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High-energy delayed γ -ray spectroscopy is a potential technique for directly assaying spent fuel assemblies and achieving the safeguards goal of quantifying nuclear material inventories for spent fuel handling, interim storage, reprocessing facilities, repository sites, and final disposal. Requirements for the γ -ray detection system, up to ∼ 6 MeV, can be summarized as follows: high efficiency at high γ -ray energies, high energy resolution, good linearity between γ -ray energy and output signal amplitude, ability to operate at very high count rates, and ease of use in industrial environments such as nuclear facilities. High Purity Germanium Detectors (HPGe) are the state of the art and provide excellent energy resolution but are limited in their count rate capability. Lanthanum Bromide (LaBr 3 ) scintillation detectors offer significantly higher count rate capabilities at lower energy resolution. Thus, LaBr 3 detectors may be an effective alternative for nuclear spent-fuel applications, where count-rate capability is a requirement. This paper documents the measured performance of a 2” (length) × 2” (diameter) of LaBr 3 scintillation detector system, coupled to a negatively biased PMT and a tapered active high voltage divider, with count-rates up to ∼ 3 Mcps. An experimental methodology was developed that uses the average current from the PMT’s anode and a dual source method to characterize the detector system at specific very high count rate values. Delayed γ -ray spectra were acquired with the LaBr 3 detector system at the Idaho Accelerator Center, Idaho State University, where samples of ∼ 3g of 235 U were irradiated with moderated neutrons from a photo-neutron source. Results of the spectroscopy characterization and analysis of the delayed γ -ray spectra acquired indicate the possible use of LaBr 3 scintillation detectors when high count rate capability may outweigh the lower energy resolution.

The delayed γ-ray and neutron fission signals utilized in active inspection techniques were measured simultaneously in order to directly compare their detection sensitivities. Fissionable and non-fissionable targets were irradiated by a 15-Hz pulsed bremsstrahlung beam operating at endpoint energies from 7 to 22 MeV. The fissionable mass detection limits for both these signals decreased approximately three orders of magnitude as the irradiation energy was increased with the delayed γ-ray limits 4.3–8.2 times smaller. The signals from the non-fissionable targets were consistent with the natural passive backgrounds for irradiation energies up to 16 MeV. At higher bremsstrahlung energies, there was a target independent active background in the delayed γ-ray signal that accounted for 35% of the gross yield. In addition, these higher irradiation energies resulted in products from 9Be(γ,p)8Li and 18O(γ,p)17N reactions interfering with the delayed γ-ray and neutron fission signals, respectively.

As indicated by leading healthcare agencies like the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), the rising antibiotic resistance of bacteria is one of the major concerns of modern healthcare, and the development of novel growth-based rapid diagnostics is a key in addressing this problem.[1,2] A common growth-based test used to help guide therapy is the antimicrobial susceptibility test (AST). Faster AST testing can lead to reduced inappropriate antibiotic use, decreased health care costs, and reduced prevalence of antimicrobial resistance.[3,4] To address this problem our group has been developing asynchronous magnetic bead rotation (AMBR) biosensors that have been used to detect the growth of individual bacterial cells.[5] Here, we introduce a radically new type of AMBR sensor that detects bacterial growth-based on the rotation of a cluster of magnetic microparticles formed through self-assembly. Using these self-assembled AMBR biosensors, the minimum inhibitory concentration (MIC) value of a uropathogenic Escherichia coli isolate was measured. While detection of the presence of pathogens is a widely covered topic in research and assay development, here we focus on demonstrating that AMBR can be used to perform AST testing. A new method for detecting the rotation of AMBR biosensors is also described, which hass enabled validation of a multi-well prototype for rapid observation of bacterial growth.

Successful detection of fissionable material contained in a variety of matrices was demonstrated by photon active interrogation of fissionable and inert target materials. Samples were irradiated with pulsed 15 MeV photons generated by a LINAC and tungsten electron/photon converter, operating at 15 Hz. Matrix materials included air (no matrix), wood, water, and lead. A unique dual mode gamma/neutron detector was used to acquire data from both fission product gamma and fission product neutron emission. Neutron emission was recorded by detecting the 478 keV capture gamma from the 10 B (n,α) 7 Li reaction, generating a photopeak in the recorded gamma spectrum. Two signatures were found to correctly differentiate between the fissionable target ( 238 U) and inert targets (lead, steel, air, and beryllium), with substantial differences in delayed gamma and neutron signatures for fissionable and inert materials in all cases. The signatures are simple to compute and are not significantly affected by system variations or interferences expected during cargo scanning.

