Showing papers on "Fluence published in 1976"

Large‐spot thermal coupling of CO2 laser radiation to metallic surfaces

Abstract: Thermal coupling of pulsed 10.6‐μm laser radiation to aluminum and titanium targets was measured as a function of incident fluence, focal‐spot size, and ambient pressure. Thermal coupling coefficients were measured with both calorimetric and fast‐response surface‐thermocouple techniques. Thermal coupling coefficients of over 0.3 were observed with the onset of a well‐developed plasma at the target surface. The thermal coupling was observed to increase slightly with increasing irradiated spot size and to decrease monotonically with increasing laser fluence. Under conditions of low ambient pressure (∼0.5 Torr) the breakdown threshold was increased by a factor of 5 and at high incident fluences the thermal coupling was roughly a factor of 2 higher than at atmospheric pressure.

Profile studies of hydrogen trapping in metals due to ion damage

Abstract: The retention of hydrogen isotopes in Mo at room temperature after ion implantation damage has been studied by measurements of H depth profiles and total D retained in the near‐surface region Implantations of He, O, Ne, and Bi in a fluence range 3×1014–2×1017/cm2 were followed by 8‐keV hydrogen bombardment to fluences of 2×1015–2×1017/cm2 Large enhancements in the amount of hydrogen retained in preimplanted samples over samples without prior implantation have been observed and are interpreted in terms of damage trapping For a given predamage ion fluence hydrogen retention increases linearly with hydrogen fluence until a saturation level is reached Increased predamage fluence results in increased saturation level for hydrogen trapping, with average concentrations as high as ∼10 at% achieved An ion‐mass dependence indicates that the lighter ions, which create fewer primary displacements, are more effective in hydrogen trapping, suggesting a dependence on the damage density in the ion cascades

Chemical trapping and crystalline amorphous transition accompanying energetic proton and deuteron bombardment of silicon and germanium

Abstract: Undoped single and polycrystalline silicon and germanium disks and internal reflection plates were bombarded with 15–38 keV H+1, D+1, H+2, and D+2 to fluences up to 2×1019 ions/cm2. The infrared and Raman local‐mode frequencies of Si–H, Si–D, Ge–H, and Ge–D were observed in the ion implanted targets. The chemical trapping efficiencies of Si and Ge for the incident ions have been determined from infrared integrated absorption intensities of the hydride and deuteride bands. The chemical trapping efficiencies for D+1 and D+2 were found to be higher than those observed for H+1 and H+2 under equal ion energy and fluence conditions. The H or D/Si and H or D/Ge atom ratios approach 2 for high fluence bombardments. A trapping mechanism to account for these observations is presented. Laser Raman scattering data showed that the surface amorphization of Si and Ge occurred to a greater extent for D+1 compared to H+1 bombardments at equal fluence and incident ion energy. Annealing of H+1, D+1, H+2, and D+2 ion implant...

The investigation of ion implanted layers by scanning electron microscopy

Abstract: SEM signals were used to image ion-implanted surfaces and to quantitatively analyze implanted layers. Silicon was used as substrate material for implantation, but some measurements on GaAs are also reported. Various ion species were implanted and the dependence of the signals upon fluence was studied. Electron backscattering and absorbed current were found to be influenced by the radiation damage rather than by the species of implanted ions. The degree of damage could be characterized by absorbed current measurements. The ion fluence necessary to produce amorphous layers was determined for N, P, and As in Si using this technique. This fluence was found to correspond to an energy deposition of 2.8×1021 keV/cm3. For the detection of very small amounts of implanted ions by characteristic X-rays, the electron energy must be fitted to the penetration depth of the ions under conditions maintaining reasonable excitation cross sections. The lowest value of the normalized detectability obtained in our measurements was 2.5×1013 Ions/cm2 for 45 keV phosphorus.

Frequency Change and Elastic Constants of Quartz Irradiated by 1 MeV Electrons

Abstract: The fractions of frequency change Δf/f for quartz resonator plates exposed to 1 MeV electrons are measured as a function of electron fluence up to 2×1017 electrons/cm2, from which the adiabatic elastic constants of c44, c14 and c66 are determined. The ratio of these fractional changes is found to have a correlation of Δc44/c44: Δ|c14|/|c14|: Δc66/c66=1.3: 3.1: 1.0. The Δf/f values against fluence are calculated for various cut angles. The result indicates that the frequency in AT-cut resonators decreases a little and then increases (~10-5), but in BT- and Y-cut resonators a drastic increase and then a decrease (10-3~10-2) are expected.

