Search

The sensitivity for identification of high-Z objects in elemental form in the massive cargos of intermodal containers with continuous bremsstrahlung radiation depends critically on discriminating the weak signal from uncollided photons from the very intense flux of scattered radiations that penetrate the cargo. We propose that this might be accomplished by rejection of detected events with E≤2–3 MeV that contain the majority of multiply scattered photons along with a correction for single-scattered photons at higher energies. Monte Carlo simulations of radiographs with a 9 MeV bremsstrahlung spectrum demonstrate that rejection of detected events with E≤3 MeV removes the majority of signals from scattered photons emerging through cargos with Z≤30 and areal densities of at least 145 g cm-2. With analytical estimates of the single-scattered intensity at higher energies, accurate estimates of linear attenuation coefficients for shielded and unshielded uranium spheres with masses as small as 0.08 kg are found. The estimated maximum dose is generally so low that reasonable order tomography of interesting portions of a container should be possible. [ABSTRACT FROM AUTHOR]

Factors affecting sensing of small quantities of fissionable material in large seagoing cargo containers by neutron interrogation and detection of β-delayed photons are explored. The propagation of variable-energy neutrons in cargos, subsequent fission of hidden nuclear material and production of the β-delayed photons, and the propagation of these photons to an external detector are considered explicitly. Detailed results of Monte Carlo simulations of these stages in representative cargos are presented. Analytical models are developed both as a basis for a quantitative understanding of the interrogation process and as a tool to allow ready extrapolation of our results to cases not specifically considered here. [ABSTRACT FROM AUTHOR]

Presents a study which examined the formation of a continuous amorphous layer by room-temperature implantation of boron into silicon. Background information on crystalline-to-amorphous conversion; Experiment; Results and discussion.

Presents a study which investigated the implantation of silicon wafers with gallium and phosphorus. Description of the activation energy for enhanced elimination of extrinsic category-II dislocation loops; Dissolution kinetics of defects; Examination of as-implanted morphology.

Discusses a study that explored the relationship between the implant conditions and the depth and nature of the amorphous layers generated in silicon. Role of amorphous layer morphology; Use of high-dose implants of arsenic, phosphorus and boron; Factors that make it more likely for residual crystal defects to be associated with lower weight ion implant distributions.

Presents a study which investigated the interaction between oxidation-induced point defects and type II dislocation loops intentionally introduced in silicon via ion implantation. Materials and methods used; Results; Conclusions.

Resonance Ionization Mass Spectrometry (RIMS) has been developed as a method to measure uranium isotope abundances. In this approach, RIMS is used as an element-selective ionization process between uranium atoms and potential isobars without the aid of chemical purification and separation. The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of the 235U/238U ratio to decrease laser-induced isotopic fractionation. In application, isotope standards are used to identify and correct bias in measured isotope ratios, but understanding laser-induced bias from first-principles can improve the precision and accuracy of experimental measurements. A rate equation model for predicting the relative ionization probability has been developed to study the effect of variations in laser parameters on the measured isotope ratio. The model uses atomic data and empirical descriptions of laser performance to estimate the laser-induced bias expected in experimental measurements of the 235U/238U ratio. Empirical corrections are also included to account for ionization processes that are difficult to calculate from first principles with the available atomic data. Development of this model has highlighted several important considerations for properly interpreting experimental results. [ABSTRACT FROM AUTHOR]

A natural uranium silicate mineral, cuprosklodowskite, has been analyzed by Resonance Ionization Mass Spectrometry (RIMS) for uranium isotope composition under a variety of experimental conditions. While no significant isobaric interferences are observed in the mass regions of interest, molecular supression is incomplete, and varies with the method of neutral U atom production. Despite low measured useful yields under the present analytical arrangement, it is possible to measure uranium isotope composition directly from the sample surface with only a minimum of sample preparation. U-RIMS is a method deserving further development for analyses of environmental materials. [ABSTRACT FROM AUTHOR]

When 239Pu and 235U undergo thermal neutron-induced fission, both produce significant numbers of beta-delayed gamma rays with energies in the several megaelectron volt range. Experiments using high energy-resolution germanium detectors have shown that it is possible to distinguish the fission of 239Pu from that of 235U. It is desirable to detect the presence of 235U or 239Pu using detectors that are less expensive and more rugged than high purity germanium detectors. To this end we demonstrate how differences in the energy spectrum and decay rates of the beta-delayed gamma rays can be used to identify 239Pu and 235U using low resolution plastic and liquid scintillator detectors. Experimental data are used to identify differences in the spectra and also to test the identification algorithms. Results to date are very promising. [ABSTRACT FROM AUTHOR]

The level structure ofKr has been investigated by measurements on chemically separated bromine samples. A total of 146 γ-rays have been assigned to the decay ofBr, and a level scheme with 58 levels is proposed. The collectivity ofKr has been investigated within the framework of the Interacting Boson Approximation (IBA-2 model). Comparison of our detailed γ-ray data and calculations have led to the establishment of a proton hole boson desciption rather than proton particle nature. This supports recent works establishing Z=40 as a major subshell closure. [ABSTRACT FROM AUTHOR]