The surface of an annealed 316L stainless steel coupon was laser shock peened and Vickers hardness measurements were subsequently taken of its surface. This Vickers hardness data was compared with measurements taken using the technique of positron annihilation Doppler broadening spectroscopy. When compared, a correlation was found between the Vickers hardness data measurements and those made using Doppler broadening spectroscopy. Although materials with a high defect density can cause the S-parameter measurements to saturate, variations in the S-parameter measurements suggest that through further research the Doppler broadening technique could be used as a viable alternative to measuring a material’s hardness. In turn, this technique, could be useful in industrial settings where surface hardness and surface defects are used to predict lifetime of components.

Active, nondestructive interrogation with neutrons and photons has seen a renaissance in recent years, owing to a broad spectrum of important applications in security, nuclear nonproliferation, contraband detection and materials analysis. Active methods are of high interest for such applications because they provide at least an order of magnitude greater sensitivity than passive methods. Accelerator-based neutron and photon active methods exploit two important factors to attain greater sensitivity: these are (i) the control of interrogating beam properties such as directionality, energy, intensity, polarization and the temporal distribution of radiation; (ii) well-founded, low energy nuclear physics that yields distinct "signatures" for elemental and isotopic content. This review addresses accelerator-based neutron and photon nondestructive testing methods and issues when applied to modern and emerging wide-ranging challenges in nondestructive detection.

The detection and identification of delayed fission γ -rays emitted between intense bremsstrahlung pulses was investigated as a method to discriminate between fissionable and non-fissionable materials. Photofission and other photonuclear reactions were induced in U 238 , Pb, Fe and Be targets using a pulsed bremsstrahlung beam for several endpoint energies between 8 and 20 MeV . In the 33 ms between irradiating pulses, correlated photon energy and time spectra were collected using a high-purity germanium detector. When non-fissionable materials were irradiated, the high-energy γ -ray yield decreased a couple orders of magnitude within the first ∼ 12 ms after the pulse. In contrast, the high-energy γ -rays emitted from the decay of fission fragments were nearly constant throughout the period between bremsstrahlung pulses. Hence, a basic energy and time discrimination provides a unique high-energy delayed γ -ray signature for fissionable material detection.

The method presented herein uses the MCNPX Monte Carlo particle transport code to track individual positrons and other particles through geometry that accounts for the detectors, backing foils, samples and sources with their actual sizes, positions and material characteristics. Polymer material, polydimethylsiloxane (PDMS), with different thickness of films served as samples. The excellent agreement between the experimental results and the MCNPX simulation of source correction effects for varied positron sources and different film thicknesses validates the applicability of the MCNPX code.

Positron Systems, Inc. applies surface and volumetric positron techniques to investigate damage in metal components in operational conditions. The width and shape of Doppler Broadening gamma peak (S-parameter) is used for defect analysis. Two techniques are commonly used: the Induced Positron Analysis - Surface technique (IPA-S) to investigate a surface and near-surface effects and the Induced Positron Analysis- Volumetric technique (IPA-V) that penetrates through the volume of the examined materials. Positron Annihilation measurements have been successfully performed in many applications. Presented below are cases demonstrating successful applications in assessing damage in gas generator fuel injector stems, aerospace parts that underwent strain and fatigue treatment, and residual stress effects in shot-peened specimens and steel plates with cracks. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The development of the APosS at the Argonne low-energy linac is discussed. We measured the yields and outline strategies to increase the number of slow positrons. We describe the measurement of the yield of slow positrons as a function of electron-beam energy and compare these results with theoretical calculations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Idaho National Laboratory, in collaboration with Idaho State University's Idaho Accelerator Center and the Los Alamos National Laboratory, continues to develop the Pulsed Photonuclear Assessment (PPA) technique for shielded nuclear material detection in large volume configurations, such as cargo containers. In recent years, the Department of Homeland Security has supported the development of a prototype PPA cargo inspection system. This PPA system integrates novel neutron and gamma-ray detectors for nuclear material detection along with a complementary and unique gray scale, density mapping component for significant shield material detection. This paper will present the developmental status of the prototype system, its detection performance using several INL Calibration Pallets, and planned enhancements to further increase its nuclear material detection capability.

