plasma focus was measured by gamma spectrometry of an activated silver target. This method results in a significant increase in accuracy over the beta‐counting method. Multiple detectors were used in order to measure the anisotropy of the fluence of neutrons. The fluence was found to be concentrated in a cone with a half‐angle of 30 deg about the axis, and to drop off rapidly outside of this cone; the anisotropy was found to depend upon the total yield of neutrons. This dependence was strongest on the axis. Neither the axial concentration of the fluence of neutrons nor its dependence on the total yield of neutrons is explained by any of the currently proposed models. Some other explanations, including the possibility of an axially distributed source, are considered.

Investigation of static electron irradiation effects in bulk Si and thin Si films at energies far below threshold

Abstract: This paper investigates the defect production in Si reported to occur at electron irradiation energies far below the theoretically predicted 170‐keV bulk‐damage threshold. In contrast to published results, our experiments show no permanent resistance changes in bulk samples at fluences as high as 2×1018 20‐keV e/cm2 (78 K). Furthermore, fundamental damage studies using EPR techniques and optical spectroscopy confirm that fewer than 10−4−10−5 atomic displacements occur per 20‐keV electron. These results disagree with previous studies of irradiation‐induced resistance changes in bulk samples and epitaxial films from which it was indirectly determined that subthreshold atomic displacements appear following ionization of a Si K‐shell electron. However, we do find large resistance decreases in p‐type Si/spinel films at fluences ≳1015 20‐keV e/cm2 (300 K) that appear similar to anomalous results in the literature. But the energy and fluence dependences of the resistance change in our experiments are correctly predicted by theory for an irradiation‐induced charge accumulation layer localized within <500 A of the irradiated Si surface. The results do not indicate the presence of a bulk‐damage mechanism. The K‐shell hypothesis is clearly ruled out since the resistance effect remains at energies well below the K‐shell ionization potential. We do not find a corresponding irradiation effect in n‐type Si/spinel films which is consistent with the fact that nearly all the previously reported subthreshold irradiation effects have appeared in p‐type Si. Our results clarify some puzzling aspects of previously reported low‐energy electron irradiation effects in Si and also indicate that the unambiguous determination of defect introduction or existing defect modification by subthreshold irradiation of semiconductors requires fundamental techniques that can be rendered immune to surface phenomena.

The Energy Spectrum of Electrons in Water Irradiated with 10- to 20- MeV Electron Beams

Abstract: The spectrum of the fluence of slowed-down electrons in a water medium irradiated by an external beam of electrons from a betatron has been measured. The measurements were made at a number of depths in the medium and for a number of incident electron energies. The approach used was to set an evacuated probe at the point of interest for a number of angular increments. Electrons traveling down the probe within an angle of θ ± 0.6° were then detected by a plastic scintillator. The measured spectra were corrected for coincidences and resolution distortion by means of a response function matrix. The fluence spectra were converted to distributions in LET.

The Energy Spectrum of Electrons in Water Irradiated with

Abstract: WEISS, H., EPP, E. R., AND HESLIN, J. The Energy Spectrum of Electrons in Water Irradiated with 10- to 20-MeV Electron Beams. Radiat. Res. 51, 211-228 (1972). The spectrum of the fluence of slowed-down electrons in a water medium irradiated by an external beam of electrons from a betatron has been measured. The measurements were made at a number of depths in the medium and for a number of incident electron energies. The approach used was to set an evacuated probe at the point of interest for a number of angular increments. Electrons traveling down the probe within an angle of 0 =t 0.60 were then detected by a plastic scintillator. The measured spectra were corrected for coincidences and resolution distortion by means of a response function matrix. The fluence spectra were converted to distributions in LET.

High fluence neutron irradiation damage in copper-10 wt % aluminum and copper†

Abstract: Annealed specimens of Cu-10 wt per cent A1 and high-purity copper were irradiated to a fast (> 0.1 MeV) fluence of about 1.3 × 1022 neutrons/cm2 at an estimated irradiation temperature of 175±25 °C. Transmission electron microscopy was used to examine the structural damage after irradiation. In the alloy, equiaxed and elongated voids were seen whose distribution was nonuniform. The voids, often associated with particles and dislocation tangles, were present in number densities of about 1011/cm3 and had sizes up to about 3500 A, (equiaxed) or about 4500 A (elongated). Arrays of tangled dislocations and small dislocation loops were also observed in the irradiated alloy. The only radiation damage seen in the high purity copper was a multitude of small dislocation loops. Explanations of these observations near the lower limit (∼0.3 Tm ) of the temperature range for void formation in face-centered cubic metals are offered in terms of differences in the starting microstructures of the specimens.

A Portable Calorimeter for X‐Ray Measurements of Fluence and Front Surface Dose Simultaneously

Abstract: A small, easily portable and highly sensitive x‐ray calorimeter has been constructed to measure fluence and front surface dose in tantalum from pulsed electron accelerations. Exceptional temperature stability and hence sensitivity is achieved by mounting the absorbers in an isothermal cavity cooled by an ice reservoir. Calibration of the x‐ray fluence absorber was made by direct exposure to a 400 Ci 137Cs γ‐ray source.

ELECTRICAL RESISTIVITY INCREASE RATES IN Fe--C AND Fe--N SOLID SOLUTIONS DURING NEUTRON IRRADIATION AT 4.5 K.

Abstract: A series of α-iron specimens containing carbon and nitrogen in interstitial solid solution were irradiated simultaneously at 4.5 K with fast neutrons to a fluence of 4.7 × 1017 n/cm2 with E > 0.1 MeV. In situ electrical resistivity measurements during irradiation showed that the apparent damage rates were dependent on the C or N concentration. In particular, the value of the initial resistivity increment per unit flux (S), passed through a minimum value at 0.1 at. per cent N and became larger than the value for pure iron above 0.14 at. per cent N. The results can be explained qualitatively as interstitial impurity-induced differences in production rates and dispersion of defects or semi-quantitatively as defect-induced deviations from Matthiessen's rule.

Small Combination X‐Ray Calorimeters

Abstract: An array of x‐ray calorimeters reading absolute energy fluence and front surface dose in tantalum was developed for measuring the areal distribution of x rays from a pulsed source. The device functions in a high electrical noise environment and in pulsed magnetic fields in excess of 1 T. The fluence detector utilizes a shielded construction to allow accurate measurements to be made in inhomogeneous radiation fields.