Effects of 33-MeV Proton Bombardent on the Performance of CdTe Gamma-Ray Detectors

Abstract: A measurement program to investigate the performance of CdTe gamma-ray detectors after bombardment with 33-MeV protons has been undertaken in order to evaluate the effects of energetic protons encountered on earth-orbiting satellites. The pulse heights and energy resolutions of the 59.6 keV line from Am241 and the 122 keV line from Co57 and the leakage currents of the detectors were monitored as a function of proton fluence. Of the two types of CdTe detectors tested, far greater radiation effects were observed in those obtained from Tyco (chlorine doped) than in the sensor obtained from Hughes (indium doped). In the former detectors the relative pulse heights appeared to decrease slowly with increasing fluence up to about (5-9) × 109 protons/cm2 beyond which the gain degraded at a faster rate. The energy resolution and leakage currents were observed to improve at moderate fluence levels near (1-3) × 109 protons/cm2. In one of the two chlorine doped sensors tested, the intrinsic detector resolution at 59.6 keV improved from 4.3 keV to 2.6 keV at a fluence of 2 × 109 protons/cm2 and subsequently degraded beyond its original performance level after 5 × 109 protons/cm2.

Variable-fluence neutron source

Abstract: A source in which the neutron fluence can be varied makes use of a (α,n) reaction between a radioactive emitter in powdered form and a target in fluid form. The emitter is immersed in the target fluid between an upper and lower filter within a closed chamber. When the emitter rests on the lower filter in the quiescent state, radiation is limited by self-absorption and the source is in the state of minimum fluence. When the target fluid is circulated by means of a booster, the emitter is fluidized and maximum fluence is achieved. Variable intermediate values can be obtained by regulating the pressure and rate of circulation of the target fluid.

Calculated Composition Changes of Some Refractory Metals in a Fusion Reactor Environment

Abstract: Composition changes that occur during neutron bombardment have been calculated for the first wall of a hypothetical fusion reactor. The first wall materials studied are pure niobium, zirconium, molybdenum, and vanadium, and some of their binary alloys. Two integrated neutron flux intensities, 3.8 x 10$sup 14$ n/(cm$sup 2$ sec) and 3.8 x 10$sup 15$ n/(cm$sup 2$ sec) up to a fluence of 3.6 x 10$sup 23$ n/cm$sup 2$, have been used in the calculations of the first three materials. In addition, the composition as a function of fluence (maximum fluence = 4.0 x 10$sup 23$ n/cm$sup 2$) has been calculated for a vanadium wall. Graphs for each material have been plotted to show the variation of composition as a function of time and/or fluence. Rates of production of hydrogen and helium have been calculated for all four materials; comparisons for niobium and vanadium walls with literature values show agreement that it is not poor. Furthermore, mixture diagrams have been constructed for two binary alloy systems, niobium- zirconium and niobium-vanadium, to relate composition at constant irradiation time to the initial composition.

Laser heating of metallic surfaces

Abstract: : Thermal coupling of pulsed 10.6 micrometer laser radiation to aluminum, copper, and titanium targets has been measured as a function of incident fluence focal spot size, and ambient pressure, using both calorimetric and fast-response surface-thermocouple techniques. A peak enhancement in thermal coupling of approximately a factor of ten was observed to occur at the onset of a well-developed plasma at the surfaces of the copper and aluminum targets. After passing through a maximum, the enhanced coupling decreased with increasing fluence and approached CW values at high incident laser fluences. For small spot sizes (area approximately equal to, or less than 0.03 sq cm), most of the enhanced absorption occurred outside the focal spot. The fraction of energy coupled to the target within the focal spot increased with increasing spot size. Under conditions of low ambient pressure (approximately 0.5 torr), the breakdown threshold was increased by a factor of 5, and at high incident fluences the thermal coupling for aluminum was roughly a factor of 2 higher than at atmospheric pressure.

Charged Particle Damage in Wrap-Around Solar Cells

Abstract: The photovoltaic parameters of wrap-around junction cells were measured with an air-mass-zero (AMO) solar simulator as a function of fluence and temperature before and after 1.0-MeV electron and 5.0-MeV proton irradiations. These cells behaved in a similar fashion to planar junction cells with respect to their radiation-dependence. Their maximum power (P/sub max/) degradation is 36% at 2 x 10/sup 15/ e/cm/sup 2/, which is slightly less than for planar junction cells. After that point they degrade at an accelerated rate of 5.2 mW/cm/sup 2/ per decade of fluence. It was also shown that sequential irradiations of protons and electrons produced equivalent gradation of all photovoltaic output parameters irrespective of the order in which the irradiations were performed. A simple first order theory was developed which accounts for the degradation in short-circuit current caused by non-uniform radiation damage distribution resulting from 5-MeV protons in silicon. Predictions by this model show adequate agreement with experimental data for short-circuit current (I/sub sc/) and relative spectral response for proton irradiation normal either to the front or to the rear of the cell.