The development of the Argonne Positron Source (APosS) at the Argonne National Laboratory low-energy electron linac is discussed. We describe the measurement of the yield of slow positrons as a function of electron-beam energy and compare these results with theoretical calculations. Those results were used to establish the optimal regime for positron production.

As many pulsed photonuclear assessment (PPA) technologies are being developed for contraband detection within cargo container configurations, the radiation safe operation of linacs for outdoor operations needs to be addressed. Idaho National Laboratory along with the Idaho Accelerator Center are conducting field operations with high energy linacs in open outdoor configurations. The relevant information pertaining to the radiation regulations and dosimetry studies for these configurations will be presented for a prototypical 10 MeV PPA nuclear material detection system.

Pulsed photonuclear interrogation environments generated by 8–24 MeV electron linac are rich with time-dependent, material-specific, radiation signatures. Nitrogen-based explosives and nuclear materials can be detected by exploiting these signatures in different delayed-time regions. Numerical and experimental results presented in this paper show the unique time and energy dependence of these signatures. It is shown that appropriate delayed-time windows are essential to acquire material-specific signatures in pulsed photonuclear assessment environments. These developments demonstrate that pulsed, high-energy, photon-inspection environments can be exploited for time-dependent, material-specific signatures through the proper operation of specialized detectors and detection methods.

The pulsed photonuclear assessment (PPA) technique, which has demonstrated the ability to detect shielded nuclear material, is based on utilizing delayed neutrons and photons between accelerator pulses. While most active interrogation systems have focused on delayed neutron and gamma-ray signatures, there is an increasing need to bring faster detection and acquisition capabilities to field inspection applications. This push for decreased interrogation times, increased sensitivity, and mitigation of false positives requires that detection systems take advantage of all available information. Collaborative research between Idaho National Lab (INL), Idaho State University’s Idaho Accelerator Center (IAC), Los Alamos National Laboratory (LANL), and Oak Ridge National Laboratory (ORNL), has focused on exploiting actively-induced, prompt radiation signatures from nuclear material within a pulsed photonuclear environment. To date, these prompt emissions have not been effectively exploited due to difficulties in detection and signal processing inherent in the prompt regime as well as an overall poor understanding of the magnitude and yields of these emissions. Exploitation of prompt radiation (defined as during an accelerator pulse/photofission event and/or immediately after (

The Idaho Accelerator Center (IAC) of Idaho State University has developed a unique radiation research facility to answer next-generation radiobiological questions. The IAC has 10 operating research accelerators. These include continuously delivered radiation beams such as a 950 keV electron beam and a 2 MeV light-ion Van de Graaff. The IAC also has a number of pulsed electron linacs which range in energy from 4 to 40 MeV. The most intense amongst them deliver peak dose rates greater than 1012 Gy/s. The operational flexibility of pulsed electron linacs allows control of peak and average dose rate, pulse separation and total dose over many orders of magnitude in these parameters. These high dose rates also allow delivery of large doses on time scales that are very small when compared to biological responses. The spectrum of particle beams that the IAC can deliver includes alphas, protons, neutrons, electrons (betas), and gammas (X-rays). Current radiobiological research at the IAC is focused upon radiation effects in unicellular organisms. The effectiveness of extremely high dose rate electron irradiation for the neutralization of microbes is being investigated. Concurrently, we are characterizing the survival mechanisms employed by microbes when exposed to these extremely high doses and dose rates. We have isolated strains from several diverse species that show increased radiation-resistance over normal populations. In addition, we were the first to demonstrate radiation-induced Bystander effects in unicellular organisms. Because of the numerous and diverse accelerators at the IAC, these and many other novel radiobiological investigations are readily attainable.

An examination of delayed neutron emission decay rates allows for the detection and identification of fissionable isotopes. The dependence of the decay rates on bremsstrahlung end-point energies and accelerator repetition rates was examined. Delayed neutrons emitted following photofission in a 238 U target were detected between bremsstrahlung pulses from a variable-energy linear electron accelerator. Bremsstrahlung end-point energies of 14, 16, 18, 20, and 22 MeV are used to show that the decay rates have no obvious dependence on the energy of the irradiating photons. Accelerator repetition rates of 3.75 , 7.5 , 15, 30, and 60 Hz demonstrate the expected frequency dependence of the decay rates.

