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Evolution of the proton induced luminescence has been examined in CdS layers and ZnSe crystals during exposure to a 1.6 MeV proton beam. The scintillation characteristics of the ZnSe and CdS measured using the identical regimes have been compared. The difference of the carrier pair generation mechanisms inherent for light and for a proton beam has been revealed. [ABSTRACT FROM AUTHOR]

Previous measurements of highly enriched uranium (HEU) storage castings performed by Oak Ridge National Laboratory (ORNL) at the Y-12 National Security Complex showed a prompt neutron time decay that is not exponential. These measurements showed that multiple time constants—originating from multiplication, time-of-flight, scattering in the assembly and room return—could be associated with this prompt neutron decay. In this work, the contribution not associated with neutron multiplication was investigated via measurements with a depleted uranium (DU) casting. The measurements at ORNL used an annular (5.0 in. OD, 3.5 in. ID, 6.0 in. H) DU casting with a time-tagged 252Cf source, centered vertically on the axis, and four closely coupled 1 × 1 × 6 in. long plastic scintillators with ¼ in. thick lead shielding adjacent to the outer surface of the casting. This setup was identical to the configuration used in the previously performed measurements with HEU castings at Y-12. The time correlation between fission events and detections in the plastic scintillators was measured, as well as the time distribution of coincidences between multiple detectors within a 512 ns time window. The measurement results were then compared with MCNP-PoliMi calculations and the previous HEU measurements. Time constants from decay fits to the HEU and DU data were compared to characterize the contributions resulting from multiplication, time-of-flight, and scattering. [ABSTRACT FROM AUTHOR]

Neutron spectroscopy has direct application to the detection of special nuclear materials (SNM) for nuclear safeguards and nonproliferation. Standard hydrogen-based liquid scintillators have been investigated for their potential nuclear safeguards and nonproliferation applications. However, deuterated liquid scintillators such as C6D6 (deuterated benzene, EJ315) show promise in providing more detailed energy information about detected neutrons than their hydrogen-based counterparts, such as BC501 and EJ309, at neutron energies greater than 2.5 MeV (1) (2). We will consider application of deuterated scintillators to neutron sources relevant to nuclear safeguards and nonproliferation (for example fission sources such as 252Cf). Detection of SNM in the presence of diverse background radiations also requires reliable pulse-shape-discrimination (PSD), and detection efficiency comparable to hydrogen-based liquid scintillators. Here, we present PSD characteristics and measured intrinsic neutron detection efficiency for a 2×2-inch cylindrical C6D6 (EJ315) scintillator. Detection efficiency is measured as a function of incident neutron energy from 0.5 to 5 MeV using a bench-top time-of-flight technique and a 252Cf source. PSD capabilities are compared with those of the hydrogen-based EJ309. In addition, simulations of efficiency versus energy for geometrically identical C6D6 and C6H6 detectors have been performed using the Monte Carlo code MCNP-PoliMi. [ABSTRACT FROM AUTHOR]

A more efficient process for developing and fielding new radiation detection technology is needed. Radiation detection systems are frequently installed and implemented based on immediate needs. This procedure often occurs without considering all the factors that will influence the ultimate effectiveness of the system. For instance, in major shipping ports portal radiation monitors are primarily made out of plastic scintillators and are placed at specific exits to detect illicit radioactive material. This placement of detectors helpfully minimizes their affect on port operations, but is it the most efficient placement and available technology for the mission? As new detector technology appears on the market, rather than simply replacing current detectors with new ones, a systems approach should be applied to most effectively deploy the new technology. Texas A&M University, through a grant from the Domestic Nuclear Detection Office, is developing a framework that will help evaluate new detection technologies and examine how to use that technology in the most efficient way. This paper focuses on the detection of shielded highly enriched uranium in large shipping containers. Our multi-disciplinary approach includes detector and transport code development and takes into account industrial engineering and socio-economic concerns. The presentation will highlight how new detector technology is incorporated into the framework and how both Monte Carlo and Deterministic simulations are used to optimize detector types and placement. Socio-economic studies will be described, as well has how best practices from the shipping port industry are taken into account. [ABSTRACT FROM AUTHOR]