Beta-delayed neutrons and high-energy γ-rays following decay ofI have been studied in order to characterize β-decay to and deexcitation of neutron-unbound levels inXe. The total γ-ray intensity observed accounts for at least 20% of the β-intensity to levels above B. No evidence was found for neutron and γ-ray emission from the same levels. With the experimental β-strength function above B detailed tests were performed on the reproduction of the experimental neutron spectrum, the level density and Porter-Thomas fluctuations of the reduced β-transition probabilities to highly excited states inXe. [ABSTRACT FROM AUTHOR]

The effect of preamorphization implantation (PAI) on boron activation in germanium was studied. It was found that following PAI, significant dynamic annealing occurred during boron implantation in germanium. For small PAI energy which leads to a thin amorphous layer, recrystallization is completed via dynamic annealing during the boron implantation. As a result, a high-temperature postimplant anneal is required to activate the remaining interstitial boron and to annihilate implantation defects. For high PAI energy, while the thick amorphous layer did not recrystallize during the dynamic annealing, it requires a high-temperature anneal in order to completely recrystallize by solid phase epitaxial regrowth (SPER). The optimized PAI energy needs to be tailored such that the surface amorphous layer not only survives dynamic annealing during boron implantation, but also completes the SPER within the designed thermal budget. Full activation of boron can then be achieved without being limited by its solid solubility in germanium. An electrically active boron concentration as high as 4.7×1020/cm3 was obtained after 400 °C rapid thermal annealing. PAI causes a similar effect in GeOI substrates. [ABSTRACT FROM AUTHOR]

Recovery of krypton from implosion of glass microballoons has been studied in the development of a radiochemical diagnostic for determination of 〈ρr)[sub fuel]. Collection onto metal surfaces following implosions performed on the OMEGA laser with 1-3 TW (1-2 kJ) of 0.35-µm light is consistent with an ion implantation mechanism. The dependence of the intrinsic collection efficiency on the energy fluence to the collector surface and its variation in implosions carried out under the same nominal conditions indicate ion energies extending to at least 0.1 MeV and energy distribution functions that are sensitive to the details of the implosion dynamics. Intrinsic sticking efficiencies approaching 0.5 can be obtained in the limit of low total energy fluence to the collector surface (≤0.1 J cm[sup -2]). [ABSTRACT FROM AUTHOR]

The final stages in the compression of microencapsulated DT fueled ICF targets require detailed characterization for meaningful comparison with predictions of hydrodynamic codes. The determination of such parameters as the fuel and shell areal densities, the average ion temperature, and the impact of implosion nonuniformities in high-density target implosions present a strong challenge. We describe several approaches utilizing the self-generated neutrons to diagnose these conditions, including neutron spectrometry, neutron activation of tracer gas and shell materials, and neutron scattering techniques. The importance of making simultaneous measurements of several core parameters to limit ambiguity in interpretation is discussed. [ABSTRACT FROM AUTHOR]

A high-efficiency, good resolution neutron spectrometer can obtain time-resolved fusion rates and mean ion energies from yield and Doppler width measurements of the 2.5 -MeV neutron peak. Gas scintillation spectrometers are capable of operating at high data rates. In order to obtain adequate energy resolution large light yields and uniform light collection over the active detector volume are required. We have measured the absolute light yield from He-Xe gas mixtures. With a high light collection efficiency the detected photon yield from the ³He(n, p)t reaction due to D-D neutrons will give adequate statistics for resolutions of a few percent. Calculations have been performed to model the detector optics in an effort to optimize the light yield and the uniformity of response. Measurements of detector resolution and time response have been made. [ABSTRACT FROM AUTHOR]

Some fundamental properties of inertial confinement fusion implosions can be measured using diagnostics based on neutron spectroscopy. An instrument that will measure neutron spectra using time-of-flight techniques with an array of "single-hit" detectors is being developed for use on Nova. Its initial application will be measurement of ion temperature. Potential other applications, such as measurement of fuel 〈ρR〈 will also be discussed. [ABSTRACT FROM AUTHOR]

We discuss two approaches to obtain neutron detectors of very high temporal resolution. In the first approach, a uranium-coated cathode is used in a streak tube configuration. Secondary electrons accompanying the fission fragments from a neutron-uranium reaction are accelerated, focused through a pinhole, and streaked. Calculations show that 20-ps time resolution can be obtained. In the second approach, a uranium-coated cathode is integrated into a transmission line. State-of-the-art technology indicates that a time resolution of 20 ps can be obtained by gating the cathode with a fast electric pulse. [ABSTRACT FROM AUTHOR]

We have constructed a detector using neutron-damaged GaAs and used it to measure subnanosecond 14 MeV neutron bursts from laser-driven deuterium-tritium filled microspheres. The detector’s very fast response (measured to be 60 ps full width at half-maximum) is due to the phenomenon of radiation-induced conductivity. The absolute sensitivity of the detector has been measured and is in excellent agreement with Monte Carlo calculations. We have also verified that the dectector is linear with neutron flux over two orders of magnitude. [ABSTRACT FROM AUTHOR]