Radiation damage and other defect studies of materials are limited to thin samples because of inherent limitations of well-established techniques such as diffraction methods and traditional positron annihilation spectroscopy (PAS) [P. Hautojarvi, et al., Positrons in Solids, Springer, Berlin, 1979, K.G. Lynn, et al., Appl. Phys. Lett. 47 (1985) 239]. This limitation has greatly hampered industrial and in-situ applications. ISU has developed new methods that use pair-production to produce positrons throughout the volume of thick samples [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427, F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262]. Unlike prior work at other laboratories that use bremsstrahlung beams to create positron beams (via pair-production) that are then directed at a sample of interest, we produce electron–positron pairs directly in samples of interest, and eliminate the intermediate step of a positron beam and its attendant penetrability limitations. Our methods include accelerator-based bremsstrahlung-induced pair-production in the sample for positron annihilation energy spectroscopy measurements (PAES), coincident proton-capture gamma-rays (where one of the gammas is used for pair-production in the sample) for positron annihilation lifetime spectroscopy (PALS), or photo-nuclear activation of samples for either type of measurement. The positrons subsequently annihilate with sample electrons, emitting coincident 511 keV gamma-rays [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427, F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262]. These gamma-ray photons are then either measured with a high-resolution germanium detector (PAES) or fast scintillators (PALS) and subsequently analyzed using standard positron data analysis methods. The high penetrability of few MeV photons allows one to study defects and characterize materials in thick samples up to hundreds of g/cm2 (approximately a meter in steel), a thickness that is completely inaccessible by any other non-destructive technique. We have demonstrated the proof-of-principle of these techniques to probe tensile strain in thick steel alloys and other metals, to measure positron lifetimes in bulk samples of lead, copper and aluminium with positron lifetime spectra that are free of the surface and source background lifetimes that complicate conventional positron lifetime measurements, and demonstrated the activation technique for damage studies of copper and single-crystal iron [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427]. We have also demonstrated the potential application of these techniques to 3-D imaging of defect density in thick structural materials [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262].

As part of the Reactor Accelerator Coupling Experiment (RACE) a set of preliminary studies were conducted to design a transport beam line that could bring a 25 MeV electron beam from a linear accelerator to a neutron-producing target inside a subcritical system. Because of the relatively low energy beam, the beam size and a relatively long beam line (implicating a possible divergence problem) different parameters and models were studied before a final design could be submitted for assembly. This report shows the results obtained from different simulations for different lattices of the transport line optics and dynamics.

A systematic set of Monte Carlo simulations of slow-positron production by energetic electrons impinging on normal and glancing incident targets are presented. The estimated slow-positron production efficiencies are found to increase linearly with incident electron energy when using converters of optimal thickness. The two targets perform equally well below 20 MeV but at higher energies the glancing incident target is more efficient. This increase in efficiency is caused by a decrease in the converter’s positron production volume, allowing a larger fraction of low-energy primary positrons to escape from the converter and impinge on the moderators. Further increases in the glancing incident target’s efficiency may be possible by altering the arrangement of the converter–moderator system.

Two techniques have recently been developed to quickly and easily apply positron annihilation spectroscopies to large structural components found in civil engineering, aviation, etc. In this paper, the authors discuss how to extend imaging capabilities to these new techniques, which will enable defect imaging similar to that obtained with positron micro-beams but at much larger sample sizes. Preliminary two-dimensional defect imaging results are presented from a highly damaged 30.5 × 30.5 cm copper plate.

The Idaho National Laboratory has developed prototype shielded nuclear material detection systems based on pulsed photonuclear assessment (PPA) techniques for the inspection of cargo containers. During this work, increased nuclear material detection capabilities have been demonstrated at higher electron beam energies than those allowed by federal regulations for cargo inspection. This paper gives a general overview of a nuclear material detection system, the PPA technique and discusses the benefits of using these higher energies. This paper also includes a summary of the numerical and test results from LINAC operations up to 24 MeV and discusses some of the federal energy limitations associated with cargo inspection.

A High Voltage Engineering K-4000 Van de Graaff ion accelerator has been acquired by Washington State University. This accelerator will be used to create a positron source through the 12 C(d, n) 13 N reaction. The accelerator is capable of accelerating protons or deuterons up to 4 MeV with a maximum beam current between 200 and 300 μA. During normal operation, a 3 MeV deuteron beam will impinge on a diamond or graphite target producing ∼3.3 Ci of 13 N from which a ∼1×10 7 s −1 monoenergetic positron beam will be created. In addition to creating positrons, the source will also emit a large number on neutrons (∼1.2×10 11 s −1 ), producing a serious radiation hazard. The development of this 13 N based positron beam is discussed including the expected positron yield and radiation shielding requirements.

