James E. Baciak

Band Gap and Structure of Single Crystal Bii3: Resolving Discrepancies In Literature

Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coefficients (α ∼ 102 cm−1) of BiI3 that renders reflection...

A Methodology for Experimental and 3-D Computational Radiation Transport Assessments of Pu-Be Neutron Sources

Plutonium-beryllium (Pu-Be) sources can be used as didactic source materials for special nuclear materials (SNM) detection evaluation protocols. Since limited specific information exists for many of the Pu-Be sources currently in service, before using a Pu-Be source for field studies, the leakage radiation of neutrons and gamma rays from the source must be fully assessed. Most Pu-Be sources have an outer stainless steel jacket and an inner tantalum jacket, with the Pu-Be homogeneously distributed throughout the inner jacket. To fully characterize the net leakage terms from our Pu-Be source, we applied three-dimensional radiation transport computations, including Monte Carlo (MCNP5) and deterministic (PENTRAN) methodologies. The transport model for our Pu-Be capsule is based on limited schematic and technical data. To define the decay history and resulting source spectrum, exothermic [alpha-neutron (α,n)] reactions are modeled using OrigenArp in the SCALE5 package. For transport modeling purposes, the intermetallic Pu-Be compound was treated as an intimate mixture of plutonium and beryllium, based on the manufacturer’s mass specifications. The net capsule leakage was derived using transport computations, and an iterative estimation of plutonium age was performed. Computational results for net leakage are in agreement with the manufacturer’s specification of neutron yield and dose rate. We also combined computational results with experimental measurement data to fully validate our computational methods. We have successfully achieved agreement between computational and experimental data for our Pu-Be source leakage, and we are using the results at the Florida Institute of Nuclear Detection and Security to evaluate a prototype SNM neutron detector array for parcel screening and national security applications.

Positive SNM Gamma Detection Achieved Through Synthetic Enhancement of Sodium Iodide Detector Spectra

We have developed a new algorithm, ASEDRA, to post-process scintillator detector spectra to render photopeaks with high accuracy. ASEDRA, or ldquoAdvanced Synthetically Enhanced Detector Resolution Algorithm,rdquo is currently applied to NaI(Tl) detectors, which are robust, but suffer from poor energy resolution. ASEDRA rapidly post-processes a NaI(Tl) detector spectrum over a few seconds on a standard laptop without prior knowledge of sources or spectrum features. ASEDRA incorporates a novel denoising algorithm based on an adaptive Chi-square methodology called ACHIP, or ldquoAdaptive Chi-quare Processed denoising.rdquo Application of ACHIP is necessary to remove stochastic noise, yet preserve fine detail, and can be used as an independent tool for general noise reduction. Following noise removal, ASEDRA sequentially employs an adaptive detector response algorithm using detector Monte Carlo data to remove the spectrum attributed to specific gammas. Tests conducted using a 2rdquo times 2rdquo NaI(Tl) detector, along with a HPGe detector demonstrate the accuracy of ASEDRA for both photopeak identification and relative yield. In this paper, we present successful results for both WGPu and natural uranium Special Nuclear Materials (SNM) sources, comparing key photopeak energies and intensities to known values. Moreover, the denoising and synthetic resolution enhancement algorithms can be adapted to any detector. ACHIP and ASEDRA are covered under a Provisional Patent, Registration Number #60/971,770, 9/12/2007.

Using Atmospheric Dispersion Theory to Inform the Design of a Short-lived Radioactive Particle Release Experiment.

AbstractAtmospheric dispersion theory can be used to predict ground deposition of particulates downwind of a radionuclide release. This paper uses standard formulations found in Gaussian plume models to inform the design of an experimental release of short-lived radioactive particles into the atmosphere. Specifically, a source depletion algorithm is used to determine the optimum particle size and release height that maximizes the near-field deposition while minimizing both the required source activity and the fraction of activity lost to long-distance transport. The purpose of the release is to provide a realistic deposition pattern that might be observed downwind of a small-scale vent from an underground nuclear explosion. The deposition field will be used, in part, to study several techniques of gamma radiation survey and spectrometry that could be used by an On-Site Inspection team investigating such an event.