In this paper we report on the design of a multilayer thermal neutron detector based on 6Li reactive foil and thin-film plastic scintillators.. The design configuration provides for a double-sided readout of the lithium foil, resulting in a doubling of the efficiency relative to a classical reactive film detector and generating a pulse height distribution with a valley between neutron and gamma signals similar to ³He tubes. The tens-of-microns thickness of the plastic scintillator limits the energy deposited by gamma rays, which provides the necessary neutron/gamma discrimination. We used MCNPX to model a multilayer lithium foil detector design and compared it with the standard high-level neutron coincidence counter HLNCC-II (18 ³He tubes operated at 4 atm). The preliminary results of the 6Li configuration show higher efficiency and one-third of the die-away time. These properties, combined with the very short dead time of the plastic scintillator, offer the potential of a very-high-performance detector. [ABSTRACT FROM AUTHOR]

A new correlation-based pulse-detection technique is presented that significantly improves the probability of detection for low signal-to-noise ratio (SNR) neutron and gamma-ray pulses generated by organic liquid scintillation detectors. Current methods utilize amplitude thresholding to detect the arrival of a pulse as well as to start acquisition of data; however, this approach faces two major limitations. First, to capture low-energy pulses the threshold amplitude should be set close to the amplitude of the noise from the electronics, and false detection is more likely to occur. Second, a large amount of measured data needs to be stored on a computer for post-processing to determine pulse characteristics—hence, the number of events that can be recorded is also limited by the hardware. In this work, we propose performing a normalized cross correlation of incoming scintillator data to a predefined pulse template, and using the correlation result to trigger the detection of a pulse. The predefined pulse template is an average of high energy pulses measured from a particular liquid scintillation detector which then can be used as a template pulse to detect low energy pulses. This technique is more robust to noise, allowing detection of lower SNR pulses. Furthermore, the cross correlation can be computed on a field programmable gate array (FPGA) using correlator hardware accelerators, so that pulses may be detected in real-time as opposed to after post-processing of acquired data. This hardware is similar to what is used for code division multiple access (CDMA) wireless communication. The correlator implemented on an FPGA also allows recognition of the pulse peaks and time-ofarrival in real-time. Therefore, we are only required to store the peak amplitude and time-ofarrival for a pulse as opposed to complete pulse waveforms—resulting in substantial reduction in required storage space. This technique is well suited for nonproliferation and homeland security applications where it is necessary to detect a large number of low SNR pulses. [ABSTRACT FROM AUTHOR]

Monitoring the amount of HEU in large (metric tons) storage arrays with passive time coincidence distribution measurements is complicated by a variety of effects. These include (1) time correlated background from HEU outside the particular localized section of the storage array being monitored, (2) cosmic radiation induced fission of nearby array materials, and (3) how large a portion of the array can be monitored with a set of detectors at a particular location in the array. Such a large storage array is typically rows of birdcages two wide and three high (metal structures that maintain safe nuclear criticality spacing between stored HEU castings) separated by 2 ft wide aisles between rows. This evaluation investigates how large an array of birdcages can be monitored by two 4 × 4 arrays of proton recoil scintillators. Monte Carlo simulations provided the time distribution of coincidences, multiplets (number of times n detection events occur in a time interval), and Feynman variance as a function of the length of the array. These calculations were performed for hypothetical arrays of 18 kg HEU standard Y-12 storage castings spaced 20 in. apart. These types of arrays are used because the spacing between HEU castings are loosely coupled in that the castings at the ends of the arrays only interact with adjacent castings; this makes monitoring of the entire array difficult with detectors at a particular location in the array. [ABSTRACT FROM AUTHOR]