This project addresses the need for improved non-destructive assay techniques for quantifying the actinide composition of spent nuclear fuel and for the independent verification of declared quantities of special nuclear materials at key stages of the fuel cycle. High-energy delayed gamma-ray spectroscopy following neutron irradiation is a potential technique for directly assaying spent fuel assemblies and achieving the safeguards goal of quantifying nuclear material inventories for spent fuel handling, interim storage, reprocessing facilities, repository sites, and final disposal. Other potential applications include determination of MOX fuel composition, characterization of nuclear waste packages, and challenges in homeland security and arms control verification.

We investigated a red to yellow color change observed in α-HgI 2 when cooled below 150 K. A phase transformation to β-HgI 2 , which has a yellow color, was ruled out by a variable temperature x-ray diffraction study. Instead the color change at cryogenic temperatures is caused by a shift in the transmission edge to shorter wavelengths which we attribute to a widening band gap at low temperatures. Using optical transmission spectroscopy the width of the band gap was measured between 10 and 330 K. At room temperature the gap was 2.11 ± 0.03 eV which is significantly smaller than the most recently published values of ~ 2.3 eV. This smaller band gap was further verified by measuring the thermoelectric current at elevated temperatures.

We report the first observation of electron momentum contributions to the Doppler broadening of radiation produced by in-flight two-photon annihilation in solids. In these experiments an approximately 2.5 MeV positron beam impinged on thin polyethylene, aluminum, and gold targets. Since energetic positrons easily penetrate the nuclear Coulomb potential and do not cause a strong charge polarization, the experimental annihilation line shapes agree well with calculations based on a simple independent-particle model. Moreover, annihilations with the deepest core electrons are greatly enhanced.

A new monoenergetic MeV positron beam source was designed and constructed at the Lawrence Livermore National Laboratory (LLNL) positron facility. The positron source provides a low emittance beam with high energy resolution for channeling, scattering, and in-flight annihilation experiments. New beam optics for extracting slow positrons from a moderator mounted in a Pelletron electrostatic accelerator is presented. A beam line was designed and constructed to energy analyze the output of the accelerator and to provide adjustable size and angular divergence beams to a target station. A post target large angle acceptance magnet separates positrons transmitted through the target from forward gamma ray emission.

As a step towards developing fast positrons as a probe to measure electron density and spin magnetic density in crystals, an experiment has been performed to measure the multiple scattering distribution of MeV positrons penetrating through thin polycrystalline films in both random and channeling directions. Image plate detectors were used to determine the angular distribution of emergent positrons. Image plate sensitivity was measured for high-energy positrons and for the background radiation encountered in our experiment. The image plates showed good sensitivity and good linearity for MeV positrons. The multiple scattering results are compared with the theory for random direction and show disagreement due to crystal effects and quantum mechanical considerations.

Energetic positrons propagating along low index directions in a crystal interact strongly with the periodic array of atoms via a process known as channeling. The trajectories of such channeled positrons can be manipulated to sample different spatial regions in the crystal. Here we report the first observations of channeling effects on in-flight annihilation radiation from positrons traversing a thin gold crystal. The channeling minimum yield for two-photon annihilation is shown to be significantly enhanced compared to single photon annihilation due to the selective interaction of well-channeled positrons with valence electrons in the interstitial regions of the crystal. Both classical and quantum calculation of annihilation yields are compared to the experimental data.

Energy-resolved two-quantum annihilation in flight of positrons with energies ranging from 10 to 71.6 keV was observed. An energy-dispersive two-detector coincidence system was used to observe the sum and difference energies of the {gamma} rays from annihilating positron-electron pairs. For positrons penetrating carbon foils the c/v dependence of the annihilation cross section is confirmed. Spectra obtained from gold-coated carbon foils show evidence of in-flight annihilation with gold M-shell electrons. (c) 1999 The American Physical Society.