PRex: An Experiment to Investigate Detection of Near-field Particulate Deposition from a Simulated Underground Nuclear Weapons Test Vent.

AbstractA radioactive particulate release experiment to produce a near-field ground deposition representative of small-scale venting from an underground nuclear test was conducted to gather data in support of treaty capability development activities. For this experiment, a CO2‐driven “air cannon” was used to inject 140La, a radioisotope of lanthanum with 1.7‐d half-life and strong gamma-ray emissions, into the lowest levels of the atmosphere at ambient temperatures. Witness plates and air samplers were laid out in an irregular grid covering the area where the plume was anticipated to deposit based on climatological wind records. This experiment was performed at the Nevada National Security Site, where existing infrastructure, radiological procedures, and support personnel facilitated planning and execution of the work. A vehicle-mounted NaI(Tl) spectrometer and a polyvinyl toluene-based backpack instrument were used to survey the deposited plume. Hand-held instruments, including NaI(Tl) and lanthanum bromide scintillators and high purity germanium spectrometers, were used to take in situ measurements. Additionally, three soil sampling techniques were investigated and compared. The relative sensitivity and utility of sampling and survey methods are discussed in the context of on-site inspection.

COMPUTATIONAL AND EXPERIMENTAL VALIDATION OF A WGPu NEUTRON LEAKAGE SOURCE USING A SHIELDED PuBe (α, n) NEUTRON SOURCE

Abstract We propose here a unique, patented shield design that transforms the complex neutron spectrum from a plutonium-beryllium (PuBe) neutron source to nearly the precise neutron signature leaking from a sphere of weapons-grade plutonium (WGPu) material. This will facilitate testing for detection of a significant quantity of weapons plutonium without the expense or risk of testing detector components with real materials. The Monte Carlo (MCNP5) and Deterministic (PENTRAN) computational codes have been used in developing the shield assembly. A nickel composite alloy shield for a PuBe capsule has been designed, built, and laboratory-tested to enable the neutron leakage spectrum from a standard 1-Ci PuBe source (mean energy of 4.6 MeV) to be transformed, through interactions in the shield, into a very close reproduction of the neutron spectrum leaking from a large, subcritical mass of WGPu metal (average neutron energy of 2.1 MeV). Nearly all current calibrations of neutron detectors use 252Cf for generation of a fission neutron spectrum, which decays with a half-life of ~2.7 yr and is very expensive to procure. By converting to this design, PuBe sources relying on 239Pu (T1/2 = 24110 yr) and lasting hundreds of years could then be used to precisely calibrate and test detectors for simulated WGPu neutrons. Alternative custom designs are also possible with further transport-based modeling.

On Neutron Spectroscopy Using Gas Proportional Detectors Optimized by Transport Theory

Abstract We explore in this study the practical limits in designing a neutron detector array to resolve the spectra from special nuclear material (SNM) neutron sources using 3He detectors. We demonstrate that radiation transport analysis yielded a spectrum unfolding strategy based on the energy structure of the BUGLE-96 cross-section library, with 47 neutron energy groups. The initial computational model used is an isotropic planar source incident on a block of high-density polyethylene moderator. Assuming 3He is diluted throughout the moderator, the 3He(n,p) reaction rate energy group matrix in the block was computed using a completely “flat” neutron source spectrum. Analyzing the energy group matrix, there are neutrons from specific collections of energy groups (energy “bands”) that induce a maximum reaction rate in specific locations; we determined that these groups cannot be further differentiated within the energy band using 3He detectors. It was determined that optimal spectral fidelity for SNM detection and characterization is achievable using four spectral bands spanning among groups 1 through 29 (31.8 keV to 17.3 MeV). Using ideal-filter materials to remove the neutrons from different regions of the spectrum, we predicted the maximum neutron spectral resolution obtainable using this approach. To demonstrate our method, we present the optimally detected spectral differences between SNM materials (plutonium and uranium), metal and oxide, using ideal-filter materials. We have also selected a number of candidate filtering materials and, by replacing the ideal filters with real materials, we exemplified our approach with a design of a neutron detector array capable of resolving the spectra from SNM neutron sources using 3He detectors.