International Atomic Energy Agency (IAEA) inspectors currently perform periodic inspections at uranium enrichment plants to verify UF6 cylinder enrichment declarations. Measurements are typically performed with handheld high-resolution sensors on a sampling of cylinders taken to be representative of the facility's entire product-cylinder inventory. As additional enrichment plants come online to support the expansion of nuclear power, reducing person-days of inspection time will take on greater importance. Pacific Northwest National Laboratory (PNNL) is developing a concept to automate the verification of enrichment plant cylinders to enable 100% product-cylinder verification and potentially, mass-balance calculations on the facility as a whole (by also measuring feed and tails cylinders). The Automated Cylinder Enrichment Verification System (ACEVS) could be located at key measurement points to positively identify each cylinder, measure its mass and enrichment, store the collected data in a secure database, and maintain continuity of knowledge on measured cylinders until IAEA inspector arrival. Given the potential for reduced inspector presence, the operational and manpower-reduction benefits of the portal concept are significant. The key technical question is whether the cylinder portal concept can meet, or potentially improve upon, the enrichment verification performance of today's technologies. PNNL's ACEVS concept utilizes sensors that can be operated in an unattended mode: moderated He-3 neutron detectors and large NaI(Tl) scintillators for gamma-ray spectroscopy. While the medium-resolution NaI(Tl) scintillators are a sacrifice in energy resolution, they do provide improved collection efficiency for signatures above 1 MeV. When compared to today's enrichment meter technologies that utilize only the weakly penetrating 185-keV line, the He-3/NaI sensor combination allows the exploitation of additional, more-penetrating signatures: neutrons produced from F-19(α,n) reactions (spawned primarily from U-234 alpha emission) and high-energy gamma rays (extending up to 8 MeV) induced by neutrons interacting in the steel cylinder. These indirect measures of U-235 require a relatively stable U-234/U-235 ratio in the product material in order to be useful for enrichment verification purposes. The hypothesis in ACEVS development is that the U-234/U-235 ratio is sufficiently constant at the specific facility where the automated system is installed to utilize neutron-related signatures as an indirect measure of U-235 enrichment. Further, these highly penetrating signatures can be combined with a modified form of "traditional" 185-keV enrichment measurements to meet target uncertainties for the verification of product cylinders. If successful, this new enrichment assay approach would provide two important benefits over today's handheld enrichment-meter method: 1) automation to enable 100% product-cylinder verification while reducing manpower requirements, and 2) assay of the entire cylinder volume rather than a small fraction of that volume. This paper focuses on the "non-traditional" enrichment assay aspects of the ACEVS concept: neutron and high-energy gamma-ray signatures, the radiation sensors designed to collect those signatures, and proof-of-principle cylinder measurements and analysis. Preliminary analysis indicates that an automated cylinder verification approach has the potential to meet target uncertainty values for 30B product cylinders (5%), assuming ore-based enrichment feed and a facility-specific calibration. Also described is the additional work needed to more definitively assess the concept's viability, most notably an improved understanding of the U-234/U-235 ratio variability in modern enrichment plants. [ABSTRACT FROM AUTHOR]

The 235U enrichment of uranium items which are thick compared to the attenuation length of the 185.7 keV gamma ray can be determined simply from the intensity of the line observed using a spectrometer calibrated against reference material measured with the same field of view. Corrections are needed to compensate for the difference in the attenuation coefficient between the assay item and the calibration item and also in the difference in the attenuation through the container wall. Traditionally these corrections have been calculated using simple analytical formulae based on Beer's exponential law for uncollided flux. However, the actual transport of gamma rays in the uranium matrix and container wall is more complex. In particular gamma rays, that scatter, suffering only a slight energy loss may enter the detector and be registered within the full energy peak which has a finite energy resolution. The effect of small angle and multiple scattering and its potential impact on enrichment meter measurements using scintillators has not to the authors knowledge been studied systematically from first principles. In this paper, we apply the general purpose Monte Carlo code MCNP™ to simulate the behavior of 235U emissions under measurement conditions representative of field measurements using the IMCN equipment. The contribution to the peak, both shape and intensity, is studied systematically as a function of wall thickness. The potential magnitude of the bias introduced by ignoring this effect over a working dynamic range is discussed along with options for correction for the effect within the framework of the traditional Region of Interest spectral analysis. Areas of follow-up work needed to further elucidate these effects are outlined. [ABSTRACT FROM AUTHOR]

Measurement of the neutron capture cross-sections of a number of short-lived isotopes would advance both pure and applied scientific research. These cross-sections are needed for calculation of criticality and waste production estimates for the Advanced Fuel Cycle Initiative, for analysis of data from nuclear weapons tests, and to improve understanding of nucleosynthesis. However, measurement of these cross-sections would require a detector with a faster signal decay time than those used in existing neutron capture experiments. Crystals of faster detector materials are not available in sufficient sizes and quantities to supply these large-scale experiments. Instead, we propose to use nanocomposite detectors, consisting of nanoscale particles of a scintillating material dispersed in a matrix material. We have successfully fabricated cerium uoride (CeF3) nanoparticles and dispersed them in a liquid matrix. We have characterized this scintillator and have measured its response to neutron capture. Results of the optical, structural, and radiation characterization will be presented. [ABSTRACT FROM AUTHOR]