Energetic, positively charged particles travelling along a low-index crystal direction undergo many highly correlated, small-angle scattering events; the effect of these interactions is to guide or ‘channel’ (refs 1,2,3,4,5,6,7,8) the particles through the lattice. Channelling effectively focuses positive particles into the interstitial regions of the crystal: nuclear collisional processes such as Rutherford backscattering are suppressed, while the number of interactions with valence electrons increases. The interaction of channelled positrons with electrons produces annihilation radiation that can in principle9,10,11,12 serve as a quantitative, spatially selective probe of electronic charge and spin densities within the crystal: in the interstitial regions, two-photon annihilation is enhanced relative to single-photon annihilation, because the latter process requires a nuclear recoil to conserve momentum. Here we report observations of single- and two-photon annihilation from channelled positrons, using a monoenergetic beam flux of 105 particles per second. Comparison of these two annihilation modes demonstrates the ability of channelled positrons to selectively sample valence electrons in a crystal. Useful practical implementation of the technique will require the development of more intense positron beams with fluxes approaching 107 particles per second.

We report on the first direct energy resolved measurements of two quantum annihilation in flight (TQAF) using monoenergetic positrons below 100 keV. A monoenergetic positron beam at Washington State University with variable kinetic energy up to 70 keV was guided onto thin aluminum targets. To clearly identify in flight positron annihilation a two dimensional Doppler broadening technique was employed utilizing two HPGe detectors. The method uniquely identifies TQAF by providing the total energy of the annihilating electron-positron pair. A basic understanding of the fundamental TQAF process has been obtained using this high resolution technique. With this fundamental understanding we believe several new avenues of experimental exploration are possible.

The subtleties of the interplay between classical and quantum pictures are revealed particularly clearly in connection with the effect of channeling. The role of this interplay is described and related to a new probe for valence electron and spin magnetic densities in thin films, which is based upon two-photon annihilation in flight of channeled MeV positrons. The channeling effect is used to steer positrons into the interstices of a crystal, where the valence electron density is high. Transmission measurements have been performed to test our quantitative understanding of the spatial and momentum distribution of channeled positrons. We calculate annihilation rates in a gold crystal and show that the probe is highly sensitive to valence electrons. Channeling states in iron are shown to be sensitive to different regions of the spin magnetic density. With a polarized beam of MeV positrons and the possibility for selective state population, we show that the spatial distribution of the magnetization density can be probed.

We report promising performance from prototype doped ZnO scintillator crystals. The scintillation properties of ZnO in powder form have long been known. Recently, Cermet Inc. has developed the capability to produce single crystals of ∼1 cm 2 area, using a high-pressure direct melting technique. Crystals of several doping species and concentrations have been produced and tested. These crystals exhibit pulse rise-times

We address the problem of MBE growth in one horizontal and one vertical direction in the presence of Schwoebel barriers. The time-independent growth equation introduced previously is shown to be identical to that for a classical particle in a potential well. We solve this equation using periodic boundary conditions and find time-independent solutions consisting of a periodic array of mounds. We derive the "dispersion relation", i.e. the amplitude as a function of wavelength for these mounds. The equation of motion is derivable from a free energy indicating that there is a most stable ground state, which is independent of the initial conditions. The mounds are marginally unstable and there is a minimum wavelength below which no mounds exist. The wavelength of the mounds coarsens slowly in time according to Λ ~ tα, with α ≈ 1/4.

Scanning tunneling microscopy studies have been performed on GaAs homoepitaxial films grown by molecular-beam epitaxy. Images show that in the earliest stages of deposition the morphology oscillates between one with two-dimensional islands and flat terraces. After the initial transient regime, the system evolves to a dynamical steady state. This state is characterized by a constant step density and as such the growth mode can be termed step flow. Comparison with reflection high-energy electron-diffrac- tion (RHEED) shows that there is a direct correspondence between the surface step density and the RHEED specular intensity. Thick films (up to 1450 monolayers) display a slowly-increasing surface roughness. Analysis of the scaling properties and comparison with theories of film growth will be made.

We consider the growth of films by molecular beam epitaxy in the presence of step-edge (Schwoebel) barriers using numerical simulation and experiments. We show that the growth of a singular surface is unstable, but that a miscut above a certain critical slope (which depends on growth conditions) leads to stable growth in a step-flow mode. For singular surfaces the instability gives rise to the formation of large mounded structures on the surface for which the slope is in the stable regime. We identify these in GaAs epitaxy using atomic force and scanning tunneling microscopy. We propose a continuum equation which exhibits these features.

This paper describes the use of intense, pulsed bremsstrahlung for inducing photofission in fissionable material. 8-MV bremsstrahlung triggers photofission in depleted uranium (DU). Particle-in-cell and Monte-Carlo codes model the source, gamma and neutron production and transport, and detector response. A variety of detectors have been fielded successfully to measure fission in DU from delayed-neutron, delayed-gamma, and prompt-neutron signals. This work represents the first clean measurement of prompt photofission neutrons in an intense, pulsed-power environment.