Improved plutonium identification and characterization results with NaI(Tl) detector using ASEDRA

The ASEDRA algorithm (Advanced Synthetically Enhanced Detector Resolution Algorithm) is a tool developed at the University of Florida to synthetically enhance the resolved photopeaks derived from a characteristically poor resolution spectra collected at room temperature from scintillator crystal-photomultiplier detector, such as a NaI(Tl) system. This work reports on analysis of a side-by-side test comparing the identification capabilities of ASEDRA applied to a NaI(Tl) detector with HPGe results for a Plutonium Beryllium (PuBe) source containing approximately 47 year old weapons-grade plutonium (WGPu), a test case of real-world interest with a complex spectra including plutonium isotopes and 241Am decay products. The analysis included a comparison of photopeaks identified and photopeak energies between the ASEDRA and HPGe detector systems, and the known energies of the plutonium isotopes. ASEDRAs performance in peak area accuracy, also important in isotope identification as well as plutonium quality and age determination, was evaluated for key energy lines by comparing the observed relative ratios of peak areas, adjusted for efficiency and attenuation due to source shielding, to the predicted ratios from known energy line branching and source isotopics. The results show that ASEDRA has identified over 20 lines also found by the HPGe and directly correlated to WGPu energies.

Iodine based compound semiconductors for room temperature gamma-ray spectroscopy

Iodine-based compound semiconductors may allow one to build a portable gamma-ray spectrometer with improved efficiency and energy resolution compared to many other portable spectrometer devices. Iodine-based semiconductors have a wide band gap that allows these detectors to operate without any cooling mechanism. Bismuth iodide (BiI3), lead iodide (PbI2) and mercuric iodide (HgI2) have theoretical gamma-ray detection efficiencies approximately 2-3 times higher than CdZnTe, the current compound semiconductor material proposed for use in several homeland/national security applications, over the range of 200-3000 keV. At 662 keV, BiI3, HgI2 and PbI2 have theoretical intrinsic photopeak efficiencies of 16.8%, 19.3% and 19.9%, respectively, while CdZnTe has a photopeak efficiency of 9.03%. In addition, gamma-ray spectrometers made from iodine-based compound semiconductor materials have demonstrated energy resolutions (FWHM) less than 2% at 662 keV. A 2% FWHM represents a significant improvement over many of todays scintillator-based radiation detectors used for homeland/national security purposes. We present some fundamental challenges in working with iodine-based semiconductors, including crystal growth issues and properties of the materials limiting radiation detector size, and the need for advanced electrode designs. Finally, we present elementary measurements illustrating the detection capabilities of iodine-based compound semiconductor materials.

3D computational and experimental radiation transport assessments of Pu-Be sources and graded moderators for parcel screening

The Florida Institute for Nuclear Detection and Security (FINDS) is currently working on the design and evaluation of a prototype neutron detector array that may be used for parcel screening systems and homeland security applications. In order to maximize neutron detector response over a wide spectrum of energies, moderator materials of different compositions and amounts are required, and can be optimized through 3-D discrete ordinates and Monte Carlo model simulations verified through measurement. Pu-Be sources can be used as didactic source materials to augment the design, optimization, and construction of detector arrays with proper characterization via transport analysis. To perform the assessments of the Pu-Be Source Capsule, 3-D radiation transport computations are used, including Monte Carlo (MCNP5) and deterministic (PENTRAN) methodologies. In establishing source geometry, we based our model on available source schematic data. Because both the MCNP5 and PENTRAN codes begin with source neutrons, exothermic (α,n) reactions are modeled using the SCALE5 code from ORNL to define the energy spectrum and the decay of the source. We combined our computational results with experimental data to fully validate our computational schemes, tools and models. Results from our computational models will then be used with experiment to generate a mosaic of the radiation spectrum. Finally, we discuss follow-up studies that highlight response optimization efforts in designing, building, and testing an array of detectors with varying moderators/thicknesses tagged to specific responses predicted using 3-D radiation transport models to augment special nuclear materials detection.