Prompt fission neutrons continue to be examined as a signature for detecting the presence of fissionable material. This technique exploits the neutron energy limitations inherent with photonuclear emissions from non‐fissionable material, allowing prompt fission neutrons to be identified and engaged for detecting nuclear material. Prompt neutron signal measurements were acquired with bremsstrahlung endpoint energies of 6 MeV for 18 targets comprised of both fissionable and non‐fissionable material; delayed neutron measurements were also collected as a reference. The 238U target was also shielded with increasing thicknesses of lead or borated polyethylene to compare the resulting detection rates of the prompt and delayed fission neutron signals.

Scanning tunneling microscopy studies have been performed on GaAs homoepitaxial films grown by molecular-beam epitaxy. Images show that in the earliest stages of deposition the morphology oscillates between one with two-dimensional islands and flat terraces. After the initial transient regime, the system evolves to a dynamical steady state. This state is characterized by a constant step density and as such the growth mode can be termed step flow. Comparison with RHEED shows that there is a direct correspondence between the surface step density and the RHEED specular intensity. Furthermore, thick films (up to 1450 monolayers) display a constant or slowly increasing surface roughness consistent with long adatom diffusion lengths and limited upward diffusion.

In intense, pulsed active detection, a single, intense pulse of radiation is used to induce photofission in fissionable material, increasing its detectability. The Mercury pulsed-power generator was converted to positive polarity (+3.7 MV, 325 kA, 50-ns FWHM) to drive an intense, pulsed radiation source based on the FIGARO active detection concept. The probing radiation source consisted of an ion-beam diode and a thick PTFE (Teflon) converter where 6–7 MeV γ-rays were produced via the 19F(p,αγ)16O reaction. A suite of radiation detectors was used both to detect the presence of irradiated fissionable material and to characterize the probing radiation source. Four types of detectors were used for the source characterization. Thermoluminescent dosimeters were used to measure the angular distribution of the dose associated with x-rays and γ-rays from the ion-beam diode. Plastic scintillator-photodiode detectors were used to characterize the time dependence of this dose. A plastic scintillator-photomultiplier detector was used to monitor the γ-ray intensity of the probing radiation source and to monitor changes in the production of background neutrons by the diode and PTFE converter. A set of rhodium foil activation counters was used to measure the absolute yield of these background neutrons. Two types of detectors with comparable sensitivities were used to measure delayed neutrons resulting from photofission: 3He proportional counters and a 6Li-loaded-glass-scintillator detector. The neutron detection rate from each detector following the probing radiation pulse was over 100 times higher with depleted uranium present than with lead.

The detection of fissile material by passive and active techniques is an area of intense interest. Many active approaches detect the products of fission induced by photon and/or neutron irradiation.1 One such approach has focused on using intense, pulsed, beam-target interactions to produce an irradiation source of 6.13-, 6.92-, and 7.13-MeV characteristic gamma rays from the 19F(p, αγ)16O reaction.2,3 Initially, work at NRL utilized the Gamble-II, 3-Ω, water-line generator with a pinch-reflex ion diode that accelerates ions to 2 MeV at ~ 0.5-TW peak ion power.3 This pulsed source also simultaneously produces neutrons from: (1) a collective ion-acceleration mechanism associated with the diode and (2) the reactions resulting from the isotopic abundance of deuterons in the ion beam, e.g., 12C(d, n)13N and 19F(d, n)20Ne. In this presentation, we review progress toward characterizing a similar mixed source using the Mercury inductive voltage adder operated in positive polarity producing ~ 4-MeV4 ion energy and ~ 0.3-TW peak ion power. An array of diagnostics is used to determine the number of gamma rays and neutrons produced by the source, including a shielded scintillator-photomultiplier for the gamma-ray time history and a rhodium-activation counter to measure the total number of neutrons produced. In addition, the gamma and neutron source strengths are inferred from delayed neutrons that result from fissions in depleted uranium and that are measured using 3He and 6Li neutron detectors.

Nuclear Resonance Fluorescence (NRF), which is possible for nuclei with atomic numbers greater than helium (Z=2), occurs when a nuclear level is excited by resonant absorption of a photon and subsequently decays by reemission of a photon. The excited nuclear states can become readily populated, provided the incident photon’s energy is within the Doppler-broadened width of the energy level being excited. Utilizing continuous energy photon spectra, as is characteristic of a bremsstrahlung photon beam, as the inspection source, ensures that at least some fraction of the impinging beam will contribute to the population of the excited energy levels in the material of interest. Upon de-excitation, either to the ground state or to a lower-energy excited state, the emitted fluorescence photon’s energy will correspond to the energy difference between the excited state and the state to which it decays. As each isotope inherently contains unique nuclear energy levels, the NRF states for each isotope are also unique. By exploiting this phenomenon, NRF photon detection provides a well-defined signature for identifying the presence of individual nuclear species. This report summarizes the second year (Fiscal Year [FY] 2009) of a collaborative research effort between Idaho National Laboratory, Idaho State University’s Idaho Accelerator Center,more » and Pacific Northwest National Laboratory. This effort focused on continuing to assess and optimize NRF-based detection techniques utilizing a slightly modified, commercially available, pulsed medical electron accelerator.« less

Shot peening is a technique used in industry to increase the fatigue life of components by creating compressive residual stresses in the near surface region. This compressive stress is pinned in the material by defects such as dislocations and monovacancies to which positrons are sensitive. Using a 22Na source S‐parameter measurements were made using positron annihilation spectroscopy (PAS) on both non‐peened and peened copper coupons. These measurements show that a correlation exists between copper coupons shot peened at different intensities that in principal can be used for verification of shot intensity. Finally a relative defect density map was produced to show that the shot peening uniformity can also be measured across the surface of a large component.

This study measures Radiation Induced Conductivity (RIC) of Low Density Polyethylene (LDPE) over temperatures ranging from {approx}110 K to {approx}350 K. RIC occurs when incident ionizing radiation deposits energy and excites electrons into the conduction band of insulators. Conductivity was measured when a voltage was applied across vacuum-baked, thin film LDPE polymer samples in a parallel plate geometry. RIC was calculated as the difference in sample conductivity under no incident radiation and under an incident {approx}4 MeV electron beam at low incident fluxes of 10{sup -4}-10{sup -1} Gr/sec. The steady-state RIC was found to agree well with the standard power law relation, {sigma}{sub RIC} = k{sub RIC}{center_dot}D ring {sup {delta}} between conductivity, {sigma} and adsorbed dose rate, D ring . Both the proportionality constant, k{sub RIC}, and the power, {delta}, were found to be temperature dependant above {approx}250 K, with behavior consistent with photoconductivity models developed for localized trap states in disordered semiconductors. Below {approx}250 K, kRIC and {delta} exhibited little change. The observed difference in temperature dependence might be related to a structural phase transition seen at T{sub {beta}}{approx}256 K in prior studies of mechanical and thermodynamic properties of LDPE.

Single neutron and gamma charge pulses were captured using a fast ADC. These data were used to populate simulated 5-µs active interrogation flash events. Analysis of these simulations revealed that long pulse tail accumulation during the flash leads to an unpredictable charge background for several microseconds after the flash. The existence of this changing background will make retrieving prompt neutron information immediately after the flash problematic. The tail accumulation is accentuated for every pulse in the flash, making the time just after the last pulse in the flash the worst possible time to measure prompt neutron data. Obtaining prompt neutron data during the flash—particularly near the beginning of the flash, where background tail accumulation is minimal—is feasible if shielding is used to control the count rates, but requires fast gamma-neutron pulse shape discrimination. A means of accentuating the pulse shape difference between neutron and gamma data is given, along with a means of compensating for the long pulse components in the dominant gamma pulses.

Nuclear resonance fluorescence (NRF) is a photon-based active interrogation approach that provides isotope-specific signatures that can be used to detect and characterize samples. As NRF systems are designed to address specific applications, an obvious first question to address is the type of photon source to be employed for the application. Our collaboration has conducted a series of NRF measurements using different photon sources to begin to examine this issue. The measurements were designed to be as similar as possible to facilitate a straight-forward comparison of the different sources. Measurements were conducted with a high-duty factor electron accelerator using bremsstrahlung photons, with a pulsed linear accelerator using bremsstrahlung photons, and with a narrow bandwidth photon source using Compton backscattered photons. We present our observations on the advantages and disadvantages of each photon source type. Issues such as signal rate, the signal-to-noise ratio, and absorbed dose are discussed.